Instability of regions containing expanded CAG repeats during replication in Escherichia coli probed by labeled oligonucleotides

DNA triplet repeat expansion and mismatch repair

DNA mismatch repair is a conserved antimutagenic pathway that maintains genomic stability through rectification of DNA replication errors and attenuation of chromosomal rearrangements. Paradoxically, mutagenic action of mismatch repair has been implicated as a cause of triplet repeat expansions that cause neurological diseases such as Huntington disease and myotonic dystrophy. This mutagenic process requires the mismatch recognition factor MutSβ and the MutLα (and/or possibly MutLγ) endonuclease, and is thought to be triggered by the transient formation of unusual DNA structures within the expanded triplet repeat element. This review summarizes the current knowledge of DNA mismatch repair involvement in triplet repeat expansion, which encompasses in vitro biochemical findings, cellular studies, and various in vivo transgenic animal model experiments. We present current mechanistic hypotheses regarding mismatch repair protein function in mediating triplet repeat expansions and discuss potential therapeutic approaches targeting the mismatch repair pathway.

Instability of repeated DNAs during transformation in Escherichia coli

Escherichia coli has provided an important model system for understanding the molecular basis for genetic instabilities associated with repeated DNA. Changes in triplet repeat length during growth following transformation in E. coli have been used as a measure of repeat instability. However, very little is known about the molecular and biological changes that may occur on transformation. Since only a small proportion of viable cells become competent, uncertainty exists regarding the nature of these transformed cells. To establish whether the process of transformation can be inherently mutagenic for certain DNA sequences, we used a genetic assay in E. coli to compare the frequency of genetic instabilities associated with transformation with those occurring in plasmid maintained in E. coli. Our results indicate that, for certain DNA sequences, bacterial transformation can be highly mutagenic. The deletion frequency of a 106 bp perfect inverted repeat is increased by as much as a factor of 2 x 10(5) following transformation. The high frequency of instability was not observed when cells stably harboring plasmid were rendered competent. Thus, the process of transformation was required to observe the instability. Instabilities of (CAG).(CTG) repeats are also dramatically elevated upon transformation. The magnitude of the instability is dependent on the nature and length of the repeat. Differences in the methylation status of plasmid used for transformation and the methylation and restriction/modification systems present in the bacterial strain used must also be considered in repeat instability measurements. Moreover, different E. coli genetic backgrounds show different levels of instability during transformation.