**5.2 miRNA biomarkers in Alzheimer's disease**

In 2014, two independent studies reported a downregulation of miR-125b in the serum of AD patients and Tan et al. correlated this with cognitive decline [142, 143]. Interestingly, a number of studies have also identified multiple miRNA panels that demonstrate diagnostic value. In a small cohort of patients a group of 7 miRNAs were shown to be differentially expressed in the plasma of AD patients [144]. In a larger study, next-generation sequencing identified 140 differentially expressed miRNAs. Subsequent validation studies using RT-PCR in a cohort of 202 patient samples demonstrated a 12-miRNA signature to differentiate between AD and control samples to a high degree of sensitivity [145]. Finally, next-generation sequencing of exosomes extracted from the blood revealed a 16-miRNA signature differentially expressed in AD patients. Validated by RT-PCR and combined with known risk factors such as age, sex and apolipoprotein ε4 allele status provided prognosis with high sensitivity [146].
