The thymine glycosylase MBD4 can bind to the product of deamination at methylated CpG sites

In addition to its well-documented effects on gene silencing, cytosine meth ylation is a prominent cause of mutations. In humans, the mutation rate fro m 5-methylcytosine (m(5)C) to thymine (T) is 10-50-fold higher(1-4) than ot her transitions and the methylated sequence CpG is consequently under-repre sented(5). Over one-third of germline point mutations associated with human genetic disease(6) and many somatic mutations leading to cancer(7,8) invol ve loss of CpG. The primary cause of mutability appears to be hydrolytic de amination. Cytosine deamination produces mismatched uracil (U), which can b e removed by uracil glycosylase(9,10), whereas m(5)C deamination generates a GT mispair that cannot be processed by this enzyme. Correction of m(5)CpG .TpG mismatches may instead be initiated by the thymine DNA glycosylase, TD G(11,12). Here we show that MBD4, an unrelated mammalian protein that conta ins a methyl-CpG binding domain(13,14), can also efficiently remove thymine or uracil from a mismatches CpG site in vitro. Furthermore, the methyl-CpG binding domain of MBD4 binds preferentially to m(5)CpG.TpG mismatches-the primary product of deamination at methyl-CpG. The combined specificities of binding and catalysis indicate that this enzyme may function to minimize m utation at methyl-CpG.