Abstract
Genome editing with CRISPR/Cas9 depends on the induction of site-specific DNA double-strand breaks (DSBs), which are repaired by distinct pathways. Non-homologous end joining (NHEJ) frequently introduces insertions or deletions (indels), resulting in mutagenic repair, whereas microhomology-mediated end joining (MMEJ) produces larger deletions due to end resection. In the presence of a donor template, homologous recombination (HR) enables precise sequence changes but is generally inefficient in many cell types. Modulating the activity of key repair factors has therefore emerged as a promising strategy to bias DSB repair outcomes toward more predictable or precise edits. Here, we employed luminescent and fluorescent reporter assays to systematically quantify the impact of PARP1 modulation on repair pathway choice. We show that PARP1 downregulation increased both NHEJ and MMEJ repair without altering HR, while PARP1 overexpression reduced NHEJ and HR but left MMEJ activity unaffected. These results highlight PARP1 as a regulator of DSB repair balance and suggest that targeted modulation of PARP1 could improve the precision of CRISPR-based genome editing.