In this cross-sectional study, we reported the overall prevalence of MCI among frail patients with CVD was 22.6%, which was similar to previous findings. We confirmed that urinary 8-oxoGsn/Cre ratios were significantly higher in MCI patients compared to NO-MCI patients (p < 0.001). Urinary 8-oxoGsn/Cre could independently and effectively evaluate MCI in frail patients with CVD.
The use of feasible biomarkers to identify patients with MCI is a challenge that needs to be addressed: These indicators would provide a more accurate detection of AD in early disease stages, when cognitive decline can still be potentially reverted. For the development of MCI related biomarkers, physiological and metabolic processes, changes in MCI status must be explored. Individuals with MCI have shown alterations in the antioxidant system, which is designed to counteract the potentially hazardous reactions initiated by oxidative stress. Nucleic acids are constantly oxidized within the cell, DNA is double stranded and contains protective proteins. Pena-Bautista et al. showed the DNA oxidation marker 8-hydroxy-2’-deoxyguanosine was able to distinguish between AD and healthy participants. However, RNA is more vulnerable to oxidative stress than DNA because it is single-stranded and lacks protective histones. RNA damage is a valid marker that may be suitable to provide useful information for early identification of MCI. The oxidatively generated modifications of RNA can be measured by urinary 8-oxoGsn levels, which is the focus of the present study.
Our study found that urinary 8-oxoGsn corrected by creatinine was independently associated with MCI. Previous studies on oxidative stress and cognitive impairment have mainly focused on brain tissue and cerebrospinal fluid rather than on urine samples. Urine is a valuable clinical specimen for early noninvasive diagnosis of diseases. However, these studies have mainly focused on the relationship between oxidative stress and dementia or AD, which are already irreversible. Nunomura et al. suggested that RNA oxidation is a prominent feature of neuronal vulnerability in patients with AD and dementia. Our previous research found that the presence of large amounts of 8-oxoGsn in the RNA could promote the secretion of pathogenic amyloid-β peptides in vivo, and the mechanism would contribute to the accumulation of amyloid-β plaques, as observed in the brains of AD patients. Consistent with previous data, Lovell et al. demonstrated increased RNA oxidative modifications in neurons undergoing early neurofibrillary tangles formation in the hippocampus/parahippocampal gyrus of MCI and late-stage AD subjects. Levels of RNA oxidative damage in MCI were comparable to those observed in late-stage AD, suggesting that oxidative damage is an early event in the pathogenesis of AD. These results indicated that 8-oxoGsn is correlated with certain late-stage chronic nervous system diseases that have a high incidence among elderly patients. MCI is an early stage of AD and can be well screened by the MMSE scale in elderly patients. A possible conclusion is therefore that 8-oxoGsn levels may be positively correlated with MCI, which also has a high incidence in the elderly patients, especially among those with CVD.
MCI reflects the transition between normal aging and dementia, and is the earliest clinical manifestation of AD. It is important to determine if oxidative stress is present in individuals early in chronic disease progression. Perez et al. found that titanium dioxide nanoparticles induced strong oxidative stress in astrocytes, cells that play key roles in neuronal homeostasis and their dysfunction can lead to MCI. Keller et al. showed significantly increased protein carbonyl formation and increased levels of lipid peroxidation in the temporal lobe of MCI subjects compared to healthy subjects. Ding et al. showed significantly elevated 8-hydroxyguanine immunoreactivity in the inferior parietal lobule of subjects early in disease progression. Our study suggests the increase of the urinary 8-oxoGsn/Cre ratio may be a useful indicator for the early screening of MCI.
The usual risk factors associated with conversion of individuals from cognitively normal status into dementia and AD are also possible risk factors for transitions into MCI. The findings that age and education levels were independently associated with MCI are consistent with the existing literature[33, 34]. It is estimated that between 10–30% of all adults aged 65 and above experience MCI. In a longitudinal study at the University of Kentucky AD Center, it was shown that age affected the ORs of individuals transitioning to MCI as well as that of dementia or death. In an analysis of six international longitudinal studies, a higher level of education was associated with a lower risk of transitioning from NO-MCI to MCI. Moreover, those with higher levels of education and socioeconomic status had longer non-impaired life expectancies.
Cardiovascular risk factors are recognized as predictors of age-related cognitive decline and dementia. MCI is an important under-researched complication of stroke and transient ischemic attack. However, it is important to note that in frail patients with CVD, no clear correlation between CVD and MCI has been identified, including previous stroke history. These findings were not consistent with our hypothesis. One possible reason for this inconsistency may be that we excluded patients with AD from our study, and only patients with mild cognitive decline were enrolled, and thus there was an insufficient number of cases with CVDs to detect early changes in cognitive function. Second, previous studies have found that some CVDs and risk factors, such as heart failure, coronary artery disease, stroke, atrial fibrillation, and obesity, are significantly associated with frailty, thus the relationship between CVDs and MCI may be weakened in frail patients. Further research on CVDs and MCI in frail patients should be performed in the future.
The strengths of the current study include the analysis of oxidized nucleosides using UPLC-MS/MS, which is considered the reference standard method due to high specificity towards the RNA forms. Furthermore, we adjusted the association between 8-oxoGsn levels and frailty by including all frail patients. Moreover, we tried to extend the relationship between 8-oxoGsn and MCI to general CVDs rather than to a specific form of CVD.
Our study has several limitations. First, the small sample size hampers the generalizability of our findings. However, the individuals included in our study were all frail patients with CVD, which may more accurately reflect the value of 8-oxoGsn in this subset of the population. Second, our cross-sectional study does not allow to draw any causative conclusions, nor can it identify the risk factors of MCI. A long-term follow-up study in this population is currently being conducted by our group, which will allow us to determine whether these patients develop dementia or AD in the future. Lastly, MMSE is a screening tool to identify MCI but it is not a diagnostic tool; thus, we failed to conduct a detailed subgroup analysis of patients with different cognitive levels. Nonetheless, the MMSE is the most widely used tool for evaluating MCI, and is supported by a high degree of popularization and application.