Human APOBEC3G suppresses homologous recombination and LIG4 ‐independent end joining in DNA double‐strand break repair

Chromosomal DNA double-strand breaks (DSBs) are repaired by two fundamentally different mechanisms: homologous recombination (HR) and nonhomologous end joining (NHEJ). While NHEJ requires the Ku70-Ku80 heterodimer (Ku) and DNA ligase IV (LIG4), recent work has established a Ku/LIG4-independent mechanism for DSB joining, referred to as alternative end-joining or theta-mediated end-joining (TMEJ). Cells lacking NHEJ and TMEJ repair DSBs in a homology-dependent manner. Here we show that APOBEC3G (A3G), a cytoplasmic antiviral protein that possesses cytidine deaminase activity on ssDNA and inhibits the replication of various types of viruses such as HIV, affects DSB repair in the human lymphocyte cell line Nalm-6. We observed that genetic deletion of A3G in LIG4-null cells resulted in (a) reduced sensitivity to replication-dependent as well as -independent DSBs, (b) enhanced frequency of HR in chromosomal and extrachromosomal reporter assays, and (c) increased frequency of TMEJ-dependent random integration as well as HR-mediated targeted integration (gene targeting). Our results suggest that human A3G, albeit mainly cytoplasmic, acts to suppress HR and TMEJ during DSB repair, particularly when NHEJ is compromised.