Oxidatively generated damage to DNA at 5-methylcytosine mispairs

Abstract

Oxidatively generated damage to DNA has been implicated as causing mutations that lead to aging and disease. The one-electron oxidation of normal DNA leads to formation of a nucleobase radical cation that hops through the DNA until it is trapped irreversibly, primarily by reaction at guanine. It has been observed that 5-methylcytosine (Cm) is a mutational "hot-spot". However, Cm in a Watson-Crick base pair with G is not especially susceptible to oxidatively induced damage. Radical cation hopping is inhibited in duplexes that contain C-A or C-T mispairs, but no reaction is detected at cytosine. In contrast, we find that the one-electron oxidation of DNA that contains Cm-A or Cm-T mispairs results primarily in reaction at Cm even in the presence of GG steps. The reaction at Cm is attributed to proton coupled electron transfer, which provides a relatively low activation barrier path for reaction at 5-methylcytosine. This enhanced reactivity of Cm in mispairs may contribute to the formation of mutational hot spots at Cm.