Endogenous APOBEC3A DNA Cytosine Deaminase Is Cytoplasmic and Nongenotoxic

APOBEC3A (A3A) is a myeloid lineage-specific DNA cytosine deaminase with a role in innate immunity to foreign DNA. Previous studies have shown that heterologously expressed A3A is genotoxic, suggesting that monocytes may have a mechanism to regulate this enzyme. Indeed, we observed no significant cytotoxicity when interferon was used to induce the expression of endogenous A3A in CD14+-enriched primary cells or the monocytic cell line THP-1. In contrast, doxycycline-induced A3A in HEK293 cells caused major cytotoxicity at protein levels lower than those observed when CD14+ cells were stimulated with interferon. Immunofluorescent microscopy of interferon-stimulated CD14+ and THP-1 cells revealed that endogenous A3A is cytoplasmic, in stark contrast to stably or transiently transfected A3A, which has a cell-wide localization. A3A constructs engineered to be cytoplasmic are also nontoxic in HEK293 cells. These data combine to suggest that monocytic cells use a cytoplasmic retention mechanism to control A3A and avert genotoxicity during innate immune responses. Background: APOBEC3A is a potentially genotoxic DNA cytosine deaminase expressed in myeloid lineage cells. Results: Exogenous APOBEC3A genotoxicity correlates with expression level and nuclear localization. Endogenous APOBEC3A is nontoxic and cytoplasmic, despite its capacity to be highly induced by interferon. Conclusion: Cytoplasmic localization prevents endogenous APOBEC3A from accessing nuclear DNA. Significance: Endogenous APOBEC3A is not genotoxic.