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Differences in genome-wide repeat sequence instability conferred by proofreading and mismatch repair defects.

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Summary

Mutation rates vary greatly across genomes. This study reveals homonucleotide run length dramatically impacts insertion/deletion (indel) mutation rates in yeast, with significant implications for genome stability.

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Area of Science:

  • Genetics
  • Molecular Biology
  • Genomics

Background:

  • Mutation rates are crucial for molecular clocks and disease association.
  • Mutation rates are non-uniform across eukaryotic genomes.
  • Insertion/deletion (indel) mutation rates vary significantly in vitro and in vivo.

Purpose of the Study:

  • To determine genome-wide rates of indel formation and repair during yeast DNA replication.
  • To investigate the impact of genomic parameters on indel mutation rates.
  • To compare the effects of proofreading and mismatch repair (MMR) defects on indel mutagenesis.

Main Methods:

  • Analysis of over 6000 indels in four mismatch repair (MMR) defective yeast strains.
  • Statistical corrections for false negatives in indel detection.
  • Genome-wide assessment of indel rates in relation to genomic features.

Main Results:

  • Indel rates increase 100,000-fold with increasing homonucleotide run length, the largest known effect on replication fidelity.
  • Long homopolymer runs are overrepresented, suggesting positive selection.
  • Proofreading defects increase indel rates in short repeats, while MMR defects dramatically increase rates in long repeats.

Conclusions:

  • Homonucleotide run length is a major determinant of indel mutation rates.
  • Proofreading and MMR pathways have distinct, quantitatively different effects on repeat sequence instability.
  • Differential impacts of these repair pathways on genome-wide repeat instability have significant biological consequences.