Huntingtin Lowering Strategies

Abstract

Trials using antisense oligonucleotide technology to lower Huntingtin levels in Huntington's disease (HD) are currently ongoing. This progress, taking place only 27 years after the identification of the Huntingtin gene (HTT) in 1993 reflects the enormous development in genetic engineering in the last decades. It is also the result of passionate basic scientific work and large worldwide registry studies that have advanced the understanding of HD. Increased knowledge of the pathophysiology of this autosomal dominantly inherited CAG-repeat expansion mediated neurodegenerative disease has led to the development of several putative treatment strategies, currently under investigation. These strategies span the whole spectrum of potential targets from genome editing via RNA interference to promoting protein degradation. Yet, recent studies revealed the importance of huntingtin RNA in the pathogenesis of the disease. Therefore, huntingtin-lowering by means of RNA interference appears to be a particular promising strategy. As a matter of fact, these approaches have entered, or are on the verge of entering, the clinical trial period. Here, we provide an overview of huntingtin-lowering approaches via DNA or RNA interference in present clinical trials as well as strategies subject to upcoming therapeutic options. We furthermore discuss putative implications for future treatment of HD patients.

Keywords: Chorea; HD; Huntington’s Disease; RNA interference; antisense oligonucleotides; disease modification; huntingtin-lowering.

Figure 1

Huntingtin-lowering strategies currently under clinical investigation in Huntington’s Disease. Allele-specific (A) and allele-unspecific (B) Antisense Oligonucleotide (ASO) strategies as well as virally mediated strategies (C) are currently being evaluated in clinical trials. ASOs (A, B) are applied via lumbar puncture (1). They are supposed to reach the cerebral cortex and deeper brain regions via diffusion along the neuraxis through cerebrospinal fluid CSF turnover. They enter brain cells and nuclei due to their lipophilic backbone modification (2). By Watson-Crick base pairing, they bind to both mutant htt and wild type htt (5, allele-unspecific) or mutant htt (3, allele-specific). This leads to nuclear RNaseH mediated degradation of pre-mRNA and mRNA and a subsequent lowering in htt protein levels of wt and mt htt (6) or mt htt, only (4). Gene-Therapeutic strategies (C) rely on a single stereotactic injection into the striatum (7) of adeno-associated viruses carrying a mi-RNA expression cassette, which is able to enter neuronal nuclei, and express a miRNA construct (8). After cleavage, this construct suppresses expression of mutant htt by RNA interference (9) and subsequent degradation by the cytosolic RISC complex (10).

Figure 2

Huntingtin- lowering approaches: Along the process from htt DNA transcription via RNA processing and translation to protein folding and function, several well-defined points of interference are possible that ultimately result in lower levels of total or mutated htt protein. Strategies that aim at the protein level (C), i.e., by means of induction of autophagy rely on the hypothesis that HD is mainly a proteinopathy that results from disturbed function and neurotoxic accumulation of mutant htt protein. In contrast, gene expression modification strategies (B) inhibit the generation of the protein, but also may be suitable to cover pathologic aspects on the RNA level that may significantly contribute to HD pathogenesis. (A) Genome editing via, i.e., CRISPR/Cas9 may permanently correct mutant HTT. Correction of the CAG Repeat expansion may have additional beneficial effects since the length of the uninterrupted CAG repeat length on DNA level is inversely correlated to disease onset (for references see text). Altering metagenomic structure by HDAC inhibition, or using Zink Finger transcription factors can inhibit transcription of mutant DNA to mRNA. PCT: PTC Therapeutics; LC3: autophagosome protein microtubule-associated protein 1A/1B light chain 3.