The Role of XPG in Processing (CAG) n/(CTG) nDNA Hairpins

Abstract

Background: During DNA replication or repair, disease-associated (CAG) n/(CTG) nexpansion can result from formation of hairpin structures in the repeat tract of the newly synthesized or nicked DNA strand. Recent studies identified a nick-directed (CAG) n/(CTG) nhairpin repair (HPR) system that removes (CAG) n/(CTG) nhairpins from human cells via endonucleolytic incisions. Because the process is highly similar to the mechanism by which XPG and XPF endonucleases remove bulky DNA lesions during nucleotide excision repair, we assessed the potential role of XPG in conducting (CAG) n/(CTG) nHPR.Results: To determine if the XPG endonuclease is involved in (CAG) n/(CTG) nhairpin removal, two XPG-deficient cell lines (GM16024 and AG08802) were examined for their ability to process (CAG) n/(CTG) nhairpins in vitro. We demonstrated that the GM16024 cell line processes all hairpin substrates as efficiently as HeLa cells, and that the AG08802 cell line is partially defective in HPR. Analysis of repair intermediates revealed that nuclear extracts from both XPG-deficient lines remove CAG/CTG hairpins via incisions, but the incision products are distinct from those generated in HeLa extracts. We also show that purified recombinant XPG protein greatly stimulates HPR in XPG-deficient extracts by promoting an incision 5' to the hairpin.Conclusions: Our results strongly suggest that 1) human cells possess multiple pathways to remove (CAG) n/(CTG) nhairpins located in newly synthesized (or nicked) DNA strand; and 2) XPG, although not essential for (CAG) n/(CTG) nhairpin removal, stimulates HPR by facilitating a 5' incision to the hairpin. This study reveals a novel role for XPG in genome-maintenance and implicates XPG in diseases caused by trinucleotide repeat expansion. © 2011 Hou et al; licensee BioMed Central Ltd.