Human MSH2 binds to trinucleotide repeat DNA structures associated with neurodegenerative diseases

The expansion of trinucleotide repeat sequences is associated with severalneurodegenerative diseases. The mechanism of this expansion is unknown butmay involve slipped-strand structures where adjacent rather than perfectcomplementary sequences of a trinucleotide repeat become paired. Here, wehave studied the interaction of the human mismatch repair protein MSH2 withslipped-strand structures formed from a triplet repeat sequence in order toaddress the possible role of MSH2 in trinucleotide expansion. Genomicclones of the myotonic dystrophy locus containing disease-relevant lengthsof (CTG)n x (CAG)n triplet repeats were examined. We have constructed twotypes of slipped-strand structures by annealing complementary strands ofDNA containing: (i) equal numbers of trinucleotide repeats (homoduplexslipped structures or S-DNA) or (ii) different numbers of repeats(heteroduplex slipped intermediates or SI-DNA). SI-DNAs having an excess ofeither CTG or CAG repeats were structurally distinct and could be separatedelectrophoretically and studied individually. Using a band-shift assay, theMSH2 was shown to bind to both S-DNA and SI-DNA in a structure- specificmanner. The affinity of MSH2 increased with the length of the repeatsequence. Furthermore, MSH2 bound preferentially to looped-out CAG repeatsequences, implicating a strand asymmetry in MSH2 recognition. Our resultsare consistent with the idea that MSH2 may participate in trinucleotiderepeat expansion via its role in repair and/or recombination.