SCN3A -related neurodevelopmental disorder: Clinical case reports and biophysical characterization

Figures

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  Figure 1.. Location of SCN3A mutations on the Nav1.3 channel schematic representation of the Nav1.3 α-subunit showing the relative positions of the identified patient mutations. The p.L209P variant is located in the S3–S4 linker of domain I, while p.N52H and p.E1809K are located on the N-terminus and C-terminus, respectively. These regions are known to play important roles in gating and channel regulation.

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  Figure 2.. Effects of SCN3A variants on current density (A–D) Representative whole-cell sodium current traces recorded from cho cells expressing WT-Nav1.3 or the indicated mutant variants (p.N52H, p.E1809K, p.L209P). (E) Quantification of peak current density at 0 mV, expressed as pA/pF, reveals a significant reduction for p.N52H compared to wt (p = 0.0037), while p.E1809K and p.L209P did not differ significantly. (F) Current–voltage relationship curves across a range of test potentials (−100 to +80 mV) show the relative current densities for wt and mutant channels. Together, these data indicate that p.N52H confers a loss-of-function effect on overall current density.

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  Figure 3.. Voltage dependence of channel activation (A–D) normalized conductance–voltage relationships for wt and each mutant channel. Both p.N52H and p.L209P produced significant hyperpolarizing shifts in the activation curve compared to wt (p < 0.05), whereas p.E1809K showed no difference. These results suggest a gain-of-function effect for p.N52H and p.L209P, enabling channel activation at more negative membrane potentials. Inset: voltage-step protocol used for activation measurements. Bar graphs show statistical information.

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  Figure 4.. Steady-state fast inactivation properties (A–D) steady-state inactivation curves for wt and mutant channels, normalized to peak current amplitude. Only p.E1809K caused a significant hyperpolarizing shift in the inactivation V1/2, consistent with a modest loss-of-function in channel availability. p.N52H and p.L209P did not differ from wt in steady-state inactivation. Inset: voltage-step protocol used for fast inactivation measurements. Bar graphs show statistical information. Bar graphs show statistical information.

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  Figure 5.. Recovery from fast inactivation (A–D) recovery from inactivation curves for wt and mutant channels, fitted with a double-exponential function. While slow recovery components did not differ among variants, the fast component was significantly accelerated in p.E1809K compared to wt (p = 0.0117). These results indicate that p.E1809K alters the kinetics of channel repriming, potentially influencing repetitive firing. Inset: voltage-step protocol used to measure recovery. Bar graphs show statistical information.

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  Figure 6.. Action potential modeling of patient-specific variants simulated neuronal action potentials based on Hodgkin–Huxley modeling incorporating wt or mutant channel gating parameters (A) Homozygous p.L209P produced increased action potential amplitude and spike number compared to wt, indicating enhanced excitability. (B) Under a heterozygous assumption (50% wt, 50% p.L209P), hyperexcitability was attenuated but remained above wt baseline. (C) Homozygous p.N52H/p.E1809K modeling revealed a subtle gain-of-function, with modestly increased spike number. (D) Heterozygous p. N52H/p.E1809K simulations closely resembled wt, suggesting reduced impact compared to p.L209P. Together, the computational predictions align with clinical severity, with case 1 (p.L209P) showing stronger excitability changes consistent with epilepsy, and case 2 (p.N52H/p.E1809K) showing subtler neuronal dysfunction without seizures.