Highly efficient base excision repair (BER) in human and rat male germ cells

Characterization of dUTPase expression in mouse postnatal development and adult neurogenesis

The enzyme dUTPase has an essential role in maintaining genomic integrity. In mouse, nuclear and mitochondrial isoforms of the enzyme have been described. Here we present the isoform-specific mRNA expression levels in different murine organs during development using RT-qPCR. In this study, we analyzed organs of 14.5-day embryos and of postnatal 2-, 4-, 10-week- and 13-month-old mice. We demonstrate organ-, sex- and developmental stage-specific differences in the mRNA expression levels of both isoforms. We found high mRNA expression level of the nuclear isoform in the embryo brain, and the expression level remained relatively high in the adult brain as well. This was surprising, since dUTPase is known to play an important role in proliferating cells, and mass production of neural cells is completed by adulthood. Thus, we investigated the pattern of the dUTPase protein expression specifically in the adult brain with immunostaining and found that dUTPase is present in the germinative zones, the subventricular and the subgranular zones, where neurogenesis occurs and in the rostral migratory stream where neuroblasts migrate to the olfactory bulb. These novel findings suggest that dUTPase may have a role in cell differentiation and indicate that accurate dTTP biosynthesis can be vital, especially in neurogenesis.

Measuring DNA modifications with the comet assay: a compendium of protocols

The comet assay is a versatile method to detect nuclear DNA damage in individual eukaryotic cells, from yeast to human. The types of damage detected encompass DNA strand breaks and alkali-labile sites (e.g., apurinic/apyrimidinic sites), alkylated and oxidized nucleobases, DNA-DNA crosslinks, UV-induced cyclobutane pyrimidine dimers and some chemically induced DNA adducts. Depending on the specimen type, there are important modifications to the comet assay protocol to avoid the formation of additional DNA damage during the processing of samples and to ensure sufficient sensitivity to detect differences in damage levels between sample groups. Various applications of the comet assay have been validated by research groups in academia, industry and regulatory agencies, and its strengths are highlighted by the adoption of the comet assay as an in vivo test for genotoxicity in animal organs by the Organisation for Economic Co-operation and Development. The present document includes a series of consensus protocols that describe the application of the comet assay to a wide variety of cell types, species and types of DNA damage, thereby demonstrating its versatility.

Assessing testicular germ cell DNA damage in the comet assay; introduction of a proof-of-concept

The in vivo comet assay is widely used to measure genotoxicity; however, the current OECD test guideline (TG 489) does not recommend using the assay to assess testicular germ cells, due to the presence of testicular somatic cells. An adapted approach to specifically assess testicular germ cells within the comet assay is certainly warranted, considering regulatory needs for germ cell-specific genotoxicity data in relation to the increasing global production of and exposure to potentially hazardous chemicals. Here, we provide a proof-of-concept to selectively analyze round spermatids and primary spermatocytes, distinguishing them from other cells of the testicle. Utilizing the comet assay recordings of DNA content (total fluorescence intensity) and DNA damage (% tail intensity) of individual comets, we developed a framework to distinguish testicular cell populations based on differences in DNA content/ploidy and appearance. Haploid round spermatid comets are identified through (1) visual inspection of DNA content distributions, (2) setting DNA content thresholds, and (3) modeling DNA content distributions using a normal mixture distribution function. We also describe an approach to distinguish primary spermatocytes during comet scoring, based on their high DNA content and large physical size. Our concept allows both somatic and germ cells to be analyzed in the same animal, adding a versatile, sensitive, rapid, and resource-efficient assay to the limited genotoxicity assessment toolbox for germ cells. An adaptation of TG 489 facilitates accumulation of valuable information regarding distribution of substances to germ cells and their potential for inducing germ cell gene mutations and structural chromosomal aberrations.

Sperm DNA integrity does play a crucial role for embryo development after ICSI, notably when good-quality oocytes from young donors are used

Based on the inconsistent literature published thus far involving infertile patients, whether intracytoplasmic sperm injection (ICSI) allows overcoming total fertilization failure due to sperm DNA fragmentation is still unclear. Related to this, female factors, which may have a significant impact on assisted reproduction outcomes, can mask male infertility. In this scenario, evaluating ICSI outcomes following cycles using healthy donor gametes could shed light on this realm, as it would avoid the influence of (un)known confounding factors present in infertile individuals. The present work, therefore, aimed to address whether single- and double-stranded sperm DNA fragmentation leads to impaired ICSI outcomes in double gamete donation cycles. The study also compared these double-gamete donation cycles to cycles in which only sperm were donated and oocytes were obtained from infertile patients. Two cohorts were included: (a) the Donor-Donor (DD) cohort, which included 27 semen donor samples used in 49 ICSI cycles with young healthy oocyte donors; and (b) the Donor-Infertile (DI) cohort, which involved 34 semen donor samples used in 57 ICSI cycles with oocytes from patients. Single- and double-stranded sperm DNA breaks were determined with alkaline and neutral Comet assays, respectively; ICSI was conducted following standard protocols and embryos were monitored through time-lapse microscopy. In the DD cohort, the percentage of sperm with high overall DNA damage correlated with fertilization rates (Rs = - 0.666; P < 0.001) and with the percentage of blastocysts per injected oocyte (Rs = - 0.414; P = 0.040). In addition, sperm DNA damage delayed the first embryo division (Rs = 0.421; P = 0.036), and development from the 8-cell to the morula stage (Rs = 0.424; P = 0.034). In contrast, double-stranded DNA breaks had no effect in this cohort. As far as the DI cohort is concerned, while overall sperm DNA damage was not found to be correlated to fertilization or blastocyst rates, pronuclei formation following ICSI was delayed when the incidence of double-stranded DNA breaks was high (Rs = 0.485; P = 0.005). In conclusion, this study, which is the first involving double donation cycles (i.e., a donor-donor cohort), supports that sperm DNA damage has a detrimental impact on fertilization rates after ICSI, and delays embryo development. Moreover, the use of oocytes from infertile individuals is suggested to hide the male-factor effect.

OXIDATIVE STRESS AND REPRODUCTIVE FUNCTION: Reactive oxygen species in the mammalian pre-implantation embryo

In brief

Reactive oxygen species are generated throughout the pre-implantation period and are necessary for normal embryo formation. However, at pathological levels, they result in reduced embryo viability which can be mediated through factors delivered by sperm and eggs at conception or from the external environment.

Abstract

Reactive oxygen species (ROS) occur naturally in pre-implantation embryos as a by-product of ATP generation through oxidative phosphorylation and enzymes such as NADPH oxidase and xanthine oxidase. Biological concentrations of ROS are required for crucial embryonic events such as pronuclear formation, first cleavage and cell proliferation. However, high concentrations of ROS are detrimental to embryo development, resulting in embryo arrest, increased DNA damage and modification of gene expression leading to aberrant fetal growth and health. In vivo embryos are protected against oxidative stress by oxygen scavengers present in follicular and oviductal fluids, while in vitro, embryos rely on their own antioxidant defence mechanisms to protect against oxidative damage, including superoxide dismutase, catalase, glutathione and glutamylcysteine synthestase. Pre-implantation embryonic ROS originate from eggs, sperm and embryos themselves or from the external environment (i.e. in vitro culture system, obesity and ageing). This review examines the biological and pathological roles of ROS in the pre-implantation embryo, maternal and paternal origins of embryonic ROS, and from a clinical perspective, we comment on the growing interest in combating increased oxidative damage in the pre-implantation embryo through the addition of antioxidants.

DNA damage in preimplantation embryos and gametes: specification, clinical relevance and repair strategies

BACKGROUND

DNA damage is a hazard that affects all cells of the body. DNA-damage repair (DDR) mechanisms are in place to repair damage and restore cellular function, as are other damage-induced processes such as apoptosis, autophagy and senescence. The resilience of germ cells and embryos in response to DNA damage is less well studied compared with other cell types. Given that recent studies have described links between embryonic handling techniques and an increased likelihood of disease in post-natal life, an update is needed to summarize the sources of DNA damage in embryos and their capacity to repair it. In addition, numerous recent publications have detailed novel techniques for detecting and repairing DNA damage in embryos. This information is of interest to medical or scientific personnel who wish to obtain undamaged embryos for use in offspring generation by ART.

OBJECTIVE AND RATIONALE

This review aims to thoroughly discuss sources of DNA damage in male and female gametes and preimplantation embryos. Special consideration is given to current knowledge and limits in DNA damage detection and screening strategies. Finally, obstacles and future perspectives in clinical diagnosis and treatment (repair) of DNA damaged embryos are discussed.

SEARCH METHODS

Using PubMed and Google Scholar until May 2021, a comprehensive search for peer-reviewed original English-language articles was carried out using keywords relevant to the topic with no limits placed on time. Keywords included ‘DNA damage repair’, ‘gametes’, ‘sperm’, ‘oocyte’, ‘zygote’, ‘blastocyst’ and ‘embryo’. References from retrieved articles were also used to obtain additional articles. Literature on the sources and consequences of DNA damage on germ cells and embryos was also searched. Additional papers cited by primary references were included. Results from our own studies were included where relevant.

OUTCOMES

DNA damage in gametes and embryos can differ greatly based on the source and severity. This damage affects the development of the embryo and can lead to long-term health effects on offspring. DDR mechanisms can repair damage to a certain extent, but the factors that play a role in this process are numerous and altogether not well characterized. In this review, we describe the multifactorial origin of DNA damage in male and female gametes and in the embryo, and suggest screening strategies for the selection of healthy gametes and embryos. Furthermore, possible therapeutic solutions to decrease the frequency of DNA damaged gametes and embryos and eventually to repair DNA and increase mitochondrial quality in embryos before their implantation is discussed.

WIDER IMPLICATIONS

Understanding DNA damage in gametes and embryos is essential for the improvement of techniques that could enhance embryo implantation and pregnancy success. While our knowledge about DNA damage factors and regulatory mechanisms in cells has advanced greatly, the number of feasible practical techniques to avoid or repair damaged embryos remains scarce. Our intention is therefore to focus on strategies to obtain embryos with as little DNA damage as possible, which will impact reproductive biology research with particular significance for reproductive clinicians and embryologists.

Application of the comet assay for the evaluation of DNA damage in mature sperm

DNA integrity is considered an important parameter of semen quality and is of significant value as a predictor of male fertility. Currently, there are several methods that can assess sperm DNA integrity. One such assay is the comet assay, or single-cell gel electrophoresis, which is a simple, sensitive, reliable, quick and low-cost technique that is used for measuring DNA strand breaks and repair at the level of individual cells. Although the comet assay is usually performed with somatic cells from different organs, the assay has the ability to detect genotoxicity in germ cells at different stages of spermatogenesis. Since the ability of sperm to remove DNA damage differs between the stages, interpretation of the results is dependent on the cells used. In this paper we give an overview on the use and applications of the comet assay on mature sperm and its ability to detect sperm DNA damage in both animals and humans. Overall, it can be concluded that the presence in sperm of significantly damaged DNA, assessed by the comet assay, is related to male infertility and seems to reduce live births. Although there is some evidence that sperm DNA damage also has a long-term impact on offspring's health, this aspect of DNA damage in sperm is understudied and deserves further attention. In summary, the comet assay can be applied as a useful tool to study effects of genotoxic exposures on sperm DNA integrity in animals and humans.

Liquid chromatography-tandem mass spectrometry reveals an active response to DNA damage in human spermatozoa

OBJECTIVE

To investigate how endogenously elevated DNA fragmentation alters the human sperm proteome, and whether this fragmentation contributes to genomic deletions.

DESIGN

Research study.

SETTING

Commercial fertility clinic.

PATIENT(S)

Men with low (0%-4%, n = 7) or high (≥16%, n = 6) sperm DNA fragmentation, as assessed by terminal deoxynucleotidyl transferase dUTP nick end labeling assay.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Global sperm proteome, single-nucleotide polymorphism genotyping array.

RESULT(S)

A total of 78 significantly differentially abundant proteins (30 decreased, 48 increased) were observed in control vs. high DNA damage samples. DNA damage resulted in robust proteomic responses, including markers of oxidative stress and apoptosis, DNA damage repair proteins, and transcription/translation and protein turnover machinery. Several key sperm functional proteins were significantly decreased in ejaculates with high DNA damage. We were unable to substantiate a link between increased DNA fragmentation and genomic deletions in human spermatozoa.

CONCLUSION(S)

Developing human spermatozoa initiate an active transcriptional response to endogenous DNA damage, which manifests as alterations in the sperm proteome.

Association of XRCC1 and ERCC2 promoters' methylation with chromatin condensation and sperm DNA fragmentation in idiopathic oligoasthenoteratozoospermic men

The aim of the study was to investigate whether the promoter methylation of XRCC1 and ERCC2 genes is associated with sperm DNA fragmentation and chromatin condensation in idiopathic oligoasthenoteratozoospermic men. This study involved 77 infertile men with idiopathic oligoasthenoteratozoospermia and 51 normozoospermic controls. The methylight method, TUNEL assay and aniline blue staining were used for the evaluation of XRCC1 and ERCC2 genes' methylation, SDF and sperm chromatin condensation, respectively. SDF (p = .004) and XRCC1 methylation (p = .0056) were found to be significantly higher in men with idiopathic OAT than in the controls, while mature spermatozoa frequency was higher in controls as compared to infertile men (p < .0001). No significant association was found between SDF and methylation of XRCC1 and ERCC2 genes (p = .9277 and p = .8257, respectively). However, compared to the cut-off point obtained by receiver operating characteristic analysis, a significant association was found between SDF and XRCC1 methylation, positive and negative methylation groups, generated according to the cut-off value for XRCC1. XRCC1 methylation was found to have a significant effect on chromatin condensation (p = .0017). No significant difference was detected among ERCC2 methylation, male infertility and SDF. In conclusion, XRCC1 methylation may have a role in sperm chromatin condensation and idiopathic OAT.

MUTYH Deficiency Is Associated with Attenuated Pulmonary Fibrosis in a Bleomycin-Induced Model

Idiopathic pulmonary fibrosis (IPF) is a progressive, irreversible lung disease of unknown etiology with limited survival. IPF incidence and prevalence increase significantly with aging, which is associated with an age-related accumulation of oxidative DNA damage. The Mutyh gene is involved in the base excision repair (BER) system, which is critical for repairing the misincorporated adenine that is opposite to the oxidized guanine base, 8-oxoguanine, and maintaining the fidelity of DNA replication. We used Mutyh knockout mice and a bleomycin-induced pulmonary fibrosis model to test the effect of MUTYH deficiency on lesion progression. Unexpectedly, a much less severe lesion of pulmonary fibrosis was observed in Mutyh^−/− than in Mutyh^+/+mice, which was supported by assay on protein levels of TGF-β1 and both fibrotic markers, α-SMA and Vimentin, in pulmonary tissues of the model animals. Mechanically, MUTYH deficiency prevented the genomic DNA of pulmonary tissue cells from the buildup of single-strand breaks (SSBs) of DNA and maintained the integrity of mtDNA. Furthermore, increased mitochondrial dynamic regulation and mitophagy were detected in pulmonary tissues of the bleomycin-induced Mutyh^−/− model mice, which could reduce the pulmonary epithelial cell apoptosis. Our results suggested that MUTYH deficiency could even induce protective responses of pulmonary tissue under severe oxidative stress.

Highly elevated base excision repair pathway in primordial germ cells causes low base editing activity in chickens

Base editing technology enables the generation of precisely genome-modified animal models. In this study, we applied base editing to chicken, an important livestock animal in the fields of agriculture, nutrition, and research through primordial germ cell (PGC)-mediated germline transmission. Using this approach, we successfully produced two genome-modified chicken lines harboring mutations in the genes encoding ovotransferrin (TF) and myostatin (MSTN); however, only 55.5% and 35.7% of genome-modified chickens had the desired base substitutions in TF and MSTN, respectively. To explain the low base-editing activity, we performed molecular analysis to compare DNA repair pathways between PGCs and the chicken fibroblast cell line DF-1. The results revealed that base excision repair (BER)-related genes were significantly elevated in PGCs relative to DF-1 cells. Subsequent functional studies confirmed that the editing activity could be regulated by modulating the expression of uracil N-glycosylase (UNG), an upstream gene of the BER pathway. Collectively, our findings indicate that the distinct DNA repair property of chicken PGCs causes low editing activity during genome modification, however, modulation of BER functions could promote the production of genome-modified organisms with the desired genotypes.

A novel F-box domain containing cyclin F like gene is required for maintaining the genome stability and survival of chicken primordial germ cells

The stability and survival of germ cells are controlled by the germline-specific genes, however, such genes are less known in the avian species. Using a microarray-based the National Center for Biotechnology Information Gene Expression Omnibus dataset, we found an unigene (Gga.9721) that upregulated in the chicken primordial germ cells (PGCs). The unigene showed 97% identities with an uncharacterized chicken cyclin F like gene. The predicted chicken cyclin F like gene was further characterized through expression and regulation in the chicken PGCs. The sequence analysis revealed that the gene shows identities with cyclin F gene and contains an F-box domain. The expression of chicken cyclin F like was detected specifically in the gonads, PGCs, and germline cells. The knockdown of cyclin F like gene resulted in DNA damage and apoptosis in the PGCs. The genes related to stemness and germness were downregulated, whereas, genes related to apoptosis and DNA damage response were increased in the PGCs after the knockdown of chicken cyclin F like. We further observed that the Nanog homeobox controlled the transcriptional activity of chicken cyclin F like gene in PGCs. Collectively, the chicken cyclin F like gene, which is not reported in any other species, is required for maintaining the genome stability of germ cells.

Deficiency of X-ray repair cross-complementing group 1 in primordial germ cells contributes to male infertility

X-ray repair cross-complementing group 1 (Xrcc1), a key DNA repair gene, plays a vital role in maintaining genomic stability and is highly expressed in the early stages of spermatogenesis, but the exact functions remain elusive. Here we generated primordial germ cell-specific Xrcc1 knockout (cXrcc1^-/-) mice to elucidate the effects of Xrcc1 on spermatogenesis. We demonstrated that Xrcc1 deficiency results in infertility in male mice due to impaired spermatogenesis. We found that cXrcc1^-/- mice exhibited smaller size of testes as well as lower sperm concentration and motility than the wild-type mice. Mechanistically, we demonstrated that Xrcc1 deficiency in primordial germ cells induced elevated levels of reactive oxygen species, mitochondria dysfunction, apoptosis, and loss of stemness of spermatogonial stem cells (SSCs) in testes. In Xrcc1-deficienct SSCs, elevated oxidative stress and mitochondrial dysfunction could be partially reversed by treatment with the antioxidant N-acetylcysteine (NAC), whereas NAC treatment did not restore the fertility or ameliorate the apoptosis caused by loss of Xrcc1. Overall, our findings provided new insights into understanding the crucial role of Xrcc1 during spermatogenesis.-Xu, C., Xu, J., Ji, G., Liu, Q., Shao, W., Chen, Y., Gu, J., Weng, Z., Zhang, X., Wang, Y., Gu, A. Deficiency of X-ray repair cross-complementing group 1 in primordial germ cells contributes to male infertility.

PEX10, SIRPA-SIRPG, and SOX5 gene polymorphisms are strongly associated with nonobstructive azoospermia susceptibility

PURPOSE

Male infertility is a multifactorial syndrome encompassing a wide variety of disorders. A previous Chinese genome-wide single-nucleotide polymorphism (SNP) association studies have identified four SNPs (rs12097821 in PRMT6 gene, rs2477686 in PEX10 gene, rs6080550 in SIRPA-SIRPG, and rs10842262 in SOX5 gene) as being significantly associated with risk factors for nonobstructive azoospermia (NOA). However, the results were not fully repeated in later studies, which calls for further investigations.

METHODS

We here performed a case-control study in a central Chinese population to explore the association between the four SNPs and male infertility, which included 631 infertile men (NOA and oligozoospermia) and 720 healthy fertile men. The genotyping was performed using the polymerase chain reaction-restriction fragment length polymorphism and confirmed by sequencing.

RESULTS

The results showed that rs12097821 and rs10842262 were strongly associated with the risk of NOA but not total male infertility or oligozoospermia, while rs2477686 and rs6080550 were not associated with the risk of total male infertility, NOA, or oligozoospermia. To improve the statistical strength, a meta-analysis was conducted. The results suggested that rs2477686, rs6080550, and rs10842262 were significantly associated with male infertility, especially with NOA, while rs12097821 was only found to be associated with total male infertility.

CONCLUSIONS

Collectively, the rs2477686, rs6080550, and rs10842262 may indeed be the genetic risk factors for NOA, which requires further investigation using larger independent sets of samples in different ethnic populations.

SNPs in ERCC1, ERCC2, and XRCC1 genes of the DNA repair pathway and risk of male infertility in the Asian populations: association study, meta-analysis, and trial sequential analysis

PURPOSE

We investigated if substitutions in the ERCC1, ERCC2, and XRCC1 genes of the DNA repair pathway correlate with non-obstructive azoospermia and male infertility.

METHODS

A total of 548 azoospermic infertile males and 410 fertile controls were genotyped for XRCC1 399A > G, 280G > A, and ERCC1 C > A 3' UTR and 541 azoospermic infertile males and 416 fertile controls were genotyped for ERCC2 751A > C using iPLEX Gold Assay. Meta-analyses were performed on XRCC1 399A > G (1022 cases and 1004 controls), ERCC1 C > A 3' UTR (879 cases and 1059 controls), and ERCC2 751A > C (914 cases and 850 controls) polymorphisms to quantitatively estimate the significance of the association between these polymorphisms and the risk of infertility.

RESULTS

Statistically significant association between ERCC2 751A > C SNP and male infertility was found using the codominant model (p = 0.03). Results of meta-analysis suggested a lack of correlation with male infertility risk, which could be due to pooling of studies from different ethnic populations. Due to limited the number of studies, a stratified analysis for different ethnic groups could not be performed.

CONCLUSION (S)

In conclusion, AA genotype of 751A > C SNP in ERCC2 correlated with a higher risk of male infertility and may contribute to an increased risk of azoospermia and male infertility in Indian men.

Transcriptome analysis of the responses to methyl methanesulfonate treatment in mouse pachytene spermatocytes and round spermatids

Spermatogenesis is threatened by DNA alkylating agents, one major category of DNA damaging agents. Currently, little is known about the alterations in transcriptome profiling of the mouse spermatogenic cells in response to DNA alkylation at distinct stages of spermatogenesis. In this study, RNA sequencing (RNA-seq) was performed in pachytene spermatocytes (PS) and round spermatids (RS) at 0 or 30min following Methyl Methanesulfonate (MMS) treatment and with untreated controls. A large number of differentially expressed genes (DEGs) were identified by comparison of the three groups in PS and RS, respectively. Functional analyses of all DEGs highlighted the protein ubiquitination pathway and DNA damage response (DDR) network being the two main biological processes in common in the two cell types. Further analyses of the DEGs with 2-fold or more changes between 30min repair and control group indicated that several cytokine signaling pathways were the most strongly affected in PS and DDR related pathways in RS, respectively. Gene ontology (GO) analyses directly showed differential biological process (BP) affected between PS and RS, with "regulation of transcription" being most overrepresented in PS and "cellular response to stress" in RS, respectively. Moreover, 374 DDR-related genes in PS and 158 in RS among all DEGs were filtered and clustered, which showed dynamic expression patterns in PS and RS. Our analyses provide a transcriptional landscape for male germ cells in response to MMS during spermatogenesis.

Protein Arginine Methyltransferase 6 Involved in Germ Cell Viability during Spermatogenesis and Down-Regulated by the Androgen Receptor

Androgens and the androgen receptor (AR) are of great importance to spermatogenesis and male fertility. AR knockout (ARKO) mice display a complete insensitivity to androgens and male infertility; however, the exact molecular mechanism for this effect remains unclear. In this study, we found that the expression levels of Prmt6 mRNA and protein were significantly up-regulated in the testes of ARKO mice compared to wild type (WT) mice. PRMT6 was principally localized to the nucleus of spermatogonia and spermatocytes by immunofluorescence staining. Furthermore, luciferase assay data showed that AR together with testosterone treatment suppressed Prmt6 transcription via binding to the androgen-responsive element (ARE) of the Prmt6 promoter. Moreover, knockdown of Prmt6 suppressed germ cells migration and promoted apoptosis. In addition, both of these cellular activities could not be enhanced by testosterone treatment. Taken together, these data indicate that PRMT6, which was down-regulated by AR and influenced cell migration and apoptosis of germ cells, could play a potentially important role in spermatogenesis.

Genotoxicity and gene expression modulation of silver and titanium dioxide nanoparticles in mice

Recently, we showed that silver nanoparticles (AgNPs) caused apoptosis, necrosis and DNA strand breaks in different cell models in vitro. These findings warranted analyses of their relevance in vivo. We investigated the genotoxic potential and gene expression profiles of silver particles of nano- (Ag20, 20 nm) and submicron- (Ag200, 200 nm) size and titanium dioxide nanoparticles (TiO2-NPs, 21 nm) in selected tissues from exposed male mice including the gonades. A single dose of 5 mg/kg bw nanoparticles was administered intravenously to male mice derived from C57BL6 (WT) and 8-oxoguanine DNA glycosylase knock-out (Ogg1(-/-) KO). Testis, lung and liver were harvested one and seven days post-exposure and analyzed for DNA strand breaks and oxidized purines employing the Comet assay with Formamidopyrimidine DNA glycosylase (Fpg) treatment, and sperm DNA fragmentation by the sperm chromatin structure assay (SCSA). Based on an initial screening of a panel of 21 genes, seven genes were selected and their expression levels were analyzed in all lung and testis tissues sampled from all animals (n = 6 mice/treatment group) using qPCR. AgNPs, in particular Ag200, caused significantly increased levels of DNA strand breaks and alkali labile sites in lung, seven days post-exposure. Fpg-sensitive lesions were significantly induced in both testis and lung. The transcript level of some key genes; Atm, Rad51, Sod1, Fos and Mmp3, were significantly induced compared to controls, particularly in lung samples from Ag200-exposed KO mice. We conclude that the Ag200 causes genotoxicity and distinct gene expression patterns in selected DNA damage response and repair related genes.

Male Rat Germ Cells Display Age-Dependent and Cell-Specific Susceptibility in Response to Oxidative Stress Challenges

For decades male germ cells were considered unaffected by aging, due to the fact that males continue to generate sperm into old age; however, evidence indicates that germ cells from aged males are of lower quality than those of young males. The current study examines the effects of aging on pachytene spermatocytes and round spermatids, and is the first study to culture these cells in isolation for an extended period. Our objective is to determine the cell-specific responses germ cells have to aging and oxidative insult. Culturing isolated germ cells from young and aged Brown Norway rats revealed that germ cells from aged males displayed an earlier decline in viability, elevated levels of reactive oxygen species (ROS), and increased spermatocyte DNA damage, compared to young males. Furthermore, oxidative insult by prooxidant 3-morpholinosydnonimine provides insight into how spermatocytes and spermatids manage excess ROS. Genome-wide microarray analyses revealed that several transcripts for antioxidants, Sod1, Cat, and Prdxs, were up-regulated in response to ROS in germ cells from young males while being expressed at lower levels in the aged. In contrast, the expression of DNA damage repair genes Rad50 and Atm were increased in the germ cells from aged animals. Our data indicate that as germ cells undergo spermatogenesis, they adapt and respond to oxidative stress differently, depending on their phase of development, and the process of aging results in redox dysfunction. Thus, even at early stages of spermatogenesis, germ cells from aged males are unable to mount an appropriate response to manage oxidative stress.

The genetic consequences of paternal acrylamide exposure and potential for amelioration

Acrylamide is a toxin that humans are readily exposed to due to its formation in many carbohydrate rich foods cooked at high temperatures. Acrylamide is carcinogenic, neurotoxic and causes reproductive toxicity when high levels of exposure are reached in mice and rats. Acrylamide induced effects on fertility occur predominantly in males. Acrylamide exerts its reproductive toxicity via its metabolite glycidamide, a product which is only formed via the cytochrome P450 detoxifying enzyme CYP2E1. Glycidamide is highly reactive and forms adducts with DNA. Chronic low dose acrylamide exposure in mice relevant to human exposure levels results in significantly increased levels of DNA damage in terms of glycidamide adducts in spermatocytes, the specific germ cell stage where Cyp2e1 is expressed. Since cells in the later stages of spermatogenesis are unable to undergo DNA repair, and this level of acrylamide exposure causes no reduction in fertility, there is potential for this damage to persist until sperm maturation and fertilisation. Cyp2e1 is also present within epididymal cells, allowing for transiting spermatozoa to be exposed to glycidamide. This could have consequences for future generations in terms of predisposition to diseases such as cancer, with growing indications that paternal DNA damage can be propagated across multiple generations. Since glycidamide is the major contributor to DNA damage, a mechanism for preventing these effects is inhibiting the function of Cyp2e1. Resveratrol is an example of an inhibitor of Cyp2e1 which has shown success in reducing damage caused by acrylamide treatment in mice.

Reference cells and ploidy in the comet assay

In the comet assay single cells are analyzed with respect to their level of DNA damage. Discrimination of the individual cell or cell type based on DNA content, with concomitant scoring of the DNA damage, is useful since this may allow analysis of mixtures of cells. Different cells can then be characterized based on their ploidy, cell cycle stage, or genome size. We here describe two applications of such a cell type-specific comet assay: (i) Testicular cell suspensions, analyzed on the basis of their ploidy during spermatogenesis; and (ii) reference cells in the form of fish erythrocytes which can be included as internal standards to correct for inter-assay variations. With standard fluorochromes used in the comet assay, the total staining signal from each cell - whether damaged or undamaged - was found to be associated with the cell's DNA content. Analysis of the fluorescence intensity of single cells is straightforward since these data are available in scoring systems based on image analysis. The analysis of testicular cell suspensions provides information on cell type specific composition, susceptibility to genotoxicants, and DNA repair. Internal reference cells, either untreated or carrying defined numbers of lesions induced by ionizing radiation, are useful for investigation of experimental factors that can cause variation in comet assay results, and for routine inclusion in experiments to facilitate standardization of methods, and comparison of comet assay data obtained in different experiments or in different laboratories. They can also be used - in combination with a reference curve - to quantify the DNA lesions induced by a certain treatment. Fish cells of a range of genome sizes, both greater and smaller than human, are suitable for this purpose, and they are inexpensive.

Genotoxic effects of two-generational selenium deficiency in mouse somatic and testicular cells

Many studies have investigated genotoxic effects of high Se diets but very few have addressed the genotoxicity of Se deprivation and its consequences in germ cells and none in somatic cells. To address these data gaps, C57BL/6 male mice were subjected to Se deprivation starting in the parental generation, i.e. before conception. Mice were given a diet of either low (0.01mg Se/kg diet) or normal (0.23mg Se/kg diet) Se content. Ogg1-deficient (Ogg1 (-/-) ) mice were used as a sensitive model towards oxidative stress due to their reduced capacity to repair oxidised purines. Ogg1 (-/-) mice also mimic the repair characteristics of human post-meiotic male germ cells which have a reduced ability to repair such lesions. The genotoxicity of Se deficiency was addressed by measuring DNA lesions with the alkaline single cell gel electrophoresis (+ Fpg to detect oxidised DNA lesions) in somatic cells (nucleated blood cells and lung cells) and male germ cells (testicular cells). Total Se concentration in liver and GPx activity in plasma and testicular cells were measured. Gene mutation was evaluated by an erythrocyte-based Pig-a assay. We found that Se deprivation of F1 from their conception and until early adulthood led to the induction of DNA lesions in testicular and lung cells expressed as significantly increased levels of DNA lesions, irrespective of the mouse genotype. In blood cells, Se levels did not appear to affect DNA lesions or mutant cell frequencies. The results suggest that the testis was the most sensitive tissue. Thus, genotoxicity induced by the low Se diet in the spermatozoal genome has potential implications for the offspring.

Mouse spermatocytes express CYP2E1 and respond to acrylamide exposure

Metabolism of xenobiotics by cytochrome P450s (encoded by the CYP genes) often leads to bio-activation, producing reactive metabolites that interfere with cellular processes and cause DNA damage. In the testes, DNA damage induced by xenobiotics has been associated with impaired spermatogenesis and adverse effects on reproductive health. We previously reported that chronic exposure to the reproductive toxicant, acrylamide, produced high levels of DNA damage in spermatocytes of Swiss mice. CYP2E1 metabolises acrylamide to glycidamide, which, unlike acrylamide, readily forms adducts with DNA. Thus, to investigate the mechanisms of acrylamide toxicity in mouse male germ cells, we examined the expression of the CYP, CYP2E1, which metabolises acrylamide. Using Q-PCR and immunohistochemistry, we establish that CYP2E1 is expressed in germ cells, in particular in spermatocytes. Additionally, CYP2E1 gene expression was upregulated in these cells following in vitro acrylamide exposure (1 µM, 18 h). Spermatocytes were isolated and treated with 1 µM acrylamide or 0.5 µM glycidamide for 18 hours and the presence of DNA-adducts was investigated using the comet assay, modified to detect DNA-adducts. Both compounds produced significant levels of DNA damage in spermatocytes, with a greater response observed following glycidamide exposure. A modified comet assay indicated that direct adduction of DNA by glycidamide was a major source of DNA damage. Oxidative stress played a small role in eliciting this damage, as a relatively modest effect was found in a comet assay modified to detect oxidative adducts following glycidamide exposure, and glutathione levels remained unchanged following treatment with either compound. Our results indicate that the male germ line has the capacity to respond to xenobiotic exposure by inducing detoxifying enzymes, and the DNA damage elicited by acrylamide in male germ cells is likely due to the formation of glycidamide adducts.

Aging genomes: a necessary evil in the logic of life

Genomes are inherently unstable because of the need for DNA sequence variation as a substrate for evolution through natural selection. However, most multicellular organisms have postmitotic tissues, with limited opportunity for selective removal of cells harboring persistent damage and deleterious mutations, which can therefore contribute to functional decline, disease, and death. Key in this process is the role of genome maintenance, the network of protein products that repair DNA damage and signal DNA damage response pathways. Genome maintenance is beneficial early in life by swiftly eliminating DNA damage or damaged cells, facilitating rapid cell proliferation. However, at later ages accumulation of unrepaired damage and mutations, as well as ongoing cell depletion, promotes cancer, atrophy, and other deleterious effects associated with aging. As such, genome maintenance and its phenotypic sequelae provide yet another example of antagonistic pleiotropy in aging and longevity.

Apurinic/apyrimidinic endonuclease immunoreactivity in germ cells of experimental varicocele-induced rat testes

Increased germ cell apoptosis is related to oxidative DNA damage; therefore, we investigated whether there was a significant change in apurinic/apyrimidinic endonuclease (APE) in varicoceles. Experimental varicoceles were created by partial ligation of the left renal vein of adult male Sprague-Dawley rats, which were sacrificed at 1, 3 and 6 weeks after varicocele creation. Testicular tissues were sampled for TUNEL, Western blotting and immunohistochemistry. There was a significant increase in apoptotic germ cells in the ipsilateral testes 6 weeks after varicocele creation. Increased activation of p53, Bax and cleaved caspase-3 in the left testes was also noted. APE increased activation until 3 weeks after varicocele creation, and then decreased at 6 weeks after varicocele surgery. The spermatocytes were immunostained for both 8-hydroxy-2'-deoxyguanosine and APE, but the spermatogonia revealed only APE immunopositivity in the defective tubules. These results suggest that repression of APE is an underlying mechanism of augmented p53-dependent apoptosis in varicocele-induced rat testes and that remaining APE in the spermatogonia plays a decisive role in regaining testicular spermatogenic function after varicocelectomy.

X-ray induced DNA damage and repair in germ cells of PARP1(-/-) male mice

Poly(ADP-ribose)polymerase-1 (PARP1) is a nuclear protein implicated in DNA repair, recombination, replication, and chromatin remodeling. The aim of this study was to evaluate possible differences between PARP1^−/− and wild-type mice regarding induction and repair of DNA lesions in irradiated male germ cells. Comet assay was applied to detect DNA damage in testicular cells immediately, and two hours after 4 Gy X-ray irradiation. A similar level of spontaneous and radiation-induced DNA damage was observed in PARP1^−/− and wild-type mice. Conversely, two hours after irradiation, a significant level of residual damage was observed in PARP1^−/− cells only. This finding was particularly evident in round spermatids. To evaluate if PARP1 had also a role in the dynamics of H2AX phosphorylation in round spermatids, in which γ-H2AX foci had been shown to persist after completion of DNA repair, we carried out a parallel analysis of γ-H2AX foci at 0.5, 2, and 48 h after irradiation in wild-type and PARP1^−/− mice. No evidence was obtained of an effect of PARP1 depletion on H2AX phosphorylation induction and removal. Our results suggest that, in round spermatids, under the tested experimental conditions, PARP1 has a role in radiation-induced DNA damage repair rather than in long-term chromatin modifications signaled by phosphorylated H2AX.

The paternal age effect: a multifaceted phenomenon

Birth rates for older fathers have increased 30% since 1980. When combined with the increased risk for genetic and multifactorial disorders in children conceived by older fathers, paternal age has become an important health issue for modern society. Laboratory research in this area has been minimal, perhaps because of significant experimental barriers, not the least of which is inadequate access to fresh, disease-free human testicular tissue. Regardless, progress has been made and intriguing models supported by experimental evidence have been proposed. The putative mechanisms range from reduced DNA repair activity, leading to increased mutagenesis, to positive selection of germ cells harboring specific disease-causing mutations. There remain many important venues for research in this increasingly relevant phenomenon that impacts future generations.

Mouse DNA polymerase kappa has a functional role in the repair of DNA strand breaks

The Y-family of DNA polymerases support of translesion DNA synthesis (TLS) associated with stalled DNA replication by DNA damage. Recently, a number of studies suggest that some specialized TLS polymerases also support other aspects of DNA metabolism beyond TLS in vivo. Here we show that mouse polymerase kappa (Polκ) could accumulate at laser-induced sites of damage in vivo resembling polymerases eta and iota. The recruitment was mediated through Polκ C-terminus which contains the PCNA-interacting peptide, ubiquitin zinc finger motif 2 and nuclear localization signal. Interestingly, this recruitment was significantly reduced in MSH2-deficient LoVo cells and Rad18-depleted cells. We further observed that Polκ-deficient mouse embryo fibroblasts were abnormally sensitive to H2O2 treatment and displayed defects in both single-strand break repair and double-strand break repair. We speculate that Polκ may have an important role in strand break repair following oxidative stress in vivo.

Enhanced genetic integrity in mouse germ cells

Genetically based diseases constitute a major human health burden, and de novo germline mutations represent a source of heritable genetic alterations that can cause such disorders in offspring. The availability of transgenic rodent systems with recoverable, mutation reporter genes has been used to assess the occurrence of spontaneous point mutations in germline cells. Previous studies using the lacI mutation reporter transgenic mouse system showed that the frequency of spontaneous mutations is significantly lower in advanced male germ cells than in somatic cell types from the same individuals. Here we used this same mutation reporter transgene system to show that female germ cells also display a mutation frequency that is lower than that in corresponding somatic cells and similar to that seen in male germ cells, indicating this is a common feature of germ cells in both sexes. In addition, we showed that statistically significant differences in mutation frequencies are evident between germ cells and somatic cells in both sexes as early as mid-fetal stages in the mouse. Finally, a comparison of the mutation frequency in a general population of early type A spermatogonia with that in a population enriched for Thy-1-positive spermatogonia suggests there is heterogeneity among the early spermatogonial population such that a subset of these cells are predestined to form true spermatogonial stem cells. Taken together, these results support the disposable soma theory, which posits that genetic integrity is normally maintained more stringently in the germ line than in the soma and suggests that this is achieved by minimizing the initial occurrence of mutations in early germline cells and their subsequent gametogenic progeny relative to that in somatic cells.

A genome-wide association study in Chinese men identifies three risk loci for non-obstructive azoospermia

Non-obstructive azoospermia (NOA) is one of the most severe forms of male infertility. Its pathophysiology is largely unknown, and few genetic influences have been defined. To identify common variants contributing to NOA in Han Chinese men, we performed a three-stage genome-wide association study of 2,927 individuals with NOA and 5,734 controls. The combined analyses identified significant (P < 5.0 × 10(-8)) associations between NOA risk and common variants near PRMT6 (rs12097821 at 1p13.3: odds ratio (OR) = 1.25, P = 5.7 × 10(-10)), PEX10 (rs2477686 at 1p36.32: OR = 1.39, P = 5.7 × 10(-12)) and SOX5 (rs10842262 at 12p12.1: OR = 1.23, P = 2.3 × 10(-9)). These findings implicate genetic variants at 1p13.3, 1p36.32 and 12p12.1 in the etiology of NOA in Han Chinese men.

Base excision repair efficiency and mechanism in nuclear extracts are influenced by the ratio between volume of nuclear extraction buffer and nuclei-implications for comparative studies

The base excision repair (BER) pathway corrects many different DNA base lesions and is important for genomic stability. The mechanism of BER cannot easily be investigated in intact cells and therefore in vitro methods that reflect the in vivo processes are in high demand. Reconstitution of BER using purified proteins essentially mirror properties of the proteins used, and does not necessarily reflect the mechanism as it occurs in the cell. Nuclear extracts from cultured cells have the capacity to carry out complete BER and can give important information on the mechanism. Furthermore, candidate proteins in extracts can be inhibited or depleted in a controlled way, making defined extracts an important source for mechanistic studies. The major drawback is that there is no standardized method of preparing nuclear extract for BER studies, and it does not appear to be a topic given much attention. Here we have examined BER activity of nuclear cell extracts from HeLa cells, using as substrate a circular DNA molecule with either uracil or an AP-site in a defined position. We show that BER activity of nuclear extracts from the same batch of cells varies inversely with the volume of nuclear extraction buffer relative to nuclei volume, in spite of identical protein concentrations in the BER assay mixture. Surprisingly, the uracil-DNA glycosylase activity (mainly UNG2), but not amount of UNG2, also correlated negatively with the volume of extraction buffer. These studies demonstrate that the method for preparation of nuclear extract is an important factor to consider for in vitro BER analysis and conditions used in comparative studies must be carefully worked out.

Possibility of mixed progenitor cells in sea star arm regeneration

In contrast to most vertebrates, invertebrate deuterostome echinoderms, such as the sea star Asterias rubens, undergo regeneration of lost body parts. The current hypothesis suggests that differentiated cells are the main source for regenerating arm in sea stars, but there is little information regarding the origin and identity of these cells. Here, we show that several organs distant to the regenerating arm responded by proliferation, most significantly in the coelomic epithelium and larger cells of the pyloric caeca. Analyzing markers for proliferating cells and parameters indicating cell ageing, such as levels of DNA damage, pigment, and lipofuscin contents as well as telomere length and telomerase activity, we suggest that cells contributing to the new arm likely originate from progenitors rather than differentiated cells. This is the first study showing that cells of mixed origin may be recruited from more distant sources of stem/progenitor cells in a sea star, and the first described indication of a role for pyloric caeca in arm regeneration. Data on growth rate during arm regeneration further indicate that regeneration is at the expense of whole animal growth. We propose a new working hypothesis for arm regeneration in sea stars involving four phases: wound healing by coelomocytes, migration of distant progenitor cells of mixed origin including from pyloric caeca, proliferation in these organs to compensate for cell loss, and finally, local proliferation in the regenerating arm.

BAX and tumor suppressor TRP53 are important in regulating mutagenesis in spermatogenic cells in mice

During the first wave of spermatogenesis, and in response to ionizing radiation, elevated mutant frequencies are reduced to a low level by unidentified mechanisms. Apoptosis is occurring in the same time frame that the mutant frequency declines. We examined the role of apoptosis in regulating mutant frequency during spermatogenesis. Apoptosis and mutant frequencies were determined in spermatogenic cells obtained from Bax-null or Trp53-null mice. The results showed that spermatogenic lineage apoptosis was markedly decreased in Bax-null mice and was accompanied by a significantly increased spontaneous mutant frequency in seminiferous tubule cells compared to that of wild-type mice. Apoptosis profiles in the seminiferous tubules for Trp53-null were similar to control mice. Spontaneous mutant frequencies in pachytene spermatocytes and in round spermatids from Trp53-null mice were not significantly different from those of wild-type mice. However, epididymal spermatozoa from Trp53-null mice displayed a greater spontaneous mutant frequency compared to that from wild-type mice. A greater proportion of spontaneous transversions and a greater proportion of insertions/deletions 15 days after ionizing radiation were observed in Trp53-null mice compared to wild-type mice. Base excision repair activity in mixed germ cell nuclear extracts prepared from Trp53-null mice was significantly lower than that for wild-type controls. These data indicate that BAX-mediated apoptosis plays a significant role in regulating spontaneous mutagenesis in seminiferous tubule cells obtained from neonatal mice, whereas tumor suppressor TRP53 plays a significant role in regulating spontaneous mutagenesis between postmeiotic round spermatid and epididymal spermatozoon stages of spermiogenesis.

In vitro evaluation of baseline and induced DNA damage in human sperm exposed to benzo[a]pyrene or its metabolite benzo[a]pyrene-7,8-diol-9,10-epoxide, using the comet assay

Exposure to genotoxins may compromise DNA integrity in male reproductive cells, putting future progeny at risk for developmental defects and diseases. To study the usefulness of sperm DNA damage as a biomarker for genotoxic exposure, we have investigated cellular and molecular changes induced by benzo[a]pyrene (B[a]P) in human sperm in vitro, and results have been compared for smokers and non-smokers. Sperm DNA obtained from five smokers was indeed more fragmented than sperm of six non-smokers (mean % Tail DNA 26.5 and 48.8, respectively), as assessed by the alkaline comet assay (P < 0.05). B[a]P-related DNA adducts were detected at increased levels in smokers as determined by immunostaining. Direct exposure of mature sperm cells to B[a]P (10 or 25 μM) caused moderate increases in DNA fragmentation which was independent of addition of human liver S9 mix for enzymatic activation of B[a]P, suggesting some unknown metabolism of B[a]P in ejaculates. In vitro exposure of samples to various doses of B[a]P (with or without S9) did not reveal any significant differences in sensitivity to DNA fragmentation between smokers and non-smokers. Incubations with the proximate metabolite benzo[a]pyrene-r-7,t-8-dihydrodiol-t9,10-epoxide (BPDE) produced DNA fragmentation in a dose-dependent manner (20 or 50 μM), but only when formamidopyrimidine DNA glycosylase treatment was included in the comet assay. These levels of DNA fragmentation were, however, low in relation to very high amounts of BPDE–DNA adducts as measured with ³²P postlabelling. We conclude that sperm DNA damage may be useful as a biomarker of direct exposure of sperm using the comet assay adapted to sperm, and as such the method may be applicable to cohort studies. Although the sensitivity is relatively low, DNA damage induced in earlier stages of spermatogenesis may be detected with higher efficiencies.

Joint effects of XRCC1 polymorphisms and polycyclic aromatic hydrocarbons exposure on sperm DNA damage and male infertility

X-ray repair cross-complementing group 1 (XRCC1) plays a role in repairing polycyclic aromatic hydrocarbons (PAHs)-induced DNA damage. We examined the effects of exposure to PAHs and XRCC1 polymorphism, alone or combined, on sperm DNA integrity and male fertility. A total of 620 idiopathic infertile subjects and 273 fertile controls were recruited in this study. PAHs exposure was indicated by urinary 1-hydroxypyrene level. Genotypes were determined by PCR-RFLP, and sperm DNA damage was detected by Tdt-mediated dUTP nick end labelling assay using flow cytometry. A positive correlation was found between PAHs exposure and sperm DNA damage (beta coefficients = 0.183, p < 0.001), whereas there was no significant association between the XRCC1 polymorphisms and sperm DNA damage. However, when the patients were dichotomized for PAHs exposure, higher sperm DNA damage was found among 399Gln allele carriers compared with the wild-type homozygotes (p = 0.033). Further analysis based on a case-control study revealed the joint effect of XRCC1-399 polymorphism and PAHs exposure on the risk of male infertility (p interaction = 0.041). These findings provided the first evidence about potential joint effects of PAHs exposure and DNA repair gene polymorphisms on male reproductive system and may be helpful in improving our understanding of the etiology of male infertility.

Parp1-XRCC1 and the repair of DNA double strand breaks in mouse round spermatids

The repair of DNA double strand breaks (DSBs) in male germ cells is slower and differently regulated compared to that in somatic cells. Round spermatids show DSB repair and are radioresistant to apoptosis induction. Mutation induction studies using ionizing irradiation, indicated a high frequency of chromosome aberrations (CA) in the next generation. Since they are in a G1 comparable stage of the cell cycle, haploid spermatids are expected to repair DSBs by the non-homologous end-joining pathway (NHEJ). However, immunohistochemical evidence indicates that not all components of the classical NHEJ pathway are available since the presence of DNA-PKcs cannot be shown. Here, we demonstrate that round spermatids, as well as most other types of male germ cells express both Parp1 and XRCC1. Therefore, we have determined whether the alternative Parp1/XRCC1 dependent NHEJ pathway is active in these nuclei and also have tested for classical NHEJ activity by a genetic method. To evaluate DSB repair in SCID mice, deficient for DNA-PKcs, and to study the involvement of the Parp1/XRCC1 dependent NHEJ pathway in round spermatids, the loss of gamma-H2AX foci after irradiation has been determined in nucleus spreads of round spermatids of SCID mice and in nucleus spreads and histological sections of Parp1-inhibited mice and their respective controls. Results show that around half of the breaks in randomly selected round spermatids are repaired between 1 and 8h after irradiation. The repair of 16% of the induced DSBs requires DNA-PKcs and 21% Parp1. Foci numbers in the Parp1-inhibited testes tend to be higher in spermatids of all epithelial stages reaching significance in stages I-III which indicates an active Parp1/XRCC1 pathway in round spermatids and a decreased repair capacity in later round spermatid stages. In Parp1-inhibited SCID mice only 14.5% of the breaks were repaired 8h after irradiation indicating additivity of the two NHEJ pathways in round spermatids.

In vitro investigations of glycidamide-induced DNA lesions in mouse male germ cells and in mouse and human lymphocytes

The industrial compound and food contaminant acrylamide (AA) is a probable human carcinogen, also known to induce male-mediated reproductive effects in animals. Most data suggest that its metabolite glycidamide (GA) is involved in the observed toxicity. We have investigated in vitro effects of AA/GA in mouse male germ cells (prior to spermatid elongation) and human and mouse peripheral blood lymphocytes, to assess inter-species and cell-type differences in susceptibility, and to explore the nature of the DNA lesion(s) as well as their potential repair. The comet assay was used in combination with the DNA-repair enzymes Fpg and hOGG1 to measure specific DNA lesions. In contrast to AA, GA induced significant levels of DNA lesions (strand breaks and alkali-labile sites) at millimolar concentrations in mouse testicular cells and human peripheral blood lymphocytes (hPBL). Using Fpg, the GA-induced DNA damage was measured at 20-50-fold higher sensitivity, in all cell types investigated. GA-induced DNA damage could not be recognised by hOGG1, suggesting that, based on the known affinities of these repair enzymes, alkylation of guanine is involved, rather than oxidation. Human lymphocytes appeared to be more susceptible to GA-induced lesions than both types of mouse cells. Mouse testicular cells and lymphocytes seemed to respond similarly to GA-induced Fpg-sensitive DNA lesions. The persistence of lesions was explored with cells from mice either proficient or deficient in Ogg1 (mouse 8-oxoguanine DNA glycosylase). Low in vitro repair of GA-induced Fpg-sensitive lesions was observed in primary male germ cells and lymphocytes from both Ogg1(+/+) and Ogg1(-/-) mice. We conclude that there may be differences between mice and humans in AA/GA-induced genotoxicity, and DNA from mouse male germ cells does not appear to be more sensitive to GA than DNA from peripheral blood lymphocytes in vitro. The usefulness of the comet assay in combination with DNA-repair enzymes is demonstrated.

Biased gene conversion and the evolution of mammalian genomic landscapes

Recombination is typically thought of as a symmetrical process resulting in large-scale reciprocal genetic exchanges between homologous chromosomes. Recombination events, however, are also accompanied by short-scale, unidirectional exchanges known as gene conversion in the neighborhood of the initiating double-strand break. A large body of evidence suggests that gene conversion is GC-biased in many eukaryotes, including mammals and human. AT/GC heterozygotes produce more GC- than AT-gametes, thus conferring a population advantage to GC-alleles in high-recombining regions. This apparently unimportant feature of our molecular machinery has major evolutionary consequences. Structurally, GC-biased gene conversion explains the spatial distribution of GC-content in mammalian genomes-the so-called isochore structure. Functionally, GC-biased gene conversion promotes the segregation and fixation of deleterious AT --> GC mutations, thus increasing our genomic mutation load. Here we review the recent evidence for a GC-biased gene conversion process in mammals, and its consequences for genomic landscapes, molecular evolution, and human functional genomics.

Reproductive consequences of paternal genotoxin exposure in marine invertebrates

Chemicals with the potential to damage DNA are increasingly present in the marine environment; yet our understanding of the long-term consequences of DNA damage for populations remains limited. We explore the impact of paternal genotoxin exposure on the reproductive biology of two ecologically important free-spawning marine invertebrates: the polychaete Arenicola marina and the mussel Mytilus edulis. Males were exposed in vivo for 72 h to methyl methanesulfonate and benzo(a)pyrene and the impact on somatic cells and sperm assessed using the Comet assay. A strong correlation between DNA damage in somatic cells and sperm was observed after 24 h exposure (P < 0.001). Recovery in sperm was significantly lower than in coelomocytes after 72 h. The fertilization success of DNA-damaged sperm was unaffected, but a significant percentage of embryos derived from sperm with induced DNA damage exhibited severe developmental abnormalities within 24 h of fertilization with potential long-term consequences for population success. Further research is required to determine the mechanism by which paternal DNA damage causes disruption of development at this early stage.

Mutagenesis is elevated in male germ cells obtained from DNA polymerase-beta heterozygous mice

Gametes carry the DNA that will direct the development of the next generation. By compromising genetic integrity, DNA damage and mutagenesis threaten the ability of gametes to fulfill their biological function. DNA repair pathways function in germ cells and serve to ameliorate much DNA damage and prevent mutagenesis. High base excision repair (BER) activity is documented for spermatogenic cells. DNA polymerase-beta (POLB) is required for the short-patch BER pathway. Because mice homozygous null for the Polb gene die soon after birth, mice heterozygous for Polb were used to examine the extent to which POLB contributes to maintaining spermatogenic genomic integrity in vivo. POLB protein levels were reduced only in mixed spermatogenic cells. In vitro short-patch BER activity assays revealed that spermatogenic cell nuclear extracts obtained from Polb heterozygous mice had one third the BER activity of age-matched control mice. Polb heterozygosity had no effect on the BER activities of somatic tissues tested. The Polb heterozygous mouse line was crossed with the lacI transgenic Big Blue mouse line to assess mutant frequency. The spontaneous mutant frequency for mixed spermatogenic cells prepared from Polb heterozygous mice was 2-fold greater than that of wild-type controls, but no significant effect was found among the somatic tissues tested. These results demonstrate that normal POLB abundance is necessary for normal BER activity, which is critical in maintaining a low germline mutant frequency. Notably, spermatogenic cells respond differently than somatic cells to Polb haploinsufficiency.

The comet assay in male reproductive toxicology

Due to our lifestyle and the environment we live in, we are constantly confronted with genotoxic or potentially genotoxic compounds. These toxins can cause DNA damage to our cells, leading to an increase in mutations. Sometimes such mutations could give rise to cancer in somatic cells. However, when germ cells are affected, then the damage could also have an effect on the next and successive generations. A rapid, sensitive and reliable method to detect DNA damage and assess the integrity of the genome within single cells is that of the comet or single-cell gel electrophoresis assay. The present communication gives an overview of the use of the comet assay utilising sperm or testicular cells in reproductive toxicology. This includes consideration of damage assessed by protocol modification, cryopreservation vs the use of fresh sperm, viability and statistics. It further focuses on in vivo and in vitro comet assay studies with sperm and a comparison of this assay with other assays measuring germ cell genotoxicity. As most of the de novo structural aberrations occur in sperm and spermatogenesis is functional from puberty to old age, whereas female germ cells are more complicated to obtain, the examination of male germ cells seems to be an easier and logical choice for research and testing in reproductive toxicology. In addition, the importance of such an assay for the paternal impact of genetic damage in offspring is undisputed. As there is a growing interest in the evaluation of genotoxins in male germ cells, the comet assay allows in vitro and in vivo assessments of various environmental and lifestyle genotoxins to be reliably determined.

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.

DNA repair in mammalian embryos

Mammalian cells have developed complex mechanisms to identify DNA damage and activate the required response to maintain genome integrity. Those mechanisms include DNA damage detection, DNA repair, cell cycle arrest and apoptosis which operate together to protect the conceptus from DNA damage originating either in parental gametes or in the embryo's somatic cells. DNA repair in the newly fertilized preimplantation embryo is believed to rely entirely on the oocyte's machinery (mRNAs and proteins deposited and stored prior to ovulation). DNA repair genes have been shown to be expressed in the early stages of mammalian development. The survival of the embryo necessitates that the oocyte be sufficiently equipped with maternal stored products and that embryonic gene expression commences at the correct time. A Medline based literature search was performed using the keywords 'DNA repair' and 'embryo development' or 'gametogenesis' (publication dates between 1995 and 2006). Mammalian studies which investigated gene expression were selected. Further articles were acquired from the citations in the articles obtained from the preliminary Medline search. This paper reviews mammalian DNA repair from gametogenesis to preimplantation embryos to late gestational stages.

On the Nature and Origin of DNA Strand Breaks in Elongating Spermatids1

Transient DNA strand breaks are generated in the whole population of elongating spermatids and are perfectly coincident with histone H4 hyperacetylation at chromatin-remodeling steps. Given the limited DNA repair capacity of elongating spermatids, chromatin remodeling may present a threat to genetic integrity of the male gamete. The nature of the DNA strand breakage, the enzymes involved, and the role of H4 hyperacetylation in the process must be determined to further investigate the potential mutagenic consequences of this important transition. We used the metachromatic dye acridine orange in combination with fluorescence-activated cell sorting to achieve separation of spermatids according to their condensation state. Using single-cell electrophoresis (comet assay), in both alkaline and neutral conditions, we demonstrated that double-stranded breaks account for most of the DNA fragmentation observed in purified elongating spermatids. DNA strand breaks were generated in round spermatids as a result of de novo histone hyperacetylation induced by trichostatin A, whereas an increase in endogenous DNA strand breaks was observed in elongating spermatids. Using a short-term culture of testicular cells, we demonstrated that DNA strand breaks in spermatids were abolished on incubation with two functionally different topoisomerase II inhibitors. Hence, topoisomerase II appears as the unique enzyme responsible for the transient double-stranded breaks in elongating spermatids but depends on histone hyperacetylation for its activity.