The effect of APOBEC3B deaminase on double-stranded DNA

Mutations mediated by the APOBEC3 (A3) family of single-strand specific cytosine deaminases can accumulate in various cancers, as strand-coordinated clusters and isolated lesions. A3-mediated mutations also occur during normal development, accounting for ~20% of heritable mutations. A3B is an archetypical member of this family and is thought to contribute to both cancer initiation and progression. A3B has a strong preference for C in a TC context and catalyzes hydrolysis of the primary amine of un-paired C to generate U. Subsequent repair generates a distinctive pattern of C-substitutions, which along with their context signify their A3B origin. Although single-stranded DNA is the preferred A3B substrate, we report here that in some instances A3B can deaminate the C of TC in a double-stranded DNA context in vitro. These include C paired to O6-methylguanine (O6meG), to an abasic (AP) site, or to a G adjacent to an AP site. AP sites are the most common lesion in DNA, and O6meG levels increase under alkylating conditions caused by environmental nitrosamines and some chemotherapeutic agents. We also show that elevated expression of A3B can enhance double-stranded breaks induced by the alkylating agent MNNG in mammalian cells, but this effect does not require A3B deaminase activity.