Mutsβ generates both expansions and contractions in a mouse model of the Fragile X-associated disorders

Fragile X-associated disorders are Repeat Expansion Diseases that result from expansion of a CGG/CCG-repeat in the FMR1 gene. Contractions of the repeat tract also occur, albeit at lower frequency. However, these contractions can potentially modulate disease symptoms or generate an allele with repeat numbers in the normal range. Little is known about the expansion mechanism and even less about contractions. We have previously demonstrated that the mismatch repair (MMR) protein MSH2 is required for expansions in a mouse model of these disorders. Here, we show that MSH3, the MSH2-binding partner in the MutS beta complex, is required for 98% of germ line expansions and all somatic expansions in this model. In addition, we provide evidence for two different contraction mechanisms that operate in the mouse model, a MutS beta-independent one that generates small contractions and a MutS beta-dependent one that generates larger ones. We also show that MutS beta complexes formed with the repeats have altered kinetics of ATP hydrolysis relative to complexes with bona fide MMR substrates and that MutS beta increases the stability of the CCG-hairpins at physiological temperatures. These data may have important implications for our understanding of the mechanism(s) of repeat instability and for the role of MMR proteins in this process.