Trinucleotide Repeat Instability is Modulated by DNA Base Lesions and DNA Base Excision Repair

Trinucleotide repeat (TNR) expansions are the cause of over 40 human neurodegenerative diseases, and are linked to DNA damage and base excision repair (BER). We explored the role of DNA damage and BER in modulating TNR instability through analysis of DNA structures, BER protein activities, and reconstitution of repair using human BER proteins and synthesized DNA containing various types of damage. We show that DNA damage and BER can modulate TNR expansions by promoting removal of a TNR hairpin through coordinated activities of BER proteins and cofactors. We found that during repair in a TNR hairpin, coordination between the 5’-flap endonuclease activity of flap endonuclease 1 (FEN1), 3’-5’ exonuclease activity of AP endonuclease 1 (APE1), and activity of DNA ligase I (LIG I) can resolve the double-flap structure produced during BER in the hairpin loop. The resolution of the double-flap structure resulted in hairpin removal and prevention or attenuation of TNR expansions and provides the first evidence that coordination among BER proteins can remove a TNR hairpin. We further explored the role of BER cofactors in modulating TNR instability and found that the repair cofactor proliferating cell nuclear antigen (PCNA) facilitates genomic stability by promoting removal of a TNR hairpin. Hairpin removal was accomplished by altering dynamic TNR structures to allow more efficient FEN1 cleavage and DNA polymerase β (pol β) synthesis and stimulating the activity of LIG I. This study provides the first evidence that a DNA repair cofactor plays an important role in modulating TNR instability. Finally, we explored the effects of sugar modifications in abasic sites on activities of BER proteins and BER efficiency during repair in a TNR tract. We found that an oxidized sugar inhibits the activities of BER enzymes, interrupting their coordination and preventing efficient repair. Inefficient repair results in accumulation of repair intermediates with DNA breaks, contributing to genomic instability. Our results indicate that DNA base lesions and BER play a crucial role in modulating TNR instability. The research presented herein provides a molecular basis for further developing BER as a target for prevention and treatment of neurodegenerative diseases caused by TNR expansion