Alzheimer’s disease (AD) is a neurodegenerative brain disorder in which the disease process may take decades until the symptoms become evident. To date, no ideal biomarker has emerged that would enable early detection of AD. Environmental and lifestyle factors are thought to affect the risk of developing AD, possibly through epigenetic mechanisms such as DNA methylation (DNAm). DNAm has been shown to differ in the blood and brain of subjects with AD compared with subjects without AD, suggesting that DNAm may be involved in the pathogenic process of AD. This study aims to detect the difference in blood DNAm at baseline between cases who later developed AD and controls who remained AD diagnosis-free during follow-up in a sample selected from a Finnish population-based cohort. Leucocyte genome-wide DNAm was profiled on approximately 850,000 CpG sites by using Infinium MethylationEPIC assay. Each CpG was regressed on the outcome of AD diagnosis during follow-up, controlling for subjects’ age at sampling, sex, smoking status, blood cell counts, working stress level, slide, and array. Specific differentially methylated positions (DMPs) were further explored using pathway analysis. Finally, the methylation level of the candidate gene (APOE) selected from the literature was compared with the sample of this study. After correction for multiple testing, the later diagnosis of AD was not significantly (adjusted p-value < 0.05) associated with methylation level at the baseline at any DNAm site. There was, however, a robust hypomethylation of DMPs among the cases, as 90.9% of the DMPs (p-value < 0.05) were hypomethylated in the case group. The 200 genes annotated by DMPs with the smallest p-values were involved in two neuronal pathways: “Axon guidance associated with semaphorins Homo sapiens” (p-value = 0.0058, adjusted p-value = 0.065) in Panther 2016 and “Semaphorin interactions Homo sapiens” (p-value = 0.00005, adjusted p-value = 0.078) in Reactome 2016. No significant difference existed in DNAm of the candidate gene (APOE) between cases and controls, while cg26190885 at the promoter region of APOE showed nominal significance (p-value = 0.04). In conclusion, no strong evidence was found to support the hypothesis that systemic changes in DNAm are involved in the pathogenesis of AD or that DNAm marks could be detected in blood before the symptoms become evident. A genome-wide pattern of hypomethylation measured by the Infinium MethylationEPIC assay was observed in the case group, serving as a venue for further investigations.

Sleep difficulties have been on the rise for the past decade. Insomnia and sleep difficulties have associations with an increased risk of overall mortality, as well as with a diverse array of complex diseases, such as coronary heart disease, major depressive disorder, fibromyalgia and Alzheimer’s disease. Epigenomics provides information on how environmental factors influence the genome via epigenetic mechanisms, such as DNA methylation. Thus far, epigenome-wide association studies looking at the effects of sleep disturbances on the methylome have provided evidence of distinctive methylation patterns in insufficient sleep, involving biological processes related to neuroplasticity and neurodegeneration. However, more knowledge is needed to determine how the severity of sleeping difficulties influence the methylome. This thesis investigates the effects of increasing sleep difficulties on DNA methylation with an epigenome-wide association study. The study sample is derived from the Health 2000 general population survey. Subjects were divided into three different groups by their self-reported level of sleeping difficulty, and methylation measurements performed from whole blood samples utilizing the Illumina Infinium MethylationEPIC kit, encompassing >850,000 CpG sites. To identify differentially methylated sites, a multivariable regression model was used with age, gender, smoking, alcohol use, cell type distribution and plate and array data as covariates. None of the differentially methylated CpG sites identified remained significant after multiple testing correction. To gain more information regarding which biological processes the methylated sites may be part of, those CpG sites with an uncorrected p-value of <0.0005 were subjected to pathway analysis. Notable significant pathways included oxytocin- and serotonin receptor-mediated signalling pathways and Alzheimer’s disease-amyloid secretase pathway. Altogether, six pathways remained significant after multiple testing correction, with a total of 12 different genes appearing in them. Furthermore, a post-hoc regression analysis was conducted between these 12 genes and their corresponding CpG sites, and health-related quality of life questionnaire responses. Significant results included associations between sleep, and discomfort and symptoms (including pain). As an additional analysis, a database search was conducted to learn more about the genes’ functionality at the level of phenotype. Results included some variant trait associations to sleep, Alzheimer’s disease and cognitive performance. The associations to Alzheimer’s disease and cognitive performance warrant further research with a similar additive model, perhaps with a larger sample.