Over the last few decades, researchers have developed innovative techniques for altering genetic sequences. More recently, scientific breakthroughs have greatly reduced the cost and complexity of making highly targeted changes in living cells. There is consensus among experts that these advances could have wide-ranging clinical applications. Although the advances may have the potential to prevent or even cure a variety of diseases when used in adults, children, and even fetuses or gametes, they also raise many important ethical and societal questions.
Tracy Hampton, PhD
To consider the promise of these advances and to address concerns about their potential misuse, the US National Academy of Sciences (NAS) and the National Academy of Medicine (NAM) launched an initiative that included a recent International Summit on Human Gene Editing held in Washington, DC (http://bit.ly/1YDu8OU). The summit, which brought together experts in science, philosophy, ethics, and politics, was co-hosted by the United Kingdom’s Royal Society and the Chinese Academy of Sciences.
While these and other experts continue to conduct research on gene editing and consider its effect on society, complex issues and differing opinions regarding human rights and ethics will make charting the way forward a challenge.
Investigators have demonstrated clear, although limited, success with somatic gene editing, which targets cells that are not transmitted to the next generation. Recently, a team from the United Kingdom treated the first patient, an infant with refractory relapsed acute B lymphoblastic leukemia, with genetically modified T cells under compassionate use and UK special therapy regulations (http://bit.ly/1XSpT1B).
The team used DNA-cutting enzymes called transcription activator–like effector nucleases (TALENs) to deactivate genes in T cells extracted from a healthy donor that would otherwise cause the donor cells to attack the recipient’s cells, thereby circumventing graft-vs-host disease. In a one-two punch strategy, the team also targeted another donor T-cell gene to protect donor cells from the effects of the lymphodepleting antileukemia drug alemtuzumab.
“We used ex vivo engineering with TALENs against 2 targets, the T-cell receptor and CD52, the target antigen for alemtuzumab. The cells were used in a time-limited manner for around 3 months for their antileukemia effects, and this mitigated against concerns for longer-term side effects,” said Waseem Qasim, MBBS, PhD, lead investigator and an immunologist at University College London. Although follow-up has been short, the treatment induced molecular remission where all other treatments had failed.
Another team reported promising results after treating 12 HIV-positive patients with modified autologous CD4+ T cells wherein the CCR5 gene, which encodes the main T-cell coreceptor for HIV, was disrupted by zinc-finger nuclease (ZFN)-mediated gene editing (Tebas P et al. N Engl J Med. 2014;370:901-910). Blood levels of HIV DNA decreased in most patients as a result of treatment with the genetically engineered T cells. Both TALENs and ZFNs rely on experimenter-designed DNA binding motifs to direct a nonspecific nuclease to cleave the genome at a specified site.
Investigators and clinicians are optimistic that another technique based on a bacterial defense mechanism—the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein (Cas) 9 system—may one day have even wider-ranging applications. The CRISPR-Cas system relies on experimenter-designed guide RNA and ribonucleotide complex formation to direct gene editing rather than protein/DNA recognition, making it much easier to design and implement.
“Zinc fingers have been under investigation for a number of years and proof-of-concept data established for a limited number of target sites, while TALENs have been around for about 5 years, with enhanced efficiency and a greater number of suitable target sites. Then in the past 3 or 4 years, there’s been an explosion in CRISPR-Cas with the possibility of targeting sites across the genome,” said Qasim.
Although the CRISPR-Cas9 gene editing system has not yet been implemented in clinical trials to disrupt or correct human somatic genes, a Cas9-mediated gene–drive approach has been used to create genetically engineered mosquitoes that are incapable of transmitting the human malaria parasite (Gantz VM et al. Proc Natl Acad Sci USA. 2015;112:E6736-E6743). This system allows for highly heritable population-level genetic modification of the malaria vector that could decrease or eliminate disease.
Much more work remains, however, to increase efficiency and improve targeting of these gene-editing tools so that they can be used safely and effectively. Nonetheless, “[f]or all these reagents, the first application in the clinic will be for ex vivo engineered cells,” said Qasim.
At the conference, George Daley, MD, PhD, the director of the Stem Cell Transplantation Program at Boston Children’s Hospital and a professor of biological chemistry and molecular pharmacology at Harvard Medical School, noted that wider use of gene editing for targeting somatic genes is imminent, with deletion of abnormal sequences entering the clinic before replacement of normal genes.
Daley also argued that applications in embryo and gamete editing are feasible, “posing then the question of whether or not they should be used in the human.” However, he stressed that the technology is much more likely to help treat monogenic human diseases, such as sickle cell anemia and cystic fibrosis, rather than polygenic conditions.
Targeting eggs, sperm, pluripotent stem cells, or very early embryos represents one of the most promising, but also worrisome, applications of gene editing because any alterations in the germline could be passed on to future generations. The strategy might prevent the transmission of certain genetic diseases to offspring, thereby potentially eliminating them, but concerns loom regarding errors and other unintended effects with unknown consequences.
“There are obvious anxieties about germline editing and the risks of undesirable side effects, but no doubt that as the technology improves, there will be enormous pressure to use these approaches to correct inherited diseases,” said Qasim. Researchers have already reported ways to reduce off-target effects and increase the specificity of the CRISPR-Cas system (Slaymaker IM et al. Science. 2016;351:84-88; Kleinstiver BP et al. Nature. doi:10.1038/nature16526 [published online January 6, 2016]).
However, before gene editing tools can be used for germline modification, experts say more knowledge is needed regarding human genetics and gene-environment interactions, as well as the pathways of disease, including the interplay between one disease and other conditions in the same patient (Baltimore D et al.Science. 2015;348:36-38).
Eric Lander, PhD, the founding director of the Broad Institute of MIT and Harvard, also questioned whether there really are many cases where germline editing would add any value over preimplantation genetic diagnosis, which allows prospective parents who carry heritable disease-causing genes to select embryos lacking those genes.
“If we really care about avoiding cases of genetic disease, germline editing is not the first, second, third, or fourth thing that we should be thinking about,” he told summit attendees.
The case for germline editing that is most compelling, he argued, is when both parents are homozygous for a disease-related gene variant, which is a rare occurrence in the grand scheme of human disease. He also questioned whether gene editing for complex polygenic diseases or enhancements would even be possible because most genes have very weak effects on their own and are often involved in a variety of physiological functions, some of which may be beneficial.
In addition, while preventions and cures are the goal of many clinical efforts, it can be argued that not every condition or disability can or should be cured, and making such distinctions can be fraught with controversy.
“If we are going to deem certain indications as permissible, can we identify a regulatory and oversight approach that will allow us to be comfortable that we can draw a line, so that we aren’t throwing out what may be very powerful, legitimate medical applications in order to stave off those which are less palatable to most of us?” Daley asked during the summit.
The potential of using germline gene editing to introduce permanent changes into the human genome that could confer characteristics ranging from intelligence or physical qualities to traits that are not even natural to humans is one reason that Hille Haker, PhD, an ethicist at Loyola University Chicago and a member of the European Group on Ethics in Sciences and New Technologies to the European Commission, supports an international ban on germline gene editing for reproductive purposes. At the summit, she argued that germline gene editing presupposes that the reproductive rights of prospective parents override the rights of future children.
John Harris, DPhil, a professor of bioethics and the director of the Institute for Science, Ethics, and Innovation at the University of Manchester, countered that all would-be parents make numerous decisions about issues that might affect their future children without thinking about their consent. He also pointed out that sexual reproduction itself carries significant risk of harm to future generations.
“If CRISPR-Cas9 and other interventions—when they’re proved to be safe enough—are not implemented, people will still reproduce and pass on heritable damage in the germline, so it won’t be the case that we will be preferring a risk-free alternative to a technology with attendant risks,” he said.
However, Haker stressed that because “almost all therapeutic uses have safer and better alternatives, the pursuit of reproductive germline gene editing only makes sense if scientists and societies were aiming at germline genetic enhancement,” adding that she “cannot see this at all as an ethically desirable or justifiable consideration.”
The summit’s 12-person organizing committee—consisting of biologists, physicians, and bioethicists whose closing statement did not represent unanimity among attendees—endorsed the use of gene editing methods for basic research that involves altering the DNA sequences of human eggs, sperm, or embryos.
The committee members noted that trying to produce a human pregnancy from such modified germ cells or embryos, however, is currently “irresponsible” due to a lack of societal consensus and safety concerns. Scientists in China recently reported editing the gene responsible for β-thalassemia in nonviable human embryos using CRISPR-Cas9, but their strategy was largely unsuccessful due to inefficiency and off-target effects (Liang P et al. Protein Cell. 2015;6:363-372).
Questions that arise when considering how to use and limit gene editing technologies will be addressed through the internal regulatory mechanisms; however, in many countries, these are lacking.
“It’s difficult to control the human impulse to seek out new treatments that appear to offer real hope for relief or even a cure, but regulatory oversight is imperfect or even entirely absent in some jurisdictions,” said R. Alta Charo, JD, a professor of law and bioethics at the University of Wisconsin at Madison and a former member of the advisory council for the National Institutes of Health’s National Center for the Advancement of Translational Sciences.
The summit highlighted just how difficult it will be to limit, let alone define, appropriate applications of gene editing and to consider the rights of everyone involved. “How will all sectors of society, whether or not they have an interest in the research per se, have a say in the collective inheritance that is the human genome?” asked Charis Thompson, PhD, a professor of gender and women’s studies at the University of California at Berkeley. “And what are the rights of future generations, if any, not to have or to have their genomes tampered with?”
An international committee appointed by the NAS and the NAM will conduct a comprehensive study of the scientific, clinical, ethical, legal, and social implications of human gene editing. Cochaired by Charo, the committee will also monitor new developments in the field and make recommendations about policy options for proceeding in this field (http://bit.ly/1K2LhJv). Its report is expected to be released late in 2016 and will represent the official views of the NAS and the NAM.
“Meetings such as the international summit and other efforts will provide a venue for ongoing evaluation of the science—both its potential benefits and its risks—as well as for discussion of how use of this science for various applications might affect public confidence in science, public health, environmental sustainability, perceptions of disability, understandings of personal autonomy and parental choice, and other broader issues,” said Charo.
The committee’s next meeting is tentatively scheduled for February 11 and 12 in Washington, DC, and it will include sessions open to the public.