Tandem MutSbeta binding to long extruded DNA trinucleotide repeats underpins pathogenic expansions

Expansion of trinucleotide repeats causes Huntingtons disease, Fragile X syndrome and over twenty other monogenic disorders1. How mismatch repair protein MutS{beta} and large repeats of CNG (N=A, T, C or G) cooperate to drive the expansion is poorly understood. Contrary to expectations, we find that MutS{beta} prefers to bind the stem of an extruded (CNG) hairpin rather than the hairpin end or hairpin-duplex junction. Structural analyses reveal that in the presence of MutS{beta}, CNG repeats with N:N mismatches adopt a B form-like pseudo-duplex, with one or two CNG repeats slipped out forming uneven bubbles that partly mimic insertion-deletion loops of mismatched DNA2. When the extruded hairpin exceeds 40-45 repeats, it can be bound by three or more MutS{beta} molecules, which are resistant to ATP-dependent dissociation. We envision that such MutS{beta}-CNG complexes recruit MutL{gamma} endonuclease to nick DNA and initiate the repeat expansion process3,4. To develop drugs against the expansion diseases, we have identified lead compounds that prevent MutS{beta} binding to CNG repeats but not to mismatched DNA.