Commentary

Opening the door to gene editing?


 

In early August, an international team of biologists reported injecting gene editing proteins into more than a hundred human embryos in Portland, Ore. The scale and success of such experimentation with human embryos is unprecedented in the United States. Given the highly experimental nature of fertility clinics in the United States and abroad, many suggest that these findings open the door to designer babies. A careful read of the report, however, indicates that the door is still quite closed, perhaps cracked open just a little.

The research team used a new method of cutting the genome, called CRISPR-Cas9. CRISPR utilizes two key components that the team combined in a test tube together: a Cas9 protein that can cut the DNA and a synthetic RNA that can guide the protein to cut a 20-letter sequence in the human genome specifically. In these experiments, the Cas9-RNA protein was designed to cut a pathogenic mutation in the MYBPC3 gene, which can cause hypertrophic cardiomyopathy. The research team could not obtain human zygotes with this mutation on both copies of the genome (a rare homozygous genotype). Such zygotes would have the most severe phenotype and be the most compelling test case for CRISPR. Instead, they focused on gene editing heterozygous human zygotes that have one normal maternal copy of the MYBPC3 gene and one pathogenic paternal copy. The heterozygous zygotes were produced by the research team via in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) using sperm donated by males carrying the pathogenic mutation (Nature. 2017 Aug 2. doi: 10.1038/nature23305).

When researchers injected the Cas9-RNA protein targeting the mutation into already fertilized zygotes, they found that 67% of the resulting embryos had two normal copies of the MYBPC3 gene. Without gene editing, approximately 50% of the embryos would have two normal copies, because the male sperm donor would produce equal numbers of sperm with normal and pathogenic genotypes. Thus, editing likely corrected only about 17% of the embryos that would have otherwise had one pathogenic paternal mutation. Thirty-six percent of embryos had additional mutations from imprecise gene editing. Further, some of the gene edits and additional mutations were mosaic, meaning that the resulting embryo harbored many different genotypes.

To overcome these challenges, the research team precisely controlled the timing of CRISPR injection to coincide with fertilization. With controlled timing, gene editing was restricted to only the paternal pathogenic mutation, resulting in 72% of all injected embryos having two normal copies of the gene in all cells without any mosaicism. Whole genome sequencing revealed no additional mutations above the detection limit of the assay. Finally, preimplantation development proceeded normally to the blastocyst stage, suggesting that the edited embryos have no functional deficits from the procedure.

A surprising finding was that new sequences could not be put into the embryo. The research team had coinjected a synthetic DNA template that differed from the normal maternal copy, but never saw this sequence incorporated into any embryo. Instead, the zygote utilized the maternal copy of the gene with the normal sequence as a template for repairing the DNA cut in the paternal copy produced by CRISPR. The biology behind this repair process is poorly understood and has not been previously reported with other human cell types. These observations suggest that we cannot easily “write” our genome. Instead, our vocabulary is limited to what is already within either the maternal or paternal copy of the genome. In other words, designer babies are not around the corner. While preimplantation genetic diagnosis (PGD) is still currently the safest way to avoid passing on autosomal dominant mutations, these new findings could enable correction of such mutations within IVF embryos, resulting in a larger pool of embryos for IVF clinics to work with.

Apart from these technical challenges, the National Academies has not given a green light to implant edited human embryos. Instead, the organization calls for several requirements to be met, including “broad societal consensus” on the need for this type of intervention. While it is not clear whether or how consensus could be achieved, it is clear that scientists, clinicians, and patients will need help from the rest of society for this research to have an impact clinically.

Dr. Krishanu Saha of the University of Wisconsin, Madison

Dr. Krishanu Saha

Dr. Saha is assistant professor of biomedical engineering at the Wisconsin Institute for Discovery at the University of Wisconsin, Madison. His lab works on gene editing of human cells. He has patent filings through the Wisconsin Alumni Research Foundation on gene editing inventions.

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