Nitrites and nitrates are commonly measured in both plasma and serum using various analytical methods. However, the presence of interfering components in the past has made these investigations difficult [14]. To address this, our study utilized the chemiluminescence detection method, which has been shown to facilitate the detection of nitrites and nitrates while excluding many interfering factors [15]. However, this method requires the removal of plasma and serum proteins. Given that plasma protein content is 3–5 g/L higher than serum protein content (Blood Proteins - an overview | ScienceDirect Topics), our positive correlations between serum nitrites and nitrates with other parameters and lack of correlation for plasma measurements could be explained.
Our findings suggest that serum nitrate and nitrite measurements may be more informative, with correlations previously found between NO metabolites and clinical activity [17] and disease severity as measured by the Expanded Disability Status Scale (EDSS) in some studies [18] being supported by our results. We observed correlations between serum nitrates and gender as well as EDSS score of patients, with females being more susceptible to MS. However, the relatively small size of our study group, resulting in a low female to male ratio, limited our ability to observe any statistical significance.
The biochemical markers chosen for our study are influenced by various environmental and dietary factors that are difficult to account for. However, if any of the biochemical markers, such as nitrate levels, correlate with the EDSS score, it could provide a valid point to assume that the given biomarker might be associated with the course of the disease progression. In our case, serum nitrate levels had a reverse correlation with the EDSS score, suggesting that patients with more severe cases of MS are expected to have decreased nitrate levels. However, due to the limitations of our study, it is currently hard to tell whether this correlation is accidental or consequential.
It has been previously reported that MS patients receiving interferon therapy exhibit higher levels of NO metabolites [8], which could be the cause of the increase in NO production revealed in our study. However, our correlation results did not show any significant association between NO, nitrites, and nitrates with disease-modifying therapy (DMT).
Although we did not find any associations of MDA levels with DMT or patient descriptors, serum MDA levels were close to significance when correlated with serum nitrates. To date, no studies have proven an association between NOx and MDA in other cases [19], [20]. A study by Violi et al. [21], which is rather ancient, is by far the only theory explaining the reverse correlation between NO metabolite levels and MDA levels. In our case, we observed a medium reverse correlation between NO levels, serum nitrate and nitrite levels, and MDA levels. Based on evidence that NO can also act as an antioxidant in lipid peroxidation, one might speculate that the overproduction of NO, and thus a higher level of its metabolites, as shown in this study, could result in lower levels of MDA. However, both nitric oxide metabolite and MDA levels are highly dependent on dietary antioxidant intake and lifestyle in general. Thus, a larger study group would be required to minimize the deviation of each sample from one another. Further investigation is required.
The XRCC1 gene SNP rs25487 has been found to deviate significantly from the Hardy-Weinberg equilibrium (HWE). Prior studies have suggested that this SNP is linked to a reduced risk of minor treatment response in esophageal cancer and an increased risk of high-grade side effects in head and neck cancer [22]. Given the increased levels of DNA damage observed in leukocytes of patients with multiple sclerosis (MS), it is possible that this SNP is also associated with DNA lesions in MS. Correlation analysis has demonstrated a significant association between XRCC1 rs25487 and DNA damage in MS, indicating that this SNP may indeed be linked to DNA damage in MS. Interestingly, XRCC1 rs25487 also exhibited a close to significant correlation with nitric oxide (NO) levels, serum nitrites, and MDA levels in both plasma and serum. However, this correlation requires further investigation as no studies have confirmed this link.
It is currently unclear whether the mutation in XRCC1 gene causes fluctuations in NO levels or if NO is inducing genotoxicity and causing point mutations. A study by Kim, 2017 found that NO has mutagenic properties and is capable of producing point mutations in certain genes. One example involved the mutagenic potency of NO produced by interferon gamma (IFN-ɣ) activated macrophages, which may be relevant to the MS case. Therefore, it is possible that NO may be the reason behind the XRCC1 mutation.
As expected, XRCC1, being a DNA repair gene, shows its SNP rs25487 association with levels of DNA damage in lymphocytes. The rare genotype TT has been found to correlate with DNA lesions in MS patients, suggesting that the C:T point mutation in XRCC1 may be caused by the mutagenic properties of NO, disrupting the XRCC1 DNA repair pathway and resulting in more evident DNA damage. However, DNA lesions can be influenced by numerous environmental factors, so additional and more specific investigations with a large sample pool are required to confirm this hypothesis.