Disruption of human meiotic telomere complex genes TERB1, TERB2 and MAJIN in men with non-obstructive azoospermia

Abstract

Non-obstructive azoospermia (NOA), the lack of spermatozoa in semen due to impaired spermatogenesis affects nearly 1% of men. In about half of cases, an underlying cause for NOA cannot be identified. This study aimed to identify novel variants associated with idiopathic NOA. We identified a nonconsanguineous family in which multiple sons displayed the NOA phenotype. We performed whole-exome sequencing in three affected brothers with NOA, their two unaffected brothers and their father, and identified compound heterozygous frameshift variants (one novel and one extremely rare) in Telomere Repeat Binding Bouquet Formation Protein 2 (TERB2) that segregated perfectly with NOA. TERB2 interacts with TERB1 and Membrane Anchored Junction Protein (MAJIN) to form the tripartite meiotic telomere complex (MTC), which has been shown in mouse models to be necessary for the completion of meiosis and both male and female fertility. Given our novel findings of TERB2 variants in NOA men, along with the integral role of the three MTC proteins in spermatogenesis, we subsequently explored exome sequence data from 1495 NOA men to investigate the role of MTC gene variants in spermatogenic impairment. Remarkably, we identified two NOA patients with likely damaging rare homozygous stop and missense variants in TERB1 and one NOA patient with a rare homozygous missense variant in MAJIN. Available testis histology data from three of the NOA patients indicate germ cell maturation arrest, consistent with mouse phenotypes. These findings suggest that variants in MTC genes may be an important cause of NOA in both consanguineous and outbred populations.

Fig. 1.

Pedigree structure of three families. a) Non-consanguineous Family 1. b) Consanguineous Family (Individual 3; M2073). c) Consanguineous Family (Individual 4; M1646). Filled symbols indicate members affected with NOA, and unfilled symbols indicate unaffected members. WES with subsequent confirmation by Sanger sequencing was performed on the individuals indicated with #, and * indicates a case in which only Sanger sequencing was performed. Pedigree information was unavailable for Individual 2.

Fig. 2.

a) Periodic acid-Schiff (PAS) staining of a control individual with normal spermatogenesis (M2211). b) PAS staining, and c) immunohistochemistry (CREM staining) of testis from individual 4 (M1646; MAJIN), indicating an arrest at round spermatid stage. Abbreviations: L=Lumen, S’gonia=Spermatogonia, SC=Sertoli cell, 1°S’cyte=Primary spermatocyte, 2°S’cyte =Secondary spermatocyte, rST= Round spermatids, eST=Elongated spermatids.

Fig. 3.

Schematic model of the connection between telomeres and the nuclear membrane in prophase I of meiosis through the MTC (TERB1-TERB2-MAJIN) in mammals. Figure adapted from the information of (Shibuya et al. 2015; Zhang et al. 2017; Wang et al. 2019).