Spermatogenomics : Correlating Testicular Gene Expression to Human Male Infertility
Spermatogenesis is a complex and coordinated process of formation of sperms from the precursor spermatogonia, occurring inside the unique environment existing in the seminiferous epithelium. This process of development, characterized by concomitant changes in the cellular morphology, metabolism and differential gene expression, can be divided into 3 distinct phases: i) proliferation of the spermatogonia through mitosis; ii) meiosis or reduction division, which commences with transformation of the type B spermatogonia into primary spermatocytes and their subsequent entry into the meiotic prophase I. These primary spermatocytes, divide to form secondary spermatocytes, and then divide again to form haploid round spermatids; (iii) spermiogenesis or differentiation and maturation of the round spermatids without further division to form the unique spermatozoa (Kerr and De Kretser, 2006, Clermont, 1966, Heller and Clermont, 1964). This complex process of division and differentiation is regulated at three distinct levels: i) The extrinsic level where the gonadotropins and testosterone regulate gene expression in the germ cells sustaining their survival and differentiation (French, 2012); ii) The interactive regulation that involves interactions between the somatic cells such as the Sertoli cells and the germ cells; iii) The intrinsic gene expression associated with each step of development of the germ cells (Eddy, 2002) wherein each stage of differentiation is accompanied by precise stage-specific differential gene expression. (Kleene, 1996, Kierszenbaum et al., 2003, Sassone-Corsi, 2002, Kleene, 2001, Sassone-Corsi, 1997). Any alterations in this gene expression pattern leads to disruption and/or arrest of spermatogenesis at various stages, causing male infertility (Zorrilla and Yatsenko, 2013, Krausz et al., 2015). Many studies have been focused on investigating the underlying molecular mechanisms governing the process of germ cell development such as self-renewal, meiotic recombination and differentiation (Hecht, 1998, Grootegoed et al., 2000, Robles et al., 2017). Analysis of differential gene expression in isolated and purified populations of different germ cells have been very useful in the understanding of the genetic regulation of human spermatogenesis by providing information about the cell type-specific gene expression and regulation. (Meistrich et al., 1973, Bellvé, 1993, Meistrich et al., 1981, Chalmel et al., 2007). However, these methods are limited by the large amounts of tissue required, which is difficult to obtain in the case of humans (Schultz et al., 2003). Large-scale gene expression studies and the “omics revolution” have also helped in identifying some of the regulators of spermatogenesis (Carrell et al., 2016). In spite of advances in the current understanding of the regulation of spermatogenesis, the exact molecular mechanisms of how the genetic and epigenetic alterations affect human spermatogenesis are still unclear (Neto et al., 2016). The present study is an attempt to investigate the human testicular gene expression pattern in the germ cells of patients with various types of azoospermia, and correlate the same to infertility. Comparative analysis of the testicular transcriptomes of infertile individuals (with arrested spermatogenesis) with the control, fertile individuals (with normal spermatogenesis) would allow identification of the cell type-specific altered genes. Analysis of these genes would provide an insight into the genetic regulation of the progress of spermatogenesis as well as allow identification of the crucial genes responsible for the arrest. The first step in this study was to ascertain the exact status of spermatogenesis in patients’ testes. Forty-four azoospermic patients were classified clinically into two major groups – obstructive (OA) and non-obstructive (NOA) azoospermia and further classified using flow cytometric analysis of the germ cells. The patients with OA exhibited presence of the diploid, tetraploid and haploid cells indicating complete spermatogenesis (Group I: DTH). The patients with NOA showed incomplete spermatogenesis with arrest at either the meiotic stage showing the presence of diploid and tetraploid cells, but not the haploid cells (Group II: DT), or at the pre-meiotic stage with only diploid cells (Group III: D). This was further verified by RT-PCR analyses for specific markers for different testicular cells. The Group I patients showed expression of markers specific for the Leydig cell (LHCGR, HSD3B2 and HSD17B3), the Sertoli cell (FSHR, KITL), spermatogonia (KIT), tetraploid cells (CCNA1, LDHC) and haploid cells (PRM1). The Group II patients showed expression of CCNA1 and LDHC, but not of PRM1. The Group III patients did not express any of the haploid or tetraploid specific markers. The germ cell pattern was further confirmed by histology where a clear difference was seen across the groups in accordance with their flow cytometric profiles. Subsequent to grouping of the patient samples based on their testicular germ-cell pattern, microarray analysis was carried out with representative samples from each group leading to identification of diploid-/tetraploid-/haploid-specific (D/T/H) genes. The enrichment, probable pathways and network interactions of these identified genes were analyzed and found to be in agreement with the classification made in this study. Further, based on their network interactions, the genes that were under multiple modes of regulation and the transcription factors that regulated multiple pathways were selected for further analysis. In absence of an in-vitro system to study germ cell differentiation, the importance of the selected genes in the progression of human spermatogenesis was analyzed from the data extrapolated from information available in the literature about expression of each gene in the human testes (wherever available), known function of the genes in various somatic cells, function in developing and adult testes of model organisms and the data from the knockout or transgenic animals where disruption of the gene/s resulted in an arrest or disruption of spermatogenesis. Expression of all the putative crucial genes was analyzed in all the patients including the control patients at the transcript level and three selected genes (one from each group- D, T and H) were further validated at the protein level using immunohistochemistry. All the genes showed a similar pattern of amplification in the different groups of patients to the pattern observed from the microarray. The diploid-specific genes (selected based on the available literature) were mainly the inhibitors or regulators of the cell cycle (CDKN1A, GADD45A, FOXM1) (Xiong et al., 1991, Jin et al., 2002, Laoukili et al., 2005) and regulators of cellular proliferation (KLFs, FOS, SRF, ATFs, SMADs) (Garrett-Sinha et al., 1996, Persengiev and Green, 2003, Angel and Karin, 1991, Ten Dijke et al., 2002). Six diploid-specific genes that were potential regulators of spermatogenesis were identified to be probable causes for the arrest of spermatogenesis at the pre-meiotic stage. CDKN1A showed elevated expression at the transcript level which suggested that DNA-damage induced proliferation check (mediated through CDKN1A) in the diploid cells probably prevented these cells from entering meiosis. This was further verified at the protein level by immuno-staining for CDKN1A. Further, GADD45A, KLF4, FOS, MCL1 and SERPINE1 were identified as genes crucial for transition from the diploid to the tetraploid stage and their aberrant expression correlated to the arrest of spermatogenesis in the Group D patients. Six tetraploid-specific genes and four haploid-specific genes were identified to be potential regulators of the tetraploid-haploid transition and responsible for the meiotic arrest. Over expression of the pro-inflammatory genes such as CCL3, IL1B and IL8 (Guazzone et al., 2009) was seen in the testis of the arrested patients which suggested that there was a potential alteration of the normal testicular micro-environment. Expression of EGR2 (a spermatogonial-maintenance gene controlling mitosis (Joseph et al., 1988)) was seen in the nucleus of spermatocytes in group DT patients which indicated its role in the meiotic arrest. To understand the role of the haploid-specific genes in the context of spermatocyte differentiation, only those genes whose expression are reported in the spermatocytes and persist till the spermatid stage were selected. Lack of expression of CST8 was identified to be potentially responsible for loss of germ cell integrity, and the loss of GGN expression in the Group DT patients seemed to be a significant contributor to the genotoxic stress in these patients. In the arrested patients RFX2 (reported to be master regulator of spermiogenesis (Wu et al., 2016)) was seen to be down regulated at the transcript level which indicated its role in the control of meiosis. This was further confirmed by IHC, where expression of RFX2 was seen to be present in the tetraploid cells of the Group DTH patients while no expression was seen in the tetraploid cells of Group DT patients. Thus, this study identified a role for RFX2 in the regulation of meiosis in humans, similar to the findings reported in rats (Horvath et al., 2009). The study also identified autophagy as a mechanism for the clearance of the arrested cells in NOA patients. IHC data using αLC3B showed that autophagy was up regulated in the arrested patients as compared to the Group DTH patients suggesting its role in cell survival and recycling of nutrients. Further, in-situ TUNEL labeling of tissue sections from the different groups (DTH, DT and D) revealed that there were no difference in the status of apoptosis in the azoospermic patients. The latter observation further corroborated with the elevated expressions of CDKN1A, GADD45A, MCL1, TNFAIP3 (reported to ensure cell survival by negatively modulating apoptosis) as seen in the NOA patients. In conclusion, this study identifies several genes that control the progression of spermatogenesis, including the genes whose alterations contribute towards an arrest in spermatogenesis, especially in azoospermia. These identified genes may be used as novel markers in the diagnosis of male infertility. The study opens up the possibility of using the identified genes as future therapeutic targets using small molecular regulators for treatment of infertility as well as targets for male contraception. The study also identifies a novel role for autophagy in patients with NOA which opens up new avenues for further investigation. Thus, this study is the beginning of understanding the complex events that regulate spermatogenesis.