Atm-dependent interactions of a mammalian chk1 homolog with meiotic chromosomes

Combined Aurora Kinase A and CHK1 Inhibition Enhances Radiosensitivity of Triple-Negative Breast Cancer Through Induction of Apoptosis and Mitotic Catastrophe Associated With Excessive DNA Damage

PURPOSE

There is an urgent need for biomarkers and new actionable targets to improve radiosensitivity of triple-negative breast cancer (TNBC) tumors. We characterized the radiosensitizing effects and underlying mechanisms of combined Aurora kinase A (AURKA) and CHK1 inhibition in TNBC.

METHODS AND MATERIALS

Different TNBC cell lines were treated with AURKA inhibitor (AURKAi, MLN8237) and CHK1 inhibitor (CHK1i, MK8776). Cell responses to irradiation (IR) were then evaluated. Cell apoptosis, DNA damage, cell cycle distribution, and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and Phosphoinositide 3-Kinase (PI3K) pathways were evaluated in vitro. Transcriptomic analysis was performed to facilitate the identification of potential biomarkers. Xenograft and immunohistochemistry were carried out to investigate the radiosensitizing effects of dual inhibition in vivo. Finally, the prognostic effect of CHEK1/AURKA in TNBC samples in the The Cancer Genome Atlas (TCGA) database and our center were analyzed.

RESULTS

AURKAi (MLN8237) induced overexpression of phospho-CHK1 in TNBC cells. The addition of MK8776 (CHK1i) to MLN8237 greatly reduced cell viability and increased radiosensitivity compared with either the control or MLN8237 alone in vitro. Mechanistically, dual inhibition resulted in inducing excessive DNA damage by prompting G2/M transition to cells with defective spindles, leading to mitotic catastrophe and induction of apoptosis after IR. We also observed that dual inhibition suppressed the phosphorylation of ERK, while activation of ERK with its agonist or overexpression of active ERK1/2 allele could attenuate the apoptosis induced by dual inhibition with IR. Additionally, dual inhibition of AURKA and CHK1 synergistically enhanced radiosensitivity in MDA-MB-231 xenografts. Moreover, we detected that both CHEK1 and AURKA were overexpressed in patients with TNBC and negatively correlated with patient survival.

CONCLUSIONS

Our findings suggested that AURKAi in combination with CHK1i enhanced TNBC radiosensitivity in preclinical models, potentially providing a novel strategy of precision treatment for patients with TNBC.

Checkpoint Kinase 1 (CHK1) Functions as Both a Diagnostic Marker and a Regulator of Epithelial-to-Mesenchymal Transition (EMT) in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is more difficult to treat and has a higher mortality rate than other subtypes. Although hormone receptor-targeted therapy is an effective treatment to increase survival rate in breast cancer patients, it is not suitable for TNBC patients. To address the issues, differentially expressed genes (DEGs) in TNBC patients from the Gene Expression Omnibus (GEO) database were analyzed. A total of 170 genes were obtained from three Genomic Spatial Events (GSEs) using the intersection of each GSE dataset and 61 DEGs were identified after validation with the gene enrichment analysis. We combined this with the degree scores from the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and protein-protein interaction (PPI) network, of which 7 genes were correlated with survival rate. Finally, a proteomics database revealed that only the CHK1 protein level was differently expressed in basal-like compared with other subtypes. We demonstrated that CHK1 expression was higher in TNBC cell lines compared with non-TNBC cell lines, and CHK1 promotes epithelial to mesenchymal transition (EMT) as well as migration and invasion ability. Our study provides new insight into the TNBC subnetwork that may be useful in the prognosis and treatment of TNBC patients.

Mitotic and DNA Damage Response Proteins: Maintaining the Genome Stability and Working for the Common Good

Cellular function is highly dependent on genomic stability, which is mainly ensured by two cellular mechanisms: the DNA damage response (DDR) and the Spindle Assembly Checkpoint (SAC). The former provides the repair of damaged DNA, and the latter ensures correct chromosome segregation. This review focuses on recently emerging data indicating that the SAC and the DDR proteins function together throughout the cell cycle, suggesting crosstalk between both checkpoints to maintain genome stability.

Investigating Molecular Determinants of Cancer Cell Resistance to Ionizing Radiation Through an Integrative Bioinformatics Approach

Eradication of cancer cells through exposure to high doses of ionizing radiation (IR) is a widely used therapeutic strategy in the clinical setting. However, in many cases, cancer cells can develop remarkable resistance to radiation. Radioresistance represents a prominent obstacle in the effective treatment of cancer. Therefore, elucidation of the molecular mechanisms and pathways related to radioresistance in cancer cells is of paramount importance. In the present study, an integrative bioinformatics approach was applied to three publicly available RNA sequencing and microarray transcriptome datasets of human cancer cells of different tissue origins treated with ionizing radiation. These data were investigated in order to identify genes with a significantly altered expression between radioresistant and corresponding radiosensitive cancer cells. Through rigorous statistical and biological analyses, 36 genes were identified as potential biomarkers of radioresistance. These genes, which are primarily implicated in DNA damage repair, oxidative stress, cell pro-survival, and apoptotic pathways, could serve as potential diagnostic/prognostic markers cancer cell resistance to radiation treatment, as well as for therapy outcome and cancer patient survival. In addition, our findings could be potentially utilized in the laboratory and clinical setting for enhancing cancer cell susceptibility to radiation therapy protocols.

Chk1 promotes non-homologous end joining in G1 through direct phosphorylation of ASF1A

The cell-cycle phase is a major determinant of repair pathway choice at DNA double strand breaks, non-homologous end joining (NHEJ), or homologous recombination (HR). Chk1 responds to genotoxic stress in S/G2 phase, but here, we report a role of Chk1 in directly promoting NHEJ repair in G1 phase. ASF1A is a histone chaperone, but it promotes NHEJ through a pathway independent of its histone-chaperone activity. Chk1 activated by ataxia telangiectasia mutated (ATM) kinase on DNA breaks in G1 promotes NHEJ through direct phosphorylation of ASF1A at Ser-166. ASF1A phosphorylated at Ser-166 interacts with the repair protein MDC1 and thus enhances MDC1's interaction with ATM and the stable localization of ATM at DNA breaks. Chk1 deficiency suppresses all steps downstream of MDC1 following a DNA break in G1, namely histone ubiquitination, 53BP1 localization to the DNA break, and NHEJ. Thus, ASF1A phosphorylation by Chk1 is essential for DNA break repair by NHEJ in G1.

Oocyte Elimination Through DNA Damage Signaling from CHK1/CHK2 to p53 and p63

Eukaryotic organisms have evolved mechanisms to prevent the accumulation of cells bearing genetic aberrations. This is especially crucial for the germline, because fecundity and fitness of progeny would be adversely affected by an excessively high mutational incidence. The process of meiosis poses unique problems for mutation avoidance because of the requirement for SPO11-induced programmed double-strand breaks (DSBs) in recombination-driven pairing and segregation of homologous chromosomes. Mouse meiocytes bearing unrepaired meiotic DSBs or unsynapsed chromosomes are eliminated before completing meiotic prophase I. In previous work, we showed that checkpoint kinase 2 (CHK2; CHEK2), a canonical DNA damage response protein, is crucial for eliminating not only oocytes defective in meiotic DSB repair (e.g., Trip13Gt mutants), but also Spo11-/- oocytes that are defective in homologous chromosome synapsis and accumulate a threshold level of spontaneous DSBs. However, rescue of such oocytes by Chk2 deficiency was incomplete, raising the possibility that a parallel checkpoint pathway(s) exists. Here, we show that mouse oocytes lacking both p53 (TRP53) and the oocyte-exclusive isoform of p63, TAp63, protects nearly all Spo11-/- and Trip13Gt/Gt oocytes from elimination. We present evidence that checkpoint kinase I (CHK1; CHEK1), which is known to signal to TRP53, also becomes activated by persistent DSBs in oocytes, and to an increased degree when CHK2 is absent. The combined data indicate that nearly all oocytes reaching a threshold level of unrepaired DSBs are eliminated by a semiredundant pathway of CHK1/CHK2 signaling to TRP53/TAp63.

ATR-CHK1 pathway as a therapeutic target for acute and chronic leukemias

Progress in cancer therapy changed the outcome of many patients and moved therapy from chemotherapy agents to targeted drugs. Targeted drugs already changed the clinical practice in treatment of leukemias, such as imatinib (BCR/ABL inhibitor) in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL), ibrutinib (Bruton's tyrosine kinase inhibitor) in chronic lymphocytic leukemia (CLL), venetoclax (BCL2 inhibitor) in CLL and acute myeloid leukemia (AML) or midostaurin (FLT3 inhibitor) in AML. In this review, we focused on DNA damage response (DDR) inhibition, specifically on inhibition of ATR-CHK1 pathway. Cancer cells harbor often defects in different DDR pathways, which render them vulnerable to DDR inhibition. Some DDR inhibitors showed interesting single-agent activity even in the absence of cytotoxic drug especially in cancers with underlying defects in DDR or DNA replication. Almost no mutations were found in ATR and CHEK1 genes in leukemia patients. Together with the fact that ATR-CHK1 pathway is essential for cell development and survival of leukemia cells, it represents a promising therapeutic target for treatment of leukemia. ATR-CHK1 inhibition showed excellent results in preclinical testing in acute and chronic leukemias. However, results in clinical trials are so far insufficient. Therefore, the ongoing and future clinical trials will decide on the success of ATR/CHK1 inhibitors in clinical practice of leukemia treatment.

From yeast to humans: Understanding the biology of DNA Damage Response (DDR) kinases

The DNA Damage Response (DDR) is a complex network of biological processes that protect cells from accumulating aberrant DNA structures, thereby maintaining genomic stability and, as a consequence, preventing the development of cancer and other diseases. The DDR pathway is coordinated by a signaling cascade mediated by the PI3K-like kinases (PIKK) ATM and ATR and by their downstream kinases CHK2 and CHK1, respectively. Together, these kinases regulate several aspects of the cellular program in response to genomic stress. Much of our understanding of these kinases came from studies performed in the 1990s using yeast as a model organism. The purpose of this review is to present a historical perspective on the discovery of the DDR kinases in yeast and the importance of this model for the identification and functional understanding of their mammalian orthologues.

CHEK1 coordinates DNA damage signaling and meiotic progression in the male germline of mice

The continuity of life depends on mechanisms in the germline that ensure the integrity of the genome. The DNA damage response/checkpoint kinases ATM and ATR are essential signaling factors in the germline. However, it remains unknown how a downstream transducer, Checkpoint Kinase 1 (CHEK1 or CHK1), mediates signaling in the male germline. Here, we show that CHEK1 has distinct functions in both the mitotic and meiotic phases of the male germline in mice. In the mitotic phase, CHEK1 is required for the resumption of prospermatogonia proliferation after birth and the maintenance of spermatogonia. In the meiotic phase, we uncovered two functions for CHEK1: one is the stage-specific attenuation of DNA damage signaling on autosomes, and the other is coordination of meiotic stage progression. On autosomes, the loss of CHEK1 delays the removal of DNA damage signaling that manifests as phosphorylation of histone variant H2AX at serine 139 (γH2AX). Importantly, CHEK1 does not have a direct function in meiotic sex chromosome inactivation (MSCI), an essential event in male meiosis, in which ATR is a key regulator. Thus, the functions of ATR and CHEK1 are uncoupled in MSCI, in contrast to their roles in DNA damage signaling in somatic cells. Our study reveals stage-specific functions for CHEK1 that ensure the integrity of the male germline.

Key mediators of somatic ATR signaling localize to unpaired chromosomes in spermatocytes

Meiotic silencing of unpaired chromatin (MSUC) occurs during the first meiotic prophase, as chromosomes that fail to pair are sequestered into a transcriptionally repressive nuclear domain. This phenomenon is exemplified by the heterologous sex chromosomes of male mammals, where the ATR DNA damage response kinase is crucial for this silencing event. However, the mechanisms underlying the initiation of MSUC remain unknown. Here, we show that essential components of ATR signaling in murine somatic cells are spatially confined to unpaired chromosomes in spermatocytes, including the ATR-dependent phosphorylation of the single-stranded DNA (ssDNA)-binding complex replication protein A (RPA) and the checkpoint kinase CHK1. These observations support a model in which ssDNA plays a central role in the recruitment of ATR during MSUC, and provide a link to meiotic progression through activation of CHK1.

Exploiting replicative stress to treat cancer

DNA replication in cancer cells is accompanied by stalling and collapse of the replication fork and signalling in response to DNA damage and/or premature mitosis; these processes are collectively known as 'replicative stress'. Progress is being made to increase our understanding of the mechanisms that govern replicative stress, thus providing ample opportunities to enhance replicative stress for therapeutic purposes. Rather than trying to halt cell cycle progression, cancer therapeutics could aim to increase replicative stress by further loosening the checkpoints that remain available to cancer cells and ultimately inducing the catastrophic failure of proliferative machineries. In this Review, we outline current and future approaches to achieve this, emphasizing the combination of conventional chemotherapy with targeted approaches.

DNA damage: a sensible mediator of the differentiation decision in hematopoietic stem cells and in leukemia

In the adult, the source of functionally diverse, mature blood cells are hematopoietic stem cells, a rare population of quiescent cells that reside in the bone marrow niche. Like stem cells in other tissues, hematopoietic stem cells are defined by their ability to self-renew, in order to maintain the stem cell population for the lifetime of the organism, and to differentiate, in order to give rise to the multiple lineages of the hematopoietic system. In recent years, increasing evidence has suggested a role for the accumulation of reactive oxygen species and DNA damage in the decision for hematopoietic stem cells to exit quiescence and to differentiate. In this review, we will examine recent work supporting the idea that detection of cell stressors, such as oxidative and genetic damage, is an important mediator of cell fate decisions in hematopoietic stem cells. We will explore the benefits of such a system in avoiding the development and progression of malignancies, and in avoiding tissue exhaustion and failure. Additionally, we will discuss new work that examines the accumulation of DNA damage and replication stress in aging hematopoietic stem cells and causes us to rethink ideas of genoprotection in the bone marrow niche.

The DNA damage checkpoint protein RAD9A is essential for male meiosis in the mouse

In mitotic cells, RAD9A functions in repairing DNA double-strand breaks (DSBs) by homologous recombination and facilitates the process by cell cycle checkpoint control in response to DNA damage. DSBs occur naturally in the germline during meiosis but whether RAD9A participates in repairing such breaks is not known. In this study, we determined that RAD9A is indeed expressed in the male germ line with a peak of expression in late pachytene and diplotene stages, and the protein was found associated with the XY body. As complete loss of RAD9A is embryonic lethal, we constructed and characterized a mouse strain with Stra8-Cre driven germ cell-specific ablation of Rad9a beginning in undifferentiated spermatogonia in order to assess its role in spermatogenesis. Adult mutant male mice were infertile or sub-fertile due to massive loss of spermatogenic cells. The onset of this loss occurs during meiotic prophase, and there was an increase in the numbers of apoptotic spermatocytes as determined by TUNEL. Spermatocytes lacking RAD9A usually arrested in meiotic prophase, specifically in pachytene. The incidence of unrepaired DNA breaks increased, as detected by accumulation of γH2AX and DMC1 foci on the axes of autosomal chromosomes in pachytene spermatocytes. The DNA topoisomerase IIβ-binding protein 1 (TOPBP1) was still localized to the sex body, albeit with lower intensity, suggesting that RAD9A may be dispensable for sex body formation. We therefore show for the first time that RAD9A is essential for male fertility and for repair of DNA DSBs during meiotic prophase I.

Roles of Chk1 in cell biology and cancer therapy

The evolutionally conserved DNA damage response (DDR) and cell cycle checkpoints preserve genome integrity. Central to these genome surveillance pathways is a protein kinase, Chk1. DNA damage induces activation of Chk1, which then transduces the checkpoint signal and facilitates cell cycle arrest and DNA damage repair. Significant progress has been made recently toward our understanding of Chk1 regulation and its implications in cancer etiology and therapy. Specifically, a model that involves both spatiotemporal and conformational changes of proteins has been proposed for Chk1 activation. Further, emerging evidence suggests that Chk1 does not appear to be a tumor suppressor; instead, it promotes tumor growth and may contribute to anticancer therapy resistance. Recent data from our laboratory suggest that activating, but not inhibiting, Chk1 in the absence of chemotherapy might represent an innovative approach to suppress tumor growth. These findings suggest unique regulation of Chk1 in cell biology and cancer etiology, pointing to novel strategies for targeting Chk1 in cancer therapy.

Maintenance of imaginal disc plasticity and regenerative potential in Drosophila by p53

Following irradiation (IR), the DNA damage response (DDR) activates p53, which triggers death of cells in which repair cannot be completed. Lost tissue is then replaced and re-patterned through regeneration. We have examined the role of p53 in co-regulation of the DDR and tissue regeneration following IR damage in Drosophila. We find that after IR, p53 is required for imaginal disc cells to repair DNA, and in its absence the damage marker, γ-H2AX is persistently expressed. p53 is also required for the compensatory proliferation and re-patterning of the damaged discs, and our results indicate that cell death is not required to trigger these processes. We identify an IR-induced delay in developmental patterning in wing discs that accompanies an animal-wide delay of the juvenile-adult transition, and demonstrate that both of these delays require p53. In p53 mutants, the lack of developmental delays and of damage resolution leads to anueploidy and tissue defects, and ultimately to morphological abnormalities and adult inviability. We propose that p53 maintains plasticity of imaginal discs by co-regulating the maintenance of genome integrity and disc regeneration, and coordinating these processes with the physiology of the animal. These findings place p53 in a role as master coordinator of DNA and tissue repair following IR.

Detection of CHK1 and CCND1 gene copy number changes in breast cancer with dual-colour fluorescence in-situ hybridization

AIMS

To investigate the correlation between CCND1 amplification and CHK1 deletion in breast cancer, and to explore their role in tumorigenesis and progression, a comparative study of the gene copy number changes of CCND1 and CHK1 was performed with dual-colour fluorescence in-situ hybridization (FISH).

METHODS AND RESULTS

Sixty-one infiltrating ductal breast carcinomas with foci of ductal carcinoma in situ (DCIS) components were selected for dual-colour FISH. A strong correlation was found between CCND1 amplification and CHK1 deletion (P<0.0001). Fourteen cases were detected that demonstrated both CCND1 amplification and CHK1 deletion. Interestingly, when comparing the infiltrating and non-invasive areas for the same tumour, we found three cases with CCND1 amplification in the infiltrating areas but not in the DCIS areas. We did not find a CHK1 gene profile difference between infiltrating and DCIS areas in the same lesions.

CONCLUSIONS

Our findings suggest that CCND1 amplification and CHK1 deletion are common events in breast cancer, and that the two genetic alterations often coexist. Our data also suggest that CHK1 deletion is an early genetic event in the development of breast cancer and can be detected at the DCIS stage, whereas CCND1 amplification is more likely to be associated with tumour progression.

Cul4A is essential for spermatogenesis and male fertility

The mammalian Cul4 genes, Cul4A and Cul4B, encode the scaffold components of the cullin-based E3 ubiquitin ligases. The two Cul4 genes are functionally redundant. Recent study indicated that mice expressing a truncated CUL4A that fails to interact with its functional partner ROC1 exhibit no developmental phenotype. We generated a Cul4A-/- strain lacking exons 4-8 that does not express any detectable truncated protein. In this strain, the male mice are infertile and exhibit severe deficiencies in spermatogenesis. The primary spermatocytes are deficient in progression through late prophase I, a time point when expression of the X-linked Cul4B gene is silenced due to meiotic sex chromosome inactivation. Testes of the Cul4A-/- mice exhibit extensive apoptosis. Interestingly, the pachytene spermatocytes exhibit persistent double stranded breaks, suggesting a deficiency in homologous recombination. Also, we find that CUL4A localizes to the double stranded breaks generated in pre-pachytene spermatocytes. The observations identify a novel function of CUL4A in meiotic recombination and demonstrate an essential role of CUL4A in spermatogenesis.

Chromatin remodeling finds its place in the DNA double-strand break response

The accurate repair of chromosomal double-strand breaks (DSBs) arising from exposure to exogenous agents, such as ionizing radiation (IR) and radiomimetic drugs is crucial in maintaining genomic integrity, cellular viability and the prevention of tumorigenesis. Eukaryotic cells have evolved efficient mechanisms that sense and respond to DSBs. The DNA DSB response is facilitated by hierarchical signaling networks that orchestrate chromatin structural changes, cell-cycle checkpoints and multiple enzymatic activities to repair the broken DNA ends. Sensors and transducers signal to numerous downstream cellular effectors which function primarily by substrate posttranslational modifications including phosphorylation, acetylation, methylation and ubiquitylation. In particular, the past several years have provided important insight into the role of chromatin remodeling and histones-specific modifications to control DNA damage detection, signaling and repair. This review summarizes recently identified factors that influence this complex process and the repair of DNA DSBs in eukaryotic cells.

Gene products of chromosome 11q and their association with CCND1 gene amplification and tamoxifen resistance in premenopausal breast cancer

INTRODUCTION

The amplification event occurring at chromosome locus 11q13, reported in several different cancers, includes a number of potential oncogenes. We have previously reported amplification of one such oncogene, namely CCND1, to be correlated with an adverse effect of tamoxifen in premenopausal breast cancer patients. Over-expression of cyclin D1 protein, however, confers tamoxifen resistance but not a tamoxifen-induced adverse effect. Potentially, co-amplification of an additional 11q13 gene, with a resulting protein over-expression, is required to cause an agonistic effect. Moreover, during 11q13 amplification a deletion of the distal 11q region has been described. In order to assess the potential impact of the deletion we examined a selected marker for this event.

METHOD

Array comparative genomic hybridization analysis was employed to identify and confirm changes in the gene expression of a number of different genes mapping to the 11q chromosomal region, associated with CCND1 amplification. The subsequent protein expression of these candidate genes was then examined in a clinical material of 500 primary breast cancers from premenopausal patients who were randomly assigned to either tamoxifen or no adjuvant treatment. The protein expression was also compared with gene expression data in a subset of 56 breast cancer samples.

RESULTS

Cortactin and FADD (Fas-associated death domain) over-expression was linked to CCND1 amplification, determined by fluorescence in situ hybridization, but was not associated with a diminished effect of tamoxifen. However, deletion of distal chromosome 11q, defined as downregulation of the marker Chk1 (checkpoint kinase 1), was associated with an impaired tamoxifen response, and interestingly with low proliferative breast cancer of low grade. For Pak1 (p21-activated kinase 1) and cyclin D1 the protein expression corresponded to the gene expression data.

CONCLUSIONS

The results indicate that many 11q13 associated gene products are over-expressed in conjunction with cyclin D1 but not linked to an agonistic effect of tamoxifen. Finally, the deletion of distal 11q, linked to 11q13 amplification, might be an important event affecting breast cancer outcome and tamoxifen response.

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.

The cellular control of DNA double-strand breaks

DNA double-strand breaks (DSBs) are the most hazardous lesions arising in the genome of eukaryotic organisms, and yet occur normally during DNA replication, meiosis, and immune system development. The efficient repair of DSBs is crucial in maintaining genomic integrity, cellular viability, and the prevention of tumorigenesis. As a consequence, eukaryotic cells have evolved efficient mechanisms that sense and respond to DSBs and ultimately repair the break. The swiftness of the DNA DSB response has paved to the identification of sensors and transducers which allowed to generate a hierarchical signaling paradigm depicting the transduction of the damage signal to numerous downstream effectors (Fig. 1). The function of such effectors involve posttranslational modifications through phosphorylation, acetylation, and methylation of the substrates. This review will address the control of DSBs in damaged eukaryotic cells, the physiological processes that require the introduction of a DSB into the genome, and the maintenance of DSBs in non-damaged cells.

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.

The role of NBS1 in the modulation of PIKK family proteins ATM and ATR in the cellular response to DNA damage

Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases have been considered the primary activators of the cellular response to DNA damage. They belong to the protein kinase family, phosphoinositide 3-kinase-related kinase (PIKKs). In human beings, deficiency of these kinases leads to hereditary diseases, namely ataxia telangiectasia (AT) with ATM deficiency and ATR-Seckel with ATR deficiency. NBS1, a component of MRE11/RAD50/NBS1 (MRN) complex, is another important player in DNA damage response (DDR). Mutations of NBS1 are responsible for Nijmegen breakage syndrome (NBS), a human hereditary disease with the characteristics that almost encompassed those of AT and ATR-Seckel. NBS1 has been conventionally thought to be a downstream substrate of ATM and ATR in DDR; however, recent studies suggest that NBS1/MRN functions upstream of both ATM and ATR by recruiting them to the proximity of DNA damage sites and activating their functions. In this mini-review, we would emphasize the requirement of NBS1 as an upstream mediator for the modulation of PIKK family proteins ATM and ATR.

Grp/DChk1 is required for G2-M checkpoint activation in Drosophila S2 cells, whereas Dmnk/DChk2 is dispensable

Cell-cycle checkpoints are signal-transduction pathways required to maintain genomic stability in dividing cells. Previously, it was reported that two kinases essential for checkpoint signalling, Chk1 and Chk2 are structurally conserved. In contrast to yeast, Xenopus and mammals, the Chk1- and Chk2-dependent pathways in Drosophila are not understood in detail. Here, we report the function of these checkpoint kinases, referred to as Grp/DChk1 and Dmnk/DChk2 in Drosophila Schneider's cells, and identify an upstream regulator as well as downstream targets of Grp/DChk1. First, we demonstrate that S2 cells are a suitable model for G(2)/M checkpoint studies. S2 cells display Grp/DChk1-dependent and Dmnk/DChk2-independent cell-cycle-checkpoint activation in response to hydroxyurea and ionizing radiation. S2 cells depleted for Grp/DChk1 using RNA interference enter mitosis in the presence of impaired DNA integrity, resulting in prolonged mitosis and mitotic catastrophe. Grp/DChk1 is phosphorylated in a Mei-41/DATR-dependent manner in response to hydroxyurea and ionizing radiation, indicating that Mei-41/ATR is an upstream component in the Grp/DChk1 DNA replication and DNA-damage-response pathways. The level of Cdc25(Stg) and phosphorylation status of Cdc2 are modulated in a Grp/DChk1-dependent manner in response to hydroxyurea and irradiation, indicating that these cell-cycle regulators are downstream targets of the Grp/DChk1-dependent DNA replication and DNA-damage responses. By contrast, depletion of Dmnk/DChk2 by RNA interference had little effect on checkpoint responses to hydroxyurea and irradiation. We conclude that Grp/DChk1, and not Dmnk/DChk2, is the main effector kinase involved in G(2)/M checkpoint control in Drosophila cells.

Mouse ovary developmental RNA and protein markers from gene expression profiling

To identify genes involved in morphogenetic events during mouse ovary development, we started with microarray analyses of whole organ RNA. Transcripts for 60% of the 15,000 gene NIA panel were detected, and about 2000 were differentially expressed in nascent newborn compared to adult ovary. Highly differentially expressed transcripts included noncoding RNAs and newly detected genes involved in transcription regulation and signal transduction. The phased pattern of newborn mouse ovary differentiation allowed us to (1) extend information on activity and stage specificity of cell type-specific genes; and (2) generate a list of candidate genes involved in primordial follicle formation, including podocalyxin (Podxl), PDGFR-beta, and a follistatin-domain-encoding gene Flst1. Oocyte-specific transcripts included many (e.g., Deltex2, Bicd2, and Zfp37) enriched in growing oocytes, as well as a novel family of untranslated RNA's (RLTR10) that is selectively expressed in early stage follicles. The results indicate that global expression profiling of whole organ RNA provides sensitive first-line information about ovarian histogenesis for which no in vitro cell models are currently available.

Checkpoint kinase 1 (CHK1) protein and mRNA expression is downregulated in aggressive variants of human lymphoid neoplasms

CHK1: gene encodes for a serine/threonine kinase involved in the regulation of cell cycle progression and DNA damage checkpoints. To determine the role of CHK1 in the pathogenesis of lymphoid neoplasms and its relationship to other DNA damage response genes, we have analyzed the gene status, protein, and mRNA expression in a series of tumors and nonneoplastic lymphoid tissues. CHK1 protein and mRNA expression levels were very low in both reactive tissues and resting lymphoid cells, whereas tumor samples showed a variable pattern of expression related to their proliferative activity. However, seven aggressive tumors showed a dissociate pattern of extremely low or negative protein expression in spite of a high proliferative activity. Four of these tumors were diffuse large B-cell lymphomas (DLCLs) with concordant reduced levels of mRNA, whereas one blastoid mantle cell lymphoma (B-MCL) and two DLCLs had relatively normal levels of mRNA. No gene mutations, deletions, or hypermethylation of the promoter region were detected in any of these cases. In all these tumors ATM, CHK2, and p53 genes were wild type. These findings suggest that CHK1 inactivation in NHLs occurs by loss of protein expression in a subset of aggressive variants alternatively to ATM, CHK2, and p53 alterations.

The role of the DNA double-strand break response network in meiosis

Organisms with sexual reproduction have two homologous copies of each chromosome. Meiosis is characterized by two successive cell divisions that result in four haploid sperms or eggs, each carrying a single copy of homologous chromosome. This process requires a coordinated reorganization of chromatin and a complex network of meiotic-specific signaling cascades. At the beginning of meiosis, each chromosome must recognize its homolog, then the two become intimately aligned along their entire lengths which allows the exchange of DNA strands between homologous sequences to generate genetic diversity. DNA double-strand breaks (DSBs) initiate meiotic recombination in a variety of organisms. Numerous studies have identified both the genomic loci of the initiating DSBs and the proteins involved in their formation. This review will summarize the activation and signaling networks required for the DSB response in meiosis.

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

Mammalian TopBP1 is a BRCT domain-containing protein whose function in mitotic cells is linked to replication and DNA damage checkpoint. Here, we study its possible role during meiosis in mice. TopBP1 foci are abundant during early prophase I and localize mainly to histone gamma-H2AX-positive domains, where DNA double-strand breaks (required to initiate recombination) occur. Strikingly, TopBP1 showed a pattern almost identical to that of ATR, a PI3K-like kinase involved in mitotic DNA damage checkpoint. In the synapsis-defective Fkbp6(-/-) mouse, TopBP1 heavily stains unsynapsed regions of chromosomes. We also tested whether Schizosaccharomyces pombe Cut5 (the TopBP1 homologue) plays a role in the meiotic recombination checkpoint, like spRad3, the ATR homologue. Indeed, we found that a cut5 mutation suppresses the checkpoint-dependent meiotic delay of a meiotic recombination defective mutant, indicating a direct role of the Cut5 protein in the meiotic checkpoint. Our findings suggest that ATR and TopBP1 monitor meiotic recombination and are required for activation of the meiotic recombination checkpoint.

Ataxia telangiectasia mutated expression and activation in the testis

Ionizing radiation (IR) and consequent induction of DNA double-strand breaks (DSBs) causes activation of the protein ataxia telangiectasia mutated (ATM). Normally, ATM is present as inactive dimers; however, in response to DSBs, the ATM dimer partners cross-phosphorylate each other on serine 1981, and kinase active ATM monomers are subsequently released. We have studied the presence of both nonphosphorylated as well as active serine 1981 phosphorylated ATM (pS1981-ATM) in the mouse testis. In the nonirradiated testis, ATM was present in spermatogonia and spermatocytes until stage VII of the cycle of the seminiferous epithelium, whereas pS1981-ATM was found only to be present in the sex body of pachytene spermatocytes. In response to IR, ATM became activated by pS1981 cross-phosphorylation in spermatogonia and Sertoli cells. Despite the occurrence of endogenous programmed DSBs during the first meiotic prophase and the presence of ATM in both spermatogonia and spermatocytes, pS1981 phosphorylated ATM did not appear in spermatocytes after treatment with IR. These results show that spermatogonial ATM and ATM in the spermatocytes are differentially regulated. In the mitotically dividing spermatogonia, ATM is activated by cross-phosphorylation, whereas during meiosis nonphosphorylated ATM or differently phosphorylated ATM is already active. ATM has been shown to be present at the synapsed axes of the meiotic chromosomes, and in the ATM knock-out mice spermatogenesis stops at pachytene stage IV of the seminiferous epithelium, indicating that indeed nonphosphorylated ATM is functional during meiosis. Additionally, ATM is constitutively phosphorylated in the sex body where its continued presence remains an enigma.

Genetic background as a possible determinant of clinical and biological features of Epstein-Barr virus infection--a hypothetical view

Epstein-Barr virus (EBV) is involved in various clinical disorders and many of the disease entities are lymphoid and epithelial malignancies. The exact mechanisms that determine the exact form of EBV-related disorder are not clear at present. Many of the clinical manifestations of these diseases are based on the biological characteristics of the target cells for EBV infection and the expression and function of EBV gene, which also perturb host immune functions. In this monograph, I propose a hypothesis regarding the mechanism involved in shaping the manifestation of EBV infection that genomic instability of EBV-infected cells and how a defective immune surveillance system against such cells plays a critical role in determining the clinical manifestation of EBV infection. Using EBV-infected B-cells from patients and carriers with ataxia telangiectasia as an example of EBV infection, I present and discuss evidence in support of the proposed hypothesis.

Analysis of 11q21-24 loss of heterozygosity candidate target genes in breast cancer: indications of TSLC1 promoter hypermethylation

Loss of heterozygosity (LOH) at the distal half of chromosome arm 11q is frequent in a variety of human tumors, including breast cancer, and is often associated with poor prognosis. In an ongoing attempt to locate and characterize the main target genes within this chromosome region, we first looked for aberrations in known genes either suggested to be involved in tumorigenesis or shown to suppress tumor formation. We examined 31 primary breast tumors showing LOH in 11q21-24 for mutations in the MRE11A, CHK1, PPP2R1B, and TSLC1 genes. The absence of intragenic alterations related to cancer led us next to evaluate possible gene silencing resulting from promoter region CpG hypermethylation, using the bisulfite sequencing technique. In addition to the four genes mentioned above, we also analyzed the ATM gene, which had been investigated for certain germline mutations in an earlier study. Only the TSLC1 promoter region exhibited aberrant methylation patterns, and altogether 33% (10/30) of the successfully analyzed tumors showed evidence of elevated levels of TSLC1 CpG methylation. Ten percent (3/30) of the tumors showed significantly increased methylation. Thus, as has been shown in lung and some other forms of cancer, hypermethylation of the TSLC1 promoter region is also frequently a second hit along with LOH in breast cancer.

ATM: genome stability, neuronal development, and cancer cross paths

One of the cornerstones of the web of signaling pathways governing cellular life and differentiation is the DNA damage response. It spans a complex network of pathways, ranging from DNA repair to modulation of numerous processes in the cell. DNA double-strand breaks (DSBs), which are formed as a result of genotoxic stress or normal recombinational processes, are extremely lethal lesions that rapidly mobilize this intricate defense system. The master controller that pilots cellular responses to DSBs is the ATM protein kinase, which turns on this network by phosphorylating key players in its various branches. ATM is the protein product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), which is characterized by neuronal degeneration, immunodeficiency, sterility, genomic instability, cancer predisposition, and radiation sensitivity. The clinical and cellular phenotype of A-T attests to the numerous roles of ATM, on the one hand, and to the link between the DNA damage response and developmental processes on the other hand. Recent studies of this protein and its effectors, combined with a thorough investigation of animal models of A-T, have led to new insights into the mode of action of this master controller of the DNA damage response. The evidence that ATM is involved in signaling pathways other than those related to damage response, particularly ones relating to cellular growth and differentiation, reinforces the multifaceted nature of this protein, in which genome stability, developmental processes, and cancer cross paths.

CHK2 kinase--a busy messenger

Checkpoint kinase 2 (Chk2) is emerging as a key mediator of diverse cellular responses to genotoxic stress, guarding the integrity of the genome throughout eukaryotic evolution. Recent studies show the fundamental role of Chk2 in the network of genome-surveillance pathways that coordinate cell-cycle progression with DNA repair and cell survival or death. Defects in Chk2 contribute to the development of both hereditary and sporadic human cancers, and earmark this kinase as a candidate tumour suppressor and an attractive target for drug discovery.

Checkpoints: chromosome pairing takes an unexpected twist

When meiotic cells complete S phase, homologous chromosomes pair, synapse and undergo recombination. A checkpoint protein is somehow required for meiotic chromosome pairing in C. elegans, thus providing a direct link between S phase and the rest of the meiotic program.

Tissue-specific regulation of Chk1 expression by p53

The regulation of Chk1, a critical protein kinase involved in G(2) phase arrest, has been a subject of recent research. Chk1 phosphorylates tumor suppressor p53 at multiple sites, while p53 has been shown to downregulate Chk1 expression under stress conditions in vitro, suggesting negative feedback between the two checkpoint proteins. Using the p53 knockout mouse model, we demonstrate by Western blot and immunohistochemistry that mChk1 expression is induced in spleen, thymus, and dermal fibroblasts and is reduced in lung and testis in p53(-/-) mice compared to p53(+/+) controls. The mChk1 protein was undetectable in heart, kidney, and skin, whereas abundant expression was observed in brain and liver in both p53(+/+) and p53(-/-) mice. These data indicate that p53 regulates Chk1 expression in a tissue-specific manner.

Chk2 tumour suppressor protein in human spermatogenesis and testicular germ-cell tumours

Chk2 is a transducer of DNA damage signals and a tumour suppressor whose germ-line mutations predispose to diverse tumour types. Unlike its downstream targets such as the p53 tumour suppressor, the expression patterns of Chk2 in tissues and tumours remain unknown. As DNA breaks occur commonly during gametogenesis, and p53 is wild-type and overexpressed in testicular cancer, we examined abundance and localisation of the Chk2 protein during normal development of human testes, and at various stages of germ-cell tumour (GCT) pathogenesis. Our results show that Chk2 is abundant in foetal germ cells and adult spermatogonia, yet only weakly expressed or lacking during the meiotic and later stages of spermatogenesis. High levels of Chk2 are detected in the majority of GCTs including all pre-invasive carcinoma-in-situ lesions, contrary to variable expression and even lack of Chk2 in subsets of invasive GCTs and some teratoma structures, respectively. Together with our analyses of cell culture models, these results indicate that downmodulation or lack of Chk2 is not simply attributable to quiescence or differentiation, they suggest a role for Chk2 in mitotic rather than meiotic divisions, support the concept of foetal origin of GCTs, and have implications for protein-based screening for tumour-associated aberrations of Chk2.

ATR-mediated checkpoint pathways regulate phosphorylation and activation of human Chk1

Chk1 is an evolutionarily conserved protein kinase that regulates cell cycle progression in response to checkpoint activation. In this study, we demonstrated that agents that block DNA replication or cause certain forms of DNA damage induce the phosphorylation of human Chk1. The phosphorylated form of Chk1 possessed higher intrinsic protein kinase activity and eluted more quickly on gel filtration columns. Serines 317 and 345 were identified as sites of phosphorylation in vivo, and ATR (the ATM- and Rad3-related protein kinase) phosphorylated both of these sites in vitro. Furthermore, phosphorylation of Chk1 on serines 317 and 345 in vivo was ATR dependent. Mutants of Chk1 containing alanine in place of serines 317 and 345 were poorly activated in response to replication blocks or genotoxic stress in vivo, were poorly phosphorylated by ATR in vitro, and were not found in faster-eluting fractions by gel filtration. These findings demonstrate that the activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345. In addition, this study implicates ATR as a direct upstream activator of Chk1 in human cells.

Caspase-2 deficiency prevents programmed germ cell death resulting from cytokine insufficiency but not meiotic defects caused by loss of ataxia telangiectasia-mutated (Atm) gene function

It is well established that programmed cell death claims up to two-thirds of the oocytes produced during gametogenesis in the developing fetal ovaries. However, the mechanisms underlying prenatal germ cell loss in females remain poorly understood. Herein we report that caspase-11 null female mice are born with a reduced number of oocyte-containing primordial follicles. This phenotype is likely due to failed cytokine processing known to occur in caspase-11 mutants since neonatal female mice lacking both interleukin (IL)-1alpha and IL-1beta also exhibit a reduced endowment of primordial follicles. In addition, germ cell death in wild-type fetal ovaries cultured ex vivo is suppressed by either cytokine, likely via ligand activation of type 1 IL-1 receptors expressed in fetal germ cells. Normal oocyte endowment can be restored in caspase-11 null female mice by simultaneous inactivation of the gene encoding the cell death executioner enzyme, caspase-2. However, caspase-2 deficiency cannot overcome gametogenic failure resulting from meiotic recombination defects in ataxia telangiectasia-mutated (Atm) null female mice. Thus, genetically distinct mechanisms exist for developmental deletion of oocytes via programmed cell death, one of which probably functions as a meiotic quality-control checkpoint that cannot be overridden.

Deletion mapping of chromosome segment 11q24-q25, exhibiting extensive allelic loss in early onset breast cancer

Frequent allelic deletions at chromosome 11q24-q25 have been described in both early and late onset breast cancers, suggesting the existence of a gene locus implicated in the initiation and/or progression of the disease. In the present study we fine mapped this region further by loss of heterozygosity (LOH) analysis in a population of early onset breast cancer cases (n = 102, 22 to 36 years old). Loss of chromosomal material was assessed for possible association with patient survival as well as Nottingham histologic grade (NHG). Additionally, we investigated the involvement of the 11q24-q25 locus in a group of familial breast cancer cases with no detectable BRCA1 or BRCA2 gene alterations (n = 32, ages 28 to 40 years). Among the consecutive patients, extensive LOH was observed for all markers at 11q24-q25, with frequencies ranging from 42% to 54%. Deletion at the D11S4125 marker was found to be associated with reduced survival (p = 0.026), whereas the adjacent D11S387 marker correlated with higher histologic grade (p = 0.042). In the familial cases, the most telomeric markers showed substantially lower proportions of LOH, ranging from 10% to 21%. Comparison of the two patient groups demonstrated that this difference in LOH frequency was statistically significant for the D11S4098, D11S968, D11S387 and D11S4125 markers (p = 0.020, p = 0.029, p = 0.0070 and p = 0.0030, respectively). We conclude that 11q25 may harbor a gene implicated in early onset breast cancer. Our data suggest that the most probable position for this locus is defined by the markers D11S387 and D11S4125 and furthermore that it may play a less significant role in familial breast cancer cases not linked to either of the BRCA genes.

Localization of two mammalian cyclin dependent kinases during mammalian meiosis

Mammalian meiotic progression, like mitotic cell cycle progression, is regulated by cyclins and cyclin dependent kinases (CDKs). However, the unique requirements of meiosis (homologous synapsis, reciprocal recombination and the dual divisions that segregate first homologues, then sister chromatids) have led to different patterns of CDK expression. Here we show that Cdk4 colocalizes with replication protein A (RPA) on the synaptonemal complexes (SCs) of newly synapsed axes of homologously pairing bivalents, but disappears from these axes by mid-pachynema. The switch from the mitotic pattern of expression occurs during the last two spermatogonial divisions. Cdk2 colocalizes with MLH1, a mismatch repair protein at sites of reciprocal recombination in mid-late pachynema. In addition Cdk2 localizes to the telomeres of chromosomal bivalents throughout meiotic prophase. The mitotic pattern of expression of Cdk2 remains unchanged throughout the spermatogonial divisions, but is altered in meiosis of the spermatocytes.

p53 down-regulates CHK1 through p21 and the retinoblastoma protein

Both fission yeast and mammalian cells require the function of the checkpoint kinase CHK1 for G2 arrest after DNA damage. The tumor suppressor p53, a well-studied stress response factor, has also been shown to play a role in DNA damage G2 arrest, although in a manner that is probably independent of CHK1. p53, however, can be phosphorylated and regulated by both CHK1 as well as another checkpoint kinase, hCds1 (also called CHK2). It was therefore of interest to determine whether reciprocally, p53 affects either CHK1 or CHK2. We found that induction of p53 either by diverse stress signals or ectopically using a tetracycline-regulated promoter causes a marked reduction in CHK1 protein levels. CHK1 downregulation by p53 occurs as a result of reduced CHK1 RNA accumulation, indicating that repression occurs at the level of transcription. Repression of CHK1 by p53 requires p21, since p21 alone is sufficient for this to occur and cells lacking p21 cannot downregulate CHK1. Interestingly, pRB is also required for CHK1 downregulation, suggesting the possible involvement of E2F-dependent transcription in the regulation of CHK1. Our results identify a new repression target of p53 and suggest that p53 and CHK1 play interdependent and complementary roles in regulating both the arrest and resumption of G2 after DNA damage.

Mutant alleles of Schizosaccharomyces pombe rad9(+) alter hydroxyurea resistance, radioresistance and checkpoint control

Schizosaccharomyces pombe rad9 mutations can render cells sensitive to hydroxyurea (HU), gamma-rays and UV light and eliminate associated checkpoint controls. In vitro mutagenesis was performed on S.pombe rad9 and altered alleles were transplaced into the genome to ascertain the functional significance of five groups of evolutionarily conserved amino acids. Most targeted regions were changed to alanines, whereas rad9-S3 encodes a protein devoid of 22 amino acids normally present in yeast but absent from mammalian Rad9 proteins. We examined whether these rad9 alleles confer radiation and HU sensitivity and whether the sensitivities correlate with checkpoint control deficiencies. One rad9 mutant allele was fully active, whereas four others demonstrated partial loss of function. rad9-S1, which contains alterations in a BH3-like domain, conferred HU resistance but increased sensitivity to gamma-rays and UV light, without affecting checkpoint controls. rad9-S2 reduced gamma-ray sensitivity marginally, without altering other phenotypes. Two alleles, rad9-S4 and rad9-S5, reduced HU sensitivity, radiosensitivity and caused aberrant checkpoint function. HU-induced checkpoint control could not be uncoupled from drug resistance. These results establish unique as well as overlapping functional domains within Rad9p and provide evidence that requirements of the protein for promoting resistance to radiation and HU are not identical.

Meiotic telomere distribution and Sertoli cell nuclear architecture are altered in Atm- and Atm-p53-deficient mice

The ataxia telangiectasia mutant (ATM) protein is an intrinsic part of the cell cycle machinery that surveys genomic integrity and responses to genotoxic insult. Individuals with ataxia telangiectasia as well as Atm(-/-) mice are predisposed to cancer and are infertile due to spermatogenesis disruption during first meiotic prophase. Atm(-/-) spermatocytes frequently display aberrant synapsis and clustered telomeres (bouquet topology). Here, we used telomere fluorescent in situ hybridization and immunofluorescence (IF) staining of SCP3 and testes-specific histone H1 (H1t) to spermatocytes of Atm- and Atm-p53-deficient mice and investigated whether gonadal atrophy in Atm-null mice is associated with stalling of telomere motility in meiotic prophase. SCP3-H1t IF revealed that most Atm(-/-) p53(-/-) spermatocytes degenerated during late zygotene, while a few progressed to pachytene and diplotene and some even beyond metaphase II, as indicated by the presence of a few round spermatids. In Atm(-/-) p53(-/-) meiosis, the frequency of spermatocytes I with bouquet topology was elevated 72-fold. Bouquet spermatocytes with clustered telomeres were generally void of H1t signals, while mid-late pachytene and diplotene Atm(-/-) p53(-/-) spermatocytes displayed expression of H1t and showed telomeres dispersed over the nuclear periphery. Thus, it appears that meiotic telomere movements occur independently of ATM signaling. Atm inactivation more likely leads to accumulation of spermatocytes I with bouquet topology by slowing progression through initial stages of first meiotic prophase and an ensuing arrest and demise of spermatocytes I. Sertoli cells (SECs), which contribute to faithful spermatogenesis, in the Atm mutants were found to frequently display numerous heterochromatin and telomere clusters-a nuclear topology which resembles that of immature SECs. However, Atm(-/-) SECs exhibited a mature vimentin and cytokeratin 8 intermediate filament expression signature. Upon IF with ATM antibodies, we observed ATM signals throughout the nuclei of human and mouse SECs, spermatocytes I, and haploid round spermatids. ATM but not H1t was absent from elongating spermatid nuclei. Thus, ATM appears to be removed from spermatid nuclei prior to the occurrence of DNA nicks which emanate as a consequence of nucleoprotamine formation.

Induction of DNA replication in the germinal vesicle of the growing mouse oocyte

Growing mouse oocytes are physiologically arrested in the G2 phase of prophase of the first meiotic division. Growing oocytes were isolated from ovaries of 9- to 12-day-old mice and fused with parthenogenetic one-cell eggs or two-cell embryos derived from fertilized eggs. Resulting hybrids were injected with Dig-11-dUTP and examined for DNA replication using immunofluorescence. Parthenogenetic one-cell eggs fused at telophase II, G1, and middle-to-late S phase, and also S-phase two-cell blastomeres, were able to trigger DNA synthesis in oocyte germinal vesicle (GV) in the majority of hybrids cultured to the end of the first cell cycle. Activation of replication in the GV occurred within 2-3 h after fusion of growing oocytes with S-phase eggs. We show indirectly that the reactivation of replication in GVs was not dependent on the breakdown of the GV envelope. Although GVs had the ability to renew DNA replication after fusion, the G2 blastomere nuclei were incapable of reinitiating DNA replication under the influence of S-phase one-cell eggs. We hypothesize that the nuclei of growing oocytes arrested in meiotic prophase are in a physiological state that is equivalent to replication-competent G1, and not G2, nuclei.

The ATM gene and breast cancer: is it really a risk factor?

The genetic determinants for most breast cancer cases remain elusive. Whilst mutations in BRCA1 and BRCA2 significantly contribute to familial breast cancer risk, their contribution to sporadic breast cancer is low. In such cases genes frequently altered in the general population, such as the gene mutated in Ataxia telangiectasia (AT), ATM may be important risk factors. The initial interest in studying ATM heterozygosity in breast cancer arose from the findings of epidemiological studies of AT families in which AT heterozygote women had an increased risk of breast cancer and estimations that 1% of the population are AT heterozygotes. One of the clinical features of AT patients is extreme cellular sensitivity to ionising radiation. This observation, together with the finding that a significant proportion of breast cancer patients show an exaggerated acute or late normal tissue reactions after radiotherapy, has lead to the suggestion that AT heterozygosity plays a role in radiosensitivity and breast cancer development. Loss of heterozygosity in the region of the ATM gene on chromosome 11, has been found in about 40% of sporadic breast tumours. However, screening for ATM mutations in sporadic breast cancer cases, showing or not adverse effects to radiotherapy, has not revealed the magnitude of involvement of the ATM gene expected. Their size and the use of the protein truncation test to identify mutations limit many of these studies. This latter parameter is critical as the profile of mutations in AT patients may not be representative of the ATM mutations in other diseases. The potential role of rare sequence variants within the ATM gene, sometimes reported as polymorphisms, also needs to be fully assessed in larger cohorts of breast cancer patients and controls in order to determine whether they represent cancer and/or radiation sensitivity predisposing mutations.

Chk2/hCds1 functions as a DNA damage checkpoint in G1 by stabilizing p53

Chk2/hcds1, the human homolog of theSaccharomyces cerevisiae RAD53/SPK1 andSchizosaccharomyces pombe cds1 DNA damage checkpoint genes, encodes a protein kinase that is post-translationally modified after DNA damage. Like its yeast homologs, the Chk2/hCds1 protein phosphorylates Cdc25C in vitro, suggesting that it arrests cells in G2 in response to DNA damage. We expressed Chk2/hCds1 in human cells and analyzed their cell cycle profile. Wild-type, but not catalytically inactive, Chk2/hCds1 led to G1 arrest after DNA damage. The arrest was inhibited by cotransfection of a dominant-negative p53 mutant, indicating that Chk2/hCds1 acted upstream of p53. In vitro, Chk2/hCds1 phosphorylated p53 on Ser-20 and dissociated preformed complexes of p53 with Mdm2, a protein that targets p53 for degradation. In vivo, ectopic expression of wild-type Chk2/hCds1 led to increased p53 stabilization after DNA damage, whereas expression of a dominant-negative Chk2/hCds1 mutant abrogated both phosphorylation of p53 on Ser-20 and p53 stabilization. Thus, in response to DNA damage, Chk2/hCds1 stabilizes the p53 tumor suppressor protein leading to cell cycle arrest in G1.