Sources to variability in circulating human miRNA signatures

There is an increasing number of studies proposing microRNAs (miRNAs) circulating in patient sera as biomarkers for a large number of human diseases including cancer, cardiovascular diseases, neurological pathologies and others. Numerous collections of biospecimens have been established throughout the world providing access to the precious samples required for biomarker development. To further explore the validation- and translational process of miRNA biomarkers it is indispensable to understand the variability of circulating miRNAs in undiseased individuals. We determined longitudinal and genome-wide miRNomes of 90 serum samples from the Janus Serum Bank in Norway, that have been stored from 23 up to 40 years at -25 degrees Celsius. Three serum samples each collected approximately 5 years apart from 30 individuals were profiled with microarrays, allowing insights into inter-individual variability, age dependent miRNA variability and the impact of storage length and pre-processing. A significant proportion of the miRNAs was affected by the age of the blood donor and a not negligible, albeit small, part of the miRNome by the storage time. A substantial part of miRNAs was differentially abundant between individuals, independent of the time when samples were collected. Stepwise filtering of the 529 miRNAs that were detected in serum samples highlighted 168 miRNAs dependent on the time point, further 56 significant between individuals and another 169 varying dependent on the sampling procedure. While the afore mentioned miRNA groups contain generally interesting and biologically important miRNAs the remaining set of 135 miRNAs with overall lowest variability between healthy individuals and across a long life span, and independent of sampling processes and storage length seems to constitute very promising biomarker candidates. Serum samples of 30 individuals collected over a time of approx. 10 years, 3 time points per individual