Coordination between Polymerase β and FEN1 Can Modulate CAG Repeat Expansion

Abstract

The oxidized DNA base 8-oxoguanine (8-oxoG) is implicated in neuronal CAG repeat expansion associated with Huntington disease, yet it is unclear how such a DNA base lesion and its repair might cause the expansion. Here, we discovered size-limited expansion of CAG repeats during repair of 8-oxoG in a wildtype mouse cell extract. This expansion was deficient in extracts from cells lacking pol ␤ and HMGB1. We demonstrate that expansion is mediated through pol ␤ multinucleotide gap-filling DNA synthesis during long-patch base excision repair. Unexpectedly, FEN1 promotes expansion by facilitating ligation of hairpins formed by strand slippage. This alternate role of FEN1 and the polymerase ␤ (pol ␤) multinucleotide gap-filling synthesis is the result of uncoupling of the usual coordination between pol ␤ and FEN1. HMGB1 probably promotes expansion by stimulating APE1 and FEN1 in forming single strand breaks and ligatable nicks, respectively. This is the first report illustrating that disruption of pol ␤ and FEN1 coordination during long-patch BER results in CAG repeat expansion.

Topoisomerase 1 and Single-Strand Break Repair Modulate Transcription-Induced CAG Repeat Contraction in Human Cells

“…First, it could be a trivial consequence of the selective detection of large contractions in our study versus the analysis of frequent events in the OGG1 studies (21,33). Thus, it is possible that defects in OGG1 promote large contractions, even as they reduce expansions, but that contractions occur at frequencies that are too low to detect by small-pool PCR.…”

“…In contrast, OGG1-APE1 leaves ends with 3Ј hydroxyls and 5Ј sugar phosphates (33). Although DNA polymerase ␤ can remove this obstruction, it can also displace the strand, allowing a CAG hairpin to form, presumably as a precursor to expansion (33). In the absence of OGG1, the troublesome ends would not be generated and the repeats would, therefore, be more stable.…”

“…The action of TOP1-TDP1 generates clean ends with 3Ј hydroxyls and 5Ј phosphates, which are conducive to religation (44); it is only when the process is interrupted that the repeat tracts are affected. In contrast, OGG1-APE1 leaves ends with 3Ј hydroxyls and 5Ј sugar phosphates (33). Although DNA polymerase ␤ can remove this obstruction, it can also displace the strand, allowing a CAG hairpin to form, presumably as a precursor to expansion (33).…”

“…It is responsible for excising 8-oxo-guanine, a common base damage caused by reactive oxygen species (53). In cell extracts and in mice, it has been shown that OGG1 normally acts to promote CAG repeat expansions (21,33). Thus, in the absence of OGG1, CAG repeats become more stable.…”

Oxidative DNA Damage Modulates Trinucleotide Repeat Instability Via DNA Base Excision Repair

“…Formation of a hairpin structure can also disrupt the coordination among DNA repair proteins that sustains an efficient DNA repair. For example, disruption of the coordination between pol β and FEN1 by the formation of a hairpin structure can promote pol β multi-nucleotide gap-filling synthesis that produces extra CAG repeats for CAG repeat expansion (115).…”