Reproductive biologist Shoukhrat Mitalipov and his team used genome editing to correct a gene that causes a potentially fatal heart condition in humans.
An international team of researchers has used CRISPR–Cas9 gene editing to correct a disease-causing mutation in a gene called MYBPC3 in dozens of viable human embryos. Such mutations cause the heart muscle to thicken — a condition known as hypertrophic cardiomyopathy that is the leading cause of sudden death in young athletes. The mutation is dominant, meaning that a child need inherit only one copy of the mutated gene to experience its effects.The study, representing a significant improvement in efficiency and accuracy over previous efforts, was published on Aug. 2nd in Nature, the embryos were not destined for implantation.
The team also tackled two safety hurdles that had clouded discussions about applying CRISPR–Cas9 to gene therapy in humans: the risk of making additional, off-target mutations and the risk of generating mosaics — in which different cells in the embryo contain different genetic sequences. The researchers say that they have found no evidence of off-target genetic changes, and generated only a single mosaic in an experiment involving 58 embryos.
The embryo experiments were conducted in the United States and led by Shoukhrat Mitalipov, a reproductive-biology specialist at the Oregon Health and Science University in Portland. Mitalipov’s team took several steps to improve the safety of the technique. Typically, researchers wishing to edit a genome will insert DNA encoding CRISPR components into cells, and then rely on the cells' machinery to generate the necessary proteins and RNA. But Mitalipov’s team instead injected the Cas9 protein itself, bound to its guide RNA, directly into the cells. Because the Cas9 protein degrades faster than the DNA that encodes it, the enzyme is left with less time to cut DNA, which leaves little time for off-target mutations to accumulate. The researchers noted that CRISPR–Cas9 error rate can vary depending on which DNA sequence is being targeted. The MYBPC3 mutation, in particular, was predicted to produce relatively few opportunities for off-target cutting.
The researchers also attempted to reduce the risk of mosaics by injecting the CRISPR–Cas9 components into the egg at the same time as they injected the sperm to fertilize it. This is earlier in development than previous human embryo editing experiments had tried, and studies in mouse embryos have shown that the technique can eliminate mosaics when the father’s genome is targeted. The low rate of mosaics and the unusually high efficiency of gene editing make the study stand out.
Several teams in China have already reported using CRISPR–Cas9 to alter disease-related genes in human embryos. Work is also under way in Sweden and the United Kingdom to use the technique to study the early stages of human embryo development. That research is aimed at understanding basic reproductive and developmental biology, as well as unpicking some of the causes of early miscarriages.
Read More:
Ma, H. et al. Correction of a pathogenic gene mutation in human embryos, Nature http://dx.doi.org/10.1038/nature23305 (2017).
Tang, L. et al. CRISPR/Cas9-mediated gene editing in human zygotes using Cas9 protein, Mol. Genet. Genomics 292, 525–533 (2017).
Suzuki, T., Asami, M. & Perry, A. C. F., Asymmetric parental genome engineering by Cas9 during mouse meiotic exit, Sci. Rep. 4, 7621 (2014).
