Integration of biological/pathophysiological contexts to help clarify genotype-phenotype mismatches in monogenetic diseases. Childhood epilepsies associated with SCN2A as a case study

When rare meets common: Treatable genetic diseases are enriched in the general psychiatric population

Mental illnesses are one of the biggest contributors to the global disease burden. Despite the increased recognition, diagnosis and ongoing research of mental health disorders, the etiology and underlying molecular mechanisms of these disorders are yet to be fully elucidated. Moreover, despite many treatment options available, a large subset of the psychiatric patient population is nonresponsive to standard medications and therapies. There has not been a comprehensive study to date examining the burden and impact of treatable genetic disorders (TGDs) that can present with neuropsychiatric features in psychiatric patient populations. In this study, we test the hypothesis that TGDs that present with psychiatric symptoms are more prevalent within psychiatric patient populations compared to the general population by performing targeted next-generation sequencing of 129 genes associated with 108 TGDs in a cohort of 2301 psychiatric patients. In total, 48 putative affected and 180 putative carriers for TGDs were identified, with known or likely pathogenic variants in 79 genes. Despite screening for only 108 genetic disorders, this study showed a two-fold (2.09%) enrichment for genetic disorders within the psychiatric population relative to the estimated 1% cumulative prevalence of all single gene disorders globally. This strongly suggests that the prevalence of these, and most likely all, genetic diseases is greatly underestimated in psychiatric populations. Increasing awareness and ensuring accurate diagnosis of TGDs will open new avenues to targeted treatment for a subset of psychiatric patients.

Potential induction of epileptic spasms by nonselective voltage-gated sodium channel blockade: Interaction with etiology

PURPOSE

Epileptic spasms are often preceded by focal (or multifocal) seizures. Based on a series of case reports suggesting that carbamazepine and oxcarbazepine may induce epileptic spasms, we set out to rigorously evaluate the potential association between exposure to voltage-gated sodium channel blockade and latency to epileptic spasms.

METHODS

We identified 50 cases (children with focal seizures and evolution to epileptic spasms) and 50 controls (children with focal seizures without evolution to epileptic spasms). For each patient, we reviewed all sequential neurology encounters between onset of epilepsy and emergence of epileptic spasms. For each encounter we recorded seizure-frequency and all anti-seizure therapy exposures. Using multivariable Cox proportional hazards regression, we evaluated the association between voltage-gated sodium channel exposure (carbamazepine, oxcarbazepine, lacosamide, or phenytoin) and latency to epileptic spasms onset, with adjustment for etiology and seizure-frequency.

RESULTS

Latency to epileptic spasms onset was independently associated with exposure to sodium channel blockade (hazard ratio = 2.4; 95% CI 1.1-5.2; P = 0.03) and high-risk etiology (hazard ratio = 2.8; 95% CI 1.5-5.1; P = 0.001). With assessment for interaction between sodium channel blockade and etiology, we identified an estimated 7-fold increased risk of epileptic spasms with the combination of sodium channel blockade and high-risk etiology (hazard ratio = 7.0, 95% CI 2.5-19.8; P < 0.001).

CONCLUSION

This study suggests that voltage-gated sodium channel blockade may induce epileptic spasms among children at risk on the basis of etiology. Further study is warranted to replicate these findings, ascertain possible drug- and dose-specific risks, and identify potential mechanisms of harm.

Identification of CNS-Penetrant Aryl Sulfonamides as Isoform-Selective NaV1.6 Inhibitors with Efficacy in Mouse Models of Epilepsy

Nonselective antagonists of voltage-gated sodium (Na_V) channels have been long used for the treatment of epilepsies. The efficacy of these drugs is thought to be due to the block of sodium channels on excitatory neurons, primarily Na_V1.6 and Na_V1.2. However, these currently marketed drugs require high drug exposure and suffer from narrow therapeutic indices. Selective inhibition of Na_V1.6, while sparing Na_V1.1, is anticipated to provide a more effective and better tolerated treatment for epilepsies. In addition, block of Na_V1.2 may complement the anticonvulsant activity of Na_V1.6 inhibition. We discovered a novel series of aryl sulfonamides as CNS-penetrant, isoform-selective Na_V1.6 inhibitors, which also displayed potent block of Na_V1.2. Optimization focused on increasing selectivity over Na_V1.1, improving metabolic stability, reducing active efflux, and addressing a pregnane X-receptor liability. We obtained compounds 30-32, which produced potent anticonvulsant activity in mouse seizure models, including a direct current maximal electroshock seizure assay.

Understanding the schizophrenia phenotype in the first patient with the full SCN2A phenotypic spectrum

The sodium voltage-gated channel α subunit 2 (SCN2A) gene encodes a subunit of sodium voltage-gated channels expressed primarily in the central nervous system that are responsible for action potential initiation and propagation in excitable cells. SCN2A mutations underlie a spectrum of distinct phenotypes, including seizure disorders, neurodevelopmental disorders, and rarer instances of episodic ataxia and schizophrenia. We report on a 38-year-old patient with adult-onset psychotic symptoms on a background of infantile-onset seizures, autistic features and episodic ataxia. Whole-exome sequencing revealed a de-novo novel SCN2A mutation (c.4966T > C, p.Ser1656Pro). This and other SCN2A mutations associated with the schizophrenia phenotype overlap those seen in neurodevelopmental disorders, suggesting a common underlying mechanism. This is the first report of a patient with the entire known SCN2A phenotypic spectrum. We highlight the importance of recognizing the psychiatric phenotypes associated with SCN2A mutations and that the phenotypic spectrum is more fluid, and less categorical, than previously thought.

Further corroboration of distinct functional features in SCN2A variants causing intellectual disability or epileptic phenotypes

BACKGROUND

Deleterious variants in the voltage-gated sodium channel type 2 (Na_v1.2) lead to a broad spectrum of phenotypes ranging from benign familial neonatal-infantile epilepsy (BFNIE), severe developmental and epileptic encephalopathy (DEE) and intellectual disability (ID) to autism spectrum disorders (ASD). Yet, the underlying mechanisms are still incompletely understood.

METHODS

To further elucidate the genotype-phenotype correlation of SCN2A variants we investigated the functional effects of six variants representing the phenotypic spectrum by whole-cell patch-clamp studies in transfected HEK293T cells and in-silico structural modeling.

RESULTS

The two variants p.L1342P and p.E1803G detected in patients with early onset epileptic encephalopathy (EE) showed profound and complex changes in channel gating, whereas the BFNIE variant p.L1563V exhibited only a small gain of channel function. The three variants identified in ID patients without seizures, p.R937C, p.L611Vfs*35 and p.W1716*, did not produce measurable currents. Homology modeling of the missense variants predicted structural impairments consistent with the electrophysiological findings.

CONCLUSIONS

Our findings support the hypothesis that complete loss-of-function variants lead to ID without seizures, small gain-of-function variants cause BFNIE and EE variants exhibit variable but profound Na_v1.2 gating changes. Moreover, structural modeling was able to predict the severity of the variant impact, supporting a potential role of structural modeling as a prognostic tool. Our study on the functional consequences of SCN2A variants causing the distinct phenotypes of EE, BFNIE and ID contributes to the elucidation of mechanisms underlying the broad phenotypic variability reported for SCN2A variants.