Nonallele-specific Silencing of Mutant and Wild-type Huntingtin Demonstrates Therapeutic Efficacy in Huntington's Disease Mice

Abstract

Huntington's disease (HD) is a fatal neurodegenerative disease caused by mutant huntingtin (htt) protein, and there are currently no effective treatments. Recently, we and others demonstrated that silencing mutant htt via RNA interference (RNAi) provides therapeutic benefit in HD mice. We have since found that silencing wild-type htt in adult mouse striatum is tolerated for at least 4 months. However, given the role of htt in various cellular processes, it remains unknown whether nonallele-specific silencing of both wild-type and mutant htt is a viable therapeutic strategy for HD. Here, we tested whether cosilencing wild-type and mutant htt provides therapeutic benefit and is tolerable in HD mice. After treatment, HD mice showed significant reductions in wild-type and mutant htt, and demonstrated improved motor coordination and survival. We performed transcriptional profiling to evaluate the effects of reducing wild-type htt in adult mouse striatum. We identified gene expression changes that are concordant with previously described roles for htt in various cellular processes. Also, several abnormally expressed transcripts associated with early-stage HD were differentially expressed in our studies, but intriguingly, those involved in neuronal function changed in opposing directions. Together, these encouraging and surprising findings support further testing of nonallele-specific RNAi therapeutics for HD.

Epigenetic Mechanisms of Neurodegeneration in Huntington's Disease

“…Harper et al [117] showed that RNA interference using adeno-associated virus-small hairpin RNA (shRNA) improves motor deficits and neuropathological phenotypes in a transgenic (N171-82Q) mouse model of HD. Most RNA interference studies using adenovirus-shRNA, lentivirus-shRNA, adeno-associated virus-miRNA, and cholesterol-conjugated siRNA have shown a reduction of mthtt aggregates, improvement of motor behavior, and reduced neuropathological sequelae (Table 1) [118][119][120][121][122][123][124][125].…”

Recent Advances in RNA Interference Therapeutics for CNS Diseases

“…HD is a gainof-function autosomal dominant disease with neuronal dysfunction occurring prior to cell death in medium spiny neurons within the striatum, as well as other brain regions. Patients exhibit involuntary hyperkinetic movements, coordination difficulties, and cognitive disturbances [50,80]. Both nonallele-specific (targeting the mutant and wild type alleles) and allele-specific (targeting only the mutant allele) approaches for HD therapy are under development.…”

“…For example, SCA1, SCA2, and SCA3 knockout mice are viable and fertile, indicating that knockdown of the wild type allele function may be tolerable [96,166,167]. Nonallele-specific silencing of HTT in HD mice resulted in a significant rescue of the HD phenotype, and 2 studies have shown that reducing levels of wild type HTT in the adult rhesus macaque striatum is safe and well tolerated for at least 6 months [80][81][82]. However, as the HD null mice are embryonic lethal, and the levels of HTT required for cell viability of adult neurons is unknown, researchers are also investigating allele-specific silencing options [83].…”

“…To detect siRNAs after delivery, Stiles et al [168] used radiolalebeled siRNAs, which allowed comparison of the volume of brain for which there was target suppression, as a function of dose and spread. For viral vectors, a common strategy to determine the distribution of transduced cells takes advantage of reporter genes, such as eGFP [80,82]. In the future, targeting the brain after systemic delivery may be possible.…”

Keystone Symposium, Neurodegenerative Diseases: New Molecular Mechanisms 17–22 February 2009

“…They also lived longer, said Davidson. Some of this work recently appeared in the February 24 Molecular Therapy online [23].…”