Involvement of APOBEC3B in mutation induction by irradiation.

To better understand the cancer risk posed by radiation and the development of radiation therapy resistant cancer cells, we investigated the involvement of the cancer risk factor, APOBEC3B, in the generation of radiation-induced mutations. Expression of APOBEC3B in response to irradiation was determined in three human cancer cell lines by real-time quantitative PCR. Using the hypoxanthine-guanine phosphoribosyl transferase (HPRT) mutation assay, mutations in the HPRT gene caused by irradiation were compared between APOBEC3B-deficient human hepatocellular carcinoma (HepG2) cells [APOBEC3B knocked out (KO) using CRISPR-Cas9 genome editing] and the parent cell line. Then, HPRT-mutated cells were individually cultured to perform PCR and DNA sequencing of HPRT exons. X-Irradiation induced APOBEC3B expression in HepG2, human cervical cancer epithelial carcinoma (HeLa) and human oral squamous cell carcinoma (SAS) cells. Forced expression of APOBEC3B increased spontaneous mutations. By contrast, APOBEC3B KO not only decreased the spontaneous mutation rate, but also strongly suppressed the increase in mutation frequency after irradiation in the parent cell line. Although forced expression of APOBEC3B in the nucleus caused DNA damage, higher levels of APOBEC3B tended to reduce APOBEC3B-induced γ-H2AX foci formation (a measure of DNA damage repair). Further, the number of γ-H2AX foci in cells stably expressing APOBEC3B was not much higher than that in controls before and after irradiation, suggesting that a DNA repair pathway may be activated. This study demonstrates that irradiation induces sustained expression of APOBEC3B in HepG2, HeLa and SAS cells, and that APOBEC3B enhances radiation-induced partial deletions.