Mac Partlin, Mary
Identification of proteins interacting with the human mismatch repair protein MLH1.
PhD thesis, University of Glasgow.
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Loss of expression of the human DNA mismatch repair (MMR) gene, hMLH1, is seen in a number of tumour cell lines resistant to a variety of cytotoxic drugs. The aim of this study was to identify other proteins that interact with hMLH1 to attempt to further elucidate its role in MMR and the engagement of downstream damage response pathways.
A yeast two-hybrid system, an in vivo system for detecting protein-protein interactions was utilised for this purpose. Fifteen known and five unknown genes were identified as encoding proteins interacting with hMLH1. These included three known hMLH1 binding proteins, hMLH3, hPMS1 and MED1. Amongst the other genes identified was the proto-oncogene c-MYC, a gene previously implicated in genetic instability and apoptosis.
Using in vitro derived mutants of c-MYC, it has been shown that hMLH1 interacts with the leucine-zipper domain of c-MYC.
The effect of elevated c-MYC expression on functional MMR was examined. An inducible c-MYC expression system, Rat-1 fibroblasts expressing c-MYCERTM, a fusion of c-MYC to the hormone binding domain of the oestrogen receptor was utilised. Elevated expression of c-MYC did not effect the mismatch specific binding complex activity in these cells as measured in EMSA experiments.
However c-MYC overexpression utilising the Rat-1 cMYCERTM system was shown to result in a mutator phenotype in these cells.
The results suggest there may be a link between the mutator phenotype, induced through overexpression of c-MYC, and loss of MMR. Overexpression of c-MYC, which is associated with many cancers, may result in the sequestration of hMLH1 preventing functional MMR. The interaction between hMLH1 and c-MYC is proposed to act in a DNA damage response pathway which is disrupted upon aberrant c-MYC expression.
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