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Microsatellite instability in yeast: dependence on repeat unit size and DNA mismatch repair genes
E A Sia1, R J Kokoska, M Dominska
1Department of Biology and Curriculum in Genetics and Molecular Biology, University of North Carolina, Chapel Hill 27599-3280, USA.
Abstract:
We examined the stability of microsatellites of different repeat unit lengths in Saccharomyces cerevisiae strains deficient in DNA mismatch repair. The msh2 and msh3 mutations destabilized microsatellites with repeat units of 1, 2, 4, 5, and 8 bp; a poly(G) tract of 18 bp was destabilized several thousand-fold by the msh2 mutation and about 100-fold by msh3. The msh6 mutations destabilized microsatellites with repeat units of 1 and 2 bp but had no effect on microsatellites with larger repeats. These results argue that coding sequences containing repetitive DNA tracts will be preferred target sites for mutations in human tumors with mismatch repair defects. We find that the DNA mismatch repair genes destabilize microsatellites with repeat units from 1 to 13 bp but have no effect on the stability of minisatellites with repeat units of 16 or 20 bp. Our data also suggest that displaced loops on the nascent strand, resulting from DNA polymerase slippage, are repaired differently than loops on the template strand.
Insights
DNA mismatch repair gene mutations significantly destabilize microsatellites, particularly shorter repeat units. This suggests repetitive DNA sequences are mutation hotspots in cancers with DNA repair defects.
Area of Science:
- Genetics
- Molecular Biology
- Genomic Instability
Background:
- Microsatellites are repetitive DNA sequences prone to instability.
- DNA mismatch repair (MMR) corrects errors during DNA replication.
- Defects in MMR are linked to various cancers.
Purpose of the Study:
- To investigate the impact of MMR gene mutations on microsatellite stability in Saccharomyces cerevisiae.
- To determine the relationship between microsatellite repeat unit length and MMR deficiency-induced instability.
- To infer the implications for mutation targets in human cancers.
Main Methods:
- Utilized Saccharomyces cerevisiae strains with targeted mutations in key MMR genes (msh2, msh3, msh6).
- Assessed the stability of microsatellites with varying repeat unit lengths (1 bp to 20 bp).
- Quantified microsatellite instability rates in wild-type versus MMR-deficient strains.
Main Results:
- msh2 and msh3 mutations destabilized microsatellites with repeat units from 1 to 8 bp, with a poly(G) tract showing thousands-fold destabilization by msh2.
- msh6 mutations destabilized short microsatellites (1-2 bp) but not longer ones.
- MMR genes impacted microsatellites up to 13 bp, but not minisatellites (16-20 bp).
- Evidence suggests differential repair of DNA polymerase slippage loops based on strand.
Conclusions:
- MMR deficiency profoundly destabilizes microsatellites, especially those with shorter repeat units.
- Repetitive DNA sequences in coding regions are likely mutation targets in human cancers with MMR defects.
- Slippage loop repair mechanisms may be strand-dependent.