Gene editing of colocalized NCF1 loci is associated with homologous recombination and chromosomal rearrangements - CAST-seq

Despite lingering genotoxicity concerns, CRISPR-based genome editing has progressively entered the gene therapy field. Here we report that interactions between nucleotide sequences that are localized on the same chromosome and exhibit a high degree of homology contribute substantially to post-editing chromosomal rearrangements. We successfully employed corrective editing strategies at the NCF1 gene, as well as at the colocalized pseudogenes, NCF1B and NCF1C, in a human cell line model of p47phox-deficient chronic granulomatous disease and in patient-derived human hematopoietic stem cells. Upon genetic manipulation, an improved droplet digital PCR-based method identified cells with altered copy numbers, spanning megabases from the edited loci. We attributed the high aberration frequency to the interaction between repetitive sequences and their predisposition to recombination events. Our findings emphasize the need for careful evaluation of the target-specific genomic context, such as the presence of homologous regions, whose genome instability can constitute a risk factor for chromosomal rearrangements upon genome editing. NCF1B and NCF1C gene loci were simultaneously targeted with Cas9- and Cas12a-RNPs in mobilized peripheral blood human CD34+ cells from a healthy donor. Genotoxicity was assesed via CAST-Seq.