ReviewTherapies targeting DNA and RNA in Huntington's disease
Introduction
All cases of Huntington's disease are caused by the same mutation, so the pathogenic processes close to this genetic cause are ideal candidates for the development of therapies. The Huntington's disease mutation—expansion of a CAG tract in the HTT gene—results in ubiquitous expression of mutant huntingtin (HTT) protein, which is thought to be the predominant toxic agent.1 Individuals who will develop Huntington's disease can be identified by genetic testing: so, in theory, intervention in the long premanifest phase could postpone or prevent symptom onset.2 Huntington's disease is an inherited autosomal dominant disorder thought to be caused by toxic gains of function;3, 4 thus, a reduction in mutant HTT production should alleviate its pathogenesis. However, evidence also suggests mutant HTT may have diminished function (ie, might cause haploinsufficiency), which might contribute to Huntington's disease,5 and could be exacerbated by a reduction in HTT production, so human trials should be designed to consider this possibility.
It is a pivotal time for therapies targeting disease-associated genes and proteins. The first clinical success for such a therapy in neurodegeneration was reported in 2016, when the US Food and Drug Administration (FDA) approved nusinersen, an antisense oligonucleotide (ASO) administered into the lumbar spinal cord, which extended survival in spinal muscular atrophy via targeted modulation of gene expression, increasing production of survival motor neuron 2 (SMN2) protein.6, 7 This FDA-approved approach restored a missing protein, rather than suppressing the function of a toxic protein. Additionally, because spinal muscular atrophy is typically more aggressive, it might be easier to show a therapeutic effect in a short efficacy trial for this disease than for Huntington's disease. Although one instance of clinical success cannot be generalised, the results of the spinal muscular atrophy trial provide evidence that neurodegeneration can be slowed in human beings by a targeted therapy modulating gene expression, which is encouraging for similar programmes in Huntington's disease.
This Review focuses on novel therapeutic strategies targeting the mutant HTT production pathway. Strategies designed to interact with HTT mRNA include ASOs and RNA interference (RNAi) compounds that accelerate degradation of the transcript, and orally bioavailable small molecules that reduce expression of HTT by altering mRNA splicing. Agents that directly interact with HTT DNA include zinc finger transcriptional repressors and clustered regularly interspaced short palindromic repeats (CRISPR) and the accompanying CRISPR-associated system (Cas) genome editing constructs (ie, CRISPR-Cas9). The term gene silencing is sometimes used for targeted reduction in gene expression, but could be interpreted as complete deactivation, which is probably neither desirable nor attainable; therefore, we prefer the term HTT lowering when referring to reduction of HTT expression. The term gene therapy is reserved for approaches in which the genetic material of living cells is modified,8 resulting in host cells manufacturing non-native mRNA and sometimes protein; these approaches have safety and regulatory implications.
Section snippets
Huntingtin and the molecular pathology of Huntington's disease
The HTT gene is highly conserved9 and its embryonic knockout in mice is lethal,10 suggesting the protein is essential for development. The function of wild-type HTT is incompletely understood, but it is ubiquitously expressed and has many interaction partners. Wild-type HTT is thought to have many roles: vesicular trafficking; mediation of endocytosis, vesicular recycling, and endosomal trafficking; coordination of cell division; and regulation of transcription and metabolism.5 The toxic
Therapies targeting RNA
mRNA is accessible in the nucleus or cytosol and, by contrast to DNA, is unprotected by repair machinery. Thus, reduction of HTT mRNA translation should be easier than modulation of transcription or alteration of the gene itself. The three main methods to reduce HTT mRNA are ASOs, RNAi compounds, and small-molecule splicing modulators (table 1). ASOs and RNAi compounds are nucleotide-based therapeutic molecules that selectively bind to mRNA through Watson-Crick complementarity and trigger RNA
References (76)
- et al.
Huntington's disease: from molecular pathogenesis to clinical treatment
Lancet Neurol
(2011) - et al.
The biology of Huntingtin
Neuron
(2016) - et al.
Treatment of infantile-onset spinal muscular atrophy with nusinersen: a phase 2, open-label, dose-escalation study
Lancet
(2016) - et al.
Targeted disruption of the Huntington's disease gene results in embryonic lethality and behavioral and morphological changes in heterozygotes
Cell
(1995) - et al.
Huntingtin controls neurotrophic support and survival of neurons by enhancing BDNF vesicular transport along microtubules
Cell
(2004) - et al.
AAV vector-mediated RNAi of mutant huntingtin expression is neuroprotective in a novel genetic rat model of Huntington's disease
Mol Ther
(2008) - et al.
Preclinical safety of RNAi-mediated HTT suppression in the Rhesus macaque as a potential therapy for Huntington's disease
Mol Ther
(2011) - et al.
Widespread suppression of huntingtin with convection-enhanced delivery of siRNA
Exp Neurol
(2012) - et al.
Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice
Cell
(1996) - et al.
RAN translation in Huntington disease
Neuron
(2015)
Sustained therapeutic reversal of Huntington's disease by transient repression of Huntingtin synthesis
Neuron
Design, characterization, and lead selection of therapeutic miRNAs targeting Huntingtin for development of gene therapy for Huntington's disease
Mol Ther Nucleic Acids
RNase H-mediated inhibition of translation by antisense oligodeoxyribonucleotides: use of backbone modification to improve specificity
Gene
Integrated safety assessment of 2′-O-methoxyethyl chimeric antisense oligonucleotides in nonhuman primates and healthy human volunteers
Mol Ther
Safety of antisense oligonucleotide and siRNA-based therapeutics
Drug Discov Today
“Huntingtin holiday”: progress toward an antisense therapy for Huntington's disease
Neuron
An antisense oligonucleotide against SOD1 delivered intrathecally for patients with SOD1 familial amyotrophic lateral sclerosis: a phase 1, randomised, first-in-man study
Lancet Neurol
Common SNP-based haplotype analysis of the 4p16.3 Huntington disease gene region
Am J Hum Genet
Huntingtin haplotypes provide prioritized target panels for allele-specific silencing in Huntington disease patients of European ancestry
Mol Ther
Single-stranded RNAs use RNAi to potently and allele-selectively inhibit mutant Huntingtin expression
Cell
Increased brain penetration and potency of a therapeutic antibody using a monovalent molecular shuttle
Neuron
Predictors of phenotypic progression and disease onset in premanifest and early-stage Huntington's disease in the TRACK-HD study: analysis of 36-month observational data
Lancet Neurol
Advances in therapeutic CRISPR/Cas9 genome editing
Transl Res
Biological and clinical changes in premanifest and early stage Huntington's disease in the TRACK-HD study: the 12-month longitudinal analysis
Lancet Neurol
Potential endpoints for clinical trials in premanifest and early Huntington's disease in the TRACK-HD study: analysis of 24 month observational data
Lancet Neurol
Neurofilament light protein in blood as a potential biomarker of neurodegeneration in Huntington's disease: a retrospective cohort analysis
Lancet Neurol
Premanifest and early Huntington's disease
CAG repeat expansion in Huntington disease determines age at onset in a fully dominant fashion
Neurology
Huntington disease: natural history, biomarkers and prospects for therapeutics
Nat Rev Neurol
FDA approves first drug for spinal muscular atrophy
Guidance for industry: guidance for human somatic cell therapy and gene therapy
Hum Gene Ther
Phylogenetic comparison of huntingtin homologues reveals the appearance of a primitive polyQ in sea urchin
Mol Biol Evol
Huntington disease
Nat Rev Dis Primers
Therapeutic interventions for disease progression in Huntington's disease
Cochrane Database Syst Rev
Inactivation of Hdh in the brain and testis results in progressive neurodegeneration and sterility in mice
Nat Genet
RNA interference improves motor and neuropathological abnormalities in a Huntington's disease mouse model
Proc Natl Acad Sci USA
Six-month partial suppression of Huntingtin is well tolerated in the adult rhesus striatum
Brain
Aberrant splicing of HTT generates the pathogenic exon 1 protein in Huntington disease
Proc Natl Acad Sci USA
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