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Isolated short CTG/CAG DNA slip-outs are repaired efficiently by hMutSbeta, but clustered slip-outs are poorly

Gagan B Panigrahi1, Meghan M Slean, Jodie P Simard

  • 1Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, ON, Canada M5G 1L7.

Proceedings of the National Academy of Sciences of the United States of America
|June 24, 2010
PubMed

Insights

DNA repair mechanisms involving mismatch repair (MMR) proteins are crucial for preventing expansions of CTG/CAG repeats that cause diseases like Huntington's. Short DNA slip-outs require hMutSbeta for repair, but clustered slip-outs can block repair, leading to disease-associated expansions.

Area of Science:

  • Molecular Biology
  • Genetics
  • Biochemistry

Background:

  • Trinucleotide repeat expansions, such as CTG/CAG, are implicated in neurodegenerative diseases and involve DNA slippage.
  • The mismatch repair (MMR) system, particularly the hMutSbeta complex (MSH2/MSH3), is known to be involved in processing these repeat expansions.

Purpose of the Study:

  • To investigate the in vitro repair requirements for DNA slip-outs of varying lengths and numbers.
  • To elucidate the role of the hMutSbeta complex in the repair of CTG/CAG repeat expansions.

Main Methods:

  • In vitro DNA repair assays were performed using varying lengths (1, 3, 20 repeats) and numbers of CTG slip-outs.
  • The requirement for human MMR proteins, specifically hMutSbeta, was assessed under different conditions.

Main Results:

  • Short slip-outs (1-3 repeats) were efficiently repaired but strictly required hMutSbeta, with optimal repair occurring at specific protein levels.
  • Long slip-outs (>3 repeats) escaped repair, and clusters of short slip-outs acted as roadblocks, hindering repair and reducing efficiency.
  • MMR deficiency exacerbated the lack of repair for clustered slip-outs, similar to observed effects with ionizing radiation damage.

Conclusions:

  • DNA repeat expansions can result from escaped repair of long slip-outs or inefficient repair of clustered short slip-outs.
  • The hMutSbeta complex plays a critical role in repairing isolated short slip-outs, but clustered slip-outs interfere with this process.
  • Interference in the repair of clustered DNA lesions, including disease-associated repeat expansions, contributes to genetic instability and disease pathogenesis.

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