Subcellular localization of the APOBEC3 proteins during mitosis and implications for genomic DNA deamination

Humans have seven APO BEC3 DNA cytosine deaminases. The activity of these enzymes allows them to restrict a variety of retroviruses and retrotransposons, but may also cause pro-mutagenic genomic uracil lesions. During interphase the APO BEC3 proteins have different subcellular localizations: cell-wide, cytoplasmic or nuclear. This implies that only a subset of APO BEC3s have contact with nuclear DNA. However, during mitosis, the nuclear envelope breaks down and cytoplasmic proteins may enter what was formerly a privileged zone. To address the hypothesis that all APO BEC3 proteins have access to genomic DNA, we analyzed the localization of the APO BEC3 proteins during mitosis. We show that APO BEC3A, APO BEC3C and APO BEC3H are excluded from condensed chromosomes, but become cell-wide during telophase. However, APO BEC3B, APO BEC3D, APO BEC3F and APO BEC3G are excluded from chromatin throughout mitosis. After mitosis, APO BEC3B becomes nuclear, and APO BEC3D, APO BEC3F and APO BEC3G become cytoplasmic. Both structural motifs as well as size may be factors in regulating chromatin exclusion. Deaminase activity was not dependent on cell cycle phase. We also analyzed APO BEC3-induced cell cycle perturbations as a measure of each enzyme's capacity to inflict genomic DNA damage. AID, APO BEC3A and APO BEC3B altered the cell cycle profile, and, unexpectedly, APO BEC3D also caused changes. We conclude that several APO BEC3 family members have access to the nuclear compartment and can impede the cell cycle, most likely through DNA deamination and the ensuing DNA damage response. Such genomic damage may contribute to carcinogenesis, as demonstrated by AID in B cell cancers and, recently, APO BEC3B in breast cancers.