Two major types of oligonucleotide drugs for gene silencing, siRNA and antisense oligonucleotide (ASO), bind to RNA by base pairing a sequence-specific manner and downregulate gene expression by inducing enzyme-dependent degradation of targeted RNA. It has now been over a decade since the discoveries of RNA interference (RNAi) and siRNAs, but despite some promising clinical trial results pharmaceutical company remains cautious about the therapeutic potential of RNAi-based drugs. ASO gapmers containing 2 to 5 chemically-modified nucleotides,locked nucleotide acid (LNA)1-3, or 2'Z-O-methoxyethyl (2'-MOE)4,5 at each terminus flanking a central 5-to10-base 'gap' of DNA, enable cleavage of target RNA by ribonuclease H (RNase H), which recognizes the DNA/RNA heteroduplex6,7. Unlike siRNAs, which tolerate only limited modifications without loosing
compatibility with the RNAi effector complex, extensive chemical modifications in ASO gapmers do not abrogate RNase H activity. Mipomersen, a 2'-MOE phosphorothioate ASO gapmer targeting the human apolipoprotein B (APOB) gene, was recently approved for treatment of familial hypercholesterolemia. Phase I clinical trial of ASO targeting SOD1 by intrathecal infusion was reported with eight patients with SOD1-positive amyotrophic lateral sclerosis (Lancet Neurol. 2013;12(5):435-42.) C9ORF was also a candidate gene for ASO.
Recently, we developed a new class of exceptionally potent heteroduplex antisense oligonucleotide. When conjugated with vitamin E to the HDO, the silencing effect of HDO was 10-50 fold greater than that of ASO for endogenous gene of the liver. We furthermore investigated a devise for penetrating blood-brain-barrier of ASO.
We will discuss the feasibility of ASO gene therapy of ALS.