CRISPR-Cas9 genome editing creates targeted double strand breaks (DSBs) in eukaryotic cells that are processed by cellular DNA repair pathways. Co-administration of single stranded oligonucleotide donor DNA (ssODN) during editing can result in high-efficiency (>20%) incorporation of ssODN sequences into the break site. This process is commonly referred to as homology directed repair (HDR) and here referred to as single stranded template repair (SSTR) to distinguish it from repair using a double stranded DNA donor (dsDonor). The high efficacy of SSTR makes it a promising avenue for the treatment of genetic diseases, but the genetic basis of SSTR editing is still unclear, leaving its use a mostly empiric process. To determine the pathways underlying SSTR in human cells, we developed a coupled knockdown-editing screening system capable of interrogating multiple editing outcomes in the context of thousands of individual gene knockdowns. Unexpectedly, we found that SSTR requires multiple components of the Fanconi Anemia (FA) repair pathway, but does not require Rad51-mediated homologous recombination, distinguishing SSTR from repair using dsDonors. Knockdown of FA genes impacts SSTR without altering break repair by non-homologous end joining (NHEJ) in multiple human cell lines and in neonatal dermal fibroblasts. Our results establish an unanticipated and central role for the FA pathway in templated repair from single stranded DNA by human cells. Therapeutic genome editing has been proposed to treat genetic disorders caused by deficiencies in DNA repair, including Fanconi Anemia. Our data imply that patient genotype and/or transcriptome profoundly impact the effectiveness of gene editing treatments and that adjuvant treatments to bias cells towards FA repair pathways could have considerable therapeutic value.

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