Hormad1 Mutation Disrupts Synaptonemal Complex Formation, Recombination, and Chromosome Segregation in Mammalian Meiosis

Abstract

Meiosis is unique to germ cells and essential for reproduction. During the first meiotic division, homologous chromosomes pair, recombine, and form chiasmata. The homologues connect via axial elements and numerous transverse filaments to form the synaptonemal complex. The synaptonemal complex is a critical component for chromosome pairing, segregation, and recombination. We previously identified a novel germ cell–specific HORMA domain encoding gene, Hormad1, a member of the synaptonemal complex and a mammalian counterpart to the yeast meiotic HORMA domain protein Hop1. Hormad1 is essential for mammalian gametogenesis as knockout male and female mice are infertile. Hormad1 deficient (Hormad1−/ −) testes exhibit meiotic arrest in the early pachytene stage, and synaptonemal complexes cannot be visualized by electron microscopy. Hormad1 deficiency does not affect localization of other synaptonemal complex proteins, SYCP2 and SYCP3, but disrupts homologous chromosome pairing. Double stranded break formation and early recombination events are disrupted in Hormad1−/ − testes and ovaries as shown by the drastic decrease in the γH2AX, DMC1, RAD51, and RPA foci. HORMAD1 co-localizes with γH2AX to the sex body during pachytene. BRCA1, ATR, and γH2AX co-localize to the sex body and participate in meiotic sex chromosome inactivation and transcriptional silencing. Hormad1 deficiency abolishes γH2AX, ATR, and BRCA1 localization to the sex chromosomes and causes transcriptional de-repression on the X chromosome. Unlike testes, Hormad1−/ − ovaries have seemingly normal ovarian folliculogenesis after puberty. However, embryos generated from Hormad1−/ − oocytes are hyper- and hypodiploid at the 2 cell and 8 cell stage, and they arrest at the blastocyst stage. HORMAD1 is therefore a critical component of the synaptonemal complex that affects synapsis, recombination, and meiotic sex chromosome inactivation and transcriptional silencing.

Genetic Evidence That Synaptonemal Complex Axial Elements Govern Recombination Pathway Choice in Mice

“…Mice have two Hop1 orthologs, HORMAD1 and HORMAD2, that become colocalized to nascent AEs ("cores") during leptonema and are removed upon synapsis, consistent with a role conserved with Hop1 and/or Red1 (Wojtasz et al 2009;Fukuda et al 2010). Additionally, Hormad1 mutants have decreased DSB levels as do hop1 yeast (Shin et al 2010), but it is not known whether Hormad1 has a role in partner choice. On the other hand, there are some differences between the organisms.…”

“…These local disruptions of the cohesin complex may compromise IH bias. A final difference with yeast, in which Tel1 and/or Mec1 phosphorylate Hop1 in response to Spo11 DSBs (Carballo et al 2008), is that mouse Hop1 (HORMAD1) appears to act upstream of ATM and ATR (Shin et al 2010). HORMAD1 is indeed phosphorylated, but the kinase remains unknown, as does the potential ortholog of the downstream effector Mek1.…”

“…Therefore, we conclude that HORMAD1 loss is not sufficient for the phenomena (rescue of Dmc1 or Trip13 mutants by Sycp3 deletion) we observe here. Conversely, SYCP3 does not require HORMAD1 to integrate into AEs (Shin et al 2010). Recently, evidence has been presented that HORMAD1 depletion depresses SPO11-induced DSBs and is involved in the oocyte checkpoint that detects asynapsis (Daniel et al 2011).…”

HORMAD2 methylation‐mediated epigenetic regulation of gene expression in thyroid cancer

“…It is recognized that HORMA‐domain containing proteins (HORMAD) plays a crucial role in the regulation of cell cycle and in mitosis and meiosis 27, 28, 29. HORMADs (HORMAD1 and HORMAD2) share homology with the meiosis‐specific HORMA‐domain proteins, which are conserved in various organisms 16…”

Clamping down on mammalian meiosis

“…As discussed below, a distinct RAD1-containing complex could perform synapsis checkpoint functions. While direct interaction of RAD1 with SC structural proteins has not been reported, the meiotic localization of RAD1, like BRCA1, TOPBP1, and ATR, but not RAD9A, is highly similar to that of the HORMA-domain-containing SC protein, HORMAD1, 15,33,34,100 and in fact, HORMAD1 is required for loading of γH2AX, BRCA1, and ATR in the XY body domain.…”