Did you know that in the 15 years since we finished reading the human genome sequence for the first time, we have developed powerful new technologies for editing a human (or any) genomes? We may not yet have jet packs or flying cars, but the ability to edit genomes precisely is now real. These new methods offer the ability to change the genome in a way that could be passed on for generations. But should we?

Some might think that, once we have decoded (that is, read out) the genome sequence of an organism, a logical next step would be to "edit" that genome - perhaps to cure a genetic disease or to alter the a trait in a plant or animal. Scientists have used laboratory methods to make changes in an organism's genome for many years, primarily to study its biology or to introduce commercially advantageous changes. For example, you might have seen "GloFish" - zebrafish that now produce a protein that makes them fluorescent. But in 2012, a new technique called CRISPR was developed and has revolutionized genome editing. The scientists who developed CRISPR recognized that they could borrow tools from the immune system of bacteria and develop "bacterial scissors" for surgically editing any DNA in any organism. CRISPR has made genome editing much simpler, faster, cheaper, and more accurate than previous methods, and the method is now used in laboratories all over the world.

Genome Editing and Human Disease

Already, we have seen some exciting early studies that use CRISPR to edit the human genome in an effort to study and, perhaps eventually, to treat a disease. Sickle cell disease causes severe pain and premature death in millions of people worldwide. Scientists have already used CRISPR to remove the mutation that causes sickle cell disease in a mouse model, and are working toward clinical trials in humans. For the latter, scientists will remove blood stem cells from a patient with sickle cell disease, edit the genome of those cells to remove the sickle cell mutation, and then re-insert the modified cells into the person's bone marrow. Since there are still no cures for sickle cell disease, such a genome-editing approach could be a major advance.

Another CRISPR application now entering clinical trials aims to combat human immunodeficiency virus (or HIV) infection. HIV enters human white blood cells and then alters those cells' genomes. Then, it makes copies of itself to infect the person's immune system, making them vulnerable to other infections. CRISPR is now being investigated for use in either cutting out the HIV-derived DNA from the genome as well as engineering a person's genome so that HIV cannot enter their cells. However, it is important to stress that these techniques are still relatively new and very much still in testing mode.

Technical Limitations of Genome Editing

CRISPR editing is not perfect and can even be inadvertently harmful. Although CRISPR is much more precise than previous genome-editing methods, it is sometimes imprecise and edits the wrong place in a genome. This means that it could randomly introduce mutations at the wrong sites rather than the genomic location being targeted. Many scientists are working to reduce this risk of imprecise editing.

Even when CRISPR precisely modifies a genome as desired, the introduced change does not always lead to the expected outcome. For example, some of the studies using CRISPR to combat HIV immediately found that on its own, the virus developed separate new mutations that allowed it to escape the CRISPR edits. These new mutations allowed the virus to start replicating again.

As scientists working on gene therapy for many years have found, there are also important technical issues to consider, especially when considering the use of CRISPR for treating a human disease. In order to get CRISPR-based genome editing to work at the right place at the right time for effective therapy, there needs to be a delivery system that works for that specific body location. This makes the bone marrow and the eye attractive for CRISPR-based therapies because they are more easily accessible; other tissues are not as easy to target with this CRISPR systems. In addition, scientists need to make sure that the human body does not mount an immune response to either the CRISPR tool itself or the components involved in its delivery.

Ethical Considerations

Beyond the technical issues, there are significant ethical issues surrounding the ability to edit the human and other genomes. Let's start with ourselves: so far, most CRISPR studies have focused on editing somatic (tissue or body) cells, so that none of the changes would get passed on to future generations. But CRISPR technologies can be used to edit germline cells, too - these make the sperm and eggs that could carry any genome edits to future children and potentially their children. On the one hand, some might think that it is a good idea to remove sickle cell disease mutations in all future generations. On the other hand, this could open the door to editing genomes to alter other traits, passing the changes on to all future generations. Where would we draw the line?

Is it okay to edit a genome to give a child better eyesight or a few inches more in height? Will we begin to create "designer babies" who might have "better" traits than their parents? As the ethicist and scientist Paul Knoepfler says in his 2015 TED talk, "what is better when we're talking about a human being?" Many groups are considering the important questions that need to be addressed, including the United States National Academies of Sciences, Engineering, and Medicine. Because of these unanswered questions, a current ban on using federal funds for conducting research on human embryos, and the unknown health risks of genome editing, creating "designer babies" is unlikely to happen in the United States anytime soon.

Like many health technologies and medications, genome editing is also likely to be quite expensive as a medical intervention, at least initially. This could worsen health disparities that already exist based on socioeconomic status. As discussed in the Social Context advance, there is also valid concern about informed consent. Can future parents of an unborn child truly give fully informed consent for that child to have their genome edited, particularly at the current time when such therapies are not fully tested or proven to be safe?

Gene Drives

Beyond human genomes ethical questions about editing other organisms' genomes are arising. Consider the mosquito and its unwelcome passengers: the malaria-causing parasite Plasmodium or viruses such as Zika or dengue. Genome-editing technologies can be used to introduce malaria-battling mutations in the mosquitoes' genome that blocks the parasite from infecting the mosquitoes in the laboratory. This might sound great, but let's think more about it. Introducing these genome-edited mosquitoes into the wild will not make much difference, because the malaria-battling mutations cannot spread through an enormous mosquito population by regular breeding alone. Our edited genome with the malaria-battling mutation will only be carried by half of the offspring from each pair of parents. After three generations, only half of the mosquito population will have the edited genomes.

Instead, it is now possible to couple the malaria-battling mutations with additional genomic changes designed to ensure a rapid spread throughout a population. The second new edit causes the malaria-battling mutation to get passed down to all offspring from that pair. So, when our new mosquitoes mate with "wild" mosquitoes, all of their babies will also have edited genomes, driving the malaria-battling mutation to spread more quickly through the population. In some instances, by thethird generation, all of the new mosquitoes could be malaria-free. This is called a "gene drive" and it has quickly become controversial.

For the first time, we can introduce genomic changes by genome editing and sweep them through populations of an entire species. On the one hand, you might feel like it's unethical not to introduce such changes if we could stop the spread of malaria. On the other hand, many scientists feel that letting such modified organisms out into the wild is too risky until we understand more about the resulting population-wide consequences. What happens if the malaria parasite develops resistance to the malaria-battling mutations, or comes up with a way to infect another organism? Who gets to decide which organisms are altered in such a large way? Again, the United States National Academies and other organizations are intensely studying the issues relating to these genome-editing technologies, and will issue recommendations as the circumstances change.

Genome editing is a method that lets scientists change the DNA of many organisms, including plants, bacteria, and animals. Editing DNA can lead to changes in physical traits, like eye color, and disease risk. Scientists use different technologies to do this.

Overview

Genome editing technologies enable scientists to make changes to DNA, leading to changes in physical traits, like eye color, and disease risk. Scientists use different technologies to do this. These technologies act like scissors, cutting the DNA at a specific spot. Then scientists can remove, add, or replace the DNA where it was cut.

The first genome editing technologies were developed in the late 1900s. More recently, a new genome editing tool called CRISPR, invented in 2009, has made it easier than ever to edit DNA. CRISPR is simpler, faster, cheaper, and more accurate than older genome editing methods. Many scientists who perform genome editing now use CRISPR.

In the Laboratory

One way that scientists use genome editing is to investigate different diseases that affect humans. They edit the genomes of animals, like mice and zebrafish, because animals have many of the same genes as humans. For example, mice and humans share about 85 percent of their genes! By changing a single gene or multiple genes in a mouse, scientists can observe how these changes affect the mouse's health and predict how similar changes in human genomes might affect human health.

Scientists at the National Human Genome Research Institute (NHGRI) are doing just this. The Burgess lab, for example, is studying zebrafish genomes. Scientists in this lab delete different genes in zebrafish one at a time using CRISPR to see how the deletion impacts the fish. The Burgess lab focuses on 50 zebrafish genes which are similar to the genes that cause human deafness so that they can better understand the genomic basis of deafness.

Treating Disease

Scientists are developing gene therapies - treatments involving genome editing - to prevent and treat diseases in humans. Genome editing tools have the potential to help treat diseases with a genomic basis, like cystic fibrosis and diabetes. There are two different categories of gene therapies: germline therapy and somatic therapy. Germline therapies change DNA in reproductive cells (like sperm and eggs). Changes to the DNA of reproductive cells are passed down from generation to generation. Somatic therapies, on the other hand, target non-reproductive cells, and changes made in these cells affect only the person who receives the gene therapy.

In 2015, scientists successfully used somatic gene therapy when a one-year old in the United Kingdom named Layla received a gene editing treatment to help her fight leukemia, a type of cancer. These scientists did not use CRISPR to treat Layla, and instead used another genome editing technology called TALENs. Doctors tried many treatments before this, but none of them seemed to work, so scientists received special permission to treat Layla using gene therapy. This therapy saved Layla's life. However, treatments like the one that Layla received are still experimental because the scientific community and policymakers still have to address technical barriers and ethical concerns surrounding genome editing.

Technical barriers

Even though CRISPR improved upon older genome editing technologies, it is not perfect. For example, sometimes genome editing tools cut in the wrong spot. Scientists are not yet sure how these errors might affect patients. Assessing the safety of gene therapies and improving upon genome editing technologies are critical steps to ensure that this technology is ready for use in patients.

Ethical concerns

Scientists and all of us should carefully consider the many ethical concerns that can emerge with genome editing, including safety. First and foremost, genome editing must be safe before it is used to treat patients. Some other ethical questions that scientists and society must consider are:

1. Is it okay to use gene therapy on an embryo when it is impossible to get permission from the embryo for treatment? Is getting permission from the parents enough?
2. What if gene therapies are too expensive and only wealthy people can access and afford them? That could worsen existing health inequalities between the rich and poor.
3. Will some people use genome editing for traits not important for health, such as athletic ability or height? Is that okay?
4. Should scientists ever be able to edit germline cells? Edits in the germline would be passed down through generations.

Most people agree that scientists should not edit the genomes of germline cells at this time because the safety and Scientific communities across the world are approaching germline therapy research with caution because edits to a germline cell would be passed down through generations. Many countries and organizations have strict regulations to prevent germline editing for this reason. The NIH, for example, does not fund research to edit human embryos.

The human genome is a 3-billion letter instruction book, using a 4-letter DNA alphabet. Just a single typo can lead to a devastating disease. Imagine the ability to, like a copy editor, quickly fix DNA spelling errors so that the genetic words and sentences read flawlessly. NIH scientists are getting close: they have developed a revolutionary new way to edit genomes precisely inside living cells, without even removing the DNA as was once necessary. Researchers are testing the value of this method, named CRISPR, for hundreds of applications. Some include creating malaria-resistant mosquitoes, and correcting gene errors in diseases known to be caused by one or just a few mutations. Recently, NIH researchers successfully edited the disease-causing mutation in blood-forming cells taken directly from people with sickle-cell disease. These gene-edited cells survived when transplanted into mice, suggesting that such a treatment might be long-lasting or possibly even curative if tested in humans. If CRISPR could be targeted effectively to cells in a living person, then the thousands of genetic diseases that currently lack treatment might be cured.

Techniques to modify DNA in the genome have existed for several decades, but the conversation about the science and ethics of genome editing has grown louder due to faster, cheaper, and more efficient technologies.

Overview

While the popular media tends to focus on the potential use of genome editing in humans, the main application of this technology has been in basic research. Editing the genomes of yeast, bacteria, mice, zebrafish, and other organisms that scientists commonly study has led to countless discoveries about how the genome is connected to physical traits, like eye color, and disease.

Researchers funded by the National Human Genome Research Institute (NHGRI) and other research institutes at the National Institutes of Health (NIH) are adopting newer techniques, such as CRISPR, to conduct their investigations. A robust understanding of how the genome gives rise to health and disease will aid the development of new treatments, including gene therapy.

Genome Editing Methods

Scientists have had the knowledge and ability to edit genomes for many years, but CRISPR technology has brought major improvements to the speed, cost, accuracy, and efficiency of genome editing. The history of genome editing technologies shows the remarkable progress in this field and also relays the critical role that basic science research plays in the development of research tools and potential disease treatments.

Homologous recombination

The earliest method scientists used to edit genomes in living cells was homologous recombination. Homologous recombination is the exchange (recombination) of genetic information between two similar (homologous) strands of DNA. Scientists began developing this technique in the late 1970s following observations that yeast, like other organisms, can carry out homologous recombination naturally.

To perform homologous recombination in the laboratory, one must generate and isolate DNA fragments bearing genome sequences similar to the portion of the genome that is to be edited. These isolated fragments can be injected into individual cells or taken up by cells using special chemicals. Once inside a cell, these DNA fragments can then recombine with the cell's DNA to replace the targeted portion of the genome.

This type of homologous recombination is limited by the fact that it is extremely inefficient in most cell types. This technique can have as low as a one-in-a-million probability of successful editing. Another weakness of homologous recombination is that it is inaccurate and has a high rate of error when the injected DNA fragments insert into an unintended part of the genome, causing what are known as off-target edits.

Zinc-finger nucleases (ZFN)

In the 1990s researchers started using zinc-finger nucleases (ZFN) to improve the specificity of genome editing and reduce off-target edits. The structures of ZFNs are engineered from naturally-occurring proteins that were discovered in eukaryotic organisms. Scientists can engineer these proteins to bind to specific DNA sequences in the genome and cut DNA. Once bound to their target DNA sequence, the ZFNs cut the genome at the specified location, allowing scientists to either delete the target DNA sequence or replace it with a new DNA sequence via homologous recombination.

Although ZFNs improved the success rate of genome editing to about 10 percent, it is difficult and time-consuming to design, construct, and produce successful zinc finger proteins, and a new ZFN must be engineered for each new target DNA sequence.

Transcription activator-like effector nucleases (TALENs)

In 2009, a new class of proteins called Transcription Activator-Like Effector Nucleases (TALENs) arrived to the genome editing scene. Similar to ZFNs, transcription activator-like effector nucleases (TALENs) are engineered from proteins found in nature and are capable of binding to specific DNA sequences.

While TALENs and ZFNs are comparable in terms of how efficiently they can create edits to the genome, TALENs bear the advantage of greater simplicity. It is much easier to engineer TALENs than it is to synthesize ZFNs.

Clustered regularly interspaced short palindromic repeats (CRISPR)

Though ZFN and TALEN technology increase the specificity and efficiency of genome editing, they are relatively expensive and complicated to use in the lab. Each edit would require the construction of a new ZFN or TALEN protein, and engineering proteins can be a difficult process that is prone to error. This is one reason why CRISPR is a game-changing technology; unlike its predecessors, CRISPR is a simple technology with little assembly required. CRISPR associated DNA sequences were first observed in bacteria in the early 1990s, but it was not until the 2000s that the scientific community understood its ability to recognize specific genome sequences and cut them via the Cas9 protein, a protein that works with CRISPR and that has DNA-cutting abilities. In nature, CRISPR is used by bacteria as an immune system to kill invading viruses, but it has now been adapted for use in the lab.

With CRISPR, researchers create a short RNA template that matches a target DNA sequence in the genome. Creating synthetic RNA sequences is much easier than engineering proteins as is those required for ZFNs and TALENs. Strands of RNA and DNA can bind to each other when they have matching sequences. The RNA portion of the CRISPR, called a guide RNA, directs Cas9 enzyme to the targeted DNA sequence. Cas9 cuts the genome at this location to make the edit. CRISPR can make deletions in the genome and/or be engineered to insert new DNA sequences. One group of scientists found that CRISPR is six times more efficient than ZFNs or TALENs in creating targeted mutations to the genome. Large-scale genomics projects that once took many years and tens of thousands of dollars can now be completed at a small fraction of time and price.

Genome editing is currently being applied to research on cancer, mental health, rare diseases, and many other disease areas.

Overview

Basic research exploring human and nonhuman genomes is critical to help scientists understand the basic biology underlying disease, as well as to discover new possible therapeutic targets. Although excitement about the potential for gene therapy has grown tremendously since the discovery of CRISPR, the vast majority of work undertaken by scientists funded by NHGRI or other NIH Institutes takes place in the petri dish and in nonhuman organisms such as mice or zebrafish. Researchers rely on genome editing tools as a way to explore the connection between genotype (genes) and phenotype (traits). A typical study might be to model human disease in mice by deleting or editing certain genes that are thought to contribute to the disease. This approach can help researchers determine if specific changes made to the genome contribute to the disease. It can also lead to the creation of "disease models," or laboratory animals that mimic human diseases and can be studied to test new therapies.

In the clinic, there are proposals to use genome editing as a treatment for disease. Many diseases from cancer to asthma have genetic bases. Through the application of genome editing technologies, physicians might eventually be able to prescribe targeted gene therapy to make corrections to patient genomes and prevent, stop, or reverse disease.

Types of Gene Therapy

There are two different categories of gene therapies: germline therapy and somatic therapy. Germline therapies can alter many cell types but by definition they also change genes in reproductive cells (like sperm and eggs). These changes would then be passed down from generation to generation. Germline therapy could potentially prevent inheritance of diseases. Somatic therapies, on the other hand, target non-reproductive cells. Changes made in these cells affect only the person who receives the gene therapy and do not pass on to future generations. Somatic therapies could be used to slow or reverse the disease process.

Of the two types of gene therapy, somatic therapies are the less controversial, and therapies using this approach are under development in research and commercial labs around the world. Treatments for HIV and cancer are being tested in clinical trials in patients. In contrast, germline therapies pose a greater number of ethical hurdles because of their ability to affect future generations. Critics have highlighted the possibility that germline therapies would pave the way for genetic enhancement, the use of genome editing to change non-medically relevant characteristics, such as athletic ability or height. More information about the ethical concerns around genome editing can be found at: What are the ethical concerns about genome editing?

Ethical concerns aside, there are still significant technical barriers that prevent genome editing therapies from entering the clinic. The research and regulatory communities have yet to begin evaluating the safety of potential treatments. The risk of off-target edits, or unintended edits, and their effects are still unknown, even for a technology like CRISPR that is many times more precise than previous techniques. Another problem to overcome is how to deliver the genome editing therapy to the right cells in the body in an effective manner.

One other challenge for gene therapy is that there is still much to learn about which genes are involved in most diseases and how different changes in these genes affect a person's risk of getting a disease. Many genes have more than one function, and in some cases editing a gene to "cure" one disease could create another. Scientists are also working to learn more about how genetic changes and environmental influences combine to result in disease and how genes interact with each other. Additionally, scientists need to learn more about how genes are controlled and how variants in these "controlling" regions of the genome are associated with disease risk. In the case of common diseases, such as diabetes, many genetic changes and environmental influences combine to result in disease. Therefore, scientists need to learn more about these changes before many gene therapies can be developed.

Most of the ethical discussions related to genome editing center around human germline because editing changes made in the germline would be passed down to future generations.

Overview

The debate about genome editing is not a new one but has regained attention following the discovery that CRISPR has the potential to make such editing more accurate and even "easy" in comparison to older technologies.

Bioethicists and researchers generally believe that human genome editing for reproductive purposes should not be attempted at this time, but that studies that would make gene therapy safe and effective should continue. Most stakeholders agree that it is important to have continuing public deliberation and debate to allow the public to decide whether or not germline editing should be permissible. As of 2014, there were about 40 countries that discouraged or banned research on germline editing, including 15 nations in Western Europe, because of ethical and safety concerns. There is also an international effort led by the US, UK, and China to harmonize regulation of the application of genome editing technologies. This effort officially launched in December 2015 with the International Summit on Human Gene Editing in Washington, DC.

NHGRI uses the term "genome editing" to describe techniques used to modify DNA in the genome. Other groups also use the term "gene editing." In general, these terms are used interchangeably.

Ethical Considerations

Safety

Due to the possibility of off-target effects (edits in the wrong place) and mosaicism (when some cells carry the edit but others do not), safety is of primary concern. Researchers and ethicists who have written and spoken about genome editing, such as those present at the International Summit on Human Gene Editing, generally agree that until germline genome editing is deemed safe through research, it should not be used for clinical reproductive purposes; the risk cannot be justified by the potential benefit. Some researchers argue that there may never be a time when genome editing in embryos will offer a benefit greater than that of existing technologies, such as preimplantation genetic diagnosis (PGD) and in-vitro fertilization (IVF).

However, scientists and bioethicists acknowledge that in some cases, germline editing can address needs not met by PGD. This includes when both prospective parents are homozygous for a disease-causing variant (they both have two copies of the variant, so all of their children would be expected to have the disease); cases of polygenic disorders, which are influenced by more than one gene; and for families who object to some elements of the PGD process.

Some researchers and bioethicists are concerned that any genome editing, even for therapeutic uses, will start us on a slippery slope to using it for non-therapeutic and enhancement purposes, which many view as controversial. Others argue that genome editing, once proved safe and effective, should be allowed to cure genetic disease (and indeed, that it is a moral imperative). They believe that concerns about enhancement should be managed through policy and regulation.

Lastly, commenters on the issue are concerned that the use of genome editing for reproductive purposes will be regulated differently inside and outside of the U.S., leading to uses considered objectionable to the American public. These arguments cite the largely self-regulated environments of the reproductive clinics that offer PGD and IVF and the existing differences in regulations among different countries.

Informed Consent

Some people worry that it is impossible to obtain informed consent for germline therapy because the patients affected by the edits are the embryo and future generations. The counterargument is that parents already make many decisions that affect their future children, including similarly complicated decisions such as PGD with IVF. Researchers and bioethicists also worry about the possibility of obtaining truly informed consent from prospective parents as long as the risks of germline therapy are unknown.

Justice and Equity

As with many new technologies, there is concern that genome editing will only be accessible to the wealthy and will increase existing disparities in access to health care and other interventions. Some worry that taken to its extreme, germline editing could create classes of individuals defined by the quality of their engineered genome.

Genome-Editing Research Involving Embryos

Many people have moral and religious objections to the use of human embryos for research. Federal funds cannot be used for any research that creates or destroys embryos. In addition, NIH does not fund any use of gene editing in human embryos.

While NIH will not fund gene editing in human embryos at this time, many bioethical and research groups believe that research using gene editing in embryos is important for myriad reasons, including to address scientific questions about human biology, as long as it is not used for reproductive purposes at this time. Some countries have already allowed genome-editing research on nonviable embryos (those that could not result in a live birth), and others have approved genome-editing research studies with viable embryos. In general, research that is conducted in embryos could use viable or nonviable embryos leftover from IVF, or embryos created expressly for research. Each case has its own moral considerations.

Patients with genetic disorders and members of the public, both in the United States and around the world, have diverse views about whether germline genome editing should be used to prevent or treat genetic disorders.

Patient and Public Perspectives

Patients, patient advocates, and families of patients with genetic disorders have diverse views on whether germline genome editing should be used to prevent or treat genetic disorders. Some patients suffering from conditions such as Huntington disease believe strongly that it should be used to prevent people from getting genetic diseases, especially ones that do not currently have treatment options. Others, such as those in the deaf community, do not consider their condition to be a disability. They worry that if human genome editing becomes widespread, persons born with genetic conditions would be less likely to be accepted in society. Many communities generally question the idea that eliminating genetic conditions will improve lives, especially given that those with disabilities often report a high quality of life.

The Pew Research Center, a nonpartisan group that polls the public on a variety of topics, conducted a survey to understand the public's feelings about gene editing for newborns that would give them a reduced risk of serious diseases during their lives. The survey noted that any change made could be passed down to future generations, which could eventually change the genetic characteristics of the population. Though they found some enthusiasm among the public about gene editing, they also found that about two-thirds of U.S. adults have worries about the technology.

Pew also found:

1. The public is about evenly divided on whether they would use gene editing for their own child
2. The public is about evenly divided on whether gene editing "crosses a line" and constitutes "meddling with nature," or is like "other ways we try to better ourselves"
3. There are religious divides in opinions about gene editing
4. Those that had prior knowledge about gene editing were more inclined to want it for their own child in the given scenario.

United States Perspectives

While the United States does not have an explicit ban on germline genome editing, it does have restrictions through legislation and NIH policy. The Appropriations Act of 2016, passed in December 2015, prohibits the Food and Drug Administration (FDA) from reviewing research that involves the creation or germline modification of embryos. The Dickey-Wicker amendment, which has been included in appropriation bills since 1996, prohibits the use of federal funds for research that involves the creation or destruction of embryos. Lastly, NIH has guidelines stipulating that the NIH Recombinant DNA Advisory Committee will not consider proposals that involve germline alteration.

In April 2015, NIH released a statement to reiterate that it will not fund any use of gene editing in human embryos due to ethical and safety concerns. The statement noted that editing the germline "has been viewed almost universally as a line that should not be crossed" and that there are not currently "compelling medical applications justifying the use of CRISPR/Cas9 in embryos."

In July 2017, using non-federal funding, scientists at Oregon Health and Science University in Portland successfully edited the genes of human embryos, avoiding many of the technical errors that similar research has faced. The embryos were only allowed to develop for a few days. This was the first such study to be carried out in the United States.

International Perspectives

In April 2015, Chinese researchers led by Junjiu Huang and Sun Yat-sen of University in Guangzhou used CRISPR on nonviable embryos from local fertility clinics in an effort to modify a gene that can cause the blood disorder beta thalassemia. The researchers only allowed the embryos (those that could not result in a live birth) to grow to about eight cells each. Even before this paper was published, researchers discussed rumors that it was taking place, and debates about the ethical implications increased. Some investigators called for a moratorium on such research until broader discussions about the ethics could take place.

The international landscape of laws and regulations that govern germline genome editing is varied, but one 2014 analysis showed that of 14 countries that allow research with embryonic stem cells, 13 ban germline modification. An expanded survey showed that of 39 countries, 29 ban germline modification and those that do not are ambiguous on the topic. In some cases, countries have stated that they will reconsider if genome editing is deemed safe. Some countries do allow preclinical research with embryos leftover from fertility treatments after appropriate review.

In the United Kingdom, it is illegal to modify the genomes of embryos used for reproduction; however, in February 2015, the UK Human Fertilisation and Embryology Authority (HFEA), a national regulatory board for fertility research, became the first to approve an application to edit the genomes of human embryos for research in developmental biology.

Because of emerging concerns about genome editing, and because the impacts of genome editing research are of international concern, the National Academies of Sciences, Engineering, and Medicine (NASEM), in collaboration with the British Royal Society and the Chinese Academy of Sciences, hosted an international summit in December 2015 to discuss concerns.

Ever since scientists realized that changes in DNA cause cancer, they have been searching for an easy way to correct those changes by manipulating DNA. Although several methods of gene editing have been developed over the years, none has really fit the bill for a quick, easy, and cheap technology.

But a game-changer occurred in 2013, when several researchers showed that a gene-editing tool called CRISPR could alter the DNA of human cells like a very precise and easy-to-use pair of scissors.

The new tool has taken the research world by storm, markedly shifting the line between possible and impossible. As soon as CRISPR made its way onto the shelves and freezers of labs around the world, cancer researchers jumped at the chance to use it.

“CRISPR is becoming a mainstream methodology used in many cancer biology studies because of the convenience of the technique,” said Jerry Li, M.D., Ph.D., of NCI’s Division of Cancer Biology.

Now CRISPR is moving out of lab dishes and into trials of people with cancer. In a small study, for example, researchers tested a cancer treatment involving immune cells that were CRISPR-edited to better hunt down and attack cancer.

Despite all the excitement, scientists have been proceeding cautiously, feeling out the tool’s strengths and pitfalls, setting best practices, and debating the social and ethical consequences of gene editing in humans.

How Does CRISPR Work?

Like many other advances in science and medicine, CRISPR was inspired by nature. In this case, the idea was borrowed from a simple defense mechanism found in some microbes, such as bacteria.

To protect themselves against invaders like viruses, these microbes capture snippets of the intruder’s DNA and store them away as segments called CRISPRs, or clustered regularly interspersed short palindromic repeats. If the same germ tries to attack again, those DNA segments (turned into short pieces of RNA) help an enzyme called Cas find and slice up the invader’s DNA.

After this defense system was discovered, scientists realized that it had the makings of a versatile gene-editing tool. Within a handful of years, multiple groups had successfully adapted the system to edit virtually any section of DNA, first in the cells of other microbes, and then eventually in human cells.

In the laboratory, the CRISPR tool consists of two main actors: a guide RNA and a DNA-cutting enzyme, most commonly one called Cas9. Scientists design the guide RNA to mirror the DNA of the gene to be edited (called the target). The guide RNA partners with Cas and—true to its name—leads Cas to the target. When the guide RNA matches up with the target gene's DNA, Cas cuts the DNA.

What happens next depends on the type of CRISPR tool that’s being used. In some cases, the target gene's DNA is scrambled while it's repaired, and the gene is inactivated. With other versions of CRISPR, scientists can manipulate genes in more precise ways such as adding a new segment of DNA or editing single DNA letters.

Scientists have also used CRISPR to detect specific targets, such as DNA from cancer-causing viruses and RNA from cancer cells. Most recently, CRISPR has been put to use as an experimental test to detect the novel coronavirus.

Why Is CRISPR a Big Deal?

Scientists consider CRISPR to be a game-changer for a number of reasons. Perhaps the biggest is that CRISPR is easy to use, especially compared with older gene-editing tools.

“Before, only a handful of labs in the world could make the proper tools [for gene editing]. Now, even a high school student can make a change in a complex genome” using CRISPR, said Alejandro Chavez, M.D., Ph.D., an assistant professor at Columbia University who has developed several novel CRISPR tools.

CRISPR is also completely customizable. It can edit virtually any segment of DNA within the 3 billion letters of the human genome, and it’s more precise than other DNA-editing tools.

And gene editing with CRISPR is a lot faster. With older methods, “it usually [took] a year or two to generate a genetically engineered mouse model, if you’re lucky,” said Dr. Li. But now with CRISPR, a scientist can create a complex mouse model within a few months, he said.

Another plus is that CRISPR can be easily scaled up. Researchers can use hundreds of guide RNAs to manipulate and evaluate hundreds or thousands of genes at a time. Cancer researchers often use this type of experiment to pick out genes that might make good drug targets.

And as an added bonus, “it’s certainly cheaper than previous methods,” Dr. Chavez noted.

What Are CRISPR’s Limitations?

With all of its advantages over other gene-editing tools, CRISPR has become a go-to for scientists studying cancer. There’s also hope that it will have a place in treating cancer, too. But CRISPR isn’t perfect, and its downsides have made many scientists cautious about its use in people.

A major pitfall is that CRISPR sometimes cuts DNA outside of the target gene—what’s known as “off-target” editing. Scientists are worried that such unintended edits could be harmful and could even turn cells cancerous, as occurred in a 2002 study of a gene therapy.

“If [CRISPR] starts breaking random parts of the genome, the cell can start stitching things together in really weird ways, and there’s some concern about that becoming cancer,” Dr. Chavez explained. But by tweaking the structures of Cas and the guide RNA, scientists have improved CRISPR’s ability to cut only the intended target, he added.

Another potential roadblock is getting CRISPR components into cells. The most common way to do this is to co-opt a virus to do the job. Instead of ferrying genes that cause disease, the virus is modified to carry genes for the guide RNA and Cas.

Slipping CRISPR into lab-grown cells is one thing; but getting it into cells in a person's body is another story. Some viruses used to carry CRISPR can infect multiple types of cells, so, for instance, they may end up editing muscle cells when the goal was to edit liver cells.

Researchers are exploring different ways to fine-tune the delivery of CRISPR to specific organs or cells in the human body. Some are testing viruses that infect only one organ, like the liver or brain. Others have created tiny structures called nanocapsules that are designed to deliver CRISPR components to specific cells.

Because CRISPR is just beginning to be tested in humans, there are also concerns about how the body—in particular, the immune system—will react to viruses carrying CRISPR or to the CRISPR components themselves.

Some wonder whether the immune system could attack Cas (a bacterial enzyme that is foreign to human bodies) and destroy CRISPR-edited cells. Twenty years ago, a patient died after his immune system launched a massive attack against the viruses carrying a gene therapy he had received. However, newer CRISPR-based approaches rely on viruses that appear to be safer than those used for older gene therapies.

Another major concern is that editing cells inside the body could accidentally make changes to sperm or egg cells that can be passed on to future generations. But for almost all ongoing human studies involving CRISPR, patients’ cells are removed and edited outside of their bodies. This “ex vivo” approach is considered safer because it is more controlled than trying to edit cells inside the body, Dr. Chavez said.

However, one ongoing study is testing CRISPR gene editing directly in the eyes of people with a genetic disease that causes blindness, called Leber congenital amaurosis.

The First Clinical Trial of CRISPR for Cancer

The first trial in the United States to test a CRISPR-made cancer therapy was launched in 2019 at the University of Pennsylvania. The study, funded in part by NCI, is testing a type of immunotherapy in which patients’ own immune cells are genetically modified to better “see” and kill their cancer.

The therapy involves making four genetic modifications to T cells, immune cells that can kill cancer. First, the addition of a synthetic gene gives the T cells a claw-like protein (called a receptor) that “sees” NY-ESO-1, a molecule on some cancer cells.

Then CRISPR is used to remove three genes: two that can interfere with the NY-ESO-1 receptor and another that limits the cells’ cancer-killing abilities. The finished product, dubbed NYCE T cells, were grown in large numbers and then infused into patients.

“We had done a prior study of NY-ESO-1–directed T cells and saw some evidence of improved response and low toxicity,” said the trial’s leader, Edward Stadtmauer, M.D., of the University of Pennsylvania. He and his colleagues wanted to see if removing the three genes with CRISPR would make the T cells work even better, he said.

The goal of this study was to first find out if the CRISPR-made treatment was safe. It was tested in two patients with advanced multiple myeloma and one with metastatic sarcoma. All three had tumors that contained NY-ESO-1, the target of the T-cell therapy.

Initial findings suggest that the treatment is safe. Some side effects did occur, but they were likely caused by the chemotherapy patients received before the infusion of NYCE cells, the researchers reported. There was no evidence of an immune reaction to the CRISPR-edited cells.

Only about 10% of the T cells used for the therapy had all four of the desired genetic edits. And off-target edits were found in the modified cells of all three patients. However, none of the cells with off-target edits grew in a way that suggested they had become cancer, Dr. Stadtmauer noted.

The treatment had a small effect on the patients’ cancers. The tumors of two patients (one with multiple myeloma and one with sarcoma) stopped growing for a while but resumed growing later. The treatment didn't work at all for the third patient.

It's exciting that the treatment initially worked for the sarcoma patient because “solid tumors have been a much more difficult nut to crack with cellular therapy," Dr. Stadtmauer said. "Perhaps [CRISPR] techniques will enhance our ability to treat solid tumors with cell therapies.”

Although the trial shows that CRISPR-edited cell therapy is possible, the long-term effects still need to be monitored, Dr. Stadtmauer continued. The NYCE cells are “safe for as long as we’ve been watching [the study participants]. Our plan is to keep monitoring them for years, if not decades,” he said.

More Studies of CRISPR Treatments to Come

While the study of NYCE T cells marked the first trial of a CRISPR-based cancer treatment, there are likely more to come.

“This [trial] was really a proof-of-principle, feasibility, and safety thing that now opens up the whole world of CRISPR editing and other techniques of [gene] editing to hopefully make the next generation of therapies,” Dr. Stadtmauer said.

Other clinical studies of CRISPR-made cancer treatments are already underway. A few trials are testing CRISPR-engineered CAR T-cell therapies, another type of immunotherapy. For example, one company is testing CRISPR-engineered CAR T cells in people with B cell cancers and people with multiple myeloma.

There are still a lot of questions about all the ways that CRISPR might be put to use in cancer research and treatment. But one thing is for certain: The field is moving incredibly fast and new applications of the technology are constantly popping up.

“People are still improving CRISPR methods,” Dr. Li said. “It’s quite an active area of research and development. I’m sure that CRISPR will have even broader applications in the future.”

December 08, 2023

Today, the U.S. Food and Drug Administration approved two milestone treatments, Casgevy and Lyfgenia, representing the first cell-based gene therapies for the treatment of sickle cell disease (SCD) in patients 12 years and older. Additionally, one of these therapies, Casgevy, is the first FDA-approved treatment to utilize a type of novel genome editing technology, signaling an innovative advancement in the field of gene therapy.

Sickle cell disease is a group of inherited blood disorders affecting approximately 100,000 people in the U.S. It is most common in African Americans and, while less prevalent, also affects Hispanic Americans. The primary problem in sickle cell disease is a mutation in hemoglobin, a protein found in red blood cells that delivers oxygen to the body’s tissues. This mutation causes red blood cells to develop a crescent or “sickle” shape. These sickled red blood cells restrict the flow in blood vessels and limit oxygen delivery to the body’s tissues, leading to severe pain and organ damage called vaso-occlusive events (VOEs) or vaso-occlusive crises (VOCs). The recurrence of these events or crises can lead to life-threatening disabilities and/or early death.

“Sickle cell disease is a rare, debilitating and life-threatening blood disorder with significant unmet need, and we are excited to advance the field especially for individuals whose lives have been severely disrupted by the disease by approving two cell-based gene therapies today,” said Nicole Verdun, M.D., director of the Office of Therapeutic Products within the FDA’s Center for Biologics Evaluation and Research. “Gene therapy holds the promise of delivering more targeted and effective treatments, especially for individuals with rare diseases where the current treatment options are limited.”

Casgevy, a cell-based gene therapy, is approved for the treatment of sickle cell disease in patients 12 years of age and older with recurrent vaso-occlusive crises. Casgevy is the first FDA-approved therapy utilizing CRISPR/Cas9, a type of genome editing technology. Patients’ hematopoietic (blood) stem cells are modified by genome editing using CRISPR/Cas9 technology.

CRISPR/Cas9 can be directed to cut DNA in targeted areas, enabling the ability to accurately edit (remove, add, or replace) DNA where it was cut. The modified blood stem cells are transplanted back into the patient where they engraft (attach and multiply) within the bone marrow and increase the production of fetal hemoglobin (HbF), a type of hemoglobin that facilitates oxygen delivery. In patients with sickle cell disease, increased levels of HbF prevent the sickling of red blood cells.

Lyfgenia is a cell-based gene therapy. Lyfgenia uses a lentiviral vector (gene delivery vehicle) for genetic modification and is approved for the treatment of patients 12 years of age and older with sickle cell disease and a history of vaso-occlusive events. With Lyfgenia, the patient’s blood stem cells are genetically modified to produce HbA^T87Q, a gene-therapy derived hemoglobin that functions similarly to hemoglobin A, which is the normal adult hemoglobin produced in persons not affected by sickle cell disease. Red blood cells containing HbA^T87Q have a lower risk of sickling and occluding blood flow. These modified stem cells are then delivered to the patient.

Both products are made from the patients’ own blood stem cells, which are modified, and are given back as a one-time, single-dose infusion as part of a hematopoietic (blood) stem cell transplant. Prior to treatment, a patients’ own stem cells are collected, and then the patient must undergo myeloablative conditioning (high-dose chemotherapy), a process that removes cells from the bone marrow so they can be replaced with the modified cells in Casgevy and Lyfgenia. Patients who received Casgevy or Lyfgenia will be followed in a long-term study to evaluate each product’s safety and effectiveness.

“These approvals represent an important medical advance with the use of innovative cell-based gene therapies to target potentially devastating diseases and improve public health,” said Peter Marks, M.D., Ph.D., director of the FDA’s Center for Biologics Evaluation and Research. “Today’s actions follow rigorous evaluations of the scientific and clinical data needed to support approval, reflecting the FDA’s commitment to facilitating development of safe and effective treatments for conditions with severe impacts on human health.”

Data Supporting Casgevy

The safety and effectiveness of Casgevy were evaluated in an ongoing single-arm, multi-center trial in adult and adolescent patients with SCD. Patients had a history of at least two protocol-defined severe VOCs during each of the two years prior to screening. The primary efficacy outcome was freedom from severe VOC episodes for at least 12 consecutive months during the 24-month follow-up period. A total of 44 patients were treated with Casgevy. Of the 31 patients with sufficient follow-up time to be evaluable, 29 (93.5%) achieved this outcome. All treated patients achieved successful engraftment with no patients experiencing graft failure or graft rejection.

The most common side effects were low levels of platelets and white blood cells, mouth sores, nausea, musculoskeletal pain, abdominal pain, vomiting, febrile neutropenia (fever and low white blood cell count), headache and itching.

Data Supporting Lyfgenia

The safety and effectiveness of Lyfgenia is based on the analysis of data from a single-arm, 24-month multicenter study in patients with sickle cell disease and history of VOEs between the ages of 12- and 50- years old. Effectiveness was evaluated based on complete resolution of VOEs (VOE-CR) between 6 and 18 months after infusion with Lyfgenia. Twenty-eight (88%) of 32 patients achieved VOE-CR during this time period.

The most common side effects included stomatitis (mouth sores of the lips, mouth, and throat), low levels of platelets, white blood cells, and red blood cells, and febrile neutropenia (fever and low white blood cell count), consistent with chemotherapy and underlying disease.

Hematologic malignancy (blood cancer) has occurred in patients treated with Lyfgenia. A black box warning is included in the label for Lyfgenia with information regarding this risk. Patients receiving this product should have lifelong monitoring for these malignancies.

Both the Casgevy and Lyfgenia applications received Priority Review, Orphan Drug, Fast Track and Regenerative Medicine Advanced Therapy designations.

The FDA granted approval of Casgevy to Vertex Pharmaceuticals Inc. and approval of Lyfgenia to Bluebird Bio Inc.