One of the risk factors for CI is CKD. The potential causes involve vascular and neurodegenerative mechanisms. The implicated factors range from traditional/non-traditional ones to direct neuronal toxicity due to uremic toxins. However, the combined effect of these factors on CI in CKD remains unknown. Phenotype presentation of a disease is the consequence of combined effect of various molecules and signaling pathways in the organism. Therefore, we applied ROC analyses to examine the combined effect of plausible clinical factors for CI prediction in patients receiving dialysis. Based on baseline comparison, we did not find significant difference in the proposed factors between CI and non-CI participants. We also selected four candidate miRNAs using next generation sequencing, miR-134, miR-182, miR-451, and miR-486 to compare between the CI and non-CI participants. The individual levels of these miRNAs did not show significant differences between the two cohorts. When we applied individual ROC analysis, Hb and age were the prominent factors, reaching AUC > 0.7. Further, we applied cumulative ROC analyses to examine the combined effect of the proposed factors. The combination of Hb, age, and miR-486 showed the best cumulative AUC for CI prediction in patients receiving dialysis. The other important combined factors included sex, homocysteine, and NSE (a nerve-injury protein). Thus, we propose that a combined effect of clinical factors contributing to mild CI in patients receiving dialysis.
It has been reported that Hb shows a U-shaped association with cross-sectional cognitive function[24, 25]. The plausible underlying mechanisms are inadequate cerebral oxygenation leading to impaired cerebral perfusion and cerebral function in low Hb concentrations. In contrast, high Hb concentrations may represent hyperviscosity, hypovolemia, polycythemia vera, and pulmonary disease. These scenarios may lead to cerebral hypoxia and cognitive impairment[24, 27]. However, there is still no evidence to show that optimal Hb concentrations prevent CI in CKD. In the present study, we found that Hb showed the highest AUC on individual ROC analysis. Moreover, a cumulated ROC analysis exhibited an acceptable AUC for CI prediction in patients receiving dialysis. Based on our findings, we propose that future studies on an optimal Hb concentration for stratified age groups of subjects are necessary for prevention or treatment of CI.
Aging is a natural process in organisms. Complex sophisticated coordinated mechanisms among tissues and organs are involved in the aging process. Commonly, aging is characterized by the progressive decline in functions of the tissues and organs. Eventually, aging contributes to the risk of disease occurrence. In the past decades, studies have investigated the molecular mechanisms of aging in different tissues[28-31]. Aging is well-recognized as the greatest risk factor for the onset of age-related neurodegenerative diseases. Age-related alterations in the brain include cell adhesion molecules, neuronal activity, and neurotransmitter and neuromodulator action. Our findings echo the aforementioned reports that aging contributes to CI in patients receiving dialysis. Nevertheless, this study is not sufficient to test our hypothesis for the plausible mechanism of age-related CI in patients receiving dialysis. An advanced study is warranted to explore the aging mechanism underlying the onset of CI in patients receiving dialysis.
Sex showed the fourth-highest AUC in our study. The cumulative AUC of Hb, age, sex, and miRNA-486 was 0.874. This finding indicates that sex plays an important role in CI of patients in Taiwan receiving dialysis. A nationwide survey in Taiwan showed that women had a higher prevalence than men for overall dementia and mild CI . The exact explanation was not provided by the authors. In our study, we did not include economic status, educational levels, and comorbidities for comparison between men and women. Therefore, we can only conclude that sex contributes to CI in patients receiving dialysis. Further studies are needed to address the plausible mechanisms underlying the above observations.
We analyzed the contribution of several uremic toxins to CI in our study. Among them, homocysteine showed the highest AUC on individual ROC analysis. Homocysteine is a protein-bound solute, and is commonly elevated in patients with CKD. On conversion to homocysteic acid, homocysteine can activate N-methyl-D-aspartate receptor, thus, leading to a direct neurotoxic effect. Homocysteine has been shown to be associated with faster rate of cognitive decline in a 6-year follow-up study on elderly subjects. Our study found homocysteine was one of cumulated top-ranked factors for prediction of mild CI in patients receiving dialysis. The cumulative AUC was 0.835 when 5 factors (Hb, age, miR-486, sex, and homocysteine) were combined. However, the role of homocysteine in causing CI in patients receiving dialysis must be validated by a large-scale population study.
Circulating miRNAs have been reported to be biomarkers of mild CI. Two sets of miRNA pairs, miR-132 and miR-134 families, were shown to differentiate patients with mild CI from age-matched controls. Next generation sequencing for miRNA expression profiling is becoming a common technology for various diseases, including those affecting the kidney and brain[36, 37]. We used this technology to profile miRNAs in patients receiving dialysis, and identified 4 candidate miRNAs: miR-134, miR-182, miR-451, and miR-486. In our study, the plasma levels of individual miRNAs were not statistically different between patients with and without CI. Using cumulative ROC analyses, we found that the combination of Hb, age, and miR-486 showed an acceptable cumulative AUC (0.897). It seems that miR-486 plays a crucial regulatory role in CI in patients receiving dialysis. One of the miR-486 target genes, GABRB3, encodes gamma-aminobutyric acid receptor subunit beta-3 (GABRB3), which is a member of the ligand-gated ion channel family. Gamma-aminobutyric acid is the major inhibitory neurotransmitter of the nervous system. The missense mutation of this gene may be associated with several nervous diseases, including epilepsy and autism. However, the regulatory roles of miRNAs are complex. Individual miRNAs can target several mRNAs, and an mRNA can be targeted by more than one miRNA. This might, at least in part, explain why we could not find a role of the CI-related miR-134 in the patients receiving dialysis. The identified miRNAs for mild CI in these patients were different from those previously reported in subjects without CKD. Currently, the evidence for differences in CI-related miRNA profiles between CKD and non-CKD cohorts is still lacking. Further studies on disease-specific miRNAs are required to clarify this issue.
A total of 152 uremic toxins have been detected, and these molecules have been shown to exert various negative effects, such as anorexia, cardiac failure, anemia, immune dysfunction, malnutrition, inflammation, and skin atrophy[23, 40]. Uremic toxins have also been suspected to have a causal relationship with CI in CKD. However, the impact and mechanism of action of each uremic toxin on cognition and cerebral nervous system in uremic state remains unknown. In our study, homocysteine and β2 microglobulin exhibited higher AUC than other uremic toxins. Guanidine compounds were found in the brain regions involved in cognition. These compounds reportedly indirectly elevate serum homocysteine in cognitive disorders. The definitive mechanisms of action of uremic toxins for the onset of CI in CKD require further investigation.
In our study, we measured blood levels of nerve-injury related proteins, such as NSE, HSP 70, and S100B; NSE was one of the top-ranked variables on cumulative ROC analyses. It is a glycolytic isoenzyme expressed in the central and peripheral neurons and neuroendocrine cells. In rats, NSE immunofluorescence signal decreased in the affected neurons 2–10 days following axonal injury. There is accumulating evidence that the level of NSE in neurons serves as a marker of axon injury, regeneration, and target reinnervation[44-46]. Accordingly, we propose that NSE might be involved in the onset of CI in patients receiving dialysis.
Limitations of the study
The present study is subject to several limitations. Firstly, our study did not include follow-up interval, and the cumulative ROC approach was limited to cross-sectional clinical data. This may ignore all possible combinations of complex interactions when the effect of time is considered. Secondly, our study did not include other possible factors affecting CI in patients receiving dialysis, such as economic and social status, comorbidities, and dialysis protocols. Thirdly, we did our best to examine the implicated uremic toxin levels in our patients. However, there are several uremic toxins that we did not examine. Therefore, their contributions to CI in these patients was not be investigated. Fourthly, the sample size was relative small, and all the participants were Taiwanese. The possible effect of ethnicity cannot be ruled out in our study. Finally, cognitive impairment was determined solely by a psychometric classification and not a clinical diagnosis. Although the CDR is considered to be a reliable indicator of cognitive status, the lack of a more thorough cognitive characterization of the sample should be pointed out. Despite the above limitations, the strength of our study is in being the first investigation on a combination of specific clinical factors predicting mild CI in dialysis patients. This approach not only provides an alternative insight into the plausible mechanisms for CI but also a method for CI prediction in such patients.