The molecular basis of human hypogonadotropic hypogonadism

Nonobstructive azoospermia: an etiologic review

ABSTRACT

Azoospermia is the complete absence of spermatozoa in the ejaculate in two or more semen analyses after centrifugation. Nonobstructive azoospermia (NOA) represents the most severe form of male factor infertility accounting for 10%-15% of cases and stems from an impairment to spermatogenesis. Understanding of the hypothalamic-pituitary-testicular axis has allowed NOA to be subcategorized by anatomic and/or pathophysiologic level. The etiologies of NOA, and therefore, the differential diagnoses when considering NOA as a cause of male factor infertility, can be subcategorized and condensed into several distinct classifications. Etiologies of NOA include primary hypogonadism, secondary hypogonadism, defects in androgen synthesis and/or response, defective spermatogenesis and sperm maturation, or a mixed picture thereof. This review includes up-to-date clinical, diagnostic, cellular, and histologic features pertaining to the multitude of NOA etiologies. This in turn will provide a framework by which physicians practicing infertility can augment their clinical decision-making, patient counseling, thereby improving upon the management of men with NOA.

MECHANISMS IN ENDOCRINOLOGY: Aberrations of the X chromosome as cause of male infertility

Male infertility is most commonly caused by spermatogenetic failure, clinically noted as oligo- or a-zoospermia. Today, in approximately 20% of azoospermic patients, a causal genetic defect can be identified. The most frequent genetic causes of azoospermia (or severe oligozoospermia) are Klinefelter syndrome (47,XXY), structural chromosomal abnormalities and Y-chromosomal microdeletions. Consistent with Ohno's law, the human X chromosome is the most stable of all the chromosomes, but contrary to Ohno's law, the X chromosome is loaded with regions of acquired, rapidly evolving genes, which are of special interest because they are predominantly expressed in the testis. Therefore, it is not surprising that the X chromosome, considered as the female counterpart of the male-associated Y chromosome, may actually play an essential role in male infertility and sperm production. This is supported by the recent description of a significantly increased copy number variation (CNV) burden on both sex chromosomes in infertile men and point mutations in X-chromosomal genes responsible for male infertility. Thus, the X chromosome seems to be frequently affected in infertile male patients. Four principal X-chromosomal aberrations have been identified so far: (1) aneuploidy of the X chromosome as found in Klinefelter syndrome (47,XXY or mosaicism for additional X chromosomes). (2) Translocations involving the X chromosome, e.g. nonsyndromic 46,XX testicular disorders of sex development (XX-male syndrome) or X-autosome translocations. (3) CNVs affecting the X chromosome. (4) Point mutations disrupting X-chromosomal genes. All these are reviewed herein and assessed concerning their importance for the clinical routine diagnostic workup of the infertile male as well as their potential to shape research on spermatogenic failure in the next years.

GnRH receptor mutations in isolated gonadotropic deficiency

GnRH and its receptor GnRHR are key regulators of the hypothalamo-pituitary axis. They modulate the secretion of LH and FSH gonadotropins and therefore, the development and maturation of gonads in fetal life as well as after birth. Congenital functional defect of this axis results in isolated hypogonadotropic hypogonadism (IHH). Several natural mutations causing IHH without anosmia have now been identified in GnRHR or GnRH genes. These mutations inactivate GnRHR or its ligand function and cause highly variable phenotypes, ranging from partial to complete gonadotropic deficiencies. The present review describes the published natural GnRHR mutations and tries to correlate them with the corresponding phenotypes according to the different steps of the GnRH system development.

Kallmann syndrome 1 gene is expressed in the marsupial gonad

Kallmann syndrome is characterized by hypogonadotrophic hypogonadism and anosmia. The syndrome can be caused by mutations in several genes, but the X-linked form is caused by mutation in the Kallmann syndrome 1 (KAL1). KAL1 plays a critical role in gonadotropin-releasing hormone (GnRH) neuronal migration that is essential for the normal development of the hypothalamic-pituitary-gonadal axis. Interestingly, KAL1 appears to be missing from the rodent X, and no orthologue has been detected as yet. We investigated KAL1 during development and in adults of an Australian marsupial, the tammar wallaby, Macropus eugenii. Marsupial KAL1 maps to an autosome within a group of genes that was added as a block to the X chromosome in eutherian evolution. KAL1 expression was widespread in embryonic and adult tissues. In the adult testis, tammar KAL1 mRNA and protein were detected in the germ cells at specific stages of differentiation. In the adult testis, the protein encoded by KAL1, anosmin-1, was restricted to the round spermatids and elongated spermatids. In the adult ovary, anosmin-1 was not only detected in the oocytes but was also localized in the granulosa cells throughout folliculogenesis. This is the first examination of KAL1 mRNA and protein localization in adult mammalian gonads. The protein localization suggests that KAL1 participates in gametogenesis not only through the development of the hypothalamic-pituitary-gonadal axis by activation of GnRH neuronal migration, but also directly within the gonads themselves. Because KAL1 is autosomal in marsupials but is X-linked in eutherians, its conserved involvement in gametogenesis supports the hypothesis that reproduction-related genes were actively recruited to the eutherian X chromosome.

Lutropin alpha, recombinant human luteinizing hormone, for the stimulation of follicular development in profoundly LH-deficient hypogonadotropic hypogonadal women: a review

Hypogonadotropic hypogonadism is defined as a medical condition with low or undetectable gonadotropin secretion, associated with a complete arrest of follicular growth and very low estradiol. The main cause can be traced back to an irregular or absent hypothalamic GnRH secretion, whereas only a minority suffers from a pituitary disorder. The choice of treatment to reverse this situation is a pulsatile GnRH application or a direct ovarian stimulation using gonadotropin injections. The goal is to achieve a proper ovarian function in these cases for a short time to allow ovulation and chance of pregnancy. Since the pulsatile GnRH treatment lost its former importance, several gonadotropins are in use to stimulate follicular growth, such as urine-derived human menopausal gonadotropin, highly purified follicle stimulating hormone (FSH) or recombinant FSH, all with different success. The introduction of recombinant luteinizing hormone (LH) and FSH provided an opportunity to investigate the distinct influences of LH and FSH alone and in combination on follicular growth in monofollicular ovulation induction cycles, and additionally on oocyte maturation, fertilization competence of the oocyte and embryo quality in downregulated IVF patients. Whereas FSH was known to be indispensable for normal follicular growth, the role of LH remained questionable. Downregulated IVF patients with this short-term gonadotropin depletion displayed no advance in stimulation success with the use of recombinant LH. Patients with hypogonadotropic hypogonadism undergoing monofollicular stimulation for ovulation induction showed clearly a specific role and need for both hormones in normal follicular growth. Therefore, a combined stimulation with FSH and LH seems to be the best treatment choice. In the first half of the stimulation cycle the FSH dosage should exceed that of LH by 2:1, with an inverse ratio for the second half.

Kisspeptins: regulators of metastasis and the hypothalamic-pituitary-gonadal axis

The kisspeptins are the peptide products of the KiSS-1 gene and the endogenous agonists for the GPR54 receptor. Although KiSS-1 was initially discovered as a metastasis suppressor gene, recent evidence suggests the kisspeptin/GPR54 system is a key regulator of the reproductive system. Disrupted GPR54 signalling causes hypogonadotrophic hypogonadism in rodents and man. Central or peripheral administration of kisspeptin potently stimulates the hypothalamic-pituitary-gonadal axis, increasing circulating gonadotrophin concentrations in a number of animal models. These effects appear likely to be mediated via the hypothalamic gonadotrophin-releasing hormone system, although kisspeptins may have direct effects on the anterior pituitary gland. Hypothalamic KiSS-1 expression is regulated by circulating sex steroids. The precise physiological role of the kisspeptin system in the regulation of reproductive function remains to be elucidated.

Screening candidate genes for mutations in patients with hypogonadotropic hypogonadism using custom genome resequencing microarrays

OBJECTIVE

The purpose of this study was to determine the consistency of calling single nucleotide polymorphisms (SNPs) by custom genome resequencing microarrays compared with capillary DNA sequencing.

STUDY DESIGN

Amplified genomic DNA from 23 patients with hypogonadotropic hypogonadism was hybridized to microarrays containing 30 kilobases of sequence from 6 different candidate genes. Capillary DNA sequencing was performed in 10 patients.

RESULTS

For 10 patients with > or =90% of bases called, 49 SNPs in 5 of 6 genes were identified. Of the 490 bases, 75 were ambiguous (read as "N"), and 415 were able to be called an A, C, G, or T. Of 415 called, 401 (96.6%) sequences were confirmed by DNA sequencing. All homozygotes (285/285) were called identically, while sequence from 89.2% (116/130) of heterozygotes agreed by both methods. The level of agreement between microarray calls and capillary DNA sequencing demonstrated substantial accuracy.

CONCLUSION

Custom genome resequencing microarrays are highly consistent with capillary sequencing in calling individual bases in genomic DNA from patients with human disease.

The product of X-linked Kallmann's syndrome gene (KAL1) affects the migratory activity of gonadotropin-releasing hormone (GnRH)-producing neurons

X-linked Kallmann's syndrome (KS) is a genetic disease characterized by anosmia and hypogonadism due to impairment in the development of olfactory axons and in the migration of gonadotropin-releasing hormone (GnRH)-producing neurons. Deletions or point mutations of a gene located at Xp22.3 (KAL1) are responsible for the disease. This gene encodes for a secreted heparin-binding protein (KAL or anosmin-1) which exhibits similarities with cell-adhesion molecules. In the present study, we show for the first time a direct action of anosmin-1 on the migratory activity of GnRH neurons. Specifically, we exposed immortalized migrating GnRH neurons (GN11 cells) to conditioned media (CM) of COS or CHO cells transiently transfected with human KAL1 gene in microchemotaxis and collagen gel assays. We found that anosmin-1-enriched media produced a cell-specific chemotactic response of GN11 cells. None of the CM enriched on three forms of anosmin-1 carrying different missense mutations (N267K, E514K and F517L) found in patients affected by X-linked KS affected the chemomigration of GN11 cells. Anosmin binds to the GN11 cell surface by interacting with the heparan sulphate proteoglycans, and the chemotactic effect of anosmin-1-enriched CM can be specifically blocked by heparin or by heparitinase pretreatment. These results strongly suggest an involvement of anosmin-1 in the control of the migratory behaviour of GnRH neurons and provide novel information on the pathogenesis of KS.

Genetic causes of human infertility

The currently characterized chromosomal disorders and gene mutations that cause infertility in humans were reviewed. Of the four arbitrary compartments, genes expressed in the gonad comprise the most common site affected by mutations causing infertility. Clinicians should be aware of the most common causes that have clinical implications: (1) women with a 45,X cell line commonly have cardiac anomalies that may pose a risk for maternal death in pregnancies achieved by donor egg IVF; (2) men with Y-chromosome deletions may produce male offspring with the same deletion, rendering them infertile; (3) CBAVD must be ascertained in men with azoospermia because of the risk for having a child with CF; and (4) some women with premature ovarian failure may be fragile X syndrome carriers, so other family members may be at risk for the full syndrome. In the future, more genes will be identified to cause infertility in humans, which will translate into clinical significance. In select cases, in which the genetic defect is known, it may be possible to use preimplantation genetic diagnosis to screen embryos prior to uterine transfer.

Molecular basis of pubertal abnormalities

The causes of abnormal pubertal development are numerous. Recent molecular investigation has increased our understanding of the genetic basis of pubertal disorders. Investigators have identified some of the genes that are critical for normal puberty and have begun to elucidate the genes and pathogenesis of genetic disorders associated with abnormal pubertal development. Identification of specific chromosomal abnormalities and gene mutations allows for diagnostic testing and enables the clinician to provide accurate counseling of the recurrence risk for relatives. In the future, knowledge of the genetic basis of these disorders will facilitate the development of novel therapies and approaches to the fertility assessment and treatment of individuals with pubertal disorders. Although great strides have been made in identifying these genes, questions remain. Why do some genetic mutations affect puberty differentially in males and females? What is the long-term impact in terms of future fertility, and what is the risk to the offspring of such patients? Further research is needed to address these issues and to identify additional genetic loci involved in pubertal development.

Contemporary issues in primary amenorrhea

Reproductive medicine has changed dramatically since the 1981 publication of the study of patients presenting with pubertal amenorrhea. The breakdown of causes likely remains unchanged, with the four most common causes of primary amenorrhea being ovarian failure (48.5%), congenital absence of the uterus and vagina (16.2%), GnRH deficiency (8.3%), and constitutional delay of puberty (6.0%). In the study of patients reported by Reindollar, 60% of patients had barriers to reproduction. Since its publication over 15 years ago, developments in assisted reproductive technologies have enabled pregnancy in many of these patients. Women with ovarian failure may gestate pregnancies from donated oocytes. Women with congenital absence of the uterus and vagina may have their fetuses carried in a surrogate uterus. During this period, the advances of molecular medicine have provided a better understanding of the etiologies of many of these disorders, including Turner's syndrome; 46,XY gonadal dysgenesis; 46,XX gonadal dysgenesis; hypogonadotropic hypogonadism; enzyme-deficient states; gonadotropin resistance; and androgen insensitivity. Contemporary issues related to these disorders involve information about molecular defects and outcome of pregnancies for patients previously considered sterile. Largely, this information has been extremely helpful and reassuring. However, the reported deaths of patients with Turner's syndrome who become pregnant by donor oocyte should remind us to proceed cautiously as new reproductive avenues are opened for these patients.

Endocrine causes of male infertility

Although endocrinopathies are not often seen in infertile men, these disorders are clinically significant; they often have potentially serious medical significance, regardless of fertility issues. Correction of these disorders represents a possible way to restore normal fertility for the male partner. Male fertility is critically dependent upon a normal hormonal milieu. The hypothalamic-pituitary-gonadal axis is quite sensitive to disruption by endocrine disorders and other generalized medical disorders. Thus, male infertility is occasionally the presenting sign for significant underlying medical disease; it is important to properly evaluate these patients.

Hypogonadotropic hypogonadism and peripheral neuropathy in Ebf2-null mice

Olf/Ebf transcription factors have been implicated in numerous developmental processes, ranging from B-cell development to neuronal differentiation. We describe mice that carry a targeted deletion within the Ebf2 (O/E3) gene. In Ebf2-null mutants, because of defective migration of gonadotropin releasing hormone-synthesizing neurons, formation of the neuroendocrine axis (which is essential for pubertal development) is impaired, leading to secondary hypogonadism. In addition, Ebf2(-/-) peripheral nerves feature defective axon sorting, hypomyelination, segmental dysmyelination and axonal damage, accompanied by a sharp decrease in motor nerve conduction velocity. Ebf2-null mice reveal a novel genetic cause of hypogonadotropic hypogonadism and peripheral neuropathy in the mouse, disclosing an important role for Ebf2 in neuronal migration and nerve development.

Human gene mutations causing infertility

The identification of gene mutations causing infertility in humans remains noticeably deficient at present. Although most males and females with infertility display normal pubertal development, nearly all of the gene mutations in humans have been characterised in people with deficient puberty and subsequent infertility. Gene mutations are arbitrarily categorised into four different compartments (I, hypothalamic; II, pituitary; III, gonadal; and IV, outflow tract). Diagnoses of infertility include hypogonadotrophic hypogonadism (compartments I and II), hypergonadotrophic hypogonadism (III), and obstructive disorders (compartment IV). Most gene mutations identified to date affect gonadal function, but it is also apparent that a large number of important genes in normal fertility have yet to be realised.

Familial gonadotropin-releasing hormone resistance and hypogonadotropic hypogonadism in a family with multiple affected individuals

OBJECTIVE

To characterize the phenotype of idiopathic hypogonadotropic hypogonadism due to compound heterozygous GnRHR gene mutations (Arg262Gln/Tyr284Cys).

DESIGN

Retrospective review.

SETTING

Tertiary medical center.

PATIENT(S)

Family containing four siblings (three female and one male) with complete idiopathic hypogonadotropic hypogonadism.

INTERVENTION(S)

Baseline and stimulated laboratory studies. One patient received GnRH treatment and one received human menopausal gonadotropins.

MAIN OUTCOME MEASURE(S)

Clinical phenotype vs. genotype is assessed by endocrine studies, karyotype, pedigree, and review of pathology slides of ovarian neoplasm.

RESULT(S)

With GnRH stimulation, two patients with idiopathic hypogonadotropic hypogonadism had maximum LH < 10 mIU/mL, and two others had peak LH > 10 mIU/mL. With repeated GnRH stimulation 24 hours later, gonadotropin levels in all patients were increased. Stimulation of thyroid-releasing hormone and tests for insulin-induced hypoglycemia were normal. One affected patient did not ovulate after GnRH treatment, but her sister ovulated with gonadotropin treatment. Another affected sibling had bilateral oophorectomy for seromucinous cystadenomas, and her hypogonadotropic state remained after castration. The man with idiopathic hypogonadotropic hypogonadism and his unaffected brother had a ring chromosome 21.

CONCLUSION(S)

All patients with complete idiopathic hypogonadotropic hypogonadism had the same GnRHR mutations, but clinical presentations and endocrinologic responses were heterogeneous. Gonadotropin levels remained low in patients with idiopathic hypogonadotropic hypogonadism after castration, and ring chromosome 21 was present, suggesting that sequences from this chromosome could affect the idiopathic hypogonadotropic hypogonadism phenotype.

The endocrine control of spermatogenesis

The hormonal regulation of spermatogenesis involves a complex interplay within the hypothalamo-pituitary-testicular axis, which commences before birth with male sexual development and continues through puberty and into adulthood. Hypothalamic gonadotrophin-releasing hormone drives these events by inducing pituitary gonadotrophin secretion, thereby stimulating testicular androgen secretion (providing virility) and spermatogenesis (providing fertility). Evidence from both animal models and man supports a need for both follicle-stimulating hormone and testosterone in achieving full spermatogenic potential, but a species difference in their relative roles exists. Clinical endocrine disorders can arise from a deficiency of hypothalamic gonadotrophin-releasing hormone and/or pituitary gonadotrophins, which results in hypogonadotrophic hypogonadism, featuring delayed/absent puberty and infertility. Physiologically-based and effective treatment with pulsatile gonadotrophin-releasing hormone or gonadotrophins can often restore fertility. Clinical conditions can also be caused by rare genetic disorders of the gonadotrophin molecules or the receptors for androgens and gonadotrophins, which result in a range of phenotypes (from male pseudohermaphroditism through to infertility); these disorders provide a unique insight into the physiology of sexual development and spermatogenesis.