Organic Chemistry Seminar

Triplet repeat sequences, such as CAG/CTG, expand in the human genome to cause several neurological disorders. This repetitive DNA sequence is known to adopt non-canonical structures such as stem-loop hairpins. Furthermore, work from other laboratories has implicated the DNA repair enzyme 8-oxo-7,8-dihydroguanine glycosylase (OGG1) in triplet repeat expansion. OGG1 initiates the base excision repair (BER) process in mammalian cells by excising the oxidatively damaged nucleobase 8-oxo-7,8-dihydroguanine (8-oxoG) from DNA. Motivated by the demonstrated involvement of 8-oxoG and OGG1 in triplet repeat expansion we first examined the susceptibility of CAG/CTG DNA to oxidative damage. We identified a hot spot for oxidative damage in the stem-loop hairpin formed by triplet repeat DNA. We next conducted a comprehensive kinetic analysis of OGG1 activity. We demonstrated that changes to the sequence composition are well tolerated whereas the structure of the DNA substrate modulates OGG1 activity. Indeed, duplexes containing 8-oxoG are efficiently repaired, whereas the stem-loop hairpin is not repaired and 8-oxoG accumulates in this non-canonical structure. Taken together, these results have allowed us to propose a toxic cycle in which BER is initiated on triplet repeat duplexes, and accumulation of damage in a stem-loop hairpin intermediate initiates a toxic cycle of damage and misrepair that result in an incremental expansion of the triplet repeat sequence. I will also describe experiments in which the triplet repeat sequence was examined in the context of a nucleosome and how these data influence our understanding of how the repetitive sequence packages in chromatin.