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

Testis-expressed gene 11 inhibits cisplatin-induced DNA damage and contributes to chemoresistance in testicular germ cell tumor

Testicular germ cell tumor (TGCT) is a rare cancer but the most common tumor among adolescent and young adult males. Patients with advanced TGCT often exhibit a worse prognosis due to the acquisition of therapeutic resistance. Cisplatin-based chemotherapy is a standard treatment for advanced TGCTs initially sensitive to cisplatin, as exemplified by embryonal carcinoma. The acquisition of cisplatin resistance, however, could be a fatal obstacle for TGCT management. To identify cisplatin resistance-related genes, we performed transcriptome analysis for cisplatin-resistant TGCT cells compared to parental cells. In two types of cisplatin-resistant TGCT cell models that we established from patient-derived TGCT cells, and from the NEC8 cell line, we found that mRNA levels of the high-mobility-group nucleosome-binding gene HMGN5 and meiosis-related gene TEX11 were remarkably upregulated compared to those in the corresponding parental cells. We showed that either HMGN5 or TEX11 knockdown substantially reduced the viability of cisplatin-resistant TGCT cells in the presence of cisplatin. Notably, TEX11 silencing in cisplatin-resistant TGCT cells increased the level of cleaved PARP1 protein, and the percentage of double-strand break marker γH2AX-positive cells. We further demonstrated the therapeutic efficiency of TEX11-specific siRNA on in vivo xenograft models derived from cisplatin-resistant patient-derived TGCT cells. Taken together, the present study provides a potential insight into a mechanism of cisplatin resistance via TEX11-dependent pathways that inhibit apoptosis and DNA damage. We expect that our findings can be applied to the improvement of cisplatin-based chemotherapy for TGCT, particularly for TEX11-overexpressing tumor.

A Mechanistic Theory of Development-Aging Continuity in Humans and Other Mammals

There is consensus among biogerontologists that aging occurs either as the result of a purposeful genome-based, evolved program or due to spontaneous, randomly occurring, maladaptive events. Neither concept has yet identified a specific mechanism to explain aging’s emergence and acceleration during mid-life and beyond. Presented herein is a novel, unifying mechanism with empirical evidence that describes how aging becomes continuous with development. It assumes that aging emerges from deterioration of a regulatory process that directs morphogenesis and morphostasis. The regulatory system consists of a genome-wide “backbone” within which its specific genes are differentially expressed by the local epigenetic landscapes of cells and tissues within which they reside, thereby explaining its holistic nature. Morphostasis evolved in humans to ensure the nurturing of dependent offspring during the first decade of young adulthood when peak parental vitality prevails in the absence of aging. The strict redundancy of each morphostasis regulatory cycle requires sensitive dependence upon initial conditions to avoid initiating deterministic chaos behavior. However, when natural selection declines as midlife approaches, persistent, progressive, and specific DNA damage and misrepair changes the initial conditions of the regulatory process, thereby compromising morphostasis regulatory redundancy, instigating chaos, initiating senescence, and accelerating aging thereafter.

Determination of c-Abl tyrosine kinase and mTERT catalytic subunit of telomerase expression level during prenatal-postnatal mouse ovary-testis development

Expression levels of genes involved in the development of germ cells vary throughout the process from bipotential gonadal period to adult gonadal formation. In mice, developments of female and male reproductive system are regulated by germ cell-specific factors and hormones, and determinative days in this regulation are very important. c-Abl is a non-receptor tyrosine kinase with cellular functions including cell proliferation, growth and development. mTERT is involved in maintaining telomerase activity and proliferation of surviving cells. We suggested that c-Abl and mTERT might be important for the healthy development of prenatal and postnatal mouse ovary and testis. We aim to demonstrate localization and expressions of c-Abl and mTERT in crucial days of ovary and testis development in prenatal and postnatal period in mouse by immunofluorescence staining and qRT-PCR, respectively. The importance of c-Abl and mTERT expressions during the healthy gonadal development is indicated in the prenatal and postnatal gonadal development. Also, protein expression levels were detected by Western Blot in only postnatal ovary and testis. Determining the functions of the c-Abl and mTERT throughout the process will be important in terms of understanding the infertility cases in the female and male with future studies.

Preimplantation Genetic Testing for Chromosomal Abnormalities: Aneuploidy, Mosaicism, and Structural Rearrangements

There is a high incidence of chromosomal abnormalities in early human embryos, whether they are generated by natural conception or by assisted reproductive technologies (ART). Cells with chromosomal copy number deviations or chromosome structural rearrangements can compromise the viability of embryos; much of the naturally low human fecundity as well as low success rates of ART can be ascribed to these cytogenetic defects. Chromosomal anomalies are also responsible for a large proportion of miscarriages and congenital disorders. There is therefore tremendous value in methods that identify embryos containing chromosomal abnormalities before intrauterine transfer to a patient being treated for infertility—the goal being the exclusion of affected embryos in order to improve clinical outcomes. This is the rationale behind preimplantation genetic testing for aneuploidy (PGT-A) and structural rearrangements (-SR). Contemporary methods are capable of much more than detecting whole chromosome abnormalities (e.g., monosomy/trisomy). Technical enhancements and increased resolution and sensitivity permit the identification of chromosomal mosaicism (embryos containing a mix of normal and abnormal cells), as well as the detection of sub-chromosomal abnormalities such as segmental deletions and duplications. Earlier approaches to screening for chromosomal abnormalities yielded a binary result of normal versus abnormal, but the new refinements in the system call for new categories, each with specific clinical outcomes and nuances for clinical management. This review intends to give an overview of PGT-A and -SR, emphasizing recent advances and areas of active development.

The Random Oligonucleotide-Primed Synthesis Assay for the Quantification of DNA Strand Breaks

DNA strand breaks arise from normal cellular processes such as replication, transcription, and DNA repair as well as spontaneous DNA damage caused by cell metabolic activities. In addition, strand breaks occur due to direct or indirect DNA damage produced by various abiotic and biotic stresses. Strand breaks are among the most problematic DNA lesions because unrepaired strand breaks may lead to cell cycle arrest, gross chromosome rearrangements, or even cell death. Thus, the measurement of the relative number of strand breaks can provide an informative picture of genome stability of a given cell, tissue, or organism. Here, we describe the use of random oligonucleotide-primed synthesis (ROPS) assay for the detection and quantification of the level of strand breaks in tissue samples. The applications of the assay for a quantitative detection of 3'OH, 3'P, or DNA strand breaks at a cleavage site of the deoxyribose residue are discussed.

Multifunctional Involvement of a C2H2 Zinc Finger Protein (PbZfp) in Malaria Transmission, Histone Modification, and Susceptibility to DNA Damage Response

In sexually reproducing organisms, meiosis is an essential step responsible for generation of haploid gametes from diploid somatic cells. The quest for understanding regulatory mechanisms of meiotic recombination in Plasmodium led to identification of a gene encoding a protein that contains 11 copies of C₂H₂ zinc fingers (ZnF). Reverse genetic approaches were used to create Plasmodium berghei parasites either lacking expression of full-length Plasmodium berghei zinc finger protein (PbZfp) (knockout [KO]) or expressing PbZfp lacking C-terminal zinc finger region (truncated [Trunc]). Mice infected with KO parasites survived two times longer (P < 0.0001) than mice infected with wild-type (WT) parasites. In mosquito transmission experiments, the infectivity of KO and Trunc parasites was severely compromised (>95% oocyst reduction). KO parasites revealed a total lack of trimethylation of histone 3 at several lysine residues (K4, K27, and K36) without any effect on acetylation patterns (H3K9, H3K14, and H4K16). Reduced DNA damage and reduced expression of topoisomerase-like Spo11 in the KO parasites with normal Rad51 expression further suggest a functional role for PbZfp during genetic recombination that involves DNA double-strand break (DSB) formation followed by DNA repair. These finding raise the possibility of some convergent similarities of PbZfp functions to functions of mammalian PRDM9, also a C₂H₂ ZnF protein with histone 3 lysine 4 (H3K4) methyltransferase activity. These functions include the major role played by the latter in binding recombination hotspots in the genome during meiosis and trimethylation of the associated histones and subsequent chromatin recruitment of topoisomerase-like Spo11 to catalyze DNA DSB formation and DMC1/Rad51-mediated DNA repair and homologous recombination.

Malaria parasites are haploid throughout their life cycle except for a brief time period when zygotes are produced as a result of fertilization between male and female gametes during transmission through the mosquito vector. The reciprocal recombination events that follow zygote formation ensure orderly segregation of homologous chromosomes during meiosis, creating genetic diversity among offspring. Studies presented in the current manuscript identify a novel C2H2 ZnF-containing protein exhibiting multifunctional roles in parasite virulence, mosquito transmission, and homologous recombination during meiosis. Understanding the transmission biology of malaria will result in the identification of novel targets for transmission-blocking intervention approaches.

Ndrg3 gene regulates DSB repair during meiosis through modulation the ERK signal pathway in the male germ cells

The N-myc downstream regulated gene (NDRG) family consists of 4 members, NDRG-1, -2, -3, -4. Physiologically, we found Ndrg3, a critical gene which led to homologous lethality in the early embryo development, regulated the male meiosis in mouse. The expression of Ndrg3 was enhanced specifically in germ cells, and reached its peak level in the pachytene stage spermatocyte. Haplo-insufficiency of Ndrg3 gene led to sub-infertility during the male early maturation. In the Ndrg3^+/- germ cells, some meiosis events such as DSB repair and synaptonemal complex formation were impaired. Disturbances on meiotic prophase progression and spermatogenesis were observed. In mechanism, the attenuation of pERK1/2 signaling was detected in the heterozygous testis. With our primary spermatocyte culture system, we found that lactate promoted DSB repair via ERK1/2 signaling in the male mouse germ cells in vitro. Deficiency of Ndrg3 gene attenuated the activation of ERK which further led to the aberrancy of DSB repair in the male germ cells in mouse. Taken together, we reported that Ndrg3 gene modulated the lactate induced ERK pathway to facilitate DSB repair in male germ cells, which further regulated meiosis and subsequently fertility in male mouse.

Pluripotent Stem Cells and DNA Damage Response to Ionizing Radiations

Pluripotent stem cells (PSCs) hold great promise in regenerative medicine, disease modeling, functional genomics, toxicological studies and cell-based therapeutics due to their unique characteristics of self-renewal and pluripotency. Novel methods for generation of pluripotent stem cells and their differentiation to the specialized cell types such as neuronal cells, myocardial cells, hepatocytes and beta cells of the pancreas and many other cells of the body are constantly being refined. Pluripotent stem cell derived differentiated cells, including neuronal cells or cardiac cells, are ideal for stem cell transplantation as autologous or allogeneic cells from healthy donors due to their minimal risk of rejection. Radiation-induced DNA damage, ultraviolet light, genotoxic stress and other intrinsic and extrinsic factors triggers a series of biochemical reactions known as DNA damage response. To maintain genomic stability and avoid transmission of mutations into progenitors cells, stem cells have robust DNA damage response signaling, a contrast to somatic cells. Stem cell transplantation may protect against radiation-induced late effects. In particular, this review focuses on differential DNA damage response between stem cells and derived differentiated cells and the possible pathways that determine such differences.

StrucBreak: A Computational Framework for Structural Break Detection in DNA Sequences

Damages or breaks in DNA may change the characteristics of genomes and causes various diseases. In this work we construct a system that incorporates the maximum likelihood-based probabilistic formula to assess the number of damages that have occurred in any DNA sequence. This approach has been progressively benchmarked by implementing simulated data sets so that the outcomes can be compared with a ground truth or reference value. At first the sequence data set order is checked through the statistical cumulative sum (STACUMSUM). The verified sequences are then estimated by prior and posterior probability to count the percentages of breaks and mutations. Maximum-likelihood estimation then finds out the exact numbers and positions of breaks and detections. In database manipulation, one factor that decides the orientation and order of the sequence is geometric distance between consecutive sequences. The geometric distance is measured for smooth representation of the genome or DNA sequences. Finally, we compared the performance of our system with DAMBE5: (A Comprehensive Software Package for Data Analysis in Molecular Biology and Evaluation), and in response to time and space complexity, StrucBreak is much faster and consumes much less space due to our algorithmic approaches.

Detecting ATM-dependent chromatin modification in DNA damage response

Loss of function or mutation of the ataxia-telangiectasia mutated gene product (ATM) results in inherited genetic disorders characterized by neurodegeneration, immunodeficiency, and cancer. Ataxia-telangiectasia mutated (ATM) gene product belongs to the PI3K-like protein kinase (PIKKs) family and is functionally implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination, cell-cycle control, and telomere maintenance. The ATM protein kinase is primarily activated in response to DNA double strand breaks (DSBs), the most deleterious form of DNA damage produced by ionizing radiation (IR) or radiomimetic drugs. It is detected at DNA damage sites, where ATM autophosphorylation causes dissociation of the inactive homodimeric form to the activated monomeric form. Interestingly, heat shock can activate ATM independent of the presence of DNA strand breaks. ATM is an integral part of the sensory machinery that detects DSBs during meiosis, mitosis, or DNA breaks mediated by free radicals. These DNA lesions can trigger higher order chromatin reorganization fuelled by posttranslational modifications of histones and histone binding proteins. Our group, and others, have shown that ATM activation is tightly regulated by chromatin modifications. This review summarizes the multiple approaches used to discern the role of ATM and other associated proteins in chromatin modification in response to DNA damage.

Ubiquitylation, neddylation and the DNA damage response

Failure of accurate DNA damage sensing and repair mechanisms manifests as a variety of human diseases, including neurodegenerative disorders, immunodeficiency, infertility and cancer. The accuracy and efficiency of DNA damage detection and repair, collectively termed the DNA damage response (DDR), requires the recruitment and subsequent post-translational modification (PTM) of a complex network of proteins. Ubiquitin and the ubiquitin-like protein (UBL) SUMO have established roles in regulating the cellular response to DNA double-strand breaks (DSBs). A role for other UBLs, such as NEDD8, is also now emerging. This article provides an overview of the DDR, discusses our current understanding of the process and function of PTM by ubiquitin and NEDD8, and reviews the literature surrounding the role of ubiquitylation and neddylation in DNA repair processes, focusing particularly on DNA DSB repair.

CHK2 kinase in the DNA damage response and beyond

The serine/threonine kinase CHK2 is a key component of the DNA damage response. In human cells, following genotoxic stress, CHK2 is activated and phosphorylates >20 proteins to induce the appropriate cellular response, which, depending on the extent of damage, the cell type, and other factors, could be cell cycle checkpoint activation, induction of apoptosis or senescence, DNA repair, or tolerance of the damage. Recently, CHK2 has also been found to have cellular functions independent of the presence of nuclear DNA lesions. In particular, CHK2 participates in several molecular processes involved in DNA structure modification and cell cycle progression. In this review, we discuss the activity of CHK2 in response to DNA damage and in the maintenance of the biological functions in unstressed cells. These activities are also considered in relation to a possible role of CHK2 in tumorigenesis and, as a consequence, as a target of cancer therapy.

DNA repair pathways in trypanosomatids: from DNA repair to drug resistance

All living organisms are continuously faced with endogenous or exogenous stress conditions affecting genome stability. DNA repair pathways act as a defense mechanism, which is essential to maintain DNA integrity. There is much to learn about the regulation and functions of these mechanisms, not only in human cells but also equally in divergent organisms. In trypanosomatids, DNA repair pathways protect the genome against mutations but also act as an adaptive mechanism to promote drug resistance. In this review, we scrutinize the molecular mechanisms and DNA repair pathways which are conserved in trypanosomatids. The recent advances made by the genome consortiums reveal the complete genomic sequences of several pathogens. Therefore, using bioinformatics and genomic sequences, we analyze the conservation of DNA repair proteins and their key protein motifs in trypanosomatids. We thus present a comprehensive view of DNA repair processes in trypanosomatids at the crossroads of DNA repair and drug resistance.

Ten years of gene discovery for meiotic event control in rice

Meiosis is the crucial process by which sexually propagating eukaryotes give rise to haploid gametes from diploid cells. Several key processes, like homologous chromosomes pairing, synapsis, recombination, and segregation, sequentially take place in meiosis. Although these widely conserved events are under both genetic and epigenetic control, the accurate details of molecular mechanisms are continuing to investigate. Rice is a good model organism for exploring the molecular mechanisms of meiosis in higher plants. So far, 28 rice meiotic genes have been characterized. In this review, we give an overview of the discovery of rice meiotic genes in the last ten years, with a particular focus on their functions in meiosis.

The oxidative damage initiation hypothesis for meiosis

The maintenance of sexual reproduction in eukaryotes is still a major enigma in evolutionary biology. Meiosis represents the only common feature of sex in all eukaryotic kingdoms, and thus, we regard it a key issue for discussing its function. Almost all asexuality modes maintain meiosis either in a modified form or as an alternative pathway, and facultatively apomictic plants increase frequencies of sexuality relative to apomixis after abiotic stress. On the physiological level, abiotic stress causes oxidative stress. We hypothesize that repair of oxidative damage on nuclear DNA could be a major driving force in the evolution of meiosis. We present a hypothetical model for the possible redox chemistry that underlies the binding of the meiosis-specific protein Spo11 to DNA. During prophase of meiosis I, oxidized sites at the DNA molecule are being targeted by the catalytic tyrosine moieties of Spo11 protein, which acts like an antioxidant reducing the oxidized target. The oxidized tyrosine residues, tyrosyl radicals, attack the phosphodiester bonds of the DNA backbone causing DNA double strand breaks that can be repaired by various mechanisms. Polyploidy in apomictic plants could mitigate oxidative DNA damage and decrease Spo11 activation. Our hypothesis may contribute to explaining various enigmatic phenomena: first, DSB formation outnumbers crossovers and, thus, effective recombination events by far because the target of meiosis may be the removal of oxidative lesions; second, it offers an argument for why expression of sexuality is responsive to stress in many eukaryotes; and third, repair of oxidative DNA damage turns meiosis into an essential characteristic of eukaryotic reproduction.

DNA double-strand break repair as determinant of cellular radiosensitivity to killing and target in radiation therapy

Radiation therapy plays an important role in the management of a wide range of cancers. Besides innovations in the physical application of radiation dose, radiation therapy is likely to benefit from novel approaches exploiting differences in radiation response between normal and tumor cells. While ionizing radiation induces a variety of DNA lesions, including base damages and single-strand breaks, the DNA double-strand break (DSB) is widely considered as the lesion responsible not only for the aimed cell killing of tumor cells, but also for the general genomic instability that leads to the development of secondary cancers among normal cells. Homologous recombination repair (HRR), non-homologous end-joining (NHEJ), and alternative NHEJ, operating as a backup, are the major pathways utilized by cells for the processing of DSBs. Therefore, their function represents a major mechanism of radiation resistance in tumor cells. HRR is also required to overcome replication stress – a potent contributor to genomic instability that fuels cancer development. HRR and alternative NHEJ show strong cell-cycle dependency and are likely to benefit from radiation therapy mediated redistribution of tumor cells throughout the cell-cycle. Moreover, the synthetic lethality phenotype documented between HRR deficiency and PARP inhibition has opened new avenues for targeted therapies. These observations make HRR a particularly intriguing target for treatments aiming to improve the efficacy of radiation therapy. Here, we briefly describe the major pathways of DSB repair and review their possible contribution to cancer cell radioresistance. Finally, we discuss promising alternatives for targeting DSB repair to improve radiation therapy and cancer treatment.

Preferential repair of DNA double-strand break at the active gene in vivo

Previous studies have demonstrated transcription-coupled nucleotide/base excision repair. We report here for the first time that DNA double-strand break (DSB) repair is also coupled to transcription. We generated a yeast strain by introducing a homing (Ho) endonuclease cut site followed by a nucleotide sequence for multiple Myc epitopes at the 3' end of the coding sequence of a highly active gene, ADH1. This yeast strain also contains the Ho cut site at the nearly silent or poorly active mating type α (MATα) locus and expresses Ho endonuclease under the galactose-inducible GAL1 promoter. Using this strain, DSBs were generated at the ADH1 and MATα loci in galactose-containing growth medium that induced HO expression. Subsequently, yeast cells were transferred to dextrose-containing growth medium to stop HO expression, and the DSB repair was monitored at the ADH1 and MATα loci by PCR, using the primer pairs flanking the Ho cut sites. Our results revealed a faster DSB repair at the highly active ADH1 than that at the nearly silent MATα locus, hence implicating a transcription-coupled DSB repair at the active gene in vivo. Subsequently, we extended this study to another gene, PHO5 (carrying the Ho cut site at its coding sequence), under transcriptionally active and inactive growth conditions. We found a fast DSB repair at the active PHO5 gene in comparison to its inactive state. Collectively, our results demonstrate a preferential DSB repair at the active gene, thus supporting transcription-coupled DSB repair in living cells.

Two SCID cases with Cernunnos-XLF deficiency successfully treated by hematopoietic stem cell transplantation

SCID affects T and B cell differentiation and functions, presenting with severe opportunistic infections in the early postnatal period. It is fatal unless stem cell transplantation is performed. RS SCID forms are caused by defects in the NHEJ pathway, the enzymatic process required for the repair of DNA double-strand breaks. Cernunnos-XLF defect is one of the defects in this pathway. Here, we present two patients with Cernunnos-XLF defect, both having microcephaly, prominent growth retardation, and T-B-NK+SCID, one of whom had AHA. These patients received hematopoietic stem cells from HLA identical related donor without conditioning regimen and recovered without any complication. Now, both of the patients are well and alive seven and one yr after transplantation, respectively. A remarkable observation was the severe diarrhea that occurred in both patients soon after transplantation.

The role of MOF in the ionizing radiation response is conserved in Drosophila melanogaster

In Drosophila, males absent on the first (MOF) acetylates histone H4 at lysine 16 (H4K16ac). This acetylation mark is highly enriched on the male X chromosome and is required for dosage compensation in Drosophila but not utilized for such in mammals. Recently, we and others reported that mammalian MOF, through H4K16ac, has a critical role at multiple stages in the DNA damage response (DDR) and double-strand break repair pathways. The goal of this study was to test whether mof is similarly required for the response to ionizing radiation (IR) in Drosophila. We report that Drosophila mof mutations in males and females, as well as mof knockdown in SL-2 cells, reduce post-irradiation survival. MOF depletion in SL-2 cells also results in an elevated frequency of metaphases with chromosomal aberrations, suggesting that MOF is involved in DDR. Mutation in Drosophila mof also results in a defective mitotic checkpoint, enhanced apoptosis, and a defective p53 response post-irradiation. In addition, IR exposure enhanced H4K16ac levels in Drosophila as it also does in mammals. These results are the first to demonstrate a requirement for MOF in the whole animal IR response and suggest that the role of MOF in the response to IR is conserved between Drosophila and mammals.

The DNA damage checkpoint allows recombination between divergent DNA sequences in budding yeast

In the early steps of homologous recombination, single-stranded DNA (ssDNA) from a broken chromosome invades homologous sequence located in a sister or homolog donor. In genomes that contain numerous repetitive DNA elements or gene paralogs, recombination can potentially occur between non-allelic/divergent (homeologous) sequences that share sequence identity. Such recombination events can lead to lethal chromosomal deletions or rearrangements. However, homeologous recombination events can be suppressed through rejection mechanisms that involve recognition of DNA mismatches in heteroduplex DNA by mismatch repair factors, followed by active unwinding of the heteroduplex DNA by helicases. Because factors required for heteroduplex rejection are hypothesized to be targets and/or effectors of the DNA damage response (DDR), a cell cycle control mechanism that ensures timely and efficient repair, we tested whether the DDR, and more specifically, the RAD9 gene, had a role in regulating rejection. We performed these studies using a DNA repair assay that measures repair by single-strand annealing (SSA) of a double-strand break (DSB) using homeologous DNA templates. We found that repair of homeologous DNA sequences, but not identical sequences, induced a RAD9-dependent cell cycle delay in the G2 stage of the cell cycle. Repair through a divergent DNA template occurred more frequently in RAD9 compared to rad9Δ strains. However, repair in rad9Δ mutants could be restored to wild-type levels if a G2 delay was induced by nocodazole. These results suggest that cell cycle arrest induced by the Rad9-dependent DDR allows repair between divergent DNA sequences despite the potential for creating deleterious genome rearrangements, and illustrates the importance of additional cellular mechanisms that act to suppress recombination between divergent DNA sequences.

The DNA damage response induces IFN

This study reveals a new complexity in the cellular response to DNA damage: activation of IFN signaling. The DNA damage response involves the rapid recruitment of repair enzymes and the activation of signal transducers that regulate cell-cycle checkpoints and cell survival. To understand the link between DNA damage and the innate cellular defense that occurs in response to many viral infections, we evaluated the effects of agents such as etoposide that promote dsDNA breaks. Treatment of human cells with etoposide led to the induction of IFN-stimulated genes and the IFN-α and IFN-λ genes. NF-κB, known to be activated in response to DNA damage, was shown to be a key regulator of this IFN gene induction. Expression of an NF-κB subunit, p65/RelA, was sufficient for induction of the human IFN-λ1 gene. In addition, NF-κB was required for the induction of IFN regulatory factor-1 and -7 that are able to stimulate expression of the IFN-α and IFN-λ genes. Cells that lack the NF-κB essential modulator lack the ability to induce the IFN genes following DNA damage. Breaks in DNA are generated during normal physiological processes of replication, transcription, and recombination, as well as by external genotoxic agents or infectious agents. The significant finding of IFN production as a stress response to DNA damage provides a new perspective on the role of IFN signaling.

Neuregulins are essential for spermatogonial proliferation and meiotic initiation in neonatal mouse testis

The transition from mitosis to meiosis is unique to germ cells. In murine embryonic ovaries and juvenile testes, retinoic acid (RA) induces meiosis via the stimulated by retinoic acid gene 8 (Stra8), but its molecular pathway requires elucidation. We present genetic evidence in vivo and in vitro that neuregulins (NRGs) are essential for the proliferation of spermatogonia and the initiation of meiosis. Tamoxifen (TAM) was injected into 14-day post-partum (dpp) Sertoli cell-specific conditional Nrg1Ser–/– mutant mice. TAM induced testis degeneration, suppressed BrdU incorporation into spermatogonia and pre-leptotene primary spermatocytes, and decreased and increased the number of STRA8-positive and TUNEL-positive cells, respectively. In testicular organ cultures from 5-6 dpp wild-type mice and cultures of their re-aggregated spermatogonia and Sertoli cells, FSH, RA [all-trans-retinoic acid (ATRA), AM580, 9-cis-RA] and NRG1 promoted spermatogonial proliferation and meiotic initiation. However, TAM treatment of testicular organ cultures from the Nrg1Ser–/– mutants suppressed spermatogonial proliferation and meiotic initiation that was promoted by FSH or AM580. In re-aggregated cultures of purified spermatogonia, NRG1, NRG3, ATRA and 9-cis-RA promoted their proliferation and meiotic initiation, but neither AM580 nor FSH did. In addition, FSH, RAs and NRG1 promoted Nrg1 and Nrg3 mRNA expression in Sertoli cells. These results indicate that in juvenile testes RA and FSH induced meiosis indirectly through Sertoli cells when NRG1 and NRG3 were upregulated, as NRG1 amplified itself and NRG3. The amplified NRG1 and NRG3 directly induced meiosis in spermatogonia. In addition, ATRA and 9-cis-RA activated spermatogonia directly and promoted their proliferation and eventually meiotic initiation.

Double-strand breaks and the concept of short- and long-term epigenetic memory

Double-strand breaks represent an extremely cytolethal form of DNA damage and thus pose a serious threat to the preservation of genetic and epigenetic information. Though it is well-known that double-strand breaks such as those generated by ionising radiation are among the principal causative factors behind mutations, chromosomal aberrations, genetic instability and carcinogenesis, significantly less is known about the epigenetic consequences of double-strand break formation and repair for carcinogenesis. Double-strand break repair is a highly coordinated process that requires the unravelling of the compacted chromatin structure to facilitate repair machinery access and then restoration of the original undamaged chromatin state. Recent experimental findings have pointed to a potential mechanism for double-strand break-induced epigenetic silencing. This review will discuss some of the key epigenetic regulatory processes involved in double-strand break (DSB) repair and how incomplete or incorrect restoration of chromatin structure can leave a DSB-induced epigenetic memory of damage with potentially pathological repercussions.

Expression of double strand DNA breaks repair genes in pterygium

PURPOSE

The alteration in the expression of some genes and proteins responsible for chosen DNA repair pathways in pterygium pathogenesis were studied. This study was focused on the examination of the expression of genes and RAD50 protein taking part in homologous recombination.

METHODS

Peripheral blood lymphocytes, samples of pterygium tissue, samples of conjunctiva of patients suffering from pterygium as well as peripheral blood lymphocytes and conjunctiva of patients from the control group were examined. In order to identify genes products from RNA, Ribonuclease Protection Assai method was applied. LIM15, RAD50, RAD54, RAD52, MRE11, XRCC2, XRCC3, RAD51, RAD51B, RAD51C, RAD51D genes transcripts were detected. Expression of RAD50 protein was analyzed immunohistochemically.

RESULTS

Peripheral blood lymphocytes analyses revealed lower level of RAD50 gene expression in the pterygium patients compared to the control group and the increased expression of XRCC2, XRCC3 and RAD51 genes in patients with pterygium, who declared the recurrence of the lesion in comparison to the patients with primary pterygium. Lower expression of the RAD54 gene in pterygium tissue comparing to conjunctiva from the eyes with pterygium was found. An expression of RAD50 gene in the conjunctiva originating from eyes with pterygium in comparison to the conjunctiva of control group was shown to be considerably higher. Expression of RAD50 protein in pterygium squamous epithelial cells was significantly higher than in conjunctiva from control group.

CONCLUSION

There may exist a relationship between pterygium pathogenesis and damages of double strand DNA, however, the elucidation of its exact nature needs further study.

DNA damage stress response in germ cells: role of c-Abl and clinical implications

Cells experiencing DNA damage undergo a complex response entailing cell-cycle arrest, DNA repair and apoptosis, the relative importance of the three being modulated by the extent of the lesion. The observation that Abl interacts in the nucleus with several proteins involved in different aspects of DNA repair has led to the hypothesis that this kinase is part of the damage-sensing mechanism. However, the mechanistic details underlying the role of Abl in DNA repair remain unclear. Here, I will review the evidence supporting our current understanding of Abl activation following DNA insults, while focusing on the relevance of these mechanisms in protecting DNA-injured germ cells. Early studies have shown that Abl transcripts are highly expressed in the germ line. Abl-deficient mice exhibit multiple abnormalities, increased perinatal mortality and reduced fertility. Recent findings have implicated Abl in a cisplatin-induced signaling pathway eliciting death of immature oocytes. A p53-related protein, TAp63, is an important immediate downstream effector of this pathway. Of note, pharmacological inhibition of Abl protects the ovarian reserve from the toxic effects of cisplatin. This suggests that the extent of Abl catalytic outputs may shift the balance between survival (likely through DNA repair) and activation of a death response. Taken together, these observations are consistent with the evolutionary conserved relationship between DNA damage and activation of the p53 family of transcription factors, while shedding light on the key role of Abl in dictating the fate of germ cells upon genotoxic insults.

Ahp2 (Hop2) function in Arabidopsis thaliana (Ler) is required for stabilization of close alignment and synaptonemal complex formation except for the two short arms that contain nucleolus organizer regions

A cytological comparative analysis of male meiocytes was performed for Arabidopsis wild type and the ahp2 (hop2) mutant with emphasis on ahp2's largely uncharacterized prophase I. Leptotene progression appeared normal in ahp2 meiocytes; chromosomes exhibited regular axis formation and assumed a typical polarized nuclear organization. In contrast, 4',6'-diamidino-2-phenylindole-stained ahp2 pachytene chromosome spreads demonstrated a severe reduction in stabilized pairing. However, transmission electron microscopy (TEM) analysis of sections from meiocytes revealed that ahp2 chromosome axes underwent significant amounts of close alignment (44% of total axis). This apparent paradox strongly suggests that the Ahp2 protein is involved in the stabilization of homologous chromosome close alignment. Fluorescent in situ hybridization in combination with Zyp1 immunostaining revealed that ahp2 mutants undergo homologous synapsis of the nucleolus-organizer-region-bearing short arms of chromosomes 2 and 4, despite the otherwise "nucleus-wide" lack of stabilized pairing. The duration of ahp2 zygotene was significantly prolonged and is most likely due to difficulties in chromosome alignment stabilization and subsequent synaptonemal complex formation. Ahp2 and Mnd1 proteins have previously been shown, "in vitro," to form a heterodimer. Here we show, "in situ," that the Ahp2 and Mnd1 proteins are synchronous in their appearance and disappearance from meiotic chromosomes. Both the Ahp2 and Mnd1 proteins localize along the chromosomal axis. However, localization of the Ahp2 protein was entirely foci-based whereas Mnd1 protein exhibited an immunostaining pattern with some foci along the axis and a diffuse staining for the rest of the chromosome.

Detection and quantification of DNA strand breaks using the ROPS (random oligonucleotide primed synthesis) assay

DNA double strand breaks (DSBs) arise from spontaneous DNA damage due to metabolic activities or from direct and indirect damaging effects of stress. DSBs are also formed transiently during such processes as replication, transcription, and DNA repair. The level of DSBs positively correlates with the activities of homologous and nonhomologous DNA repair pathways, which in turn inversely correlate with methylation levels and chromatin structure. Thus, measurement of strand breaks can provide an informative picture of genome stability of a given cell. The use of random oligonucleotide-primed synthesis for the analysis of DSB levels is described. Applications of the assay for quantitative detection of 3'OH, 3'P, or DNA strand breaks at a cleavage site of the deoxyribose residue are discussed.

MOF and histone H4 acetylation at lysine 16 are critical for DNA damage response and double-strand break repair

The human MOF gene encodes a protein that specifically acetylates histone H4 at lysine 16 (H4K16ac). Here we show that reduced levels of H4K16ac correlate with a defective DNA damage response (DDR) and double-strand break (DSB) repair to ionizing radiation (IR). The defect, however, is not due to altered expression of proteins involved in DDR. Abrogation of IR-induced DDR by MOF depletion is inhibited by blocking H4K16ac deacetylation. MOF was found to be associated with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), a protein involved in nonhomologous end-joining (NHEJ) repair. ATM-dependent IR-induced phosphorylation of DNA-PKcs was also abrogated in MOF-depleted cells. Our data indicate that MOF depletion greatly decreased DNA double-strand break repair by both NHEJ and homologous recombination (HR). In addition, MOF activity was associated with general chromatin upon DNA damage and colocalized with the synaptonemal complex in male meiocytes. We propose that MOF, through H4K16ac (histone code), has a critical role at multiple stages in the cellular DNA damage response and DSB repair.

Multiple roles of BRIT1/MCPH1 in DNA damage response, DNA repair, and cancer suppression

Mammalian cells are frequently at risk of DNA damage from both endogenous and exogenous sources. Accordingly, cells have evolved the DNA damage response (DDR) pathways to monitor and assure the integrity of their genome. In cells, the intact and effective DDR is essential for the maintenance of genomic stability and it acts as a critical barrier to suppress the development of cancer in humans. Two central kinases for the DDR pathway are ATM and ATR, which can phosphorylate and activate many downstream proteins for cell cycle arrest, DNA repair, or apoptosis if the damages are irreparable. In the last several years, we and others have made significant progress to this field by identifying BRIT1 (also known as MCPH1) as a novel key regulator in the DDR pathway. BRIT1 protein contains 3 breast cancer carboxyl terminal (BRCT) domains which are conserved in BRCA1, MDC1, 53BP1, and other important molecules involved in DNA damage signaling, DNA repair, and tumor suppression. Our in vitro studies revealed BRIT1 to be a chromatinbinding protein required for recruitment of many important DDR proteins (ATM, MDC1, NBS1, RAD51, BRCA2) to the DNA damage sites. We recently also generated the BRIT1 knockout mice and demonstrated its essential roles in homologous recombination DNA repair and in maintaining genomic stability in vivo. In humans, BRIT1 is located on chromosome 8p23.1, where loss of hetero-zigosity is very common in many types of cancer. In this review, we will summarize the novel roles of BRIT1 in DDR, describe the relationship of BRIT1 deficiency with cancer development, and also discuss the use of synthetic lethality approach to target cancers with HR defects due to BRIT1 deficiency.

BRIT1/MCPH1 is essential for mitotic and meiotic recombination DNA repair and maintaining genomic stability in mice

BRIT1 protein (also known as MCPH1) contains 3 BRCT domains which are conserved in BRCA1, BRCA2, and other important molecules involved in DNA damage signaling, DNA repair, and tumor suppression. BRIT1 mutations or aberrant expression are found in primary microcephaly patients as well as in cancer patients. Recent in vitro studies suggest that BRIT1/MCPH1 functions as a novel key regulator in the DNA damage response pathways. To investigate its physiological role and dissect the underlying mechanisms, we generated BRIT1^−/− mice and identified its essential roles in mitotic and meiotic recombination DNA repair and in maintaining genomic stability. Both BRIT1^−/− mice and mouse embryonic fibroblasts (MEFs) were hypersensitive to γ-irradiation. BRIT1^−/− MEFs and T lymphocytes exhibited severe chromatid breaks and reduced RAD51 foci formation after irradiation. Notably, BRIT1^−/− mice were infertile and meiotic homologous recombination was impaired. BRIT1-deficient spermatocytes exhibited a failure of chromosomal synapsis, and meiosis was arrested at late zygotene of prophase I accompanied by apoptosis. In mutant spermatocytes, DNA double-strand breaks (DSBs) were formed, but localization of RAD51 or BRCA2 to meiotic chromosomes was severely impaired. In addition, we found that BRIT1 could bind to RAD51/BRCA2 complexes and that, in the absence of BRIT1, recruitment of RAD51 and BRCA2 to chromatin was reduced while their protein levels were not altered, indicating that BRIT1 is involved in mediating recruitment of RAD51/BRCA2 to the damage site. Collectively, our BRIT1-null mouse model demonstrates that BRIT1 is essential for maintaining genomic stability in vivo to protect the hosts from both programmed and irradiation-induced DNA damages, and its depletion causes a failure in both mitotic and meiotic recombination DNA repair via impairing RAD51/BRCA2's function and as a result leads to infertility and genomic instability in mice.

The repair of DNA breaks in cells is critical for maintaining genomic integrity and suppressing tumor development. DNA breaks can arise from exogenous agents such as ionizing radiation (IR) or can form during the process of germ cell (sperm and egg) generation. BRIT1 protein (also known as MCPH1) is a recently identified DNA damage responding protein, and its mutations or reduced expression are found in primary microcephaly (small brain) patients, as well as in cancer patients. To investigate BRIT1's physiological functions and dissect the underlying molecular mechanism, we used a genetic approach (gene targeting technology) to delete BRIT1 gene in mice and generated a mouse model with BRIT1 deficiency (called BRIT1-knockout mice). Here, we showed that BRIT1 knockout mice are more sensitive to IR due to their inability to repair the IR-induced DNA breaks. These mice are also infertile, and their DNA repair during the process of germ cell generation was impaired substantially. Thus, in this study, we generated a novel mouse model (BRIT1 knockout mice) with striking phenotypes related to defective DNA repair and clearly demonstrated the essential role of BRIT1 in DNA repair at organism level.

The DNA-damage response in human biology and disease

The prime objective for every life form is to deliver its genetic material, intact and unchanged, to the next generation. This must be achieved despite constant assaults by endogenous and environmental agents on the DNA. To counter this threat, life has evolved several systems to detect DNA damage, signal its presence and mediate its repair. Such responses, which have an impact on a wide range of cellular events, are biologically significant because they prevent diverse human diseases. Our improving understanding of DNA-damage responses is providing new avenues for disease management.

Predicting meiotic pathways in human fetal oogenesis

Gene function prediction has proven valuable in formulating testable hypotheses. It is particularly useful for exploring biological processes that are experimentally intractable, such as meiotic initiation and progression in the human fetal ovary. In this study, we developed the first functional gene network for the human fetal ovary, HFOnet, by probabilistically integrating multiple genomic features using a naïve Bayesian model. We demonstrated that this network could accurately recapture known functional connections between genes, as well as predict new connections. Our findings suggest that known meiosis-specific genes (i.e., with functions only in meiotic processes in the germ cells) make either no or a few functional connections but are highly clustered with neighbor genes. In contrast, known nonspecific meiotic genes (i.e., with functions in both meiotic and nonmeiotic processes in the germ cells and somatic cells) exhibit numerous connections but low clustering coefficients, indicating their role as central modulators of diverse pathways, including those in meiosis. We also predicted novel genes that may be involved in meiotic initiation and DNA repair. This global functional network provides a much-needed framework for exploring gene functions and pathway components in early human female meiosis that are difficult to tackle by traditional in vivo mammalian genetics.

Division of labor: DNA repair and the cell cycle specific functions of the Mre11 complex

Genomic integrity is maintained via the concerted action of proteins that coordinate and control DNA replication and those that respond to DNA damage. The Mre11 complex is a mediator of the DNA damage response through its functions in DNA double strand break (DSB) sensing, checkpoint activation and recombinational DNA repair. The complex responds to mitotic and meiotic DSBs, and is also activated in cells experiencing DNA replication stress. The Mre11 complex's role in recombinational repair primarily concerns the promotion of homologous recombination (HR), but it is also implicated in non-homologous end joining (NHEJ)--a DSB repair mechanism prevalent in non-dividing cells. We recently characterized deletion of the Mre11 complex member, Rad50, in a number of postmitotic and proliferative tissues of the mouse. These studies indicated that the complex is dispensable in postmitotic tissues, but loss of Rad50 in proliferating cells resulted in accumulation of unrepaired, DNA replication-dependent lesions. The data suggest that the Mre11 complex is not a major contributor to NHEJ and support the interpretation that its role in recombinational DNA repair is largely restricted to dividing cells, in which repair involving sister chromatids predominates. An exception to this concept is manifest in previous work from our laboratory revealing that the mammalian Mre11 complex promotes meiotic DSB repair, an event involving recombination between sister chromatids of homologous chromosomes and taking place in cells not undergoing replication. Together these studies highlight the importance of cell cycle and cell type specific modulation of the Mre11 complex's repair activities in vivo.

Tetrahymena meiotic nuclear reorganization is induced by a checkpoint kinase-dependent response to DNA damage

In the ciliate Tetrahymena, meiotic micronuclei (MICs) undergo extreme elongation, and meiotic pairing and recombination take place within these elongated nuclei (the "crescents"). We have previously shown that elongation does not occur in the absence of Spo11p-induced DNA double-strand breaks (DSBs). Here we show that elongation is restored in spo11Delta mutants by various DNA-damaging agents including ones that may not cause DSBs to a notable extent. MIC elongation following Spo11p-induced DSBs or artificially induced DNA lesions is probably a DNA-damage response mediated by a phosphokinase signal transduction pathway, since it is suppressed by the ATM/ATR kinase inhibitors caffeine and wortmannin and by knocking out Tetrahymena's ATR orthologue. MIC elongation occurs concomitantly with the movement of centromeres away from the telomeric pole of the MIC. This DNA damage-dependent reorganization of the MIC helps to arrange homologous chromosomes alongside each other but is not sufficient for exact pairing. Thus, Spo11p contributes to bivalent formation in two ways: by creating a favorable spatial disposition of homologues and by stabilizing pairing by crossovers. The polarized chromosome orientation inside the crescent resembles the conserved meiotic bouquet, and crescent and bouquet also share the putative function of aiding meiotic pairing. However, they are regulated differently because in Tetrahymena, DSBs are required for entering rather than exiting this stage.

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.

Detecting ATM-dependent chromatin modification in DNA damage and heat shock response

The ataxia telangiectasia-mutated gene product (ATM), whose loss of function is responsible for ataxia telangiectasia (A-T), is a protein kinase that interacts with several substrates and is implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination, cell-cycle control and telomere maintenance (Pandita, Expert Reviews in Molecular Medicine 5:1-21, 2003; Pandita, Oncogene 21:611-618, 2002; Matsuoka et al., Science 316:1160-1166, 2007). The ATM protein kinase is primarily activated in response to DNA double-strand breaks (DSBs) caused by ionizing radiation (IR) or radiomimetic drugs (Pandita et al., Oncogene 19:1386-1391, 2000). ATM is also activated by heat shock, which occurs independent of DNA damage (Hunt et al., Can Res 69:3010-3017, 2007). ATM is observed at the sites of DNA damage, where it is autophosphorylated and is dissociated from its non-active dimeric form to the active monomeric form (Bakkenist and Kastan, Nature 421:499-506, 2003). The ATM protein appears to be a part of the sensory machinery that detects DSBs during meiosis or mitosis, or breaks consequent to the damage by free radicals. Recent studies support the argument that ATM activation is regulated by chromatin modifications (Gupta, Mol Cell Biol 25:5292-5305, 2005). This review summarizes the multiple approaches used to discern the role of ATM in chromatin modification in response to DNA damage as well as heat shock.

Stra8 and its inducer, retinoic acid, regulate meiotic initiation in both spermatogenesis and oogenesis in mice

In eukaryotes, diploid cells give rise to haploid cells via meiosis, a program of two cell divisions preceded by one round of DNA replication. Although key molecular components of the meiotic apparatus are highly conserved among eukaryotes, the mechanisms responsible for initiating the meiotic program have diverged substantially among eukaryotes. This raises a related question in animals with two distinct sexes: Within a given species, are similar or different mechanisms of meiotic initiation used in the male and female germ lines? In mammals, this question is underscored by dramatic differences in the timing of meiotic initiation in males and females. Stra8 is a vertebrate-specific, cytoplasmic factor expressed by germ cells in response to retinoic acid. We previously demonstrated that Stra8 gene function is required for meiotic initiation in mouse embryonic ovaries. Here we report that, on an inbred C57BL/6 genetic background, the same factor is also required for meiotic initiation in germ cells of juvenile mouse testes. In juvenile C57BL/6 males lacking Stra8 gene function, the early mitotic development of germ cells appears to be undisturbed. However, these cells then fail to undergo the morphological changes that define meiotic prophase, and they do not display the molecular hallmarks of meiotic chromosome cohesion, synapsis and recombination. We conclude that, in mice, Stra8 regulates meiotic initiation in both spermatogenesis and oogenesis. Taken together with previous observations, our present findings indicate that, in both the male and female germ lines, meiosis is initiated through retinoic acid induction of Stra8.

Adriamycin induces H2AX phosphorylation in human spermatozoa

AIM

To investigate whether adriamycin induces DNA damage and the formation of gammaH2AX (the phosphorylated form of histone H2AX) foci in mature spermatozoa.

METHODS

Human spermatozoa were treated with adriamycin at different concentrations. gammaH2AX was analyzed by immunofluorescent staining and flow cytometry and double-strand breaks (DSB) were detected by the comet assay.

RESULTS

The neutral comet assay revealed that the treatment with adriamycin at 2 microg/mL for different times (0.5, 2, 8 and 24 h), or for 8 h at different concentrations (0.4, 2 and 10 microg/mL), induced significant DSB in spermatozoa. Immunofluorent staining and flow cytometry showed that the expression of gH2AX was increased in a dose-dependent and time-dependant manner after the treatment of adriamycin. Adriamycin also induced the concurrent appearance of DNA maintenance/repair proteins RAD50 and 53BP1 with gammaH2AX in spermatozoa. Wortmannin, an inhibitor of the phosphatidylinositol 3-kinase (PI3K) family, abolished the co-appearance of these two proteins with gammaH2AX.

CONCLUSION

Human mature spermatozoa have the same response to DSB-induced H2AX phosphorylation and subsequent recruitment of DNA maintenance/repair proteins as somatic cells.

ZIP4H (TEX11) deficiency in the mouse impairs meiotic double strand break repair and the regulation of crossing over

We have recently shown that hypomorphic Mre11 complex mouse mutants exhibit defects in the repair of meiotic double strand breaks (DSBs). This is associated with perturbation of synaptonemal complex morphogenesis, repair and regulation of crossover formation. To further assess the Mre11 complex's role in meiotic progression, we identified testis-specific NBS1-interacting proteins via two-hybrid screening in yeast. In this screen, Zip4h (Tex11), a male germ cell specific X-linked gene was isolated. Based on sequence and predicted structural similarity to the S. cerevisiae and A. thaliana Zip4 orthologs, ZIP4H appears to be the mammalian ortholog. In S. cerevisiae and A. thaliana, Zip4 is a meiosis-specific protein that regulates the level of meiotic crossovers, thus influencing homologous chromosome segregation in these organisms. As is true for hypomorphic Nbs1 (Nbs1^ΔB/ΔB) mice, Zip4h^−/Y mutant mice were fertile. Analysis of spermatocytes revealed a delay in meiotic double strand break repair and decreased crossover formation as inferred from DMC1 and MLH1 staining patterns, respectively. Achiasmate chromosomes at the first meiotic division were also observed in Zip4h^−/Y mutants, consistent with the observed reduction in MLH1 focus formation. These results indicate that meiotic functions of Zip4 family members are conserved and support the view that the Mre11 complex and ZIP4H interact functionally during the execution of the meiotic program in mammals.

The process of meiosis is initiated by the formation of programmed DNA double strand breaks. The subsequent repair of these breaks is critical to the formation of egg and sperm in mammals. Numerous proteins that function in repair in somatic cells also have essential activities during meiotic repair. Examples of such proteins include the Mre11 complex, which is a detector of double strand breaks and plays a role in promoting their repair, as well as the kinase ATM, which governs cell cycle checkpoint responses in somatic cells. In this study, we report the isolation of a human, testis-specific Mre11 complex interacting protein, ZIP4H, and the establishment of a ZIP4H-deficient mouse. Sequence similarity, and phenotypic characterization of mice harboring a mutation in this gene indicate it is the mammalian ortholog of the meiosis-specific Zip4 proteins of S. cerevisiae and A. thaliana. Like its budding yeast and plant counterparts, mouse ZIP4H is required for efficient crossover formation between homologs. In the future, understanding the relationship between the Mre11 complex and ZIP4H will provide new insight to how Mre11 complex activities are adapted to the specialized framework of meiotic double strand break repair.

Differential contribution of poly(ADP-ribose)polymerase-1 and -2 (PARP-1 and -2) to the poly(ADP-ribosyl)ation reaction in rat primary spermatocytes

Poly(ADP-ribose)polymerases (PARP-1 and -2) are activated by DNA strand breaks to synthesize protein-bound ADP-ribose polymers from NAD+. The two enzymes are overexpressed in rat spermatocytes and are likely to play a role in meiosis. Indeed parp-2-/- mice, but not parp-1 knockouts, show hypofertility. Aside, PARP-1 and PARP-2 are both involved in DNA damage repair and signalling, but their relative contributions to such processes remain as yet unknown, largely because of the lack of PARP isoform-specific inhibitors that has precluded in vivo studies. Here, we used permeabilized rat primary spermatocytes or isolated spermatocyte nuclei and radiolabelled NAD+ to investigate potential isoform-specific effects on basic features of the poly(ADP-ribosyl)ation reaction, including size of ADP-ribose polymers at different NAD+ concentrations, extent of auto- versus etheromodification, and modulation of such reactions by the PARP inhibitor, PJ34. We found that PARP-1 automodification prevailed over PARP-2 modification. In addition, over 50% of cellular poly(ADP-ribose) was covalently bound to histones H1 and H2. The inhibitory effect of PJ34 appeared to be targeted mainly to the elongation step of the reaction. We propose that a different propensity of PARP-1 and PARP-2 to undergo automodification and/or catalyze etheromodification, both in terms of number of enzyme molecules being involved and amount of bound poly(ADP-ribose), may underlie distinct roles in the regulation of spermatocyte functions.

Apoptosis in mouse fetal and neonatal oocytes during meiotic prophase one

Background

The vast majority of oocytes formed in the fetal ovary do not survive beyond birth. Possible reasons for their loss include the elimination of non-viable genetic constitutions arising through meiosis, however, the precise relationship between meiotic stages and prenatal apoptosis of oocytes remains elusive. We studied oocytes in mouse fetal and neonatal ovaries, 14.5–21 days post coitum, to examine the relationship between oocyte development and programmed cell death during meiotic prophase I.

Results

Microspreads of fetal and neonatal ovarian cells underwent immunocytochemistry for meiosis- and apoptosis-related markers. COR-1 (meiosis-specific) highlighted axial elements of the synaptonemal complex and allowed definitive identification of the stages of meiotic prophase I. Labelling for cleaved poly-(ADP-ribose) polymerase (PARP-1), an inactivated DNA repair protein, indicated apoptosis. The same oocytes were then labelled for DNA double strand breaks (DSBs) using TUNEL. 1960 oocytes produced analysable results.

Oocytes at all stages of meiotic prophase I stained for cleaved PARP-1 and/or TUNEL, or neither. Oocytes with fragmented (19.8%) or compressed (21.2%) axial elements showed slight but significant differences in staining for cleaved PARP-1 and TUNEL to those with intact elements. However, fragmentation of axial elements alone was not a good indicator of cell demise. Cleaved PARP-1 and TUNEL staining were not necessarily coincident, showing that TUNEL is not a reliable marker of apoptosis in oocytes.

Conclusion

Our data indicate that apoptosis can occur throughout meiotic prophase I in mouse fetal and early postnatal oocytes, with greatest incidence at the diplotene stage. Careful selection of appropriate markers for oocyte apoptosis is essential.

A role for the HOXB7 homeodomain protein in DNA repair

Homeobox genes encode transcription factors which function in body axis patterning in the developing embryo. Recent evidence suggests that the maintenance of specific HOX expression patterns is necessary for regulating the homeostasis of adult tissues as well. In this study, HOXB7 transformed human mammary epithelial cells, MCF10A, to grow in minimally supplemented medium, to form colonies in Matrigel, and display resistance to ionizing radiation. Searching for protein partners of HOXB7 that might contribute to resistance to ionizing radiation, we identified four HOXB7-binding proteins by GST pull-down/affinity chromatography and confirmed their interactions by coimmunoprecipitation in vivo. Interestingly, all four HOXB7-binding proteins shared functions as genomic caretakers and included members of the DNA-dependent protein kinase holoenzyme (Ku70, Ku80, DNA-PK(cs)) responsible for DNA double-strand break repair by nonhomologous end joining pathway and poly(ADP) ribose polymerase. Exogenous and endogenous expression of HOXB7 enhanced nonhomologous end joining and DNA repair functions in vitro and in vivo, which were reversed by silencing HOXB7. This is the first mechanistic study providing definitive evidence for the involvement of any HOX protein in DNA double-strand break repair.

Evolution of DNA strand-breaks in cultured spermatocytes: The Comet Assay reveals differences in normal and γ-irradiated germ cells

In reproductive toxicity assessment, in vitro systems can be used to determine mechanisms of action of toxicants. However, they generally investigate the immediate effects of toxicants, on isolated germ cells or spermatozoa. We report here the usefulness of in vitro cultures of rat spermatocytes and Sertoli cells, in conjunction with the Comet Assay to analyze the evolution of DNA strand-breaks and thus to determine DNA damage in germ cells. We compared cultures of normal and gamma-irradiated germ cells. In non-irradiated spermatocytes, the Comet Assay revealed the presence of DNA strand-breaks, which numbers decreased with the duration of the culture, suggesting the involvement of DNA repair mechanisms related to the meiotic recombination. In irradiated cells, the evolution of DNA strand-breaks was strongly modified. Thus our model is able to detect genotoxic lesions and/or DNA repair impairment in cultured spermatocytes. We propose this model as an in vitro tool for the study of genotoxic injuries on spermatocytes.

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.

Cellular response to DNA damage

Eukaryotic cells, from yeast to man, possess evolutionarily conserved mechanisms to accurately and efficiently repair the overwhelming majority of DNA damage, thereby ensuring genomic integrity. Important repair pathways include base excision repair, nucleotide excision repair, mismatch repair, non-homologous end-joining, and homologous recombination. Defects in DNA repair processes generally result in susceptibility to cancer and, often, abnormalities in multiple organ systems. While signal transduction pathways have been intensely studied, epigenetic changes occurring in response to DNA damage are rapidly increasing in importance. Effective radiation and chemotherapy sensitization could result from selective inhibition of DNA repair in tumor cells. DNA damage repair is a dynamic field of research where the fruits of basic research often have important clinical implications.