HUMAN MSH2 BINDS TO TRINUCLEOTIDE REPEAT DNA STRUCTURES ASSOCIATED WITH NEURODEGENERATIVE DISEASES

The expansion of trinucleotide repeat sequences is associated with sev eral neurodegenerative diseases. The mechanism of this expansion is un known but may involve slipped-strand structures where adjacent rather than perfect complementary sequences of a trinucleotide repeat become paired, Here, we have studied the interaction of the human mismatch re pair protein MSH2 with slipped-strand structures formed from a triplet repeat sequence in order to address the possible role of MSH2 in trin ucleotide expansion, Genomic clones of the myotonic dystrophy locus co ntaining disease-relevant lengths of (CTG)(n).(CAG)(n) triplet repeats were examined, We have constructed two types of slipped-strand struct ures by annealing complementary strands of DNA containing: (i) equal n umbers of trinucleotide repeats (homoduplex slipped structures or S-DN A) or (ii) different numbers of repeats (heteroduplex slipped intermed iates or SI-DNA), SI-DNAs having an excess of either CTG or CAG repeat s were structurally distinct and could be separated electrophoreticall y and studied individually, Using a band-shift assay, the MSH2 was sho wn to bind to both S-DNA and SI-DNA in a structure-specific manner, Th e affinity of MSH2 increased with the length of the repeat sequence, F urthermore, MSH2 bound preferentially to looped-out CAG repeat sequenc es, implicating a strand asymmetry in MSH2 recognition. Our results ar e consistent with the idea that MSH2 may participate in trinucleotide repeat expansion Via its role in repair and/or recombination.