Grants and Contracts Details
Description
The long-term goal of the project is to understand the mechanisms by which cells maintain
trinucleotide repeat (TNR) stability. Expansions of TNR sequences, e.g., (CAG)~ and (CTG)~, are
tightly associated with progression of certain human neurological and neurodegenerative diseases
including Huntington's disease (HD) and myotonic dystrophy. However, the mechanisms and factors
that promote/prevent TNR expansions are unknown. Because CAG and CTG repeats form thermo-
stable hairpins with multiple A-A and T-T mispairs in the hairpin stem, respectively, DNA mismatch
repair (MMR) and TNR hairpin repair have been proposed to play major roles in TNR maintenance.
Surprisingly, previous studies in transgenic mice suggest that mismatch recognition protein MSH2-
MSH3 heterodimer (also called MutSt3) promotes (CAG)~ expansions by binding to (CAG)~-formed
hairpins and inhibiting their repair. Our recent studies have shown that human cells catalyze error-free
repair of (CAG)25 and (CTG)25 hairpins in a nick-directed PCNA-dependent manner. The repair targets
the nicked strand for incisions at the repeat sequences, followed by repair DNA synthesis using the
continuous strand as a template, thereby ensuring TNR stability. However, MutSI3 does not inhibit,
stimulates (CAG)25 or (CTG)25 hairpin repair. Interestingly, our preliminary studies have shown that
cell lines derived from HD patients are defective in (CAG)~ hairpin repair, hinting possible
pathogenesis for HD. In this application, Specific Aim 1 is to evaluate the model that the MMR
system promotes (CAG)~ expansion. Human and animal cell lines with or without MutSt3
overexpression will be examined for their ability to repair (CAG)~ hairpins in vitro and to replicate CAG
repeats in vivo. Determination of TNR hairpin repair activity and (CAG)~ stability in these cells will
clarify if the MMR system is responsible for (CAG)~ expansions in human cells. Specific Aim 2 is to
further test the hypothesis that hairpin repair defects are associated with TNR diseases, by
screening hairpin repair proficiency in cell lines derived from HD patients. Specific Aim 3 is to purify
and characterize a protein required for (CAG)~ hairpin repair but defective in an HD cell line. A
successful completion of the proposed work will provide significant insight into the mechanisms of
TNR expansions and the etiology of TNR expansion-associated diseases.
Status | Finished |
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Effective start/end date | 6/1/10 → 7/31/15 |
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