Sequencing of a ‘mouse azoospermia’ gene panel in azoospermic men: identification of RNF212 and STAG3 mutations as novel genetic causes of meiotic arrest

STUDY QUESTION

What is the diagnostic potential of next generation sequencing (NGS) based on a ‘mouse azoospermia’ gene panel in human non-obstructive azoospermia (NOA)?

SUMMARY ANSWER

The diagnostic performance of sequencing a gene panel based on genes associated with mouse azoospermia was relatively successful in idiopathic NOA patients and allowed the discovery of two novel genes involved in NOA due to meiotic arrest.

WHAT IS KNOWN ALREADY

NOA is a largely heterogeneous clinical entity, which includes different histological pictures. In a large proportion of NOA, the aetiology remains unknown (idiopathic NOA) and yet, unknown genetic factors are likely to play be involved. The mouse is the most broadly used mammalian model for studying human disease because of its usefulness for genetic manipulation and its genetic and physiological similarities to man. Mouse azoospermia models are available in the Mouse Genome Informatics database (MGI: http://www.informatics.jax.org/).

STUDY DESIGN, SIZE, DURATION

The first step was to design of a ‘mouse azoospermia’ gene panel through the consultation of MGI. The second step was NGS analysis of 175 genes in a group of highly selected NOA patients (n = 33). The third step was characterization of the discovered gene defects in human testis tissue, through meiotic studies using surplus testicular biopsy material from the carriers of the RNF212 and STAG3 pathogenic variants. The final step was RNF212 and STAG3 expression analysis in a collection of testis biopsies.

PARTICIPANTS/MATERIALS, SETTING, METHODS

From a total of 1300 infertile patients, 33 idiopathic NOA patients were analysed in this study, including 31 unrelated men and 2 brothers from a consanguineous family. The testis histology of the 31 unrelated NOA patients was as follows: 20 Sertoli cell-only syndrome (SCOS), 11 spermatogenic arrest (6 spermatogonial arrest and 5 spermatocytic arrest). The two brothers were affected by spermatocytic arrest. DNA extracted from blood was used for NGS on Illumina NextSeq500 platform. Generated sequence data was filtered for rare and potentially pathogenic variants. Functional studies in surplus testicular tissue from the carriers included the investigation of meiotic entry, XY body formation and metaphases by performing fluorescent immunohistochemical staining and immunocytochemistry. mRNA expression analysis through RT-qPCR of RNF212 and STAG3 was carried out in a collection of testis biopsies with different histology.

MAIN RESULTS AND THE ROLE OF CHANCE

Our approach was relatively successful, leading to the genetic diagnosis of one sporadic NOA patient and two NOA brothers. This relatively high diagnostic performance is likely to be related to the stringent patient selection criteria i.e. all known causes of azoospermia were excluded and to the relatively high number of patients with rare testis histology (spermatocytic arrest). All three mutation carriers presented meiotic arrest, leading to the genetic diagnosis of three out of seven cases with this specific testicular phenotype. For the first time, we report biallelic variants in STAG3, in one sporadic patient, and a homozygous RNF212 variant, in the two brothers, as the genetic cause of NOA. Meiotic studies allowed the detection of the functional consequences of the mutations and provided information on the role of STAG3 and RNF212 in human male meiosis.

LIMITATIONS, REASONS FOR CAUTION

All genes, with the exception of 5 out of 175, included in the panel cause azoospermia in mice only in the homozygous or hemizygous state. Consequently, apart from the five known dominant genes, heterozygous variants (except compound heterozygosity) in the remaining genes were not taken into consideration as causes of NOA. We identified the genetic cause in approximately half of the patients with spermatocytic arrest. The low number of analysed patients can be considered as a limitation, but it is a very rare testis phenotype. Due to the low frequency of this specific phenotype among infertile men, our finding may be considered of low clinical impact. However, at an individual level, it does have relevance for prognostic purposes prior testicular sperm extraction.

WIDER IMPLICATIONS OF THE FINDINGS

Our study represents an additional step towards elucidating the genetic bases of early spermatogenic failure, since we discovered two new genes involved in human male meiotic arrest. We propose the inclusion of RNF212 and STAG3 in a future male infertility diagnostic gene panel. Based on the associated testis phenotype, the identification of pathogenic mutations in these genes also confers a negative predictive value for testicular sperm retrieval. Our meiotic studies provide novel insights into the role of these proteins in human male meiosis. Mutations in STAG3 were first described as a cause of female infertility and ovarian cancer, and Rnf212 knock out in mice leads to male and female infertility. Hence, our results stimulate further research on shared genetic factors causing infertility in both sexes and indicate that genetic counselling should involve not only male but also female relatives of NOA patients.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by the Spanish Ministry of Health Instituto Carlos III-FIS (grant number: FIS/FEDER-PI14/01250; PI17/01822) awarded to CK and AR-E, and by the European Commission, Reproductive Biology Early Research Training (REPROTRAIN, EU-FP7-PEOPLE-2011-ITN289880), awarded to CK, WB, and AE-M. The authors have no conflict of interest.

Association of serum testosterone levels and testicular volume in adult patients

A retrospective observational study was undertaken to gain new insight into the relationship between total testicular volume and levels of serum testosterone, luteinising hormone, follicle-stimulating hormone, prolactin and clinical variables. A total of 312 men with sexual dysfunction or infertility were divided into groups A and B (156 each) on the basis of basal plasma testosterone ≤5 nmol/L of ≥12 nmol/L respectively. Group A was subclassified in A₁ (primary hypogonadism) and A₂ (secondary hypogonadism). There were significant differences in total testicular volume between group A (15.33 ± 11.94 ml) and group B (36.74 ± 6.9; p < .001) and also between subgroup A₁ (11.07 ± 8.49 ml) and subgroup A₂ (23.62 ± 13.04 ml; p < .001). Only 13.5% of patients in group B had a total testicular volume <30 ml. Differences in all studied parameters were found between group A and group B. There were no variations when comparing age, body mass index and testosterone in groups A₁ and A₂ . The use of total testicular volume and body mass index together for predicting testosterone levels yields a sensitivity and specificity of 85.3% and 86.5% respectively. Logistic regression analysis, univariate and multivariate models, using the measurement of total testicular volume resulted in a high capacity to predict testosterone levels.

[Surgical treatment of fracture of the corpus cavernosum and the urethra: review of the literature and report of 2 cases]

OBJECTIVES

Rupture of the corpora cavernosa, also known as fracture of the penis, is uncommon. To date, only 200 cases approximately have been reported. This lesion may be associated with rupture of the urethra in 25% of the cases. The diagnostic methods, management and the results achieved in these patients are discussed.

METHODS

We describe two cases that had been treated at our institution and review the literature.

RESULTS

Preservation of micturition and sexual function was achieved with early surgical management in these two patients with a follow-up of more than one year.

CONCLUSIONS

An associated urethral lesion should be strongly suspected in the presence of urethrorrhagia. A negative urine sediment discards the presence of this lesion. If urethral injury associated with penile rupture is suspected, low pressure urethrography is the diagnostic method of choice following sonographic evaluation of the corpora cavernosa. Cavernosal integrity can only be established by cavernosography in equivocal cases and is not required when extravasation is demonstrated by urethrography. Conservative management carries a high risk of penile curvature and painful erection; therefore, whenever possible, surgery should be performed within a few hours following trauma and after careful evaluation. The cavernosal defect is repaired and end-to-end anastomosis of the urethra is performed. The patient should be informed of the risk of painful erection, penile curvature and urethral stenosis.

[Endoscopy surgery of utriculocele]

Of 12 patients with utriculocele, 6 consulted for sterility and were submitted to endoscopic surgery. Transurethral resection of the utricular capsule removed distal obstruction of the seminal duct in all cases (increased ejaculate volume), in half of the cases sperm density and motility improved, and in 1 case pregnancy was achieved with insemination of capacitated semen (ICA).