Oligonucleotide-Based Therapy for FTD/ALS Caused by the C9orf72 Repeat Expansion: A Perspective

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive and lethal disease of motor neuron degeneration, leading to paralysis of voluntary muscles and death by respiratory failure within five years of onset. Frontotemporal dementia (FTD) is characterised by degeneration of frontal and temporal lobes, leading to changes in personality, behaviour, and language, culminating in death within 5-10 years. Both of these diseases form a clinical, pathological, and genetic continuum of diseases, and this link has become clearer recently with the discovery of a hexanucleotide repeat expansion in the C9orf72 gene that causes the FTD/ALS spectrum, that is, c9FTD/ALS. Two basic mechanisms have been proposed as being potentially responsible for c9FTD/ALS: loss-of-function of the protein encoded by this gene (associated with aberrant DNA methylation) and gain of function through the formation of RNA foci or protein aggregates. These diseases currently lack any cure or effective treatment. Antisense oligonucleotides (ASOs) are modified nucleic acids that are able to silence targeted mRNAs or perform splice modulation, and the fact that they have proved efficient in repeat expansion diseases including myotonic dystrophy type 1 makes them ideal candidates for c9FTD/ALS therapy. Here, we discuss potential mechanisms and challenges for developing oligonucleotide-based therapy for c9FTD/ALS.

Figure 1

Structure of the C9orf72 locus and its three primary transcripts. Two alternative first exons are used, 1a and 1b, and both of these lie upstream of the translation start site. Between exons 1a and 1b lies the hexanucleotide expansion region. Note that two putative CpG islands lie either side of the expansion. The shorter of these regions, CpG 20, overlaps with much of the sequence of exon 1a. Although there are three transcripts, the position of the ATG start codon in exon 2 means that only two protein isoforms are translated: isoform A is 481 amino acids long, while the shorter isoform B is only 222 amino acids in length.

Figure 2

Chemical structures of commonly used oligonucleotides. Nucleic acids can bind to RNA targets that are complementary to their own sequence and trigger target degradation. However, unmodified DNAs/RNAs are subject to endonuclease degradation. Thus, to target RNAs, modifications to this primary structure are needed. Modifications to the nucleic acid backbone can lead to structures that can better interact with the target and are resistant to endonucleases. Modifications in the sugar ring are also possible and give rise to nucleic acids that mimic RNA, have better targeting, and are also resistant to endonuclease action.

Figure 3

Strategy for exon 51-skipping in Duchenne muscular dystrophy. Exon 51-skipping by appropriate PMO or 2′OMePS, indicated by a blue line, can restore the reading frame of dystrophin in a DMD patient, who lacks exon 52 in the mRNA of the DMD gene, leading to out-of-frame products. Dot line and Ex indicate introns and exons, respectively.

Figure 4

Strategy for ASO-based knocking down of the C9orf72 gene in a c9FTD/ALS patient. Using antisense oligonucleotides (ASO) targeting the GGGGCC repeat expansion can lead to selective knocking down of the mutant allele which could otherwise cause the formation of RNA foci. It also preserves the expression of the normal allele that may have an important function for cell survival. ASO is indicated by a blue line, dot line indicates introns, and squares indicate exons.