Molecular correlates of age-dependent seizures in an inherited neonatal-infantile epilepsy

Abstract

Many idiopathic epilepsy syndromes have a characteristic age dependence, the underlying molecular mechanisms of which are largely unknown. Here we propose a mechanism that can explain that epileptic spells in benign familial neonatal-infantile seizures occur almost exclusively during the first days to months of life. Benign familial neonatal-infantile seizures are caused by mutations in the gene SCN2A encoding the voltage-gated Na(+) channel Na(V)1.2. We identified two novel SCN2A mutations causing benign familial neonatal-infantile seizures and analysed the functional consequences of these mutations in a neonatal and an adult splice variant of the human Na(+) channel Na(V)1.2 expressed heterologously in tsA201 cells together with beta1 and beta2 subunits. We found significant gating changes leading to a gain-of-function, such as an increased persistent Na(+) current, accelerated recovery from fast inactivation or altered voltage-dependence of steady-state activation. Those were restricted to the neonatal splice variant for one mutation, but more pronounced for the adult form for the other, suggesting that a differential developmental splicing does not provide a general explanation for seizure remission. We therefore analysed the developmental expression of Na(V)1.2 and of another voltage-gated Na(+) channel, Na(V)1.6, using immunohistochemistry and real-time reverse transcription-polymerase chain reaction in mouse brain slices. We found that Na(V)1.2 channels are expressed early in development at axon initial segments of principal neurons in the hippocampus and cortex, but their expression is diminished and they are gradually replaced as the dominant channel type by Na(V)1.6 during maturation. This finding provides a plausible explanation for the transient expression of seizures that occur due to a gain-of-function of mutant Na(V)1.2 channels.

The Fibroblast Growth Factor 14·Voltage-gated Sodium Channel Complex Is a New Target of Glycogen Synthase Kinase 3 (GSK3)

“…Coronal sections (5-10 m) were serially cut using a crytostat, mounted on Superfrost glass microscope slides (Fisher Scientific, Waltham, MA), and left to dry overnight at room temperature. Sections were then fixed in fresh acetone at Ϫ20°C for 8 -10 min and dried at room temperature for 30 min, as described previously (24). Samples were washed three times with TBS, blocked for 1 h in TBS containing 0.3% Triton X-100 and 3% normal goat serum, and incubated overnight at 4°C with the following primary antibodies: mouse anti-FGF14 (monoclonal 1:100; Sigma) and rabbit anti-PanNav (polyclonal 1:100; Sigma) dissolved in TBS containing 1% normal goat serum and 0.3% Triton X-100.…”

Genetic Epilepsy Syndromes Without Structural Brain Abnormalities: Clinical Features and Experimental Models

“…2) is particularly important for the excitability of the axon initial segment in glutamatergic neurons early in development. Mutations causing BFNIS have been studied in both transfected neocortical neurons and cell lines identifying gain of function effects [63,64] consistent with hyperexcitability of excitatory neurons. Remission may depend on a developmental switch between Nav1.2 and Nav1.6 in myelinated axons that occur at early developmental stages [63,64].…”

Epileptic Encephalopathies: New Genes and New Pathways

“…Similar observations have been made for other genes causing epileptic encephalopathies. Mutations of the SCN2A gene were originally described in association with benign familial neonatal-infantile seizures (BFNIS) [106][107][108][109][110], but patients with more severe infantile onset epileptic encephalopathy phenotypes have been reported [111,112]. It was recently determined that the gene STXBP1, responsible for one-third of cases of the early infantile epileptic encephalopathy Ohtahara syndrome [8,113,114], also causes early onset epileptic encephalopathy that does not have the pathognomonic EEG abnormality of Ohtahara syndrome [115].…”