Genetic evidence that synaptonemal complex axial elements govern recombination pathway choice in mice

TI17, a novel compound, exerts anti-MM activity by impairing Trip13 function of DSBs repair and enhancing DNA damage

BACKGROUND

Thyroid hormone receptor interacting protein 13 (Trip13) is an AAA-ATPase that regulates the assembly or disassembly protein complexes and mediates Double-strand breaks (DSBs) repair. Overexpression of Trip13 has been detected in many cancers and is associated with myeloma progression, disease relapse and poor prognosis inmultiple myeloma (MM).

METHODS

We have identified a small molecular, TI17, through a parallel compound-centric approach, which specifically targets Trip13. To identify whether TI17 targeted Trip13, pull-down and nuclear magnetic resonance spectroscopy (NMR) assays were performed. Cell counting kit-8, clone formation, apoptosis and cell cycle assays were applied to investigate the effects of TI17. We also utilized a mouse model to investigate the effects of TI17 in vivo.

RESULTS

TI17 effectively inhibited the proliferation of MM cells, and induced the cycle arrest and apoptosis of MM cells. Furthermore, treatment with TI17 abrogates tumor growth and has no apparent side effects in mouse xenograft models. TI17 specifically impaired Trip13 function of DSBs repair and enhanced DNA damage responses in MM. Combining with melphalan or HDAC inhibitor panobinostat triggers synergistic anti-MM effect.

CONCLUSIONS

Our study suggests that TI17 could be acted as a specific inhibitor of Trip13 and supports a preclinical proof of concept for therapeutic targeting of Trip13 in MM.

Meiotic Chromosome Structure, the Synaptonemal Complex, and Infertility

In meiosis, homologous chromosome synapsis is mediated by a supramolecular protein structure, the synaptonemal complex (SC), that assembles between homologous chromosome axes. The mammalian SC comprises at least eight largely coiled-coil proteins that interact and self-assemble to generate a long, zipper-like structure that holds homologous chromosomes in close proximity and promotes the formation of genetic crossovers and accurate meiotic chromosome segregation. In recent years, numerous mutations in human SC genes have been associated with different types of male and female infertility. Here, we integrate structural information on the human SC with mouse and human genetics to describe the molecular mechanisms by which SC mutations can result in human infertility. We outline certain themes in which different SC proteins are susceptible to different types of disease mutation and how genetic variants with seemingly minor effects on SC proteins may act as dominant-negative mutations in which the heterozygous state is pathogenic.

TRIP 13-dependent pathways promote the development of gastric cancer

TRIP13 is highly expressed in various human tumors and promotes tumorigenesis. We aimed to explore the biological effect of TRIP13 on gastric cancer. The RNA sequence data were retrieved from TCGA to evaluate TRIP13 mRNA expression in gastric cancer. Paired formalin-fixed paraffin-embedded blocks were further analyzed to verify the relationship between TRIP13 expression and carcinogenic status. The functions of TRIP13 on the proliferation of gastric malignancy were investigated by MTT, flow cytometry, colony formation experiment, and nude mouse tumor formation experiment. Finally, microarray analysis of TRIP13-related pathways was performed to identify the potential underlying mechanism of TRIP13 in gastric cancer. TRIP13 was found to have high expression in tumor samples. TRIP13 expression status was significantly subjective to tumor-node-metastasis (TNM) staging and poor survival. The downregulation of TRIP13 promoted apoptosis and inhibited tumor growth. TRIP13-dependent JAK/STAT and NF-κB signaling cascade were found as two key pathways in the carcinogenesis of GC. In conclusion, TRIP13 participates in the carcinogenesis of stomach cancer, and its overexpression in the cancerous tissues dovetail with advanced stage and survival. Moreover, TRIP13 functions as an upstream regulator of the JAK/STAT and p53 signaling pathways, which play critical roles in developing various malignancies.

Rad51-mediated interhomolog recombination during budding yeast meiosis is promoted by the meiotic recombination checkpoint and the conserved Pif1 helicase

During meiosis, recombination between homologous chromosomes (homologs) generates crossovers that promote proper segregation at the first meiotic division. Recombination is initiated by Spo11-catalyzed DNA double strand breaks (DSBs). 5' end resection of the DSBs creates 3' single strand tails that two recombinases, Rad51 and Dmc1, bind to form presynaptic filaments that search for homology, mediate strand invasion and generate displacement loops (D-loops). D-loop processing then forms crossover and non-crossover recombinants. Meiotic recombination occurs in two temporally distinct phases. During Phase 1, Rad51 is inhibited and Dmc1 mediates the interhomolog recombination that promotes homolog synapsis. In Phase 2, Rad51 becomes active and functions with Rad54 to repair residual DSBs, making increasing use of sister chromatids. The transition from Phase 1 to Phase 2 is controlled by the meiotic recombination checkpoint through the meiosis-specific effector kinase Mek1. This work shows that constitutive activation of Rad51 in Phase 1 results in a subset of DSBs being repaired by a Rad51-mediated interhomolog recombination pathway that is distinct from that of Dmc1. Strand invasion intermediates generated by Rad51 require more time to be processed into recombinants, resulting in a meiotic recombination checkpoint delay in prophase I. Without the checkpoint, Rad51-generated intermediates are more likely to involve a sister chromatid, thereby increasing Meiosis I chromosome nondisjunction. This Rad51 interhomolog recombination pathway is specifically promoted by the conserved 5'-3' helicase PIF1 and its paralog, RRM3 and requires Pif1 helicase activity and its interaction with PCNA. This work demonstrates that (1) inhibition of Rad51 during Phase 1 is important to prevent competition with Dmc1 for DSB repair, (2) Rad51-mediated meiotic recombination intermediates are initially processed differently than those made by Dmc1, and (3) the meiotic recombination checkpoint provides time during prophase 1 for processing of Rad51-generated recombination intermediates.

Chromosome-nuclear envelope tethering - a process that orchestrates homologue pairing during plant meiosis?

During prophase I of meiosis, homologous chromosomes pair, synapse and exchange their genetic material through reciprocal homologous recombination, a phenomenon essential for faithful chromosome segregation. Partial sequence identity between non-homologous and heterologous chromosomes can also lead to recombination (ectopic recombination), a highly deleterious process that rapidly compromises genome integrity. To avoid ectopic exchange, homology recognition must be extended from the narrow position of a crossover-competent double-strand break to the entire chromosome. Here, we review advances on chromosome behaviour during meiotic prophase I in higher plants, by integrating centromere- and telomere dynamics driven by cytoskeletal motor proteins, into the processes of homologue pairing, synapsis and recombination. Centromere–centromere associations and the gathering of telomeres at the onset of meiosis at opposite nuclear poles create a spatially organised and restricted nuclear state in which homologous DNA interactions are favoured but ectopic interactions also occur. The release and dispersion of centromeres from the nuclear periphery increases the motility of chromosome arms, allowing meiosis-specific movements that disrupt ectopic interactions. Subsequent expansion of interstitial synapsis from numerous homologous interactions further corrects ectopic interactions. Movement and organisation of chromosomes, thus, evolved to facilitate the pairing process, and can be modulated by distinct stages of chromatin associations at the nuclear envelope and their collective release.

Summary: We review plant meiosis, including chromosome tethering at the nuclear periphery that, we propose, defines chromosome dynamics-facilitating DNA sequence-based pairing of homologues

Dynamic localization of SPO11-1 and conformational changes of meiotic axial elements during recombination initiation of maize meiosis

Meiotic double-strand breaks (DSBs) are generated by the evolutionarily conserved SPO11 complex in the context of chromatin loops that are organized along axial elements (AEs) of chromosomes. However, how DSBs are formed with respect to chromosome axes and the SPO11 complex remains unclear in plants. Here, we confirm that DSB and bivalent formation are defective in maize spo11-1 mutants. Super-resolution microscopy demonstrates dynamic localization of SPO11-1 during recombination initiation, with variable numbers of SPO11-1 foci being distributed in nuclei but similar numbers of SPO11-1 foci being found on AEs. Notably, cytological analysis of spo11-1 meiocytes revealed an aberrant AE structure. At leptotene, AEs of wild-type and spo11-1 meiocytes were similarly curly and discontinuous. However, during early zygotene, wild-type AEs become uniform and exhibit shortened axes, whereas the elongated and curly AEs persisted in spo11-1 mutants, suggesting that loss of SPO11-1 compromised AE structural maturation. Our results reveal an interesting relationship between SPO11-1 loading onto AEs and the conformational remodeling of AEs during recombination initiation.

Meiosis is essential during sexual reproduction to produce haploid gametes. Recombination is the most crucial step during meiotic prophase I. It enables pairing of homologous chromosomes prior to their reductional division and generates new combinations of genetic alleles for transmission to the next generation. Meiotic recombination is initiated by generating DNA double-strand breaks (DSBs) via SPO11, a topoisomerase-related enzyme. The activity, timing and location of this DSB machinery must be controlled precisely, but how this is achieved remains obscure. Here, we show dynamic localization of SPO11-1 on chromatin during meiotic initiation in maize, yet a similar number of SPO11-1 is able to load onto axial elements (AEs), which accompanies a structural change of the AEs of wild-type meiotic chromosomes. Interestingly, loss of SPO11-1 not only affects DSB formation but also impairs structural alterations of AEs, resulting in abnormally long and curly AEs during early meiosis. Our study provides new insights into SPO11-1 localization during recombination initiation and suggests an intimate relationship between DSB formation and AE structural changes.

Meiosis: the chromosomal foundation of reproduction

Meiosis is the chromosomal foundation of reproduction, with errors in this important process leading to aneuploidy and/or infertility. In this review celebrating the 50th anniversary of the founding of the Society for the Study of Reproduction, the important chromosomal structures and dynamics contributing to genomic integrity across generations are highlighted. Critical unsolved biological problems are identified, and the advances that will lead to their ultimate resolution are predicted.

Insights into a Crucial Role of TRIP13 in Human Cancer

Thyroid Hormone Receptor Interacting Protein 13 (TRIP13) plays a key role in regulating mitotic processes, including spindle assembly checkpoint and DNA repair pathways, which may account for Chromosome instability (CIN). As CIN is a predominant hallmark of cancer, TRIP13 may act as a tumor susceptibility locus. Amplification of TRIP13 has been observed in various human cancers and implicated in several aspects of malignant transformation, including cancer cell proliferation, drug resistance and tumor progression. Here, we discussed the functional significance of TRIP13 in cell progression, highlighted the recent findings on the aberrant expression in human cancers and emphasized its significance for the therapeutic potential.

Evolution and meiotic organization of heteromorphic sex chromosomes

The evolution of heteromorphic sex chromosomes has occurred independently many times in different lineages. The differentiation of sex chromosomes leads to dramatic changes in sequence composition and function and guides the evolutionary trajectory and utilization of genes in pivotal sex determination and reproduction roles. In addition, meiotic recombination and pairing mechanisms are key in orchestrating the resultant impact, retention and maintenance of heteromorphic sex chromosomes, as the resulting exposure of unpaired DNA at meiosis triggers ancient repair and checkpoint pathways. In this review, we summarize the different ways in which sex chromosome systems are organized at meiosis, how pairing is affected, and differences in unpaired DNA responses. We hypothesize that lineage specific differences in meiotic organization is not only a consequence of sex chromosome evolution, but that the establishment of epigenetic changes on sex chromosomes contributes toward their evolutionary conservation.

A conserved filamentous assembly underlies the structure of the meiotic chromosome axis

The meiotic chromosome axis plays key roles in meiotic chromosome organization and recombination, yet the underlying protein components of this structure are highly diverged. Here, we show that 'axis core proteins' from budding yeast (Red1), mammals (SYCP2/SYCP3), and plants (ASY3/ASY4) are evolutionarily related and play equivalent roles in chromosome axis assembly. We first identify 'closure motifs' in each complex that recruit meiotic HORMADs, the master regulators of meiotic recombination. We next find that axis core proteins form homotetrameric (Red1) or heterotetrameric (SYCP2:SYCP3 and ASY3:ASY4) coiled-coil assemblies that further oligomerize into micron-length filaments. Thus, the meiotic chromosome axis core in fungi, mammals, and plants shares a common molecular architecture, and likely also plays conserved roles in meiotic chromosome axis assembly and recombination control.

In vitro maturation affects chromosome segregation, spindle morphology and acetylation of lysine 16 on histone H4 in horse oocytes

Implantation failure and genetic developmental disabilities in mammals are caused by errors in chromosome segregation originating mainly in the oocyte during meiosis I. Some conditions, like maternal ageing or in vitro maturation (IVM), increase the incidence of oocyte aneuploidy. Here oocytes from adult mares were used to investigate oocyte maturation in a monovulatory species. Experiments were conducted to compare: (1) the incidence of aneuploidy, (2) the morphology of the spindle, (3) the acetylation of lysine 16 on histone H4 (H4K16) and (4) the relative amount of histone acetyltransferase 1 (HAT1), K(lysine) acetyltransferase 8 (KAT8, also known as MYST1), histone deacetylase 1 (HDAC1) and NAD-dependent protein deacetylase sirtuin 1 (SIRT1) mRNA in metaphase II stage oocytes that were in vitro matured or collected from peri-ovulatory follicles. The frequency of aneuploidy and anomalies in spindle morphology was increased following IVM, along with a decrease in H4K16 acetylation that was in agreement with our previous observations. However, differences in the amount of the transcripts investigated were not detected. These results suggest that the degradation of transcripts encoding for histone deacetylases and acetyltransferases is not involved in the changes of H4K16 acetylation observed following IVM, while translational or post-translational mechanisms might have a role. Our study also suggests that epigenetic instabilities introduced by IVM may affect the oocyte and embryo genetic stability.

The DNA Damage Checkpoint Eliminates Mouse Oocytes with Chromosome Synapsis Failure

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.

Environmental influences on ovarian dysgenesis - developmental windows sensitive to chemical exposures

A woman's reproductive health and ability to have children directly affect numerous aspects of her life, from personal well-being and socioeconomic standing, to morbidity and lifespan. In turn, reproductive health depends on the development of correctly functioning ovaries, a process that starts early during fetal life. Early disruption to ovarian programming can have long-lasting consequences, potentially manifesting as disease much later in adulthood. A growing body of evidence suggests that exposure to chemicals early in life, including endocrine-disrupting chemicals, can cause a range of disorders later in life, such as those described in the ovarian dysgenesis syndrome hypothesis. In this Review, we discuss four specific time windows during which the ovary is particularly sensitive to disruption by exogenous insults: gonadal sex determination, meiotic division, follicle assembly and the first wave of follicle recruitment. To date, most evidence points towards the germ cell lineage being the most vulnerable to chemical exposure, particularly meiotic division and follicle assembly. Environmental chemicals and pharmaceuticals, such as bisphenols or mild analgesics (including paracetamol), can also affect the somatic cell lineages. This Review summarizes our current knowledge pertaining to environmental chemicals and pharmaceuticals, and their potential contributions to the development of ovarian dysgenesis syndrome. We also highlight knowledge gaps that need addressing to safeguard female reproductive health.

Superresolution microscopy reveals the three-dimensional organization of meiotic chromosome axes in intact Caenorhabditis elegans tissue

When cells enter meiosis, their chromosomes reorganize as linear arrays of chromatin loops anchored to a central axis. Meiotic chromosome axes form a platform for the assembly of the synaptonemal complex (SC) and play central roles in other meiotic processes, including homologous pairing, recombination, and chromosome segregation. However, little is known about the 3D organization of components within the axes, which include cohesin complexes and additional meiosis-specific proteins. Here, we investigate the molecular organization of meiotic chromosome axes in Caenorhabditis elegans through STORM (stochastic optical reconstruction microscopy) and PALM (photo-activated localization microscopy) superresolution imaging of intact germ-line tissue. By tagging one axis protein (HIM-3) with a photoconvertible fluorescent protein, we established a spatial reference for other components, which were localized using antibodies against epitope tags inserted by CRISPR/Cas9 genome editing. Using 3D averaging, we determined the position of all known components within synapsed chromosome axes to high spatial precision in three dimensions. We find that meiosis-specific HORMA domain proteins span a gap between cohesin complexes and the central region of the SC, consistent with their essential roles in SC assembly. Our data further suggest that the two different meiotic cohesin complexes are distinctly arranged within the axes: Although cohesin complexes containing the kleisin REC-8 protrude above and below the plane defined by the SC, complexes containing COH-3 or -4 kleisins form a central core, which may physically separate sister chromatids. This organization may help to explain the role of the chromosome axes in promoting interhomolog repair of meiotic double-strand breaks by inhibiting intersister repair.

Coordination of Double Strand Break Repair and Meiotic Progression in Yeast by a Mek1-Ndt80 Negative Feedback Loop

During meiosis, homologous chromosomes are physically connected by crossovers and sister chromatid cohesion. Interhomolog crossovers are generated by the highly regulated repair of programmed double strand breaks (DSBs). The meiosis-specific kinase Mek1 is critical for this regulation. Mek1 downregulates the mitotic recombinase Rad51, indirectly promoting interhomolog strand invasion by the meiosis-specific recombinase Dmc1. Mek1 also promotes the formation of crossovers that are distributed throughout the genome by interference and is the effector kinase for a meiosis-specific checkpoint that delays entry into Meiosis I until DSBs have been repaired. The target of this checkpoint is a meiosis-specific transcription factor, Ndt80, which is necessary to express the polo-like kinase CDC5 and the cyclin CLB1 thereby allowing completion of recombination and meiotic progression. This work shows that Mek1 and Ndt80 negatively feedback on each other such that when DSB levels are high, Ndt80 is inactive due to high levels of Mek1 activity. As DSBs are repaired, chromosomes synapse and Mek1 activity is reduced below a threshold that allows activation of Ndt80. Ndt80 transcription of CDC5 results in degradation of Red1, a meiosis-specific protein required for Mek1 activation, thereby abolishing Mek1 activity completely. Elimination of Mek1 kinase activity allows Rad51-mediated repair of any remaining DSBs. In this way, cells do not enter Meiosis I until recombination is complete and all DSBs are repaired.

Prophase I: Preparing Chromosomes for Segregation in the Developing Oocyte

Formation of an oocyte involves a specialized cell division termed meiosis. In meiotic prophase I (the initial stage of meiosis), chromosomes undergo elaborate events to ensure the proper segregation of their chromosomes into gametes. These events include processes leading to the formation of a crossover that, along with sister chromatid cohesion, forms the physical link between homologous chromosomes. Crossovers are formed as an outcome of recombination. This process initiates with programmed double-strand breaks that are repaired through the use of homologous chromosomes as a repair template. The accurate repair to form crossovers takes place in the context of the synaptonemal complex, a protein complex that links homologous chromosomes in meiotic prophase I. To allow proper execution of meiotic prophase I events, signaling processes connect different steps in recombination and synapsis. The events occurring in meiotic prophase I are a prerequisite for proper chromosome segregation in the meiotic divisions. When these processes go awry, chromosomes missegregate. These meiotic errors are thought to increase with aging and may contribute to the increase in aneuploidy observed in advanced maternal age female oocytes.

Double-strand break repair on sex chromosomes: challenges during male meiotic prophase

During meiotic prophase, DNA double-strand break (DSB) repair-mediated homologous recombination (HR) occurs for exchange of genetic information between homologous chromosomes. Unlike autosomes or female sex chromosomes, human male sex chromosomes X and Y share little homology. Although DSBs are generated throughout male sex chromosomes, homologous recombination does not occur for most regions and DSB repair process is significantly prolonged. As a result, male sex chromosomes are coated with many DNA damage response proteins and form a unique chromatin structure known as the XY body. Interestingly, associated with the prolonged DSB repair, transcription is repressed in the XY body but not in autosomes, a phenomenon known as meiotic sex chromosome inactivation (MSCI), which is critical for male meiosis. Here using mice as model organisms, we briefly summarize recent progress on DSB repair in meiotic prophase and focus on the mechanism and function of DNA damage response in the XY body.

Histone H2AFX Links Meiotic Chromosome Asynapsis to Prophase I Oocyte Loss in Mammals

Chromosome abnormalities are common in the human population, causing germ cell loss at meiotic prophase I and infertility. The mechanisms driving this loss are unknown, but persistent meiotic DNA damage and asynapsis may be triggers. Here we investigate the contribution of these lesions to oocyte elimination in mice with chromosome abnormalities, e.g. Turner syndrome (XO) and translocations. We show that asynapsed chromosomes trigger oocyte elimination at diplonema, which is linked to the presence of phosphorylated H2AFX (γH2AFX). We find that DNA double-strand break (DSB) foci disappear on asynapsed chromosomes during pachynema, excluding persistent DNA damage as a likely cause, and demonstrating the existence in mammalian oocytes of a repair pathway for asynapsis-associated DNA DSBs. Importantly, deletion or point mutation of H2afx restores oocyte numbers in XO females to wild type (XX) levels. Unexpectedly, we find that asynapsed supernumerary chromosomes do not elicit prophase I loss, despite being enriched for γH2AFX and other checkpoint proteins. These results suggest that oocyte loss cannot be explained simply by asynapsis checkpoint models, but is related to the gene content of asynapsed chromosomes. A similar mechanistic basis for oocyte loss may operate in humans with chromosome abnormalities.

Chromosome abnormalities, such as aneuploidies and structural variants (i.e. translocations, inversions), are strikingly common in the human population, causing disorders such as Down syndrome and Turner syndrome. One important consequence of chromosome abnormalities in mammals is errors during meiosis, the specialized cell division that generates sperm and eggs for reproduction. As a result of these meiotic errors, patients with chromosome abnormalities oftentimes suffer from infertility due to loss of developing germ cells. The precise molecular mechanism for germ cell losses and infertility due to chromosome abnormalities is not well understood, but is hypothesized to result from a surveillance mechanism, which has evolved to prevent aneuploidies from developing from abnormal germ cells. In mammals, meiotic surveillance mechanisms have been hypothesized to monitor for unrepaired DNA double-strand breaks (DSB) and/or chromosome pairing/synapsis errors. Here we test these hypotheses using a variety of chromosomally variant mouse models. We find that germ cell loss in female mice with chromosome abnormalities is dependent on phosphorylation of the histone variant H2AFX, an epigenetic mark involved in the transcriptional silencing of asynapsed chromosomes during meiosis. These data inform a silencing-based mechanism of germ cell loss in patients with chromosome abnormalities and for the prophase I surveillance system which safeguards against aneuploidy.

Genomic Complexity Profiling Reveals That HORMAD1 Overexpression Contributes to Homologous Recombination Deficiency in Triple-Negative Breast Cancers

Triple-negative breast cancers (TNBC) are characterized by a wide spectrum of genomic alterations, some of which might be caused by defects in DNA repair processes such as homologous recombination (HR). Despite this understanding, associating particular patterns of genomic instability with response to therapy has been challenging. Here, we show that allelic-imbalanced copy-number aberrations (AiCNA) are more prevalent in TNBCs that respond to platinum-based chemotherapy, thus providing a candidate predictive biomarker for this disease. Furthermore, we show that a high level of AiCNA is linked with elevated expression of a meiosis-associated gene, HORMAD1. Elevated HORMAD1 expression suppresses RAD51-dependent HR and drives the use of alternative forms of DNA repair, the generation of AiCNAs, as well as sensitizing cancer cells to HR-targeting therapies. Our data therefore provide a mechanistic association between HORMAD1 expression, a specific pattern of genomic instability, and an association with response to platinum-based chemotherapy in TNBC.

SIGNIFICANCE

Previous studies have shown correlation between mutational "scars" and sensitivity to platinums extending beyond associations with BRCA1/2 mutation, but do not elucidate the mechanism. Here, a novel allele-specific copy-number characterization of genome instability identifies and functionally validates the inappropriate expression of the meiotic gene HORMAD1 as a driver of HR deficiency in TNBC, acting to induce allelic imbalance and moderate platinum and PARP inhibitor sensitivity with implications for the use of such "scars" and expression of meiotic genes as predictive biomarkers.

TRIP13 promotes error-prone nonhomologous end joining and induces chemoresistance in head and neck cancer

Head and neck cancer (SCCHN) is a common, aggressive, treatment-resistant cancer with a high recurrence rate and mortality, but the mechanism of treatment-resistance remains unclear. Here we describe a mechanism where the AAA-ATPase TRIP13 promotes treatment-resistance. Overexpression of TRIP13 in non-malignant cells results in malignant transformation. High expression of TRIP13 in SCCHN leads to aggressive, treatment-resistant tumors and enhanced repair of DNA damage. Using mass spectrometry, we identify DNA-PKcs complex proteins that mediate non homologous end joining (NHEJ), as TRIP13 binding partners. Using repair-deficient reporter systems, we show that TRIP13 promotes NHEJ, even when homologous recombination is intact. Importantly, overexpression of TRIP13 sensitizes SCCHN to an inhibitor of DNA-PKcs. Thus, this study defines a new mechanism of treatment resistance in SCCHN and underscores the importance of targeting NHEJ to overcome treatment failure in SCCHN and potentially in other cancers that overexpress TRIP13.

Down-regulation of Rad51 activity during meiosis in yeast prevents competition with Dmc1 for repair of double-strand breaks

Interhomolog recombination plays a critical role in promoting proper meiotic chromosome segregation but a mechanistic understanding of this process is far from complete. In vegetative cells, Rad51 is a highly conserved recombinase that exhibits a preference for repairing double strand breaks (DSBs) using sister chromatids, in contrast to the conserved, meiosis-specific recombinase, Dmc1, which preferentially repairs programmed DSBs using homologs. Despite the different preferences for repair templates, both Rad51 and Dmc1 are required for interhomolog recombination during meiosis. This paradox has recently been explained by the finding that Rad51 protein, but not its strand exchange activity, promotes Dmc1 function in budding yeast. Rad51 activity is inhibited in dmc1Δ mutants, where the failure to repair meiotic DSBs triggers the meiotic recombination checkpoint, resulting in prophase arrest. The question remains whether inhibition of Rad51 activity is important during wild-type meiosis, or whether inactivation of Rad51 occurs only as a result of the absence of DMC1 or checkpoint activation. This work shows that strains in which mechanisms that down-regulate Rad51 activity are removed exhibit reduced numbers of interhomolog crossovers and noncrossovers. A hypomorphic mutant, dmc1-T159A, makes less stable presynaptic filaments but is still able to mediate strand exchange and interact with accessory factors. Combining dmc1-T159A with up-regulated Rad51 activity reduces interhomolog recombination and spore viability, while increasing intersister joint molecule formation. These results support the idea that down-regulation of Rad51 activity is important during meiosis to prevent Rad51 from competing with Dmc1 for repair of meiotic DSBs.

Meiotic recombination in mammals: localization and regulation

During meiosis, a programmed induction of DNA double-strand breaks (DSBs) leads to the exchange of genetic material between homologous chromosomes. These exchanges increase genome diversity and are essential for proper chromosome segregation at the first meiotic division. Recent findings have highlighted an unexpected molecular control of the distribution of meiotic DSBs in mammals by a rapidly evolving gene, PR domain-containing 9 (PRDM9), and genome-wide analyses have facilitated the characterization of meiotic DSB sites at unprecedented resolution. In addition, the identification of new players in DSB repair processes has allowed the delineation of recombination pathways that have two major outcomes, crossovers and non-crossovers, which have distinct mechanistic roles and consequences for genome evolution.

Mouse HORMAD1 is a meiosis i checkpoint protein that modulates DNA double- strand break repair during female meiosis

Oocytes in embryonic ovaries enter meiosis I and arrest in the 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 in 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 Embryonic Day 16.5 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.

Dissociation of the X chromosome from the synaptonemal complex in the XY body of the rodent Galea musteloides

The XY body from spermatocytes of the rodent Galea musteloides shows progressive changes of the synaptonemal complex (SC) axes and the X-chromatin during pachynema. There is a gross thickening of the X-axis and the formation of a large X chromosome loop at mid and late pachytene stages. The SC proteins synaptonemal complex protein 3 (SYCP3), synaptonemal complex protein 1, and synaptonemal complex central element protein 3 and the proteins breast cancer 1, MutL homolog 1 (MLH1), and radiation-repair 51 (related to meiotic processes), the cohesin structural maintenance of chromosome 3, the centromeric protein (with CREST antibody), and the silenced chromatin (with phosphorylated (139ph) H2A histone family, member X (γ-H2AX) antibody) were analyzed in this XY body. The thick X-axis, including the interstitial loop, becomes formed by four to six laminae showing a cross-striation with a periodicity of about 20 nm. The whole length of the gross X-axis shows no significant changes during pachynema, but the interstitial chromatin of the X chromosome and the X centromere are included in the large loop, and it becomes separated from the SC. A conventional SC formed by the Y-axis, a central region and a thin lateral element originally corresponding to the X-axis, remains undisturbed up to the end of pachynema. A single MLH1 focus develops either at the distal or the proximal region of the loop end attached to the conventional SC. The chromatin surrounding the thickened axis is labeled with γ-H2AX. It is shown that most of the SYCP3 protein associated with the X chromosome loop is not involved in the SC maintenance, but it is located with the cohesin axis separated from the SC proper.

Rad54 is required for the normal development of male and female germ cells and contributes to the maintainance of their genome integrity after genotoxic stress

Rad54 is an important factor in the homologous recombination pathway of DNA double-strand break repair. However, Rad54 knockout (KO) mice do not exhibit overt phenotypes at adulthood, even when exposed to radiation. In this study, we show that in Rad54 KO mouse the germline is actually altered. Compared with the wild-type (WT) animals, these mice have less premeiotic germ cells. This germ cell loss is found as early as in E11.5 embryos, suggesting an early failure during mutant primordial germ cells development. Both testicular and ovarian KO germ cells exhibited high radiation sensitivity leading to a long-term gametogenesis defect at adulthood. The KO female germline was particularly affected displaying decreased litter size or sterility. Spermatogenesis recovery after irradiation was slower and incomplete in Rad54 KO mice compared with that of WT mice, suggesting that loss of germ stem cell precursors is not fully compensated along the successive rounds of spermatogenesis. Finally, spermatogenesis recovery after postnatal irradiation is in part regulated by glial-cell-line-derived neurotrophic factor (GDNF) in KO but not in irradiated WT mice, suggesting that Sertoli cell GDNF production is stimulated upon substantial germ cell loss only. Our findings suggest that Rad54 has a key function in maintaining genomic integrity of the developing germ cells.

The cohesion protein SOLO associates with SMC1 and is required for synapsis, recombination, homolog bias and cohesion and pairing of centromeres in Drosophila Meiosis

Cohesion between sister chromatids is mediated by cohesin and is essential for proper meiotic segregation of both sister chromatids and homologs. solo encodes a Drosophila meiosis-specific cohesion protein with no apparent sequence homology to cohesins that is required in male meiosis for centromere cohesion, proper orientation of sister centromeres and centromere enrichment of the cohesin subunit SMC1. In this study, we show that solo is involved in multiple aspects of meiosis in female Drosophila. Null mutations in solo caused the following phenotypes: 1) high frequencies of homolog and sister chromatid nondisjunction (NDJ) and sharply reduced frequencies of homolog exchange; 2) reduced transmission of a ring-X chromosome, an indicator of elevated frequencies of sister chromatid exchange (SCE); 3) premature loss of centromere pairing and cohesion during prophase I, as indicated by elevated foci counts of the centromere protein CID; 4) instability of the lateral elements (LE)s and central regions of synaptonemal complexes (SCs), as indicated by fragmented and spotty staining of the chromosome core/LE component SMC1 and the transverse filament protein C(3)G, respectively, at all stages of pachytene. SOLO and SMC1 are both enriched on centromeres throughout prophase I, co-align along the lateral elements of SCs and reciprocally co-immunoprecipitate from ovarian protein extracts. Our studies demonstrate that SOLO is closely associated with meiotic cohesin and required both for enrichment of cohesin on centromeres and stable assembly of cohesin into chromosome cores. These events underlie and are required for stable cohesion of centromeres, synapsis of homologous chromosomes, and a recombination mechanism that suppresses SCE to preferentially generate homolog crossovers (homolog bias). We propose that SOLO is a subunit of a specialized meiotic cohesin complex that mediates both centromeric and axial arm cohesion and promotes homolog bias as a component of chromosome cores.

Sexual reproduction entails an intricate 2-step division called meiosis in which homologous chromosomes and sister chromatids are sequentially segregated to yield gametes (eggs and sperm) with exactly one copy of each chromosome. The Drosophila meiosis protein SOLO is essential for cohesion between sister chromatids. SOLO localizes to centromeres throughout meiosis where it collaborates with the conserved cohesin complex to enable sister centromeres to orient properly – to the same pole during the first division and to opposite poles during the second division. In solo mutants, sister chromatids become disconnected early in meiosis and segregate randomly through both meiotic divisions generating gametes with random (and mostly wrong) numbers of chromosomes. In this study we show that SOLO also localizes to chromosome arms where it is required to construct stable synaptonemal complexes that connect homologs while they recombine. In addition, SOLO is required to prevent crossovers between sister chromatids, as only homolog crossovers are useful for forming the interhomolog connections (chiasmata) needed for homolog segregation. SOLO collaborates with cohesin for these tasks as well. We propose that SOLO is a subunit of a specialized meiotic cohesin complex and a multi-purpose cohesion protein that regulates several meiotic processes needed for proper chromosome segregation.

Differentiation of spermatogonial stem cell-like cells from murine testicular tissue into haploid male germ cells in vitro

In vitro differentiation of spermatogonial stem cells (SSCs) promotes the understanding of the mechanism of spermatogenesis. The purpose of this study was to isolate spermatogonial stem cell-like cells from murine testicular tissue, which then were induced into haploid germ cells by retinoic acid (RA). The spermatogonial stem cell-like cells were purified and enriched by a two-step plating method based on different adherence velocities of SSCs and somatic cells. Cell colonies were present after culture in M1-medium for 3 days. Through alkaline phosphatase, RT-PCR and indirect immunofluorescence cell analysis, cell colonies were shown to be SSCs. Subsequently, cell colonies of SSCs were cultured in M2-medium containing RA for 2 days. Then the cell colonies of SSCs were again cultured in M1-medium for 6-8 days, RT-PCR and indirect immunofluorescence cell analysis were chosen to detect haploid male germ cells. It could be demonstrated that 10(-7) mol l(-1) of RA effectively induced the SSCs into haploid male germ cells in vitro.

Effects of testicular interstitial fluid on the proliferation of the mouse spermatogonial stem cells in vitro

Spermatogenesis is a process in adult male mammals supported by spermatogonial stem cells (SSCs). The cultivation of SSCs has potential value, for example for the treatment of male infertility or spermatogonial transplantation. Testicular interstitial fluid was added to culture medium to a final concentration of 5, 10, 20, 30 or 40%, in order to investigate its effects on proliferation of mouse SSCs in vitro, Alkaline phosphatase (AKP) assay, reverse transcription polymerase chain reaction (RT-PCR) analysis and indirect immunofluorescence of cells were performed to identify SSCs, and the proliferation rate and diameters of the SSCs colonies were measured. The results showed that the optimal addition of testicular interstitial fluid to culture medium was 30%. When medium supplemented with 30% testicular interstitial fluid was used to culture mouse SSCs, the optimum proliferation rate and diameter of the cell colonies were 72.53% and 249 μm, respectively, after 8 days in culture, values that were significant higher than those found for other groups (P < 0.05). In conclusion, proliferation of mouse SSCs could be promoted significantly by supplementation of the culture medium with 30% testicular interstitial fluid. More research is needed to evaluate and understand the precise physiological role of testicular interstitial fluid during cultivation of SSCs.

Mitosis is a source of potential markers for screening and survival and therapeutic targets in cervical cancer

The effect of preventive human papillomavirus (HPV) vaccination on the reduction of the cervical cancer (CC) burden will not be known for 30 years. Therefore, it's still necessary to improve the procedures for CC screening and treatment. The objective of this study was to identify and characterize cellular targets that could be considered potential markers for screening or therapeutic targets. A pyramidal strategy was used. Initially the expression of 8,638 genes was compared between 43 HPV16-positive CCs and 12 healthy cervical epitheliums using microarrays. A total of 997 genes were deregulated, and 21 genes that showed the greatest deregulation were validated using qRT-PCR. The 6 most upregulated genes (CCNB2, CDC20, PRC1, SYCP2, NUSAP1, CDKN3) belong to the mitosis pathway. They were further explored in 29 low-grade cervical intraepithelial neoplasias (CIN1) and 21 high-grade CIN (CIN2/3) to investigate whether they could differentiate CC and CIN2/3 (CIN2+) from CIN1 and controls. CCNB2, PRC1, and SYCP2 were mostly associated with CC and CDC20, NUSAP1, and CDKN3 were also associated with CIN2/3. The sensitivity and specificity of CDKN3 and NUSAP1 to detect CIN2+ was approximately 90%. The proteins encoded by all 6 genes were shown upregulated in CC by immunohistochemistry. The association of these markers with survival was investigated in 42 CC patients followed up for at least 42 months. Only CDKN3 was associated with poor survival and it was independent from clinical stage (HR = 5.9, 95%CI = 1.4-23.8, p = 0.01). CDKN3 and NUSAP1 may be potential targets for the development of screening methods. Nevertheless, further studies with larger samples are needed to define the optimal sensitivity and specificity. Inhibition of mitosis is a well-known strategy to combat cancers. Therefore, CDKN3 may be not only a screening and survival marker but a potential therapeutic target in CC. However, whether it's indispensable for tumor growth remains to be demonstrated.

Multiple barriers to nonhomologous DNA end joining during meiosis in Drosophila

Repair of meiotic double-strand breaks (DSBs) uses the homolog and recombination to yield crossovers while alternative pathways such as nonhomologous end joining (NHEJ) are suppressed. Our results indicate that NHEJ is blocked at two steps of DSB repair during meiotic prophase: first by the activity of the MCM-like protein MEI-218, which is required for crossover formation, and, second, by Rad51-related proteins SPN-B (XRCC3) and SPN-D (RAD51C), which physically interact and promote homologous recombination (HR). We further show that the MCM-like proteins also promote the activity of the DSB repair checkpoint pathway, indicating an early requirement for these proteins in DSB processing. We propose that when a meiotic DSB is formed in the absence of both MEI-218 and SPN-B or SPN-D, a DSB substrate is generated that can enter the NHEJ repair pathway. Indeed, due to its high error rate, multiple barriers may have evolved to prevent NHEJ activity during meiosis.

The spatial regulation of meiotic recombination hotspots: are all DSB hotspots crossover hotspots?

A key step for the success of meiosis is programmed homologous recombination, during which crossovers, or exchange of chromosome arms, take place. Crossovers increase genetic diversity but their main function is to ensure accurate chromosome segregation. Defects in crossover number and position produce aneuploidies that represent the main cause of miscarriages and chromosomal abnormalities such as Down's syndrome. Recombination is initiated by the formation of programmed double strand breaks (DSBs), which occur preferentially at places called DSB hotspots. Among all DSBs generated, only a small fraction is repaired by crossover, the other being repaired by other homologous recombination pathways. Crossover maps have been generated in a number of organisms, defining crossover hotspots. With the availability of genome-wide maps of DSBs as well as the ability to measure genetically the repair outcome at several hotspots, it is becoming more and more clear that not all DSB hotspots behave the same for crossover formation, suggesting that chromosomal features distinguish different types of hotspots.

Meiotic genetics moves forward with SPATA22 (repro42)

This commentary provides a summary of existing meiotic mutants affecting the synaptonemal complex and meiotic recombination in order to contextualize the recent discovery of SPATA22/repro42 through ENU mutagenesis.

Phosphorylation of chromosome core components may serve as axis marks for the status of chromosomal events during mammalian meiosis

Meiotic recombination and chromosome synapsis between homologous chromosomes are essential for proper chromosome segregation at the first meiotic division. While recombination and synapsis, as well as checkpoints that monitor these two events, take place in the context of a prophase I-specific axial chromosome structure, it remains unclear how chromosome axis components contribute to these processes. We show here that many protein components of the meiotic chromosome axis, including SYCP2, SYCP3, HORMAD1, HORMAD2, SMC3, STAG3, and REC8, become post-translationally modified by phosphorylation during the prophase I stage. We found that HORMAD1 and SMC3 are phosphorylated at a consensus site for the ATM/ATR checkpoint kinase and that the phosphorylated forms of HORMAD1 and SMC3 localize preferentially to unsynapsed chromosomal regions where synapsis has not yet occurred, but not to synapsed or desynapsed regions. We investigated the genetic requirements for the phosphorylation events and revealed that the phosphorylation levels of HORMAD1, HORMAD2, and SMC3 are dramatically reduced in the absence of initiation of meiotic recombination, whereas BRCA1 and SYCP3 are required for normal levels of phosphorylation of HORMAD1 and HORMAD2, but not of SMC3. Interestingly, reduced HORMAD1 and HORMAD2 phosphorylation is associated with impaired targeting of the MSUC (meiotic silencing of unsynapsed chromatin) machinery to unsynapsed chromosomes, suggesting that these post-translational events contribute to the regulation of the synapsis surveillance system. We propose that modifications of chromosome axis components serve as signals that facilitate chromosomal events including recombination, checkpoint control, transcription, and synapsis regulation.

Meiosis in mice without a synaptonemal complex

The synaptonemal complex (SC) promotes fusion of the homologous chromosomes (synapsis) and crossover recombination events during meiosis. The SC displays an extensive structural conservation between species; however, a few organisms lack SC and execute meiotic process in a SC-independent manner. To clarify the SC function in mammals, we have generated a mutant mouse strain (Sycp1(-/-)Sycp3(-/-), here called SC-null) in which all known SC proteins have been displaced from meiotic chromosomes. While transmission electron microscopy failed to identify any remnants of the SC in SC-null spermatocytes, neither formation of the cohesion axes nor attachment of the chromosomes to the nuclear membrane was perturbed. Furthermore, the meiotic chromosomes in SC-null meiocytes achieved pre-synaptic pairing, underwent early homologous recombination events and sustained a residual crossover formation. In contrast, in SC-null meiocytes synapsis and MLH1-MLH3-dependent crossovers maturation were abolished, whereas the structural integrity of chromosomes was drastically impaired. The variable consequences that SC inactivation has on the meiotic process in different organisms, together with the absence of SC in some unrelated species, imply that the SC could have originated independently in different taxonomic groups.