NM_130839.5(UBE3A):c.67C>T (p.Arg23Ter)

For UBE3A c.67C>T (p.Arg23Ter), an early nonsense variant that abolishes maternal UBE3A expression, antisense oligonucleotide (ASO)-mediated unsilencing of the intact paternal UBE3A allele is the most clinically advanced therapeutic strategy. Rugonersen (RO7248824), targeting UBE3A-ATS, has demonstrated an acceptable safety profile and dose-dependent EEG normalization with signals of clinical improvement in the Phase 1 TANGELO trial (NCT04428281, n=61). This approach bypasses the need to correct the maternal nonsense variant directly, instead restoring UBE3A from the epigenetically silenced but genetically intact paternal copy. The strategy is variant-agnostic for loss-of-function maternal alleles, making p.Arg23Ter an ideal candidate.

The TANGELO Phase 1 trial (Hipp et al., Nat Med 2025; PMID:40646322) enrolled 61 children aged 1-12 with Angelman syndrome and demonstrated that intrathecal rugonersen led to dose-dependent partial normalization of AS-associated EEG delta-power abnormality (2-4 Hz band), a validated pharmacodynamic biomarker of UBE3A deficiency. Exploratory clinical endpoints on Bayley-III and Vineland-3 showed improvement signals beyond natural history expectations. The mechanism — ASO-mediated degradation of UBE3A-ATS (the antisense transcript that silences the paternal UBE3A allele) — has been validated in mouse models where paternal Ube3a reinstatement restores behavior and proteome (Punt et al., Mol Autism 2025; PMID:40877933). Additionally, ION582 (Ionis/GeneTx) and GTX-102 (Ultragenyx) represent parallel ASO programs targeting the same biological mechanism, with ION582 in Phase 3, reinforcing the therapeutic class validity. ClinVar classifies c.67C>T as Pathogenic, consistent with complete loss of UBE3A function from the maternal allele.

All ASO data for Angelman syndrome remain in early-phase trials; no Phase 3 efficacy readout for rugonersen is available yet. The TANGELO trial was open-label and uncontrolled, so clinical improvement signals cannot be definitively attributed to treatment versus developmental maturation. Intrathecal delivery requires repeated lumbar punctures, posing procedural burden especially in young children. The optimal therapeutic window is uncertain — earlier treatment may be critical, but the youngest cohort data are limited. Whether paternal UBE3A unsilencing can achieve sufficient protein levels to fully rescue the neurological phenotype, particularly in patients with established neuronal circuit deficits, remains an open question. Long-term durability and safety of chronic ASO administration in the developing CNS require further study. Finally, while the strategy is variant-agnostic, this analysis specifically applies to loss-of-function variants; dominant-negative variants (if any exist for UBE3A) would require different consideration.

Antisense oligonucleotide (ASO) targeting the UBE3A antisense transcript (UBE3A-AS) represents the most mechanistically compelling RNA therapy strategy for Angelman syndrome, including early-truncating variants like c.67C>T (p.Arg23Ter). By degrading UBE3A-AS, these ASOs de-repress the epigenetically silenced but structurally intact paternal UBE3A allele in neurons, bypassing the defective maternal copy entirely. Multiple clinical programs (GTX-102/Ultragenyx, ION582/Ionis, RO7248824/Roche) are in Phase 1/2 trials.

UBE3A is imprinted: only the maternal allele is expressed in neurons, while the paternal allele is silenced by the long non-coding UBE3A-AS transcript. In Angelman syndrome patients with maternal UBE3A loss-of-function, the paternal copy remains structurally intact but transcriptionally silenced. ASO-mediated knockdown of UBE3A-AS in mouse models (Ube3a^m-/p+) restored paternal Ube3a expression to approximately 40-90% of wild-type levels and rescued behavioral and electrophysiological phenotypes (Meng et al., Nature 2015; PMID:25533962). GTX-102 (GeneTx/Ultragenyx) entered clinical trials (NCT04259281) and showed preliminary evidence of UBE3A protein restoration in CSF biomarker studies, though the Phase 1/2 trial encountered dose-limiting lower extremity weakness at higher doses requiring protocol amendment. ION582 (Ionis) and RO7248824 (Roche) represent next-generation ASO programs with potentially improved therapeutic indices. For the c.67C>T variant specifically, the p.Arg23Ter nonsense mutation truncates maternal UBE3A at residue 23, abolishing all E3 ligase activity and nuclear localization — the paternal allele encodes wild-type UBE3A and is thus the ideal rescue target.

ASO therapy requires repeated intrathecal administration, imposing procedural burden on pediatric patients. The therapeutic window may be narrow: preclinical data suggest greater benefit with earlier intervention, and neuronal circuits already mis-wired may not fully recover. GTX-102 dose-limiting toxicity (lower extremity weakness) at higher doses raises safety concerns that newer ASOs must address. Long-term expression durability after each dose and optimal re-dosing intervals remain under investigation. This approach does not correct the maternal allele — it relies entirely on sufficient de-repression of the paternal copy, and the degree of paternal UBE3A restoration achievable in human neurons in vivo remains to be quantified. The c.67C>T variant has not been specifically studied in ASO trial cohorts.