Antisense oligonucleotides (ASOs), typically 15-20 bases in length, bind sequence specifically to the target RNA and address aberrant RNA processing. Due to the low copy number of mRNAs, a small number of ASOs can directly modulate the cellular proteome. This principle of molecular recognition provides ASOs with great flexibility in targeting and expedites their development a as class of safe and effective treatment strategy for the vast array of rare genetic diseases.
Mechanism
Aberrant protein production is underlying multiple rare diseases caused by a single underlying genetic mutation. By targeting the disease-causing RNA with ASOs, corrections of protein levels have been achievable.
ASOs regulate RNA function through several mechanisms that can be broadly categorized as either promoting RNA fragmentation and degradation, or mass occupancy/steric arrest. The exact mechanism by which ASO regulates RNA depends on ASO chemistry and design, where the ASO binds, and how the target RNA functions.
The alternative splicing mechanism is the widely adopted mechanism for CNS targeted individualized ASOs and occurs in nusinersen. The ASO would bind to precursor mRNA and block splice sites, inducing exon inclusion or skipping. This mechanism is used in treating diseases such as spinal muscular atrophy (SMA) and Duchenne muscular dystrophy (DMD). RNase H-mediated degradation is where the RNase H1 enzyme recognizes and cleaves RNA-DNA hybrid structures and is not commonly applied in individualized ASOs.
Chemical Modifications
Even unmodified ASOs have been used to target RNA with moderate success. However, the phosphodiester bond is susceptible to nuclease degradation, and the large size and charged nature of these bonds are restricting access to intracellular targets.
Chemically modified ASOs have been invented with enhanced efficacy and stability, decreased immune response and limited off-target toxicity. For individualized ASOs intended for CNS dosing, the only FDA permitted modifications are the phosphorothioate and 2′-O-methyl and 2′-O-methoxyethyl modifications. These are modifications with extensive clinical experience supporting their safe administration.
Advantages of ASOs
Uptake of antisense oligonucleotides are independent of the cell cycle. Hence ASOs can enter both proliferating and non-proliferating cells. ASOs do not contain viral sequences, so will not be triggering any immune responses, nor integrating into the patient genome.