TopBP1 and ATR colocalization at meiotic chromosomes: role of TopBP1/Cut5 in the meiotic recombination checkpoint

Initiation and resolution of interhomolog connections: crossover and non-crossover sites along mouse synaptonemal complexes

Programmed double-strand breaks at prophase of meiosis acquire immunologically detectable RAD51-DMC1 foci or early nodules (ENs) that are associated with developing chromosome core segments; each focus is surrounded by a gammaH2AX-modified chromosome domain. The 250-300 ENs per nucleus decline in numbers during the development of full-length cores and the remaining foci are relatively evenly distributed along the mature cores (gamma distribution of nu=2.97). The ENs become transformed nodules (TNs) by the acquisition of RPA, BLM, MSH4 and topoisomerases that function in repair and Holliday junction resolution. At the leptotene-zygotene transition, TNs orient to positions between the aligned cores where they initiate structural interhomolog contacts prior to synaptonemal complex (SC) formation, possibly future crossover sites. Subsequently, TNs are associated with SC extension at the synaptic forks. Dephosphorylation of TN-associated histone gammaH2AX chromatin suggests annealing of single strands or repair of double-strand breaks DSBs at this time. Some 200 TNs per pachytene nucleus are distributed proportional to SC length and are evenly distributed along the SCs (nu= approximately 4). At this stage, gammaH2AX-modified chromatin domains are associated with transcriptionally silenced sex chromosomes and autosomal sites. Immunogold electron microscope evidence shows that one or two TNs of the 10-15 TNs per SC acquire MLH1 protein, the hallmark of reciprocal recombination, whereas the TNs that do not acquire MLH1 protein relocate from their positions along the midline of the SCs to the periphery of the SCs. Relocation of TNs may be associated with the conversion of potential crossovers into non-crossovers.

Phosphorylation of ATR-interacting protein on Ser239 mediates an interaction with breast-ovarian cancer susceptibility 1 and checkpoint function

The signaling of DNA damage and replication stress involves a multitude of proteins, including the kinases ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR), and proteins with BRCA1 COOH-terminal (BRCT) domains. The BRCT domain-containing proteins facilitate the phosphorylation of ATM/ATR substrates and can be coimmunoprecipitated with ATM or ATR. However, their mode of interaction with the ATM/ATR kinases remains elusive. Here, we show that breast-ovarian cancer susceptibility 1 (BRCA1) interacts directly with ATR-interacting protein (ATRIP), an obligate partner of ATR. The interaction involves the BRCT domains of BRCA1 and Ser(239) of ATRIP, a residue that is phosphorylated in both irradiated and nonirradiated cells. Consistent with a role of BRCA1 in ATR signaling, substitution of Ser(239) of ATRIP with Ala leads to a G(2)-M checkpoint defect. We propose that a direct physical interaction between BRCA1 and ATRIP is required for the checkpoint function of ATR.

Mouse pachytene checkpoint 2 (trip13) is required for completing meiotic recombination but not synapsis

In mammalian meiosis, homologous chromosome synapsis is coupled with recombination. As in most eukaryotes, mammalian meiocytes have checkpoints that monitor the fidelity of these processes. We report that the mouse ortholog (Trip13) of pachytene checkpoint 2 (PCH2), an essential component of the synapsis checkpoint in Saccharomyces cerevisiae and Caenorhabditis elegans, is required for completion of meiosis in both sexes. TRIP13-deficient mice exhibit spermatocyte death in pachynema and loss of oocytes around birth. The chromosomes of mutant spermatocytes synapse fully, yet retain several markers of recombination intermediates, including RAD51, BLM, and RPA. These chromosomes also exhibited the chiasmata markers MLH1 and MLH3, and okadaic acid treatment of mutant spermatocytes caused progression to metaphase I with bivalent chromosomes. Double mutant analysis demonstrated that the recombination and synapsis genes Spo11, Mei1, Rec8, and Dmc1 are all epistatic to Trip13, suggesting that TRIP13 does not have meiotic checkpoint function in mice. Our data indicate that TRIP13 is required after strand invasion for completing a subset of recombination events, but possibly not those destined to be crossovers. To our knowledge, this is the first model to separate recombination defects from asynapsis in mammalian meiosis, and provides the first evidence that unrepaired DNA damage alone can trigger the pachytene checkpoint response in mice.

It is critical that the chromosomes carried by sperm and eggs contain faithful representations of the genome of the individual that produced them. During the process of meiosis, the maternal and paternal copies of each chromosome “synapse” with each other (become tightly associated), exchange genetic material via the process of recombination, then separate into daughter cells in the first of two meiotic cell divisions. The intricate chromosome behavior is subject to errors, so most organisms have evolved meiotic “checkpoints” that monitor fidelity of chromosome synapsis and repair of DNA damage. These checkpoints cause defective cells to self destruct rather than generate defective sperm or eggs. We studied the effects of deleting mouse Trip13, a gene that in distant organisms plays a key role in meiotic checkpoint control. These experiments revealed that instead of having a checkpoint role, Trip13 is required for one of the two major classes of recombination in meiosis that is required for repairing broken DNA molecules. The chromosomes still synapsed normally, but animals were sterile due to massive death of oocytes and spermatocytes. These results indicate that, in addition to a checkpoint that responds to failed synapsis, one exists to specifically detect unrepaired DNA damage that is due to failed recombination.

TopBP1 contains a transcriptional activation domain suppressed by two adjacent BRCT domains

TopBP1 has eight BRCT [BRCA1 (breast-cancer susceptibility gene 1) C-terminus] domains and is involved in initiating DNA replication, and DNA damage checkpoint signalling and repair. Several BRCT-domain-containing proteins involved in mediating DNA repair have transcriptional regulatory domains, and as demonstrated for BRCA1 these regulatory domains are important in mediating the functions of these proteins. These transcriptional regulatory processes involve modification of chromatin, and recent evidence has clearly demonstrated that the ability to modify chromatin plays an important role in regulating DNA damage signalling and repair. Here we report the identification of a TopBP1 transcriptional activation domain that is rich in hydrophobic residues, interspersed with acidic amino acids, characteristics that are typical of transcriptional activation domains identified previously. Two adjacent repressor domains encoded by BRCT2 and BRCT5 silence this activator and experiments suggest that these repressors actively recruit repressor complexes. Both the activator and BRCT2 repressor domains function in yeast. The present study identifies several chromatin modification domains encoded by TopBP1, and the implications of these findings are discussed in the context of the DNA damage response and the understanding of TopBP1 function.

Rad4TopBP1, a scaffold protein, plays separate roles in DNA damage and replication checkpoints and DNA replication

Rad4TopBP1, a BRCT domain protein, is required for both DNA replication and checkpoint responses. Little is known about how the multiple roles of Rad4TopBP1 are coordinated in maintaining genome integrity. We show here that Rad4TopBP1 of fission yeast physically interacts with the checkpoint sensor proteins, the replicative DNA polymerases, and a WD-repeat protein, Crb3. We identified four novel mutants to investigate how Rad4TopBP1 could have multiple roles in maintaining genomic integrity. A novel mutation in the third BRCT domain of rad4+TopBP1 abolishes DNA damage checkpoint response, but not DNA replication, replication checkpoint, and cell cycle progression. This mutant protein is able to associate with all three replicative polymerases and checkpoint proteins Rad3ATR-Rad26ATRIP, Hus1, Rad9, and Rad17 but has a compromised association with Crb3. Furthermore, the damaged-induced Rad9 phosphorylation is significantly reduced in this rad4TopBP1 mutant. Genetic and biochemical analyses suggest that Crb3 has a role in the maintenance of DNA damage checkpoint and influences the Rad4TopBP1 damage checkpoint function. Taken together, our data suggest that Rad4TopBP1 provides a scaffold to a large complex containing checkpoint and replication proteins thereby separately enforcing checkpoint responses to DNA damage and replication perturbations during the cell cycle.

TopBP1 activates the ATR-ATRIP complex

ATR is a key regulator of checkpoint responses to incompletely replicated and damaged DNA, but the mechanisms underlying control of its kinase activity are unknown. TopBP1, the vertebrate homolog of yeast Cut5/Dbp11, has dual roles in initiation of DNA replication and regulation of checkpoint responses. We show that recombinant TopBP1 induces a large increase in the kinase activity of both Xenopus and human ATR. The ATR-activating domain resides in a conserved segment of TopBP1 that is distinct from its numerous BRCT repeats. The isolated ATR-activating domain from TopBP1 induces ectopic activation of ATR-dependent signaling in both Xenopus egg extracts and human cells. Furthermore, Xenopus egg extracts containing a version of TopBP1 with an inactivating point mutation in the ATR-activating domain are defective in checkpoint regulation. These studies establish that activation of ATR by TopBP1 is a crucial step in the initiation of ATR-dependent signaling processes.

The diverse roles of transverse filaments of synaptonemal complexes in meiosis

In most eukaryotes, homologous chromosomes (homologs) are closely apposed during the prophase of the first meiotic division by a ladderlike proteinaceous structure, the synaptonemal complex (SC) [Fawcett, J Biophys Biochem Cytol 2:403-406, 1956; Moses, J Biophys Biochem Cytol 2:215-218, 1956]. SCs consist of two proteinaceous axes, which each support the two sister chromatids of one homolog, and numerous transverse filaments (TFs), which connect the two axes. Organisms that assemble SCs perform meiotic recombination in the context of these structures. Although much information has accumulated about the composition of SCs and the pathways of meiotic crossing over, several questions remain about the role of SCs in meiosis, in particular, about the role of the TFs. In this review, we focus on possible role(s) of TFs. The interest in TF functions received new impulses from the recent characterization of TF-deficient mutants in a number of species. Intriguingly, the phenotypes of these mutants are very different, and a variety of TF functions appear to be hidden behind a façade of morphological conservation. However, in all TF-deficient mutants a specific class of crossovers that display interference is affected. TFs appear to create suitable preconditions for the formation of these crossovers in most species, but are most likely not directly involved in the interference process itself. Furthermore, TFs are important for full-length homolog alignment.

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.

Surveillance of different recombination defects in mouse spermatocytes yields distinct responses despite elimination at an identical developmental stage

Fundamentally different recombination defects cause apoptosis of mouse spermatocytes at the same stage in development, stage IV of the seminiferous epithelium cycle, equivalent to mid-pachynema in normal males. To understand the cellular response(s) that triggers apoptosis, we examined markers of spermatocyte development in mice with different recombination defects. In Spo11(-)(/)(-) mutants, which lack the double-strand breaks (DSBs) that initiate recombination, spermatocytes express markers of early to mid-pachynema, forming chromatin domains that contain sex body-associated proteins but that rarely encompass the sex chromosomes. Dmc1(-)(/)(-) spermatocytes, impaired in DSB repair, appear to arrest at or about late zygonema. Epistasis analysis reveals that this earlier arrest is a response to unrepaired DSBs, and cytological analysis implicates the BRCT-containing checkpoint protein TOPBP1. Atm(-)(/)(-) spermatocytes show similarities to Dmc1(-)(/)(-) spermatocytes, suggesting that ATM promotes meiotic DSB repair. Msh5(-)(/)(-) mutants display a set of characteristics distinct from these other mutants. Thus, despite equivalent stages of spermatocyte elimination, different recombination-defective mutants manifest distinct responses, providing insight into surveillance mechanisms in male meiosis.

Mutant meiotic chromosome core components in mice can cause apparent sexual dimorphic endpoints at prophase or X-Y defective male-specific sterility

Genetic modifications causing germ cell death during meiotic prophase in the mouse frequently have sexually dimorphic phenotypes where oocytes reach more advanced stages than spermatocytes. To determine to what extent these dimorphisms are due to differences in male versus female meiotic prophase development, we compared meiotic chromosome events in the two sexes in both wild-type and mutant mice. We report the abundance and time course of appearance of structural and recombination-related proteins of fetal oocyte nuclei. Oocytes at successive days post coitus show rapid, synchronous meiotic prophase development compared with the continuous spermatocyte development in adult testis. Consequently, a genetic defect requiring 2-3 days from the onset of prophase to reach arrest registers pachytene as the developmental endpoint in oocytes. Pachytene spermatocytes, on the other hand, which normally accumulate during days 4-10 after the onset of prophase, will be rare, giving the appearance of an earlier endpoint than in oocytes. We conclude that these different logistics create apparent sexually dimorphic endpoints. For more pronounced sexual dimorphisms, we examined meiotic prophase of mice with genetic modifications of meiotic chromosome core components that cause male but not female sterility. The correlations between male sterility and alterations in the organization of the sex chromosome cores and X-Y chromatin may indicate that impaired signals from the XY domain (XY chromosome cores, chromatin, dense body and sex body) may interfere with the progression of the spermatocyte through prophase. Oocytes, in the absence of the X-Y pair, do not suffer such defects.

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.

Meiotic S-phase damage activates recombination without checkpoint arrest

Checkpoints operate during meiosis to ensure the completion of DNA synthesis and programmed recombination before the initiation of meiotic divisions. Studies in the fission yeast Schizosaccharomyces pombe suggest that the meiotic response to DNA damage due to a failed replication checkpoint response differs substantially from the vegetative response, and may be influenced by the presence of homologous chromosomes. The checkpoint responses to DNA damage during fission yeast meiosis are not well characterized. Here we report that DNA damage induced during meiotic S-phase does not activate checkpoint arrest. We also find that in wild-type cells, markers for DNA breaks can persist at least to the first meiotic division. We also observe increased spontaneous S-phase damage in checkpoint mutants, which is repaired by recombination without activating checkpoint arrest. Our results suggest that fission yeast meiosis is exceptionally tolerant of DNA damage, and that some forms of spontaneous S-phase damage can be repaired by recombination without activating checkpoint arrest.

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.

TopBP1 localises to centrosomes in mitosis and to chromosome cores in meiosis

Topoisomerase IIbeta binding protein 1 (TopBP1), previously shown to localise to sites of DNA damage and to stalled replication forks, has been implicated in DNA replication and in DNA damage response. In this work we showed that TopBP1 was localised in structures other than stalled replication forks. In late mitosis TopBP1 localises to centrosomes in a manner similar to other DNA damage response proteins such as BRCA1 and p53. Spindle checkpoint activation does not affect this centrosomal localisation. Moreover, in the testis, we detected high levels of TopBP1 associated with meiotic prophase chromosome cores and the X-Y pair. Together, these data suggest a direct role of TopBP1 during both mitosis and meiotic prophase I.