Taking a Break From Huntingtin
Authors: Beverly L Davidson
Journal: Molecular Therapy (2012)

Three groups have forged an important collaboration to test the utility of antisense oligonucleotides (ASOs) as a therapeutic modality for the treatment of Huntington's disease (HD), a fatal neurodegenerative disease manifesting in ~75,000 individuals in the United States and Europe alone.1Shoulson I Young AB Milestones in huntington disease.Mov Disord. 2011; 26: 1127-1133Crossref PubMed Scopus (56) Google Scholar HD is one of a group of dominantly inherited neurodegenerative diseases caused by polyglutamine repeat expansion. In the case of HD, the mutation induces a toxic gain of function in the huntingtin protein, perturbing numerous pathways at the cellular level and inducing motor and psychiatric symptoms. The groups, led by Don Cleveland of UC San Diego, Seng Cheng at Genzyme, and Frank Bennett at Isis Pharmaceuticals, screened a series of ASOs targeting mouse and/or human huntingtin and then rigorously evaluated them in three established animal models of HD and in naive nonhuman primates.2Kordasiewicz HB Stanek LM Wancewicz EV Mazur C McAlonis MM Pytel KA et al.Sustained therapeutic reversal of Huntington's disease by transient repression of huntingtin synthesis.Neuron. 2012; 74: 1031-1044Abstract Full Text Full Text PDF PubMed Scopus (530) Google Scholar The use of ASOs is a strategy to target huntingtin mRNAs for destruction, thereby reducing huntingtin protein levels. In mice, the ASOs were delivered via pumps into the ventricles of the brain, and the effects of the ASOs on behavior and huntingtin protein levels were monitored. The infused ASOs distributed widely in rodent brain, lowering huntingtin mRNA and protein. In an acute, short-lived HD model, ASO delivery extended life span by several days and increased brain mass. In two additional models expressing full-length mutant transgenic alleles, the ASOs improved motor function. One of the crucial and intriguing findings of the study was that the clinical benefit outlasted the duration of huntingtin protein suppression. The respite from the mutant protein, even in the setting of reduced levels of wild-type huntingtin, was remarkably beneficial. In the nonhuman primates, ASOs infused into the spinal canal resulted in notable bioactivity in specific cortical regions as well as cervical spinal cord, and, again, knockdown of huntingtin persisted well beyond the ASO delivery period. This work complements prior rodent studies demonstrating the efficacy of silencing huntingtin using RNA interference-based approaches3Harper SQ Staber PD He X Eliason SL Martins IH Mao Q et al.RNA interference improves motor and neuropathological abnormalities in a Huntington's disease mouse model.Proc Natl Acad Sci USA. 2005; 102: 5820-5825Crossref PubMed Scopus (573) Google Scholar,4Boudreau RL McBride JL Martins I Shen S Xing Y Carter BJ et al.Nonallele-specific silencing of mutant and wild-type huntingtin demonstrates therapeutic efficacy in Huntington's disease mice.Mol Ther. 2009; 17: 1053-1063Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar,5Machida Y Okada T Kurosawa M Oyama F Ozawa K Nukina N rAAV-mediated shRNA ameliorated neuropathology in Huntington disease model mouse.Biochem Biophys Res Commun. 2006; 343: 190-197Crossref PubMed Scopus (117) Google Scholar,6Drouet V Perrin V Hassig R Dufour N Auregan G Alves S et al.Sustained effects of nonallele-specific Huntingtin silencing.Ann Neurol. 2009; 65: 276-285Crossref PubMed Scopus (177) Google Scholar,7Rodriguez-Lebron E Denovan-Wright EM Nash K Lewin AS Mandel RJ Intrastriatal rAAV-mediated delivery of anti-huntingtin shRNAs induces partial reversal of disease progression in R6/1 Huntington's disease transgenic mice.Mol Ther. 2005; 12: 618-633Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar and the short-term safety of reducing huntingtin expression in the nonhuman primate brain.8McBride JL Pitzer MR Boudreau RL Dufour B Hobbs T Ojeda SR et al.Preclinical safety of RNAi-mediated HTT suppression in the rhesus macaque as a potential therapy for Huntington's disease.Mol Ther. 2011; 19: 2152-2162Abstract Full Text Full Text PDF PubMed Scopus (184) Google Scholar In those earlier works, sustained expression of inhibitory RNAs targeting mutant huntingtin alone3Harper SQ Staber PD He X Eliason SL Martins IH Mao Q et al.RNA interference improves motor and neuropathological abnormalities in a Huntington's disease mouse model.Proc Natl Acad Sci USA. 2005; 102: 5820-5825Crossref PubMed Scopus (573) Google Scholar or both mutant and the wild-type alleles4Boudreau RL McBride JL Martins I Shen S Xing Y Carter BJ et al.Nonallele-specific silencing of mutant and wild-type huntingtin demonstrates therapeutic efficacy in Huntington's disease mice.Mol Ther. 2009; 17: 1053-1063Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar,6Drouet V Perrin V Hassig R Dufour N Auregan G Alves S et al.Sustained effects of nonallele-specific Huntingtin silencing.Ann Neurol. 2009; 65: 276-285Crossref PubMed Scopus (177) Google Scholar from recombinant viral vectors provided therapeutic benefit throughout the striatum and significantly improved motor phenotypes. Cumulatively, the data show that reduction of both the mutant and wild-type alleles (in rodents) and normal huntingtin (in the adult nonhuman primate brain) is well tolerated. Another critical issue that had not previously been addressed was the reversibility of the disease process using these huntingtin-lowering approaches. Studies in transgenic models suggested that this was possible, however. Experiments in Renee Hen's lab by Ai Yamamoto9Yamamoto A Lucas JJ Hen R Reversal of neuropathology and motor dysfunction in a conditional model of Huntington's disease.Cell. 2000; 101: 57-66Abstract Full Text Full Text PDF PubMed Scopus (910) Google Scholar showed that suppression of mutant huntingtin after onset of histological and behavioral phenotypes improved these same disease manifestations. “Turning off” the disease allele even after there was obvious striatal loss was also beneficial.10Díaz-Hernández M Torres-Peraza J Salvatori-Abarca A Morán MA Gómez-Ramos P Alberch J et al.Full motor recovery despite striatal neuron loss and formation of irreversible amyloid-like inclusions in a conditional mouse model of Huntington's disease.J Neurosci. 2005; 25: 9773-9781Crossref PubMed Scopus (66) Google Scholar Here, the ASOs, which are distributed broadly in the rodent brain after delivery into the ventricles, also improved symptoms when delivered after disease onset. Importantly, they provided for benefit well after mRNA and protein levels returned to normal. Similar findings would presumably result from knocking down the mutant allele only, which may be accomplished by targeting the CAG-repeat (encoding polyQ) expansion.11Davidson BL Monteys AM Singles engage the RNA interference pathway.Cell. 2012; 150: 873-875Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar,12Yu D Pendergraff H Liu J Kordasiewicz HB Cleveland DW Swayze EE et al.Single-stranded RNAs use RNAi to potently and allele-selectively inhibit mutant Huntingtin expression.Cell. 2012; 150: 895-908Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar As this technology transitions into the clinic for HD therapy, the major hurdle remains delivery and distribution to a large primate brain. ASO distribution using immunohistochemical methods appears to be broad after a 21-day intrathecal infusion into the nonhuman primate cerebrospinal fluid. However, the brain areas showing the most robust knockdown were the spinal cord, followed by the posterior and anterior cortex. There was very modest (nonsignificant) knockdown in the caudate (~20%), a region dramatically affected in HD. It remains to be tested whether the ~20% reduction of huntingtin in the caudate will provide clinical benefit. The putamen also suffers from extensive cell loss in HD, but no data were presented as to whether the ASOs achieve sufficient knockdown in this region. Future studies in emerging models of HD in sheep13Jacobsen JC Bawden CS Rudiger SR McLaughlan CJ Reid SJ Waldvogel HJ et al.An ovine transgenic Huntington's disease model.Hum Mol Genet. 2010; 19: 1873-1882Crossref PubMed Scopus (137) Google Scholar or nonhuman primates14Yang SH Cheng PH Banta H Piotrowska-Nitsche K Yang JJ Cheng EC et al.Towards a transgenic model of Huntington's disease in a non-human primate.Nature. 2008; 453: 921-924Crossref PubMed Scopus (370) Google Scholar will help address the utility of this exciting approach to treating HD.