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Postreplication Repair Inhibits CAG · CTG Repeat Expansions in Saccharomyces cerevisiae

Eppley Institute for Research in Cancer and Allied Diseases,² Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, Nebraska 68198-6805¹

Received 28 June 2006/ Returned for modification 20 July 2006/ Accepted 2 October 2006

Trinucleotide repeats (TNRs) are unique DNA microsatellites that can expand to cause human disease. Recently, Srs2 was identified as a protein that inhibits TNR expansions in Saccharomyces cerevisiae. Here, we demonstrate that Srs2 inhibits CAG · CTG expansions in conjunction with the error-free branch of postreplication repair (PRR). Like srs2 mutants, expansions are elevated in rad18 and rad5 mutants, as well as the PRR-specific PCNA alleles pol30-K164R and pol30-K127/164R. Epistasis analysis indicates that Srs2 acts upstream of these PRR proteins. Also, like srs2 mutants, the pol30-K127/164R phenotype is specific for expansions, as this allele does not alter mutation rates at dinucleotide repeats, at nonrepeating sequences, or for CAG · CTG repeat contractions. Our results suggest that Srs2 action and PRR processing inhibit TNR expansions. We also investigated the relationship between PRR and Rad27 (Fen1), a well-established inhibitor of TNR expansions that acts at 5' flaps. Our results indicate that PRR protects against expansions arising from the 3' terminus, presumably replication slippage events. This work provides the first evidence that CAG · CTG expansions can occur by 3' slippage, and our results help define PRR as a key cellular mechanism that protects against expansions.

Published ahead of print on 23 October 2006.

This article has been cited by other articles:

• Dhar, A., Lahue, R. S. (2008). Rapid unwinding of triplet repeat hairpins by Srs2 helicase of Saccharomyces cerevisiae. Nucleic Acids Res 36: 3366-3373 [Abstract] [Full Text]