The Atr and Atm protein kinases associate with different sites along meiotically pairing chromosomes

Bloom syndrome DNA helicase mitigates mismatch repair-dependent apoptosis

Generation of O6-methylguanine (O6-meG) by DNA-alkylating agents such as N-methyl N-nitrosourea (MNU) activates the multiprotein mismatch repair (MMR) complex and the checkpoint response involving ATR/CHK1 and ATM/CHK2 kinases, which may in turn trigger cell cycle arrest and apoptosis. The Bloom syndrome DNA helicase BLM interacts with the MMR complex, suggesting functional relevance to repair and checkpoint responses. We observed a strong interaction of BLM with MMR proteins in HeLa cells upon treatment with MNU as evidenced by co-immunoprecipitation as well as colocalization in the nucleus as revealed by dual immunofluorescence staining. Knockout of BLM sensitized HeLa MR cells to MNU-induced cell cycle disruption and enhanced expression of the apoptosis markers cleaved caspase-9 and PARP1. MNU-treated BLM-deficient cells also exhibited a greater number of 53BP1 foci and greater phosphorylation levels of H2AX at S139 and RPA32 at S8, indicating the accumulation of DNA double-strand breaks. These findings suggest that BLM prevents double-strand DNA breaks during the MMR-dependent DNA damage response and mitigates O6-meG-induced apoptosis.

ATM signaling modulates cohesin behavior in meiotic prophase and proliferating cells

Cohesins are ancient and ubiquitous regulators of chromosome architecture and function, but their diverse roles and regulation remain poorly understood. During meiosis, chromosomes are reorganized as linear arrays of chromatin loops around a cohesin axis. This unique organization underlies homolog pairing, synapsis, double-stranded break induction, and recombination. We report that axis assembly in Caenorhabditis elegans is promoted by DNA-damage response (DDR) kinases that are activated at meiotic entry, even in the absence of DNA breaks. Downregulation of the cohesin-destabilizing factor WAPL-1 by ATM-1 promotes axis association of cohesins containing the meiotic kleisins COH-3 and COH-4. ECO-1 and PDS-5 also contribute to stabilizing axis-associated meiotic cohesins. Further, our data suggest that cohesin-enriched domains that promote DNA repair in mammalian cells also depend on WAPL inhibition by ATM. Thus, DDR and Wapl seem to play conserved roles in cohesin regulation in meiotic prophase and proliferating cells.

MRNIP interacts with sex body chromatin to support meiotic progression, spermatogenesis, and male fertility in mice

Meiosis has a principal role in sexual reproduction to generate haploid gametes in both sexes. During meiosis, the cell nucleus hosts a dynamic environment where some genes are transcriptionally activated, and some are inactivated at the same time. This becomes possible through subnuclear compartmentalization. The sex body, sequestering X and Y chromosomes during male meiosis and creating an environment for the meiotic sex chromosome inactivation (MSCI) is one of the best known and studied subnuclear compartments. Herein, we show that MRNIP forms droplet-like accumulations that fuse together to create a distinct subnuclear compartment that partially overlaps with the sex body chromatin during diplotene. We demonstrate that Mrnip-/- spermatocytes have impaired DNA double-strand break (DSB) repair, they display reduced sex body formation and defective MSCI. We show that Mrnip-/- undergoes critical meiocyte loss at the diplotene stage. Furthermore, we determine that DNA DSBs (induced by SPO11) and synapsis initiation (facilitated by SYCP1) precede Mrnip expression in testes. Altogether, our findings indicate that in addition to an emerging role in DNA DSB repair, MRNIP has an essential function in spermatogenesis during meiosis I by forming drop-like accumulations interacting with the sex body.

Multiple 9-1-1 complexes promote homolog synapsis, DSB repair, and ATR signaling during mammalian meiosis

DNA damage response mechanisms have meiotic roles that ensure successful gamete formation. While completion of meiotic double-strand break (DSB) repair requires the canonical RAD9A-RAD1-HUS1 (9A-1-1) complex, mammalian meiocytes also express RAD9A and HUS1 paralogs, RAD9B and HUS1B, predicted to form alternative 9-1-1 complexes. The RAD1 subunit is shared by all predicted 9-1-1 complexes and localizes to meiotic chromosomes even in the absence of HUS1 and RAD9A. Here, we report that testis-specific disruption of RAD1 in mice resulted in impaired DSB repair, germ cell depletion, and infertility. Unlike Hus1 or Rad9a disruption, Rad1 loss in meiocytes also caused severe defects in homolog synapsis, impaired phosphorylation of ATR targets such as H2AX, CHK1, and HORMAD2, and compromised meiotic sex chromosome inactivation. Together, these results establish critical roles for both canonical and alternative 9-1-1 complexes in meiotic ATR activation and successful prophase I completion.

ARID1A Mutation in Metastatic Breast Cancer: A Potential Therapeutic Target

Distant metastasis is the principal cause of mortality for breast cancer patients. Targeting specific mutations that have been acquired during the evolution process of advanced breast cancer is a potential means of enhancing the clinical efficacy of treatment strategies. In metastatic breast cancer, ARID1A is the most prevalent mutation of the SWI/SNF complex, which regulates DNA repair, recombination, and gene transcription. The low expression of ARID1A is associated with poor disease-free survival and overall survival of patients with luminal A or HER2-rich breast cancer. In addition, ARID1A plays a prominent role in maintaining luminal characteristics and has an advantage for identifying responses to treatment, including endocrine therapies, HDAC inhibitors and CDK4/6 inhibitors. The therapeutic vulnerabilities initiated by ARID1A alterations encourage us to explore new approaches to cope with ARID1A mutant-related drug resistance or metastasis. In this review, we describe the mutation profiles of ARID1A in metastatic breast cancer and the structure and function of ARID1A and the SWI/SNF complex as well as discuss the potential mechanisms of ARID1A-mediated endocrine resistance and therapeutic potential.

Phospho-Regulation of Meiotic Prophase

Germ cells undergoing meiosis rely on an intricate network of surveillance mechanisms that govern the production of euploid gametes for successful sexual reproduction. These surveillance mechanisms are particularly crucial during meiotic prophase, when cells execute a highly orchestrated program of chromosome morphogenesis and recombination, which must be integrated with the meiotic cell division machinery to ensure the safe execution of meiosis. Dynamic protein phosphorylation, controlled by kinases and phosphatases, has emerged as one of the main signaling routes for providing readout and regulation of chromosomal and cellular behavior throughout meiotic prophase. In this review, we discuss common principles and provide detailed examples of how these phosphorylation events are employed to ensure faithful passage of chromosomes from one generation to the next.

The capacity of oocytes for DNA repair

Female fertility and offspring health are critically dependent on the maintenance of an adequate supply of high-quality oocytes. Like somatic cells, oocytes are subject to a variety of different types of DNA damage arising from endogenous cellular processes and exposure to exogenous genotoxic stressors. While the repair of intentionally induced DNA double strand breaks in gametes during meiotic recombination is well characterised, less is known about the ability of oocytes to repair pathological DNA damage and the relative contribution of DNA repair to oocyte quality is not well defined. This review will discuss emerging data suggesting that oocytes are in fact capable of efficient DNA repair and that DNA repair may be an important mechanism for ensuring female fertility, as well as the transmission of high-quality genetic material to subsequent generations.

ATR is a multifunctional regulator of male mouse meiosis

Meiotic cells undergo genetic exchange between homologs through programmed DNA double-strand break (DSB) formation, recombination and synapsis. In mice, the DNA damage-regulated phosphatidylinositol-3-kinase-like kinase (PIKK) ATM regulates all of these processes. However, the meiotic functions of the PIKK ATR have remained elusive, because germline-specific depletion of this kinase is challenging. Here we uncover roles for ATR in male mouse prophase I progression. ATR deletion causes chromosome axis fragmentation and germ cell elimination at mid pachynema. This elimination cannot be rescued by deletion of ATM and the third DNA damage-regulated PIKK, PRKDC, consistent with the existence of a PIKK-independent surveillance mechanism in the mammalian germline. ATR is required for synapsis, in a manner genetically dissociable from DSB formation. ATR also regulates loading of recombinases RAD51 and DMC1 to DSBs and recombination focus dynamics on synapsed and asynapsed chromosomes. Our studies reveal ATR as a critical regulator of mouse meiosis.

DNA damage response protein TOPBP1 regulates X chromosome silencing in the mammalian germ line

Meiotic synapsis and recombination between homologs permits the formation of cross-overs that are essential for generating chromosomally balanced sperm and eggs. In mammals, surveillance mechanisms eliminate meiotic cells with defective synapsis, thereby minimizing transmission of aneuploidy. One such surveillance mechanism is meiotic silencing, the inactivation of genes located on asynapsed chromosomes, via ATR-dependent serine-139 phosphorylation of histone H2AFX (γH2AFX). Stimulation of ATR activity requires direct interaction with an ATR activation domain (AAD)-containing partner. However, which partner facilitates the meiotic silencing properties of ATR is unknown. Focusing on the best-characterized example of meiotic silencing, meiotic sex chromosome inactivation, we reveal this AAD-containing partner to be the DNA damage and checkpoint protein TOPBP1. Conditional TOPBP1 deletion during pachynema causes germ cell elimination associated with defective X chromosome gene silencing and sex chromosome condensation. TOPBP1 is essential for localization to the X chromosome of silencing "sensors," including BRCA1, and effectors, including ATR, γH2AFX, and canonical repressive histone marks. We present evidence that persistent DNA double-strand breaks act as silencing initiation sites. Our study identifies TOPBP1 as a critical factor in meiotic sex chromosome silencing.

Meiotic Silencing in Mammals

Meiosis is essential for reproduction in sexually reproducing organisms. A key stage in meiosis is the synapsis of maternal and paternal homologous chromosomes, accompanied by exchange of genetic material to generate crossovers. A decade ago, studies found that when chromosomes fail to synapse, the many hundreds of genes housed within them are transcriptionally inactivated. This process, meiotic silencing, is conserved in all mammals studied to date, but its purpose is not yet defined. Here, I review the molecular genetics of meiotic silencing and consider the many potential functions that it could serve in the mammalian germ line. In addition, I discuss how meiotic silencing influences sex differences in meiotic infertility and the profound impact that meiotic silencing has had on the evolution of mammalian sex chromosomes.

Unique sex chromosome systems in Ellobius: How do male XX chromosomes recombine and undergo pachytene chromatin inactivation?

Most mammalian species have heteromorphic sex chromosomes in males, except for a few enigmatic groups such as the mole voles Ellobius, which do not have the Y chromosome and Sry gene. The Ellobius (XX ♀♂) system of sex chromosomes has no analogues among other animals. The structure and meiotic behaviour of the two X chromosomes were investigated for males of the sibling species Ellobius talpinus and Ellobius tancrei. Their sex chromosomes, despite their identical G-structure, demonstrate short synaptic fragments and crossover-associated MLH1 foci in both telomeric regions only. The chromatin undergoes modifications in the meiotic sex chromosomes. SUMO-1 marks a small nucleolus-like body of the meiotic XX. ATR and ubiH2A are localized in the asynaptic area and the histone γH2AFX covers the entire XX bivalent. The distribution of some markers of chromatin inactivation differentiates sex chromosomes of mole voles from those of other mammals. Sex chromosomes of both studied species have identical recombination and meiotic inactivation patterns. In Ellobius, similar chromosome morphology masks the functional heteromorphism of the male sex chromosomes, which can be seen at meiosis.

Escape of X-linked miRNA genes from meiotic sex chromosome inactivation

Past studies have indicated that transcription of all X-linked genes is repressed by meiotic sex chromosome inactivation (MSCI) during the meiotic phase of spermatogenesis in mammals. However, more recent studies have shown an increase in steady-state levels of certain X-linked miRNAs in pachytene spermatocytes, suggesting that either synthesis of these miRNAs increases or that degradation of these miRNAs decreases dramatically in these cells. To distinguish between these possibilities, we performed RNA-FISH to detect nascent transcripts from multiple miRNA genes in various spermatogenic cell types. Our results show definitively that Type I X-linked miRNA genes are subject to MSCI, as are all or most X-linked mRNA genes, whereas Type II and III X-linked miRNA genes escape MSCI by continuing ongoing, active transcription in primary spermatocytes. We corroborated these results by co-localization of RNA-FISH signals with both a corresponding DNA-FISH signal and an immunofluorescence signal for RNA polymerase II. We also found that X-linked miRNA genes that escape MSCI locate non-randomly to the periphery of the XY body, whereas genes that are subject to MSCI remain located within the XY body in pachytene spermatocytes, suggesting that the mechanism of escape of X-linked miRNA genes from MSCI involves their relocation to a position outside of the repressive chromatin domain associated with the XY body. The fact that Type II and III X-linked miRNA genes escape MSCI suggests an immediacy of function of the encoded miRNAs specifically required during the meiotic stages of spermatogenesis.

Multiple facets of histone variant H2AX: a DNA double-strand-break marker with several biological functions

In the last decade, many papers highlighted that the histone variant H2AX and its phosphorylation on Ser 139 (γH2AX) cannot be simply considered a specific DNA double-strand-break (DSB) marker with a role restricted to the DNA damage response, but rather as a ‘protagonist’ in different scenarios. This review will present and discuss an up-to-date view regarding the ‘non-canonical’ H2AX roles, focusing in particular on possible functional and structural parts in contexts different from the canonical DNA DSB response. We will present aspects concerning sex chromosome inactivation in male germ cells, X inactivation in female somatic cells and mitosis, but will also focus on the more recent studies regarding embryonic and neural stem cell development, asymmetric sister chromosome segregation in stem cells and cellular senescence maintenance. We will discuss whether in these new contexts there might be a relation with the canonical DNA DSB signalling function that could justify γH2AX formation. The authors will emphasize that, just as H2AX phosphorylation signals chromatin alteration and serves the canonical function of recruiting DSB repair factors, so the modification of H2AX in contexts other than the DNA damage response may contribute towards creating a specific chromatin structure frame allowing ‘non-canonical’ functions to be carried out in different cell types.

Impact of the number of Robertsonian chromosomes on germ cell death in wild male house mice

Previous studies in the house mouse have shown that the presence of Robertsonian (Rb) metacentric chromosomes in heterozygous condition affects the process of spermatogenesis. This detrimental effect mainly depends on the number of metacentrics involved and the complexity of the resulting meiotic figures. In this study, we aimed at elucidating the relationship between the chromosomal composition and spermatogenesis impairment in mice present in an area of chromosomal polymorphism (the so-called Barcelona system BRbS) in which Rb mice are surrounded by all acrocentric animals, no established metacentric races are present and the level of structural heterozygosity is relatively low. Using the terminal deoxinucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay, we report higher frequency of apoptotic spermatogenetic cells in mice carrying six pairs of metacentrics at the homozygous state than in those carrying two or three fusions at the heterozygous state. Specifically, we detected a higher frequency of TUNEL-positive (T+) tubules and of T+ cells per tubule cross section and also a lower spermatid/spermatocyte ratio. These results indicate that the number of metacentrics at the homozygous state is more influential in determining apoptotic germ cell death than that of moderate chromosome heterozygosity. The percentage of germ cell death lower than 50 % found in our samples and the geographic distribution of the set of metacentrics within the BRbS indicate that although the spermatogenic alterations detected in this area could act as a partial barrier to gene flow, they are not sufficient to prevent Rb chromosomes from spreading in nature.

Clamping down on mammalian meiosis

The RAD9A-RAD1-HUS1 (9-1-1) complex is a PCNA-like heterotrimeric clamp that binds damaged DNA to promote cell cycle checkpoint signaling and DNA repair. While various 9-1-1 functions in mammalian somatic cells have been established, mounting evidence from lower eukaryotes predicts critical roles in meiotic germ cells as well. This was investigated in 2 recent studies in which the 9-1-1 complex was disrupted specifically in the mouse male germline through conditional deletion of Rad9a or Hus1. Loss of these clamp subunits led to severely impaired fertility and meiotic defects, including faulty DNA double-strand break repair. While 9-1-1 is critical for ATR kinase activation in somatic cells, these studies did not reveal major defects in ATR checkpoint pathway signaling in meiotic cells. Intriguingly, this new work identified separable roles for 9-1-1 subunits, namely RAD9A- and HUS1-independent roles for RAD1. Based on these studies and the high-level expression of the paralogous proteins RAD9B and HUS1B in testis, we propose a model in which multiple alternative 9-1-1 clamps function during mammalian meiosis to ensure genome maintenance in the germline.

Programmed phosphorylation of histone H2AX precedes a phase of DNA double-strand break-independent synapsis in mouse meiosis

Accurate homologue synapsis during meiosis is essential for faithful chromosome segregation and formation of viable gametes. The finding of Spo11-dependent gamma-H2AX (γH2AX) formation during leptotene and data on mutant mice have led to the notion that synapsis in mammals depends on meiotic DNA double-stranded break (DSB) repair. A second wave of ataxia telangiectasia mutated (ATM) and Rad3-related (ATR)-dependent γH2AX formation has been observed in Atm-null mice during zygotene, suggesting that this wave of phosphorylation also occurs in normal mice. Here I aimed to confirm and to analyse in deep this wave of phosphorylation. Immunostaining of spread spermatocytes shows that γH2AX accumulates on the short last axis stretches to pair. This accumulation appears within all the nuclei undergoing a specific step of late zygotene and disappears from every spermatocyte immediately after pairing completion. This γH2AX signal co-localises with ATR, is Spo11-independent and does not co-localise with free DNA 3'-end labelling. I conclude that ATR/γH2AX asynapsis signalling at the end of zygotene belongs to a physiologically programmed pathway operating at a specific meiotic step, and I propose that this pathway is involved in the triggering of a phase of DSB-independent chromosome pairing that leads to synapsis completion in normal mouse meiosis.

Meiotic DNA double-strand breaks and chromosome asynapsis in mice are monitored by distinct HORMAD2-independent and -dependent mechanisms

Meiotic crossover formation involves the repair of programmed DNA double-strand breaks (DSBs) and synaptonemal complex (SC) formation. Completion of these processes must precede the meiotic divisions in order to avoid chromosome abnormalities in gametes. Enduring key questions in meiosis have been how meiotic progression and crossover formation are coordinated, whether inappropriate asynapsis is monitored, and whether asynapsis elicits prophase arrest via mechanisms that are distinct from the surveillance of unrepaired DNA DSBs. We disrupted the meiosis-specific mouse HORMAD2 (Hop1, Rev7, and Mad2 domain 2) protein, which preferentially associates with unsynapsed chromosome axes. We show that HORMAD2 is required for the accumulation of the checkpoint kinase ATR along unsynapsed axes, but not at DNA DSBs or on DNA DSB-associated chromatin loops. Consistent with the hypothesis that ATR activity on chromatin plays important roles in the quality control of meiotic prophase, HORMAD2 is required for the elimination of the asynaptic Spo11(-/-), but not the asynaptic and DSB repair-defective Dmc1(-/-) oocytes. Our observations strongly suggest that HORMAD2-dependent recruitment of ATR to unsynapsed chromosome axes constitutes a mechanism for the surveillance of asynapsis. Thus, we provide convincing evidence for the existence of a distinct asynapsis surveillance mechanism that safeguards the ploidy of the mammalian germline.

ATM protein kinase: the linchpin of cellular defenses to stress

ATM is the most significant molecule involved in monitoring the genomic integrity of the cell. Any damage done to DNA relentlessly challenges the cellular machinery involved in recognition, processing and repair of these insults. ATM kinase is activated early to detect and signal lesions in DNA, arrest the cell cycle, establish DNA repair signaling and faithfully restore the damaged chromatin. ATM activation plays an important role as a barrier to tumorigenesis, metabolic syndrome and neurodegeneration. Therefore, studies of ATM-dependent DNA damage signaling pathways hold promise for treatment of a variety of debilitating diseases through the development of new therapeutics capable of modulating cellular responses to stress. In this review, we have tried to untangle the complex web of ATM signaling pathways with the purpose of pinpointing multiple roles of ATM underlying the complex phenotypes observed in AT patients.

The Ddc2/ATRIP checkpoint protein monitors meiotic recombination intermediates

During meiosis, accurate segregation of intact chromosomes is essential for generating healthy gametes. Defects in recombination and/or chromosome synapsis activate the pachytene checkpoint, which delays meiotic cell cycle progression to avoid aberrant chromosome segregation and formation of defective gametes. Here, we characterize the role of the conserved DNA damage checkpoint protein Ddc2/ATRIP in this meiotic surveillance mechanism. We show that deletion of DDC2 relieves the checkpoint-dependent meiotic block that occurs in Saccharomyces cerevisiae mutants defective in various aspects of meiotic chromosome dynamics and results in the generation of faulty meiotic products. Moreover, production of the Ddc2 protein is induced during meiotic prophase, accumulates in checkpoint-arrested mutants and localizes to distinctive chromosomal foci. Formation of meiotic Ddc2 foci requires the generation of Spo11-dependent DNA double-strand breaks (DSBs), and is impaired in an RPA mutant. Chromatin immunoprecipitation analysis reveals that Ddc2 accumulates at meiotic DSB sites, indicating that Ddc2 senses the presence of meiotic recombination intermediates. Furthermore, pachytene checkpoint signaling is defective in the ddc2 mutant. In addition, we show that mammalian ATRIP colocalizes with ATR, TopBP1 and RPA at unsynapsed regions of mouse meiotic chromosomes. Thus, our results point to an evolutionary conserved role for Ddc2/ATRIP in monitoring meiotic chromosome metabolism.

Meiotic homologue alignment and its quality surveillance are controlled by mouse HORMAD1

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.

DNA mismatch repair and infertility

PURPOSE OF REVIEW

To highlight the significance of the abnormal DNA repair mechanism in male infertility.

RECENT FINDINGS

DNA repair defects cause a variety of spermatogenic defects in mouse models. Evidence is accumulating to demonstrate the importance of DNA repair defects in human nonobstructive azoospermia. Epigenetic changes may also play a crucial role in infertility.

SUMMARY

The DNA in the cell needs to be constantly repaired to ensure fidelity of DNA replication, to maintain genome stability and to ensure propagation of species. The DNA repair and recombination machineries are highly conserved across the species and inactivation of these pathways may lead to replication and recombination errors. This review summarizes the different types of DNA lesions and DNA repair pathways, particularly focusing on highly conserved meiotic regulators, the DNA mismatch repair proteins. Targeted deletions of some of these proteins result in infertility and predisposes to tumor in mutant mouse models. There is evidence for loss of some of these proteins in human male infertility. Because defective DNA repair is associated with a mutator phenotype, the risk of transmission to the offspring of these otherwise infertile men conceived using an assisted reproductive technology needs further evaluation.

Reproductive isolation in hybrid mice due to spermatogenesis defects at three meiotic stages

Early in the process of speciation, reproductive failures occur in hybrid animals between genetically diverged populations. The sterile hybrid animals are often males in mammals and they exhibit spermatogenic disruptions, resulting in decreased number and/or malformation of mature sperms. Despite the generality of this phenomenon, comparative study of phenotypes in hybrid males from various crosses has not been done, and therefore the comprehensive genetic basis of the disruption is still elusive. In this study, we characterized the spermatogenic phenotype especially during meiosis in four different cases of reproductive isolation: B6-ChrX(MSM), PGN-ChrX(MSM), (B6 × Mus musculus musculus-NJL/Ms) F(1), and (B6 × Mus spretus) F(1). The first two are consomic strains, both bearing the X chromosome of M. m. molossinus; in B6-ChrX(MSM), the genetic background is the laboratory strain C57BL/6J (predominantly M. m. domesticus), while in PGN-ChrX(MSM) the background is the PGN2/Ms strain purely derived from wild M. m. domesticus. The last two cases are F(1) hybrids between mouse subspecies or species. Each of the hybrid males exhibited cell-cycle arrest and/or apoptosis at either one or two of three distinct meiotic stages: premeiotic stage, zygotene-to-pachytene stage of prophase I, and metaphase I. This study shows that the sterility in hybrid males is caused by spermatogenic disruptions at multiple stages, suggesting that the responsible genes function in different cellular processes. Furthermore, the stages with disruptions are not correlated with the genetic distance between the respective parental strains.

Vpr and its interactions with cellular proteins

Like most viral regulatory proteins, HIV-1 Vpr and homologous proteins from primate lentiviruses are small and multifunctional. They are associated with a plethora of effects and functions, including induction of cell cycle arrest in the G(2) phase, induction of apoptosis, transactivation, enhancement of the fidelity of reverse transcription, and nuclear import of viral DNA in macrophages and other nondividing cells. This review focuses on the cellular proteins that have been reported to interact with Vpr and their significance with respect to the known functions and effects of Vpr on cells and on viral replication.

Mouse HORMAD1 and HORMAD2, two conserved meiotic chromosomal proteins, are depleted from synapsed chromosome axes with the help of TRIP13 AAA-ATPase

Meiotic crossovers are produced when programmed double-strand breaks (DSBs) are repaired by recombination from homologous chromosomes (homologues). In a wide variety of organisms, meiotic HORMA-domain proteins are required to direct DSB repair towards homologues. This inter-homologue bias is required for efficient homology search, homologue alignment, and crossover formation. HORMA-domain proteins are also implicated in other processes related to crossover formation, including DSB formation, inhibition of promiscuous formation of the synaptonemal complex (SC), and the meiotic prophase checkpoint that monitors both DSB processing and SCs. We examined the behavior of two previously uncharacterized meiosis-specific mouse HORMA-domain proteins—HORMAD1 and HORMAD2—in wild-type mice and in mutants defective in DSB processing or SC formation. HORMADs are preferentially associated with unsynapsed chromosome axes throughout meiotic prophase. We observe a strong negative correlation between SC formation and presence of HORMADs on axes, and a positive correlation between the presumptive sites of high checkpoint-kinase ATR activity and hyper-accumulation of HORMADs on axes. HORMADs are not depleted from chromosomes in mutants that lack SCs. In contrast, DSB formation and DSB repair are not absolutely required for depletion of HORMADs from synapsed axes. A simple interpretation of these findings is that SC formation directly or indirectly promotes depletion of HORMADs from chromosome axes. We also find that TRIP13 protein is required for reciprocal distribution of HORMADs and the SYCP1/SC-component along chromosome axes. Similarities in mouse and budding yeast meiosis suggest that TRIP13/Pch2 proteins have a conserved role in establishing mutually exclusive HORMAD-rich and synapsed chromatin domains in both mouse and yeast. Taken together, our observations raise the possibility that involvement of meiotic HORMA-domain proteins in the regulation of homologue interactions is conserved in mammals.

Generation of haploid gametes in most organisms requires that homologues become connected via crossovers during meiosis. Efficient formation of crossovers depends on HORMA-domain proteins in diverse taxa. These proteins ensure that programmed meiotic DSBs are preferentially repaired from homologues, rather than from sister chromatids. This inter-homologue bias is crucial for homology search and crossovers formation. HORMA-domain proteins have been also implicated in DSB formation, in suppression of synaptonemal complex formation between non-homologous chromosomes, and in the meiotic prophase checkpoint that monitors DSB repair. Despite the importance of HORMA-domain proteins in various organisms, a role for these proteins in mammalian meiosis hasn't been reported. We examined the behaviour of meiotic mouse HORMA-domain proteins—HORMAD1 and HORMAD2—in wild-type and meiotic mutants. HORMAD1/2 preferentially accumulate on unsynapsed chromosome axes. Our data suggest that HORMAD1/2 depletion from chromosomes is a response to synaptonemal complex formation and it that is a conserved process supported by TRIP13/Pch2 AAA-ATPase. Assuming that HORMA-domain functions are conserved in mammals, we speculate that depletion of HORMADs from axes might contribute to the down-regulation of inter-homologue bias and the prophase checkpoint once homology search is completed and synaptonemal complexes form between aligned homologues.

CDK2 is required for proper homologous pairing, recombination and sex-body formation during male mouse meiosis

Cyclin-dependent kinase 2 (CDK2) was assumed to be essential in the mammalian cell cycle both at the G1-S transition and throughout the S phase. Interestingly, ablation of Cdk2 in mice does not have substantial consequences for embryonic or postnatal development, but both males and females are infertile. In the present study, we have analysed the meiotic alterations leading to infertility in Cdk2-/- male mice. We have studied the distribution and dynamics of several proteins related to meiosis progression, such as synaptonemal complex proteins, cohesin complexes, and centromere-, telomere- and recombination-related proteins. Cdk2-/- spermatocytes show an incomplete chromosome pairing, an extensive non-homologous synapsis and arrest at a pachytene-like stage with unrepaired programmed double-strand breaks. In these spermatocytes, some telomeres do not attach to the nuclear envelope, and sex chromosomes do not form a sex body. Our data demonstrate an unpredicted participation of CDK2 in the accurate pairing and recombination between homologues during mammalian meiosis.

The consequences of asynapsis for mammalian meiosis

During mammalian meiosis, synapsis of paternal and maternal chromosomes and the generation of DNA breaks are needed to allow reshuffling of parental genes. In mammals errors in synapsis are associated with a male-biased meiotic impairment, which has been attributed to a response to persisting DNA double-stranded breaks in the asynapsed chromosome segments. Recently it was discovered that the chromatin of asynapsed chromosome segments is transcriptionally silenced, providing new insights into the connection between asynapsis and meiotic impairment.

Impaired spermatogenesis and elevated spontaneous tumorigenesis in xeroderma pigmentosum group A gene (Xpa)-deficient mice

We have reported that xeroderma pigmentosum group A (Xpa) gene-knockout mice [Xpa (-/-) mice] are deficient in nucleotide excision repair (NER) and highly sensitive to UV-induced skin carcinogenesis. Although xeroderma pigmentosum group A patients show growth retardation, immature sexual development, and neurological abnormalities as well as a high incidence of UV-induced skin tumors, Xpa (-/-) mice were physiologically and behaviorally normal. In the present study, we kept Xpa (-/-) mice for 2 years under specific pathogen-free (SPF) conditions and found that the testis diminished in an age-dependent manner, and degenerating seminiferous tubules and no spermatozoa were detected in the 24-month-old Xpa (-/-) mice. In addition, a higher incidence of spontaneous tumorigenesis was observed in the 24-month-old Xpa (-/-) mice compared to Xpa (+/+) controls. Xpa (-/-) mice provide a useful model for investigating the aging and internal tumor formation in XPA patients.

Extensive meiotic asynapsis in mice antagonises meiotic silencing of unsynapsed chromatin and consequently disrupts meiotic sex chromosome inactivation

Chromosome synapsis during zygotene is a prerequisite for the timely homologous recombinational repair of meiotic DNA double-strand breaks (DSBs). Unrepaired DSBs are thought to trigger apoptosis during midpachytene of male meiosis if synapsis fails. An early pachytene response to asynapsis is meiotic silencing of unsynapsed chromatin (MSUC), which, in normal males, silences the X and Y chromosomes (meiotic sex chromosome inactivation [MSCI]). In this study, we show that MSUC occurs in Spo11-null mouse spermatocytes with extensive asynapsis but lacking meiotic DSBs. In contrast, three mutants (Dnmt3l, Msh5, and Dmc1) with high levels of asynapsis and numerous persistent unrepaired DSBs have a severely impaired MSUC response. We suggest that MSUC-related proteins, including the MSUC initiator BRCA1, are sequestered at unrepaired DSBs. All four mutants fail to silence the X and Y chromosomes (MSCI failure), which is sufficient to explain the midpachytene apoptosis. Apoptosis does not occur in mice with a single additional asynapsed chromosome with unrepaired meiotic DSBs and no disturbance of MSCI.

Maintenance of mitochondrial DNA by the Caenorhabditis elegans ATR checkpoint protein ATL-1

Here we show that inactivation of the ATR-related kinase ATL-1 results in a significant reduction in mitochondrial DNA (mtDNA) copy numbers in Caenorhabditis elegans. Although ribonucleotide reductase (RNR) expression and the ATP/dATP ratio remained unaltered in atl-1 deletion mutants, inhibition of RNR by RNAi or hydroxyurea treatment caused further reductions in mtDNA copy number. These results suggest that ATL-1 functions to maintain mtDNA independently of RNR.

Both ATM and ATR promote the efficient and accurate processing of programmed meiotic double-strand breaks

The ATM and ATR protein kinases play central roles in the cellular response to double-strand breaks (DSBs) by regulating DNA repair, cell-cycle arrest and apoptosis. During meiosis, SPO11-dependent DSBs are generated, initiating recombination between homologous chromosomes. Previous studies in mice and plants have shown that defects in ATM result in the appearance of abnormally fragmented chromosomes. However, the role of ATR in promoting normal meiosis has not yet been elucidated. Employing null Arabidopsis mutants of ATR and ATM, we demonstrate here that although atr mutants display no obvious defects in any phase of meiotic progression, the combination of defects in atr and atm exacerbates the fragmentation observed in the atm single mutant, prevents complete synapsis of chromosomes, and results in extensive and persistent interactions between non-homologous DNAs. The observed non-homologous interactions require the induction of programmed breaks: the combination of either the atm single or the atr atm double mutant with a spo11 defect eliminates the ectopic interactions observed in the double mutant, as well as significantly reducing the fragmentation seen in atm or in atr atm. Our results suggest that ATM is required for the efficient processing of SPO11-dependent DSBs during meiosis. They also indicate that ATM and ATR act redundantly to inhibit sustained interactions between non-homologous chromatids, and that these ectopic interactions require SPO11 activity.

Role of the Saccharomyces cerevisiae Rad53 checkpoint kinase in signaling double-strand breaks during the meiotic cell cycle

DNA double-strand breaks (DSBs) can arise at unpredictable locations after DNA damage or in a programmed manner during meiosis. DNA damage checkpoint response to accidental DSBs during mitosis requires the Rad53 effector kinase, whereas the meiosis-specific Mek1 kinase, together with Red1 and Hop1, mediates the recombination checkpoint in response to programmed meiotic DSBs. Here we provide evidence that exogenous DSBs lead to Rad53 phosphorylation during the meiotic cell cycle, whereas programmed meiotic DSBs do not. However, the latter can trigger phosphorylation of a protein fusion between Rad53 and the Mec1-interacting protein Ddc2, suggesting that the inability of Rad53 to transduce the meiosis-specific DSB signals might be due to its failure to access the meiotic recombination sites. Rad53 phosphorylation/activation is elicited when unrepaired meiosis-specific DSBs escape the recombination checkpoint. This activation requires homologous chromosome segregation and delays the second meiotic division. Altogether, these data indicate that Rad53 prevents sister chromatid segregation in the presence of unrepaired programmed meiotic DSBs, thus providing a salvage mechanism ensuring genetic integrity in the gametes even in the absence of the recombination checkpoint.

The management of DNA double-strand breaks in mitotic G2, and in mammalian meiosis viewed from a mitotic G2 perspective

DNA double-strand breaks (DSBs) are extremely hazardous lesions for all DNA-bearing organisms and the mechanisms of DSB repair are highly conserved. In the eukaryotic mitotic cell cycle, DSBs are often present following DNA replication while, in meiosis, hundreds of DSBs are generated as a prelude to the reshuffling of the maternally and paternally derived genomes. In both cases, the DSBs are repaired by a process called homologous recombinational repair (HRR), which utilises an intact DNA molecule as the repair template. Mitotic and meiotic HRR are managed by 'checkpoints' that inhibit cell division until DSB repair is complete. Here we attempt to summarise the substantial recent progress in understanding the checkpoint management of HRR in mitosis (focussing mainly on mammals) and then go on to use this information as a framework for understanding the presumed checkpoint management of HRR in mammalian meiosis.

Analysis of programmed cell death in mouse fetal oocytes

We report a short-term culture system that allows to define novel characteristic of programmed cell death (PCD) in fetal oocytes and to underscore new aspects of this process. Mouse fetal oocytes cultured in conditions allowing meiotic prophase I progression underwent apoptotic degeneration waves as revealed by TUNEL staining. TEM observations revealed recurrent atypical apoptotic morphologies characterized by the absence of chromatin margination and nuclear fragmentation; oocytes with autophagic and necrotic features were also observed. Further characterization of oocyte death evidenced DNA ladder, Annexin V binding, PARP cleavage, and usually caspase activation (namely caspase-2). In the aim to modulate the oocyte death process, we found that the addition to the culture medium of the pan-caspase inhibitors Z-VAD or caspase-2-specific inhibitor Z-VDVAD resulted in a partial and transient prevention of this process. Oocyte death was significantly reduced by the antioxidant agent NAC and partly prevented by KL and IGF-I growth factors. Finally, oocyte apoptosis was reduced by calpain inhibitor I and increased by rapamycin after prolonged culture. These results support the notion that fetal oocytes undergo degeneration mostly by apoptosis. This process is, however, often morphologically atypical and encompasses other forms of cell death including caspase-independent apoptosis and autophagia. The observation that oocyte death occurs mainly at certain stages of meiosis and can only be attenuated by typical anti-apoptotic treatments favors the notion that it is controlled at least in part by stage-specific oocyte-autonomous meiotic checkpoints and when activated is little amenable to inhibition being the oocyte able to switch back and forth among different death pathways.

p53 and ATM/ATR regulate 7,12-dimethylbenz[a]anthracene-induced immunosuppression

The tumor suppressor protein p53 is a transcription factor that regulates apoptotic responses produced by genotoxic agents. Previous studies have reported that 7,12-dimethylbenz[a]anthracene (DMBA)-induced bone marrow toxicity is p53-dependent in vivo. Our laboratory has shown that DMBA-induced splenic immunosuppression is CYP1B1- and microsomal epoxide hydrolase (mEH)-dependent, demonstrating that the DMBA-3,4-dihydrodiol-1,2-epoxide metabolite (DMBA-DE) is probably responsible for DMBA-induced immunosuppression. DMBA-DE is known to bind to DNA leading to strand breaks. Therefore, we postulated that a p53 pathway is required for DBMA-induced immunosuppression. In the present studies, our data show that activated p53 accumulated in the nuclei of spleen cells in WT and AhR-null mice after DMBA treatment, but not in CYP1B1-null or mEH-null mice. These results suggest that DMBA activates p53 in a CYP1B1- and mEH-dependent manner in vivo but is not AhR-dependent. Ataxia telangiectasia mutated (ATM) and ATM and Rad3-related protein (ATR) are sensors for DNA damage that signal p53 activation. Increased ATM, phospho-ATM (Ser(1987)), and ATR levels were observed after DMBA treatment in WT, p53-null, and AhR-null mice but not in CYP1B1-null or mEH-null mice. Therefore, ATM and ATR seem to act upstream of p53 as sensors of DNA damage. Ex vivo immune function studies demonstrated that DMBA-induced splenic immunosuppression is p53-dependent at doses of DMBA that produce immunosuppression in the absence of cytotoxicity. High-dose DMBA cytotoxicity may be associated with p53-independent pathways. This study provides new insights into the requirement of genotoxicity for DMBA-induced immunosuppression in vivo and highlights the roles of ATM/ATR in signaling p53.

p300/CREB-binding protein interacts with ATR and is required for the DNA replication checkpoint

The highly related acetyltransferases, p300 and CREB-binding protein (CBP) are coactivators of signal-responsive transcriptional activation. In addition, recent evidence suggests that p300/CBP also interacts directly with complexes that mediate DNA replication and repair. In this report, we show that loss of p300/CBP in mammalian cells results in a defect in the cell cycle arrest induced by stalled DNA replication. We demonstrate that complexes containing p300/CBP and ATR can be detected in mammalian cells, and that the downstream kinase CHK1 fails to be phosphorylated in response to stalled DNA replication in cells that lack p300/CBP. These observations broaden the roles for the p300/CBP acetyltransferases to include the modulation of chromatin structure and function during DNA metabolic events as well as for transcription.

Drosophila dCBP is involved in establishing the DNA replication checkpoint

The CBP/p300 family of proteins comprises related acetyltransferases that coactivate signal-responsive transcription. Recent evidence suggests that p300/CBP may also interact directly with complexes that mediate different aspects of DNA metabolism such as replication and repair. In this report, we show that loss of dCBP in Drosophila cells and eye discs results in a defect in the cell cycle arrest induced by stalled DNA replication. We show that dCBP and the checkpoint kinase Mei-41 can be found together in a complex and, furthermore, that dCBP has a genetic interaction with mei-41 in the response to stalled DNA replication. These observations suggest a broader role for the p300/CBP acetyltransferases in the modulation of chromatin structure and function during DNA metabolic events as well as for transcription.

ATM-dependent DNA damage surveillance in T-cell development and leukemogenesis: the DSB connection

The immune system is capable of recognizing and eliminating an enormous array of pathogens due to the extremely diverse antigen receptor repertoire of T and B lymphocytes. However, the development of lymphocytes bearing receptors with unique specificities requires the generation of programmed double strand breaks (DSBs) coupled with bursts of proliferation, rendering lymphocytes susceptible to mutations contributing to oncogenic transformation. Consequently, mechanisms responsible for monitoring global genomic integrity must be activated during lymphocyte development to limit the oncogenic potential of antigen receptor locus recombination. Mutations in ATM (ataxia-telangiectasia mutated), a kinase that coordinates DSB monitoring and the response to DNA damage, result in impaired T-cell development and predispose to T-cell leukemia. Here, we review recent evidence providing insight into the mechanisms by which ATM promotes normal lymphocyte development and protects from neoplastic transformation.

Sex chromosomes, synapsis, and cohesins: a complex affair

During first meiotic prophase, homologous chromosomes are held together by the synaptonemal complex, a tripartite proteinaceous structure that extends along the entire length of meiotic bivalents. While this feature is applicable for autosomes, sex chromosomes often escape from this rule. Many species present sex chromosomes that differ between them in their morphology, length, and gene content. Moreover, in some species, sex chromosomes appear in a single dose in one of the sexes. In all of these cases, the behavior of sex chromosomes during meiosis is conspicuously affected, and this includes the assembly and dynamics of the synaptonemal complex. We review in this study the structure of the synaptonemal complex in the sex chromosomes of three groups of organisms, namely: mammals, orthopterans, and hemipterans, which present different patterns of sex chromosome structure and behavior. Of special interest is the analysis of the organization of the axial/lateral elements of the synaptonemal complex in relation to other axial structures organized along meiotic chromosomes, mainly the cohesin axis. The differences found in the behavior of both axial structures reveal that while the organization of a cohesin axis along sex chromosomes is a conserved feature in most organisms and it shows very little morphological variations, the axial/lateral elements of the synaptonemal complex present a wide range of structural modifications on these chromosomes.

Genetic analysis of chromosome pairing, recombination, and cell cycle control during first meiotic prophase in mammals

Meiosis is a double-division process that is preceded by only one DNA replication event to produce haploid gametes. The defining event in meiosis is prophase I, during which chromosome pairs locate each other, become physically connected, and exchange genetic information. Although many aspects of this process have been elucidated in lower organisms, there has been scant information available until now about the process in mammals. Recent advances in genetic analysis, especially in mice and humans, have revealed many genes that play essential roles in meiosis in mammals. These include cell cycle-regulatory proteins that couple the exit from the premeiotic DNA synthesis to the progression through prophase I, the chromosome structural proteins involved in synapsis, and the repair and recombination proteins that process the recombination events. Failure to adequately repair the DNA damage caused by recombination triggers meiotic checkpoints that result in ablation of the germ cells by apoptosis. These analyses have revealed surprising sexual dimorphism in the requirements of different gene products and a much less stringent checkpoint regulation in females. This may provide an explanation for the 10-fold increase in meiotic errors in females compared with males. This review provides a comprehensive analysis of the use of genetic manipulation, particularly in mice, but also of the analysis of mutations in humans, to elucidate the mechanisms that are required for traverse through prophase I.

Checking Your Breaks: Surveillance Mechanisms of Meiotic Recombination

Numerous DNA double-strand breaks (DSBs) are introduced into the genome in the course of meiotic recombination. This poses a significant hazard to the genomic integrity of the cell. Studies in a number of organisms have unveiled the existence of surveillance mechanisms or checkpoints that couple the formation and repair of DSBs to cell cycle progression. Through these mechanisms, aberrant meiocytes are delayed in their meiotic progression, thereby facilitating repair of meiotic DSBs, or are culled through programmed cell death, thereby protecting the germline from aneuploidies that could lead to spontaneous abortions, birth defects and cancer predisposition in the offspring. Here we summarize recent progress in our understanding of these checkpoints. This review focuses on the surveillance mechanisms of the budding yeast S. cerevisiae, where the molecular details are best understood, but will frequently compare and contrast these mechanisms with observations in other organisms.

The evolution of meiosis: recruitment and modification of somatic DNA-repair proteins

Several DNA-damage detection and repair mechanisms have evolved to repair double-strand breaks induced by mutagens. Later in evolutionary history, DNA single- and double-strand cuts made possible immune diversity by V(D)J recombination and recombination at meiosis. Such cuts are induced endogenously and are highly regulated and controlled. In meiosis, DNA cuts are essential for the initiation of homologous recombination, and for the formation of joint molecule and crossovers. Many proteins that function during somatic DNA-damage detection and repair are also active during homologous recombination. However, their meiotic functions may be altered from their somatic roles through localization, posttranslational modifications and/or interactions with meiosis-specific proteins. Presumably, somatic repair functions and meiotic recombination diverged during evolution, resulting in adaptations specific to sexual reproduction. (c) 2005 Wiley Periodicals, Inc.

SPO11 is required for sex-body formation, and Spo11 heterozygosity rescues the prophase arrest ofAtm-/- spermatocytes

SPO11 introduces double-strand breaks (DSBs) that trigger the phosphorylation of H2AX during meiotic prophase. In mice, SPO11 is strictly required for initiation of meiotic recombination and synapsis, yet SPO11 is still considered to be dispensable for sex-body formation in mouse spermatocytes. We provide conclusive evidence showing that functional SPO11, and consequently recombination and synapsis, are required for phosphorylation of H2AX in the X-Y chromatin and for sex-body formation in mouse spermatocytes. We investigated the role in meiosis of the three kinases [ATM (ataxia telangiectasia mutated), ATR (ataxia-telangiectasia- and Rad-3-related) and DNA-PKcs (DNA-dependent-protein-kinase catalytic subunit)] known to phosphorylate H2AX in mitotic cells. We found that DNA-PKcs can be ruled out as an essential kinase in this process, whereas ATM is strictly required for the chromatin-wide phosphorylation of H2AX occurring in leptotene spermatocytes in response to DSBs. Remarkably, we discovered that Spo11 heterozygosity can rescue the prophase-I-arrest characteristic of ATM-deficient spermatocytes. Characterization of the rescued Atm-/-Spo11+/- mutant indicates that ATM is dispensable for sex-body formation and phosphorylation of H2AX in this subnuclear domain. The co-localization of ATR, phosphorylated H2AX and the sex chromatin observed in the Atm-/-Spo11+/- mutant, along with ATR transcription kinetics during the first wave of spermatogenesis, confirm and expand recent findings indicating that ATR is the kinase involved in H2AX phosphorylation in the sex body.

Chromosome 18 pairing behavior in human trisomic oocytes. Presence of an extra chromosome extends bouquet stage

Little is known about the first meiotic prophase stages in the human female because these occur during fetal life, and only a few studies have addressed aneuploid human oocytes. In this paper, the synaptic process in the meiotic prophase in three 47, XX + 18 cases is analyzed. A complete study of the dynamics of centromeres and telomeres, cohesin core and synapsis development in aneuploid female meiosis was performed. Investigation of chromosome dynamics in prophase of trisomy 18 oocytes show that these events follow the major patterns seen earlier in euploid oocytes. However, there is a significant delay in the resolution of bouquet topology which could relate to the presence of a surplus chromosome 18 axial element in zygotene oocytes. Pachytene oocytes displayed normal synapsis among the three chromosome 18s. However, in some oocytes the surplus chromosome 18 core was aligned to the bivalent 18. As ataxia telangiectasia and Rad3 related kinase (ATR) has been described as a marker for late-pairing chromosomes in mice, ATR distribution was analyzed in human meiocytes –spermatocytes, euploid oocytes and trisomic oocytes. In contrast to the observations made in mice, no preferential staining for late-pairing chromosomes was observed in humans. In the cases studied, bivalent synapses progressed as in a normal ovary, contrasting with the hypothesis that a surplus chromosome can modify pairing of other chromosomes.

Sex Chromosome Inactivation: The Importance of Pairing

In mammals, the process of making sperm is marked by inactivation of sex chromosomes. Why and how does this happen? The answer apparently lies in whether a chromosome finds a pairing partner. Similar mechanisms in mold and worms reveal a surprising and recurrent theme throughout evolution.

BRCA1, histone H2AX phosphorylation, and male meiotic sex chromosome inactivation

In mammalian spermatogenesis, the X and Y chromosomes are transcriptionally silenced during the pachytene stage of meiotic prophase (meiotic sex chromosome inactivation, MSCI), forming a condensed chromatin domain termed the sex or XY body. The nucleosomal core histone H2AX is phosphorylated within the XY chromatin domain just prior to MSCI, and it has been hypothesized that this triggers the chromatin condensation and transcriptional repression. Here, we show that the kinase ATR localizes to XY chromatin at the onset of MSCI and that this localization is disrupted in mice with a mutant form of the tumor suppressor protein BRCA1. In the mutant pachytene cells, ATR is usually present at nonsex chromosomal sites, where it colocalizes with aberrant sites of H2AX phosphorylation; in these cells, there is MSCI failure. In rare pachytene cells, ATR does locate to XY chromatin, H2AX is then phosphorylated, a sex body forms, and MSCI ensues. These observations highlight an important role for BRCA1 in recruiting the kinase ATR to XY chromatin at the onset of MSCI and provide compelling evidence that it is ATR that phosphorylates H2AX and triggers MSCI.

Distinct DNA-damage-dependent and -independent responses drive the loss of oocytes in recombination-defective mouse mutants

Defects in meiotic recombination in many organisms result in arrest because of activation of a meiotic checkpoint(s). The proximal defect that triggers this checkpoint in mammalian germ cells is not understood, but it has been suggested to involve either the presence of DNA damage in the form of unrepaired recombination intermediates or defects in homologous chromosome pairing and synapsis independent of DNA damage per se. To distinguish between these possibilities in the female germ line, we compared mouse oocyte development in a mutant that fails to form the double-strand breaks (DSBs) that initiate meiotic recombination (Spo11-/-) to mutants with defects in processing DSBs when they are formed (Dmc1-/- and Msh5-/-), and we examined the epistasis relationships between these mutations. Absence of DSB formation caused a partial defect in follicle formation, whereas defects in DSB repair caused earlier and more severe meiotic arrest, which could be suppressed by eliminating DSB formation. Therefore, our analysis reveals that there are both DNA-damage-dependent and -independent responses to recombination errors in mammalian oocytes. By using these findings as a paradigm, we also examined oocyte loss in mutants lacking the DNA-damage checkpoint kinase ATM. The absence of ATM caused defects in folliculogenesis that were similar to those in Dmc1 mutants and that could be suppressed by Spo11 mutation, implying that oocyte death in Atm-deficient animals is a response to defective DSB repair.

Silencing of unsynapsed meiotic chromosomes in the mouse

In Neurospora, DNA unpaired in meiosis both is silenced and induces silencing of all DNA homologous to it. This process, called meiotic silencing by unpaired DNA, is thought to protect the host genome from invasion by transposable elements. We now show that silencing of unpaired (unsynapsed) chromosome regions also takes place in the mouse during both male and female meiosis. The tumor suppressor protein BRCA1 is implicated in this silencing, mirroring its role in the meiotic silencing of the X and Y chromosomes in normal male meiosis. These findings impact on the interpretation of the relationship between synaptic errors and sterility in mammals and extend our understanding of the biology of Brca1.

Ubiquitin ligase Rad18Sc localizes to the XY body and to other chromosomal regions that are unpaired and transcriptionally silenced during male meiotic prophase

In replicative damage bypass (RDB) in yeast, the ubiquitinconjugating enzyme RAD6 interacts with the ubiquitin ligase RAD18. In the mouse, these enzymes are represented by two homologs of RAD6, HR6a and HR6b, and one homolog of RAD18, Rad18Sc. Expression of these genes and the encoded proteins is ubiquitous, but there is relatively high expression in the testis. We have studied the subcellular localization by immunostaining Rad18Sc and other RDB proteins in mouse primary spermatocytes passing through meiotic prophase in spermatogenesis. The highest Rad18Sc protein level is found at pachytene and diplotene, and the protein localizes mainly to the XY body, a subnuclear region that contains the transcriptionally inactivated X and Y chromosomes. In spermatocytes that carry translocations for chromosomes 1 and 13, Rad18Sc protein concentrates on translocation bivalents that are not fully synapsed. The partly synapsed bivalents are often localized in the vicinity of the XY body, and show a very low level of RNA polymerase II, indicating that the chromatin is in a silent configuration similar to transcriptional silencing of the XY body. Thus, Rad18Sc localizes to unsynapsed and silenced chromosome segments during the male meiotic prophase. All known functions of RAD18 in yeast are related to RDB. However, in contrast to Rad18Sc, expression of UBC13 and polη, known to be involved in subsequent steps of RDB, appears to be diminished in the XY body and regions containing the unpaired translocation bivalents. Taken together, these observations suggest that the observed subnuclear localization of Rad18Sc may involve a function outside the context of RDB. This function is probably related to a mechanism that signals the presence of unsynapsed chromosomal regions and subsequently leads to transcriptional silencing of these regions during male meiotic prophase.