The mouse meiotic mutation mei1 disrupts chromosome synapsis with sexually dimorphic consequences for meiotic progression

Initiation of meiotic recombination in mammals

Meiotic recombination is initiated by the induction of programmed DNA double strand breaks (DSBs). DSB repair promotes homologous interactions and pairing and leads to the formation of crossovers (COs), which are required for the proper reductional segregation at the first meiotic division. In mammals, several hundred DSBs are generated at the beginning of meiotic prophase by the catalytic activity of SPO11. Currently it is not well understood how the frequency and timing of DSB formation and their localization are regulated. Several approaches in humans and mice have provided an extensive description of the localization of initiation events based on CO mapping, leading to the identification and characterization of preferred sites (hotspots) of initiation. This review presents the current knowledge about the proteins known to be involved in this process, the sites where initiation takes place, and the factors that control hotspot localization.

The use of transgenic mouse models in the study of male infertility

Over the past few decades with the rapid advances in embryo and embryonic stem cell manipulation techniques, transgenic mouse models have emerged as a powerful tool for the study of gene function and complex diseases including male infertility. In this review we give a brief history of the development of tools for the production of transgenic mouse models. This spans the advances from early pronuclear injection to the use of targeted embryonic stem cells to produce gene targeted, conditional, and inducible knockout mouse models. Lastly we provide a few examples to illustrate the utility of mouse models in the study of male infertility.

Functional conservation of Mei4 for meiotic DNA double-strand break formation from yeasts to mice

Meiotic recombination is initiated by the programmed induction of DNA double-strand breaks (DSBs) catalyzed by the evolutionarily conserved Spo11 protein. Studies in yeast have shown that DSB formation requires several other proteins, the role and conservation of which remain unknown. Here we show that two of these Saccharomyces cerevisiae proteins, Mei4 and Rec114, are evolutionarily conserved in most eukaryotes. Mei4(-/-) mice are deficient in meiotic DSB formation, thus showing the functional conservation of Mei4 in mice. Cytological analyses reveal that, in mice, MEI4 is localized in discrete foci on the axes of meiotic chromosomes that do not overlap with DMC1 and RPA foci. We thus propose that MEI4 acts as a structural component of the DSB machinery that ensures meiotic DSB formation on chromosome axes. We show that mouse MEI4 and REC114 proteins interact directly, and we identify conserved motifs as required for this interaction. Finally, the unexpected, concomitant absence of Mei4 and Rec114, as well as of Mnd1, Hop2, and Dmc1, in some eukaryotic species (particularly Neurospora crassa, Drosophila melanogaster, and Caenorhabditis elegans) suggests the existence of Mei4-Rec114-dependent and Mei4-Rec114-independent mechanisms for DSB formation, and a functional relationship between the chromosome axis and DSB formation.

Meiosis in flowering plants and other green organisms

Sexual eukaryotes generate gametes using a specialized cell division called meiosis that serves both to halve the number of chromosomes and to reshuffle genetic variation present in the parent. The nature and mechanism of the meiotic cell division in plants and its effect on genetic variation are reviewed here. As flowers are the site of meiosis and fertilization in angiosperms, meiotic control will be considered within this developmental context. Finally, we review what is known about the control of meiosis in green algae and non-flowering land plants and discuss evolutionary transitions relating to meiosis that have occurred in the lineages giving rise to the angiosperms.

Oocyte maturation failure: a syndrome of bad eggs

To show that disruption of meiotic competence results in cell cycle arrest, and the production of immature oocytes that are not capable of fertilization. Through an extensive review of animal studies and clinical case reports, we define the syndrome of oocyte maturation failure as a distinct oocyte disorder, present a classification system based on clinical parameters, and discuss the potential molecular origins for the disease.

A high throughput genetic screen identifies new early meiotic recombination functions in Arabidopsis thaliana

Meiotic recombination is initiated by the formation of numerous DNA double-strand breaks (DSBs) catalysed by the widely conserved Spo11 protein. In Saccharomyces cerevisiae, Spo11 requires nine other proteins for meiotic DSB formation; however, unlike Spo11, few of these are conserved across kingdoms. In order to investigate this recombination step in higher eukaryotes, we took advantage of a high-throughput meiotic mutant screen carried out in the model plant Arabidopsis thaliana. A collection of 55,000 mutant lines was screened, and spo11-like mutations, characterised by a drastic decrease in chiasma formation at metaphase I associated with an absence of synapsis at prophase, were selected. This screen led to the identification of two populations of mutants classified according to their recombination defects: mutants that repair meiotic DSBs using the sister chromatid such as Atdmc1 or mutants that are unable to make DSBs like Atspo11-1. We found that in Arabidopsis thaliana at least four proteins are necessary for driving meiotic DSB repair via the homologous chromosomes. These include the previously characterised DMC1 and the Hop1-related ASY1 proteins, but also the meiotic specific cyclin SDS as well as the Hop2 Arabidopsis homologue AHP2. Analysing the mutants defective in DSB formation, we identified the previously characterised AtSPO11-1, AtSPO11-2, and AtPRD1 as well as two new genes, AtPRD2 and AtPRD3. Our data thus increase the number of proteins necessary for DSB formation in Arabidopsis thaliana to five. Unlike SPO11 and (to a minor extent) PRD1, these two new proteins are poorly conserved among species, suggesting that the DSB formation mechanism, but not its regulation, is conserved among eukaryotes.

Persistence of histone H2AX phosphorylation after meiotic chromosome synapsis and abnormal centromere cohesion in poly (ADP-ribose) polymerase (Parp-1) null oocytes

In spite of the impact of aneuploidy on human health little is known concerning the molecular mechanisms involved in the formation of structural or numerical chromosome abnormalities during meiosis. Here, we provide novel evidence indicating that lack of PARP-1 function during oogenesis predisposes the female gamete to genome instability. During prophase I of meiosis, a high proportion of Parp-1((-/-)) mouse oocytes exhibit a spectrum of meiotic defects including incomplete homologous chromosome synapsis or persistent histone H2AX phosphorylation in fully synapsed chromosomes at the late pachytene stage. Moreover, the X chromosome bivalent is also prone to exhibit persistent double strand DNA breaks (DSBs). In striking contrast, such defects were not detected in mutant pachytene spermatocytes. In fully-grown wild type oocytes at the germinal vesicle stage, PARP-1 protein associates with nuclear speckles and upon meiotic resumption, undergoes a striking re-localization towards spindle poles as well as pericentric heterochromatin domains at the metaphase II stage. Notably, a high proportion of in vivo matured Parp-1((-/-)) oocytes show lack of recruitment of the kinetochore-associated protein BUB3 to centromeric domains and fail to maintain metaphase II arrest. Defects in chromatin modifications in the form of persistent histone H2AX phosphorylation during prophase I of meiosis and deficient sister chromatid cohesion during metaphase II predispose mutant oocytes to premature anaphase II onset upon removal from the oviductal environment. Our results indicate that PARP-1 plays a critical role in the maintenance of chromosome stability at key stages of meiosis in the female germ line. Moreover, in the metaphase II stage oocyte PARP-1 is required for the regulation of centromere structure and function through a mechanism that involves the recruitment of BUB3 protein to centromeric domains.

The consequences of asynapsis for mammalian meiosis

During mammalian meiosis, synapsis of paternal and maternal chromosomes and the generation of DNA breaks are needed to allow reshuffling of parental genes. In mammals errors in synapsis are associated with a male-biased meiotic impairment, which has been attributed to a response to persisting DNA double-stranded breaks in the asynapsed chromosome segments. Recently it was discovered that the chromatin of asynapsed chromosome segments is transcriptionally silenced, providing new insights into the connection between asynapsis and meiotic impairment.

Integration of CREB and bHLH transcriptional signaling pathways through direct heterodimerization of the proteins: role in muscle and testis development

The cAMP response element binding protein/activating transcription factor (CREB/ATF) family of transcription factors is hormone responsive and critical for nearly all mammalian cell types. The basic helix-loop-helix (bHLH) family of transcription factors is important during the development and differentiation of a wide variety of cell types. Independent studies of the role of the bHLH protein scleraxis in testicular Sertoli cells and paraxis in muscle development using yeast-2-hybrid screens provided the novel observation that bHLH proteins can directly interact with ATF/CREB family members. Analysis of the interactions demonstrated the helix-loop-helix domain of bHLH proteins directly interacts with the leucine zipper (ZIP) region of CREB2/ATF4 to form heterodimers. The direct bHLH-CREB2 binding interactions were supported using co-immunoprecipitation of recombinant proteins. Structural analysis of bHLH and ATF4 heterodimer using previous crystal structures demonstrated the heterodimer likely involves the HLH and Zip domains and has the potential capacity to bind DNA. Transfection assays demonstrated CREB2/ATF4 over-expression blocked stimulatory actions of scleraxis or paraxis. CREB1 inhibited MyoD induced myogenic conversion of C3H10T1/2 cells. CREB2/ATF4 and scleraxis are expressed throughout embryonic and postnatal testis development, with scleraxis specifically expressed in Sertoli cells. ATF4 and scleraxis null mutant mice both had similar adult testis phenotypes of reduced spermatogenic capacity. In summary, bHLH and CREB family members were found to directly heterodimerize and inhibit the actions of bHLH dimers on Sertoli cells and myogenic precursor cells. The observations suggest a mechanism for direct cross-talk between cAMP induced and bHLH controlled cellular differentiation.

Successful delivery after the transfer of embryos obtained from a cohort of incompletely in vivo matured oocytes at retrieval time

OBJECTIVE

To report a case of successful delivery of a healthy baby after intracytoplasmic sperm injection (ICSI) in a patient with no mature oocytes at the time of oocyte retrieval.

DESIGN

Case report.

SETTING

Department of Reproductive Medicine.

PATIENT

After a controlled ovarian hyperstimulation cycle, a total of seven immature oocytes were collected.

INTERVENTION(S)

Medical management.

MAIN OUTCOME MEASURE(S)

The timing of polar body extrusion was checked every 2 hours, and ICSI was performed as soon as the first polar body was extruded.

RESULT(S)

Following incubation in culture medium, five oocytes reached the metaphase II stage within 8-8.5 hours. Three oocytes were fertilized after ICSI, and two of three cleaved embryos were transferred on day 3. The embryo transfer was followed by a single pregnancy and the delivery of a healthy baby.

CONCLUSION

This case report demonstrates that embryos obtained from in vitro matured oocytes retain the developmental competence for full-term. It illustrates the importance of regularly monitoring the polar body extrusion when all collected oocytes are immature.

Spo11 and the Formation of DNA Double-Strand Breaks in Meiosis

Meiotic recombination is carried out through a specialized pathway for the formation and repair of DNA double-strand breaks made by the Spo11 protein, a relative of archaeal topoisomerase VI. This review summarizes recent studies that provide insight to the mechanism of DNA cleavage by Spo11, functional interactions of Spo11 with other proteins required for break formation, mechanisms that control the timing of recombination initiation, and evolutionary conservation and divergence of these processes.

A dominant, recombination-defective allele of Dmc1 causing male-specific sterility

DMC1 is a meiosis-specific homolog of bacterial RecA and eukaryotic RAD51 that can catalyze homologous DNA strand invasion and D-loop formation in vitro. DMC1-deficient mice and yeast are sterile due to defective meiotic recombination and chromosome synapsis. The authors identified a male dominant sterile allele of Dmc1, Dmc1^Mei11, encoding a missense mutation in the L2 DNA binding domain that abolishes strand invasion activity. Meiosis in male heterozygotes arrests in pachynema, characterized by incomplete chromosome synapsis and no crossing-over. Young heterozygous females have normal litter sizes despite having a decreased oocyte pool, a high incidence of meiosis I abnormalities, and susceptibility to premature ovarian failure. Dmc1^Mei11 exposes a sex difference in recombination in that a significant portion of female oocytes can compensate for DMC1 deficiency to undergo crossing-over and complete gametogenesis. Importantly, these data demonstrate that dominant alleles of meiosis genes can arise and propagate in populations, causing infertility and other reproductive consequences due to meiotic prophase I defects.

About 10%–15% of couples are infertile due to defects in meiosis (the process by which egg or sperm cells containing a single copy of each chromosome are produced). Because studying the genetics of meiosis in humans is difficult, we performed genetic screens in mice and identified a novel mutation in Dmc1 that causes male-specific infertility due to defects in meiosis. Dmc1 encodes a key protein required for meiotic recombination; the mutation causes a single amino acid change that prevents genetic exchange, or crossing-over, in males, abolishes its recombination activity, and abrogates the production of sperm. Though heterozygous females are fertile, they have fewer oocytes due to a high incidence of meiosis I abnormalities, and show susceptibility to premature ovarian failure. Importantly, these data demonstrate that dominant alleles of meiosis genes can arise and propagate in populations, and produce meiotic prophase I defects that cause infertility and other reproductive abnormalities.

Meiosis occurs in both the male and female germ lines; here the authors describe a mutation that affects only male meiosis, causing sterility.

AtPRD1 is required for meiotic double strand break formation in Arabidopsis thaliana

The initiation of meiotic recombination by the formation of DNA double-strand breaks (DSBs) catalysed by the Spo11 protein is strongly evolutionary conserved. In Saccharomyces cerevisiae, Spo11 requires nine other proteins for meiotic DSB formation, but, unlike Spo11, few of these proteins seem to be conserved across kingdoms. In order to investigate this recombination step in higher eukaryotes, we have isolated a new gene, AtPRD1, whose mutation affects meiosis in Arabidopsis thaliana. In Atprd1 mutants, meiotic recombination rates fall dramatically, early recombination markers (e.g., DMC1 foci) are absent, but meiosis progresses until achiasmatic univalents are formed. Besides, Atprd1 mutants suppress DSB repair defects of a large range of meiotic mutants, showing that AtPRD1 is involved in meiotic recombination and is required for meiotic DSB formation. Furthermore, we showed that AtPRD1 and AtSPO11-1 interact in a yeast two-hybrid assay, suggesting that AtPRD1 could be a partner of AtSPO11-1. Moreover, our study reveals similarity between AtPRD1 and the mammalian protein Mei1, suggesting that AtPRD1 could be a Mei1 functional homologue.

Origins of New Male Germ-line Functions from X-Derived Autosomal Retrogenes in the Mouse

Recent literature demonstrates that retrogenes tend to leave the X chromosome and integrate onto the autosomes and evolve male-biased expression patterns. Several selection-based evolutionary mechanisms have been proposed to explain this observation. Testing these selection-based models requires examining the evolutionary history and functional properties of new retrogenes, particularly those that show evidence of directional movement between the X and the autosomes (X-related retrogenes). This includes autosomal retrogenes with parental paralogs on the X chromosome (X-derived autosomal retrogenes) and those retrogenes integrated onto the X chromosomes (X-linked retrogenes). In order to understand why retrogenes tend to move nonrandomly in genomes, we examined the expression patterns and evolutionary mechanisms concerning gene pairs having young retrogenes--originating less than 20 MYA (after mouse-rat split). We demonstrate that these X-derived autosomal retrogenes evolved a more restricted male-biased expression pattern: they are expressed exclusively or predominantly in the testis, in particular, during the late stages of spermatogenesis. In contrast, the parental counterparts have relatively broad expression patterns in various tissues and spermatogenetic stages. We further observed that positive selection is targeting these X-derived autosomal retrogenes with novel male-biased expression patterns. This suggests that such retrogenes evolved new male germ-line functions that may be complementary to the functions of the parental paralogs, which themselves contribute little during spermatogenesis. Such evolutionary changes may be beneficial to the populations. Furthermore, most identified X-related retrogenes have recruited novel adjacent sequences as their untranslated regions (UTRs), suggesting that these UTRs, acquired de novo, may play an important role in establishing new regulatory mechanisms to carry out the new male germ-line functions.

Mutation in mouse hei10, an e3 ubiquitin ligase, disrupts meiotic crossing over

Crossing over during meiotic prophase I is required for sexual reproduction in mice and contributes to genome-wide genetic diversity. Here we report on the characterization of an N-ethyl-N-nitrosourea-induced, recessive allele called mei4, which causes sterility in both sexes owing to meiotic defects. In mutant spermatocytes, chromosomes fail to congress properly at the metaphase plate, leading to arrest and apoptosis before the first meiotic division. Mutant oocytes have a similar chromosomal phenotype but in vitro can undergo meiotic divisions and fertilization before arresting. During late meiotic prophase in mei4 mutant males, absence of cyclin dependent kinase 2 and mismatch repair protein association from chromosome cores is correlated with the premature separation of bivalents at diplonema owing to lack of chiasmata. We have identified the causative mutation, a transversion in the 5′ splice donor site of exon 1 in the mouse ortholog of Human Enhancer of Invasion 10 (Hei10; also known as Gm288 in mouse and CCNB1IP1 in human), a putative B-type cyclin E3 ubiquitin ligase. Importantly, orthologs of Hei10 are found exclusively in deuterostomes and not in more ancestral protostomes such as yeast, worms, or flies. The cloning and characterization of the mei4 allele of Hei10 demonstrates a novel link between cell cycle regulation and mismatch repair during prophase I.

Human infertility and reproductive complications have devastating social and monetary costs. Errors in meiosis during reproduction may lead to birth defects, spontaneous abortion, or infertility. Many of the genes essential for meiosis function in DNA repair and mutations in several of these genes have been shown to contribute to cancer. The identification of the genes necessary for normal meiosis is an important goal and will potentially influence the fields of reproductive and cancer biology. In this study, genetic screens in mice have generated the mutation mei4. mei4 causes male and female sterility by disrupting meiosis and altering the function of the DNA repair system known as mismatch repair. We have identified the causative mutation behind the mei4 phenotype in a gene called Human Enhancer of Invasion 10 or Hei10. This work demonstrates that Hei10 is essential for the completion of meiosis and that it functions to coordinate the DNA repair system and the progression of the cell cycle during meiosis.

Resolving RAD51C function in late stages of homologous recombination

DNA double strand breaks are efficiently repaired by homologous recombination. One of the last steps of this process is resolution of Holliday junctions that are formed at the sites of genetic exchange between homologous DNA. Although various resolvases with Holliday junctions processing activity have been identified in bacteriophages, bacteria and archaebacteria, eukaryotic resolvases have been elusive. Recent biochemical evidence has revealed that RAD51C and XRCC3, members of the RAD51-like protein family, are involved in Holliday junction resolution in mammalian cells. However, purified recombinant RAD51C and XRCC3 proteins have not shown any Holliday junction resolution activity. In addition, these proteins did not reveal the presence of a nuclease domain, which raises doubts about their ability to function as a resolvase. Furthermore, oocytes from infertile Rad51C mutant mice exhibit precocious separation of sister chromatids at metaphase II, a phenotype that reflects a defect in sister chromatid cohesion, not a lack of Holliday junction resolution. Here we discuss a model to explain how a Holliday junction resolution defect can lead to sister chromatid separation in mouse oocytes. We also describe other recent in vitro and in vivo evidence supporting a late role for RAD51C in homologous recombination in mammalian cells, which is likely to be resolution of the Holliday junction.

Gender effects on the incidence of aneuploidy in mammalian germ cells

Aneuploidy occurs in 0.3% of newborns, 4% of stillbirths, and more than 35% of all human spontaneous abortions. Human gametogenesis is uniquely and gender-specific susceptible to errors in chromosome segregation. Overall, between 1% and 4% of sperm and as many as 20% of human oocytes have been estimated by molecular cytogenetic analysis to be aneuploid. Maternal age remains the paramount aetiological factor associated with human aneuploidy. The majority of extra chromosomes in trisomic offspring appears to be of maternal origin resulting from nondisjunction of homologous chromosomes during the first meiotic division. Differences in the recombination patterns between male and female meiosis may partly account for the striking gender- and chromosome-specific differences in the genesis of human aneuploidy, especially in aged oocytes. Nondisjunction of entire chromosomes during meiosis I as well as premature separation of sister chromatids or homologues prior to meiotic anaphase can contribute to aneuploidy. During meiosis, checkpoints at meiotic prophase and the spindle checkpoint at M-phase can induce meiotic arrest and/or cell death in case of disturbances in pairing/recombination or spindle attachment of chromosomes. It has been suggested that gender differences in aneuploidy may result from more permissive checkpoints in females than males. Furthermore, age-related loss of chromosome cohesion in oocytes as a cause of aneuploidy may be female-specific. Comparative data about the susceptibility of human male and female germ cells to aneuploidy-causing chemicals is lacking. Increases of aneuploidy frequency in sperm have been shown after exposure to therapeutic drugs, occupational agents and lifestyle factors. Conversely, data on oocyte aneuploidy caused by exogenous agents is limited because of the small numbers of oocytes available for analysis combined with potential maternal age effects. The vast majority of animal studies on aneuploidy induction in germ cells represent cause and effect data. Specific studies designed to evaluate possible gender differences in induction of germ cell aneuploidy have not been found. However, the comparison of rodent data available from different laboratories suggests that oocytes are more sensitive than male germ cells when exposed to chemicals that effect the meiotic spindle. Only recently, in vitro experiments, analyses of transgenic animals and knockdown of expression of meiotic genes have started to address the molecular mechanisms underlying chromosome missegregation in mammalian germ cells whereby striking differences between genders could be shown. Such information is needed to clarify the extent and the mechanisms of gender effects, including possible differential susceptibility to environmental agents.

RAD51C deficiency in mice results in early prophase I arrest in males and sister chromatid separation at metaphase II in females

RAD51C is a member of the RecA/RAD51 protein family, which is known to play an important role in DNA repair by homologous recombination. In mice, it is essential for viability. Therefore, we have generated a hypomorphic allele of Rad51c in addition to a null allele. A subset of mice expressing the hypomorphic allele is infertile. This infertility is caused by sexually dimorphic defects in meiotic recombination, revealing its two distinct functions. Spermatocytes undergo a developmental arrest during the early stages of meiotic prophase I, providing evidence for the role of RAD51C in early stages of RAD51-mediated recombination. In contrast, oocytes can progress normally to metaphase I after superovulation but display precocious separation of sister chromatids, aneuploidy, and broken chromosomes at metaphase II. These defects suggest a possible late role of RAD51C in meiotic recombination. Based on the marked reduction in Holliday junction (HJ) resolution activity in Rad51c-null mouse embryonic fibroblasts, we propose that this late function may be associated with HJ resolution.

The Mre11 complex influences DNA repair, synapsis, and crossing over in murine meiosis

The Mre11 complex (consisting of MRE11, RAD50, and NBS1/Xrs2) is required for double-strand break (DSB) formation, processing, and checkpoint signaling during meiotic cell division in S. cerevisiae. Whereas studies of Mre11 complex mutants in S. pombe and A. thaliana indicate that the complex has other essential meiotic roles , relatively little is known regarding the functions of the complex downstream of meiotic break formation and processing or its role in meiosis in higher eukaryotes. We analyzed meiotic events in mice harboring hypomorphic Mre11 and Nbs1 mutations which, unlike null mutants, support viability . Our studies revealed defects in the temporal progression of meiotic prophase, incomplete and aberrant synapsis of homologous chromosomes, persistence of strand exchange proteins, and alterations in both the frequency and placement of MLH1 foci, a marker of crossovers. A unique sex-dependent effect on MLH1 foci and chiasmata numbers was observed: males exhibited an increase and females a decrease in recombination levels. Thus, our findings implicate the Mre11 complex in meiotic DNA repair and synapsis in mammals and indicate that the complex may contribute to the establishment of normal sex-specific differences in meiosis.

Gene expression profiles of Spo11-/- mouse testes with spermatocytes arrested in meiotic prophase I

Spo11, a meiosis-specific protein, introduces double-strand breaks on chromosomal DNA and initiates meiotic recombination in a wide variety of organisms. Mouse null Spo11 spermatocytes fail to synapse chromosomes and progress beyond the zygotene stage of meiosis. We analyzed gene expression profiles in Spo11(-/ -)adult and juvenile wild-type testis to describe genes expressed before and after the meiotic arrest resulting from the knocking out of Spo11. These genes were characterized using the Gene Ontology data base. To focus on genes involved in meiosis, we performed comparative gene expression analysis of Spo11(-/ -)and wild-type testes from 15-day mice, when spermatocytes have just entered pachytene. We found that the knockout of Spo11 causes dramatic changes in the level of expression of genes that participate in meiotic recombination (Hop2, Brca2, Mnd1, FancG) and in the meiotic checkpoint (cyclin B2, Cks2), but does not affect genes encoding protein components of the synaptonemal complex. Finally, we discovered unknown genes that are affected by the disruption of the Spo11 gene and therefore may be specifically involved in meiosis and spermatogenesis.

Lsh is required for meiotic chromosome synapsis and retrotransposon silencing in female germ cells

Lymphoid specific helicase (Lsh) is a major epigenetic regulator that is essential for DNA methylation and transcriptional silencing of parasitic elements in the mammalian genome. However, whether Lsh is involved in the regulation of chromatin-mediated processes during meiosis is not known. Here, we show that Lsh is essential for the completion of meiosis and transcriptional repression of repetitive elements in the female gonad. Oocytes from Lsh knockout mice exhibit demethylation of transposable elements and tandem repeats at pericentric heterochromatin, as well as incomplete chromosome synapsis associated with persistent RAD51 foci and gammaH2AX phosphorylation. Failure to load crossover-associated foci results in the generation of non-exchange chromosomes. The severe oocyte loss observed and lack of ovarian follicle formation, together with the patterns of Lsh nuclear compartmentalization in the germ line, demonstrate that Lsh has a critical and previously unidentified role in epigenetic gene silencing and maintenance of genomic stability during female meiosis.

Uncovering the uncharacterized and unexpected: unbiased phenotype-driven screens in the mouse

Phenotype-based chemical mutagenesis screens for mouse mutations have undergone a transformation in the past five years from a potential approach to a practical tool. This change has been driven by the relative ease of identifying causative mutations now that the complete genome sequence is available. These unbiased screens make it possible to identify genes, gene functions and processes that are uniquely important to mammals. In addition, because chemical mutagenesis generally induces point mutations, these alleles often uncover previously unappreciated functions of known proteins. Here we provide examples of the success stories from forward genetic screens, emphasizing the examples that illustrate the discovery of mammalian-specific processes that could not be discovered in other model organisms. As the efficiency of sequencing and mutation detection continues to improve, it is likely that forward genetic screens will provide an even more important part of the repertoire of mouse genetics in the future.

Fast forward to new genes in mammalian reproduction

The study of reproductive genetics in mammals has lagged behind that of simpler and more tractable model organisms, such as D. melanogaster, C. elegans and various yeast models. Although much valuable information has been generated using these organisms, they do not model the genetic and biological complexity of mammalian reproduction. Thus, the majority of genes required for gametogenesis in mammals remain unidentified. To expand on the existing knowledge of mammalian reproductive genetics, we have carried out forward genetic screens in mice to identify infertility mutants and the underlying mutant genes. Two different approaches were used: mutagenesis of the germline in whole mice, and mutagenesis of embryonic stem cells. This was followed by two- or three-generation breeding schemes to identify pedigrees segregating infertility mutations, which were then phenotypically characterized, genetically mapped, and in some cases, positionally cloned. This whole-genome approach has generated a wide collection of mutants with defects ranging from problems with germ cell development to abnormal sperm morphology. These models have allowed us to study the genetics, as well as the physiology, of reproduction in mammals. This review focuses on describing some of the genes identified in these screens and the ongoing effort to characterize additional mutants.

Mutations that affect meiosis in male mice influence the dynamics of the mid-preleptotene and bouquet stages

Meiosis pairs and segregates homologous chromosomes and thereby forms haploid germ cells to compensate the genome doubling at fertilization. Homologue pairing in many eukaryotic species depends on formation of DNA double strand breaks (DSBs) during early prophase I when telomeres begin to cluster at the nuclear periphery (bouquet stage). By fluorescence in situ hybridization criteria, we observe that mid-preleptotene and bouquet stage frequencies are altered in male mice deficient for proteins required for recombination, ubiquitin conjugation and telomere length control. The generally low frequencies of mid-preleptotene spermatocytes were significantly increased in male mice lacking recombination proteins SPO11, MEI1, MLH1, KU80, ubiquitin conjugating enzyme HR6B, and in mice with only one copy of the telomere length regulator Terf1. The bouquet stage was significantly enriched in Atm(-/-), Spo11(-/-), Mei1(m1Jcs/m1Jcs), Mlh1(-/-), Terf1(+/-) and Hr6b(-/-) spermatogenesis, but not in mice lacking recombination proteins DMC1 and HOP2, the non-homologous end-joining DNA repair factor KU80 and the ATM downstream effector GADD45a. Mice defective in spermiogenesis (Tnp1(-/-), Gmcl1(-/-), Asm(-/-)) showed wild-type mid-preleptotene and bouquet frequencies. A low frequency of bouquet spermatocytes in Spo11(-/-)Atm(-/-) spermatogenesis suggests that DSBs contribute to the Atm(-/-)-correlated bouquet stage exit defect. Insignificant changes of bouquet frequencies in mice with defects in early stages of DSB repair (Dmc1(-/-), Hop2(-/-)) suggest that there is an ATM-specific influence on bouquet stage duration. Altogether, it appears that several pathways influence telomere dynamics in mammalian meiosis.

Initiation of meiotic recombination by formation of DNA double-strand breaks: mechanism and regulation

Homologous recombination is essential for accurate chromosome segregation during meiosis in most sexual organisms. Meiotic recombination is initiated by the formation of DSBs (DNA double-strand breaks) made by the Spo11 protein. We review here recent findings pertaining to protein–protein interactions important for DSB formation, the mechanism of an early step in the processing of Spo11-generated DSBs, and regulation of DSB formation by protein kinases.

Detection of quantitative trait loci causing abnormal spermatogenesis and reduced testis weight in the small testis (Smt) mutant mouse

The small testis (Smt) mutant mouse is characterized by a small testis of one third to one half the size of a normal testis, and its spermatogenesis is mostly arrested at early stages of meiosis, although a small number of spermatocytes at the late prophase of meiosis and a few spermatids can sometimes be seen. We performed quantitative trait locus (QTL) analysis of these spermatogenic traits and testis weight using 221 F2 males obtained from a cross between Smt and MOM (Mus musculus molossinus) mice. At the genome-wide 5% level, we detected two QTLs affecting meiosis on chromosomes 4 and 13, and two QTLs for paired testis weight as a percentage of body weight on chromosomes 4 and X. In addition, we found several QTLs for degenerated germ cells and multinuclear giant cells on chromosomes 4, 7 and 13. Interestingly, for cell degeneration, the QTL on chromosome 13 interacted epistatically with the QTL on chromosome 4. These results reveal polygenic participation in the abnormal spermatogenesis and small testis size in the Smt mutant.

Genetic analysis of chromosome pairing, recombination, and cell cycle control during first meiotic prophase in mammals

Meiosis is a double-division process that is preceded by only one DNA replication event to produce haploid gametes. The defining event in meiosis is prophase I, during which chromosome pairs locate each other, become physically connected, and exchange genetic information. Although many aspects of this process have been elucidated in lower organisms, there has been scant information available until now about the process in mammals. Recent advances in genetic analysis, especially in mice and humans, have revealed many genes that play essential roles in meiosis in mammals. These include cell cycle-regulatory proteins that couple the exit from the premeiotic DNA synthesis to the progression through prophase I, the chromosome structural proteins involved in synapsis, and the repair and recombination proteins that process the recombination events. Failure to adequately repair the DNA damage caused by recombination triggers meiotic checkpoints that result in ablation of the germ cells by apoptosis. These analyses have revealed surprising sexual dimorphism in the requirements of different gene products and a much less stringent checkpoint regulation in females. This may provide an explanation for the 10-fold increase in meiotic errors in females compared with males. This review provides a comprehensive analysis of the use of genetic manipulation, particularly in mice, but also of the analysis of mutations in humans, to elucidate the mechanisms that are required for traverse through prophase I.

Checking Your Breaks: Surveillance Mechanisms of Meiotic Recombination

Numerous DNA double-strand breaks (DSBs) are introduced into the genome in the course of meiotic recombination. This poses a significant hazard to the genomic integrity of the cell. Studies in a number of organisms have unveiled the existence of surveillance mechanisms or checkpoints that couple the formation and repair of DSBs to cell cycle progression. Through these mechanisms, aberrant meiocytes are delayed in their meiotic progression, thereby facilitating repair of meiotic DSBs, or are culled through programmed cell death, thereby protecting the germline from aneuploidies that could lead to spontaneous abortions, birth defects and cancer predisposition in the offspring. Here we summarize recent progress in our understanding of these checkpoints. This review focuses on the surveillance mechanisms of the budding yeast S. cerevisiae, where the molecular details are best understood, but will frequently compare and contrast these mechanisms with observations in other organisms.

Cytogenetic analysis of azoospermic patients: karyotype comparison of peripheral blood lymphocytes and testicular tissue

OBJECTIVE

The objective was to compare the results of a complete chromosomal, genetic and histological investigation in 13 azoospermic men with the results of the intracytoplasmic sperm injection (ICSI) procedure.

STUDY DESIGN

Peripheral blood samples were used for the measurement of follicle-stimulating hormone (FSH) levels, chromosomal analysis, microdeletions in the azoospermia factor (AZF) region of the Y chromosome and cystic fibrosis transmembrane conductance regulator (CFTR) mutation analysis. Testicular tissue was used for histological scoring and cytogenetic evaluation.

RESULTS

Peripheral blood cytogenetic analysis revealed a normal male karyotype in all cases. Chromosomal analysis from testicular tissue revealed a mosaicism for the terminal deletion of chromosome 22 with a breakpoint site at 22q13 in one patient with congenital bilateral absence of the vas deferens (CBAVD). Deletions in the AZFa, ATFb, and AZFc regions were not detected. The CFTR mutational analysis showed normal results in all patients.

CONCLUSIONS

Cytogenetic evaluation of testicular tissue should be performed in non-obstructive and obstructive azoospermic patients as well as in patients with multiple failed IVF and recurrent spontaneous abortion.

Cell cycle regulation in mammalian germ cells

Meiosis is a unique form of cellular division by which a diploid cell produces genetically distinct haploid gametes. Initiation and regulation of mammalian meiosis differs between the sexes. In females, meiosis is initiated during embryo development and arrested shortly after birth during prophase I. In males, spermatogonial stem cells initiate meiosis at puberty and proceed through gametogenesis with no cell cycle arrest. Mouse genes required for early meiotic cell cycle events are being identified by comparative analysis with other eukaryotic systems, by virtue of gene knockout technology and by mouse mutagenesis screens for reproductive defects. This review focuses on mouse reproductive biology and describes the available mouse mutants with defects in the early meiotic cell cycle and prophase I regulatory events. These research tools will permit rapid advances in such medically relevant research areas as infertility, embryo lethality and developmental abnormalities.

Recombination in men with Klinefelter syndrome

Klinefelter syndrome (KS: 47,XXY), occurs in one in 1000 male births. Men with KS are infertile and have higher rates of aneuploidies in sperm compared with normal fertile men. In the course of analyzing recombination in a population of infertile men, we observed that four men in our study presented with KS. We examined whether these men differed in recombination parameters among themselves and relative to normal men. Even though the number of men with KS analyzed was small, we observed remarkable variation in spermatogenesis. In spite of the fact that the men had the same genetic cause for infertility, two of four KS patients had few or no spermatogenic cells that progressed through meiosis to the pachytene stage, whereas the other two men produced abundant pachytene cells that had recombination frequencies comparable with those of fertile men, although one had a significant reduction in fidelity of synapsis. Moreover, regardless of histological appearance, examination of outcomes of assisted reproduction indicated that sperm were extracted from testis biopsies in all four cases, and when used in assisted reproductive practices chromosomally normal babies were born. These results reinforce that: (i) men with the same underlying genetic cause for infertility do not present with uniform pathology, (ii) the checkpoint machinery that might arrest spermatogenesis in the face of chromosomal abnormalities does not prevent pockets of complete spermatogenesis in men with KS, and (iii) aneuploidy, in some cases, is compatible with birth of a chromosomally normal child, suggesting that sperm produced from a background of aneuploidy can be normal in men with KS.

Not all germ cells are created equal: Aspects of sexual dimorphism in mammalian meiosis

The study of mammalian meiosis is complicated by the timing of meiotic events in females and by the intermingling of meiotic sub-stages with somatic cells in the gonad of both sexes. In addition, studies of mouse mutants for different meiotic regulators have revealed significant differences in the stringency of meiotic events in males versus females. This sexual dimorphism implies that the processes of recombination and homologous chromosome pairing, while being controlled by similar genetic pathways, are subject to different levels of checkpoint control in males and females. This review is focused on the emerging picture of sexual dimorphism exhibited by mammalian germ cells using evidence from the broad range of meiotic mutants now available in the mouse. Many of these mouse mutants display distinct differences in meiotic progression and/or dysfunction in males versus females, and their continued study will allow us to understand the molecular basis for the sex-specific differences observed during prophase I progression.

Mammalian oocyte therapies

In assisted human reproduction, the cytoplasm of oocytes recovered from follicles is often abnormal. Its lower quality, especially in older patients, may be responsible for certain chromosomal abnormalities or developmental arrest. Thus, the deficiency of some vital molecules, which are necessary for oocyte maturation, can be the cause of infertility in women. Moreover, mutated mitochondrial DNA (mtDNA) that is located in the oocyte cytoplasm might be transmitted to offspring. With the advance of new micromanipulation techniques like the oocyte nucleus replacement or cytoplasmic transfer, some of these abnormalities could be theoretically eliminated. In this review, we briefly discuss some of these approaches and their potential use in assisted human reproduction.

Mouse genetic models for aneuploidy induction in germ cells

Rodents have been successfully used as models to identify risks of chemical exposures or age to aneuploidy induction in germ cells, which may be transmitted to the progeny. For this administration in vivo as well as exposures to in vitro maturing germ cells have been useful. Genetic models involving mice with structural chromosomal rearrangements and transgenic animals have the potential to model conditions predisposing to aneuploidy in one or both sexes, and in this way to identify potential targets for aneugens and gender-effects. The review provides an overview of mouse genetic models for aneuploidy induction in mammalian germ cells and discusses perspectives for combining genetic with experimental approaches in aneuploidy research.

Arrest of spermatogenesis at the early meiotic stage in the small testis mutant (Smt) mice

A spontaneous mutant having small testes was found in a laboratory mouse strain of mixed origins. The testis size was 1/3-1/2 of normal size, but no significant difference was seen in body mass and weight of organs such as kidney and seminal vesicle, which are influenced by androgen. Small testis males were found to be infertile by the mating test, although formation of a vaginal plug was normally observed in their female partners. Histological and air-dried specimens revealed degeneration of zygotene or early pachytene spermatocytes and very few numbers of pachytene and diplotene spermatocytes, round and elongate spermatids and mature spermatozoa in the mutant testis. Therefore, it was concluded that spermatogenesis is disrupted at the zygotene to early pachytene stages of meiosis in the mutant males. Segregation ratios of normal and mutant males were in accord with the assumption that the small testis character is caused by an autosomal recessive mutation. This mutant may be useful for research that would contribute to the elucidation of genetic mechanisms controlling the process of spermatogenesis and as a model animal for male infertility in humans.

Random mutagenesis of proximal mouse chromosome 5 uncovers predominantly embryonic lethal mutations

A region-specific ENU mutagenesis screen was conducted to elucidate the functional content of proximal mouse Chr 5. We used the visibly marked, recessive, lethal inversion Rump White (Rw) as a balancer in a three-generation breeding scheme to identify recessive mutations within the approximately 50 megabases spanned by Rw. A total of 1003 pedigrees were produced, representing the largest inversion screen performed in mice. Test-class animals, homozygous for the ENU-mutagenized proximal Chr 5 and visibly distinguishable from nonhomozygous littermates, were screened for fertility, hearing, vestibular function, DNA repair, behavior, and dysmorphology. Lethals were identifiable by failure to derive test-class animals within a pedigree. Embryonic lethal mutations (total of 34) were overwhelmingly the largest class of mutants recovered. We characterized them with respect to the time of embryonic death, revealing that most act at midgestation (8.5-10.5) or sooner. To position the mutations within the Rw region and to guide allelism tests, we performed complementation analyses with a set of new and existing chromosomal deletions, as well as standard recombinational mapping on a subset of the mutations. By pooling the data from this and other region-specific mutagenesis projects, we calculate that the mouse genome contains approximately 3479-4825 embryonic lethal genes, or about 13.7%-19% of all genes.

Mei1 is epistatic to Dmc1 during mouse meiosis

The Mei1(m1Jcs) allele contains a point mutation in a novel gene required for normal meiosis in male and female mice. We previously hypothesized that Mei1 is likely required for the formation of genetically programmed double-strand breaks (DSBs), the initiating event of meiotic recombination because in mutant spermatocytes (1) RAD51 foci are greatly reduced at zygonema; (2) RAD51 foci can be restored by cisplatin-induced DNA damage; and (3) phosphorylated H2AX is greatly reduced at leptonema. If this hypothesis is correct, Mei1 would act upstream of genes required for repair of DSBs by homologous recombination. To test this, we examined meiosis in Mei(m1Jcs)/Mei1(m1Jcs) (Mei1(-/-)) and Dmc1(tm1Jcs)/Dmc1(tm1Jcs) (Dmc1(-/-)) mice and mice homozygous at both loci (Dmc1(-/-) Mei1(-/-)), exploiting the fact that oogenesis is much more severely affected by the absence of DMC1 than by the absence of MEI1. The phenotypes of both male and female double mutants were identical to that of Mei1(-/-) animals. Therefore, Mei1 can be positioned upstream of Dmc1 in the genetic pathway that operates during mammalian meiosis. Furthermore, this epistatic interaction provides additional evidence in support of the hypothesis that Mei1 is required for the initiating events of meiotic recombination.

Genetic manipulations to study reproduction

Fertility disorders affect approximately 15% of individuals worldwide. With the imminent completion of the human and mouse genome sequence, it will be more feasible to identify the relevant genes underlying many fertility disorders. Already, the mouse has been utilized extensively as a genetic tool for the dissection of gene function, often providing significant insights into the relationship between gene and disease. In fact, there are over 200 mouse models that display reproductive defects. However, the available mouse mutant resources provide functional information for a mere 10% of the total number of genes in the mouse or human genomes at best. The improvement of available genome annotations together with more powerful techniques to manipulate the mouse genome provide substantial improvements in our ability to identify genes involved in reproduction, and in the future will likely benefit patients with fertility problems.

Positional cloning and characterization of mouse mei8, a disrupted allelle of the meiotic cohesin Rec8

A novel mutation, mei8, was isolated in a forward genetic screen for infertility mutations induced by chemical mutagenesis of ES cells. Homozygous mutant mice are sterile. Mutant females exhibit ovarian dysgenesis and lack ovarian follicles at reproductive maturity. Affected males have small testes due to arrest of spermatogenesis during meiotic prophase I. Genetic mapping and positional cloning of mei8 led to the identification of a mutation in Rec8, a homolog of the yeast meiosis-specific cohesin gene REC8. Analysis of meiosis in Rec8(mei8)/Rec8(mei8) spermatocytes showed that, while initiation of recombination and synapsis occurs, REC8 is required for the completion and/or maintenance of synapsis, cohesion of sister chromatids, and the formation of chiasmata, as it is in other organisms. However, unlike yeast and Caenorhabditis elegans, localization of REC8 on meiotic chromosomes is not required for the assembly of axial elements.

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

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

Antiviral protein Ski8 is a direct partner of Spo11 in meiotic DNA break formation, independent of its cytoplasmic role in RNA metabolism

Meiotic recombination initiates with double-strand breaks (DSBs) catalyzed by Spo11 in conjunction with accessory proteins whose roles are not understood. Two-hybrid analysis reveals a network of interactions connecting the yeast DSB proteins to one another. Of these proteins, Ski8 was known to function in cytoplasmic RNA metabolism, suggesting that its role in recombination might be indirect. However, obligate partners of Ski8 in RNA metabolism are dispensable for recombination and Ski8 relocalizes to the nucleus and associates with chromosomes specifically during meiosis. Interaction of Ski8 with Spo11 is essential for DSB formation and Ski8 relocalization. Thus, Ski8 plays distinct roles in RNA metabolism and, as a direct partner of Spo11, in DSB formation. Ski8 works with Spo11 to recruit other DSB proteins to meiotic chromosomes, implicating Ski8 as a scaffold protein mediating assembly of a multiprotein complex essential for DSB formation.

Spindles, mitochondria and redox potential in ageing oocytes

Studies of human oocytes obtained from women of advanced reproductive age revealed that spindles are frequently aberrant, with chromosomes sometimes failing to align properly at the equator during meiosis I and II. Chromosomal analyses of donated and spare human oocytes and cytogenetic and molecular studies on the origin of trisomies collectively suggest that errors in chromosome segregation during oogenesis increase with advancing maternal age and as the menopause approaches. Disturbances in the fidelity of chromosome segregation, especially at anaphase I, leading to aneuploidy are prime causes of reduced developmental competence of embryos in assisted reproduction, as well as being responsible for the genesis of genetic disease. This review provides an overview of spindle formation and chromosome behaviour in mammalian oocytes. Evidence of a link between abnormal mitochondrial function in oocytes and somatic follicular cells, and finally disturbances in chromosome cohesion and segregation, and cell cycle control in aged mammalian oocytes, are also discussed.

Activating Transcription Factor 4 Is Required for the Differentiation of the Lamina Propria Layer of the Vas Deferens1

Activating transcription factor 4 (ATF4/CREB2) is a member of the cyclic-AMP response element-binding (CREB) family. These proteins have been shown to regulate cell proliferation and differentiation in a broad number of tissues during embryo development. Here we report that male ATF4(-/-) mice are subfertile, despite the fact that they produce sufficient sperm and are able to fertilize wild-type eggs in vitro. An analysis of the ejaculatory ducts revealed abnormal constrictions in the lumen of the vas deferens. The lamina propria layer of the vas deferens was significantly thicker in the ATF4(-/-) mice and the cells that make up this layer were rounder and more abundant than in the ATF4(+/+) littermates. The change in the morphology of the lamina propria was associated with sexual maturation. A histologic analysis of the lamina propria revealed a reduction in the production of elastic fibers and interstitial cells of Cajal, as judged by the expression of neuron-specific enolase. These observations predict that ATF4 is required for the normal differentiation of the lamina propria layer of the vas deferens at sexual maturation. The morphology of the ATF4(-/-) lamina propria and the constriction of the lumen are consistent with an obstruction in the vas deferens contributing to the subfertility of the ATF4(-/-) males.

Positional cloning and characterization of Mei1, a vertebrate-specific gene required for normal meiotic chromosome synapsis in mice

The mouse meiotic mutant Mei1 was isolated in a screen for infertile mice descended from chemically mutagenized embryonic stem cells. Homozygotes of both sexes are sterile due to meiotic arrest caused by defects in chromosome synapsis. Notably, RAD51 protein does not load onto Mei1 mutant meiotic chromosomes, suggesting that there is a defect in either recombinational repair or the production of double-strand breaks (DSBs) that require such repair. Here, we show that treatment of mutant males with cisplatin restores RAD51 loading, suggesting that mutant spermatocytes have intact recombinational repair mechanisms. Levels of histone H2AX phosphorylation (gammaH2AX) at leptonema are significantly reduced compared with wild-type controls but comparable to that seen in animals deficient for SPO11, the molecule required for catalyzing DSB formation during meiosis. These observations provide evidence that genetically programmed DSB induction is defective in Mei1 leptotene spermatocytes. We also report the positional cloning of Mei1, which encodes a product without significant homology to any known protein. Expressed almost exclusively in gonads, Mei1 has no apparent homologs in yeast, worms, or flies. However, Mei1 orthologs are present in the genomes of mammals, chickens, and zebrafish. Thus, Mei1 is required for vertebrate meiosis. To our knowledge, Mei1 is the first meiosis-specific mutation identified by forward genetic approaches in mammals.

BRCA2 deficiency in mice leads to meiotic impairment and infertility

The role of Brca2 in gametogenesis has been obscure because of embryonic lethality of the knockout mice. We generated Brca2-null mice carrying a human BAC with the BRCA2 gene. This construct rescues embryonic lethality and the mice develop normally. However, there is poor expression of the transgene in the gonads and the mice are infertile, allowing examination of the function of BRCA2 in gametogenesis. BRCA2-deficient spermatocytes fail to progress beyond the early prophase I stage of meiosis. Observations on localization of recombination-related and spermatogenic-related proteins suggest that the spermatocytes undergo early steps of recombination (DNA double strand break formation), but fail to complete recombination or initiate spermiogenic development. In contrast to the early meiotic prophase arrest of spermatocytes, some mutant oocytes can progress through meiotic prophase I, albeit with a high frequency of nuclear abnormalities, and can be fertilized and produce embryos. Nonetheless, there is marked depletion of germ cells in adult females. These studies provide evidence for key roles of the BRCA2 protein in mammalian gametogenesis and meiotic success.

Toward the genetics of mammalian reproduction: induction and mapping of gametogenesis mutants in mice

The genetic control of mammalian gametogenesis is inadequately characterized because of a lack of mutations causing infertility. To further the discovery of genes required for mammalian gametogenesis, phenotype-driven screens were performed in mice using random chemical mutagenesis of whole animals and embryonic stem cells. Eleven initial mutations are reported here that affect proliferation of germ cells, meiosis, spermiogenesis, and spermiation. Nine of the mutations have been mapped genetically. These preliminary studies provide baselines for estimating the number of genes required for gametogenesis and offer guidance in conducting new genetic screens that will accelerate and optimize mutant discovery. This report demonstrates the efficacy and expediency of mutagenesis to identify new genes required for mammalian gamete development.

Somatic cell haploidization: an update

Oocyte donation is the only method of treating female sterility caused by complete absence of oocytes, with the loss of genetic motherhood. Genetic fatherhood of males with complete absence of spermatozoa can only be restored by assisted reproduction treatment if sperm precursor cells belonging to the male germline can still be recovered from the testis. Otherwise, sperm donation is the only available solution. Somatic nucleus haploidization after injection into previously enucleated donor oocytes (diploid-to-haploid reduction) might enable the reconstruction of new oocytes carrying the complete nuclear genome of female patients lacking their own oocytes. Such newly formed oocytes could subsequently be fertilized by spermatozoa from the patient's husband. In cases of male infertility with complete absence of the germline, the patient's somatic cell nuclei could be injected into the oocytes without previous enucleation, and somatic nucleus haploidization would occur in the presence of the original female nucleus (triploid-to-diploid reduction), hopefully leading to the formation of a diploid embryo. Both interventions differ substantially from cloning because embryos are formed by syngamy with the male and female genomes originating from the two genetic parents, as in natural fertilization. Ultrastructural remodelling of mouse somatic cell nucleoli can be achieved in enucleated metaphase II mouse oocytes. Haploidization has also been attempted with Sertoli cells and with fibroblasts, both of which are also available in male patients. Experiments are currently under way to assess the regularity of chromatid segregation during somatic nucleus haploidization.

Mouse models of male infertility

Spermatogenesis is a complex process that involves stem-cell renewal, genome reorganization and genome repackaging, and that culminates in the production of motile gametes. Problems at all stages of spermatogenesis contribute to human infertility, but few of them can be modelled in vitro or in cell culture. Targeted mutagenesis in the mouse provides a powerful method to analyse these steps and has provided new insights into the origins of male infertility.