Investigating the regulation of APOBEC3A and APOBEC3B expression in keratinocytes

APOBEC3A (A3A) and APOBEC3B (A3B) can disrupt viral and retrotransposon replication through deamination, while off-target activity represents a major source of mutations in epithelial cancers. A3A additionally has an emerging role as an RNA editor within immune cells. To explore their function and regulation in normal epithelia, we used near-normal human keratinocyte cells (NIKS) in which CRISPR-Cas9 editing had been used to delete endogenous A3A, or to epitope-tag A3A or A3B. Having observed that A3A induction by known inducers (phorbol ester and interferon-) occurs in a subset of cells and is sensitive to contact inhibition, we hypothesised cell cycle-dependent regulation and discovered striking induction upon cell cycle re-entry induced by Epidermal Growth Factor (EGF) in NIKS, primary keratinocytes (NHEK), and breast mammary epithelial cells (MCF10A). We found A3B was also cell-cycle related and specifically upregulated during G2/M phase of proliferating cells. Surprisingly, high A3A levels appear to persist through S-phase without overt genotoxic effects. Instead, we observe high levels of A3A-dependent RNA editing, the significance of which remains to be determined. A3A induction is blunted in cancer cell lines and in NIKS expressing human papillomavirus oncogenes, and A3A deletion significantly increased proliferation of NIKS. Together, our findings uncover EGF signalling as a potent inducer of A3A in epithelial cells with implications for cell cycle regulation and carcinogenesis.