Incision-dependent and error-free repair of (CAG) n /(CTG) n hairpins in human cell extracts

• Published in: Nature structural & molecular biology Vol. 16; no. 8; pp. 869 - 875
• Main Authors: Chan, Nelson L S, Li, Guo-Min, Gu, Liya, Hou, Caixia
• Format: Journal Article
• Language: English
• Published: New York Nature Publishing Group US 01-08-2009; Nature Publishing Group
• Subjects: Biochemistry; Biological Microscopy; Biomedical and Life Sciences; Cellular biology; Deoxyribonucleic acid; DNA; Etiology; Life Sciences; Membrane Biology; Molecular biology; Molecular structure; Neurological disorders; Protein Structure
• ISSN: 1545-9993; 1545-9985
• DOI: 10.1038/nsmb.1638

CAG/CTG trinucleotide repeat expansion is linked to disorders such as Huntington's disease. These repeats tend to form stable hairpins that contribute to expansion. Removal of such DNA hairpins in human cell extracts is now found to occur via endonucleolytic incisions in an error-free manner followed by DNA synthesis.
Expansion of CAG/CTG trinucleotide repeats is associated with certain familial neurological disorders, including Huntington's disease. Increasing evidence suggests that formation of a stable DNA hairpin within CAG/CTG repeats during DNA metabolism contributes to their expansion. However, the molecular mechanism(s) by which cells remove CAG/CTG hairpins remain unknown. Here we demonstrate that human cell extracts can catalyze error-free repair of CAG/CTG hairpins in a nick-directed manner. The repair system specifically targets CAG/CTG tracts for incisions in the nicked DNA strand, followed by DNA resynthesis using the continuous strand as a template, thereby ensuring CAG/CTG stability. Proliferating cell nuclear antigen (PCNA) is required for the incision step of the hairpin removal, which uses distinct endonuclease activities for individual CAG/CTG hairpins depending on their strand locations and/or secondary structures. We discuss the implications of these data for understanding the etiology of neurological diseases and trinucleotide repeat instability.