Septins are members of the GTPase superfamily, which has been implicated in diverse cellular functions including cytokinesis and morphogenesis. Septin 12 (SEPT12) is a testis-specific gene critical for the terminal differentiation of male germ cells. We report the identification of two missense SEPT12 mutations, c.266C>T/p.Thr89Met and c.589G>A/p.Asp197Asn, in infertile men. Both mutations are located inside the GTPase domain and may alter the protein structure as suggested by in silico modeling. The p.Thr89Met mutation significantly reduced guanosine-5'-triphosphate (GTP) hydrolytic activity, and the p.Asp197Asn mutation (SEPT12(D197N)) interfered with GTP binding. Both mutant SEPT12 proteins restricted the filament formation of the wild-type SEPT12 in a dose-dependent manner. The patient carrying SEPT12(D197N) presented with oligoasthenozoospermia, whereas the SEPT12(T89M) patient had asthenoteratozoospermia. The characteristic sperm pathology of the SEPT12(D197N) patient included defective annulus with bent tail and loss of SEPT12 from the annulus of abnormal sperm. Our finding suggests loss-of-function mutations in SEPT12 disrupted sperm structural integrity by perturbing septin filament formation.

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Single-strand conformation polymorphism analysis of exon-containing genomic DNA segments of the deleted-in-azoospermia-like (DAZL) gene was performed in 160 infertile Taiwanese men presenting with severe oligozoospermia and nonobstructive azoospermia. An A-->G transition at nucleotide 386 in exon 3 was identified. The mutation is located within the RNA-recognition motif (aa 32-117) domain of the DAZL protein and will lead to Thr54-->Ala change (T54A) of DAZL protein. Analysis of cDNA from testicular tissue of infertile carriers showed absence of expression for the T54A allele, implying that the allele carrying T54A polymorphism is hardly, if ever, expressed. The frequencies of T54A allele in patients and the control group were 7.39% and 0.86%, respectively (P = 0.0003). The phenotypes varied significantly in cases with heterozygous T54A polymorphism, ranging from hypospermatogenesis and maturation arrest to Sertoli cell-only syndrome. A combination of DAZ gene deletion and T54A polymorphism did not worsen the phenotype. Our findings provide strong evidence for the role of the autosomal DAZL gene in human spermatogenesis.

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