Abstract
Deamination of 5-methyl cytosine is a major cause of cancer-driver mutations in inflammation-associated cancers. The deaminase APOBEC3B is expressed in these cancers and causes mutations under replication stress; however, the mechanisms by which APOBEC3B mediates deamination and its association with genomic disorders are still unclear. Here, we show that APOBEC3B is stabilized to induce deamination reaction in response to DNA double-strand breaks (DSBs), resulting in the formation of long-lasting DSBs. Uracil, the major deamination product, is subsequently targeted by base excision repair (BER) through uracil-DNA glycosylase 2 (UNG2); hence late-onset DSBs arise as by-products of BER. The frequency of these delayed DSBs was increased by treatment of cells with a PARP inhibitor, and was suppressed following knock-down of UNG2. The late-onset DSBs were induced in an ATR-dependent manner. Those secondary DSBs were persistent, unlike DSBs directly caused by γ-ray irradiation. Overall, these results suggest that the deaminase APOBEC3B is induced in response to DSBs, leading to long-lasting DSB formation in addition to mutagenic 5me-C>T transition induction.
Highlights
Cancer development is associated with mutations and genomic instability [1,2]
While deamination of 5me-cytosine is induced only at epigenetically-methylated DNA loci, deamination of cytosine should be induced widely; this causes C > U transitions that subsequently induce uracil-DNA glycosylase 2 (UNG2)-initiated base excision repair (BER) to eliminate uracil from the DNA strands in the nucleus [10]
To determine the deamination status of cellular APOBEC3B in response to DNA damage, double-strand break (DSB) that appeared as by-products of BER were analyzed in irradiated cells expressing an APOBEC3B-containing or negative control vector
Summary
In cancers that develop under conditions of chronic inflammation, deamination-associated C > T mutations are induced massively in association with expression of deaminase APOBEC3A and B [3]. These C > T mutations occur widely in epigenetically-methylated CpG islands; this is because deamination of 5me-cytosine yields thymine (5me-C > T transition) [4]. While deamination at unpaired ssDNA loci could result in G/T-mismatches that are repairable by base excision repair (BER) (via the thymine DNA glycosylases TDG and MBD4) [8,9], deamination at exposed ssDNA sites could lead to mutational 5me-C > T transitions in addition to major C > U transitions that are repairable through UNG-mediated BER [10]. Deamination-mediated mutagenesis, i.e., 5me-C > T transition, is elevated in APOBEC3B-expressing cells [5] and is a major pathway that results in cancer-driver mutations, such as those present in the p53 and APC genes [11]
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