Clinical management of epileptic encephalopathies of childhood and infancy

Both gain-of-function and loss-of-function de novo CACNA1A mutations cause severe developmental epileptic encephalopathies in the spectrum of Lennox-Gastaut syndrome

OBJECTIVE

Developmental epileptic encephalopathies (DEEs) are genetically heterogeneous severe childhood-onset epilepsies with developmental delay or cognitive deficits. In this study, we explored the pathogenic mechanisms of DEE-associated de novo mutations in the CACNA1A gene.

METHODS

We studied the functional impact of four de novo DEE-associated CACNA1A mutations, including the previously described p.A713T variant and three novel variants (p.V1396M, p.G230V, and p.I1357S). Mutant cDNAs were expressed in HEK293 cells, and whole-cell voltage-clamp recordings were conducted to test the impacts on Ca_V 2.1 channel function. Channel localization and structure were assessed with immunofluorescence microscopy and three-dimensional (3D) modeling.

RESULTS

We find that the G230V and I1357S mutations result in loss-of-function effects with reduced whole-cell current densities and decreased channel expression at the cell membrane. By contrast, the A713T and V1396M variants resulted in gain-of-function effects with increased whole-cell currents and facilitated current activation (hyperpolarized shift). The A713T variant also resulted in slower current decay. 3D modeling predicts conformational changes favoring channel opening for A713T and V1396M.

SIGNIFICANCE

Our findings suggest that both gain-of-function and loss-of-function CACNA1A mutations are associated with similarly severe DEEs and that functional validation is required to clarify the underlying molecular mechanisms and to guide therapies.

Treatment of Seizures Associated with Lennox-Gastaut and Dravet Syndromes: A Focus on Cannabidiol Oral Solution

Cannabidiol oral solution for seizures associated with Lennox-Gastaut and Dravet syndromes.

A novel epileptic encephalopathy mutation in KCNB1 disrupts Kv2.1 ion selectivity, expression, and localization

A missense mutation in the pore-forming α subunit of a delayed rectifier Kv channel is associated with epileptic encephalopathy, alters the cation selectivity of voltage-gated currents, and disrupts channel expression and localization.

The epileptic encephalopathies are a group of highly heterogeneous genetic disorders. The majority of disease-causing mutations alter genes encoding voltage-gated ion channels, neurotransmitter receptors, or synaptic proteins. We have identified a novel de novo pathogenic K⁺ channel variant in an idiopathic epileptic encephalopathy family. Here, we report the effects of this mutation on channel function and heterologous expression in cell lines. We present a case report of infantile epileptic encephalopathy in a young girl, and trio-exome sequencing to determine the genetic etiology of her disorder. The patient was heterozygous for a de novo missense variant in the coding region of the KCNB1 gene, c.1133T>C. The variant encodes a V378A mutation in the α subunit of the Kv2.1 voltage-gated K⁺ channel, which is expressed at high levels in central neurons and is an important regulator of neuronal excitability. We found that expression of the V378A variant results in voltage-activated currents that are sensitive to the selective Kv2 channel blocker guangxitoxin-1E. These voltage-activated Kv2.1 V378A currents were nonselective among monovalent cations. Striking cell background–dependent differences in expression and subcellular localization of the V378A mutation were observed in heterologous cells. Further, coexpression of V378A subunits and wild-type Kv2.1 subunits reciprocally affects their respective trafficking characteristics. A recent study reported epileptic encephalopathy-linked missense variants that render Kv2.1 a tonically activated, nonselective cation channel that is not voltage activated. Our findings strengthen the correlation between mutations that result in loss of Kv2.1 ion selectivity and development of epileptic encephalopathy. However, the strong voltage sensitivity of currents from the V378A mutant indicates that the loss of voltage-sensitive gating seen in all other reported disease mutants is not required for an epileptic encephalopathy phenotype. In addition to electrophysiological differences, we suggest that defects in expression and subcellular localization of Kv2.1 V378A channels could contribute to the pathophysiology of this KCNB1 variant.

Management of genetic epilepsies: From empirical treatment to precision medicine

Despite the over 20 antiepileptic drugs (AEDs) now licensed for epilepsy treatment, seizures can be effectively controlled in about ∼70% of patients. Thus, epilepsy treatment is still challenging in about one third of patients and this may lead to a severe medically, physically, and socially disabling condition. However, there is clear evidence of heterogeneity of response to existing AEDs and a significant unmet need for effective intervention. A number of studies have shown that polymorphisms may influence the poor or inadequate therapeutic response as well as the occurrence of adverse effects. In addition, the new frontier of genomic technologies, including chromosome microarrays and next-generation sequencing, improved our understanding of the genetic architecture of epilepsies. Recent findings in some genetic epilepsy syndromes provide insights into mechanisms of epileptogenesis, unrevealing the role of a number of genes with different functions, such as ion channels, proteins associated to the vesical synaptic cycle or involved in energy metabolism. The rapid progress of high-throughput genomic sequencing and corresponding analysis tools in molecular diagnosis are revolutionizing the practice and it is a fact that for some monogenic epilepsies the molecular confirmation may influence the choice of the treatment. Moreover, the novel genetic methods, that are able to analyze all known genes at a reasonable price, are of paramount importance to discover novel therapeutic avenues and individualized (or precision) medicine.