Synthetic, chemically modified single-stranded nucleic acid polymers of 15 to 25 nucleotides engineered to hybridize by Watson–Crick base-pairing to a specific pre-mRNA, mature mRNA, or non-coding RNA in human cells. First demonstrated in 1978 by Paul Zamecnik and Mary Stephenson using a 13-mer synthetic oligodeoxynucleotide complementary to Rous sarcoma virus RNA to block viral replication and translation, ASOs now represent a major pharmacological class alongside siRNAs for genetic, neurodegenerative, and cardiometabolic disorders.
Class
Synthetic single-stranded therapeutic oligodeoxynucleotide / ribonucleotide analog
Unmodified phosphodiester oligodeoxynucleotides are destroyed within minutes by serum and intracellular deoxyribonucleases and ribonucleases; clinical ASOs are therefore manufactured by automated solid-phase phosphoramidite synthesis with backbone and sugar-ring modifications.
First-generation backbone chemistry: Substituting a non-bridging oxygen atom in the phosphate linkage with sulfur creates a phosphorothioate (PS) backbone, conferring nuclease resistance and promoting reversible binding to plasma albumin that prevents rapid renal filtration and facilitates cellular uptake without lipid nanoparticles.
Second- and third-generation sugar chemistries: 2'-O-methyl (2'-OMe), 2'-O-methoxyethyl (2'-MOE), locked nucleic acid (LNA), and phosphorodiamidate morpholino oligomer (PMO) modifications dramatically increase RNA hybridization melting temperature and reduce innate immune Toll-like receptor activation.
Mechanism of Action
As categorized by Stanley Crooke (2017), clinical ASOs act through two distinct post-hybridization mechanisms dictated by their chemical architecture. RNase H1-dependent gapmers feature a central 8-to-10-nucleotide window ("gap") of deoxyribonucleotides flanked by 2'-MOE or LNA "wings"; when the DNA gap hybridizes to target RNA in the nucleus or cytoplasm, endogenous RNase H1 recognizes the DNA:RNA heteroduplex and hydrolyzes the RNA strand. By contrast, fully modified steric-blocking ASOs (2'-MOE or PMO throughout the entire strand) do not form an RNase H1 substrate; instead, they mask splice sites or intronic splicing silencers to redirect the spliceosome, or sterically block the ribosome or microRNAs.
Biological & Regulatory Roles
Because RNase H1 is abundant in the nucleus as well as the cytosol, gapmer ASOs readily degrade nuclear-retained transcripts, intronic sequences, and chromatin-bound long non-coding RNAs that cytoplasmic siRNAs reach less efficiently.
Interactions & Complexes
Intrathecally administered ASOs distribute broadly across neurons and glia of the central nervous system and dorsal root ganglia (where they have been investigated to knock down Nav1.7 in chronic pain) without a carrier vehicle, whereas hepatocyte-targeted gapmers (such as eplontersen) conjugate triantennary GalNAc to enter the liver via the asialoglycoprotein receptor.
Therapeutic & Biotechnology Applications
Splice-switching ASOs: Nusinersen (2'-MOE PS) promotes exon 7 inclusion in SMN2 pre-mRNA to treat spinal muscular atrophy, while eteplirsen, golodirsen, and casimersen (PMOs) induce exon skipping in the dystrophin (DMD) transcript to restore the reading frame in Duchenne muscular dystrophy.
RNase H1 gapmers: Mipomersen (targeting APOB / ApoB-100 mRNA), inotersen and eplontersen (targeting TTR mRNA in hereditary transthyretin amyloidosis), and tofersen (targeting mutant SOD1 mRNA in amyotrophic lateral sclerosis) silence pathogenic genes by catalytic transcript degradation.