Instability of (CTG)n？(CAG)n trinucleotide repeats and DNA synthesis

Expansion of (CTG)n？(CAG)n trinucleotide repeat (TNR) sequences at distinct chromosomal loci is the mutation common to multiple neurological diseases including myotonic dystrophy type 1 (DM1), Huntington disease (HD), Huntington disease-like 2 (HDL2), dentatorubral-pallidoluysian atrophy (DRPLA), spinal and bulbar muscular atrophy (SBMA), and several forms of spinocerebellar ataxia (SCA). The polyglutamine diseases HD, DRPLA, SBMA, and SCA1, 3, 6, 7, 17 result from increases of (CAG)n repeats in the coding (nontemplate) strand for mRNA synthesis of the cognate genes ((CAG)n in RNA) to produce mutant polyglutamine proteins with toxic gain-of-function [1]. In contrast, (CTG)n？(CAG)n expansion at the DMPK 3' UTR alters the chromatin structure of the region, downregulates transcription of the locus and, as at the JPH3 gene produce poly-(CUG) pre-mRNAs respectively in DM1 and HDL2 patients that sequester the MBNL (CUG) binding proteins, leading to trans-dominant interference with the normal splicing of multiple RNAs. Finally, bidirectional transcription at the SCA8 locus can result in expression of both a polyglutamine protein and a (CUG)n expansion transcript, which may represent a toxic gain-of-function at both the protein and RNA levels.Trinucleotide repeat expansion requires DNA synthesis, either during DNA replication or repair. The effects of replication origin proximity, replication polarity, and replication inhibition support replication-based models of TNR instability in mitotic cells [2-9]. Hairpin formation by DNA polymerase slippage is a likely mechanism for changes in TNR repeat length [10-12]. Hairpin structure formation by DNA polymerase slippage at (CTG)n？(CAG)n sequences has been well documented in vitro [13,14] and can result in either insertion or deletion mutations. However, hairpins have also been postulated to arise during replication fork reversal and postreplication repair [2,15,16], Okazaki fragment maturation [17-19], base excision repair [20],