Meiosis and sex chromosome aneuploidy: how meiotic errors cause aneuploidy; how aneuploidy causes meiotic errors

Meiosis through three centuries

Meiosis is the specialized cellular program that underlies gamete formation for sexual reproduction. It is therefore not only interesting but also a fundamentally important subject for investigation. An especially attractive feature of this program is that many of the processes of special interest involve organized chromosomes, thus providing the possibility to see chromosomes "in action". Analysis of meiosis has also proven to be useful in discovering and understanding processes that are universal to all chromosomal programs. Here we provide an overview of the different historical moments when the gap between observation and understanding of mechanisms and/or roles for the new discovered molecules was bridged. This review reflects also the synergy of thinking and discussion among our three laboratories during the past several decades.

Clinical evaluation of noninvasive prenatal testing for sex chromosome aneuploidies in 9,176 Korean pregnant women: a single-center retrospective study

BACKGROUND

To evaluate the clinical significance of noninvasive prenatal testing (NIPT) for detecting fetal sex chromosome aneuploidies (SCAs) in Korean pregnant women.

METHODS

We retrospectively analyzed NIPT data from 9,176 women with singleton pregnancies referred to the CHA Biotech genome diagnostics center. Cell-free fetal DNA (cffDNA) was extracted from maternal peripheral blood, and high-throughput massively parallel sequencing was conducted. Subsequently, the positive NIPT results for SCA were validated via karyotype and chromosomal microarray analyses.

RESULTS

Overall, 46 cases were SCA positive after NIPT, including 20, 12, 8, and 6 for Turner, triple X, Klinefelter, and Jacob syndromes, respectively. Among 37 women with invasive prenatal diagnosis, 19 had true positive NIPT results. The overall positive predictive value (PPV) of NIPT for detecting SCAs was 51.35%. The PPV was 18.75% for Turner, 88.89% for triple X, 71.43% for Klinefelter, and 60.00% for Jacob's syndromes. NIPT accuracy for detecting sex chromosome trisomies was higher than that for sex chromosome monosomy (P = 0.002). No significant correlation was observed between fetal SCA incidence and maternal age (P = 0.914), except for the borderline significance of Jacob's syndrome (P = 0.048). No significant differences were observed when comparing NIPT and karyotyping validation for fetal SCA according to pregnancy characteristics.

CONCLUSION

Our data suggest that NIPT can reliably screen for SCAs, and it performed better in predicting sex chromosome trisomies compared with monosomy X. No correlation was observed between maternal age and fetal SCA incidence, and no association was observed between different pregnancy characteristics. The accuracy of these findings requires improvements; however, our study provides an important reference for clinical genetic counseling and further management. Larger scale studies, considering confounding factors, are required for accurate evaluation.

Sex chromosomes: How to make a hermaphrodite

The existence of sex chromosomes complicates the evolution of cosexuality (hermaphroditism). Four new genomic studies from haploid-dominant plants show commonalities and differences in mechanisms of the evolution of cosexuality, raising questions about the genetics of sexual dimorphism and the fate of cosexual lineages.

Sex-specific meiosis responses to Gsdf in medaka (Oryzias latipes)

The meiotic entry of undifferentiated germ cells is sexually specific and strictly regulated by the testicular or ovarian environment. Germline stem cells with a set of abnormal sex chromosomes and associated autosomes undergo defective meiotic processes and are eventually eliminated by yet to be defined post-transcriptional modifications. Herein, we report the role of gsdf, a member of BMP/TGFβ family uniquely found in teleost, in the regulation of meiotic entry in medaka (Oryzias latipes) via analyses of gametogenesis in gsdf-deficient XX and XY gonads in comparison with their wild-type siblings. Several differentially expressed genes, including the FKB506-binding protein 7 (fkbp7), were significantly upregulated in pubertal gsdf-deficient gonads. The increase in alternative pre-mRNA isoforms of meiotic synaptonemal complex gene sycp3 was visualized using Integrative Genomics Viewer and confirmed by real-time qPCR. Nevertheless, immunofluorescence analysis showed that Sycp3 protein products reduced significantly in gsdf-deficient XY oocytes. Transmission electron microscope observations showed that normal synchronous cysts were replaced by asynchronous cysts in gsdf-deficient testis. Breeding experiments showed that the sex ratio deviation of gsdf^-/- XY gametes in a non-Mendelian manner might be due to the non-segregation of XY chromosomes. Taken together, our results suggest that gsdf plays a role in the proper execution of cytoplasmic and nuclear events through receptor Smad phosphorylation and Sycp3 dephosphorylation to coordinate medaka gametogenesis, including sex-specific mitotic divisions and meiotic recombination.

Satellite DNAs and human sex chromosome variation

Satellite DNAs are present on every chromosome in the cell and are typically enriched in repetitive, heterochromatic parts of the human genome. Sex chromosomes represent a unique genomic and epigenetic context. In this review, we first report what is known about satellite DNA biology on human X and Y chromosomes, including repeat content and organization, as well as satellite variation in typical euploid individuals. Then, we review sex chromosome aneuploidies that are among the most common types of aneuploidies in the general population, and are better tolerated than autosomal aneuploidies. This is demonstrated also by the fact that aging is associated with the loss of the X, and especially the Y chromosome. In addition, supernumerary sex chromosomes enable us to study general processes in a cell, such as analyzing heterochromatin dosage (i.e. additional Barr bodies and long heterochromatin arrays on Yq) and their downstream consequences. Finally, genomic and epigenetic organization and regulation of satellite DNA could influence chromosome stability and lead to aneuploidy. In this review, we argue that the complete annotation of satellite DNA on sex chromosomes in human, and especially in centromeric regions, will aid in explaining the prevalence and the consequences of sex chromosome aneuploidies.

The Hypothesis of the Prolonged Cell Cycle in Turner Syndrome

Turner syndrome (TS) is a chromosomal disorder that is caused by a missing or structurally abnormal second sex chromosome. Subjects with TS are at an increased risk of developing intrauterine growth retardation, low birth weight, short stature, congenital heart diseases, infertility, obesity, dyslipidemia, hypertension, insulin resistance, type 2 diabetes mellitus, metabolic syndrome, and cardiovascular diseases (stroke and myocardial infarction). The underlying pathogenetic mechanism of TS is unknown. The assumption that X chromosome-linked gene haploinsufficiency is associated with the TS phenotype is questioned since such genes have not been identified. Thus, other pathogenic mechanisms have been suggested to explain this phenotype. Morphogenesis encompasses a series of events that includes cell division, the production of migratory precursors and their progeny, differentiation, programmed cell death, and integration into organs and systems. The precise control of the growth and differentiation of cells is essential for normal development. The cell cycle frequency and the number of proliferating cells are essential in cell growth. 45,X cells have a failure to proliferate at a normal rate, leading to a decreased cell number in a given tissue during organogenesis. A convergence of data indicates an association between a prolonged cell cycle and the phenotypical features in Turner syndrome. This review aims to examine old and new findings concerning the relationship between a prolonged cell cycle and TS phenotype. These studies reveal a diversity of phenotypic features in TS that could be explained by reduced cell proliferation. The implications of this hypothesis for our understanding of the TS phenotype and its pathogenesis are discussed. It is not surprising that 45,X monosomy leads to cellular growth pathway dysregulation with profound deleterious effects on both embryonic and later stages of development. The prolonged cell cycle could represent the beginning of the pathogenesis of TS, leading to a series of phenotypic consequences in embryonic/fetal, neonatal, pediatric, adolescence, and adulthood life.

Usefulness of combined NGS and QF‐PCR analysis for product of conception karyotyping

Purpose

Since chromosomal abnormalities can be detected in more than half of miscarriages, cytogenetic testing of the product of conception (POC) can provide important information when preparing for a subsequent pregnancy. Conventional karyotyping is the common diagnostic method for a POC but can be problematic due to the need for cell culture.

Methods

We here conducted shallow whole‐genome sequencing (sWGS) using next‐generation sequencing (NGS) for alternative POC cytogenomic analysis. Since female euploidy samples can include 69,XXX triploidy, additional QF‐PCR was performed in these cases.

Results

We here analyzed POC samples from miscarriages in 300 assisted reproductive technology (ART) pregnancies and detected chromosomal abnormalities in 201 instances (67.0%). Autosomal aneuploidy (151 cases, 50.3%) was the most frequent abnormality, consistent with prior conventional karyotyping data. Mosaic aneuploidy was detected in seven cases (2.0%). Notably, the frequency of triploidy was 2.3%, 10‐fold lower than the reported frequency in non‐ART pregnancies. Structural rearrangements were identified in nine samples (3%), but there was no case of segmental mosaicism.

Conclusions

These data suggest that NGS‐based sWGS, with the aid of QF‐PCR, is a viable alternative karyotyping procedure that does not require cell culture. This method could also assist with genetic counseling for couples who undergoes embryo selection based on PGT‐A data.

Generation of the Y-chromosome linked red fluorescent protein transgenic mouse model and sexing at the preimplantation stage

In mammals, sexual fate is determined by the chromosomes of the male and female gametes during fertilization. Males (XY) or females (XX) are produced when a sperm containing a Y or X-chromosome respectively fertilizes an X-chromosome-containing unfertilized egg. However, sexing of preimplantation stage embryos cannot be conducted visually. To address this, transgenic male mouse models with the ubiquitously expressed green fluorescent protein (GFP) transgene on X- (X-GFP) or Y-chromosomes (Y-GFP) have been established. However, when crossed with wild-type females, sexing of the preimplantation stage embryos by observing the GFP signal is problematic in some cases due to X-inactivation, loss of Y-chromosome (LOY), or loss of transgene fluorescence. In this study, a mouse model with the ubiquitously expressed red fluorescent protein (RFP) transgene on the Y-chromosome was generated since RFP is easily distinguishable from GFP signals. Unfortunately, the ubiquitously expressed tdTomato RFP transgene on the Y-chromosome (Y-RFP) mouse showed the lethal phenotype after birth. No lethal phenotypes were observed when the mitochondrial locating signal N-terminal of tdTomato (mtRFP) was included in the transgene construct. Almost half of the collected fertilized eggs from Y-mtRFP male mice crossed with wild-type females had an RFP signal at the preimplantation stage (E1.5). Therefore, XY eggs were recognized as RFP-positive embryos at the preimplantation stage. Furthermore, 100% sexing was observed at the preimplantation stage using the X-linked GFP/Y-linked RFP male mouse. The established Y-mtRFP mouse models may be used to study sex chromosome related research.

Cytogenetic Screening in Couples with Recurrent Pregnancy Loss: A Single-Center Study and Review of Literature

Context:

Recurrent pregnancy loss (RPL) is a devastating reproductive problem that affects more than 2% of couples who are trying to conceive. Chromosomal rearrangements in either carrier are a major cause of clinically recognized abortion.

Aims:

The purpose of this study is to report the prevalence of chromosome abnormalities in RPL and provide clinical characteristics of couples with two and more miscarriages.

Settings and Design:

Genetic counseling in laboratory of histology housed in a Faculty of Medicine of Sfax.

Materials and Methods:

Karyotype was generated from the peripheral blood lymphocyte cultures and the cytogenetic analysis was performed using R-bands after heat denaturation and Giemsa (RHG) banding. A multiplex polymerase chain reaction wherever necessary was done.

Statistical Analysis Used:

SPSS version 17.

Results:

A total of 104 couples with RPL were carried out in this study. The frequency of chromosomal rearrangements was 11.5%, three times more prevalent in men than women (P = 0.08). In addition, the prevalence of chromosomal anomalies increases according to the number of miscarriages (from 4.8% to 7.6%, with 2 or ≥3 miscarriages, respectively; P = 0.9). Finally, a particular familial adverse reproductive background was found in these carriers (P = 0.03).

Conclusions:

These data highlight that an RPL evaluation is appropriate after the second miscarriage and that cytogenetic evaluation is necessary for an accurate approach to elucidate the causes of RPL. Moreover, familial adverse reproductive backgrounds have an impact of being carrier of chromosome abnormalities and a larger study is mandatory to define reproductive characteristics of carriers.

Molecular basis of reproductive senescence: insights from model organisms

PURPOSE

Reproductive decline due to parental age has become a major barrier to fertility as couples have delayed having offspring into their thirties and forties. Advanced parental age is also associated with increased incidence of neurological and cardiovascular disease in offspring. Thus, elucidating the etiology of reproductive decline is of clinical importance.

METHODS

Deciphering the underlying processes that drive reproductive decline is particularly challenging in women in whom a discrete oocyte pool is established during embryogenesis and may remain dormant for tens of years. Instead, our understanding of the processes that drive reproductive senescence has emerged from studies in model organisms, both vertebrate and invertebrate, that are the focus of this literature review.

CONCLUSIONS

Studies of reproductive aging in model organisms not only have revealed the detrimental cellular changes that occur with age but also are helping identify major regulator proteins controlling them. Here, we discuss what we have learned from model organisms with respect to the molecular mechanisms that maintain both genome integrity and oocyte quality.

Impact of parental origin of X-chromosome on clinical and biochemical profile in Turner syndrome

Objectives To evaluate if the parental origin of X-chromosome has an impact on the phenotype and biochemical profile in Turner syndrome (TS). Result of the previous studies have been equivocal and could be attributable to the multicentric study design with different experts examining heterogeneous TS population of various ethnic background. Methods A cross-sectional single center study from Northern India. Fifty nine diagnosed subjects of TS and their parents participated in the study. Parental origin of intact X-chromosome was determined using 12 highly polymorphic short tandem repeats (STR) on X-chromosome. For the evaluation of parent-of-origin effects, typical phenotypic traits including congenital malformations, anthropometry, body composition by dual energy X-ray absorptiometry (DXA) and biochemical profile were compared. Clinical stigmata of TS in all subjects were examined by a single expert. Results The intact X-chromosome was of maternal origin (Xm) in 49.1% subjects while 50.9% had paternal origin (Xp). Skeletal anomalies were more common in Xm group, out of which prevalence of short neck and short fourth metatarsal reached statistical significance (p=0.04 and 0.01 respectively). A strong correlation was observed between subject's baseline height standard deviation score (Ht SDS) and paternal height (r=0.593, p<0.001), maternal height (r=0.564, p<0.001) and mid-parental height (MPH) (r=0.372, p=0.047) in Xp group. This effect was not seen in Xm subjects whose baseline Ht SDS showed no significant correlation with maternal height, paternal height or MPH. No differences were detected between the groups with regard to biochemical profile or body composition. Conclusions We speculate that the differences in skeletal anomalies and height correlations between Xm and Xp groups could be due to the modifying effect of epigenetic signature on short stature homeobox (SHOX) gene of Xm. SHOX gene is not modified on Xp thereby explaining the paucity of skeletal changes and height correlations in Xp subjects.

Karyotyping and prenatal diagnosis of 47,XX,+ 8[67]/46,XX [13] Mosaicism: case report and literature review

BACKGROUND

Trisomy 8 mosaicism has a wide phenotypic variability, ranging from mild dysmorphic features to severe malformations. This report concluded a female pregnant woman with trisomy 8 mosaicism, and carefully cytogenetic diagnoses were performed to give her prenatal diagnostic information. This report also provides more knowledge about trisomy 8 mosaicism and the prenatal diagnostic for clinicians.

CASE PRESENTATION

In this present study, we reported one case of pregnancy woman with trisomy 8 mosaicism. Noninvasive prenatal testing prompted an abnormal Z-score, but further three dimension color ultrasound result suggested a single live fetus with no abnormality. The phenotypic of the pregnant woman was normal. Based on our results, there were no abnormal initial myeloid cells (< 10^- 4), which suggested that the patient had no blood diseases. The peripheral blood karyotype of the patient was 47,XX,+ 8[67]/46,XX [13], and karyotype of amniotic fluid was 46, XX. The next generation sequencing (NGS) result suggested that the proportions of trisomy 8 in different tissues were obviously different; and 0% in amniotic fluid. Last, the chromosomes of the patient and her baby were confirmed using chromosome microarray analysis (CMA), and the results were arr[GRCh37](8) × 3,11p15.5p13(230750-33,455,733) × 2 hmz and normal.

CONCLUSIONS

This pregnancy woman was trisomy 8 mosaicism, but the phenotypic was normal, and also the fetus was normal. Carefully cytogenetic diagnoses should be performed for prenatal diagnose.

Update On The Clinical Perspectives And Care Of The Child With 47,XXY (Klinefelter Syndrome)

47,XXY (Klinefelter syndrome [KS]) is the most common sex chromosomal aneuploidy (1:660), yet, despite this, only 25% of the males are ever diagnosed. Males with 47,XXY present with characteristic symptoms throughout their lifetime with typical physical and neurodevelopmental manifestations focused in growth, cognitive development, endocrine function, and reproduction. Studies have demonstrated that optimal outcomes are dependent on early detection combined with consistent and targeted neurodevelopmental treatment throughout the lifespan. During infancy and into the preschool years, individuals with 47,XXY commonly face deficits in growth and development in the areas of early hormonal, motor, speech, and behavioral development. As they transition into school, the primary neurodevelopmental concerns include language difficulty, executive dysfunction, behavior, and learning and reading deficits. Adults with 47,XXY often present with taller than average height, low levels of fertility, azoospermia, and elevated gonadotropin levels. These presentations may persist from early childhood through adulthood but can be mitigated by appropriate interventions. Early neurodevelopmental and hormonal treatment has been shown to have a minimizing effect on the physical and neurodevelopmental manifestations in individuals with 47,XXY. With innovative and current research studies, the features common to the neurodevelopmental profile of 47,XXY have been further expanded and defined. Further research is necessary to elucidate and understand the relationship between the brain, behavior, and the phenotypic profile of 47,XXY.

A method for determining sex and chromosome copy number: sex-by-sequencing reveals the first two species of marine mammals with XXY chromosome condition

Molecular assays of sex determination typically rely on qualitative evaluation of sex-linked markers, which can lead to uncertainty when results contradict morphological identifiers of sex. To investigate whether disagreement between phenotypic and genotypic assays of sex could be underpinned by variation in sex chromosome copy number, we developed a quantitative genetic method to determine sex that is broadly applicable to mammals with XY sex determination. We sequenced a region of the zinc-finger gene ZF, which has fixed genetic differences between the X and Y chromosomes, and screened 173 cetacean specimens for ZFX–ZFY haplotype identity and read depth. Using a subset of 90 male specimens, we demonstrate that haplotype read depth is an accurate estimator of chromosome copy number. We identified three specimens representing two different cetacean species that had external female morphological traits, Y chromosome haplotypes, and ratios of ZFX:ZFY haplotypes that were above the 1:1 value expected for genetic males. These results provide the first evidence of XXY aneuploidy in cetaceans. Investigation of the reproductive tract of one specimen, a True’s beaked whale (Mesoplodon mirus), revealed an intersex phenotype; despite having external characteristics typically diagnostic for the female sex, a penis and testes were present. Our results suggest that intersex phenotypes may be associated with XXY aneuploidy, and that this phenomenon may be underestimated due to it not being detectable by qualitative assays for determining sex.

How do genes that escape from X-chromosome inactivation contribute to Turner syndrome?

X-chromosome inactivation generally results in dosage equivalence for expression of X-linked genes between 46,XY males and 46,XX females. The 20-30% of genes that escape silencing are thus candidates for having a role in the phenotype of Turner syndrome. Understanding which genes escape from silencing, and how they avoid this chromosome-wide inactivation is therefore an important step toward understanding Turner Syndrome. We have examined the mechanism of escape using a previously reported knock-in of a BAC containing the human escape gene RPS4X in mouse. We now demonstrate that escape from inactivation for RPS4X is already established by embryonic Day 9.5, and that both silencing and escape are faithfully maintained across the lifespan. No overt abnormalities were observed for transgenic mice up to 1 year of age despite robust transcription of the human RPS4X gene with no detectable downregulation of the mouse homolog. However, there was no significant increase in protein levels, suggesting translational compensation in the mouse. Finally, while many of the protein-coding genes have been assessed for their inactivation status, less is known about the X-linked RNA genes, and we propose that for many microRNA genes their inactivation status can be predicted as they are intronic to genes for which the inactivation status is known.

Recombination hotspots in an extended human pseudoautosomal domain predicted from double-strand break maps and characterized by sperm-based crossover analysis

The human X and Y chromosomes are heteromorphic but share a region of homology at the tips of their short arms, pseudoautosomal region 1 (PAR1), that supports obligate crossover in male meiosis. Although the boundary between pseudoautosomal and sex-specific DNA has traditionally been regarded as conserved among primates, it was recently discovered that the boundary position varies among human males, due to a translocation of ~110 kb from the X to the Y chromosome that creates an extended PAR1 (ePAR). This event has occurred at least twice in human evolution. So far, only limited evidence has been presented to suggest this extension is recombinationally active. Here, we sought direct proof by examining thousands of gametes from each of two ePAR-carrying men, for two subregions chosen on the basis of previously published male X-chromosomal meiotic double-strand break (DSB) maps. Crossover activity comparable to that seen at autosomal hotspots was observed between the X and the ePAR borne on the Y chromosome both at a distal and a proximal site within the 110-kb extension. Other hallmarks of classic recombination hotspots included evidence of transmission distortion and GC-biased gene conversion. We observed good correspondence between the male DSB clusters and historical recombination activity of this region in the X chromosomes of females, as ascertained from linkage disequilibrium analysis; this suggests that this region is similarly primed for crossover in both male and female germlines, although sex-specific differences may also exist. Extensive resequencing and inference of ePAR haplotypes, placed in the framework of the Y phylogeny as ascertained by both Y microsatellites and single nucleotide polymorphisms, allowed us to estimate a minimum rate of crossover over the entire ePAR region of 6-fold greater than genome average, comparable with pedigree estimates of PAR1 activity generally. We conclude ePAR very likely contributes to the critical crossover function of PAR1.

95% of our genome is contained in 22 pairs of chromosomes shared by all humans. However, women and men differ in their sex chromosomes: while women have two X chromosomes, men have an X and a smaller, sex-determining Y chromosome. To ensure correct partition of X and Y into sperm, genetic exchange (crossover) must occur between these very different chromosomes in a short, shared region. The location of the boundary of this region was thought to have been conserved since before the divergence from old world monkeys at least 27 million years ago, but recently it has been shown that some human males carry an extended version on their Y chromosomes, thanks to the transposition of a piece of DNA from the X chromosome. Here, we asked if genetic exchange occurs in this newly extended region. To do this, we used previously published information that signposted the positions within the X chromosome segment which exhibit the hallmarks of crossover initiation. We then sought direct evidence of crossover in the sperm of men carrying the extension. This work showed that the signposts were accurate, pointing to frequent crossover in this novel shared sex-chromosomal domain.

Inheritance of Chromosomes, Sex Determination, and the Human Genome

Who is the determining factor for the sex of the offspring—mother, father, or both parents? This fundamental hypothesis proposes a new model of sex determination, challenging the existing dogma that the male Y chromosome of the father is the sole determinant of the sex of the offspring. According to modern science, the 3 X chromosomes (male XY and female XX) are assumed to be similar, and the sex of the offspring is determined after the zygote is formed. In contrast to this, the new hypothesis based on theoretical research proposes that the 3 X chromosomes can be differentiated, based on the presence of Barr bodies. The first X in female XX chromosomes and X in male XY chromosomes are similar as they lack Barr body and are hereby denoted as ‘X’ and referred to as ancestral chromosomes. The second X chromosome in the female cells which is a Barr body, denoted as X, is different. This X chromosome along with the Y chromosome are referred to as parental chromosomes. Sperm with a Y chromosome can only fuse with an ovum containing the ‘X’ chromosome. Similarly, sperm with the ‘X’ chromosome can only fuse with an ovum containing the X chromosome. Cell biology models of gametogenesis and fertilization were simulated with the new hypothesis model and assessed. Only chromosomes that participated in recombination could unite to form the zygote. This resulted in a paradigm shift in our understanding of sex determination, as both parents were found to be equally responsible for determining the sex of the offspring. The gender of the offspring is determined during the prezygotic stage itself and is dependent on natural selection. A new dimension has been given to inheritance of chromosomes. This new model also presents a new nomenclature for pedigree charts. This work of serendipity may contribute to future research in cell biology, gender studies, genome analysis, and genetic disorders including cancer.

Mammalian X Chromosome Dosage Compensation: Perspectives From the Germ Line

Sex chromosomes are advantageous to mammals, allowing them to adopt a genetic rather than environmental sex determination system. However, sex chromosome evolution also carries a burden, because it results in an imbalance in gene dosage between females (XX) and males (XY). This imbalance is resolved by X dosage compensation, which comprises both X chromosome inactivation and X chromosome upregulation. X dosage compensation has been well characterized in the soma, but not in the germ line. Germ cells face a special challenge, because genome wide reprogramming erases epigenetic marks responsible for maintaining the X dosage compensated state. Here we explain how evolution has influenced the gene content and germ line specialization of the mammalian sex chromosomes. We discuss new research uncovering unusual X dosage compensation states in germ cells, which we postulate influence sexual dimorphisms in germ line development and cause infertility in individuals with sex chromosome aneuploidy.

Epigenetics in Turner syndrome

Background

Monosomy of the X chromosome is the most frequent genetic abnormality in human as it is present in approximately 2% of all conceptions, although 99% of these embryos are spontaneously miscarried. In postnatal life, clinical features of Turner syndrome may include typical dysmorphic stigmata, short stature, sexual infantilism, and renal, cardiac, skeletal, endocrine and metabolic abnormalities.

Main text

Turner syndrome is due to a partial or total loss of the second sexual chromosome, resulting in the development of highly variable clinical features. This phenotype may not merely be due to genomic imbalance from deleted genes but may also result from additive influences on associated genes within a given gene network, with an altered regulation of gene expression triggered by the absence of the second sex chromosome. Current studies in human and mouse models have demonstrated that this chromosomal abnormality leads to epigenetic changes, including differential DNA methylation in specific groups of downstream target genes in pathways associated with several clinical and metabolic features, mostly on autosomal chromosomes. In this article, we begin exploring the potential involvement of both genetic and epigenetic factors in the origin of X chromosome monosomy. We review the dispute between the meiotic and post-zygotic origins of 45,X monosomy, by mainly analyzing the findings from several studies that compare gene expression of the 45,X monosomy to their euploid and/or 47,XXX trisomic cell counterparts on peripheral blood mononuclear cells, amniotic fluid, human fibroblast cells, and induced pluripotent human cell lines. From these studies, a profile of epigenetic changes seems to emerge in response to chromosomal imbalance. An interesting finding of all these studies is that methylation-based and expression-based pathway analyses are complementary, rather than overlapping, and are correlated with the clinical picture displayed by TS subjects.

Conclusions

The clarification of these possible causal pathways may have future implications in increasing the life expectancy of these patients and may provide informative targets for early pharmaceutical intervention.

Sex chromosome aneuploidies

Sex chromosome aneuploidies comprise a relatively common group of chromosome disorders characterized by the loss or gain of one or more sex chromosomes. We discuss five of the better-known sex aneuploidies: Turner syndrome (XO), Klinefelter syndrome (XXY), trisomy X (XXX), XYY, and XXYY. Despite their prevalence in the general population, these disorders are underdiagnosed and the specific genetic mechanisms underlying their phenotypes are poorly understood. Although there is considerable variation between them in terms of associated functional impairment, each disorder has a characteristic physical, cognitive, and neurologic profile. The most common cause of sex chromosome aneuploidies is nondisjunction, which can occur during meiosis or during the early stages of postzygotic development. The loss or gain of genetic material can affect all daughter cells or it may be partial, leading to tissue mosaicism. In both typical and atypical sex chromosome karyotypes, there is random inactivation of all but one X chromosome. The mechanisms by which a phenotype results from sex chromosome aneuploidies are twofold: dosage imbalance arising from a small number of genes that escape inactivation, and their endocrinologic consequences.

Generation of integration-free induced pluripotent stem cells (GZHMUi001-A) by reprogramming peripheral blood mononuclear cells from a 47, XXX syndrome patient

47, XXX syndrome is one of several sex-chromosomal aneuploidies, and it has an incidence of approximately 1/1000 in newborn females. Because of heterogeneity in X-inactivation, these patients may exhibit a variety of clinical symptoms. Here, we report the generation of an integration-free human induced pluripotent stem cell line (GZHMUi001-A) by using Sendai virus to reprogram peripheral blood mononuclear cells from a 47, XXX syndrome patient with premature ovarian failure. This 47, XXX iPS cell line has characteristics of pluripotent stem cells and is a useful tool for the investigation of this X chromosome aneuploid disease.

Reduced MAD2 levels dampen the apoptotic response to non-exchange sex chromosomes and lead to sperm aneuploidy

In meiosis, non-exchange homologous chromosomes are at risk for mis-segregation and should be monitored by the spindle assembly checkpoint (SAC) to avoid formation of aneuploid gametes. Sex chromosome mis-segregation is particularly common and can lead to sterility or to aneuploid offspring (e.g. individuals with Turner or Klinefelter syndrome). Despite major implications for health and reproduction, modifiers of meiotic SAC robustness and the subsequent apoptotic response in male mammals remain obscure. Levels of SAC proteins, e.g. MAD2, are crucial for normal checkpoint function in many experimental systems, but surprisingly, apparently not in male meiosis, as indicated by the lack of chromosome segregation defects reported earlier in Mad2^+/- spermatocytes. To directly test whether MAD2 levels impact the meiotic response to mis-segregating chromosomes, we used Spo11β-only^mb mice that are prone to non-exchange X-Y chromosomes. We show that reduced MAD2 levels attenuate the apoptotic response to mis-segregating sex chromosomes and allow the formation of aneuploid sperm. These findings demonstrate that SAC protein levels are crucial for the efficient elimination of aberrant spermatocytes.

Non-Canonical and Sexually Dimorphic X Dosage Compensation States in the Mouse and Human Germline

Somatic X dosage compensation requires two mechanisms: X inactivation balances X gene output between males (XY) and females (XX), while X upregulation, hypothesized by Ohno and documented in vivo, balances X gene with autosomal gene output. Whether X dosage compensation occurs in germ cells is unclear. We show that mouse and human germ cells exhibit non-canonical X dosage states that differ from the soma and between the sexes. Prior to genome-wide reprogramming, X upregulation is present, consistent with Ohno's hypothesis. Subsequently, however, it is erased. In females, erasure follows loss of X inactivation, causing X dosage excess. Conversely, in males, erasure leads to permanent X dosage decompensation. Sex chromosomally abnormal models exhibit a “sex-reversed” X dosage state: XX males, like XX females, develop X dosage excess, while XO females, like XY males, develop X dosage decompensation. Thus, germline X dosage compensation states are determined by X chromosome number, not phenotypic sex. These unexpected differences in X dosage compensation states between germline and soma offer unique perspectives on sex chromosome infertility.

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X dosage compensation in germ cells is reset during GWR

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PGCs exhibit X upregulation before GWR, in keeping with Ohno's hypothesis

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X upregulation is lost during GWR

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Mouse and human germ cells exhibit X dosage states that are sexually dimorphic

Zinc availability during germline development impacts embryo viability in Caenorhabditis elegans

Zinc is an essential metal that serves as a cofactor in a variety of cellular processes, including meiotic maturation. Cellular control of zinc uptake, availability and efflux is closely linked to meiotic progression in rodent and primate reproduction where large fluctuations in zinc levels are critical at several steps in the oocyte-to-embryo transition. Despite these well-documented roles of zinc fluxes during meiosis, only a few of the genes encoding key zinc receptors, membrane-spanning transporters, and downstream signaling pathway factors have been identified to date. Furthermore, little is known about analogous roles for zinc fluxes in the context of a whole organism. Here, we evaluate whether zinc availability regulates germline development and oocyte viability in the nematode Caenorhabditis elegans, an experimentally flexible model organism. We find that similar to mammals, mild zinc limitation in C. elegans profoundly impacts the reproductive axis: the brood size is significantly reduced under conditions of zinc limitation where other physiological functions are not perturbed. Zinc limitation in this organism has a more pronounced impact on oocytes than sperm and this leads to the decrease in viable embryo production. Moreover, acute zinc limitation of isolated zygotes prevents extrusion of the second polar body during meiosis and leads to aneuploid embryos. Thus, the zinc-dependent steps in C. elegans gametogenesis roughly parallel those described in meiotic-to-mitotic transitions in mammals.

The epigenetic processes of meiosis in male mice are broadly affected by the widely used herbicide atrazine

BACKGROUND

Environmental factors such as pesticides can cause phenotypic changes in various organisms, including mammals. We studied the effects of the widely used herbicide atrazine (ATZ) on meiosis, a key step of gametogenesis, in male mice.

METHODS

Gene expression pattern was analysed by Gene-Chip array. Genome-wide mapping of H3K4me3 marks distribution was done by ChIP-sequencing of testis tissue using Illumina technologies. RT-qPCR was used to validate differentially expressed genes or differential peaks.

RESULTS

We demonstrate that exposure to ATZ reduces testosterone levels and the number of spermatozoa in the epididymis and delays meiosis. Using Gene-Chip and ChIP-Seq analysis of H3K4me3 marks, we found that a broad range of cellular functions, including GTPase activity, mitochondrial function and steroid-hormone metabolism, are affected by ATZ. Furthermore, treated mice display enriched histone H3K4me3 marks in regions of strong recombination (double-strand break sites), within very large genes and reduced marks in the pseudoautosomal region of X chromosome.

CONCLUSIONS

Our data demonstrate that atrazine exposure interferes with normal meiosis, which affects spermatozoa production.

Ovotesticular disorder of sex development with unusual karyotype: patient report

BACKGROUND

Ovotesticular disorder of sex development (OT-DSD) (true hermaphroditism) is an anatomopathological diagnosis based on the findings of testicular and ovarian tissues in the same subject, in the same gonad (ovotestis), or in separate gonads. OT-DSD is a rare cause of sex ambiguity, and the most common karyotype is 46,XX; mosaics and chimeras are found only in 10%-20%.

AIM

To report a case of an OT-DSD patient with a rare karyotype constitution.

CASE REPORT

A 2-month-old child with male sex assignment was referred to our clinic for investigation of sex ambiguity. He was the second child of healthy unrelated parents; pregnancy and labor were uneventful. On physical examination, he had a 2.3-cm phallus and perineal hypospadias (Prader grade III); the right gonad was in the labioscrotal fold and the left was found in the inguinal channel. Karyotype was 46,XX/47,XXY/48,XXYY. Anatomopathological examination of gonads revealed right testis and left ovotestis. The male sex assignment was maintained; the child underwent left gonadectomy, removal of Mullerian structures and urethroplasty.

CONCLUSION

A thorough revision of literature revealed a single case of OT-DSD with the same chromosome constitution. Gonadal biopsy is necessary to establish diagnosis in cases of sex chromosome mosaicism.

Double trisomy 48,XXX,+18 with multiple dysmorphic features

BACKGROUND

Chromosomal abnormality is a common cause of congenital anomalies, psychiatric disorders, and mental retardation. However, the double trisomy 48,XXX,+18 is a rare chromosome abnormality.

METHODS

Case report and literature review.

RESULTS

A 7-hour-old girl presented to our unit because of poor response after birth. She presented with multiple dysmorphic features, including small for gestational age infant, flat nasal bridge, widely-spaced eyes, the left thumb deformities, flat facial profile, raised sternum, ventricular septal defect, the third lateral brain ventricle enlargement, and small liver. This case expands the spectrum of malformations reported in association with the double trisomy 48,XXX,+18. The literature on 16 fetuses or infants with the 48,XXX,+18 were also reviewed.

CONCLUSION

These data suggested that in patients with clinical features similar to trisomy 18, especially with anomalies of the ears and/or reproductive malformations, double trisomy (48,XXX,+18) should be considered and karyotyping should be performed although it is a rare disease.

Fitness of hatchery-reared salmonids in the wild

Accumulating data indicate that hatchery fish have lower fitness in natural environments than wild fish. This fitness decline can occur very quickly, sometimes following only one or two generations of captive rearing. In this review, we summarize existing data on the fitness of hatchery fish in the wild, and we investigate the conditions under which rapid fitness declines can occur. The summary of studies to date suggests: nonlocal hatchery stocks consistently reproduce very poorly in the wild; hatchery stocks that use wild, local fish for captive propagation generally perform better than nonlocal stocks, but often worse than wild fish. However, the data above are from a limited number of studies and species, and more studies are needed before one can generalize further. We used a simple quantitative genetic model to evaluate whether domestication selection is a sufficient explanation for some observed rapid fitness declines. We show that if selection acts on a single trait, such rapid effects can be explained only when selection is very strong, both in captivity and in the wild, and when the heritability of the trait under selection is high. If selection acts on multiple traits throughout the life cycle, rapid fitness declines are plausible.

Globally Divergent but Locally Convergent X- and Y-Chromosome Influences on Cortical Development

Owing to their unique evolutionary history, modern mammalian X- and Y-chromosomes have highly divergent gene contents counterbalanced by regulatory features, which preferentially restrict expression of X- and Y-specific genes. These 2 characteristics make opposing predictions regarding the expected dissimilarity of X- vs. Y-chromosome influences on biological structure and function. Here, we quantify this dissimilarity using in vivo neuroimaging within a rare cohort of humans with diverse sex chromosome aneuploidies (SCAs). We show that X- and Y-chromosomes have opposing effects on overall brain size but exert highly convergent influences on local brain anatomy, which manifest across biologically distinct dimensions of the cerebral cortex. Large-scale online meta-analysis of functional neuroimaging data indicates that convergent sex chromosome dosage effects preferentially impact centers for social perception, communication, and decision-making. Thus, despite an almost complete lack of sequence homology, and opposing effects on overall brain size, X- and Y-chromosomes exert congruent effects on the proportional size of cortical systems involved in adaptive social functioning. These convergent X-Y effects (i) track the dosage of those few genes that are still shared by X- and Y-chromosomes, and (ii) may provide a biological substrate for the link between SCA and increased rates of psychopathology.

Detection of fetal sex chromosome aneuploidy by massively parallel sequencing of maternal plasma DNA: initial experience in a Chinese hospital

OBJECTIVES

To evaluate the performance of a massively parallel sequencing (MPS)-based test in detecting fetal sex chromosome aneuploidy (SCA) and to present a comprehensive clinical counseling protocol for SCA-positive patients.

METHODS

This was a retrospective study in a large patient cohort of 5950 singleton pregnancies which underwent MPS-based testing as a prenatal screening test for trisomies 21, 18 and 13, with X and Y chromosomes as secondary findings, in Southwest Hospital in China. MPS-based SCA-positive women were offered the choice of knowing whether their SCA results were positive and those who did commenced a two-stage post-test clinical counseling protocol. In Stage 1, general information about SCA was given, and women were given the option of invasive testing for confirmation of findings; in Stage 2, those who had chosen to undergo invasive testing were informed about the specific SCA affecting their fetus and their management options.

RESULTS

Thirty-three cases were classified as SCA-positive by MPS-based testing. After Stage 1 of the two-stage post-test clinical counseling session, 33 (100%) of these pregnant women chose to know the screening test results, and 25 (75.76%) underwent an invasive diagnostic procedure and karyotype analysis, in one of whom karyotyping failed. In thirteen cases, karyotyping confirmed the MPS-based test results (two X0 cases, seven XXX cases, three XXY cases and one XYY case), giving a positive predictive value of 54.17% (13/24 cases confirmed by karyotyping). After post-test clinical counseling session Stage 2, seven women chose to terminate the pregnancy: one X0 case, two XXX cases, the three XXY cases and the single XYY case. Six women decided to continue with pregnancy: one X0 case and five XXX cases.

CONCLUSION

Our study showed the feasibility of clinical application of the MPS-based test in the non-invasive detection of fetal SCA. Together with a two-stage post-test clinical counseling protocol, it leads to a well-informed decision-making procedure.

Meiotic recombination analyses of individual chromosomes in male domestic pigs (Sus scrofa domestica)

For the first time in the domestic pig, meiotic recombination along the 18 porcine autosomes was directly studied by immunolocalization of MLH1 protein. In total, 7,848 synaptonemal complexes from 436 spermatocytes were analyzed, and 13,969 recombination sites were mapped. Individual chromosomes for 113 of the 436 cells (representing 2,034 synaptonemal complexes) were identified by immunostaining and fluorescence in situ hybridization (FISH). The average total length of autosomal synaptonemal complexes per cell was 190.3 µm, with 32.0 recombination sites (crossovers), on average, per cell. The number of crossovers and the lengths of the autosomal synaptonemal complexes showed significant intra- (i.e. between cells) and inter-individual variations. The distributions of recombination sites within each chromosomal category were similar: crossovers in metacentric and submetacentric chromosomes were concentrated in the telomeric regions of the p- and q-arms, whereas two hotspots were located near the centromere and in the telomeric region of acrocentrics. Lack of MLH1 foci was mainly observed in the smaller chromosomes, particularly chromosome 18 (SSC18) and the sex chromosomes. All autosomes displayed positive interference, with a large variability between the chromosomes.

Klinefelter syndrome and TESE-ICSI

Until few years ago, Klinefelter syndrome with a homogenous 47,XXY karyotype was considered a model of absolute male sterility. We will discuss first the potential fertility following Testicular Sperm Injection, then the physiopathology of spermatogenic failure and the origin of focal spermatogenesis and risk of aneuploidy in offspring, and third the advantage of searching spermatozoa earlier instead of adult age. The rate of positive sperm extraction seems to be better for younger patients. During childhood, there is a low rate of spermatogonia. The spermagonia, which completes the spermatogenesis, seems resulting from a rare clone of 46,XY gonia, having lost their extra X chromosome. Several arguments suggest that this focal spermatogenesis decreases with age. In addition, androgen treatment, frequently prescribed in case of Klinefelter syndrome, carries a risk of decreasing focal spermatogenesis by lowering gonadotropins. The question arises if it is necessary to expect the sperm cryopreservation before introducing androgen treatment. Further studies are necessary to determine the best age of sperm retrieval in case of Klinefelter syndrome.

Aneuploidy frequency in spermatozoa of Egyptian men with normal and abnormal semen parameters using fluorescence in situ hybridisation

Chromosome anomalies were suggested to be more frequent in infertile males so our case-control study aimed at evaluating the incidence of spermatic aneuploidies in forty males with severe oligoasthenoteratozoospermia (OAT) and comparing it with that in another forty males having normal semen parameters. Semen samples were collected and analysed in the Clinical Pathology Department according to criteria of the World Health Organization (WHO laboratory manual for the examination and processing of human semen, 2010, WHO Press). Fluorescence in situ hybridisation (FISH) was performed on decondensed spermatozoa from fresh semen ejaculates, using dual coloured chromosome-specific DNA probes labelled with fluorochromes to study sperm aneuploidies in chromosomes 13, 21, X and Y. There was no statistical significant difference between cases and controls regarding disomy frequencies for chromosomes 13, 21 or both combined. However, 13, 21 diploidy frequency was significantly higher among OAT cases. Regarding chromosomes X and Y, both cases and controls showed similar results for disomy/diploidy frequency for both chromosomes; however, there was a statistical significant increase in YY disomy/diploidy frequency among OAT patients. X chromosome-bearing spermatozoa were found to be significantly higher among controls. Patients with severe OAT have a higher total sperm aneuploidy rate, regarding chromosomes 13, 21, X and Y but without a statistical significant difference.

Fertility in Turner syndrome

There is increasing interest in fertility and use of assisted reproductive technologies for women with Turner syndrome (TS). Current parenting options include adoption, surrogacy, and spontaneous and assisted reproduction. For women with TS, specific risks of pregnancy include higher than usual rates of spontaneous abortion, foetal anomaly, maternal morbidity and mortality. Heterologous fertility assistance using oocytes from related or unrelated donors is an established technique for women with TS. Homologous fertility preservation includes cryopreservation of the patient's own gametes prior to the progressive ovarian atresia known to occur: preserving either mature oocytes or ovarian tissue containing primordial follicles. Mature oocyte cryopreservation requires ovarian stimulation and can be performed only in postpubertal individuals, when few women with TS have viable oocytes. Ovarian tissue cryopreservation, however, can be performed in younger girls prior to ovarian atresia - over 30 pregnancies have resulted using this technique, however, none in women with TS. We recommend consideration of homologous fertility preservation techniques in children only within specialized centres, with informed consent using protocols approved by a research or clinical ethics board. It is essential that further research is performed to improve maternal and foetal outcomes for women with TS.

Suppression of genetic recombination in the pseudoautosomal region and at subtelomeres in mice with a hypomorphic Spo11 allele

BACKGROUND

Homologous recombination is the key process that generates genetic diversity and drives evolution. SPO11 protein triggers recombination by introducing DNA double stranded breaks at discreet areas of the genome called recombination hotspots. The hotspot locations are largely determined by the DNA binding specificity of the PRDM9 protein in human, mice and most other mammals. In budding yeast Saccharomyces cerevisae, which lacks a Prdm9 gene, meiotic breaks are formed opportunistically in the regions of accessible chromatin, primarily at gene promoters. The genome-wide distribution of hotspots in this organism can be altered by tethering Spo11 protein to Gal4 recognition sequences in the strain expressing Spo11 attached to the DNA binding domain of the Gal4 transcription factor. To establish whether similar re-targeting of meiotic breaks can be achieved in PRDM9-containing organisms we have generated a Gal4BD-Spo11 mouse that expresses SPO11 protein joined to the DNA binding domain of yeast Gal4.

RESULTS

We have mapped the genome-wide distribution of the recombination initiation sites in the Gal4BD-Spo11 mice. More than two hundred of the hotspots in these mice were novel and were likely defined by Gal4BD, as the Gal4 consensus motif was clustered around the centers in these hotspots. Surprisingly, meiotic DNA breaks in the Gal4BD-Spo11 mice were significantly depleted near the ends of chromosomes. The effect is particularly striking at the pseudoautosomal region of the X and Y chromosomes - normally the hottest region in the genome.

CONCLUSIONS

Our data suggest that specific, yet-unidentified factors influence the initiation of meiotic recombination at subtelomeric chromosomal regions.

An idic(15) associated with POF (premature ovarian failure): molecular cytogenetic definition of a case and review of the literature

We report on a 36-year-old infertile woman, presenting a premature ovarian failure with an otherwise normal female phenotype. Cytogenetic analyses showed the presence of a supernumerary marker chromosome, that was characterized by FISH (fluorescent in situ hybridization) and array CGH (comparative genomic hybridization). This marker chromosome was derived from chromosome 15, and contained only heterochromatic material. The Prader Willi/Angelman region was not present. No duplications of the 15q regions were detected by array CGH. Supernumerary markers of chromosome 15 have been reported in cases of infertility and amenorrhea, that is also described in cases with marker derived by other acrocentric chromosomes. The case here presented constitutes a further example that etiology of POF is not always associated with a defective gene, but in some cases oocytes atresia can be the consequence of the abnormal meiotic pairing of chromosomes.

Tetrasomy 9p mosaicism associated with a normal phenotype in two cases

Tetrasomy 9p is a rare chromosomal syndrome and about 30% of known cases exhibit mosaicism. Approximately 50 of the reported cases with tetrasomy 9p mosaicism show a characteristic facial appearance, growth failure, and developmental delay. However, 3 patients with mosaicism for isochromosome 9p and a normal phenotype have also been reported. We report 2 additional cases of clinically normal young females with tetrasomy 9p mosaicism, one of whom also exhibited X chromosome aneuploidy mosaicism leading to an overall of 6 different cell lines. STR analysis performed on this complex mosaic case indicated that the extra isochromosome was of maternal origin while the X chromosome aneuploidy was of paternal origin, indicating a postzygotic event.

Human male meiotic sex chromosome inactivation

In mammalian male gametogenesis the sex chromosomes are distinctive in both gene activity and epigenetic strategy. At first meiotic prophase the heteromorphic X and Y chromosomes are placed in a separate chromatin domain called the XY body. In this process, X,Y chromatin becomes highly phosphorylated at S139 of H2AX leading to the repression of gonosomal genes, a process known as meiotic sex chromosome inactivation (MSCI), which has been studied best in mice. Post-meiotically this repression is largely maintained. Disturbance of MSCI in mice leads to harmful X,Y gene expression, eventuating in spermatocyte death and sperm heterogeneity. Sperm heterogeneity is a characteristic of the human male. For this reason we were interested in the efficiency of MSCI in human primary spermatocytes. We investigated MSCI in pachytene spermatocytes of seven probands: four infertile men and three fertile controls, using direct and indirect in situ methods. A considerable degree of variation in the degree of MSCI was detected, both between and within probands. Moreover, in post-meiotic stages this variation was observed as well, indicating survival of spermatocytes with incompletely inactivated sex chromosomes. Furthermore, we investigated the presence of H3K9me3 posttranslational modifications on the X and Y chromatin. Contrary to constitutive centromeric heterochromatin, this heterochromatin marker did not specifically accumulate on the XY body, with the exception of the heterochromatic part of the Y chromosome. This may reflect the lower degree of MSCI in man compared to mouse. These results point at relaxation of MSCI, which can be explained by genetic changes in sex chromosome composition during evolution and candidates as a mechanism behind human sperm heterogeneity.

Function of the sex chromosomes in mammalian fertility

The sex chromosomes play a highly specialized role in germ cell development in mammals, being enriched in genes expressed in the testis and ovary. Sex chromosome abnormalities (e.g., Klinefelter [XXY] and Turner [XO] syndrome) constitute the largest class of chromosome abnormalities and the commonest genetic cause of infertility in humans. Understanding how sex-gene expression is regulated is therefore critical to our understanding of human reproduction. Here, we describe how the expression of sex-linked genes varies during germ cell development; in females, the inactive X chromosome is reactivated before meiosis, whereas in males the X and Y chromosomes are inactivated at this stage. We discuss the epigenetics of sex chromosome inactivation and how this process has influenced the gene content of the mammalian X and Y chromosomes. We also present working models for how perturbations in sex chromosome inactivation or reactivation result in subfertility in the major classes of sex chromosome abnormalities.

Semi-automated scoring of triple-probe FISH in human sperm: methods and further validation

Although the frequency and consequence of sperm chromosomal abnormalities are considerable, few epidemiologic studies in large samples have been conducted to investigate etiologic risk factors. This is, in part, attributable to the labor intensive demands of manual sperm fluorescence in situ hybridization (FISH) scoring. As part of an epidemiologic study investigating environmental risk factors for aneuploidy among men attending a hospital-based fertility clinic, a semi-automated method of slide scoring was further validated and used to estimate sex chromosome sperm disomy frequency in a large number of samples. Multiprobe FISH for chromosomes X, Y, and 18 was used to determine sex chromosome disomy in sperm nuclei. Semi-automated scoring methods were used to quantify X disomy (sperm FISH genotype XX18), Y disomy (YY18), and XY disomy (XY18). The semi-automated results were compared with the results from manual scoring in 10 slides. The semi-automated method was then used to estimate sex chromosome disomy frequency in 60 men. Of 10 slides scored, significant differences between the manual and semi-automated results were seen primarily in one slide that was of poor quality because of over swollen nuclei. Among 60 men analyzed using the semi-automated method, median total sex chromosome disomy frequency was 1.65%, which is higher than seen among normal men but within range with reports from fertility clinic populations. These results further validate that semi-automated methods can be used to score sperm disomy with results comparable to manual methods. This is the largest study to date to provide estimates of sex chromosome disomy among men attending fertility clinics. These methods should be replicated in larger clinic populations to arrive at stable estimates of aneuploidy frequency in men who are members of subfertile couples. © 2011 International Society for Advancement of Cytometry.