Compound heterozygous variants of the DMRTC2 gene are associated with non-obstructive azoospermia in a patient

Dear Editor, Male infertility comprises 50% of infertility cases globally, and one of the most severe forms is non-obstructive azoospermia (NOA), which affects approximately 10%–15% of infertile men.1 Genetic factors play an important role in the pathogenesis of NOA. Recent research progress has increasingly elucidated the pathogenesis of NOA. Multiple studies, particularly those using whole-exome sequencing, have identified an increasing number of pathogenic genes, including meiosis specific with OB-fold (MEIOB), telomere repeat binding bouquet formation protein 1 (TERB1), and ubiquitin-specific peptidase 26 (USP26)2–4 associated with NOA. These findings have greatly advanced our understanding of the molecular mechanisms underlying the failure of spermatogenesis. However, the genetic etiology of NOA for a considerable number of patients remains to be identified. DMRT-like family C2 (DMRTC2; also known as DMRT7 in mice) is located on human chromosome 19, which is specifically expressed in testis tissue in humans and mice.5 Previous research has shown that Dmrtc2−/− male mice are infertile, and they exhibit phenotypes of meiotic arrest.6 However, there have been no clinical reports of this gene variant causing NOA in humans. Our study reported the novel pathogenic variants of DMRTC2 found in a NOA patient. The methodologies are available in Supplementary Participants and Methods. This study has been approved by the Ethics Committee of the National Institute of Family Planning (Beijing, China; Approval No. 2021010), and all participants in this study have signed written informed consent forms, agreeing that their anonymized data would be used for the analysis and publication of the results of this study. In this study, a 35-year-old man from a Chinese family had a 2-year history of infertility without contraception (Figure 1a). Bilateral, small-sized testes (12 ml; reference range: >14 ml) were detected, with no nodules in the bilateral epididymis and no varicose veins in the bilateral spermatic cord. Ultrasound examinations showed no abnormalities in the testes and epididymis. Semen examination showed complete azoospermia. A hormonal analysis showed normal blood concentrations of testosterone (12.43 nmol l−1; reference range: 10.4–34.7 nmol l−1) and follicle-stimulating hormone (7.24 IU l−1; reference range: 2.0–7.6 IU l−1) with an elevation of luteinizing hormone (8.18 IU l−1; reference range: 1.8–5.2 IU l−1). The histology of the testis showed spermatogenic arrest. The seminiferous tubules of the patient were found to contain spermatocytes and spermatids with the absence of spermatozoa (Johnsen score = 5), which manifested maturation arrest (Figure 1b). The patient showed a normal karyotype, and no microdeletion was identified on the Y chromosome.Figure 1: Identification of the novel DMRTC2 variants in a patient with NOA from a Chinese family. (a) Pedigree for the family with the variants in DMRTC2 gene. The black solid symbols represent individuals with NOA, and the arrow points to the proband (II-1). (b) Testicular histological analysis of the patient showed dysplasia of spermatogenic cells. The image shows seminiferous tubule structures where no spermatozoa is observed, but spermatocytes and spermatids are present. Scale bar = 100 µm (left) and 25 µm (right). (c) Sanger sequencing validated WES results. The red boxes indicate the locations of the DMRTC2 variants. As shown, the proband carries two mutations, while each of his unaffected parents carries only one of them. (d) Conservation analysis of the p.R46H variant. The black box indicates the mutated amino acid. Conservation analysis showed that the variant occurred at highly conserved residues through evolution across several species. DMRTC2: DMRT-like family C2; NOA: non-obstructive azoospermia; WES: whole-exome sequencing.We identified the pathogenic variants carried through the patient by whole-exome sequencing. After bioinformatic analysis and screening, we identified that the patient carried two heterozygous variants located in the DMRTC2 gene on chromosome 19: c.G137A(p.R46H) and c.862_863insGGCAGCTGCAGCAAGA(p.E296Afs*4). Among the genes in the shortlist (Supplementary Table 1), the DMRTC2 gene is specifically expressed in male testis tissue cells, and Dmrtc2−/− male mice display a phenotype characterized by spermiogenesis arrest.5Supplementary Table 1: Candidate pathogenic variations and in silico bioinformatics prediction in the affected individualThe result of Sanger sequencing showed that the two variants carried by the patient were inherited separately from his parents (Figure 1c). In the gnomAD database, the overall frequency and the frequency in the East Asian population of the missense variant c.G137A are both sufficiently low. Conservation assessment shows that the amino acid residue is highly conserved across different species (Figure 1d). Based on the analysis results of polymorphism phenotyping, version 2 (PolyPhen-2, possibly damaging), MutationTaster (disease causing), and combined annotation-dependent depletion (CADD, score 24.4), this variant is predicted to have a deleterious impact on the structure and function of DMRTC2 (Supplementary Table 2). The other variant c.862_863insGGCAGCTGCAGCAAGA was not available in the gnomAD database, indicating its rarity. The variant results in an insertion of 16 bases, causing a frameshift and premature termination codon, potentially yielding a truncated protein. Based on the above results, we infer that the compound heterozygous variants in DMRTC2 are the pathogenic variants for NOA in this patient.Supplementary Table 2: Bioinformatics prediction of DMRT-like family C2 variants identified in the non-obstructive azoospermia patientDMRT genes encode a highly conserved family of transcription factors. In a variety of vertebrates, DMRT genes play crucial roles in spermiogenesis.7 Prior investigations have demonstrated the pathogenicity of Dmrtc2 in mice; however, DMRTC2 variants have not been identified in humans.6 In the Dmrtc2−/− mice, most Dmrtc2 mutant germ cells undergo apoptosis after being arrested during the pachytene stage of meiosis. Although a few of the mutant cells can survive to diplonema, they show defects in sex chromosome epigenetics during the subsequent transition from the pachytene stage to the diplotene stage, which leads to spermatogenic arrest in the Dmrtc2−/− mice.8 Similar to some genes mentioned in a recent study on NOA-related genes, DMRTC2 is speculated to cause meiotic arrest in humans.9 The patient of the current study exhibited spermatogenic arrest at meiosis and the absence of spermatozoa. Although testicular sperm extraction was not performed on the patient, the analysis of similar cases suggests that the testicular extraction result would also be negative.9 The testicular pathological phenotype of this patient aligns with the phenotypes observed in the Dmrtc2−/− mice. In the seminiferous tubules of Dmrtc2−/− males, spermatocytes and spermatogonia were visible with a few round spermatids and no elongating spermatids.10 The result of bioinformatics prediction showed that both variants carried by the patient may be deleterious. This suggests that the reason for the block in meiotic progression caused by the lack of DMRTC2 may be the same between humans and mice. It is important to note that while knockout mouse models can provide valuable functional insights, not all human variants result in phenotypes in model organisms. Therefore, this study has certain limitations, and the current results need further evaluation in functional studies in appropriate biological models. In the present study, testicular biopsy was only performed in a specific area of the right testis and may not fully reflect the pathological status of the entire testis. Because of the multifocal nature of testicular histopathological findings, this sampling method may limit a comprehensive assessment of testicular pathological changes. In conclusion, we demonstrated that compound heterozygous variants in DMRTC2 represent a novel genetic cause of NOA in men, characterized by spermatogenic failure. Our findings expand the current evidence regarding genetic causes of NOA and provide valuable insights that may advance the development of improved clinical diagnostic strategies. AUTHOR CONTRIBUTIONS XYG, YS, and BBW contributed to the study design. XBL, MY, and XYG collected and interpreted the patient data. YM, TYL, and LXR analyzed the data of whole-exome sequencing and Sanger sequencing. YM drafted the manuscript. All authors read and approved the final manuscript. COMPETING INTERESTS All authors declare no competing interests. Supplementary Information is linked to the online version of the paper on the Asian Journal of Andrology website.