Keratoconus (KC), as the most common corneal ectasia, is a significant clinical issue, affecting individuals worldwide. It leads to corneal protrusion and progressive thinning, resulting in irregular astigmatism, visual impairment, and ultimately becoming a leading cause of corneal transplantation (Andreanos et al., 2017; Arnal et al., 2011). This disorder usually manifests as a bilateral but asymmetric condition. It typically manifests during adolescence to early adulthood and tends to stabilize or slow down after the age of 40 (Gomes et al., 2015). The global prevalence of KC is 1.38 per 1,000, impacting over 50 million individuals across 15 different countries. Notably, in some regions, including the Middle East, KC is more common, affecting up to 5% of the population. In Iran, the KC prevalence is 40 in 1000 individuals (Gomes et al., 2022; Hashemi et al., 2020). The interplay between genetic susceptibility and environmental factors is noteworthy. Those who are genetically predisposed to the disorder might require environmental triggers for its onset. Elevated environmental stressors can induce oxidative damage to KC corneas, which are unable to effectively process reactive oxygen species (ROS). This leads to corneal thinning, biomechanical and geometrical corneal changes, and visual problems (Gordon-Shaag et al., 2015). Considering that the cornea absorbs around 80% of incident ultraviolet B radiation, the potential for ROS production is heightened (Cantemir et al., 2016).

As mentioned, ROS overproduction significantly contributes to the development and progression of KC (Liu and Yan, 2021). As ROS accumulates in the cells, it interacts with lipid membranes, proteins, nuclear and mitochondrial DNA, ultimately causing apoptosis or necrosis and cellular damage. Various factors contribute to ROS production, with the mitochondrial electron transport chain, xanthine oxidase, uncoupled nitric oxide (NO) synthase, and NADPH oxidase being the primary contributors (Fan Gaskin et al., 2021). However, the cornea possesses an enzymatic antioxidant system, including catalase (CAT), glutathione peroxidase (GPX), glutathione reductase, and superoxide dismutase (SOD), which could mitigate severe ROS damage (Arnal et al., 2011).

SOD, a cornerstone of the enzymatic antioxidant system, converts superoxide anion radicals (O2°) into hydrogen peroxide (H2O2) (Cantemir et al., 2016). Human SODs, metalloenzymes, comprise three isoenzymes found in the cytosol and mitochondrial intermembrane (SOD1), the mitochondrial matrix and inner membrane (SOD2), and the extracellular compartment (SOD3). SOD1, which are widely distributed in human cells, are responsible for the majority of SOD activity, and can play a critical role in corneal physiology by neutralizing O2°-. Interestingly, their distribution differs between normal and KC corneas (Lopes et al., 2020; Rosa et al., 2021; Yagi-Yaguchi et al., 2020). The human SOD1 gene, approximately 9307 base pairs in length, is located on chromosome 21q22.11 and contains five exons and four introns. Several SOD1 polymorphisms affect SOD1 gene expression and enzyme activity. One of the most studied is the SNP (single nucleotide polymorphism) rs2234694 (+35A/C) polymorphism, located at the junction site between the intron and exon 3 and it has been associated with the increase in SOD1 enzyme activity (Zelko et al., 2002). In contrast, another polymorphism (rs36232792), characterized by deletion of a 50 bp fragment, is located 1684 bp upstream of the ATG start codon in the promoter region of the SOD1 gene, and it is associated with the reduction promoter activity of the gene, that is associated with decreased mRNA levels, and subsequently, it may alter the level of ROS detoxification. Thus, 50 bp deletion in the SOD1 gene is associated with decreased promoter activity and lower mRNA levels which is caused by by the loss of two specificity protein 1 (Sp1) binding sites (Milani et al., 2012). Sp1 is a zinc finger family transcription factor that directly binds to GC-rich motifs in gene promoters and regulates gene expression (Safe et al., 2014). Due to the high interaction of ROS with DNA, the deletion genetic polymorphism may play an important role in inter-individual differences in maintaining the genome integrity (Tripathi et al., 2020).

Given the distinct corneal susceptibility to ROS and the critical role of SOD1 in oxidative damage and KC; therefore, this study aimed to investigate the association of the 50 bp deletion in the SOD1 gene and the enzymatic activity with KC in the Iranian population. Additionally, we compared plasma levels of malondialdehyde (MDA), a lipid peroxidation marker, and the ferric-reducing ability of plasma (FRAP), indicative of total antioxidant capacity (TAC), between patient and control groups. This study demonstrated the intricate interplay between genetics, oxidative stress (OS), and KC pathology.