A research group from Sun Yat-sen University in Guangzhou (China) have edited the genomes of human embryos for the first time.
The results were recently published in Protein & Cell and confirmed widespread rumours that such experiments had been conducted. The rumours sparked a high-profile debate about the ethical implications of such work. In the paper, researchers led by Junjiu Huang tried to keep such concerns at bay by utilizing ‘non-viable’ embryos obtained from local fertility clinics, which cannot result in a live birth.
The team attempted to modify the gene responsible for Î²-thalassaemia, a potentially fatal blood disorder, using the gene-editing technique CRISPR/Cas9. “I believe this is the first report of CRISPR/Cas9 applied to human pre-implantation embryos and as such the study is a landmark, as well as a cautionary tale,” commented George Daley, stem cell biologist at Harvard Medical School (MA, USA). “Their study should be a stern warning to any practitioner who thinks the technology is ready for testing to eradicate disease genes.”
Some scientists are of the opinion that gene editing in embryos may have a bright future as it has the potential to eradicate devastating genetic diseases during the development of the embryo. Others however, say that such work crosses an ethical line: germline modification (genetic changes to embryos) are heritable and could have an unpredictable effect on future generations. Researchers have also expressed concerns that any gene-editing research on human embryos could lead to unsafe or unethical uses of the technique.
The technique used by Huang’s team involved injecting embryos with the enzyme complex CRISPR/Cas9, which binds and splices DNA at specific locations. The complex can be programmed to target the disease-causing gene, which is then replaced or repaired by another molecule introduced at the same time. The system is well studied in human adult cell and in animal embryos but there had been no published reports of its use in human embryos.
The study aimed to see whether the procedure could replace a gene in a single-cell fertilized human embryo; and in principle, all cells produced as the embryo developed would then have the repaired gene. The embryos they obtained from the fertility clinics had been created for use in in vitro fertilization but had an extra set of chromosomes, following fertilization by two sperm preventing the embryos from resulting in a live birth, though they do undergo the first stages of development.
Huang’s group studied the ability of the CRISPR/Cas9 system to edit HBB, the gene that encodes the human Î²-globin protein – mutations in the gene are responsible for Î²-thalassaemia. Of the 86 injected embryos, 71 embryos survived and 54 were genetically tested. The results revealed that just 28 were successfully spliced, and that only a fraction of those contained the replacement genetic material. “If you want to do it in normal embryos, you need to be close to 100%,” explained Huang. “That’s why we stopped. We still think it’s too immature.”
His team also found a surprising number of ‘off-target’ mutations which were most likely introduced by the CRISPR/Cas9 complex acting on other parts of the genome. This effect is one of the main safety concerns surrounding germline gene editing due to such unintended mutations which could be harmful. The rate of such mutations was much higher than those observed in previous gene-editing studies of mouse embryos or human adult cells. Huang noted that his team likely only detected a subset of these unintended mutations because they only looked at the exome (a portion of the genome). “If we did the whole genome sequence, we would get many more,” he added.
Critics of the paper have noted that the low efficiencies and high number of off-target mutations could be specific to the abnormal embryos used in the study. Huang acknowledges the critique, but because there are no examples of gene editing in normal embryos he says that there is no way to know if the gene-editing technique operates differently in them.
Still, he maintains that the embryos allow for a more meaningful model than an animal model or one using adult human cells. “We wanted to show our data to the world so people know what really happened with this model, rather than just talking about what would happen without data,” he explains.
The research team plans to work out on how to decrease the number of off-target mutations using adult human cells or animal models by using different strategies including an alternative gene editing technique known as TALEN.
It looks like this paper will reignite the debate on human-embryo editing, and it will probably continue for some time. CRISPR/Cas9 is known for its ease of use and there are reports that other groups in China are also experimenting on human embryos. Edward Lanphier who is president of Sangamo Biosciences in Richmond (CA, USA) which applies gene-editing techniques to adult human cells, fears that more scientists will now start to work towards improving on Huang’s paper. “The ubiquitous access to and simplicity of creating CRISPRs,” commented Lanphier, “creates opportunities for scientists in any part of the world to do any kind of experiments they want.”
Liang P, Xu Y, Zhang X et al. CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes.
Protein & Cell, doi:10.1007/s13238-015-0153-5 (2015).