Purpose Recent studies have shown that genetic abnormalities may be responsible for most unknown cases of male infertility. Human Nsun7 gene, which is located on chromosome4, has a role in sperm motility by encoding the putative methyltransferase Nsun7 protein. The aim of the present study was to investigate the mutations of exon4 in the Nsun7 gene, which is associated with sperm motility defect. Methods Semen samples including those of fertile normospermic (normal), infertile oligospermic (with normal sperm motility), and infertile asthenospermic (with reduced sperm motility) men were collected from the Omid and Fatemezahra IVF centres (Babol, Iran). These samples were then analysed on the basis of World Health Organization guidelines using the general phenol–chloroform DNA extraction method. Exon4 was amplified using Sun-F/Sun-R primers. Samples from asthenospermic men, which showed different patterns of movement onsingle-strand conformation polymorphism compared with normal and oligospermic samples, were identified and subjected to sequencing for further identification of possible mutations. Results Analysis of extracted sperm proteins showed that the rate of Nsun7 decreased. Likewise, direct sequencing of PCR products, along with their analysis, confirmed the deletion mutation of adenine in location 11337 of the Nsun7 gene in asthenospermic men. Comparison of normal and mutant proteinstructuresofNsun7indicated that theA11337-deletionof theexon4resultedinthevalineresidues-157withGTA-codon in normospermic replaced with TAG-early stop codon in asthenospermic samples, causing an abortive protein product withaminoacidsequenceshorterthannormal.Thesecondary structure of the protein, the protein folding, and ligand binding sites were changed, indicating the impairment of the protein function. Conclusions Because the Nsun7 gene products have a role in sperm motility, it will lead to impairment in the activity of the protein and motility of sperm flagella as well as male
