Expression levels of blood microRNAs as biomarker of cognitive decline due to Alzheimer's disease

Studies investigating differential miRNAs expression levels in patients with Alzheimer’s disease (AD) abounded in the last decades and catalysed the interest towards miRNAs as novel non-invasive biomarkers of AD. Chapter 1 provides an overview of AD’s pathogenesis, discusses the evolution of the disease’s definition, and introduces miRNAs. In Chapter 2, a systematic review and a P-value based meta-analysis of 107 studies investigate miRNA expression levels in AD patients. This leads to a prioritisation of 25, 32 and 5 dysregulated miRNAs at study-wide significance in the brain, the blood and the cerebrospinal fluid (CSF) of AD patients, respectively. A pathway enrichment analysis for the top dysregulated miRNAs in the brain confirms their role in regulating biological functions implicated in AD. In Chapter 3, expression levels of the 32 dysregulated miRNAs in the blood and 6 top dysregulated miRNAs in the brain of AD patients, are assessed using real-time quantitative polymerase chain reaction in the blood of cognitively healthy individuals from the CHARIOT-PRO cohort. Low performers on the total Repeatable Battery for the Assessment of Neuropsychological Status scale show downregulation of six miRNAs (hsa-miR-128-3p, hsa-miR-144-5p, hsa-miR-146a-5p, hsa-miR-26a-5p, hsa-miR-29c-3p and hsa-miR-363-3p). Pathway enrichment analysis highlights involvement in pathways initiating early pathogenetic changes in AD. Finally, in chapter 4, whole-genome sequencing data from the Alzheimer’s Disease Neuroimaging Initiative is used to perform an association analysis between polymorphisms within the six miRNAs’ genes and CSF biomarkers of neurodegeneration. A functional annotation of significant variants highlights expression quantitative trait loci, location in enhancer regions and alterations in the binding sites of transcription factors regulating neuronal function. The association of variants located within the same miRNA gene with different markers of neurodegeneration reveals a positive correlation between members of the amyloid cascade and microglial activation in the CSF. The final chapter highlights the clinical relevance of these findings and discusses future perspectives.Open Acces