(CRISPR)-associated system (Cas) have been in the news almost daily off-late; it is widely used to modify genes in model systems including animal zygotes and human cells, and hold tremendous promise for both basic research and clinical applications.
Until now, we have not clearly understood the DNA repair mechanisms in human early embryos, and in the efficiency and potential off-target effects of using technologies such as CRISPR/Cas9 in human pre-implantation embryos.
Scientists from China have now genetically modified human embryos. They used tripronuclear (3PN) zygotes (non-viable embryos) to further investigate CRISPR/Cas9-mediated gene editing in human cells. These embryos cannot result in a live birth, and were obtained from local fertility clinics. The study published in Protein & Cell shows that CRISPR/Cas9 could effectively cleave the endogenous β-globin gene (HBB), the gene responsible for β-thalassaemia.
However, the efficiency of homologous recombination directed repair (HDR) of HBB was low and the edited embryos were mosaic. Off-target cleavage was also apparent in these zygotes. Furthermore, the endogenous delta-globin gene (HBD), which is homologous to HBB, competed with exogenous donor oligos to act as the repair template, leading to untoward mutations.
Taken together, their work highlights the pressing need to further improve the fidelity and specificity of the CRISPR/Cas9 platform. Overall, their study talks about how they tried to genetically modify human embryos, but were not extremely successful in the process.
“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,” says George Daley, a stem-cell biologist at Harvard Medical School in Boston, Massachusetts. “Their study should be a stern warning to any practitioner who thinks the technology is ready for testing to eradicate disease genes.”
There has been a lot of concerns raised by researchers all around the world on this study. They have expressed that work crosses an ethical line: there were several warnings by researchers that because the genetic changes to embryos, known as germline modification, are heritable, they could have an unpredictable effect on future generations. Researchers have also expressed concerns that any gene-editing research on human embryos could be a slippery slope towards unsafe or unethical uses of the technique. This latest paper has reignited the debate on human-embryo editing and there are reports that other groups in China are also experimenting on human embryos.
Huang (the corresponding author of the paper) says that the paper was rejected by Nature and Science, in part because of ethical objections, however both journals declined to comment on the claim. He adds that 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 technique operates differently in them.
Still, he maintains that the embryos allow for a more meaningful model — and one closer to a normal human embryo — than an animal model or one using adult human cells, which is true in an eerie way.