M05 Prevention of Aggregate Formation by RNAI Knockdown of Huntingtin in the Q175 HD Mouse Model

Abstract

Background Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by a highly polymorphic CAG trinucleotide repeat expansion in the huntingtin (Htt) gene. Major hallmarks of HD are aggregation of mutant Htt and gradual loss of neurons, predominantly in the striatum. Knock-in mouse models recapitulating phenotypic features of HD have proven to be useful tools to study disease mechanisms and potential therapeutic approaches. Aims We aimed to monitor and quantify pathological processes in the Q175 HD mouse model. Furthermore, we determined striatal aggregate load in Q175 mice upon shRNA-mediated knockdown of several target proteins. Methods/techniques Here we apply an automated high content imaging platform to characterise the Q175 HD mouse strain. We established a panel of immunohistochemical, as well as multiplex RNA in situ hybridization readouts and analysed brain samples from mice of different age up to 12 months of age. In addition, we used intrastriatal stereotaxic AAV injections to induce RNAi knockdown of several target proteins. Results/outcome We found a progressive increase in mutant Htt aggregate load within the striatum starting at an age of 3–4 months. Despite the rise in aggregate load, we did not observe a concomitant degeneration of medium spiny neurons, nor signs of inflammation as assessed by number, morphology and size of astrocytes and microglia, even several months after the reported onset of behavioural symptoms. In case of Htt knockdown, we observed an efficient prevention of aggregate formation when applied at 2 months of age, prior to aggregate appearance in Q175 heterozygous mice. In contrast, Htt knockdown when Htt aggregates have manifested at 6 months of age had no impact on amount or size of aggregates suggesting an inefficient reduction of aggregate formation and inefficient clearance of existing aggregates. Conclusions Altogether these finding demonstrate the value of our imaging platform and the Q175 mouse model for the validation of HD aggregate lowering strategies in terms of timing and efficiency.