Functional rescue of a fatal ERAD mutation via alternative splicing.

Endoplasmic reticulum (ER)-associated degradation (ERAD) is essential for cellular proteostasis, with the SEL1L-HRD1 protein complex targeting misfolded proteins in the ER for proteasomal degradation. Disruption of this pathway underlies a recently identified infant-onset neurodevelopmental disorder (ENDI syndrome), characterized by profound developmental delay, microcephaly, and immune deficiency. Its most severe form, ENDI with agammaglobulinemia (ENDI-A), is driven by a bi-allelic SEL1L Cys141Tyr (C141Y) mutation within the fibronectin II (FNII) domain, for which no treatment currently exists. Here, we serendipitously uncover a striking mechanism of intrinsic rescue in knock-in mouse models of the C141Y mutation: enhanced usage of an alternative splice donor site within exon 4 bypasses the mutant FNII-encoding region, restoring ERAD activity and rescuing key disease phenotypes including perinatal lethality, growth retardation, B cell deficiency, and neurodevelopmental defects. Building on this discovery, we demonstrate that antisense oligonucleotide (ASO)-mediated exon skipping in patient-derived fibroblasts generates a truncated yet functional SEL1L protein, fully rescuing ERAD function and ER proteostasis. These results establish RNA splicing modulation as a viable therapeutic strategy for ERAD deficiency and extend the clinical potential of exon-skipping therapy to diseases of protein misfolding. ONE-SENTENCE SUMMARY: This study reports the discovery of using antisense oligonucleotides (ASOs) to rescue the biallelic SEL1L C141Y variant, offering a potential therapeutic strategy.