Progress and Challenges in RNA Interference Therapy for Huntington Disease

Aberrantly splicedHTT,a new player in Huntington’s disease pathogenesis

“…One major approach currently under development in HD therapeutics is targeting HTT RNA for degradation using either antisense oligonucleotides or RNAi technologies. [40][41][42] Therapies targeting HTT RNA downstream of exon 1 will only reduce levels of a full-length HTT transcript leaving the exon 1 misspliced transcript untouched. We suggest an optimal strategy for HD therapeutics will target the RNA at the 5′ UTR or in exon 1 in order to reduce levels of both the full-length and the short mis-spliced exon 1 HTT transcript.…”

Using non-coding small RNAs to develop therapies for Huntington's disease

“…In a series of RNAi applications in different HD transgenic rodent models, it was proven that silencing the mutant HD transgene can significantly inhibit neurodegeneration, improving motor control, and in some cases, it extended the lifespan of HD mice ( Figure 1). [2][3][4][5][6][7][8][9][10][11][12][13][14][15] More recently, strategies have been devised to specifically silence the mutant allele, whereas preserving expression of its wild-type counterpart. Because of the lack of a transgenic animal model expressing both alleles of human wild-type and mutant HD gene, allele-specific silencing is conducted in cells from HD patients currently.…”

“…As neurodegeneration proceeds, particularly quickly in transgenic mice expressing the 5¢-fragment of human HD protein (e.g., HD-N171-82Q, CAG140, R6/2, R6/1 and HD190qG), these animals were the first to be used for the proof-ofprinciple experiments with RNAi-based therapies ( Figure 1). [2][3][4] Most of these studies used RNAi targeted at a human HD transgene. If these RNAi eventually go to clinical trials, this interfering small RNA molecule will silence both the mutant and the wild-type huntingtin alleles.…”

“…In some studies, RNAi was designed to target both the human transgene and the endogenous mouse HD gene to evaluate whether the body can tolerate losing a certain percentage of endogenous wild-type huntingtin at the same time mutant HD is silenced. [2][3][4] To determine the potential clinical efficacy of RNAi at silencing mutant HD and reducing its toxicity, mice were made to take up short hairpin RNA (shRNA) or synthetic siRNA. There are two strategies for introducing RNAi in vivo (Figure 2).…”

“…One is to use a viral vector-mediated shRNA and miRNA (or shRNA imbedded in an miRNA scaffold) constructed from lentivirus, adenovirus, or adenoassociated virus adeno-associated virus (AAV)1, AAV2/1, or rAAV5 ( Figure 2). [2][3][4][6][7][8][10][11][12][13][14] The second is to use synthetic siRNA introduced with Lipofectamine 2000 (Invitrogen, Carlsad, CA, USA) or cholesterol ( Figure 2). 5,9 Synthetic double-stranded RNA and virus-borne shRNA were designed to silence the mutant human HD allele present in transgenic mice.…”