Antisense Oligonucleotide Therapies for Neurodegenerative Diseases

Abstract

Antisense oligonucleotides represent a novel therapeutic platform for the discovery of medicines that have the potential to treat most neurodegenerative diseases. Antisense drugs are currently in development for the treatment of amyotrophic lateral sclerosis, Huntington's disease, and Alzheimer's disease, and multiple research programs are underway for additional neurodegenerative diseases. One antisense drug, nusinersen, has been approved for the treatment of spinal muscular atrophy. Importantly, nusinersen improves disease symptoms when administered to symptomatic patients rather than just slowing the progression of the disease. In addition to the benefit to spinal muscular atrophy patients, there are discoveries from nusinersen that can be applied to other neurological diseases, including method of delivery, doses, tolerability of intrathecally delivered antisense drugs, and the biodistribution of intrathecal dosed antisense drugs. Based in part on the early success of nusinersen, antisense drugs hold great promise as a therapeutic platform for the treatment of neurological diseases.

Keywords: Huntington's disease; RNA; amyotrophic lateral sclerosis; antisense oligonucleotides; spinal muscular atrophy.

Figure 1.

Common chemical modifications and designs used for ASO therapeutics. Common chemical modifications include 2′-sugar modifications such as 2′-O-methyl, 2′-O- MOE), and 2′fluoro; 2′,4′-bridged sugar modifications such as LNA and cEt BNA; the phosphate modification phosphorothioate, which can be synthesized as the (Sp) and (Rp) isomers; and the sugar-phosphate replacement morpholino. ASOs can be designed as single-stranded ASOs that utilize the RNase H mechanism (gapmer), uniformly modified to promote splicing, or as double-stranded designs that work through the RISC pathway (siRNA). Abbreviations: ASO, antisense oligonucleotide; cEt BNA, constrained ethyl bicyclic nucleic acid; fluoro, fluorine; LNA, locked nucleic acid; MOE, methoxyethyl; RISC, RNA-induced silencing complex; siRNA, small interfering RNA.

Figure 2.

Mechanisms of action for ASOs. ASOs can modulate RNA function by a variety of mechanisms, including degradation of the pre-mRNA in the nucleus or mature RNA in the cytoplasm by RNase H1, and degradation of RNA in the cytoplasm by the RISC complex (Ago2) or ribozymes or DNAzymes. ASOs can also modulate RNA functiona by nondegradative mechanisms such as splicing or polyadenylation modulation in the nucleus and decrease or increase protein translation in the cytoplasm. Abbreviations: Ago2, argonaute 2; ASO, antisense oligonucleotide; mRNA, messenger RNA; pre-mRNA, precursor mRNA; RISC, RNA-induced silencing complex; uORF, upstream open reading frame.

Figure 3.

Mechanism of nusinersen action. (a) Humans express two SMN genes, SMN1 and SMN2, that differ by a few nucleotides due to a duplication in chromosome 5. A cytosine to thymine transition in exon 7 of SMN2 weakens the RNA splicing signal such that most transcripts derived from SMN2 skip exon 7, creating a truncated protein that is rapidly degraded. SMA patients have a deletion mutation in the SMN1 gene, resulting in an SMN protein deficiency. (b) Nusinersen binds to a site in intron 7 of the SMN2 premRNA, displacing the splicing suppressor proteins hnRNP A1/A2 and allowing U1 small nuclear ribonucleoproteins to bind to the 5′-splice site, promoting inclusion of SMN2 exon 2 into the mRNA. Abbreviations: mRNA, messenger RNA; premRNA, precursor mRNA; SMA, spinal muscular atrophy.