Coordinated Processing of 3′ Slipped (CAG)n/(CTG)n Hairpins by DNA Polymerases β and δ Preferentially Induces Repeat Expansions

Abstract

Background: Expansion of CAG/CTG repeats causes familial neurological disorders, but the molecular basis is unknown. Results: DNA polymerases ␤ and ␦ effectively incorporate nucleotides to a CAG/CTG hairpin primer, leading to hairpin retention. Conclusion: Coordinated actions by polymerases ␤ and ␦ on hairpin primers during DNA synthesis promotes CAG/CTG repeat expansions. Significance: The work discovers a novel mechanism for CAG/CTG repeat expansions.

Repeat instability during DNA repair: Insights from model systems

“…Instead Polβ adds a few nucleotides to the 3′ end of the hairpin thus generating an effective primer for Polδ-catalyzed DNA synthesis that occurs without hairpin excision. The net result is that the hairpin becomes fixed in the nascent strand and can result in a repeat expansion (Chan et al ., 2013) (Figure 3B, left pathway). In either case, since the hairpins formed by CAG and CTG repeats are hot spots for oxidation, and since OGG1 has a reduced affinity for 8-oxoG in these hairpins and excises them at a significantly lower rate compared with duplexes (Jarem et al ., 2011), hairpin formation could result in a ‘‘toxic oxidation cycle’’ in which the repair of one lesion would increase the opportunity for the generation of additional oxidized bases.…”

The Repeat Expansion Diseases: The dark side of DNA repair

“…The second sub-pathway involves Polβ acting without Polδ or Polε to carry out a more limited gap-filling reaction that involves the synthesis of fewer nucleotides [104]. There is in vitro data to support the idea that expansions could arise in the Polβ/Polδ/Polε-dependent pathway if strand-slippage occurred on the nascent strand, an event that would be facilitated by secondary structure formation, and if priming by Polβ then occurred from the slipped position [105]. Expansions may also arise by structure formation on the displaced strand that prevents proper flap processing.…”

“…(a) Repair of the nick may proceed via an LP BER pathway that involves Polβ, Polδ and perhaps Polε [111]. Strand-slippage/hairpin formation at the 3’ terminus of the nascent strand arising during strand displacement synthesis by Polδ/Polε could result in expansion if the hairpin is not removed because Polβ synthesis prevents proof-reading by Polδ/Polε [105]. Formation of a secondary structure on a displaced flap could also result in expansion if proper processing were blocked.…”

Replication stalling and DNA microsatellite instability

“…The BER polymerase beta has been shown to expand nascent strand CTG trinucleotide repeats at gaps generated by OGG1/APE1, similar to the structure of gaps expected from repeat-induced stalling and dissociation of the lagging strand polymerase [147–149]. Microsatellite expansion catalyzed by OGG1/APE1/Pol beta has engendered the “toxic oxidation cycle” model to account for microsatellite expansion induced by reactive oxygen species [147].…”