Should you edit your children’s genes?


In the fierce debate about CRISPR gene editing, it’s time to give patients a voice.

Ruthie Weiss walks home from playing basketball with her father (centre) and uncle. Her sight problems have not prevented her from excelling at sport.

Ruthie Weiss’s basketball team seemed to be minutes away from its fourth straight loss. But even as she stood on the sidelines for a brief rest, the nine-year-old had not given up. She convinced the coach to put her back in the game. Then, she charged out onto the court, caught a pass from a teammate and drove straight to the basket. Swish! Ruthie scored a quick two points, putting her team in the lead. As the game clock wound down, she scored again, clinching the victory. The team had earned its first win of the season, and celebrated as if it had just taken the national championship. A couple of parents from the opposing team even stopped by to congratulate Ruthie, who had scored all of her team’s 13 points: “Wow, she’s unbelievable!” they told her mum and dad.

What makes Ruthie’s performance even more extraordinary is her DNA. Because of a misspelling in one of her genes, she has albinism: her body produces very little of the pigment melanin, which means that her skin and hair are fair, and that she is legally blind. Her visual acuity is ten times worse than average. She is still learning to read and will probably never be able to drive a car, but she can make out the basket and her teammates well enough to shoot, pass and play.

In January, Ruthie’s dad Ethan asked her whether she wished that her parents had corrected the gene responsible for her blindness before she was born. Ruthie didn’t hesitate before answering — no. Would she ever consider editing the genes of her own future children to help them to see? Again, Ruthie didn’t blink — no.

The answer made Ethan Weiss, a physician–scientist at the University of California, San Francisco, think. Weiss is well aware of the rapid developments in gene-editing technologies — techniques that could, theoretically, prevent children from being born with deadly disorders or with disabilities such as Ruthie’s. And he believes that if he had had the option to edit blindness out of Ruthie’s genes before she was born, he and his wife would have jumped at the chance. But now he thinks that would have been a mistake: doing so might have erased some of the things that make Ruthie special — her determination, for instance. Last season, when Ruthie had been the worst player on her basketball team, she had decided on her own to improve, and unbeknownst to her parents had been practising at every opportunity. Changing her disability, he suspects, “would have made us and her different in a way that we would have regretted”, he says. “That’s scary.”

Ethan and Ruthie are not the only people pondering these kinds of questions. The emergence of a powerful gene-editing technology, known as CRISPR–Cas9, has elicited furious debate about whether and how it might be used to modify the genomes of human embryos. The changes to their genomes would almost certainly be passed down to subsequent generations, breaching an ethical line that has typically been considered uncrossable.

But emerging technologies are already testing the margins of what people deem acceptable. Parents today have unprecedented control over what they pass on to their children: they can use prenatal genetic screening to check for conditions such as Down’s syndrome, and choose whether or not to carry a fetus to term. Preimplantation genetic diagnosis allows couples undergoing in vitro fertilization to select embryos that do not have certain disease-causing mutations. Even altering the heritable genome — as might be done if CRISPR were used to edit embryos — is acceptable to some. Mitochondrial replacement therapy, which replaces a very small number of genes that a mother passes on with those from a donor, was approved last year in the United Kingdom for people who are at risk of certain genetic disorders.

Many safety, technical and legal barriers still stand in the way of editing DNA in human embryos. But some scientists and ethicists say that it is important to think through the implications of embryo editing now — before these practical hurdles are overcome. What sort of world would these procedures create for those currently living with disease and for future generations?

So far, little has been heard from the people who could be first affected by the technology — but speaking with these communities reveals a diverse set of views. Some are impatient, and say that there is a duty to use genome editing quickly to eliminate serious, potentially fatal conditions. Some doubt that society will embrace it to the degree that many have feared, or hoped. Above all, people such as Ethan Weiss caution that if policymakers do not consult people with disabilities and their families, the technology could be used unthinkingly, in ways that harm patients and society, today and in the future.

“Hearing the voices of people who live with these conditions is really important,” says Tom Shakespeare, a medical sociologist at the University of East Anglia in Norwich, UK.

The cases for

John Sabine, now 60, was once described as one of the brightest legal minds of his generation in England. Now, he is in the advanced stages of Huntington’s disease: he cannot walk or talk, is incontinent and requires constant care. Charles Sabine, his younger brother, carries the same genetic glitch that causes Huntington’s disease, and therefore knows that, like his brother and his father before him, he is destined to undergo the same deterioration of brain and body.

Charles and his brother have five children between them, each of whom as a 50% chance of having inherited the mutation that causes Huntington’s disease. To Charles — and to many others who live with the mutation that causes Huntington’s — there is no legitimate ethical argument about whether gene editing should be used, either to treat people living with the condition now or to spare their children from it.

“Anyone who has to actually face the reality of one of these diseases is not going to have a remote compunction about thinking that there is any moral issue at all,” Sabine says. “If there was a room somewhere where someone said, ‘Look, you can go in there and have your DNA changed,’ I would be there breaking the door down.”

Matt Wilsey, a technology entrepreneur in San Francisco, would be there too. His daughter Grace was one of the first people in the world to be diagnosed with a disease caused by a mutation in the gene NGLY1, which makes it difficult for her cells to get rid of misshapen proteins. Grace, now six years old, has severe movement and developmental disabilities. She can barely walk and cannot talk. Because her condition is new to medicine, doctors cannot even predict how long she might live.

Wilsey is bullish on CRISPR. He says that if he had had the chance to detect and fix the mutation in Grace’s genome before she was born, he would have. But he is frustrated that the debate over editing embryos seems to have monopolized discussions about the technology. He is hopeful that a gene-therapy-like approach using CRISPR, which would be free of the ethical concerns about altering the genes she passes on, could help Grace within several years. And he wonders whether a temporary moratorium on embryo editing might allow the field to focus on such approaches sooner.

“As a parent with an incredibly sick child, what are we supposed to do — sit by on the sidelines while my child dies? There’s zero chance of that,” Wilsey says. “CRISPR is a bullet train that has left the station — there’s no stopping it, so how can we harness it for good?”

A meeting convened in December 2015 by the US national academies of sciences and medicine, the Chinese Academy of Sciences and the Royal Society of London recommended such a moratorium in light of multiple safety and ethical concerns. Still, many bioethicists and scientists have argued that if defects in single genes causing fatal and debilitating conditions could be corrected in an embryo, then they should be. Shakespeare notes that embryo editing for conditions that cause major disability and death are likely to raise less concern and criticism in the long term. But, he says: “As soon as you get away from the archetypal terrible condition, then you’ve got a debate about whether a condition makes life unbearably hard.”

Social consequences

Many people are concerned about where that line would be drawn. Although it may seem now that only a few, very severe conditions should be subject to gene editing, disability activists point out that the list of conditions considered as illnesses, and possibly subject to medical treatment, is expanding. “More and more, people think of obesity or predisposition to alcoholism as disease,” says Carol Padden, a linguist at the University of California, San Diego. She herself is deaf, and points out that many deaf people do not consider it a disability. This stance has led to controversy when, for instance, deaf parents decline technologies such as cochlear implants for their children, or even go so far as to select, through processes such as preimplantation genetic diagnosis, children who will be deaf.

Like Padden, some disability-studies researchers do not oppose the idea of gene editing, but do think that society needs to understand that it is not possible to eliminate all disability, and that humans might lose something important if they try to do so.

“These kinds of interactions significantly change our attitudes about what kinds of people matter in the world.”

Padden points out that accommodations originally intended for people with disabilities often end up benefiting everyone. For example, the development of closed captioning — subtitles for the hearing-impaired on television — required major advocacy from the deaf community and legislative action to get off the ground in the United States in the 1970s. Today, people rely on it in ways that no one could have foreseen, such as in noisy airports and sports bars. Some people use it to learn to read or to learn a language.

Rosemarie Garland-Thomson, a literature scholar and co-director of the Disability Studies Initiative at Emory University in Atlanta, Georgia, adds that legislative mandates, such as the 1990 Americans with Disabilities Act in the United States, have helped to integrate people with disabilities into society — in workplaces, schools and other public spaces. As a result, the world is much more humane for everyone, says Garland-Thomson. “These kinds of interactions significantly change our attitudes about what kinds of people matter in the world.”

The idea that parents should edit out characteristics that are considered debilitating goes against this drive towards inclusion, Garland-Thomson warns, and could create a harsher social climate for everyone. The experience of disability, she adds, is universal; all people inevitably experience sickness, accidents and age-related decline. “At our peril, we are right now trying to decide what ways of being in the world ought to be eliminated,” she says.

Padden says that ethicists, patients and disabilities-studies researchers must work urgently to make a broad societal case in favour of greater acceptance of diversity. This has been a long-fought battle, and many see evidence of progress — for instance, in the ‘neurodiversity’ movement, which champions the idea that medical conditions such as autism are part of the spectrum of human variation. “We do have to start coming up with more arguments for diversity, and quickly, because CRISPR is coming upon us faster than some of us are thinking about this issue,” she says.

Making choices

The prospect of editing the genome of a human embryo is still in its early stages, but the ability to prevent the inheritance of some conditions already exists. Prenatal screening, which has advanced to the point that doctors can sample a developing fetus’s DNA through its mother’s blood, has given parents the option to terminate pregnancies when a disease or disability is diagnosed. This has already started to show limited effects on the population.

In Europe, for example, the prevalence of a Down’s syndrome diagnosis during pregnancy has risen from 20 cases per 10,000 in 1990 to 23 cases per 10,000 today, as the average age of women having babies has increased. But the number of children born with the syndrome has stayed level at about 11 per 10,000, because many women whose fetuses are diagnosed with the condition terminate their pregnancies. In the United States, pregnancies in which a Down’s syndrome diagnosis is made are terminated in 67–85% of cases.

By surveying women whose fetuses and babies are diagnosed with Down’s syndrome, and by compiling similar surveys from around the world, medical geneticist Brian Skotko of the Massachusetts General Hospital in Boston has found that doctors sometimes advise women to terminate or give up for adoption babies diagnosed prenatally with Down’s syndrome. They can influence the decision by using phrases such as “I’m sorry”, or “I have some bad news to share”1. For instance, 34% of 71 Dutch women who terminated their pregnancy after a Down’s syndrome diagnosis said that their doctors did not even mention the possibility of carrying the pregnancy to term when discussing their options2.

Mark Leach, a lawyer in Louisville, Kentucky, whose 11-year-old daughter has Down’s syndrome, says that he and his wife have been asked many times — especially when his wife was pregnant with their second child — whether they “knew beforehand” that Juliet would be born with Down’s. (They didn’t.) Some people are simply curious, Leach says, but for others, there’s judgement in that question. “The ability to do something beforehand imposes a sense of, ‘You should do not only what’s right for you, but what’s right for society’,” Leach says. It bothers him, he says, that although government and private health insurers routinely pay for prenatal diagnosis, he recently learned that his school system is discontinuing support for the learning specialist who had been helping Juliet to thrive in mathematics and reading.

Dorothy Roberts, a professor of law and sociology at the University of Pennsylvania in Philadelphia, says that this kind of pressure is troubling and that it could get worse if embryo editing were to become readily available. “Women should not be given the responsibility of ensuring the genetic fitness of their children based on lack of support for children with disabilities.”

Leach knows that children with disabilities can live rich lives. Juliet likes ballet dancing and horse-riding, and she is especially attuned to the names of people and animals whom she knows. And Leach says that she helps to remind other people how to care for others. “The main thing that would be lost if Down’s syndrome continues to diminish is a diminishment in the amount of compassion that is shown in this world,” he says.

Even among people who already have life-threatening conditions, many choose not to interfere with the way the genetic cards are dealt. Edward Wild, a neurologist who cares for people with Huntington’s disease at University College London, estimates that fewer than 5% of patients in the United Kingdom use preimplantation genetic screening to select embryos that lack the disease-causing mutation and so avoid passing it to their children. Some people do not know that they have the mutation; some decide against screening because of the costs or risks involved; some have personal or moral objections to the technique; and some just have a sense that 50–50 odds of passing down the disease are not so bad.

“Having kids the fun way is still much more popular than having kids the science way, even though the latter is how you guarantee the kid is free of Huntington’s disease,” Wild says.

“Prediction: my grandchildren will be embryo-screened, germline-edited. Won’t ‘change what it means to be human’. It’ll be like vaccination.”

Even if gene editing were safe, effective and everyone opted to use it, it would not eliminate genetic diseases, because researchers still have a long way to go to understand the genes involved. Even Huntington’s, which is fairly well characterized, is no easy target. The glitch that causes it is a repeat of a particular genetic sequence; the more repeats, the more severe the symptoms, and repeats are added with each successive generation. New families are diagnosed with Huntington’s all the time, either because the disease is misdiagnosed in older generations or because symptoms worsen, and become recognizable, in subsequent generations. Although he is working on genetic techniques to treat Huntingon’s, Wild doesn’t hold out high hopes for a future free of the disease. “Although it’s nice to think about, it’s little more than a dream,” he says.

Human biology can complicate things in other ways. Padden notes, for instance, that some mutations that predispose to genetic disease, such as the sickle-cell mutation, confer population-level benefits, such as resistance to malaria. So editing out one disease could backfire by increasing the risk of another. She argues that very little is known about the potential benefits of other mutations associated with disease, and applying genome editing too freely could have unintended consequences.

And if it were adopted, the technology would almost certainly be applied unevenly around the world. Aleksa Owen, a sociologist at the University of Illinois at Chicago, predicts that genome editing would be used first in countries that approve of and support assisted reproductive technologies, such as the United Kingdom, some other European Union countries, China and Israel. But it would probably be too expensive for many people in developing countries.

Uneven access

Still, some scientists predict that editing human embryos could have transformative effects. During the National Academies’ summit on gene editing in December, Harvard University geneticist Dan MacArthur tweeted, “Prediction: my grandchildren will be embryo-screened, germline-edited. Won’t ‘change what it means to be human’. It’ll be like vaccination.”

Sandy Sufian, a historian of medicine and disability at the University of Illinois, agrees with MacArthur that CRISPR has the potential to become widely adopted, both because of the perception that it would save money that would otherwise be spent caring for disabled people and because of people’s fear of disability. But she questions the idea that eliminating such conditions will necessarily improve human life. Sufian has cystic fibrosis, a disease caused by mutations that render her lung cells more vulnerable to infection and disease. She spends 40 hours a week inhaling medicine to clear her lungs of mucus, exercising and undergoing physical therapy; others have to quit their jobs to make sufficient time for treatments. Yet given the option to edit cystic fibrosis out of her bloodline, Sufian wouldn’t do it. “There are some great things that come from having a genetic illness,” she says.

Garland-Thomson echoes that sentiment; she has one and a half arms and six fingers because of a condition called limb-reduction disorder. She says that she values traits in herself that she may have developed as adaptations to the condition: she is very sociable and wonders if that is because she’s had to learn to work hard to make others feel comfortable around her. “Any kinds of restrictions or limitations have created the opportunity for me to develop work-arounds,” Garland-Thomson says.

Shakespeare, who has achondroplasia, a genetic condition that causes shorter than average stature, says that people with disabilities are just as able to attain life satisfaction as others. “I have achieved everything I hoped for in life, despite having restricted growth: career, children, friendship and love.” He wouldn’t want to have altered his own genes to be taller, he says.

Disability rights

People without disabilities consistently underestimate the life satisfaction of those with them. Although people with disabilities report a slightly lower overall quality of life than those without, the difference is small. One study3 found that half of people with serious disabilities ranked their quality of life as ‘good’ or ‘excellent’.

People also overestimate how severely health affects their happiness compared with other factors, such as economic or social support. One 1978 study4, for instance, compared people who had recently become paralysed as a result of accidents with people who had recently won between US$50,000 and $1 million in a state lottery. Although people who had had accidents ranked their happiness lower than lottery winners, both groups predicted that their future happiness would be roughly equal, and people who had accidents derived more pleasure from everyday activities, such as eating breakfast or talking to a friend.

“A lot of this terrific science and technology has to take into account that the assumption of what life is like for people who are different is based on prejudice against disability,” says Lennard Davis, a disability-studies researcher at the University of Illinois, who was raised by two deaf parents.

There is a common saying among people in the disability-rights community: “Nothing about us without us.” People with disabilities argue that scientists, policymakers and bioethicists should take steps to ensure that the CRISPR debate reflects what is best for patients and their families, to ensure its most humane use now and for future generations.

At a minimum, they say, the investment in developing CRISPR should be matched by investments in innovations to help people who are already living with conditions that cause disability. And it is essential that people with the conditions that are up for consideration as possible CRISPR targets should be included in the decision-making processes.

For their part, Ruthie Weiss and her dad have already made up their minds. Ruthie must work harder than her classmates to do some routine activities. But when she is dominating the basketball court, or practising the piano, or skiing down a mountain, Ethan Weiss doesn’t see a child with a disability. He sees his daughter making the most of her life, given all her strengths and challenges. And he knows that he wouldn’t change a thing.

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CRISPR: gene editing is just the beginning


The real power of the biological tool lies in exploring how genomes work.

Molecular biologists are riding a wave of new technologies made possible by CRISPR.

Whenever a paper about CRISPR–Cas9 hits the press, the staff at Addgene quickly find out. The non-profit company is where study authors often deposit molecular tools that they used in their work, and where other scientists immediately turn to get them. It is also where other scientists immediately turn to get their hands on these reagents. “We get calls within minutes of a hot paper publishing,” says Joanne Kamens, executive director of the company in Cambridge, Massachusetts.

Addgene’s phones have been ringing a lot since early 2013, when researchers first reported1, 2, 3 that they had used the CRISPR–Cas9 system to slice the genome in human cells at sites of their choosing. “It was all hands on deck,” Kamens says. Since then, molecular biologists have rushed to adopt the technique, which can be used to alter the genome of almost any organism with unprecedented ease and finesse. Addgene has sent 60,000 CRISPR-related molecular tools — about 17% of its total shipments — to researchers in 83 countries, and the company’s CRISPR-related pages were viewed more than one million times in 2015.

Much of the conversation about CRISPR–Cas9 has revolved around its potential for treating disease or editing the genes of human embryos, but researchers say that the real revolution right now is in the lab. What CRISPR offers, and biologists desire, is specificity: the ability to target and study particular DNA sequences in the vast expanse of a genome. And editing DNA is just one trick that it can be used for. Scientists are hacking the tools so that they can send proteins to precise DNA targets to toggle genes on or off, and even engineer entire biological circuits — with the long-term goal of understanding cellular systems and disease.

“For the humble molecular biologist, it’s really an extraordinarily powerful way to understand how the genome works,” says Daniel Bauer, a haematologist at the Boston Children’s Hospital in Massachusetts. “It’s really opened the number of questions you can address,” adds Peggy Farnham, a molecular biologist at the University of Southern California, Los Angeles. “It’s just so fun.”

Here, Nature examines five ways in which CRISPR–Cas9 is changing how biologists can tinker with cells.

Broken scissors

There are two chief ingredients in the CRISPR–Cas9 system: a Cas9 enzyme that snips through DNA like a pair of molecular scissors, and a small RNA molecule that directs the scissors to a specific sequence of DNA to make the cut. The cell’s native DNA repair machinery generally mends the cut — but often makes mistakes.

That alone is a boon to scientists who want to disrupt a gene to learn about what it does. The genetic code is merciless: a minor error introduced during repair can completely alter the sequence of the protein it encodes, or halt its production altogether. As a result, scientists can study what happens to cells or organisms when the protein or gene is hobbled.

But there is also a different repair pathway that sometimes mends the cut according to a DNA template. If researchers provide the template, they can edit the genome with nearly any sequence they desire at nearly any site of their choosing.

In 2012, as laboratories were racing to demonstrate how well these gene-editing tools could cut human DNA, one team decided to take a different approach. “The first thing we did: we broke the scissors,” says Jonathan Weissman, a systems biologist at the University of California, San Francisco (UCSF).

Weissman learned about the approach from Stanley Qi, a synthetic biologist now at Stanford University in California, who mutated the Cas9 enzyme so that it still bound DNA at the site that matched its guide RNA, but no longer sliced it. Instead, the enzyme stalled there and blocked other proteins from transcribing that DNA into RNA. The hacked system allowed them to turn a gene off, but without altering the DNA sequence4.

The team then took its ‘dead’ Cas9 and tried something new: the researchers tethered it to part of another protein, one that activates gene expression. With a few other tweaks, they had built a way to turn genes on and off at will5.

Several labs have since published variations on this method; many more are racing to harness it for their research6 (see ‘Hacking CRISPR’). One popular application is to rapidly generate hundreds of different cell lines, each containing a different guide RNA that targets a particular gene. Martin Kampmann, another systems biologist at UCSF, hopes to screen such cells to learn whether flipping certain genes on or off affects the survival of neurons exposed to toxic protein aggregates — a mechanism that is thought to underlie several neurodegenerative conditions, including Alzheimer’s disease. Kampmann had been carrying out a similar screen with RNA interference (RNAi), a technique that also silences genes and can process lots of molecules at once, but which has its drawbacks. “RNAi is a shotgun with well-known off-target effects,” he says. “CRISPR is the scalpel that allows you to be more specific.”

Nik Spencer/Nature

Weissman and his colleagues, including UCSF systems biologist Wendell Lim, further tweaked the method so that it relied on a longer guide RNA, with motifs that bound to different proteins. This allowed them to activate or inhibit genes at three different sites all in one experiment7. Lim thinks that the system can handle up to five operations at once. The limit, he says, may be in how many guide RNAs and proteins can be stuffed into a cell. “Ultimately, it’s about payload.”

That combinatorial power has drawn Ron Weiss, a synthetic biologist at the Massachusetts Institute of Technology (MIT) in Cambridge, into the CRISPR–Cas9 frenzy. Weiss and his colleagues have also created multiple gene tweaks in a single experiment8, making it faster and easier to build complicated biological circuits that could, for example, convert a cell’s metabolic machinery into a biofuel factory. “The most important goal of synthetic biology is to be able to program complex behaviour via the creation of these sophisticated circuits,” he says.

CRISPR epigenetics

When geneticist Marianne Rots began her career, she wanted to unearth new medical cures. She studied gene therapy, which targets genes mutated in disease. But after a few years, she decided to change tack. “I reasoned that many more diseases are due to disturbed gene-expression profiles, not so much the single genetic mutations I had been focused on,” says Rots, at the University Medical Center Groningen in the Netherlands. The best way to control gene activity, she thought, was to adjust the epigenome, rather than the genome itself.

The epigenome is the constellation of chemical compounds tacked onto DNA and the DNA-packaging proteins called histones. These can govern access to DNA, opening it up or closing it off to the proteins needed for gene expression. The marks change over time: they are added and removed as an organism develops and its environment shifts.

In the past few years, millions of dollars have been poured into cataloguing these epigenetic marks in different human cells, and their patterns have been correlated with everything from brain activity to tumour growth. But without the ability to alter the marks at specific sites, researchers are unable to determine whether they cause biological changes. “The field has met a lot of resistance because we haven’t had the kinds of tools that geneticists have had, where they can go in and directly test the function of a gene,” says Jeremy Day, a neuroscientist at the University of Alabama at Birmingham.

CRISPR–Cas9 could turn things around. In April 2015, Charles Gersbach, a bioengineer at Duke University in Durham, North Carolina, and his colleagues published9 a system for adding acetyl groups — one type of epigenetic mark — to histones using the broken scissors to carry enzymes to specific spots in the genome.

The team found that adding acetyl groups to proteins that associate with DNA was enough to send the expression of targeted genes soaring, confirming that the system worked and that, at this location, the epigenetic marks had an effect. When he published the work, Gersbach deposited his enzyme with Addgene so that other research groups could use it — and they quickly did. Gersbach predicts that a wave of upcoming papers will show a synergistic effect when multiple epigenetic markers are manipulated at once.

The tools need to be refined. Dozens of enzymes can create or erase an epigenetic mark on DNA, and not all of them have been amenable to the broken-scissors approach. “It turned out to be harder than a lot of people were expecting,” says Gersbach. “You attach a lot of things to a dead Cas9 and they don’t happen to work.” Sometimes it is difficult to work out whether an unexpected result arose because a method did not work well, or because the epigenetic mark simply doesn’t matter in that particular cell or environment.

Rots has explored the function of epigenetic marks on cancer-related genes using older editing tools called zinc-finger proteins, and is now adopting CRISPR–Cas9. The new tools have democratized the field, she says, and that has already had a broad impact. People used to say that the correlations were coincidental, Rots says — that if you rewrite the epigenetics it will have no effect on gene expression. “But now that it’s not that difficult to test, a lot of people are joining the field.”

CRISPR code cracking

Epigenetic marks on DNA are not the only genomic code that is yet to be broken. More than 98% of the human genome does not code for proteins. But researchers think that a fair chunk of this DNA is doing something important, and they are adopting CRISPR–Cas9 to work out what that is.

Some of it codes for RNA molecules — such as microRNAs and long non-coding RNAs — that are thought to have functions apart from making proteins. Other sequences are ‘enhancers’ that amplify the expression of the genes under their command. Most of the DNA sequences linked to the risk of common diseases lie in regions of the genome that contain non-coding RNA and enhancers. But before CRISPR–Cas9, it was difficult for researchers to work out what those sequences do. “We didn’t have a good way to functionally annotate the non-coding genome,” says Bauer. “Now our experiments are much more sophisticated.”

Farnham and her colleagues are using CRISPR–Cas9 to delete enhancer regions that are found to be mutated in genomic studies of prostate and colon cancer. The results have sometimes surprised her. In one unpublished experiment, her team deleted an enhancer that was thought to be important, yet no gene within one million bases of it changed expression. “How we normally classify the strength of a regulatory element is not corresponding with what happens when you delete that element,” she says.

“I wish I had had this technology sooner. My postdoc would have been a lot shorter.”

More surprises may be in store as researchers harness CRISPR–Cas9 to probe large stretches of regulatory DNA. Groups led by geneticists David Gifford at MIT and Richard Sherwood at the Brigham and Women’s Hospital in Boston used the technique to create mutations across a 40,000-letter sequence, and then examined whether each change had an effect on the activity of a nearby gene that made a fluorescent protein10. The result was a map of DNA sequences that enhanced gene expression, including several that had not been predicted on the basis of gene regulatory features such as chromatin modifications.

Delving into this dark matter has its challenges, even with CRISPR–Cas9. The Cas9 enzyme will cut where the guide RNA tells it to, but only if a specific but common DNA sequence is present near the cut site. This poses little difficulty for researchers who want to silence a gene, because the key sequences almost always exist somewhere within it. But for those who want to make very specific changes to short, non-coding RNAs, the options can be limited. “We cannot take just any sequence,” says Reuven Agami, a researcher at the Netherlands Cancer Institute in Amsterdam.

Researchers are scouring the bacterial kingdom for relatives of the Cas9 enzyme that recognize different sequences. Last year, the lab of Feng Zhang, a bioengineer at the Broad Institute of MIT and Harvard in Cambridge, characterized a family of enzymes called Cpf1 that work similarly to Cas9 and could expand sequence options11. But Agami notes that few alternative enzymes found so far work as well as the most popular Cas9. In the future, he hopes to have a whole collection of enzymes that can be targeted to any site in the genome. “We’re not there yet,” he says.

CRISPR sees the light

Gersbach’s lab is using gene-editing tools as part of an effort to understand cell fate and how to manipulate it: the team hopes one day to grow tissues in a dish for drug screening and cell therapies. But CRISPR–Cas9’s effects are permanent, and Gersbach’s team needed to turn genes on and off transiently, and in very specific locations in the tissue. “Patterning a blood vessel demands a high degree of control,” he says.

Gersbach and his colleagues took their broken, modified scissors — the Cas9 that could now activate genes — and added proteins that are activated by blue light. The resulting system triggers gene expression when cells are exposed to the light, and stops it when the light is flicked off12. A group led by chemical biologist Moritoshi Sato of the University of Tokyo rigged a similar system13, and also made an active Cas9 that edited the genome only after it was hit with blue light14.

Others have achieved similar ends by combining CRISPR with a chemical switch. Lukas Dow, a cancer geneticist at Weill Cornell Medical College in New York City, wanted to mutate cancer-related genes in adult mice, to reproduce mutations that have been identified in human colorectal cancers. His team engineered a CRISPR–Cas9 system in which a dose of the compound doxycycline activates Cas9, allowing it to cut its targets15.

The tools are another step towards gaining fine control over genome editing. Gersbach’s team has not patterned its blood vessels just yet: for now, the researchers are working on making their light-inducible system more efficient. “It’s a first-generation tool,” says Gersbach.

Model CRISPR

Cancer researcher Wen Xue spent the first years of his postdoc career making a transgenic mouse that bore a mutation found in some human liver cancers. He slogged away, making the tools necessary for gene targeting, injecting them into embryonic stem cells and then trying to derive mice with the mutation. The cost: a year and US$20,000. “It was the rate-limiting step in studying disease genes,” he says.

A few years later, just as he was about to embark on another transgenic-mouse experiment, his mentor suggested that he give CRISPR–Cas9 a try. This time, Xue just ordered the tools, injected them into single-celled mouse embryos and, a few weeks later — voilá. “We had the mouse in one month,” says Xue. “I wish I had had this technology sooner. My postdoc would have been a lot shorter.”

Researchers who study everything from cancer to neurodegeneration are embracing CRISPR-Cas9 to create animal models of the diseases. It lets them engineer more animals, in more complex ways, and in a wider range of species. Xue, who now runs his own lab at the University of Massachusetts Medical School in Worcester, is systematically sifting through data from tumour genomes, using CRISPR–Cas9 to model the mutations in cells grown in culture and in animals.

Researchers are hoping to mix and match the new CRISPR–Cas9 tools to precisely manipulate the genome and epigenome in animal models. “The real power is going to be the integration of those systems,” says Dow. This may allow scientists to capture and understand some of the complexity of common human diseases.

Take tumours, which can bear dozens of mutations that potentially contribute to cancer development. “They’re probably not all important in terms of modelling a tumour,” says Dow. “But it’s very clear that you’re going to need two or three or four mutations to really model aggressive disease and get closer to modelling human cancer.” Introducing all of those mutations into a mouse the old-fashioned way would have been costly and time-consuming, he adds.

Bioengineer Patrick Hsu started his lab at the Salk Institute for Biological Studies in La Jolla, California, in 2015; he aims to use gene editing to model neurodegenerative conditions such as Alzheimer’s disease and Parkinson’s disease in cell cultures and marmoset monkeys. That could recapitulate human behaviours and progression of disease more effectively than mouse models, but would have been unthinkably expensive and slow before CRISPR–Cas9.

Even as he designs experiments to genetically engineer his first CRISPR–Cas9 marmosets, Hsu is aware that this approach may be only a stepping stone to the next. “Technologies come and go. You can’t get married to one,” he says. “You need to always think about what biological problems need to be solved.”

CRISPR, the disruptor


A powerful gene-editing technology is the biggest game changer to hit biology since PCR. But with its huge potential come pressing concerns.

Three years ago, Bruce Conklin came across a method that made him change the course of his lab.

Conklin, a geneticist at the Gladstone Institutes in San Francisco, California, had been trying to work out how variations in DNA affect various human diseases, but his tools were cumbersome. When he worked with cells from patients, it was hard to know which sequences were important for disease and which were just background noise. And engineering a mutation into cells was expensive and laborious work. “It was a student’s entire thesis to change one gene,” he says.

Then, in 2012, he read about a newly published technique1 called CRISPR that would allow researchers to quickly change the DNA of nearly any organism — including humans. Soon after, Conklin abandoned his previous approach to modelling disease and adopted this new one. His lab is now feverishly altering genes associated with various heart conditions. “CRISPR is turning everything on its head,” he says.

The sentiment is widely shared: CRISPR is causing a major upheaval in biomedical research. Unlike other gene-editing methods, it is cheap, quick and easy to use, and it has swept through labs around the world as a result. Researchers hope to use it to adjust human genes to eliminate diseases, create hardier plants, wipe out pathogens and much more besides. “I’ve seen two huge developments since I’ve been in science: CRISPR and PCR,” says John Schimenti, a geneticist at Cornell University in Ithaca, New York. Like PCR, the gene-amplification method that revolutionized genetic engineering after its invention in 1985, “CRISPR is impacting the life sciences in so many ways,” he says.

“This power is so easily accessible by labs — you don’t need a very expensive piece of equipment and people don’t need to get many years of training to do this,” says Stanley Qi, a systems biologist at Stanford University in California. “We should think carefully about how we are going to use that power.”

Research revolution

Biologists have long been able to edit genomes with molecular tools. About ten years ago, they became excited by enzymes called zinc finger nucleases that promised to do this accurately and efficiently. But zinc fingers, which cost US$5,000 or more to order, were not widely adopted because they are difficult to engineer and expensive, says James Haber, a molecular biologist at Brandeis University in Waltham, Massachusetts. CRISPR works differently: it relies on an enzyme called Cas9 that uses a guide RNA molecule to home in on its target DNA, then edits the DNA to disrupt genes or insert desired sequences. Researchers often need to order only the RNA fragment; the other components can be bought off the shelf. Total cost: as little as $30. “That effectively democratized the technology so that everyone is using it,” says Haber. “It’s a huge revolution.”

CRISPR methodology is quickly eclipsing zinc finger nucleases and other editing tools (see ‘The rise of CRISPR’). For some, that means abandoning techniques they had taken years to perfect. “I’m depressed,” says Bill Skarnes, a geneticist at the Wellcome Trust Sanger Institute in Hinxton, UK, “but I’m also excited.” Skarnes had spent much of his career using a technology introduced in the mid-1980s: inserting DNA into embryonic stem cells and then using those cells to generate genetically modified mice. The technique became a laboratory workhorse, but it was also time-consuming and costly. CRISPR takes a fraction of the time, and Skarnes adopted the technique two years ago.

Publications: Scopus; Patents: The Lens; Funding: NIH RePORTER.

Researchers have traditionally relied heavily on model organisms such as mice and fruit flies, partly because they were the only species that came with a good tool kit for genetic manipulation. Now CRISPR is making it possible to edit genes in many more organisms. In April, for example, researchers at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, reported using CRISPR to study Candida albicans, a fungus that is particularly deadly in people with weakened immune systems, but had been difficult to genetically manipulate in the lab3. Jennifer Doudna, a CRISPR pioneer at the University of California, Berkeley, is keeping a list of CRISPR-altered creatures. So far, she has three dozen entries, including disease-causing parasites called trypanosomes and yeasts used to make biofuels.

Yet the rapid progress has its drawbacks. “People just don’t have the time to characterize some of the very basic parameters of the system,” says Bo Huang, a biophysicist at the University of California, San Francisco. “There is a mentality that as long as it works, we don’t have to understand how or why it works.” That means that researchers occasionally run up against glitches. Huang and his lab struggled for two months to adapt CRISPR for use in imaging studies. He suspects that the delay would have been shorter had more been known about how to optimize the design of guide RNAs, a basic but important nuance.

By and large, researchers see these gaps as a minor price to pay for a powerful technique. But Doudna has begun to have more serious concerns about safety. Her worries began at a meeting in 2014, when she saw a postdoc present work in which a virus was engineered to carry the CRISPR components into mice. The mice breathed in the virus, allowing the CRISPR system to engineer mutations and create a model for human lung cancer4. Doudna got a chill; a minor mistake in the design of the guide RNA could result in a CRISPR that worked in human lungs as well. “It seemed incredibly scary that you might have students who were working with such a thing,” she says. “It’s important for people to appreciate what this technology can do.”

Andrea Ventura, a cancer researcher at Memorial Sloan Kettering Cancer Center in New York and a lead author of the work, says that his lab carefully considered the safety implications: the guide sequences were designed to target genome regions that were unique to mice, and the virus was disabled such that it could not replicate. He agrees that it is important to anticipate even remote risks. “The guides are not designed to cut the human genome, but you never know,” he says. “It’s not very likely, but it still needs to be considered.”

Editing out disease

Last year, bioengineer Daniel Anderson of the Massachusetts Institute of Technology in Cambridge and his colleagues used CRISPR in mice to correct a mutation associated with a human metabolic disease called tyrosinaemia5. It was the first use of CRISPR to fix a disease-causing mutation in an adult animal — and an important step towards using the technology for gene therapy in humans (see ‘A brief history of CRISPR’).

The idea that CRISPR could accelerate the gene-therapy field is a major source of excitement in scientific and biotechnology circles. But as well as highlighting the potential, Anderson’s study showed how far there is to go. To deliver the Cas9 enzyme and its guide RNA into the target organ, the liver, the team had to pump large volumes of liquid into blood vessels — something that is not generally considered feasible in people. And the experiments corrected the disease-causing mutation in just 0.4% of the cells, which is not enough to have an impact on many diseases.

Over the past two years, a handful of companies have sprung up to develop CRISPR-based gene therapy, and Anderson and others say that the first clinical trials of such a treatment could happen in the next one or two years. Those first trials will probably be scenarios in which the CRISPR components can be injected directly into tissues, such as those in the eye, or in which cells can be removed from the body, engineered in the lab and then put back. For example, blood-forming stem cells might be corrected to treat conditions such as sickle-cell disease or β-thalassaemia. It will be a bigger challenge to deliver the enzyme and guide RNA into many other tissues, but researchers hope that the technique could one day be used to tackle a wider range of genetic diseases.

Yet many scientists caution that there is much to do before CRISPR can be deployed safely and efficiently. Scientists need to increase the efficiency of editing, but at the same time make sure that they do not introduce changes elsewhere in the genome that have consequences for health. “These enzymes will cut in places other than the places you have designed them to cut, and that has lots of implications,” says Haber. “If you’re going to replace somebody’s sickle-cell gene in a stem cell, you’re going to be asked, ‘Well, what other damage might you have done at other sites in the genome?’”

Keith Joung, who studies gene editing at Massachusetts General Hospital in Boston, has been developing methods to hunt down Cas9’s off-target cuts. He says that the frequency of such cuts varies widely from cell to cell and from one sequence to another: his lab and others have seen off-target sites with mutation frequencies ranging from 0.1% to more than 60%. Even low-frequency events could potentially be dangerous if they accelerate a cell’s growth and lead to cancer, he says.

With so many unanswered questions, it is important to keep expectations of CRISPR under control, says Katrine Bosley, chief executive of Editas, a company in Cambridge, Massachusetts, that is pursuing CRISPR-mediated gene therapy. Bosley is a veteran of commercializing new technologies, and says that usually the hard part is convincing others that an approach will work. “With CRISPR it’s almost the opposite,” she says. “There’s so much excitement and support, but we have to be realistic about what it takes to get there.”

CRISPR on the farm

While Anderson and others are aiming to modify DNA in human cells, others are targeting crops and livestock. Before the arrival of gene-editing techniques, this was generally done by inserting a gene into the genome at random positions, along with sequences from bacteria, viruses or other species that drive expression of the gene. But the process is inefficient, and it has always been fodder for critics who dislike the mixing of DNA from different species or worry that the insertion could interrupt other genes. What is more, getting genetically modified crops approved for use is so complex and expensive that most of those that have been modified are large commodity crops such as maize (corn) and soya beans.

Illustration by Sébastien Thibault

With CRISPR, the situation could change: the ease and low cost may make genome editing a viable option for smaller, speciality crops, as well as animals. In the past few years, researchers have used the method to engineer petite pigs and to make disease-resistant wheat and rice. They have also made progress towards engineering dehorned cattle, disease-resistant goats and vitamin-enriched sweet oranges. Doudna anticipates that her list of CRISPR-modified organisms will grow. “There’s an interesting opportunity to consider doing experiments or engineering pathways in plants that are not as important commercially but are very interesting from a research perspective — or for home vegetable gardens,” she says.

CRISPR’s ability to precisely edit existing DNA sequences makes for more-accurate modifications, but it also makes it more difficult for regulators and farmers to identify a modified organism once it has been released. “With gene editing, there’s no longer the ability to really track engineered products,” says Jennifer Kuzma, who studies science policy at North Carolina State University in Raleigh. “It will be hard to detect whether something has been mutated conventionally or genetically engineered.”

That rings alarm bells for opponents of genetically modified crops, and it poses difficult questions for countries trying to work out how to regulate gene-edited plants and animals. In the United States, the Food and Drug Administration has yet to approve any genetically modified animal for human consumption, and it has not yet announced how it will handle gene-edited animals.

Under existing rules, not all crops made by genome editing would require regulation by the US Department of Agriculture (see Nature 500, 389390; 2013). But in May, the agriculture department began to seek input on how it can improve regulation of genetically modified crops — a move that many have taken as a sign that the agency is re-evaluating its rules in light of technologies such as CRISPR. “The window has been cracked,” says Kuzma. “What goes through the window remains to be seen. But the fact that it’s even been cracked is pretty exciting.”

Engineered ecosystems

Beyond the farm, researchers are considering how CRISPR could or should be deployed on organisms in the wild. Much of the attention has focused on a method called gene drive, which can quickly sweep an edited gene through a population. The work is at an early stage, but such a technique could be used to wipe out disease-carrying mosquitoes or ticks, eliminate invasive plants or eradicate herbicide resistance in pigweed, which plagues some US farmers.

Usually, a genetic change in one organism takes a long time to spread through a population. That is because a mutation carried on one of a pair of chromosomes is inherited by only half the offspring. But a gene drive allows a mutation made by CRISPR on one chromosome to copy itself to its partner in every generation, so that nearly all offspring will inherit the change. This means that it will speed through a population exponentially faster than normal (see ‘Gene drive’) — a mutation engineered into a mosquito could spread through a large population within a season. If that mutation reduced the number of offspring a mosquito produced, then the population could be wiped out, along with any malaria parasites it is carrying.

Publications: Scopus; Patents: The Lens; Funding: NIH RePORTER.

But many researchers are deeply worried that altering an entire population, or eliminating it altogether, could have drastic and unknown consequences for an ecosystem: it might mean that other pests emerge, for example, or it could affect predators higher up the food chain. And researchers are also mindful that a guide RNA could mutate over time such that it targets a different part of the genome. This mutation could then race through the population, with unpredictable effects.

“It has to have a fairly high pay-off, because it has a risk of irreversibility — and unintended or hard-to-calculate consequences for other species,” says George Church, a bioengineer at Harvard Medical School in Boston. In April 2014, Church and a team of scientists and policy experts wrote a commentary in Science6 warning researchers about the risks and proposing ways to guard against accidental release of experimental gene drives.

At the time, gene drives seemed a distant prospect. But less than a year later, developmental biologist Ethan Bier of the University of California, San Diego, and his student Valentino Gantz reported that they had designed just such a system in fruit flies7. Bier and Gantz had used three layers of boxes to contain their flies and adopted lab safety measures usually used for malaria-carrying mosquitoes. But they did not follow all the guidelines urged by the authors of the commentary, such as devising a method to reverse the engineered change. Bier says that they were conducting their first proof-of-principle experiments, and wanted to know whether the system worked at all before they made it more complex.

For Church and others, this was a clear warning that the democratization of genome editing through CRISPR could have unexpected and undesirable outcomes. “It is essential that national regulatory authorities and international organizations get on top of this — really get on top of it,” says Kenneth Oye, a political scientist at the Massachusetts Institute of Technology and lead author of the Science commentary. “We need more action.” The US National Research Council has formed a panel to discuss gene drives, and other high-level discussions are starting to take place. But Oye is concerned that the science is moving at lightning speed, and that regulatory changes may happen only after a high-profile gene-drive release.

The issue is not black and white. Micky Eubanks, an insect ecologist at Texas A&M University in College Station, says that the idea of gene drives shocked him at first. “My initial gut reaction was ‘Oh my god, this is terrible. It’s so scary’,” he says. “But when you give it more thought and weigh it against the environmental changes that we have already made and continue to make, it would be a drop in the ocean.”

Some researchers see lessons for CRISPR in the arc of other new technologies that prompted great excitement, concern and then disappointment when teething troubles hit. Medical geneticist James Wilson of the University of Pennsylvania in Philadelphia was at the centre of booming enthusiasm over gene therapy in the 1990s — only to witness its downfall when a clinical trial went wrong and killed a young man. The field went into a tailspin and has only recently begun to recover. The CRISPR field is still young, Wilson says, and it could be years before its potential is realized. “It’s in the exploration stage. These ideas need to ferment.”

Then again, Wilson has been bitten by the CRISPR bug. He says that he was sceptical of all the promises being made about it until his own lab began to play with the technique. “It’s ultimately going to have a role in human therapeutics,” he says. “It’s just really spectacular.”

CRISPR tweak may help gene-edited crops bypass biosafety regulation


Technique deletes plant genes without adding foreign DNA.

These lettuce-plantlets have had their genomes edited with CRISPR/Cas9, but do not contain foreign DNA.

A twist on a revolutionary gene-editing technique may make it possible to modify plant genomes while sidestepping national biosafety regulations, South Korean researchers say.

Plant scientists have been quick to experiment with the popular CRISPR/Cas9 technique, which uses an enzyme called Cas9, guided by two RNA strands, to precisely cut segments of DNA in a genome. By disabling specific genes in wheat and rice, for example, researchers hope to make disease-resistant strains of the crops.

But the process can introduce bits of foreign DNA into plant genomes. And some jurisdictions, such as the European Union, could decide to classify such plants as genetically modified organisms (GMOs)1 — making their acceptance by regulatory bodies contentious, says geneticist Jin-Soo Kim of Seoul National University.

Kim and his team tweaked the technique so that it can delete specific plant genes without introducing foreign DNA, creating plants that he and his colleagues think “might be exempt from current GMO regulations”2.

“In terms of science, our approach is just another improvement in the field of genome editing. However, in terms of regulations and public acceptance, our method could be path-breaking,” says Kim.

DNA-free CRISPR

Conventionally, researchers get CRISPR/Cas9 working in a plant cell by first shuttling in the gene that codes for the Cas9 enzyme. The gene is introduced on a plasmid — a circular packet of DNA — which is usually carried into a plant by the bacterial pest Agrobacterium tumefaciens. As a result, Agrobacterium DNA can end up in the plant’s genome. Even if the pest is not used, fragments of the Cas9 gene may themselves be incorporated into the plant’s genome.

To get around this problem, Kim and his colleagues avoid gene-shuttling altogether. They report a recipe to assemble the Cas9 enzyme together with its guide RNA sequences (which the enzyme requires to find its target) outside the plant, and use solvents to get the resulting protein complex into the plant. The technique works efficiently to knock out selected genes in tobacco plants, rice, lettuce and thale cress, they say, reporting their results in Nature Biotechnology2.

“I think this is a milestone work for plant science,’ says bioethicist Tetsuya Ishii at Hokkaido University in Sapporo, Japan, who has extensively studied the framework of regulation surrounding genetic engineering in plants.

Kim wants to use the technique to edit the banana; the crop’s most popular cultivar, the Cavendish variety, is struggling to combat a devastating soil fungus and may go extinct. Gene editing could, for example, be used to knock out the receptor that the fungus uses to invade cells, without any need, in Kim’s view, to classify the resulting banana as a GMO. “We will save the banana so that our children and grandchildren can still enjoy the fruit,” he says.

Skirting regulations

Other scientists have recently achieved similar results with different genome-editing techniques. Jeffrey Wolt, a specialist in risk analysis of plant biotechnology at Iowa State University in Ames, points out that some researchers have introduced gene-editing protein complexes called TALENs directly into plants, for example3; others have used nanoparticles to usher in different gene-editing proteins4. To his mind, Kim’s paper is just one more tool in plant breeders’ arsenals — although many researchers say that CRISPR is cheaper and easier to use than other tools.

Jens Boch, a plant geneticist at Martin Luther University of Halle-Wittenberg in Germany who helped to develop TALEN, says that he hopes that workarounds that avoid Agrobacterium will not be necessary. When plants reproduce sexually, their genes are remixed, so they produce some offspring that do not have the offending bacterial DNA; breeding these Agrobacterium-free plants should appease regulators, he hopes. Agrobacterium “is just too easy to use, and this is going to be the method of choice”, he says. “I don’t believe that plant breeders will use Kim’s method.” (Still, Kim points out that some plants, such as the banana, do not reproduce sexually, so would not lose an Agrobacterium gene if it were lodged in their genome.)

It is unclear what stance regulatory authorities will take on CRISPR-edited plants. The European Commission is currently debating regulations to take into account the latest techniques, and it is conceivable that it will still classify plants as GMOs even if they lack foreign DNA.

In the United States, editing plants with Agrobacterium is currently a trigger for regulation by the Animal and Plant Health Inspection Service, yet plants edited in other ways have bypassed regulations. But rules may change there too: in July, the White House launched a multiyear initiative to review federal regulations on agricultural biotechnology.

If regulations on CRISPR plants do turn out to be severe, Boch says, “the method proposed by Kim is a very good one to circumvent some of the possible criticisms”.

Genetic mutation blocks prion disease


Unknown mechanism helped some people in Papua New Guinea escape historic, deadly outbreak.

A genetic variant protected some practitioners of cannibalism from prion disease.

Scientists who study a rare brain disease that once devastated entire communities in Papua New Guinea have described a genetic variant that appears to stop misfolded proteins known as prions from propagating in the brain1.

Kuru was first observed in the mid-twentieth century among the Fore people of Papua New Guinea. At its peak in the late 1950s, the disease killed up to 2% of the group’s population each year. Scientists later traced the illness to ritual cannibalism2, in which tribe members ate the brains and nervous systems of their dead. The outbreak probably began when a Fore person consumed body parts from someone who had sporadic Creutzfeldt-Jakob disease (CJD), a prion disease that spontaneously strikes about one person in a million each year.

Scientists have noted previously that some people seem less susceptible to prion diseases if they have an amino-acid substitution in a particular region of the prion protein — codon 1293. And in 2009, a team led by John Collinge — a prion researcher at University College London who is also the lead author of the most recent analysis — found another protective mutation among the Fore, in codon 1274.

The group’s latest work, reported on 10 June in Nature1, shows that the amino-acid change that occurs at this codon, replacing a glycine with a valine, has a different and more powerful effect than the substitution at codon 129. The codon 129 variant confers some protection against prion disease only when it is present on one of the two copies of the gene that encodes the protein. But transgenic mice with the codon-127 mutation were completely resistant to kuru and CJD regardless of whether they bore one or two copies of it.

The researchers say that the mutation in codon 127 appears to confer protection by preventing prion proteins from becoming misshapen.

“It is a surprise,” says Eric Minikel, a prion researcher at the Broad Institute in Cambridge, Massachusetts. “This was a story I didn’t expect to have another chapter.”

Collinge and his colleagues are now continuing their work, to figure out the mutant protein’s structure and how it shields against illness.

Deadly animal prion disease appears in Europe


How brain disorder related to mad-cow disease spread to Norway is a mystery.

Reindeer have never before been found to have chronic wasting disease in the wild.

A highly contagious and deadly animal brain disorder has been detected in Europe for the first time. Scientists are now warning that the single case found in a wild reindeer might represent an unrecognized, widespread infection.

Chronic wasting disease (CWD) was thought to be restricted to deer, elk (Cervus canadensis) and moose (Alces alces) in North America and South Korea, but on 4 April researchers announced that the disease had been discovered in a free-ranging reindeer (Rangifer tarandus tarandus) in Norway. This is both the first time that CWD has been found in Europe and the first time that it has been found in this species in the wild anywhere in the world.

“It’s worrying — of course, especially for animals. It’s a nasty disease,” says Sylvie Benestad, an animal-disease researcher at the Norwegian Veterinary Institute in Oslo who, along with colleague Turid Vikøren, diagnosed the diseased reindeer.

A key question now is whether this is a rare — even unique — case, or if the disease is widespread but so far undetected in Europe.

“If it’s similar to our prion disease in the United States and Canada, the disease is subtle and it would be easy to miss,” says Christina Sigurdson, a pathologist at the University of California, San Diego, who has shown that reindeer can contract CWD in a laboratory environment1.

Mysterious origins

Like both bovine spongiform encephalopathy — also known as mad-cow disease — and variant Creutzfeldt-Jakob disease in humans, CWD occurs when cellular proteins called prions bend into an abnormal shape, inducing neighbouring, healthy proteins to do the same. The misfolded proteins aggregate in the brain and sometimes in other tissue, causing weight loss, coordination problems and behaviour changes. There is no cure or vaccine; as far as scientists know, CWD is always fatal.

Although the disease is not known to be transmissible to humans, it is highly contagious among deer, elk and related animals, which can shed infectious misfolded prion proteins in their saliva, urine and faeces. Animals infected with CWD have been found in more than 20 states in the United States and 2 provinces in Canada. The disease has also been detected in captive animals in South Korea, which imported CWD with a shipment of live elk brought into the country for farming in the late 1990s.

The infected reindeer ended up on Vikøren’s necropsy table thanks to scientists with the Norwegian Institute for Nature Research in Trondheim. They found it as they used a helicopter to track a free-ranging herd from the Nordfjella population in the alpine regions of southern Norway. Their goal was to capture adult female reindeer and collar them for satellite tracking — but when the researchers landed, they discovered a sick animal that could not move and soon died.

During the necropsy, Benestad tested for the abnormally folded proteins as a matter of routine. Eventually, a total of three different antibody-based tests all confirmed the presence of prions.

“I was very afraid,” Benestad says. During her long career as a prion researcher she has heard scientists from the United States and Canada discuss CWD, how contagious it is and how hard it is to stamp out.

It is a mystery how this disease arrived on a mountaintop in Norway. Benestad and Vikøren think it unlikely that it was it imported. They suspect that it might have arisen spontaneously, or jumped the species barrier from a prion disease in sheep called scrapie, although such a jump has never been seen before.

“The $64,000 question is what is the origin of this case of CWD in Europe,” says Glenn Telling, a prion-disease researcher at Colorado State University in Fort Collins. “What we do know is that once CWD is detected in new locations, it typically takes a foothold in that location, and is difficult to eradicate.”

Devastating wheat fungus appears in Asia for first time


Scientists race to determine origin of  Bangladesh outbreak, which they warn could spread farther afield.

Wheat in northern India could be threatened by an outbreak of fungal disease in Bangladesh.

Update: On 26 April, a team led by microbial population geneticist Daniel Croll, who is at the Swiss Federal Institute of Technology in Zurich, reported on github.com that the Bangladeshi wheat-blast strain is closely related to those collected in Brazilian wheat fields and on nearby weeds. His team’s analysis, which uses the data on the website Open Wheat Blast, reveals that the sample is not closely related to known rice-blast-causing strains of M. oryzae. Croll’s team concludes that wheat blast was probably introduced to Bangladesh from Brazil, and warns that other Asian countries that import Brazilian wheat, including Thailand, the Philippines and Vietnam, should be on the lookout for the disease.

Fields are ablaze in Bangladesh, as farmers struggle to contain Asia’s first outbreak of a fungal disease that periodically devastates crops in South America. Plant pathologists warn that wheat blast could spread to other parts of south and southeast Asia, and are hurrying to trace its origins.

“It’s important to know what the strain is,” says Sophien Kamoun, a biologist at the Sainsbury Laboratory in Norwich, UK, who has created a website, Open Wheat Blast (go.nature.com/bkczwf), to encourage researchers to share data.

Efforts are also under way to find wheat genes that confer resistance to the disease.

First detected in February and confirmed with genome sequencing by Kamoun’s lab this month, the wheat-blast outbreak has already caused the loss of more than 15,000 hectares of crops in Bangladesh. “It’s really an explosive, devastating disease,” says plant pathologist Barbara Valent of Kansas State University in Manhattan, Kansas. “It’s really critical that it be controlled in Bangladesh.”

After rice, wheat is the second most cultivated grain in Bangladesh, which has a population of 156 million people. More broadly, inhabitants of south Asia grow 135 million tonnes of wheat each year.

Wheat blast is caused by the fungus Magnaporthe oryzae. Since 1985, when scientists discovered it in Brazil’s Paraná state, the disease has raced across South America.

The fungus is better known as a pathogen of rice. But unlike in rice, where M. oryzae attacks the leaves, the fungus strikes the heads of wheat, which are difficult for fungicides to reach. A 2009 outbreak in wheat cost Brazil one-third of that year’s crop. “There are regions in South America where they don’t grow wheat because of the disease,” Valent says. Wheat blast was spotted in Kentucky in 2011, but vigorous surveillance helped to stop it spreading in the United States.

In South America, the disease tends to take hold in hot and humid spells. Such conditions are present in Bangladesh, and the disease could migrate across south and southeast Asia, say plant pathologists. In particular, it could spread over the Indo-Gangetic Plain through Bangladesh, northern India and eastern Pakistan, warn scientists at the Bangladesh Agricultural Research Institute (BARI) in Nashipur.

Bangladeshi officials are burning government-owned wheat fields to contain the fungus, and telling farmers not to sow seeds from infected plots. The BARI hopes to identify wheat varieties that are more tolerant of the fungus and agricultural practices that can keep it at bay, such as crop rotation and seed treatment.

Wheat blast strikes the heads of wheat, which are difficult for fungicides to reach.

It is unknown how wheat blast got to Bangladesh. One possibility is that a wheat-infecting strain was brought in from South America, says Nick Talbot, a plant pathologist at the University of Exeter, UK. Another is that an M. oryzae strain that infects south Asian grasses somehow jumped to wheat, perhaps triggered by an environmental shift: that is what happened in Kentucky, when a rye-grass strain infected wheat.

To tackle the question, this month Kamoun’s lab sequenced a fungus sample from Bangladesh. The strain seems to be related to those that infect wheat in South America, says Kamoun, but data from other wheat-infecting strains and strains that plague other grasses are needed to pinpoint the outbreak’s origins conclusively.

The Open Wheat Blast website might help. Kamoun has uploaded the Bangladeshi data, and Talbot has deposited M. oryzae sequences from wheat in Brazil. Talbot hopes that widely accessible genome data could help to combat the outbreak. Researchers could use them to screen seeds for infection or identify wild grasses that can transmit the fungus to wheat fields.

Rapid data sharing is becoming more common in health emergencies, such as the outbreak of Zika virus in the Americas. Kamoun and Talbot say that their field should follow suit. “The plant-pathology community has a responsibility to allow data to be used to combat diseases that are happening now, and not worry too much about whether they may or may not get a Nature paper out of it,” says Talbot.

Last month, Valent’s team reported the first gene variant known to confer wheat-blast resistance (C. D. Cruz et al. Crop Sci. http://doi.org/bfk7; 2016), and field trials of crops that bear the resistance gene variant have begun in South America. But plant pathologists say that finding one variant is not enough: wheat strains must be bred with multiple genes for resistance, to stop M. oryzae quickly overcoming their defences.

The work could help in the Asian crisis, says Talbot. “What I would hope for out of this sorry situation,” he says, “is that there will be a bigger international effort to identify resistance genes.”

Deadly new wheat disease threatens Europe’s crops


Researchers caution that stem rust may have returned to world’s largest wheat-producing region.

The stem rust fungus damages wheat, leaving a characteristic brown stain.

An infection that struck wheat crops in Sicily last year is a new and unusually devastating strain of fungus, researchers say — and its spores may spread to infect this year’s harvests in Europe, the world’s largest wheat-producing region.

“We have to be careful of shouting wolf too loudly. But this could be the largest outbreak that we have had in Europe for many, many years,” says Chris Gilligan, an epidemiologist at the University of Cambridge, UK, who leads a team that has modelled the probable spread of the fungus’s spores.

In alerts released on 2 February, researchers revealed the existence of TTTTF, a kind of stem rust — named for the characteristic brownish stain it lays down as it destroys wheat leaves and stems. The alarm was raised by researchers at the Global Rust Reference Center (GRRC), which is part of Aarhus University in Denmark, and the International Maize and Wheat Improvement Center (CIMMYT), headquartered in Texcoco, Mexico.

Last year, the stem rust destroyed tens of thousands of hectares of crops in Sicily. What’s particularly troubling, the researchers say, is that GRRC tests suggest the pathogen can infect dozens of laboratory-grown strains of wheat, including hardy varieties that are usually highly resistant to disease. The team is now studying whether commercial crops are just as susceptible.

Adding further concern, the centres say that two new strains of another wheat disease, yellow rust, have been spotted over large areas for the first time — one in Europe and North Africa, and the other in East Africa and Central Asia. The potential effects of the yellow-rust fungi aren’t yet clear, but the pathogens seem to be closely related to virulent strains that have previously caused epidemics in North America and Afghanistan.

The Food and Agriculture Organization of the United Nations (FAO) in Rome issued similar alerts about the three diseases on 3 February.

Severe wheat damage in Europe could affect food prices, inflation and the region’s economic stability, says James Brown, a plant pathologist at the John Innes Centre in Norwich, UK.

But researchers hope that by putting out alerts before European wheat crops have started to grow this year, they will give farmers enough warning to monitor fields and apply fungicides, halting the disease’s spread. Plant breeders can also start to ramp up efforts to produce resistant varieties. “Timely action is crucial,” says Fazil Dusunceli, a plant pathologist at the FAO.

Return of stem rust

In the mid-twentieth century, devastation caused by stem rust spurred efforts to breed wheat strains that could resist the fungi. That research — led by agronomist Norman Borlaug — famously led to the Green Revolution in agriculture, increasing crop yields around the world.

But stem rust returned in the late 1990s and 2000s, with a variety called Ug99 that spread through Africa and parts of the Middle East. It ruined harvests and caused international concern because, says Dusunceli, more than 90% of wheat crops were susceptible to it. So far, however, it hasn’t hit large wheat-producing regions such as Europe, China and North America. Researchers are developing resistant crops.

Stem rust epidemics haven’t been seen in Europe since the 1950s, says Mogens Hovmøller, who leads the GRRC’s testing team. “It’s not a challenge plant breeders have faced for many years,” agrees Brown.

But the outbreak that hit Sicily in 2016 suggests that the disease has now returned. Unusually, even the hardy durum wheat, used to make pasta, is susceptible to it, says Hovmøller. But it’s too early to say whether the new infection could be as devastating as Ug99.

Models based on wind and weather patterns, conducted by Gilligan’s team at Cambridge University together with CIMMYT and the UK’s Met Office in Exeter, suggest that stem-rust spores released during the Sicilian outbreak may well have been deposited throughout the Mediterranean region. That doesn’t mean the infection will spread — the spores may not have survived the winter, for example — but it is worrying enough for researchers to raise the alarm.

The yellow-rust strains are also a concern, says Hovmøller. For Europe, perhaps the most alarming is one provisionally called Pst(new), which was spotted in Sicily, Morocco, Italy and northern Europe in 2016. The fungus is related to a virulent strain that hit North America in the 2000s, but it is not clear how aggressive it is.

Early-warning system

Researchers are accustomed to finding one or two new wheat-rust strains each year in Europe; these must be guarded against but are not usually dangerously virulent. But since 2010, the region has experienced a greater influx of wheat pathogens, says Hovmøller.

He doesn’t know why, but speculates that it could be down to warmer autumns and milder winters attributable to climate change, combined with changes in farming practices, such as sowing wheat earlier in the season. Increases in international travel — potentially spreading spores on clothing — could also be a factor, speculates Brown.

Hovmøller and others will in the next few weeks ask the European Research Council for funds to establish an early-warning system. That will help partners including breeders, scientists and agrochemical companies in Europe to share diagnostic facilities and information about potential outbreaks.

Dusunceli thinks that such a network might have helped to mitigate the Sicily outbreak, which in turn would have meant that fewer spores could spread to other parts of the continent. “I wouldn’t question the necessity for an early-warning system,” he says.

‘Gene drive’ mosquitoes engineered to fight malaria


Mutant mozzies could rapidly spread through wild populations.

The Anopheles stephensi mosquito can spread the malaria parasite to humans.

Mutant mosquitoes engineered to resist the parasite that causes malaria could wipe out the disease in some regions — for good.

Humans contract malaria from mosquitoes that are infected by parasites from the genus Plasmodium. Previous work had shown that mosquitoes could be engineered to rebuff the parasite P. falciparum1, but researchers lacked a way to ensure that the resistance genes would spread rapidly through a wild population.

In work published on 23 November in the Proceedings of the National Academy of Sciences, researchers used a controversial method called ‘gene drive’ to ensure that an engineered mosquito would pass on its new resistance genes to nearly all of its offspring2 — not just half, as would normally be the case.

The result: a gene that could spread through a wild population like wildfire.

“This work suggests that we’re a hop, skip and jump away from actual gene-drive candidates for eventual release,” says Kevin Esvelt, an evolutionary engineer at Harvard University in Cambridge, Massachusetts, who studies gene drive in yeast and nematodes.

For Anthony James, a molecular biologist at the University of California, Irvine, and an author of the paper, such a release would spell the end of a 30-year quest to use mozzie genetics to squash malaria.

James and his laboratory have painstakingly built up the molecular tools to reach this goal. They have worked out techniques for creating transgenic mosquitoes — a notoriously challenging endeavour — and isolated genes that could confer resistance to P. falciparum. But James lacked a way to ensure that those genes would take hold in a wild population.

Fast forward

The concept of engineering a gene drive has been around for about a decade, and James’s laboratory had tried to produce them in the past. The process was agonizingly slow.

Then, in January, developmental biologists Ethan Bier and Valentino Gantz at the University of California, San Diego, contacted James with a stunning finding: they had engineered a gene drive in fruit flies, and wondered whether the same system might work in mosquitoes. James jumped at the opportunity to find out.

Bier and Gantz had used a gene-editing system called CRISPR–Cas9 to engineer a gene drive. They inserted genes encoding the components of the system that were designed to insert a specific mutation in their fruit flies. The CRISPR–Cas9 system then copied that mutation from one chromosome to the other3. James used that system in mosquitoes to introduce two genes that his past work showed would cause resistance to the malaria pathogen.

The resulting mosquitoes passed on the modified genes to more than 99% of their offspring. Although the researchers stopped short of confirming that all the insects were resistant to the parasite, they did show that the offspring expressed the genes.

“It’s a very significant development,” says Kenneth Oye, a political scientist who studies emerging technologies at the Massachusetts Institute of Technology in Cambridge. “Things are moving rapidly in this field.”

Other teams are developing gene drives that could control malaria. A team at Imperial College London has developed a CRISPR-based gene drive in Anopheles gambiae, the mosquito species that transmits malaria in sub-Saharan Africa. The group’s gene drive inactivates genes involved in egg production in female mosquitoes, which could be used to reduce mosquito populations, according to team member Austin Burt, an evolutionary geneticist. Their results will be published in Nature Biotechnology next month, Burt says.

Oye notes that such technological advances are outpacing the regulatory and policy discussions that surround the use of gene drive to engineer wild populations. Gene drives are controversial because of the potential that they hold for altering entire ecosystems.

Before testing gene drive in the field, Oye hopes that researchers will study the long-term consequences of the changes, such as their stability and potential to spread to other species, as well as methods to control them. “I’m less worried about malevolence than getting something wrong,” he says.

Esvelt says that the US-based researchers made a wise decision in selecting a non-native mosquito species for their experiments. (The team worked with Anopheles stephensi, which is native to the Indian subcontinent.) “Even if they escaped the lab, there’d be no one to mate with and spread the drive,” Esvelt says.

James predicts that it will take his team less than a year to prepare mosquitoes that would be suitable for field tests, but he is in no rush to release them. “It’s not going to go anywhere until the social science advances to the point where we can handle it,” he says. “We’re not about to do anything foolish.”

Splice of life


Researchers, bioethicists and regulators must contribute to transparent discussions on the risks and ethics of editing human embryos.

The news last month that scientists had edited the genomes of human embryos induced a predictable sharp intake of breath . The work is notable because it altered the germ line, meaning that in a viable embryo, the genetic changes would have been passed on to all future offspring. What should be society’s response to such research? How should the scientific community view other current and foreseeable experiments along similar lines, and what should it do about them?

Gene-editing tools have evolved to the point at which targeted changes to a genome can be made with unprecedented ease. In theory, gene editing allows specific genetic traits to be changed. The potential clinical applications, in which babies are engineered so that they no longer carry faulty, disease-causing genes that run in the family, might be attractive to many. But even such potentially legitimate clinical applications remain some way off. There are also longer-term ethical concerns that germline gene therapy might creep beyond eliminating deadly or debilitating heritable disorders to include disabilities, less serious conditions, and cosmetic and other supposed enhancements — leading to ‘designer babies’ and raising the spectre of eugenics.

Now is a good time for a public debate about such human germline editing — gene editing in sperm, eggs or embryos applied in ways that would allow changes to propagate to subsequent generations. Not only should voices from civil society outside the closeted worlds of science, bioethics and regulation be heard, but their highly diverse viewpoints must also help to set the terms of the debate. The accumulated knowledge and experience of the relevant academic disciplines and regulators needs to be taken into account. Ultimately, such debates should be resolved with international discussion, and regulation at national levels.

Practical considerations

The latest research, published in Protein & Cell, demonstrates the issues (P. Liang et al. Protein Cell http://doi.org/34q; 2015). The researchers deliberately used embryos that were products of eggs fertilized by two sperm, and so could never grow into a baby. The details of the work highlight why attempting human germline gene therapy using editing techniques any time soon would be a terrible mistake. The efficiency of genetic modification turned out to be poor, and the technique generated many unintended mutations. It could be a long time before researchers can demonstrate that the benefits of the procedure would outweigh the risks. Until such a time, it is clear that no sensible laboratory, regulator or nation should even consider any attempt to implant and develop to birth an edited embryo.

The potential power and relative ease of gene editing offer compelling reasons to support such research, however. The latest work, for example, aimed to edit a gene that when mutated is responsible for the blood disorder β-thalassaemia. (The gene also helps to protect against Plasmodium falciparum malaria.) Extending the research could help us to understand and treat the blood disorder, forms of which can be severe. There is also a strong basic-science incentive for such experiments, which can help us to understand human development and perhaps be used to produce useful cell lines. A total ban on research would therefore seem counterproductive.

But there is also a need to keep germline gene therapy in perspective. Preimplantation genetic diagnosis and selection of healthy embryos during in vitro fertilization already provides a safer alternative for avoiding genetic disease in newborns — as can prenatal screening and abortion. The diseases for which gene editing would be superior are few.

“There is a need to keep germline gene therapy in perspective.”

Many countries ban or restrict research that involves the destruction of human embryos, and moreover bar human germ­line modification. Even in countries with more-liberal laws, there is a de facto ban on gene editing as part of a human-reproduction technology, because the safety and efficacy of such work would not meet existing clinical-trial standards. Debates on other genetic-engineering topics such as recombinant DNA and somatic cloning have touched on many of the issues relevant to germline editing. What usually emerges from such discussions is a green light for properly regulated research, with tight restrictions on how that research could be applied. The same outcome seems the most sensible here, and probably the most likely, in light of the embryo-editing work.

But all involved must actively work to make that happen, and not passively assume that the field will simply evolve towards best practice. How should a more general discussion proceed? Whether in collaboration or separately, national governments need to step up on this issue. Scientists, companies and ethicists are already voicing their views and setting up further meetings.

Also helpful might be an official forum of experts to assist emergent policy discussions — an international meeting of scientists, regulators, ethicists and representatives of civil society, perhaps convened under the auspices of the World Health Organization (WHO). Such a meeting could take stock of the state of the science of gene editing, and articulate the regulatory and ethical landscapes. It could then quickly move to help close any gaps in legislation, and develop a regulatory framework for the inevitable germline-related advances in gene-editing techniques.

A model perhaps is a similar meeting convened by the WHO last year to rapidly assess the ethics of emergency clinical trials of Ebola drugs and vaccines that had not been fully tested for their safety. As with Ebola, any meeting on germline gene editing should also be given access to the unpublished results of ongoing experiments.

Transparent and inclusive discussion of issues raised by gene-editing technologies that could open the door to germline gene therapy is a must. Scientifically and ethically informed contributions would remind people that for the foreseeable future, science-fiction scenarios of ‘designer babies’ remain just that, while providing an articulation of the limitations of our scientific understanding.

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