The deoxyribose phosphate lyase of DNA polymerase β suppresses a processive DNA synthesis to prevent trinucleotide repeat instability

Abstract

Trinucleotide repeat (TNR) instability is associated with over 42 neurodegenerative diseases and cancer, for which the molecular mechanisms remain to be elucidated. We have shown that the DNA base excision repair (BER) pathway and its central component, DNA polymerase β (pol β), in particular, its polymerase activity plays an active role in regulating somatic TNR instability. Herein, we revealed a unique role of the pol β dRP lyase in preventing somatic TNR instability. We found that deficiency of pol β deoxyribose phosphate (dRP) lyase activity locked the pol β dRP lyase domain to a dRP group, and this ‘tethered’ pol β to its template forcing the polymerase to perform a processive DNA synthesis. This subsequently promoted DNA strand slippage allowing pol β to skip over a template loop and causing TNR deletion. We showed that the effects were eliminated by complementation of the dRP lyase deficiency with wild-type pol β protein. The results indicate that pol β dRP lyase activity restrained the pol β-dRP interaction to suppress a pol β processive DNA synthesis, thereby preventing TNR deletion. This further implicates a potential of pol β dRP lyase inhibition as a novel treatment of TNR-expansion diseases.

On the wrong DNA track: Molecular mechanisms of repeat-mediated genome instability

“…It is possible that during BER, CAG repeats may fold into a hairpin or a loop-out structure, which, if incorporated into the nascent strand, would lead to a repeat expansion. MutS␤ likely stabilizes this structure, thus enhancing the chances for expansion (449,452) (Fig. 9A).…”

“…Importantly, purified MutS␤ complex additionally promotes Pol␤-mediated repeat instability (449). Furthermore, in vivo, MutS␤ and Pol ␤ colo-calize as a response to DNA damage (451) and together accumulate at CAG repeats during DNA replication (449). Overall, it appears that BER and MMR cooperate together to promote CAG repeat instability (449,451).…”

“…What is the molecular mechanism of BER-promoted repeat instability? At least in vitro, Pol ␤ is inherently slippery when synthesizing over CAG repeats and often adds (449) or skips over (450, 451) a portion of the repeat. Importantly, purified MutS␤ complex additionally promotes Pol␤-mediated repeat instability (449).…”

“…At least in vitro, Pol ␤ is inherently slippery when synthesizing over CAG repeats and often adds (449) or skips over (450, 451) a portion of the repeat. Importantly, purified MutS␤ complex additionally promotes Pol␤-mediated repeat instability (449). Furthermore, in vivo, MutS␤ and Pol ␤ colo-calize as a response to DNA damage (451) and together accumulate at CAG repeats during DNA replication (449).…”

“…Furthermore, in vivo, MutS␤ and Pol ␤ colo-calize as a response to DNA damage (451) and together accumulate at CAG repeats during DNA replication (449). Overall, it appears that BER and MMR cooperate together to promote CAG repeat instability (449,451). It is possible that during BER, CAG repeats may fold into a hairpin or a loop-out structure, which, if incorporated into the nascent strand, would lead to a repeat expansion.…”