Researchers from around the Tri-State area came together in 2020 to examine the effectiveness of the Crispr-Cas9 double stranded DNA break (DSB) induction on human embryos to repair a chromosomal mutation. The study, which was published in Cell, began with sperm from a mutated male patient at the EYS locus, which causes retinitis pigmentosa blindness. The researchers then attempted to use CRISPR-Cas9 technology to repair the blindness gene in a number of fertilized embryonic stem cells that carried the EYS mutation. The results showed that about half of the breaks in the experiment went unrepaired, which resulted in an undetectable paternal allele. After mitosis, the loss of one or both the chromosomal arms was also common. This study shows that using CRISPR-Cas9 technology is still in its early days, and needs to be further vetted before it is used to treat patients.
Instead of correctly and consistently editing the genome of the embryos, the Crispr-Cas9 wreaked havoc, leaving behind chromosomal trauma. The data shows that the embryos started to tear apart and get rid of big pieces of the chromosome that had the EYS mutation, some losing the entire chromosome. The promise of Crispr technology is about changing one gene, but how can that be done when a larger, untargeted part of the genome is also being altered? Dr. Egli, the paper’s main author, brought up a more likely use for the Crispr editing: deploying it as a form of “moleculure bomb”, sent in to shred unwanted chromosomes. An important part of using gene editing is the ability to consistently predict the outcome, However, the resulting “mosaicism prevents inferring the genotype of the fetus from a biopsy and is thus incompatible for clinical use”, according to the Cell authors.
There were many rarities that appeared in the alleles of the embryos used. With a small sample size, due to the difficulty to acquire human embryos, there was no ability to rule out rare events. Although there were combinations of maternal and paternal alleles that showed interhomolog events, they occurred after the two-cell-stage injections, all mosaic. A single Cas9-induced break can result in outcomes in the human embryo that suggest species-specific differences in repair. In on-target sequencing of the cells, the detection of only a wild-type maternal allele might have been because of the unrepaired breaks and the loss of the chromosomal arm or the loss of the entire chromosome. This study shines light on the dangers of Crispr gene editing. The quotes from researchers, doctors, and genealogists all echo the same risk, we must walk before we can run. Testing and ensuring the safety of using Crispr on an embryo before the first round of DNA replication happens is crucial to the ultimate promise of gene repair. If it can’t be done safely with no off target effects, then Crispr “would be deeply unethical”, according to Dr Faraheny from Duke University.