Abstract #32

Efficient double-strand break repair is required to prevent expansion and breakage of structure-forming CAG/CTG repeats and promote survival of cells with expanded repeats. Rangapriya Sundararajan, Rachel Zunder, Catherine H. Freudenreich. Department of Biology, Tufts University, Medford, MA.
   Expansion of CAG/CTG trinucleotide repeats causes 13 genetic diseases including Huntington’s disease. Expanded CAG/CTG repeats are prone to expansions, contractions, and exhibit increasing fragility with increasing tract length. In this study, we determined the role of double-strand break repair proteins in preventing repeat fragility and repeat expansions and contractions. CAG-70 and CAG-155 were highly fragile in HR and NHEJ mutants, indicating that both DSB repair pathways are important for repairing lesions at CAG/CTG trinucleotide repeats. The absence of either the HR or NHEJ proteins had opposing effects on repeat stability. Deletion of proteins in the RAD52-dependent HR pathway resulted in a significant 10-fold increase in expansion frequency, while deletion of Dnl4 ligase that is required for NHEJ resulted primarily in contractions. CAG/CTG tract breaks and expansions were also dramatically increased in the absence of Mre11p, and comparisons to nuclease-deficient Mre11 and Tel1D strains suggests that the primary function of Mre11 at CAG/CTG tracts is to sense lesions. We also report that repair choice at a repeat tract is length dependent, and suggest that the initial lesions formed at CAG-70 repeats are usually single strand breaks, whereas a CAG-155 tract causes replication fork stalling. Our results have general implications for how structure-forming lesions are repaired as well as for how repeat expansions occur in humans. Using microcolony assays to monitor cell survival, we show that strains that carry expanded CAG tracts and are deficient for DSB repair pathways undergo fewer cell divisions compared to repair-deficient strains without expanded repeats. These results show for the first time that defective DSB repair of structure-forming repeats has consequences for cell survival, which could have implications for progression of trinucleotide repeat diseases since patient tissues containing long repeat tracts frequently exhibit cell death.

Return to YGM 2006 Home at SGD