Mutation of the ATPase Domain of MutS Homolog-5 (MSH5) Reveals a Requirement for a Functional MutSγ Complex for All Crossovers in Mammalian Meiosis

During meiosis, induction of DNA double strand breaks (DSB) leads to recombination between homologous chromosomes, resulting in crossovers (CO) and non-crossovers (NCO). In the mouse, only 10% of DSBs resolve as COs, mostly through a class I pathway dependent on MutSg (MSH4/ MSH5) and MutLg (MLH1/MLH3), the latter representing the ultimate marker of these CO events. A second Class II CO pathway accounts for only a few COs, but is not thought to involve MutSg/ MutLg, and is instead dependent on MUS81-EME1. For class I events, loading of MutLg is thought to be dependent on MutSg, however MutSg loads very early in prophase I at a frequency that far exceeds the final number of class I COs. Moreover, loss of MutSg in mouse results in apoptosis before CO formation, preventing analysis of its CO function. We generated a mutation in the ATP binding domain of Msh5 ( Msh5 GA ). While this mutation was not expected to affect MutSg complex formation, MutSg foci do not accumulate during prophase I. However, most spermatocytes from Msh5 GA/GA mice progress to late pachynema and beyond, considerably further than meiosis in Msh5 -/- animals. At pachynema, Msh5 GA/GA spermatocytes show persistent DSBs, incomplete homolog pairing, and fail to accumulate MutLg. Unexpectedly, Msh5 GA/GA diakinesis-staged spermatocytes have no chiasmata at all from any CO pathway, indicating that a functional MutSg complex is critical for all CO events regardless of their mechanism of generation.