Background: The genetic mutation in Huntington's disease (HD) is a CAG repeat expansion in the coding region of the huntingtin (Htt) gene. RNAi strategies have proven effective in substantially down-regulating Htt mRNA in the striatum through delivery of siRNAs or viral vectors based on whole tissue assays, but the extent of htt mRNA lowering in individual neurons is unknown.
Objective: Here we characterize the effect of an AAV9-GFP-miRHtt vector on Htt mRNA levels in striatal neurons of Q140/Q140 knock-in mice.
Methods: HD mice received bilateral striatal injections of AAV9-GFP-miRHtt or AAV9-GFP at 6 or 12 weeks and striata were evaluated at 6 months of age for levels of Htt mRNA and protein and for mRNA signal within striatal neurons using RNAscope multiplex fluorescence in situ hybridization.
Results: Compared to controls, the striatum of 6-month old mice treated at 6 or 12 weeks of age with AAV9-GFP-miRHtt showed a reduction of 40-50% in Htt mRNA and lowering of 25-40% in protein levels. The number of Htt mRNA foci in medium spiny neurons (MSNs) of untreated Q140/Q140 mice varied widely per cell (0 to 34 per cell), with ∼10% of MSNs devoid of foci. AAV9-GFP-miRHtt treatment shifted the distribution toward lower numbers and the percentage of cells without foci increased to 14-20%. The average number of Htt mRNA foci per MSN was reduced by 43%.
Conclusions: The findings here show that intrastriatal infusion of an AAV9-GFP-miRHtt vector lowers mRNA expression of Htt in striatum by ∼50%, through a partial reduction in the number of copies of mutant Htt mRNAs per cell. These findings demonstrate at the neuronal level the variable levels of Htt mRNA expression in MSNs and the neuronal heterogeneity of RNAi dependent Htt mRNA knockdown.
Keywords: AAV; Htt; Huntington; RNAscope; huntingtin; in situ hybridization; mRNA; microRNA.