Expansion of CTG repeats from human disease genes is dependent upon replication mechanisms in Escherichia coli: the effect of long patch mismatch repair revisited.

Many human hereditary disease genes have been recently associated with the expansion of CTG/GAC repeats. We have used a plasmid-based assay in Escherichia coli to investigate the instability of a (CTG/GAC) insert containing 64 repeats. Using this assay, expansions were biochemically detected and subsequently quantified. We show that the occurence of expansions within these trinucleotide repeats is dependent upon replicative mechanisms. Expansions of up to 30 repeats and deletions of almost all possible sizes occured regardless of the orientation of the insert relative to the replication origin. In contradiction to a previous report, the mismatch repair pathway was found to strongly stabilize these repeat stretches.