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The DNA Damage Checkpoint Eliminates Mouse Oocytes with Chromosome Synapsis Failure. Rinaldi VD et al. (2017) Pairing and synapsis of homologous chromosomes during meiosis is crucial for producing genetically normal gametes and is dependent upon repair of SPO11-induced double-strand breaks (DSBs) by homologous recombination. To prevent transmission of genetic defects, diverse organisms have evolved mechanisms to eliminate meiocytes containing unrepaired DSBs or unsynapsed chromosomes. Here we show that the CHK2 (CHEK2)-dependent DNA damage checkpoint culls not only recombination-defective mouse oocytes but also SPO11-deficient oocytes that are severely defective in homolog synapsis. The checkpoint is triggered in oocytes that accumulate a threshold level of spontaneous DSBs (∼10) in late prophase I, the repair of which is inhibited by the presence of HORMAD1/2 on unsynapsed chromosome axes. Furthermore, Hormad2 deletion rescued the fertility of oocytes containing a synapsis-proficient, DSB repair-defective mutation in a gene (Trip13) required for removal of HORMADs from synapsed chromosomes, suggesting that many meiotic DSBs are normally repaired by intersister recombination in mice.//////////////////
HORMAD1-dependent checkpoint/surveillance mechanism eliminates asynaptic oocytes. Kogo H et al. Meiotic pachytene checkpoints monitor the failure of homologous recombination and synapsis to ensure faithful chromosome segregation during gamete formation. To date, the molecular basis of the mammalian pachytene checkpoints has remained largely unknown. We here report that mouse HORMAD1 is required for a meiotic prophase checkpoint that eliminates asynaptic oocytes. Hormad1-deficient mice are infertile and show an extensive failure of homologous pairing and synapsis, consistent with the evolutionarily conserved function of meiotic HORMA domain proteins. Unexpectedly, Hormad1-deficient ovaries contain a normal number of oocytes despite asynapsis and consequently produce aneuploid oocytes, indicating a checkpoint failure. By the analysis of Hormad1/Spo11 double mutants, the Hormad1 deficiency was found to abrogate the massive oocyte loss in the Spo11-deficient ovary. The Hormad1 deficiency also causes the eventual loss of pseudo sex body in the Spo11-deficient ovary and testis. These results suggest the involvement of HORMAD1 in the repressive chromatin domain formation that is proposed to be important in the meiotic prophase checkpoints. We also show the extensive phosphorylation of HORMAD1 in the Spo11-deficient testis and ovary, suggesting an involvement of novel DNA damage-independent phosphorylation signaling in the surveillance mechanism. Our present results provide clues to HORMAD1-dependent checkpoint in response to asynapsis in mammalian meiosis. //////
Mouse HORMAD1 Is a Meiosis I Checkpoint Protein That Modulates DNA Double Strand Break Repair During Female Meiosis. Shin YH et al. Oocytes in embryonic ovaries enter meiosis I, and arrest in diplonema stage. Perturbations in meiosis I such as abnormal double strand break (DSB) formation and repair, adversely affect oocyte survival. We previously discovered that HORMAD1 is a critical component of the synaptonemal complex but not essential for oocyte survival. No significant differences were observed in the number of primordial, primary, secondary and developing follicles between wild-type and Hormad1(-/-) newborn, 8-day and 80-day ovaries. Meiosis I progression in Hormad1(-/-)embryonic ovaries was normal through the zygotene stage, and oocytes arrested in diplonema, however, we did not visualize oocytes with completely synapsed chromosomes. We investigated effects of HORMAD1 deficiency on the kinetics of DNA DSB formation and repair in the mouse ovary. We irradiated E16.5 wild-type and Hormad1(-/-) ovaries and monitored DSB repair using gammaH2AX, RAD51 and DMC1 immunofluorescence. Our results showed a significant drop in unrepaired DSBs in the irradiated Hormad1(-/-) zygotene oocytes as compared to the wild-type oocytes. Moreover, Hormad1 deficiency rescued Dmc1(-/-) oocytes. These results indicate that Hormad1 deficiency promotes DMC1-independent DSB repairs, which in turn helps asynaptic Hormad1(-/-)oocytes resist perinatal loss.
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Comment |
Meiotic homologue alignment and its quality surveillance are controlled by mouse HORMAD1. Daniel K et al. Meiotic crossover formation between homologous chromosomes (homologues) entails DNA double-strand break (DSB) formation, homology search using DSB ends, and synaptonemal-complex formation coupled with DSB repair. Meiotic progression must be prevented until DSB repair and homologue alignment are completed, to avoid the formation of aneuploid gametes. Here we show that mouse HORMAD1 ensures that sufficient numbers of processed DSBs are available for successful homology search. HORMAD1 is needed for normal synaptonemal-complex formation and for the efficient recruitment of ATR checkpoint kinase activity to unsynapsed chromatin. The latter phenomenon was proposed to be important in meiotic prophase checkpoints in both sexes. Consistent with this hypothesis, HORMAD1 is essential for the elimination of synaptonemal-complex-defective oocytes. Synaptonemal-complex formation results in HORMAD1 depletion from chromosome axes. Thus, we propose that the synaptonemal complex and HORMAD1 are key components of a negative feedback loop that coordinates meiotic progression with homologue alignment: HORMAD1 promotes homologue alignment and synaptonemal-complex formation, and synaptonemal complexes downregulate HORMAD1 function, thereby permitting progression past meiotic prophase checkpoints.
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Mutations |
1 mutations
Species: mouse
Mutation name: None
type: null mutation
fertility: infertile - ovarian defect
Comment: Hormad1 mutation disrupts synaptonemal complex formation, recombination, and chromosome segregation in Mammalian meiosis. Shin YH et al. 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 ?H2AX, DMC1, RAD51, and RPA foci. HORMAD1 co-localizes with ?H2AX to the sex body during pachytene. BRCA1, ATR, and ?H2AX co-localize to the sex body and participate in meiotic sex chromosome inactivation and transcriptional silencing. Hormad1 deficiency abolishes ?H2AX, 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.
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