Chromosome axis protein SYCP2 recruits HORMAD2 to enable meiotic synapsis quality control in mice

Abstract

Faithful chromosome segregation during meiosis depends on accurate recombination and synapsis of homologous chromosomes. These processes are monitored in mammals by checkpoint mechanisms involving the meiotic HORMA-domain proteins HORMAD1 and HORMAD2, which bind unsynapsed chromosome axes and promote activation of the DNA damage-response kinase ATR independently of DNA double-strand breaks (DSBs). However, no mechanism for axis recruitment of HORMAD1 or HORMAD2 had been demonstrated, nor had its role in checkpoint function been tested. We establish that a putative HORMAD-interacting region--the closure motif (CM)--within the chromosome-axis component SYCP2 is selectively required for HORMAD2, but not HORMAD1, localization. Deletion of the SYCP2 CM disrupts SYCP2-HORMAD2 complexes and prevents HORMAD2 axis binding without affecting axis assembly or recombination. Consequently, ATR accumulation and signaling on unsynapsed axes are reduced, and the prophase checkpoint malfunctions in a sexually dimorphic manner--causing aberrant elimination of synapsis-proficient spermatocytes and persistence of asynaptic oocytes. The phenotypes of SYCP2-CM-deficient and HORMAD2-null mice are indistinguishable, establishing the requirement for HORMAD2 axis recruitment in synapsis surveillance. We propose that axial recruitment generates a HORMAD2 scaffold that drives clustering-mediated ATR network activation independently of DSBs, thereby linking chromosome-axis architecture to synapsis quality control in mammalian meiosis.