Structure-Dependent DNA Damage and Repair in a Trinucleotide Repeat Sequence

摘要

Triplet repeat sequences, such as CAG/CTG, expand in the human genome to cause severalnneurological disorders. As part of the expansion process the formation of non-B DNA conformations by thenrepeat sequence has previously been proposed. Furthermore, the base excision repair enzyme 7,8-dihydro-n8-oxoguanine glycosylase (OGG1) has recently been implicated in the repeat expansion [Kovtun, I. V., Liu, Y.,nBjoras, M., Klugland, A., Wilson, S. H., and McMurray, C. T. (2007) Nature 447, 447-452]. In this work wenhave found that the non-B conformation adopted by (CAG)10, a hairpin, is hypersusceptible toDNA damagenrelative to the (CAG)10/(CTG)10 duplex and, in particular, that a hot spot for DNA damage exists.nSpecifically, we find that a single guanine in the loop of the hairpin is susceptible to modification bynperoxynitrite. Interestingly, we find that human OGG1 (hOGG1) is able to excise 7,8-dihydro-8-oxoguaninen(8-oxoG) from the loop of a hairpin substrate, albeit with a marked decrease in efficiency relative to duplexnsubstrates; thehOGG1 enzyme removes 8-oxoG from the loop of a hairpin with a rate that is∼700-fold slowernthan that observed for DNA duplex. Thus, while damage is preferentially generated in the loop of the hairpin,nDNA repair is less efficient. These observed structure-dependent patterns of DNA damage and repair mayncontribute to the OGG1-dependent mechanism of trinucleotide repeat expansion.