Human thymine DNA glycosylase (TDG) and methyl-CpG-binding protein 4 (MBD4) excise thymine glycol (Tg) from a Tg:G mispair

The repair enzymes thymine DNA glycosylase (TDG) and methyl-CpG-binding protein 4 (MBD4) remove thymines from T:G mismatches resulting from deamination of 5-methylcytosine. Thymine glycol, a common DNA lesion produced by oxidative stress, can arise from oxidation of thymine or from oxidative deamination of 5-methylcytosine, and is then present opposite adenine or opposite guanine, respectively. Here we have used oligonucleotides with thymine glycol incorporated into different sequence contexts and paired with adenine or guanine. We show that TDG and MBD4 can remove thymine glycol when present opposite guanine but not when paired with adenine. The efficiency of these enzymes for removal of thymine glycol is about half of that for removal of thymine in the same sequence context. The two proteins may have evolved to act specifically on DNA mismatches produced by deamination and by oxidation-coupled deamination of 5-methylcytosine. This repair pathway contributes to mutation avoidance at methylated CpG dinucleotides.