Residency · Residency · Medical Genetics Genomics

Epilepsy Genetics: From Channelopathies to Precision Treatment

Introduction

Epilepsy affects approximately 1% of the population, and genetic factors contribute to at least 70-80% of cases. The genetic landscape ranges from monogenic channelopathies and developmental-epileptic encephalopathies (DEEs) to complex polygenic susceptibility in common epilepsy syndromes. Precision medicine approaches are increasingly enabling genotype-guided treatment selection, particularly in severe early-onset epilepsies.

Genetic Architecture of Epilepsy

Monogenic Epilepsies

Monogenic forms account for the majority of developmental and epileptic encephalopathies, with over 200 genes now associated with monogenic epilepsy. Most severe forms are caused by de novo dominant variants, though autosomal recessive and X-linked forms are also recognized.

Complex/Polygenic Epilepsy

The genetic generalized epilepsies (GGE), including juvenile myoclonic epilepsy and childhood absence epilepsy, follow a complex inheritance pattern. Common variants with small effect sizes collectively increase risk. GWAS have identified susceptibility loci near ion channel and synaptic genes, with heritability estimated at 70-80% for GGE.

Somatic Mosaicism

Brain somatic mutations in mTOR pathway genes cause focal cortical dysplasia. These variants are detectable only in affected brain tissue, not in blood. Key genes include MTOR, PIK3CA, AKT3, and TSC1/TSC2.

Ion Channel Genes (Channelopathies)

Sodium Channels

SCN1A causes Dravet syndrome (severe myoclonic epilepsy of infancy) through loss-of-function variants. SCN2A produces early-onset DEE through gain-of-function variants or later-onset epilepsy/ASD through loss-of-function variants. SCN8A causes severe early-onset DEE with gain-of-function variants.

Potassium Channels

KCNQ2 is associated with both benign familial neonatal epilepsy (mild form) and severe DEE. KCNQ3 causes benign familial neonatal epilepsy. KCNA2 produces episodic ataxia with epilepsy. KCNT1 causes epilepsy of infancy with migrating focal seizures through gain-of-function mechanisms.

Other Channels

CACNA1A is associated with absence epilepsy and episodic ataxia type 2. GABA-A receptor subunit genes (GABRA1, GABRG2, GABRB3) are implicated in absence and febrile seizures. NMDA receptor subunits GRIN2A and GRIN2B are associated with the epilepsy-aphasia spectrum.

Non-Channel Monogenic Epilepsies

Synaptic and Signaling Genes

STXBP1 (Munc18-1) causes early-onset DEE and is one of the most commonly identified causes. SYNGAP1 produces ID with epilepsy characterized by eyelid myoclonia with absences. CDKL5 causes early-onset seizures with stereotypies and predominantly affects females. PCDH19 causes seizure clusters with fever affecting heterozygous females through a cellular interference model. SLC6A1 is associated with myoclonic-atonic epilepsy.

mTOR Pathway

TSC1 and TSC2 cause tuberous sclerosis complex with cortical tubers and subependymal nodules. Focal cortical dysplasia type II results from somatic mutations in MTOR, DEPDC5, and NPRL3.

Metabolic Epilepsies

ALDH7A1 causes pyridoxine-dependent epilepsy, treatable with vitamin B6. PNPO causes pyridoxal phosphate-responsive epilepsy. SLC2A1 (GLUT1) deficiency presents with absence seizures and movement disorder, treated with the ketogenic diet. FOLR1 causes cerebral folate deficiency, treatable with folinic acid.

Precision Treatment Based on Genotype

SCN1A (Dravet Syndrome)

Sodium channel blockers (carbamazepine, phenytoin, lamotrigine, oxcarbazepine) are contraindicated as they worsen seizures. Preferred agents include valproate, clobazam, stiripentol, fenfluramine, and cannabidiol.

SCN2A and SCN8A (Gain-of-Function)

In contrast to SCN1A, sodium channel blockers (phenytoin, carbamazepine) may be beneficial for gain-of-function variants. Distinguishing gain-of-function from loss-of-function is critical for treatment selection.

KCNQ2

KCNQ2-related epilepsy responds to carbamazepine and other sodium channel blockers. Phenobarbital may be less effective.

KCNT1

Quinidine, a potassium channel blocker, represents a theoretical precision therapy with variable clinical efficacy.

TSC1/TSC2

Everolimus, an mTOR inhibitor, is FDA-approved for TSC-associated seizures and subependymal giant cell astrocytomas. Vigabatrin is first-line for infantile spasms in TSC.

SLC2A1 (GLUT1 Deficiency)

The ketogenic diet provides alternative brain fuel through ketone bodies, bypassing the defective glucose transport. Valproate should be avoided as it inhibits fatty acid oxidation and counteracts the ketogenic diet.

Genetic Testing Strategy

When to Test

Genetic testing has the highest yield in DEE and early-onset severe epilepsy (30-50%), epilepsy with intellectual disability or dysmorphic features, epilepsy with a family history suggesting monogenic inheritance, treatment-resistant epilepsy, and epilepsy with associated neuroimaging findings such as cortical malformations or tubers.

Testing Approach

Epilepsy gene panels (100-500+ genes) are first-line in many centers. Exome sequencing offers a broader approach when panels are negative or the phenotype is atypical. Genome sequencing captures structural variants and non-coding regions. Chromosomal microarray is indicated for epilepsy with ID or developmental delay. Somatic testing of resected brain tissue is appropriate for focal cortical dysplasia.

Diagnostic Yield

Comprehensive genetic testing achieves a diagnostic yield of 30-50% in DEE, less than 5% for monogenic causes in GGE (common epilepsy), and up to 30% for somatic mTOR variants in surgically resected FCD type II tissue.

Counseling Considerations

Most severe DEEs are caused by de novo variants, carrying a recurrence risk of approximately 1-2%. Gonadal mosaicism is a recognized phenomenon, particularly for SCN1A and CDKL5. PCDH19 has unique inheritance where heterozygous females are affected while hemizygous males are typically unaffected. A genetic diagnosis may influence reproductive decision-making and prenatal testing options.

Clinical Pearls

SCN1A is the single most important gene in epilepsy genetics -- sodium channel blockers are contraindicated in Dravet syndrome and can precipitate status epilepticus. Distinguishing gain-of-function from loss-of-function variants in channel genes such as SCN2A is essential for precision treatment selection. Metabolic epilepsies (pyridoxine-dependent epilepsy, GLUT1 deficiency) are treatable conditions, and an empiric vitamin B6 trial should always be considered in neonatal seizures of unclear etiology. Genetic diagnosis changes management in approximately 30-40% of patients with DEE, making early testing cost-effective.

References

1. Symonds JD, Zuberi SM, Stewart K, et al. Incidence and phenotypes of childhood-onset genetic epilepsies: a prospective population-based national cohort. Brain. 2019;142(8):2303-2318.
2. Scheffer IE, Berkovic S, Capovilla G, et al. ILAE classification of the epilepsies: position paper of the ILAE Commission for Classification and Terminology. Epilepsia. 2017;58(4):512-521.
3. Brunklaus A, Ellis R, Reavey E, Forbes GH, Zuberi SM. Prognostic, clinical and demographic features in SCN1A mutation-positive Dravet syndrome. Brain. 2012;135(8):2329-2336.
4. Koenig MK, Bell CS, Hebert AA, et al. Efficacy and safety of everolimus for subependymal giant cell astrocytomas associated with tuberous sclerosis complex. JAMA Neurol. 2024;81(2):178-186.

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