New CRISPR Technique Causes Few Unintended Mutations in somatic cells of Fruit Flies

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"... Single cuts are useful when only one version of the gene is defective, a common occurrence in many genetic disorders. CRISPR-Nickase can remove the faulty copy, and the cell’s own DNA repair machinery can then copy the normal version in its place. This differs from traditional CRISPR-Cas9, where an experimentally introduced DNA template is required to repair the damage of a double-strand break—a process that can be error-prone ...

Repairing nicks, however, is commonplace in cells. Each time a gene is copied or expressed, the cell nicks DNA to unwind it and repairs it in order to recondense. “This happens thousands and thousands of times a day,” ...

Because cells naturally repair nicks, the researchers hypothesized that CRISPR-Nickase could be an efficient way to edit single copies of a gene. In research published by other scientists, Nickase has shown promise in fruit fly sperm and eggs, which only carry one copy of each gene. This study shows that it also can work in somatic cells, which carry two. ..."

From the abstract:

"Repair of double-strand breaks (DSBs) in somatic cells is primarily accomplished by error-prone nonhomologous end joining and less frequently by precise homology-directed repair preferentially using the sister chromatid as a template. Here, a Drosophila system performs efficient somatic repair of both DSBs and single-strand breaks (SSBs) using intact sequences from the homologous chromosome in a process we refer to as homologous chromosome-templated repair (HTR). Unexpectedly, HTR-mediated allelic conversion at the white locus was more efficient (40 to 65%) in response to SSBs induced by Cas9-derived nickases D10A or H840A than to DSBs induced by fully active Cas9 (20 to 30%). Repair phenotypes elicited by Nickase versus Cas9 differ in both developmental timing (late versus early stages, respectively) and the production of undesired mutagenic events (rare versus frequent). Nickase-mediated HTR represents an efficient and unanticipated mechanism for allelic correction, with far-reaching potential applications in the field of gene editing."

New CRISPR Technique Causes Few Unintended Mutations in Fruit Flies | The Scientist Magazine® A study finds that CRISPR-Nickase, which changes just one allele of a given gene, improves gene editing efficiency compared with CRISPR-Cas9.

Cas9/Nickase-induced allelic conversion by homologous chromosome-templated repair in Drosophila somatic cells (open access)