Evaluation of Potential Biomarkers and Risk Factors for Cognitive Impairment

Abstract

Cognitive impairment (CI) is a neuropathological condition that include deficit of visuospatial skills, thoughts, attention, learning, language and memory. CI has recently emerged as one of the most important health threat to old age individuals. The cognitive abilities are influenced by various factors, such as, genetics, environment, diet, age and life style. Human exposure to heavy metals and high fat diet (HFD) consumption are potential risk factors for developing CI. It is necessary to establish reliable serum based biomarkers and the genetic determinants, for CI and its progression, hence to comprehend the underlying mechanism of the disease. Therefore, this study aimed to investigate the possible association between the concentration of heavy metals and the extent of cognitive impairment. We also assessed serum based biomarkers including high density lipoprotein (HDL), low density lipoprotein (LDL), triglycerides, total cholesterol, total-tau and serum amyloid β-42 protein. The genetic determinants including APOE polymorphism and mutations of exon 16 and 17 of the amyloid precursor protein (APP) gene were studied. We examined 183 patients diagnosed with cognitive impairment; mild (n=72), moderate (n=86) and severe (n=25) based on their mini mental state examination (MMSE) score and compared them with age-matched healthy controls (n=90). All the subjects were interviewed on a specially designed performa to obtain demographic data, history and co-morbidities. The level of Copper(Cu), Lead(Pb), Aluminum(Al), Zinc(Zn), Cadmium(Cd) and Manganese(Mn) were measured in blood samples, after microwave assisted acid digestion, using atomic absorption spectrophotometry. Results showed that all the aforementioned elements were significantly higher in the cognitively impaired patients and increasing concentration was observed with the increase in severity of the XXIV disease. The correlation study has shown that among the studied metals, Al and Cu were strongly associated with the CI. The results of serum biomarkers have shown that severity of the disease increases with decrease in the concentration of HDL cholesterol and amyloid β-42. On the other hand, increase in LDL cholesterol, triglyceride, total cholesterol and total-tau were associated with the disease progression. Correlation studies revealed strong association between amyloid β-42, HDL cholesterol and total-tau with MMSE score. Receiver Operating Characteristic (ROC) showed the cutoff values of total-tau and amyloid β-42 with sensitivity and specificity; depicted that serum level of these proteins can be used as a predictive marker for CI. The genotyping results showed association of APOE4 allele with CI and a higher association was observed with severe CI group. However the sequence analysis of exon 16 and 17 of APP revealed no mutations. To further validate our results we examined the effect of metals and HFD in animal model to evaluate the neuropathological changes in young brain and compared it with untreated young mice (8-11 weeks = 2-3 months) and aged mice (12 months) to understand the underlying molecular mechanisms. Mice were given 300ppm of Al, Cu, Pb and Cd in drinking water and HFD feed (40% of the feed weight was animal fat)for 42 days. Metals+HFD treated mice were subjected to behavior tests, such as, Morris water maze, elevated plus maze, fear condition and contextual memory to evaluate memory levels. Spatial memory, contextual memory and fear memory were significantly impaired in metals+HFD group compared to young mice. The extent of neurodegeneration with metals+HFD co-exposure was considerably high in hippocampus and cortex, compared to aged mice brain and untreated young mice. Increased oxidative stress was recorded in the cortex, hippocampus and amygdala of metals+HFD group. The acetylcholine XXV concentration was decreased in cortex, hippocampus and amygdala of metals+HFD group, explaining the cholinergic deficits that caused cognitive impairment. Among the studied metals, Al was found to be highly accumulated in cortex, hippocampus and amygdala followed by Pb, Cu and Cd. Hippocampus showed greater accumulation of metals than cortex and amygdala. These data provided novel evidences that combined administration of metals and HFD enhanced aging process, caused memory impairment, cholinergic hypofunction, elevated oxidative stress and neurodegeneration in young mice. This study suggested the need for a decrease in metal exposure to humans from environment, food and industries. Also reported for the first time, is the association of total and fractional cholesterol, total-tau and amyloid β-42 as serum biomarkers andAPOE4 allele as a risk factor for cognitively impaired patients from Pakistan.