Using Divalent siRNA for Targeted Post-Transcriptional Silencing in Huntington’s Disease

Abstract

Huntington’s Disease (HD) is a progressive, autosomal dominant disorder, which affects psychiatric, motor, and cognitive function. The progression of Huntington’s Disease is divided into 3 stages: presymptomatic, prodromal, and manifest. The age of onset for the disease is 35-44 years. There is currently no treatment or cure for Huntington's disease that alters, regresses, or otherwise reverses the progression of the disease. HD carries an approximately 100% mortality rate upon diagnosis, with most deaths occurring in the middle ages between 50-60 years old. HD is caused by a CAG repeat expansion in the HTT gene. This repeat expansion leads to the production of mutant Huntingtin protein (mHTT). The mHTT can disrupt important processes involved in protein transport, neuronal signaling, and apoptosis, leading to cell death. HD affects neurons in the cortex as well as in the striatum, affecting mainly medium spiny neurons (MSN) in the striatum. The treatment for this experiment will be divalent small interfering RNA (di-siRNA). Divalent siRNA is designed to be effectively delivered to target cells to control gene expression. Once injected into the target cells, it degrades mRNA, effectively decreasing the amount of production of the mutant HTT protein. This experiment uses both wild-type (WT) mice and Q175 mice, of both sexes, to test the effect of divalent small interfering RNA (di-siRNA) therapy on lowering levels of mHTT expression. Both di-siRNA NTC (Non-targeting control) and di-siRNA HTT are used as treatments, which are administered via intracranial injections. The Q175 mouse model of HD mimics the prodromal stage of HD and allows us to study both motor performance and motivation in mice. The effectiveness of the therapy is being measured using the tapered beam test, 5mm beam test, and open field test, all of which quantify the motor coordination of each mouse. With the experiment ongoing, data from cohorts 1, 2, and 3 suggest nonresponsiveness to the treatment. Based on data collected so far, there may be an alternate mechanism by which HD occurs, but further research is needed to draw definitive conclusions.