Ana Gámez-Valero, PhD

, Eulàlia Martí, PhD

Plasma extracellular small RNAs as early biomarkers of Huntington’s disease and indicators ofdynamic changes in disease progression

In Huntington’s disease (HD) the expanded CAG repeat in the Huntingtin (HTT ) gene is pathogenic through mechanisms involving both, the mutant protein, and the CAG repeat RNA derived from the transcribed gene. In addition, small RNAs (sRNAs) produced in HD have recently been uncovered as toxic drivers. Many of these species, including micro RNAs and tRNA- derived fragments, are bioactive compounds essential for the normal brain function which are strongly and early perturbed in HD.

In different neurodegenerative diseases, including HD, the deregulation of sRNAs expression have been shown to occur early before clinical symptoms start. These changes can be peripherally detected in plasma, where sRNAs circulate either free, bound to proteins/lipids, and/or encapsulated within extracellular vesicles (EVs). EVs are released by all cells and its content, including sRNAs, may reflect the physiological and pathological state of the cells of origin. Therefore, the analysis of EVs- sRNA content offers an exciting source of biomarkers to detect the earliest alterations. Moreover, different studies have reported that the vast majority of sRNAs are found outside EVs in the extravesicular milieu, but their biomarker potential has been little explored.

We propose that circulating sRNAs in vesicular and extravesicular plasma compartments offer complementary layers of information that will help in defining premanifest and prodromal changes in HD. Here, we will deeply characterize sRNAs found in EVs and extravesicular plasma sub- fractions, both at premanifest and manifest stages, compared to healthy individuals, and in a longitudinal follow- up, using exhaustively characterized cohorts. This study should contribute to overcome the challenge for physician and drug developers to predict the course of HD. Our results will be important for a better patients’ management and clinical care, and may facilitate patients’ stratification during preclinical trials, being an important step for the future advancement of therapeutic development and for identifying optimal treatment windows.