Synergistic modulation of cyclobutane pyrimidine dimer photoproduct formation and deamination at a T^mCG site over a full helical DNA turn in a nucleosome core particle

Sunlight-induced C to T mutation hotspots in skin cancers occur primarily at methylated CpG sites that coincide with sites of UV-induced cyclobutane pyrimidine dimer (CPD) formation. The C or 5-methyl-C in CPDs are not stable and deaminate to U and T, respectively, which leads to the insertion of A by DNA polymerase η and defines a probable mechanism for the origin of UV-induced C to T mutations. We have now determined the photoproduct formation and deamination rates for 10 consecutive T=^mCG CPDs over a full helical turn at the dyad axis of a nucleosome and find that whereas photoproduct formation and deamination is greatly inhibited for the CPDs closest to the histone surface, it is greatly enhanced for the outermost CPDs. Replacing the G in a T=^mCG CPD with A greatly decreased the deamination rate. These results show that rotational position and flanking sequence in a nucleosome can significantly and synergistically modulate CPD formation and deamination that contribute to C to T mutations associated with skin cancer induction and may have influenced the evolution of the human genome.