SDF is a condition that is linked to male infertility and can have negative effects on reproductive sequels. One major cause of SDF is oxidative stress, which is an increase in reactive oxygen species in the seminal fluid that can damage sperm DNA, and is considered a major contributing factor in male subfertility [16]. The current study aimed to investigate whether certain SNPs in the antioxidant system genes are associated with elevated SDF in a group of Palestinian men.
The study revealed significant differences in several seminal parameters. The liquefaction time of cases was found to be significantly longer. It was also found that the total motility and the normal form of sperm in the cases were significantly lower. Studies on relation between routine semen parameters and SDF reported that abnormal SDF is correlated with lower sperm motility, lower normal form, and longer liquefaction time, findings that are consistent with our results [17–19] and implicate that SDF correlates with crucial sperm parameters.
Genetic variation in genes involved in oxidative stress response can influence seminal fluid quality and fertility [20]. The current work found that the genotype frequencies of NOS3 (rs1799983 G > T) (GT), SOD2 (rs4880 A > G) (AA and AG), EPHX2 (rs1042064 T > C) (CC and CT), and AHR (rs2066853 G > A) (GA and GG) are significantly different between SDF-positive males and controls. Additionally, the allele frequencies of SOD2 (rs4880 A > G) (G-allele), and EPHX2 (rs1042064 T > C) (T-allele) disclosed significant differences between the two groups.
The nitric oxide synthase 3 (NOS3) gene expression is involved in normal spermatogenesis regulation in testis. In the investigated rs1799983 G/T SNP, glutamic acid is substituted with aspartic acid at codon 298. Evidence showed that this variation affects the enzyme function [21]. In support to our findings, earlier studies reported significant association between this SNP and sperm motility, negative semen parameters, and male infertility in men from diverse populations (e.g.,[22–24]).
SOD2 is a mitochondrial matrix enzyme that plays a pivotal role in the response to oxidative stress, and protection of sperm from oxidation [25]. Our SDF group differed significantly in terms of SOD2 (rs4880 A > G) genotypes (AA and AG) and the G-allele. The SOD2 G-allele (Ala) variant, as compared to the A-allele (Val) generates more H2O2 that might cause increased sperm DNA damage [26], and this may explain the association observed between this variant and elevated SDF.
Genotypic and allelic frequencies of EPHX2 rs1042064 T > C showed high significant difference between cases and controls. EPHX2 (epoxide hydrolase) is involved in the metabolism of various exogenous epoxides, such as heterocyclic amines, polyaromatic hydrocarbons and N-nitroso compounds [27]. The rs1042064 T > C SNP is located in the 3′ UTR of EPHX2 mRNA and was associated with decreased semen quality (oligozoospermia and asthenospermia) and male infertility [28]. This polymorphism seems to alter the expression level of EPHX2, by disrupting the binding of miRNA(s), and hence modifies the susceptibility of sperm to damaging epoxides.
Results of the current study also revealed a statistically significant association between aryl hydrocarbon receptor (AHR) rs2066853 SNP and SDF. The AHR polymorphic genotypes GG and GA were more common in the study cases. AHR mediates the toxicity of a variety of environmental chemicals, such as polycyclic aromatic hydrocarbons (PAHs) and dioxins. Genetic variants of AHR may result in significant differences in sensitivity to toxic effects of those contaminants and contribute to elevated SDF and male infertility [29, 30].
Regarding the rest of the investigated genes variants, no significant differences were observed between study cases and controls. In this context, discrepant results have been reported by authors from other populations. Inconsistency in results of candidate gene polymorphism-disease association studies is largely due to population homogeneity, rate of consanguinity, ethnicity and genetic background.
Seminal fluid and sperm cells are endowed with an array of antioxidant enzymes to neutralize the adverse impacts of endogenous and environmental oxidants and protect the sperm from DNA damage. Interindividual variation in terms of functional polymorphism in oxidative stress genes can play a pivotal role in elevating sperm DNA damage and idiopathic male infertility. Results of this work confirmed that common polymorphisms in NOS3, SOD2, EPHX2, and ARH genes are associated with elevated SDF. According to MDR analysis, the “NOS3, SOD2, EPHX2” SNPs combination has the highest impact on SDF.