Lena Charlotte Schülke’s research while affiliated with University of Münster and other places

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.

Study question Can cryptozoospermic patients be sub classified based on clinical parameters? Summary answer Cryptozoospermic patients can be subdivided in two subgroups based on the histological phenotype of their testicular tissues, testicular volume and FSH levels. What is known already Cryptozoospermia is a severe form of oligozoospermia in which patients present with a sperm concentration of ≤ 0.1 million per milliliter. Due to such low sperm concentration, cryptozoospermic patients depend on the surgical procedure of testicular sperm extraction (TESE) to retrieve sperm for intracytoplasmic sperm injection (ICSI). The general etiology underlying cryptozoospermia remains hitherto unknown, likely due to the high heterogeneity within this patient cohort regarding histological and endocrine parameters. Study design, size, duration We retrospectively selected 132 cryptozoospermic patients (Crypto) who underwent TESE during their fertility treatment. Exclusion parameters were genetic diseases, a history of cryptorchidism or tumors and hormonal treatment. As controls, we selected 160 patients with obstructive azoospermia and normal spermatogenesis. All 292 patients were used for principal component analysis (PCA) followed by hierarchical clustering on principal components (HCPC). Participants/materials, setting, methods PCA and hierarchical clustering were performed considering age, testicular volume, hormonal values (FSH, LH, testosterone and free testosterone), ejaculate parameters (Ejaculate pH and volume, sperm concentration and total sperm count) and histological information. Histological analyses were performed on two PAS-stained sections from two independent biopsies per testis and patient. The percentage of seminiferous tubules containing elongated spermatids, round spermatids, spermatocytes or spermatogonia, as well as Sertoli cell only and hyalinized tubules was assessed. Main results and the role of chance The PCA and hierarchical clustering subdivided the patient cohort into 3 different clusters. Cluster 1 included all the controls (n = 160) and 2 Crypto patients. Remaining Crypto patients were equally subdivided between cluster 2 (n = 65) and cluster 3 (n = 65). The control patients in cluster 1 were characterized by the highest percentages of tubules with elongated spermatids (>80%), normal testicular volume and hormonal values. Characteristic of Crypto patients in cluster 2 was the arrest of germ cell differentiation at the level of spermatocytes in the majority of seminiferous tubules. In contrast, the majority of seminiferous tubules in Crypto patients in cluster 3 showed a Sertoli cell only phenotype or even tubular shadows. Interestingly, the more severe histological phenotype of the cluster 3 patients was accompanied by higher FSH and LH levels as well as lower testicular volume compared to cluster 2 patients. Despite this, the percentage of tubules with full spermatogenesis was similar in the two Crypto groups and no difference was found in the sperm retrieval rate after TESE in the two patient groups. Limitations, reasons for caution Uncovering the existence of subgroups within the cohort of Crypto-patients is the prerequisite for unveiling potentially distinct etiologies. Comprehensive genetic and scRNA-seq analyses of these Crypto-subgroups remain to be performed to validate the findings. Wider implications of the findings Overall, this study offers novel approaches towards the understanding of the etiology underlying the reduced sperm formation in Crypto patients and the division into two subgroups will facilitate the strategic search for underlying causes. Moreover, this approach could be applied to any infertility type in order to identify hidden subgroups. Trial registration number not applicable