APOBEC3A-Induced DNA Damage Drives Polymerase Theta Dependency and Synthetic Lethality in Cancer

APOBEC3 cytidine deaminases drive cancer evolution. There is an unmet need to target cancer cells with APOBEC3 activity. Here, we identify error-prone theta-mediated end joining (TMEJ) as the main pathway for repairing APOBEC3-induced double-strand breaks (DSBs). Using fluorescent DSB repair reporters and a novel biochemical assay, we demonstrate that APOBEC3A competes with replication protein A (RPA) for single-stranded DNA overhangs, thereby exposing microhomologous sequences and shifting DSB repair towards error-prone TMEJ. Genomic analysis of clinical tumor samples confirmed the co-occurrence and proximity between APOBEC3-induced mutational footprints, microhomology-mediated deletions (MMDs), and TMEJ-associated chromosomal instability signatures. Crucially, inhibition of DNA polymerase theta synergizes with APOBEC3A-induced DSBs to induce synthetic lethality in vitro and in vivo. Collectively, our findings identify TMEJ as the preferred mechanism for repairing APOBEC3A-induced DSBs and establish polymerase theta inhibition as a novel, promising strategy to eliminate cancer cells with APOBEC3A activity.