Chromosome 14 MicroRNA Cluster Members in Brain-Derived Extracellular Vesicles Can Be Genotype-Associated Biomarker Candidates for Alzheimer’s Disease and Frontotemporal Dementia

Background: The chromosome 14 microRNA cluster (C14MC) is the second largest microRNA (miRNA) cluster in the human genome, which is tightly regulated during early development and dysregulated in many disease conditions. Members of this cluster are reported to have important temporal and spatial roles in early development, especially in neurogenesis. Although extensively studied as cancer biomarkers, their potential as molecular signatures of neurodegenerative diseases (NDs) remains poorly understood. Given the importance of non-invasive or less-invasive biomarkers of dementia, brain-derived small extracellular vesicles (BDsEVs) represent a promising source of disease-associated miRNAs. BDsEVs can traverse the blood–brain barrier and potentially become a surrogate for molecular changes within the brain in accessible body fluids. We investigated whether BDsEV-associated C14MC miRNAs exhibit shared or disease-specific expression patterns in Alzheimer’s disease (AD) and genetically defined subtypes of frontotemporal dementia (FTD). Methods: BDsEVs were isolated from post-mortem frontal cortex tissue of neurologically healthy controls, AD, and FTD cases carrying pathogenic GRN, MAPT, or C9orf72 mutations. Seven candidate C14MC miRNAs were quantified by RT-qPCR, while small RNA-sequencing data were analysed to characterise C14MC expression patterns. Results: Among the RT-qPCR candidates, miR-323a-5p and miR-758-5p showed significant differences between diagnostic groups. MiR-323a-5p demonstrated disease-associated upregulation in FTD-MAPT compared with all other groups (fold change = 6.68, adjusted p < 0.001) in the relative-expression analysis, with ΔCt analysis supporting all comparisons except AD. Exploratory small RNA-sequencing identified nominal expression differences in FTD-MAPT (miR-1197, miR-381-3p, miR-299-3p) and FTD-C9orf72 (miR-485-3p, miR-381-3p) compared to controls, alongside nominal differences observed in AD against FTD mutation groups (miR-1247-5p), with recurrent alterations across multiple comparisons. Further nominal differences were observed between underlying proteinopathies. Conclusions: Collectively, these findings identify disease- and genotype-associated patterns of C14MC miRNAs within BDsEVs, although the sequencing findings remain exploratory and require validation in larger independent cohorts. Thus, our results highlight the potential of individual C14MC members as BDsEV-derived molecular signatures and candidate biomarkers for molecular stratification of dementia.