Homozygous mutations in C14orf39/SIX6OS1 cause non-obstructive azoospermia and premature ovarian insufficiency in humans.

Human infertility is a multifactorial disease that affects 8%-12% of reproductive-aged couples worldwide. However, the genetic causes of human infertility are still poorly understood. Synaptonemal complex (SC) is a conserved tripartite structure that holds homologous chromosomes together and plays an indispensable role in the meiotic progression. Here, we identified three homozygous mutations in the SC coding gene C14orf39/SIX6OS1 in infertile individuals from different ethnic populations by whole-exome sequencing (WES). These mutations include a frameshift mutation (c.204_205del [p.His68Glnfs∗2]) from a consanguineous Pakistani family with two males suffering from non-obstructive azoospermia (NOA) and one female diagnosed with premature ovarian insufficiency (POI) as well as a nonsense mutation (c.958G>T [p.Glu320∗]) and a splicing mutation (c.1180-3C>G) in two unrelated Chinese men (individual P3907 and individual P6032, respectively) with meiotic arrest. Mutations in C14orf39 resulted in truncated proteins that retained SYCE1 binding but exhibited impaired polycomplex formation between C14ORF39 and SYCE1. Further cytological analyses of meiosis in germ cells revealed that the affected familial males with the C14orf39 frameshift mutation displayed complete asynapsis between homologous chromosomes, while the affected Chinese men carrying the nonsense or splicing mutation showed incomplete synapsis. The phenotypes of NOA and POI in affected individuals were well recapitulated by Six6os1 mutant mice carrying an analogous mutation. Collectively, our findings in humans and mice highlight the conserved role of C14ORF39/SIX6OS1 in SC assembly and indicate that the homozygous mutations in C14orf39/SIX6OS1 described here are responsible for infertility of these affected individuals, thus expanding our understanding of the genetic basis of human infertility.

In silico analysis of a novel pathogenic variant c.7G > A in C14orf39 gene identified by WES in a Pakistani family with azoospermia.

Infertility is a multifactorial disorder that affects approximately 12% of couples of childbearing ages worldwide. Few studies have been conducted to understand the genetic causes of infertility in depth. The synaptonemal complex (SC), which is essential for the progression of meiosis, is a conserved tripartite structure that binds homologous chromosomes together and is thus required for fertility. This study investigated genetic causes of infertility in a Pakistani consanguineous family containing two patients suffering from non-obstructive azoospermia (NOA). We performed whole-exome sequencing, followed by Sanger sequencing, and identified a novel pathogenic variant (c.7G > A [p.D3N]) in the SC coding gene C14orf39, which was recessively co-segregated with NOA. In silico analysis revealed that charges on wild-type residues were lost, which may result in loss of interactions with other molecules and residues, and a reduction in protein stability occurred, which was caused by the p.D3N mutation. The novel variant generated the mutant protein C14ORF39D3N, and homozygous mutations in C14orf39 resulted in NOA. The transcriptome profile of C14ORF39 shows that it is specifically expressed in early brain development, which suggests that research in this area is required to study other functions of C14ORF39 in addition to its role in the germline. This research highlights the conserved role of C14orf39/SIX6OS1 in assembly of the SC and its indispensable role in facilitating genetic diagnosis in patients with infertility, which may enable the development of future treatments.

Variations of C14ORF39 and SYCE1 Identified in Idiopathic Premature Ovarian Insufficiency and Nonobstructive Azoospermia.

CONTEXT: Premature ovarian insufficiency (POI) and nonobstructive azoospermia (NOA) are the most severe diseases causing irreversible infertility in females and males, respectively. The contribution of synaptonemal complex (SC) gene variations in the pathogenesis of sporadic patients with POI and NOA has not been systematically illustrated. OBJECTIVE: To investigate the role of SC genes in the pathogenesis of sporadic POI and NOA. DESIGN: Genetic and functional study. SETTING: University-based reproductive medicine center. PATIENT(S): A total of 1030 patients with sporadic POI and 400 patients with sporadic NOA. INTERVENTION(S): The variations of SC genes were filtered in the in-house database of whole exome sequencing performed in 1030 patients with sporadic POI and 400 patients with sporadic NOA. The pathogenic or likely pathogenic variations following recessive inheritance mode were selected according to American College of Medical Genetics and Genomics (ACMG) guidelines and confirmed by Sanger sequencing. The pathogenic effects of the variations were verified by functional studies. MAIN OUTCOME MEASURE(S): ACMG classification and functional characteristics. RESULT(S): Two homozygous variations of C14ORF39 and 2 recessive variations of SYCE1 were first identified in sporadic patients with POI and NOA, respectively. Functional studies showed the C14ORF39 variations significantly accelerated the protein degradation and the variations in SYCE1 disrupted its interaction with SYCP1 or C14ORF39, both of which affected SC assembly and meiosis. CONCLUSION(S): Our study identified novel pathogenic variations of C14ORF39 and SYCE1 in sporadic patients with POI or NOA, highlighting the essential role of SC genes in the maintenance of ovarian and testicular function.