THE GENETICS OF MALE INFERTILITY: A FIELD OF STUDY WHOSE TIME IS NOW

Idiopathic male infertility is often associated with genetic and epigenetic abnormalities. Such abnormalities include chromosome translocations and aneuploidies, Y chromosome microdeletions, and mutations of the CFTR gene. The unraveling of the human genome and ongoing animal transgenic studies have identified numerous other genes likely to be associated with male infertility. Initial reports from human studies have identified several candidate genes, including the protamine genes, SPO11, EIF5A2, USP26, ACT, and others. In addition to gene mutations and polymorphisms, damage to the chromatin resulting in single and double strand DNA breaks affects fertility. Recent studies are highlighting the role of such abnormalities in male infertility, and point to protamine defects as one cause of DNA damage. Epigenetic abnormalities also are being investigated, including the role of residual sperm mRNA in embryogenesis, and the effects of abnormal spermatogenesis on gene imprinting. These studies are pointing to complex etiologies and clinical ramifications in many infertile men.     

精子发生障碍的遗传学研究进展

不孕不育影响着约10%～15%的夫妇,其中男性因素约占一半。精子发生障碍是男性不育的主要病因,主要表现为无精子症或少弱精子症。大量研究已经确定Y染色体及Yq微缺失与男性不育的相关性。X染色体因其在男性仅有单拷贝而在精子生成过程中表达特殊,对精子发生障碍有重要意义。对畸精子症和弱精子症患者的研究表明,位于常染色体的4个基因（SPATA16,PICK1,CATSPER和AURKC）可能与精子发生障碍有关。虽然经全球范围的努力,预期在不久的将来可提供新的基因检测技术来检测精子发生障碍的遗传学原因,但目前可用于精子发生障碍的基因测试仍然有限。     

Key functional genes of spermatogenesis identified by microarray analysis

At present many couples face difficulties when trying to conceive that may have a genetic basis. The male factor is the cause of infertility as often as the female. Therefore it is important to identify key genes involved in spermatogenesis which may be linked to male infertility. This review discusses the identification of a range of genes associated with male fertility using microarrays. Based on differences in gene expression profiles between fertile and infertile male subgroups or between fetal and adult male gonads, many genes important for spermatogenesis have been discovered. Genes that are critical at particular stages of spermatogenesis were defined and can be considered as potential male fertility biomarkers. The studies described showed that microarrays may be potentially used as a diagnostic platform to increase the efficacy of diagnosis and perhaps treatment of infertile males.     

Key functional genes of spermatogenesis identified by microarray analysis

At present many couples face difficulties when trying to conceive that may have a genetic basis. The male factor is the cause of infertility as often as the female. Therefore it is important to identify key genes involved in spermatogenesis which may be linked to male infertility. This review discusses the identification of a range of genes associated with male fertility using microarrays. Based on differences in gene expression profiles between fertile and infertile male subgroups or between fetal and adult male gonads, many genes important for spermatogenesis have been discovered. Genes that are critical at particular stages of spermatogenesis were defined and can be considered as potential male fertility biomarkers. The studies described showed that microarrays may be potentially used as a diagnostic platform to increase the efficacy of diagnosis and perhaps treatment of infertile males.     

Genetic screening of infertile men

Male infertility is an extraordinarily common medical condition, affecting 1 in 20 men. According to the World Health Organization, this condition is now considered to be a complex disease involving physical, genetic and environmental factors. With continuing advances in our understanding of male reproductive physiology and endocrinology, together with the availability of the complete sequence of the human genome and powerful functional genomic techniques, the stage is now set to identify the genes that are essential for spermatogenesis. Given that the process of spermatogenesis, from the germ cell to mature sperm, is complex, the challenge for research is to develop the strategies for identifying new genetic causes of idiopathic male infertility and defining genotypes associated with specific defects in semen parameters and testicular pathologies. Such information will form the basis of new genetic tests that will allow the clinician to make an accurate diagnosis of the male partner and a more informed decision about treatment options for the couple.     

The search for SNPs,CNVs, and epigenetic variants associated with the complex disease of male infertility

Understanding the genetic basis of idiopathic male infertility has long been the focus of many researchers. Numerous recent studies have attempted to identify relevant single nucleotide polymorphisms (SNPs) through medical re-sequencing studies in which candidate genes are sequenced in large numbers of cases and controls in the search for risk or causative polymorphisms. Two major characteristics have limited the utility of the re-sequencing studies. First, reported SNPs have only accounted for a small percentage of idiopathic male infertility. Second, SNPs reported to have an association with male infertility based on gene re-sequencing studies often fail validation in follow-up studies. Recent advances in the tools available for genetic studies have enabled interrogation of the entire genome in search of common, and more recently, rare variants. In this review, we discuss the progress of studies on genetic and epigenetic variants of male infertility as well as future directions that we predict will be the most productive in identifying the genetic basis for male factor infertility based on our current state of knowledge in this field as well as lessons learned about the genetic basis for complex diseases from other disease models.     

The search for SNPs, CNVs, and epigenetic variants associated with the complex disease of male infertility

Understanding the genetic basis of idiopathic male infertility has long been the focus of many researchers. Numerous recent studies have attempted to identify relevant single nucleotide polymorphisms (SNPs) through medical re-sequencing studies in which candidate genes are sequenced in large numbers of cases and controls in the search for risk or causative polymorphisms. Two major characteristics have limited the utility of the re-sequencing studies. First, reported SNPs have only accounted for a small percentage of idiopathic male infertility. Second, SNPs reported to have an association with male infertility based on gene re-sequencing studies often fail validation in follow-up studies. Recent advances in the tools available for genetic studies have enabled interrogation of the entire genome in search of common, and more recently, rare variants. In this review, we discuss the progress of studies on genetic and epigenetic variants of male infertility as well as future directions that we predict will be the most productive in identifying the genetic basis for male factor infertility based on our current state of knowledge in this field as well as lessons learned about the genetic basis for complex diseases from other disease models.     

MOLECULAR BIOLOGY OF MALE INFERTILITY

About 15% of couples have reduced fertility and in approximately one-half of all cases the reason is male infertility, usually of genetic origin. Thus, in the context of research in genes involved in reproduction and sex determination, genetic anomalies in gametogenesis are being extensively studied. The most frequent pathogenic causes of male infertility are Y-chromosomal microdeletions (8-15%) in the long arm of the Y chromosome, which, by loss of specific DNA segments, leads to loss of vital genes for sperm production. Infertile men, who attend infertility clinics, rise to 15% among those with azoospermia or spermatogenesis problem. The new technique of intracytoplasmic sperm injection has allowed many infertile men to achieve their dreams of fatherhood. However, the spermatogenic defect is genetic anomalies, which might be a potential risk of transmitting this defect to future offspring. Therefore, genetic counseling of all couples with the diagnosis of male infertility is recommended before their enrolment in intrauterine insemination, in vitro fertilization, and intracytoplasmic sperm injection. The important role of genetic abnormalities in the causation of human male infertility is increasingly recognized. While much remains to be learned in this fast-moving field, considerable progress has been made in the clinical delineation of genetic forms of male infertility and in the characterization of the responsible genes and their mutations or deletions. This review should provide insight into the understanding of parthenogenesis of male infertility in the human.     

无精子症患者基因检测分析

目的:利用全外显子测序技术,筛选无精子症患者相关基因,丰富男性不育基因库。方法:抽取20例无精子症患者外周血,提取DNA,使用杂交捕获方法构建DNA文库,采用高通量测序技术检测人类全外显子组中20099个基因的外显子区及旁侧内含子区(20bp),将测序数据与人类基因组hg19参考序列进行比对,筛选变异基因,变异位点进行Sanger测序验证。结果:共计筛选出26个基因43个变异位点,排除15个常染色体隐性遗传的单个变异位点及13个与精子运动相关的变异位点,剩余11个基因的15个变异位点可能与精子发生障碍相关,其中包括FAM71B、STARD9、CLTCL1、PCBP3、S100PBP等5个在睾丸组织高表达基因,SYCE3、EFCAB6、DDX4、KDM5D、RGS22、MTL5等6个基因可能与精子发生障碍相关。结论:通过研究筛选到可能影响男性不育的基因,为男性不育的基因诊断研究提供参考。     

CONTROL OF SPERMATOGENESIS IN PRIMATE AND PROSPECT OF MALE CONTRACEPTION

The present review is a summary of mechanisms of spermatogenesis in primates with emphasis on anti-spermatogenesis of testosterone (T), gossypol, and “testicular heat stress” for development of male contraception. Both FSH and testosterone stimulate all phases of spermatogenesis. FSH is capable of amplifying the population of the differentiated spermatogonia (B1, B2, B3 and B4) and controls the spermatogonia production rate, and, in synergy with testosterone, regulating spermatogenesis in adult monkeys. Pituitary FSH beta gene expression is governed by a feedback of Beta inhibin, which is a major component of the testicular negative feedback signals. Beta inhibin secreted by Sertoli cells is in turn inhibited by testosterone from Leydig cells under the control of LH. Disturbance of the normal interaction of pituitary FSH with Sertoli cell Beta inhibin is responsible for azoospermia or oligozoospermia induced by exogenous T. Three possible regimens of T, gossypol and “heat stress” have been suggested for male contraception. They act on different sites and stages of spermatogenesis in testis or sperm activity in epididymis. Apoptosis induced by testosterone occurs mainly at stages VII–VIII of spermatogenesis while that by testicular “heat stress” mostly occurs at stages I–IV and X–XII. Low dose of gossypol mainly influences the sperm activity in the epididymis although it also acts on testicular spermatids.     

Genetics of osteoporosis

Osteoporosis is a common disease with a strong genetic component characterised by reduced bone mass and an increased risk of fragility fractures. Twin and family studies have shown that genetic factors contribute to osteoporosis by influencing bone mineral density (BMD), and other phenotypes that are associated with fracture risk, although the heritability of fracture itself is modest. Linkage studies have identified several quantitative trait loci that regulate BMD but most causal genes remain to be identified. In contrast, linkage studies in monogenic bone diseases have been successful in gene identification, and polymorphisms in many of these genes have been found to contribute to the regulation of bone mass in the normal population. Population-based studies have identified polymorphisms in several candidate genes that have been associated with bone mass or osteoporotic fracture, although individually these polymorphisms only account for a small amount of the genetic contribution to BMD regulation. Environmental factors such as diet and physical activity are also important determinants of BMD, and in some cases specific nutrients have been found to interact with genetic polymorphisms to regulate BMD. From a clinical standpoint, advances in knowledge about the genetic basis of osteoporosis are likely to be important in increasing the understanding of the pathophysiology of the disease; providing new genetic markers with which to assess fracture risk and in identifying genes and pathways that form molecular targets for the design of the next generation of drug treatments.     

Association of heat shock proteins,heat shock factors and male infertility

It has been well established that heat shock proteins (HSP) and heat shock factors (HSF) are involved in wide varieties of physiological regulation process and signal pathway. Numerous members of heat shock family exhibit a cell-type-specific expression pattern during spermatogenesis and play crucial roles in germ cell development. This led to the emerging studies to reveal the association between heat shock family and male infertility. Aberrant expressions of HSP/HSFs observed in sterile men and animal models indicate the two opposite effects, both protective and harmful, of heat shock family on male fertility. Moreover, HSP/HSFs are also involved in the two major causes of male infertility. It seems that different behaviors of HSP/HSFs patients with varicocele and Chlamydia trachomatis infection lead to distinct outcomes of male fertility. In addition, emerging evidence has demonstrated that the altered expression of HSP/HSFs may be responsible for the abnormal germ cell apoptosis and subsequently results in impaired spermatogenesis. Therefore, heat shock family may play an important role in the quality-control of germ cells during spermatogenesis, raising the prospect of their utility for novel treatment targets in male infertility.     

Proteins Involved in Meiotic Recombination: A Role in Male Infertility?

Meiotic recombination results in the formation of crossovers, by which genetic information is exchanged between homologous chromosomes during prophase I of meiosis. Recombination is a complex process involving many proteins. Alterations in the genes involved in recombination may result in infertility. Molecular studies have improved our understanding of the roles and mechanisms of the proteins and protein complexes involved in recombination, some of which have function in mitotic cells as well as meiotic cells. Human gene sequencing studies have been performed for some of these genes and have provided further information on the phenotypes observed in some infertile individuals. However, further studies are needed to help elucidate the particular role(s) of a given protein and to increase our understanding of these protein systems. This review will focus on our current understanding of proteins involved in meiotic recombination from a genomic perspective, summarizing our current understanding of known mutations and single nucleotide polymorphisms that may affect male fertility by altering meiotic recombination.     

Proteins involved in meiotic recombination: a role in male infertility?

Meiotic recombination results in the formation of crossovers, by which genetic information is exchanged between homologous chromosomes during prophase I of meiosis. Recombination is a complex process involving many proteins. Alterations in the genes involved in recombination may result in infertility. Molecular studies have improved our understanding of the roles and mechanisms of the proteins and protein complexes involved in recombination, some of which have function in mitotic cells as well as meiotic cells. Human gene sequencing studies have been performed for some of these genes and have provided further information on the phenotypes observed in some infertile individuals. However, further studies are needed to help elucidate the particular role(s) of a given protein and to increase our understanding of these protein systems. This review will focus on our current understanding of proteins involved in meiotic recombination from a genomic perspective, summarizing our current understanding of known mutations and single nucleotide polymorphisms that may affect male fertility by altering meiotic recombination.     

HOXA10基因表达对子宫内膜异位症患者胚胎着床的作用

同源异型盒（HOX）蛋白质是高度保守的转录因子，决定着包括人类在内的许多物种的部分生物性状的表达。 HOXA10基因通过调节下游基因及雌激素、孕激素的表达直接影响子宫的胚胎发育和着床。根据体内激素的调控作用，HOXA10基因在子宫内膜上呈周期性表达，其表达高峰发生在植入窗期。HOXA10基因在子宫内膜异位症（EMs）病变中表达对子宫内膜组织的“从头开始”发育过程是必要的。EMs患者的子宫内膜存在影响胚胎着床的诸多不利因素，提示胚胎着床失败可能是EMs患者不孕的主要因素之一。EMs患者通常不会出现HOXA10表达水平在黄体中期上升的现象，可以解释EMs是造成不孕的原因之一。     

HOXA10基因表达对子宫内膜异位症患者胚胎着床的作用

同源异型盒（HOX）蛋白质是高度保守的转录因子,决定着包括人类在内的许多物种的部分生物性状的表达。HOXA10基因通过调节下游基因及雌激素、孕激素的表达直接影响子宫的胚胎发育和着床。根据体内激素的调控作用,HOXA10基因在子宫内膜上呈周期性表达,其表达高峰发生在植入窗期。HOXA10基因在子宫内膜异位症（EMs）病变中表达对子宫内膜组织的“从头开始”发育过程是必要的。EMs患者的子宫内膜存在影响胚胎着床的诸多不利因素,提示胚胎着床失败可能是EMs患者不孕的主要因素之一。EMs患者通常不会出现HOXA10表达水平在黄体中期上升的现象,可以解释EMs是造成不孕的原因之一。     

Initiative for standardization of reporting genetics of male infertility

The number of publications on research of male infertility is increasing. Technologies used in research of male infertility generate complex results and various types of data that need to be appropriately managed, arranged, and made available to other researchers for further use. In our previous study, we collected over 800 candidate loci for male fertility in seven mammalian species. However, the continuation of the work towards a comprehensive database of candidate genes associated with different types of idiopathic human male infertility is challenging due to fragmented information, obtained from a variety of technologies and various omics approaches. Results are published in different forms and usually need to be excavated from the text, which hinders the gathering of information. Standardized reporting of genetic anomalies as well as causative and risk factors of male infertility therefore presents an important issue. The aim of the study was to collect examples of diverse genomic loci published in association with human male infertility and to propose a standardized format for reporting genetic causes of male infertility. From the currently available data we have selected 75 studies reporting 186 representative genomic loci which have been proposed as genetic risk factors for male infertility. Based on collected and formatted data, we suggested a first step towards unification of reporting the genetics of male infertility in original and review studies. The proposed initiative consists of five relevant data types: 1) genetic locus, 2) race/ethnicity, number of participants (infertile/controls), 3) methodology, 4) phenotype (clinical data, disease ontology, and disease comorbidity), and 5) reference. The proposed form for standardized reporting presents a baseline for further optimization with additional genetic and clinical information. This data standardization initiative will enable faster multi-omics data integration, database development and sharing, establishing more targeted hypotheses, and facilitating biomarker discovery.