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 MutS gamma (MSH4/MSH5) and MutL gamma (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 MutS gamma/MutL gamma, and is instead dependent on MUS81-EME1. For class I events, loading of MutL gamma is thought to be dependent on MutS gamma, however MutS gamma loads very early in prophase I at a frequency that far exceeds the final number of class I COs. Moreover, loss of MutS gamma in mouse results in apoptosis before CO formation, preventing the 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, MutS gamma foci do not accumulate during prophase I. However, most spermatocytes from Msh5(GA/G)A 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 MutL gamma. 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.