Coordinated processing of 3′ slipped (CAG)n/(CTG)n hairpins by DNA polymerases preferentially induces repeat expansions

Abstract

Expansion of CAG/CTG trinucleotide repeats causes certain familial neurological disorders. Hairpin formation in the nascent strand during DNA synthesis is considered a majorpath for (CAG)n/(CTG)nexpansion. However, the underlying mechanism is unclear. We show here that removal or retention of a nascent strand hairpin depends on hairpin structures and DNA polymerases involved. Pol delta alone removesthe 3′slipped hairpin using its proofreading activity when it contains no immediate 3′complementary sequences. However, in the presence of pol beta, pol delta preferentially facilitates hairpin retention regardless of hairpin structures. In this reaction, pol beta incorporates several nucleotides to the hairpin 3′end, which serves as an effective primer for the continuous DNA synthesis by pol delta, thereby leading to repeat expansion. These findings strongly support a mechanism that involves polymerase switching between pol delta and pol beta in coping with 3′slipped (CAG)n/(CTG)nhairpins during DNA synthesis.