This is a mechanism module, not a specific disease. Disorder entries reference the relevant paradigm node via conforms_to. Use "antisense_oligonucleotide_therapy#Pathogenic mRNA Accumulation" for RNase H-knockdown targets (SOD1-ALS/tofersen, ATTR amyloidosis/inotersen or eplontersen, familial hypercholesterolemia/mipomersen, familial chylomicronemia syndrome/volanesorsen or olezarsen, hereditary angioedema/donidalorsen, FUS-ALS/jacifusen). Use "antisense_oligonucleotide_therapy#Aberrant Pre-mRNA Splicing" for splice-site occlusion targets (SMA/nusinersen, DMD exon-skipping/eteplirsen, golodirsen, viltolarsen, casimersen). Use "antisense_oligonucleotide_therapy#Pathogenic Viral mRNA Translation" for steric blockade targets (CMV retinitis/fomivirsen). The conforming pathophysiology node in a disorder file should describe the disease-specific RNA defect, referencing the causal gene and transcript. Since ASO design relies principally on knowledge of mRNA sequence, this module also captures the expedient bench-to-bedside pipeline enabled by RNA targeting — the same sequence logic that makes individualized ASOs (e.g., milasen for a single CLN7 patient) feasible within months.
Pathogenic mRNA Accumulation
trigger
In dominant gain-of-function or toxic protein overexpression diseases, the causal mRNA is transcribed and translated at a level that produces harmful amounts of the encoded protein. The transcript itself is structurally normal but its abundance or the toxicity of its protein product is the pathogenic driver — making mRNA level reduction, rather than protein inhibition, the upstream therapeutic target. Examples include SOD1 mRNA in SOD1-ALS, TTR mRNA in hereditary transthyretin amyloidosis, APOB mRNA in homozygous familial hypercholesterolemia, APOC3 mRNA in familial chylomicronemia syndrome, KLKB1 (prekallikrein) mRNA in hereditary angioedema, and FUS mRNA in FUS-ALS. RNase H-dependent ASOs bind the target mRNA and recruit endogenous RNase H1 to cleave the heteroduplex, degrading the message and reducing the pathogenic protein.
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RNase H-Mediated Transcript Degradation
The ASO base-pairs with the target mRNA, forming a DNA:RNA heteroduplex that recruits endogenous RNase H1 to cleave the RNA strand.
Reduction of Pathogenic Protein
consequence
RNase H-mediated mRNA cleavage reduces ribosomal substrate availability, producing a sustained, dose-dependent reduction in the pathogenic protein. The magnitude of protein reduction is the primary pharmacodynamic readout: tofersen reduces CSF SOD1; inotersen and eplontersen reduce serum TTR; mipomersen reduces LDL-C via APOB reduction; volanesorsen and olezarsen reduce APOC3 and serum triglycerides; donidalorsen reduces prekallikrein. Protein reduction translates to disease modification when the targeted protein is the proximate pathogenic driver.
Aberrant Pre-mRNA Splicing
trigger
In diseases caused by splice-disrupting mutations or regulatory splicing defects, the pre-mRNA is transcribed but incorrectly processed. Two distinct defect types drive the two splice-modulating ASO strategies: (1) mutation-bearing exons disrupt the reading frame — most prominently in Duchenne muscular dystrophy, where out-of-frame deletion of one or more exons prevents dystrophin production; exon-skipping ASOs restore the reading frame by forcing the spliceosome to bypass the mutant exon, producing a truncated but partially functional protein. (2) Intronic silencer elements suppress exon inclusion — in spinal muscular atrophy, ISS-N1 in SMN2 intron 7 causes exon 7 skipping and loss of full-length SMN protein; nusinersen blocks ISS-N1 to restore exon 7 inclusion. Both strategies employ sterically-acting ASOs (no RNase H recruitment) with uniformly-modified backbones (2'-MOE or PMO chemistry).
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ASO-Directed Splice Redirection
The ASO occupies a splice regulatory element on the pre-mRNA, physically blocking spliceosome access and redirecting exon choice.
ASO-Directed Splice Redirection
effector
A steric-blocking ASO (uniformly modified; no RNase H recruitment) binds a splice regulatory element on the pre-mRNA — a splice acceptor, splice donor, exonic splicing silencer, or intronic silencing sequence — and physically prevents spliceosome recognition. The spliceosome bypasses the blocked element, either skipping a mutation-bearing exon (exon-skipping paradigm) or including a normally silenced exon (exon-inclusion paradigm). Phosphorodiamidate morpholino oligomer (PMO) chemistry, used in the DMD exon-skipping ASOs, is uncharged and cell-penetrant without lipid formulation, while 2'-MOE chemistry (used in nusinersen) supports intrathecal delivery to motor neurons.
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Restored Protein Reading Frame
Successful splice redirection yields an in-frame mRNA that can be translated into a truncated but partially functional protein, or restores full-length protein when an inclusion event is achieved.
Restored Protein Reading Frame
consequence
Splice redirection produces an mRNA with a corrected reading frame that is translated into a truncated but partially functional protein, or in inclusion paradigms, into full-length protein. In DMD exon-skipping, the resulting Becker-like dystrophin — shorter than wild-type but in-frame — partially compensates for the absent full-length protein. In SMA, nusinersen-driven SMN2 exon 7 inclusion produces full-length SMN protein that rescues motor neuron survival. The degree of clinical benefit scales with the amount of functional protein restored and the residual burden of out-of-frame or truncated product.
Pathogenic Viral mRNA Translation
trigger
In viral infections where the pathogenic protein is virus-encoded rather than host-encoded, an ASO can sterically block translation of the viral mRNA without recruiting RNase H. Fomivirsen (Vitravene, FDA-approved 1998, withdrawn 2006 due to market factors), the first FDA-approved ASO, targeted CMV UL123/IE2 mRNA by steric translation blockade, reducing viral immediate-early protein production and halting viral replication in CMV retinitis. This paradigm differs from host-gene RNase H knockdown: the target is an exogenous transcript whose elimination does not require endogenous RNase H, and the therapeutic window relies on viral-sequence specificity rather than gene-expression selectivity.
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Steric Viral mRNA Translation Blockade
The ASO base-pairs with the viral mRNA target site, physically impeding ribosome scanning or 48S complex assembly without cleaving the transcript.
Steric Viral mRNA Translation Blockade
effector
The ASO:viral mRNA duplex physically impedes ribosomal access to the translation initiation region or coding sequence without recruiting RNase H. Because the RNA strand is not cleaved, this mechanism requires sustained ASO occupancy at the target site. The steric blockade reduces viral immediate-early gene product levels, disrupting the transcriptional amplification cascade essential for productive viral replication. This paradigm is now largely superseded by RNase H knockdown and small-molecule antivirals for most viral targets, but established the proof of concept for ASO therapeutics.