Andrology of two hidden clinical subgroups among men with idiopathic cryptozoospermia.

STUDY QUESTION: Are there subgroups among patients with cryptozoospermia pointing to distinct etiologies?

SUMMARY ANSWER: We reveal two distinct subgroups of cryptozoospermic (Crypto) patients based on testicular tissue composition, testicular volume, and FSH levels.

WHAT IS KNOWN ALREADY: Cryptozoospermic patients present with a sperm concentration below 0.1 million/ml. While the etiology of the severely impaired spermatogenesis remains largely unknown, alterations of the spermatogonial compartment have been reported including a reduction of the reserve stem cells in these patients.

STUDY DESIGN, SIZE, DURATION: To assess whether there are distinct subgroups among cryptozoospermic patients, we applied the statistical method of cluster analysis. For this, we retrospectively selected 132 cryptozoospermic patients from a clinical database who underwent a testicular biopsy in the frame of fertility treatment at a university hospital. As controls (Control), we selected 160 patients with obstructive azoospermia and full spermatogenesis. All 292 patients underwent routine evaluation for endocrine, semen, and histological parameters (i.e. the percentage of tubules with elongated spermatids). Moreover, outcome of medically assisted reproduction (MAR) was assessed for cryptozoospermic (n = 73) and Control patients (n = 87), respectively. For in-depth immunohistochemical and histomorphometrical analyses, representative tissue samples from cryptozoospermic (n = 27) and Control patients (n = 12) were selected based on cluster analysis results and histological parameters.

PARTICIPANTS/MATERIALS, SETTING, METHODS: This study included two parts: firstly using clinical parameters of the entire cohort of 292 patients, we performed principal component analysis (PCA) followed by hierarchical clustering on principal components (i.e. considering hormonal values, ejaculate parameters, and histological information). Secondly, for histological analyses seminiferous tubules were categorized according to the most advanced germ cell type present in sections stained with Periodic acid Schif. On the selected cohort of 39 patients (12 Control, 27 cryptozoospermic), we performed immunohistochemistry for spermatogonial markers melanoma-associated antigen 4 (MAGEA4) and piwi like RNA-mediated gene silencing 4 (PIWIL4) followed by quantitative analyses. Moreover, the morphologically defined Adark spermatogonia, which are considered to be the reserve stem cells, were quantified.

MAIN RESULTS AND THE ROLE OF CHANCE: The PCA and hierarchical clustering revealed three different clusters, one of them containing all Control samples. The main factors driving the sorting of patients to the clusters were the percentage of tubules with elongated spermatids (Cluster 1, all Control patients and two cryptozoospermic patients), the percentage of tubules with spermatocytes (Cluster 2, cryptozoospermic patients), and tubules showing a Sertoli cells only phenotype (Cluster 3, cryptozoospermic patients). Importantly, the percentage of tubules containing elongated spermatids was comparable between Clusters 2 and 3. Additional differences were higher FSH levels (P < 0.001) and lower testicular volumes (P < 0.001) in Cluster 3 compared to Cluster 2. In the spermatogonial compartment of both cryptozoospermic Clusters, we found lower numbers of MAGEA4+ and Adark spermatogonia but higher proportions of PIWIL4+ spermatogonia, which were significantly correlated with a lower percentage of tubules containing elongated spermatids. In line with this common alteration, the outcome of MAR was comparable between Controls as well as both cryptozoospermic Clusters.

LIMITATIONS, REASONS FOR CAUTION: While we have uncovered the existence of subgroups within the cohort of cryptozoospermic patients, comprehensive genetic analyses remain to be performed to unravel potentially distinct etiologies.

WIDER IMPLICATIONS OF THE FINDINGS: The novel insight that cryptozoospermic patients can be divided into two subgroups will facilitate the strategic search for underlying genetic etiologies. Moreover, the shared alterations of the spermatogonial stem cell compartment between the two cryptozoospermic subgroups could represent a general response mechanism to the reduced output of sperm, which may be associated with a progressive phenotype. This study therefore offers novel approaches towards the understanding of the etiology underlying the reduced sperm formation in cryptozoospermic patients.

STUDY FUNDING/COMPETING INTEREST(S): German research foundation CRU 326 (grants to: SDP, NN). Moreover, we thank the Faculty of Medicine of the University of Münster for the financial support of Lena Charlotte Schülke through the MedK-program. We acknowledge support from the Open Access Publication Fund of the University of Münster. The authors have no potential conflicts of interest.

TRIAL REGISTRATION NUMBER: N/A.