Mutations in the chromosome pairing gene FKBP6 are not a common cause of non-obstructive azoospermia

Genome-wide association study implicates testis-sperm specific FKBP6 as a susceptibility locus for impaired acrosome reaction in stallions

Impaired acrosomal reaction (IAR) of sperm causes male subfertility in humans and animals. Despite compelling evidence about the genetic control over acrosome biogenesis and function, the genomics of IAR is as yet poorly understood, providing no molecular tools for diagnostics. Here we conducted Equine SNP50 Beadchip genotyping and GWAS using 7 IAR–affected and 37 control Thoroughbred stallions. A significant (P<6.75E-08) genotype–phenotype association was found in horse chromosome 13 in FK506 binding protein 6 (FKBP6). The gene belongs to the immunophilins FKBP family known to be involved in meiosis, calcium homeostasis, clathrin-coated vesicles, and membrane fusions. Direct sequencing of FKBP6 exons in cases and controls identified SNPs g.11040315G>A and g.11040379C>A (p.166H>N) in exon 4 that were significantly associated with the IAR phenotype both in the GWAS cohort (n = 44) and in a large multi-breed cohort of 265 horses. All IAR stallions were homozygous for the A-alleles, while this genotype was found only in 2% of controls. The equine FKBP6 was exclusively expressed in testis and sperm and had 5 different transcripts, of which 4 were novel. The expression of this gene in AC/AG heterozygous controls was monoallelic, and we observed a tendency for FKBP6 up-regulation in IAR stallions compared to controls. Because exon 4 SNPs had no effect on the protein structure, it is likely that FKBP6 relates to the IAR phenotype via regulatory or modifying functions. In conclusion, FKBP6 was considered a susceptibility gene of incomplete penetrance for IAR in stallions and a candidate gene for male subfertility in mammals. FKBP6 genotyping is recommended for the detection of IAR–susceptible individuals among potential breeding stallions. Successful use of sperm as a source of DNA and RNA propagates non-invasive sample procurement for fertility genomics in animals and humans.

Impaired acrosomal reaction (IAR) of sperm causes male subfertility in humans and animals, and currently the molecular causes of the condition are not known. Here we report the mapping, identification, and functional analysis of a susceptibility locus for IAR in stallions. The candidate region was mapped to horse chromosome 13 by SNP genotyping and GWAS of 7 IAR affected and 44 control Thoroughbred stallions. Re-sequencing and case-control analysis of functionally relevant candidate genes in the region identified FKBP6 gene as a significantly associated locus. The association was confirmed by genotyping 265 male horses of multiple breeds. FKBP6 belongs to the immunophilins FKBP family known to be involved in meiosis, calcium homeostasis, clathrin-coated vesicles, and membrane fusions. We showed that the equine FKBP6 is exclusively and monoallelically expressed in testis and sperm and has 5 different transcripts, of which 4 were novel. Overall, FKBP6 was considered a susceptibility gene of incomplete penetrance for IAR in stallions and a candidate gene for male subfertility in other mammals. Successful use of sperm as a source of DNA and RNA propagates non-invasive sample procurement for fertility genomics in animals and humans.

Genetic causes of spermatogenic failure

Approximately 10%-15% of couples are infertile, and a male factor is involved in almost half of these cases. This observation is due in part to defects in spermatogenesis, and the underlying causes, including genetic abnormalities, remain largely unknown. Until recently, the only genetic tests used in the diagnosis of male infertility were aimed at detecting the presence of microdeletions of the long arm of the Y chromosome and/or chromosomal abnormalities. Various other single-gene or polygenic defects have been proposed to be involved in male fertility. However, their causative effects often remain unproven. The recent evolution in the development of whole-genome-based techniques and the large-scale analysis of mouse models might help in this process. Through knockout mouse models, at least 388 genes have been shown to be associated with spermatogenesis in mice. However, problems often arise when translating this information from mice to humans.

Meiotic recombination and male infertility: from basic science to clinical reality?

Infertility is a common problem that affects approximately 15% of the population. Although many advances have been made in the treatment of infertility, the molecular and genetic causes of male infertility remain largely elusive. This review will present a summary of our current knowledge on the genetic origin of male infertility and the key events of male meiosis. It focuses on chromosome synapsis and meiotic recombination and the problems that arise when errors in these processes occur, specifically meiotic arrest and chromosome aneuploidy, the leading cause of pregnancy loss in humans. In addition, meiosis-specific candidate genes will be discussed, including a discussion on why we have been largely unsuccessful at identifying disease-causing mutations in infertile men. Finally clinical applications of sperm aneuploidy screening will be touched upon along with future prospective clinical tests to better characterize male infertility in a move towards personalized medicine.

No significantly increased frequency of the inversion polymorphism at the WBS-critical region 7q11.23 in German parents of patients with Williams-Beuren syndrome as compared to a population control

Background

Typical Williams-Beuren syndrome (WBS) is commonly caused by a ~1.5 Mb - ~1.8 Mb heterozygous deletion of contiguous genes at chromosome region 7q11.23. The majority of WBS cases occurs sporadically but few familial cases of autosomal dominant inheritance have been reported. Recent data demonstrated the existence of the paracentric inversion polymorphism at the WBS critical region in 7q11.23 in some of the progenitors transmitting the chromosome which shows the deletion in the affected child. In parents having a child affected by WBS the prevalence of such a structural variant has been reported to be much higher (~25- ~30%) than in the general population (~1- ~6%). However, in these previously reported studies only a limited number of randomly selected patients and non transmitting parents of WBS patients were used as controls, but without specification of any clinical data. Therefore we have undertaken a German population-based molecular cytogenetic investigation. We evaluated the incidence of the paracentric inversion polymorphism at 7q11.23 analyzing interphase nuclei of lymphocytes using a three color fluorescence in situ hybridization (FISH) probe.

Results

FISH analysis was carried out on couples with a child affected by WBS as compared to a population sample composed of different normal individuals: Control group I: couples with two healthy children, control group II: couples with fertility problems, planning ICSI and control group III: couples with two healthy children and one child with a chromosome aberration, not involving region 7q11.23. The three color FISH assay showed that the frequency of the paracentric inversion polymorphism at 7q11.23 in couples with a child affected by WBS was 20.8% (5 out of 24 pairs) as compared to 8.3% (2 out of 24 pairs, control group I), 25% (4 out of 16 pairs, control group II) and 9.1% (1 out of 11 pairs, control group III), respectively (total 7 out of 51 pairs, 13.8%). The frequencies differed between the groups, but this was statistically not significant (p > 0.05, Fisher's test).

Conclusion

Our results do not support the hypothesis that the paracentric inversion polymorphism at 7q11.23 is a major predisposing factor for the WBS deletion.

Screening of genes involved in chromosome segregation during meiosis I: toward the identification of genes responsible for infertility in humans

Prophase I of male meiosis during early spermatogenesis involves dynamic chromosome segregation processes, including synapsis, meiotic recombination and cohesion. Genetic defects in the genes that participate in these processes consistently cause reproduction failure in mice. To identify candidate genes responsible for infertility in humans, we performed gene expression profiling of mouse spermatogenic cells undergoing meiotic prophase I. Cell fractions enriched in spermatogonia, leptotene/zygotene spermatocytes or pachytene spermatocytes from developing mouse testis were separately isolated by density gradient sedimentation and subjected to microarray analysis. A total of 726 genes were identified that were upregulated in leptotene/zygotene spermatocytes. To evaluate the screening efficiency for meiosis-specific genes, we randomly selected 12 genes from this gene set and characterized each gene product using reverse transcription (RT)-PCR of RNA from gonadal tissues, in situ hybridization on testicular tissue sections and subcellular localization analysis of the encoded protein. Four of the 12 genes were confirmed as genes expressed in meiotic stage and 2 of these 4 genes were novel, previously uncharacterized genes. Among the three confirmation methods that were used, RT-PCR appeared to be the most efficient method for further screening. These 726 candidates for human infertility genes might serve as a useful resource for next-generation sequencing combined with exon capture by microarray.

Alterations in the steroid hormone receptor co-chaperone FKBPL are associated with male infertility: a case-control study

BACKGROUND

Male infertility is a common cause of reproductive failure in humans. In mice, targeted deletions of the genes coding for FKBP6 or FKBP52, members of the FK506 binding protein family, can result in male infertility. In the case of FKBP52, this reflects an important role in potentiating Androgen Receptor (AR) signalling in the prostate and accessory glands, but not the testis. In infertile men, no mutations of FKBP52 or FKBP6 have been found so far, but the gene for FKBP-like (FKBPL) maps to chromosome 6p21.3, an area linked to azoospermia in a group of Japanese patients.

METHODS

To determine whether mutations in FKBPL could contribute to the azoospermic phenotype, we examined expression in mouse and human tissues by RNA array blot, RT-PCR and immunohistochemistry and sequenced the complete gene from two azoospermic patient cohorts and matching control groups. FKBPL-AR interaction was assayed using reporter constructs in vitro.

RESULTS

FKBPL is strongly expressed in mouse testis, with expression upregulated at puberty. The protein is expressed in human testis in a pattern similar to FKBP52 and also enhanced AR transcriptional activity in reporter assays. We examined sixty patients from the Japanese patient group and found one inactivating mutation and one coding change, as well as a number of non-coding changes, all absent in fifty-six controls. A second, Irish patient cohort of thirty showed another two coding changes not present in thirty proven fertile controls.

CONCLUSIONS

Our results describe the first alterations in the gene for FKBPL in azoospermic patients and indicate a potential role in AR-mediated signalling in the testis.

Unravelling the genetics of spermatogenic failure

Subfertility, defined as the inability to conceive within 1 year of unprotected intercourse, affects 10–15% of couples. In up to 55% of couples, the male partner is diagnosed with spermatogenic failure, i.e. one or more semen parameters fall below the WHO criteria for normozoospermia. In these cases, assisted reproductive technology is usually used to achieve pregnancy. Both genetic and environmental factors are thought to underlie spermatogenic failure. Despite years of research, only few genetic factors have clearly been shown to cause spermatogenic failure, and the identification of additional genetic causes or risk factors has proven to be extremely difficult. In this review, we will present an overview of established genetic causes of spermatogenic failure, describe pitfalls in searching for novel genetic factors and discuss research opportunities for the future.

Gene polymorphisms/mutations relevant to abnormal spermatogenesis

Despite the identification of an increasing number of candidate genes involved in spermatogenesis, the armamentarium of diagnostic genetic tests in male infertility remains extremely limited. A number of new causative mutations have been reported for hypogonadotrophic hypogonadism but still the genetic diagnosis in this pathological condition is made only in about 20% of cases. The sole molecular genetic test that is routinely proposed in severe spermatogenic disturbances is screening for Yq microdeletion. The search for causative mutations in the Y chromosome, and in autosomal and X-linked genes, has mostly been unsuccessful. The paucity of gene mutations raises questions about the appropriateness of the currently used screening approaches. Among the proposed genetic risk factors, gr/gr deletion of the Y chromosome seems to be the most promising polymorphism. Other polymorphisms are awaiting further confirmation, whereas for some (POLG, DAZL, USP26, FSHR) a lack of association with abnormal spermatogenesis has now been ascertained. It is likely that some polymorphisms lead to testicular dysfunction only when in association with a specific genetic background or with environmental factors. Future large-scale studies with stringent study design may provide a more efficient way to identify clinically relevant genetic factors of male infertility.

Absence of the H2AX mutations in idiopathic infertile men with spermatogenic impairment

H2AX is a histone H2A variant and one of the evolutionarily conserved fertility factors involved in DNA repair to maintain the genomic integrity and ensure the proper meiotic process. Male H2ax mutant mice are infertile and display defective meiosis. To investigate the possible association of variations of the H2AX gene with spermatogenic impairment in humans, mutation screening of the entire coding region of this gene was carried out in 302 patients with azoospermia or severe oligospermia along with 198 normospermic controls. No mutations or other sequence variants were identified in the 500 subjects tested. This suggests that it is unlikely that the H2AX mutations are a common genetic cause of spermatogenic impairment in idiopathic infertile men.

Mutation screening of the FKBP6 gene and its association study with spermatogenic impairment in idiopathic infertile men

Fkbp6 has been proved to be a new component of synaptonemal complexes and be involved in homologous chromosomes pairing and male infertility in mice. To explore the possible association between variations in the FKBP6 gene and impaired spermatogenesis in human, mutation screening of all the eight exons and the intron/exon boundaries of the gene was performed in 323 patients with azoospermia or severe oligozoospermia and 205 fertile controls by denatured HPLC and DNA sequencing. As a result, four novel and one known single nucleotide transitions were identified, including c.58-2A>G, c.111C>T, c.156G>T, c.594G>A, and c.216C>A (rs3750075). The frequencies of genotype CA, allele A of c.216C>A and haplotype ‘GAG’ consisting of c.156G>T, c.216C>A, and c.594G>A were significantly lower in infertile patients than those in controls. These findings suggest that the FKBP6 gene may play a role in modifying the susceptibility to idiopathic spermatogenic impairment in human and propose that the allele A of c.216C>A seems to be a protective factor for the development of male infertility.

Is a genetic defect in Fkbp6 a common cause of azoospermia in humans?

FK506-binding protein 6 (Fkbp6) is a member of a gene family containing a prolyl isomerase/FK506-binding domain and tetratricopeptide protein-protein interaction domains. Recently, the targeted inactivation of Fkbp6 in mice has been observed to result in aspermic males and the absence of normal pachytene spermatocytes. The loss of Fkbp6 results in abnormal pairing and a misalignment of the homologous chromosomes, and in non-homologous partner switches and autosynapsis of the X chromosome cores in meiotic spermatocytes. In this study, we analyzed whether human FKBP6 gene defects might be associated with human azoospermia. We performed a mutation analysis in all the coding regions of the human FKBP6 gene in 19 patients with azoospermia resulting from meiotic arrest. The expression of the human FKBP6 gene was specific to the testis, and a novel polymorphism site, 245C --> G (Y60X) could be found in exon 3. Our findings suggest that the human FKBP6 gene might be imprinted in the testis based on an analysis using two polymorphism sites.