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Cis-elements governing trinucleotide repeat instability in Saccharomyces cerevisiae.
M L Rolfsmeier1, M J Dixon, L Pessoa-Brandão
1Eppley Institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, Omaha, Nebraska 68198-6805, USA.
Genetics
|April 6, 2001
Summary
Yeast genetic assays revealed that trinucleotide repeat (TNR) instability has a minimal repeat length threshold and is sequence-specific, similar to humans. These findings demonstrate conserved mechanisms of TNR instability across species.
Area of Science:
- Genetics
- Molecular Biology
- Genomic Instability
Background:
- Trinucleotide repeat (TNR) instability in humans is influenced by specific cis-elements, including a minimal repeat length threshold and sequence specificity.
- The hairpin-forming ability of CNG sequences is hypothesized to drive TNR instability by interfering with DNA repair.
- The molecular basis of these cis-elements remains unclear due to a lack of direct demonstration in model systems.
Purpose of the Study:
- To investigate the conserved nature of TNR instability cis-elements by monitoring expansions and contractions in yeast.
- To determine if a minimal repeat length threshold for TNR mutations exists in a model organism.
- To assess the role of sequence composition and flap endonuclease in TNR instability.
Main Methods:
- Utilized yeast genetic assays to monitor trinucleotide repeat expansions and contractions.
- Analyzed mutations across various CTG tract lengths and sequence compositions (CNG vs. A-T rich).
- Examined the impact of flap endonuclease (Rad27p) deficiency on TNR expansion thresholds.
Main Results:
- A threshold of approximately 15-17 repeats was identified for both CTG expansions and contractions in yeast.
- CNG/GNC sequences exhibited high mutation rates, while A-T rich sequences were more stable, supporting a hairpin-mediated instability model.
- The absence of flap endonuclease (Rad27p) did not significantly alter the expansion threshold, suggesting flap processing is not critical for threshold determination.
- Expansions and contractions occurred at similar rates for CTG tracts between 15 and 25 repeats.
Conclusions:
- Key cis-elements governing human TNR instability, including repeat length thresholds and sequence specificity, are functional in yeast.
- The findings support a hairpin-mediated mechanism for TNR instability, conserved across species.
- Expansions constitute a significant portion of TNR mutations in yeast, particularly within the identified threshold range.