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Preprint WatchMildSeptember 14th, 2026

A De Novo, Intragenic Modifier in Cis Rescues Scn8a-N1768D Epilepsy and Defines a Therapeutic Window Bounded by Gain- and Loss-of-Function

Bahramnejad, E.; Hammer, M. F.; Hiller, V. J.

An intragenic cis-acting frameshift that reduces Scn8a messenger RNA by roughly 30 percent rescues gain-of-function epilepsy in Scn8a-N1768D mice, while 70 percent reduction is lethal, defining a bounded therapeutic window for NaV1.6 lowering.

Mild contradiction

1 prior failure

One documented clinical failure (Phase 1 or 2) overlaps with the claimed mechanism.

This preprint reports that a de novo two base pair frameshift in cis with the Scn8a-N1768D gain-of-function allele titrates NaV1.6 expression across a narrow range, with no Scn8a messenger RNA reduction in short-lived mice, 30 percent reduction in long-lived mice that lose spontaneous tonic-clonic seizures, and 70 percent reduction in mice that die of hindlimb paralysis. The authors read this as validation of Scn8a lowering together with a bounded therapeutic dose window. Claidex holds one prior failure on this target, nbi-921352-scn8a-dee13-sponsor-shutdown, in which Neurocrine terminated the open-label extension of NBI-921352, the only NaV1.6-selective asset to reach the clinic, at eight enrolled participants. That termination was recorded as a sponsor decision rather than an efficacy failure, so the preprint does not contradict a negative clinical result. It does sharpen the design question the prior failure left open, because a therapeutic window bounded on both sides implies that a selective NaV1.6 inhibitor needs exposure-response titration and a pharmacodynamic readout of channel lowering, neither of which a small open-label extension in eight participants could deliver. Severity is set to MILD on one matching claim with a non-efficacy archetype.

Abstract excerpt

Gain-of-function variants in SCN8A, encoding NaV1.6, cause developmental and epileptic encephalopathy with early-onset, drug-resistant seizures. SCN8A is a Goldilocks gene: both excess and deficit of NaV1.6 are deleterious, so therapies that lower NaV1.6 must operate within a narrow window. Here we characterize a congenic C3H/HeJ.C57BL/6J colony carrying Scn8a-N1768D and identify a tightly linked, cis-acting modifier (L) that converts a severe phenotype into graded outcomes. Among N1768D homozygotes on a 98.4% C3H/HeJ.C57BL/6J background, three phenotypes segregate: short-lived (SL) mice with early lethal tonic-clonic seizures (TCS), long-lived (LL) mice with delayed onset and reduced seizure burden, and hindlimb-paralysis (HP) mice with early death. Heterozygotes separate into D/+ SL mice, which develop spontaneous TCS and die of epilepsy, and D/+ LL mice, which lack spontaneous TCS and epilepsy-related death. Genetic analysis places a second locus (L) tightly linked to Scn8a and co-segregating with phenotype. Hippocampal RNA-seq shows dose-dependent Scn8a mRNA reduction: none in SL (D+/D+), 30% in LL (DL/D+), and 70% in HP (DL/DL). Long-read trio DNA sequencing identifies a de novo 2-bp frameshift in coding exon 15 of Scn8a, in cis with N1768D, that introduces a premature stop codon ~26 kb upstream of the exon--exon junction, predicted to trigger nonsense-mediated decay. Allele-specific read counting confirms selective, cis-acting loss of DL transcripts with preserved D+ output. Thus, a de novo, intragenic frameshift variant titrates NaV1.6 through a Goldilocks window in which intermediate reduction rescues Gain-of-function epilepsy, whereas insufficient or excessive reduction remains lethal, validating Scn8a-lowering and its therapeutic dose window.

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This is an automated contradiction flag, not an editorial judgment on the preprint's quality. Flags identify where the preclinical literature and the clinical failure record diverge.