Introduction
The genesis of gametes containing an intact, haploid genome is critical for the prevention of birth defects, and is highly dependent upon the fidelity of chromosome dynamics before the first meiotic division. Homologous chromosomes must pair, synapse, undergo recombination, and segregate properly to opposite poles. Recombination, which repairs repair double strand breaks (DSBs) that are genetically induced in leptonema, is coupled with synapsis in budding yeast and mammals. While our knowledge of the assembly and nature of recombination machinery is extensive, little is known about the disassembly of recombination intermediates, recruitment of DNA replication machinery during recombinational repair, and how the choice between different repair pathways is made.
Defects in recombination can preclude homologous chromosome pairing, leave unrepaired chromosome breaks, and cause aneuploidy by abrogating crossing over. To avoid such deleterious outcomes, surveillance systems (“checkpoints”) exist to sense meiotic errors and eliminate cells containing unresolved defects. In many organisms, including S. cerevisiae, Drosophila melanogaster, C. elegans, and mice [1–4], meiocytes with defects in recombination and/or chromosome synapsis trigger meiotic arrest in the pachytene stage of meiotic prophase I. This response to meiotic defects is referred to as the “pachytene checkpoint” (reviewed in [5]). Genetic experiments in S. cerevisiae have identified elements of the pachytene checkpoint machinery (reviewed in [5]). In addition to meiosis-specific proteins, these include factors that play roles in DNA damage signaling in mitotic cells [6–10]. Arabidopsis thaliana does not appear to have a pachytene checkpoint akin to that in yeast [11], nor do male Drosophila.
The pachytene checkpoint is known to monitor two aspects of meiotic chromosome metabolism in S. cerevisiae and C. elegans: (1) DSB repair and (2) chromosome synapsis [2,12]. In mice, both spermatocytes and oocytes harboring mutations that disrupt DSB repair (such as Dmc1, Msh5, and Atm) are efficiently eliminated in pachynema, but spermatocytes are much more sensitive to DSB repair–independent synapsis defects than oocytes [13–15]. However, because recombination is required for synapsis in mice (mutations in recombination genes such as Dmc1 cause extensive asynapsis [16]), it has remained formally uncertain whether there is a distinct pachytene checkpoint that responds to defects in meiotic recombination, and if so, whether it would be identical to that used in somatic cells. The mechanisms of putative pachytene checkpoint control remain unknown in mammals, since no mutations have been identified that abolish it.
PCH2, encoding a nucleolar-localized AAA-ATPase that was originally identified in an S. cerevisiae genetic screen for mutants that relieve pachytene arrest of asynaptic zip1 mutants [8], was recently determined to be an essential component of the pachytene synapsis (but not DSB repair) checkpoint in yeast and worms [2,12]. PCH2 orthologs are present in organisms that undergo synaptic meiosis, but not asynaptic meiosis, prompting the suggestion that a Pch2-dependent checkpoint evolved to monitor synaptonemal complex (SC) defects from yeast to humans [12]. Here, we generated mice deficient for the Trip13, the ortholog of PCH2, and evaluated whether it also plays a role in the pachytene checkpoint. Surprisingly, while we found no evidence for checkpoint function, we did uncover a potential role for this protein in noncrossover (NCO) repair of meiotic DSBs.