RNA Interference Therapy for Machado-Joseph Disease: Long-Term Safety Profile of Lentiviral Vectors Encoding Short Hairpin RNAs Targeting Mutant Ataxin-3

Machado-Joseph disease (MJD) or spinocerebellar ataxia type 3 is a neurodegenerative disorder caused by an abnormal repetition of a CAG codon in the MJD1 gene. This expansion translates into a long polyglutamine tract, leading to the misfolding of the mutant protein ataxin-3, which abnormally accumulates in the nucleus, thus leading to neurodegeneration in specific brain regions. No treatment able to modify the progression of the disease is available. However, it has previously been shown that specific silencing of mutant ataxin-3 by RNA interference with viral vectors is a promising therapeutic strategy for MJD. Nevertheless, reports of cytotoxic effects of this technology led to the safety profile of the previously tested lentiviral vectors encoding short hairpin (sh)RNAs (LV-shmutatx3) targeting mutant ataxin-3 upon brain injection being investigated. For this purpose, the vectors were injected in the mouse striata, and neuronal dysfunction, degeneration, gliosis, off-target effects, and saturation of the RNA interference machinery were evaluated. It was found that: (1) LV-shmutatx3 mediated stable and long-term expression of the shRNA in neurons of the mouse striatum; (2) neuronal dysfunction evaluated by darpp-32, NeuN, and cresyl violet staining, initially more pronounced, became indistinguishable from the phosphate-buffered saline group at 8 weeks and resolved within 20 weeks; (3) astrocytic activation was present, which resolved within 8 weeks; (4) microglial activity and proinflammatory cytokines release were present, which resolved and normalized within 20 weeks; and (5) there were no off-target effects or saturation of the endogenous RNA interference processing machinery in the mouse striatum. The data show that injection of lentiviral vectors encoding a shRNA targeting mutant ataxin-3 in the mouse brain induce transient dysfunctions, which resolve within 20 weeks. Importantly, long-term expression (up to 20 weeks post injection) of this shRNA (driven by H1 promoter) led to no toxic effect in vivo. This study thus constitutes an additional step in a future translation of gene silencing as a therapy for MJD.