Antisense therapy in neurology

Abstract

Antisense therapy is an approach to fighting diseases using short DNA-like molecules called antisense oligonucleotides. Recently, antisense therapy has emerged as an exciting and promising strategy for the treatment of various neurodegenerative and neuromuscular disorders. Previous and ongoing pre-clinical and clinical trials have provided encouraging early results. Spinal muscular atrophy (SMA), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), Duchenne muscular dystrophy (DMD), Fukuyama congenital muscular dystrophy (FCMD), dysferlinopathy (including limb-girdle muscular dystrophy 2B; LGMD2B, Miyoshi myopathy; MM, and distal myopathy with anterior tibial onset; DMAT), and myotonic dystrophy (DM) are all reported to be promising targets for antisense therapy. This paper focuses on the current progress of antisense therapies in neurology.

Figure 1

Chemical structure of biological and synthetic oligonucleotides. (A) DNA; (B) RNA; (C) 2'O-methylphosphorothioate (2'O-MePS); (D) Morpholino (PMO); (E) 2'-methoxyethoxy (2'-MOE); (F) PMO with peptide conjugate (PPMO); (G) Locked nucleic acid (LNA); (H) Vivo-morpholino (vPMO); (I) Peptide nucleic acid (PNA); (J) Boranophosphate-oligodeoxy-nucleoside (BH3-ODN); (K) Oxetane-modified AO.

Figure 2

Mechanism of exon skipping therapy for Duchenne muscular dystrophy (DMD). Nonsense mutations in the DMD gene can create a novel STOP codon which results in the loss of DMD protein. Exon skipping corrects this error when exons (black) that are bound to antisense oligos (green) are spliced out of the pre-mRNA, and the resulting exon sequences “fit together”, i.e., are in-frame (denoted by the shape of each exon—ends that fit together are in-frame). Out-of-frame mutations caused by the loss of exonic sequences, through deletion or splice site mutations, can also be corrected through exon skipping, which removes exons adjacent to the mutation site so that the remaining exons are in-frame. The result is a truncated yet partly functional protein, as in the case of Becker muscular dystrophy (BMD).

Figure 3

Strategy of antisense therapy for Fukuyama dystrophy. Retrotransposon insertion in the FCMD gene leads to aberrant splicing. An antisense vivo-morpholino cocktail (A3, E3 and D5) restores normal splicing.

Figure 4

Mechanism of antisense silencing via RNase H1 activity. Myotonic dystrophy (DM1) is caused by RNA gain-of-function due to an expanded CUG repeat in the dystrophia myotonica-protein kinase (DMPK) gene transcript.RNase H1-mediated degradation of target nucleic acids is facilitated by AO “gapmers”, composed of a central gap region which supports RNase H1 activity and flanking nucleotides at the 5' and 3'-ends which are resistant to RNase H1 degradation and display strong binding affinity for target RNA.

Figure 5

Mechanism of antisense exon 7 inclusion in SMN2. Spinal muscular atrophy (SMA) is caused by a loss-of-function mutation in the SMN1 gene. Within the SMN2 gene, a paralogue of SMN1, a single nucleotide substitution in exon 7 interferes with an exonic splicing enhancer, impairing production of normal SMN protein. AOs targeted to the intronic splice silencer site (ISS) in intron 7 of SMN2 facilitate the retention of exon 7 within the mature mRNA, increasing the production of functional SMN protein.