Influence of DNA structure on hypoxanthine and 1,N-6-ethenoadenine removalhy murine 3-methyladenine DNA glycosylase

3-Methyladenine DNA glycosylases initiate base excision repair by flipping the nucleotide bearing the target base out of doable-stranded DNA into an a ctive site pocket for glycosylic bond cleavage and base release. Substrate bases for th murine 3-methyladenine DNA glycosylase (other than 3-methylade nine) include hypoxanthine and 1,N-6-ethenoadenine, two mutagenic adducts f ormed by both endogenous and exogenous agents. Using double-stranded DNA ol igonucleotides containing damaged bases at specific sites, we studied the r elative removal rates for these two adducts when located in different seque nce contexts. One of the sequence contests was an A:T tract, chosen because DNA secondary structure is known to change along the length of this tract, due to a progressive narrowing of the minor groove. Here we report that re moval rates for hypoxanthine, but not for 1,N-6-ethenoadenine, are dramatic ally affected by its location within the A:T tract. In addition, the remova l rates of hypoxanthine and 1,N-6-ethenoadenine when paired opposite thymin e or cytosine were examined, and in each sequence context hypoxanthine remo val decreased by at least 20-fold when paired opposite cytosine versus thym ine, In contrast, 1,N-6-ethenoadenine removal was unaffected by the identit y of the opposing pyrimidine. We conclude that the removal of certain bases by the mouse 3-methyladenine DNA glycosylase can be modulated by both adja cent and opposing sequence contests. The influence of DNA sequence context upon DNA repair rates, such as those described here, may contribute to the creation of mutational hot spots in mammalian cells.