CRISPR is an exciting new way to edit any gene in the genome of any animal (so long as the sequence of the gene is known). Scientists have used CRISPR to prevent muscular dystrophy in mouse embryos and treat liver disease in adult mice. Now a different group has used CRISPR to remove a cataract-causing mutation from mouse sperm cells.
They completely prevented baby mice from inheriting cataracts from their parents. If this works in humans too, it will mean that people who know they have a disease-causing mutation, for example a mutation in the BRCA gene that can cause breast cancer or a mutation in the presenilin-1 gene that can cause early onset Alzheimer’s disease, can treat their eggs or their sperm before they make a baby. Now, all scientists can do is create embryos through in vitro fertilisation and screen those embryos for the mutation.
For this study, the scientists used mice that had a cataract-causing mutation in the Crygc gene. All the mutant Crygc mice had cataracts. The scientists took a specialised type of stem cell that turns into sperm cells from the testes of the mutant mice. They treated the sperm stem cells with CRISPR to edit out the mutation and replace it with the normal Crygc gene sequence. They included a red fluorescent protein in the treated cells so they could easily see which cells had been treated. They then picked red treated cells and grew them on plates in the lab. They tested 24 groups of cells and found that 12 had the corrected Crygc gene. So CRISPR in this application had a 50% success rate.
One problem with CRISPR is that it can cause changes in DNA outside of the gene you are trying to edit. So the scientists sequenced the whole genome in treated cells and found 185 mutations. This sounds like a lot but it’s actually the same number found in untreated cells. This is because the cells had divided 20-23 times on the plate to create mini colonies of cells before the scientists sequenced them. Each time a cell divides, it has to copy all its DNA to give to the daughter cell. Each time this happens it makes an average of 4-5 mistakes in the DNA sequence. These mistakes are mutations. The scientists also checked with CRISPR treatment changed the epigenetic marks on cells.
So CRISPR could be used to edit out the Crygc mutation in sperm stem cells, with no major side effect to the cell. But, more importantly, could these corrected sperm stem cells make healthy mouse babies? To answer that, the scientists took the treated sperm stem cells and injected them into the testes of infertile male mice. They waited two months and collected mature sperm cells. They then used these cells for in vitro fertilisation and implanted the resulting CRISPRed embryos into female mice. The implanted mice gave birth to 39 live pups, all of which had corrected Crygc gene. 100% success rate.
The Scientific Paper (free and open access!):