Genetic variants of eNOS gene as a risk factor to idiopathic male infertility

Abstract

About 15% of the causes of male infertility are genetic and include chromosomal aberrations and single gene mutations. However, in about 50% the cause remains unknown (idiopathic). Oxidative stress is a major issue associated with decreased motility of sperm, male infertility and assisted reproduction outcomes, and in recent years, considerable attention has been focused on the deleterious effects of reactive oxygen species (ROS) on sperm function. An excessive level of ROS is harmful to sperm and leads to DNA and plasma membrane damage through lipid peroxidation. Once the sperm discard most of their cytoplasm during the final stage of spermatogenesis, the availability of cytoplasmic enzymes defense is limited and therefore these cells are especially susceptible to ROS. Increased lipid peroxidation of sperm plasma membrane can lead to alteration of membrane fluidity, which can make the dysfunctional sperm through impaired metabolism, acrosome reactivity and capacity of sperm to fuse with the oocyte. This can result in abnormal sperm concentration, motility loss and abnormal morphology, leading to loss of fertility.Nitric oxide (NO) is a messenger in a wide range of biological processes and modulates sexual and reproductive functions in mammals as well as being a powerful antioxidant in seminal plasma. There are three distinct isoforms of NO synthase (NOS) called induced (iNOS), endothelial (eNOS) and neuronal (nNOS). The endothelial isoform of NOS (eNOS) is encoded by the gene NOS3 and is the main source of nitric oxide.Three types of polymorphisms of the NOS3 gene have been described. The G894T polymorphism (rs1799983, Glu298Asp) is an exchange of glutamic acid for aspartic acid at codon 298 of exon 7 and there is a tendency for decreased activity of the enzyme in the presence of eNOS 894T allele compared with GG homozygotes. Another polymorphism represents a point mutation of thymine (T) to cytosine (C) at nucleotide -786 in the 5 'region of the gene (786C-T, rs2070744). The-786C allele is associated with reduced promoter activity, suppression of transcription of eNOS and decrease of nitric oxide. There is also a polymorphism variation in the number of tandem repeats (VNTR) in intron 4 gene (polymorphism 4A/4B). This polymorphism is responsible for more than 25% of basal plasma generation of nitric oxide, suggesting that this gene may play an important role in their physiology.Previous studies evaluated the possible association between polymorphisms of endothelial nitric oxide synthase and idiopathic infertility, with conflicting results. Thus, we hypothesized that eNOS gene polymorphisms may be associated with idiopathic male infertility. There are only three reports linking polymorphisms of these genes in male infertility and no studies in the Brazilian population. Therefore, the objective of this study is to investigate the frequency of polymorphisms (T-786C, G894T, and 4a/b) in men with idiopathic infertility and control subjects, correlating the data with the motility of men with severe idiopathic oligozoospermia, and perform haplotype analysis, correlating the findings set of polymorphisms with male infertility. (AU)