Expression and possible functions of DNA lesion bypass proteins in spermatogenesis

Flurochloridone induced abnormal spermatogenesis by damaging testicular Sertoli cells in mice

BACKGROUND

Flurochloridone (FLC), a selective herbicide used on a global scale, has been reported to have male reproductive toxicity whose evidence is limited, but its mechanism remains unclear. The present study was conducted to systematically explore the male reproductive toxicity of FLC, including sperm quality, spermatogenesis, toxicity targets, and potential mechanisms.

METHODS

Male C57BL/6 mice aged 6-7 weeks received gavage administration of FLC (365/730 mg/kg/day) for 28 consecutive days. Then, the tissue and sperm of mice were collected for analysis. We measured the gonadosomatic index and analyzed sperm concentration, motility, malformation rate, and mitochondrial membrane potential (MMP). Spermatocyte immunofluorescence staining was performed to analyze meiosis. We also performed pathological staining on the testis and epididymis tissue and TUNEL staining, immunohistochemical analysis, and ultrastructural observation on the testicular tissue.

RESULTS

Results showed that FLC caused testicular weight reduction, dysfunction, and architectural damage in mice, but no significant adverse effect was found in the epididymis. The exposure interfered with spermatogonial proliferation and meiosis, affecting sperm concentration, motility, kinematic parameters, morphology, and MMP, decreasing sperm quality. Furthermore, mitochondrial damage and apoptosis of testicular Sertoli cells were observed in mice treated with FLC.

CONCLUSION

We found that FLC has significant adverse effects on spermatogonial proliferation and meiosis. Meanwhile, apoptosis and mitochondrial damage may be the potential mechanism of Sertoli cell damage. Our study demonstrated that FLC could induce testicular Sertoli cell damage, leading to abnormal spermatogenesis, which decreased sperm quality. The data provided references for the toxicity risk and research methods of FLC application in the environment.

Chromoanasynthesis: another way for the formation of complex chromosomal abnormalities in human reproduction

Chromoanasynthesis has been described as a novel cause of massive constitutional chromosomal rearrangements. Based on DNA replication machinery defects, chromoanasynthesis is characterized by the presence of chromosomal duplications and triplications locally clustered on one single chromosome, or a few chromosomes, associated with various other types of structural rearrangements. Two distinct mechanisms have been described for the formation of these chaotic genomic disorders, i.e. the fork stalling and template switching and the microhomology-mediated break-induced replication. Micronucleus-based processes have been evidenced as a causative mechanism, thus, highlighting the close connection between segregation errors and structural rearrangements. Accumulating data indicate that chromoanasynthesis is operating in human germline cells and during early embryonic development. The development of new tools for quantifying chromoanasynthesis events should provide further insight into the impact of this catastrophic cellular phenomenon in human reproduction.

Somatic hypermutation of immunoglobulin genes in Rad18 knockout mice

Somatic hypermutation (SHM) of immunoglobulin (Ig) genes is triggered by the activity of activation-induced cytidine deaminase (AID). AID induces DNA lesions in variable regions of Ig genes, and error-prone DNA repair mechanisms initiated in response to these lesions introduce the mutations that characterize SHM. Error-prone DNA repair in SHM is proposed to be mediated by low-fidelity DNA polymerases such as those that mediate trans-lesion synthesis (TLS); however, the mechanism by which these enzymes are recruited to AID-induced lesions remains unclear. Proliferating cell nuclear antigen (PCNA), the sliding clamp for multiple DNA polymerases, undergoes Rad6/Rad18-dependent ubiquitination in response to DNA damage. Ubiquitinated PCNA promotes the replacement of the replicative DNA polymerase stalled at the site of a DNA lesion with a TLS polymerase. To examine the potential role of Rad18-dependent PCNA ubiquitination in SHM, we analyzed Ig gene mutations in Rad18 knockout (KO) mice immunized with T cell-dependent antigens. We found that SHM in Rad18 KO mice was similar to wild-type mice, suggesting that Rad18 is dispensable for SHM. However, residual levels of ubiquitinated PCNA were observed in Rad18 KO cells, indicating that Rad18-independent PCNA ubiquitination might play a role in SHM.

Differential expression of speckled POZ protein, SPOP: putative regulation by miR-145

The speckle POZ protein, SPOP, is an adaptor of the Cul3-based ubiquitination process, and has been implicated in the carcinogenesis process. Despite recent elucidation of biological functions, regulation of SPOP gene expression has not been reported. In this study, the mRNA levels of the mouse SPOP (mSPOP) gene were first shown to vary noticeably in different tissues. However, the SPOP protein was detected in high abundance only in Purkinje cells of the cerebellum and seminiferous tubule of the testis, echoing previous reports of involvement of ubiquitination in neuron cells and in spermatogenesis. In other mouse tissues and human cancer cell lines analysed, only low SPOP protein levels were detected. The 3'-untranslated regions of both the mSPOP and human SPOP transcripts harbor a conserved putative miR-145 binding site (BS). In some tissues and cell lines, miR-145 and SPOP protein levels were in an inverse relationship suggesting miR-145 regulation. Luciferase assays of deletion and point mutation constructs of the miR-145 BS, and miR-145 induction by serum starvation that resulted in reduced endogenous SPOP levels provided further evidence that miR-145 is likely involved in post-transcriptional regulation of SPOP expression in selected tissues, and possibly with the participation of other miRNA species.

Expression of DNA-PKcs and BRCA1 as prognostic indicators in nasopharyngeal carcinoma following intensity-modulated radiation therapy

The mechanisms of radiation-induced effects in cancer mainly involve double-strand breaks (DSBs) which are important in maintaining the stability of genes. The DNA repair genes breast cancer 1 (BRCA1) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) are capable of maintaining genetic stability through two distinct and complementary repair mechanisms for DNA DSBs, known as repair-homologous recombination (HR) and non-homologous end joining (NHEJ). DNA-PKcs is a member of the phosphatidylinositol 3-kinase (PI3K) family. The PI3K/AKT cell signaling pathway is implicated in cell migration and invasion. The BRCA1 protein is implicated in multiple complex cellular processes that are related to chromosome sensitivity to mutagens. To determine the protein expression and clinical implications of DNA-PKcs and BRCA1 in nasopharyngeal carcinoma (NPC) and cancer progression, we evaluated its expression status by immunohistochemistry in 87 patients who received intensity-modulated radiation therapy (IMRT). In NPC, negative expression of DNA-PKcs was detected in 35 of the 87 (40.2%) cancer types and was significantly associated with poor patient survival (P<0.05). The overexpression of DNA-PKcs and BRCA1 also led to significantly improved distant metastasis-free survival compared with patients who did not overexpress both genes, although the expression level of BRCA1 and distant metastasis-free survival were not closely correlated. In addition, multivariate analysis indicated that DNA-PKcs status is a predictive marker of distant metastasis-free survival. In conclusion, lower expression of DNA-PKcs may be correlated with higher distant metastasis in patients with NPC. DNA-PKcs may be a predictive marker of distant metastasis after IMRT, independent of the classical prognostic marker. BRCA1 may additionally exert a synergistic effect to predict distant metastasis-free survival.

From meiosis to postmeiotic events: homologous recombination is obligatory but flexible

Sexual reproduction depends on the success of faithful chromosome transmission during meiosis to yield viable gametes. Central to meiosis is the process of recombination between paternal and maternal chromosomes, which boosts the genetic diversity of progeny and ensures normal homologous chromosome segregation. Imperfections in meiotic recombination are the source of de novo germline mutations, abnormal gametes, and infertility. Thus, not surprisingly, cells have developed a variety of mechanisms and tight controls to ensure sufficient and well-distributed recombination events within their genomes, the details of which remain to be fully elucidated. Local and genome-wide studies of normal and genetically engineered cells have uncovered a remarkable stochasticity in the number and positioning of recombination events per chromosome and per cell, which reveals an impressive level of flexibility. In this minireview, we summarize our contemporary understanding of meiotic recombination and its control mechanisms, and address the seemingly paradoxical and poorly understood diversity of recombination sites. Flexibility in the distribution of meiotic recombination events within genomes may reside in regulation at the chromatin level, with histone modifications playing a recently recognized role.

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.

The critical mutagenic translesion DNA polymerase Rev1 is highly expressed during G(2)/M phase rather than S phase

The Rev1 protein lies at the root of mutagenesis in eukaryotes. Together with DNA polymerase zeta (Rev3/7), Rev1 function is required for the active introduction of the majority of mutations into the genomes of eukaryotes from yeast to humans. Rev1 and polymerase zeta are error-prone translesion DNA polymerases, but Rev1's DNA polymerase catalytic activity is not essential for mutagenesis. Rather, Rev1 is thought to contribute to mutagenesis principally by engaging in crucial protein-protein interactions that regulate the access of translesion DNA polymerases to the primer terminus. This inference is based on the requirement of the N-terminal BRCT (BRCA1 C-terminal) domain of Saccharomyces cerevisiae Rev1 for mutagenesis and the interaction of the C-terminal region of mammalian Rev1 with several other translesion DNA polymerases. Here, we report that S. cerevisiae Rev1 is subject to pronounced cell cycle control in which the levels of Rev1 protein are approximately 50-fold higher in G(2) and throughout mitosis than during G(1) and much of S phase. Differential survival of a rev1Delta strain after UV irradiation at various points in the cell cycle indicates that this unanticipated regulation is physiologically relevant. This unexpected finding has important implications for the regulation of mutagenesis and challenges current models of error-prone lesion bypass as a process involving polymerase switching that operates mainly during S phase to rescue stalled replication forks.

Silencing of unpaired chromatin and histone H2A ubiquitination in mammalian meiosis

During meiotic prophase in male mammals, the X and Y chromosomes are incorporated in the XY body. This heterochromatic body is transcriptionally silenced and marked by increased ubiquitination of histone H2A. This led us to investigate the relationship between histone H2A ubiquitination and chromatin silencing in more detail. First, we found that ubiquitinated H2A also marks the silenced X chromosome of the Barr body in female somatic cells. Next, we studied a possible relationship between H2A ubiquitination, chromatin silencing, and unpaired chromatin in meiotic prophase. The mouse models used carry an unpaired autosomal region in male meiosis or unpaired X and Y chromosomes in female meiosis. We show that ubiquitinated histone H2A is associated with transcriptional silencing of large chromatin regions. This silencing in mammalian meiotic prophase cells concerns unpaired chromatin regions and resembles a phenomenon described for the fungus Neurospora crassa and named meiotic silencing by unpaired DNA.