Structure-guided inhibition of the cancer DNA-mutating enzyme APOBEC3A

The normally antiviral enzyme APOBEC3A^1-4 is an endogenous mutagen in many different human cancers^5-7, where it becomes hijacked to fuel tumor evolvability. APOBEC3A's single-stranded DNA C-to-U editing activity^1,8 results in multiple mutagenic outcomes including signature single-base substitution mutations (isolated and clustered), DNA breakage, and larger-scale chromosomal aberrations^5-7. Transgenic expression in mice demonstrates its tumorigenic potential⁹. APOBEC3A inhibitors may therefore comprise a novel class of anti-cancer agents that work by blocking mutagenesis, preventing tumor evolvability, and lessening detrimental outcomes such as drug resistance and metastasis. Here we reveal the structural basis of competitive inhibition of wildtype APOBEC3A by hairpin DNA bearing 2'-deoxy-5-fluorozebularine in place of the cytidine in the TC recognition motif that is part of a three-nucleotide loop. The nuclease-resistant phosphorothioated derivatives of these inhibitors maintain nanomolar in vitro potency against APOBEC3A, localize to the cell nucleus, and block APOBEC3A activity in human cells. These results combine to suggest roles for these inhibitors to study A3A activity in living cells, potentially as conjuvants, leading toward next-generation, combinatorial anti-mutator and anti-cancer therapies.