SHOC1 is a ERCC4-(HhH) ₂-like protein, integral to the formation of crossover recombination intermediates during mammalian meiosis

Abstract

Chromosome segregation errors during meiosis result in the formation of aneuploid gametes and are the leading cause of pregnancy loss and birth defects in humans. Proper chromosome segregation requires pairwise associations of maternal and paternal homologous chromosomes. Chiasmata, which are the cytological manifestations of crossovers (COs), provide a physical link that holds the homologs together as a pair, facilitating their orientation on the spindle at meiosis I. Although CO-promoting activities ensure a balanced number and position of COs, their identity and mechanism of action in mammals remain understudied. Previous work in yeast and Arabidopsis has shown that Zip2 and Shoc1 are ortholog proteins with an important role in promoting the formation of COs. Our work is the first study in mammals showing the in vivo and in vitro function of mouse and human SHOC1. We show that purified recombinant human SHOC1, an XPF/MUS81 family member, preferentially binds branched DNA molecules but apparently lacks in vitro endonuclease activity, despite its conserved ERCC4-(HhH) ₂ core structure. Cytological observations suggest that initial steps of recombination are normal in a majority of spermatocytes from SHOC1 hypomorphic mice. However, late stages of recombination appear abnormal, as chromosomal localization of MLH1 is reduced. In agreement, chiasma formation is reduced, and cells arrest at metaphase I with a few lagging chromosomes and subsequent apoptosis. This analysis of SHOC1-deficient mice and the selective localization of SHOC1 to a subset of recombination sites show that SHOC1 acts at key mid-stage steps of the CO formation process. The formation of chromosome axial elements and homologous pairing are apparently normal, but synapsis is altered with SYCP1 frequently failing to extend the full length of the chromosome axes. Finally, we describe that SHOC1 interacts with TEX11, another protein important for the formation of COs, connecting SHOC1 to chromosome axis and structure.

Author summary

Meiosis is a specialized type of cell division in which haploid gametes are generated to counterbalance the doubling of the chromosome number occurring at fertilization. Proper chromosome segregation requires pairwise associations of the maternal and paternal homologous chromosomes. This is provided by chiasmata, which are generated as a result of homologous recombination-mediated repair of double-strand DNA breaks (DSBs). The association of meiotic chromosomes in pairs through chiasmata guarantees that each gamete receives only one copy of each chromosome. For this reason, errors in recombination are commonly linked to spontaneous abortions, aneuploid-based birth defects, and in some cases, infertility. We observed that the absence of SHOC1 results in deficient recombination and incomplete meiosis-specific modification of chromosome structure. Our findings thus indicate that the mouse SHOC1 protein is required for normal progression of important aspects of meiosis that act to ensure that the correct numbers of chromosomes are transferred to the next generation.