Harvard University researchers have smashed the record for the most genetic alterations made in a single cell. The researchers used the CRISPR gene editing tool to make 13,200 changes to the DNA in a single human cell. The previous record was 62
CRISPR is most commonly used to make a single change to the DNA, although the tool can be used to make many changes and to perform edits on a much larger scale. These large-scale changes would likely make so many changes that it would result in a new species being created.
In 2017, researchers made huge numbers of changes to the Y chromosome in mice, which resulted in the chromosome being totally eradicated in vitro. A similar technique could be used to remove defective genes in humans, which could serve as a novel treatment for Down syndrome. Down syndrome involves an additional chromosome – a third copy of chromosome 21. The eradication of that additional chromosome could potentially serve as a cure. Researchers at the University of Washington did succeed in eradicating the additional chromosome in cell cultures in 2012, without affecting other parts of the DNA.
To set the record, the researchers used CRISPR to make changes to a sequence of DNA called LINE-1, which is a repetitive section of DNA known as a transposon. Transposons are found in multiple locations across the human genome and account for 17% of DNA. Transposons are able to copy themselves and change position in the DNA, either creating or changing mutations. Studies have suggested that transposons are not required and are junk DNA, but they can lead to dangerous mutations developing.
One of the problems of making such large-scale edits of the DNA is it can result in cell death. Previous attempts to make large-scale changes to LINE-1 elements did not end well and resulted in cell death. The researchers at Harvard got around the problem by using a form of CRISPR that does not make double stranded cuts to the DNA, instead it swaps base pairs, for example changing cytosine for thymine or adenine for guanine.
Using this form of CRISPR, the researchers were able to make around 13,200 edits without introducing genome-wide instability and killing the cells. The 13,200 edits corresponded to approximately half of all transposons in the cells they were working on. They are now working on increasing the number of edits to remove all transposons in the cells.
Performing such extensive edits could be used to eliminate all copies of viral DNA from a genome, thus preventing the reactivation of viruses. That study was the previous record holder, involving 62 edits to the genomes of pigs to remove retrovirus DNA. Another potential application is to make large numbers of changes to the DNA of pigs to enable their organs to be used in human transplants or simply to eradicate transposons to reduce the risk of genetic mutations.
The research is detailed in the paper – Enabling large-scale genome editing by reducing DNA nicking – which was recently published in the journal bioRxiv. DOI: 10.1101/574020