We identified 2 plausible pathogenic de novo SCN2A
mutations, E1211K and I1473M, in 2 sporadic intractable childhood epilepsy cases. Together with the de novo R102X,18
which we previously discovered in a patient with sporadic intractable childhood epilepsy, these mutations indicate that SCN2A
is a plausible etiologic candidate gene underlying intractable childhood epilepsies.
One de novo mutation, E1211K, was identified in a patient with sporadic infantile spasms that progressed into severe symptomatic generalized epilepsy. Although the patient was born with mild asphyxia, we surmise that it would be insufficient to cause the sporadic infantile spasms and favor the notion that the de novo E1211K is the most possible primary cause. A respiratory arrest caused by a status epilepticus at age 17 years plausibly accounts for the rapid deterioration of his physical and mental condition.
The electrophysiologic analyses revealed that E1211K significantly altered the functional properties of Nav1.2 channel. E1211K caused a large hyperpolarizing shift of the voltage dependence of activation, affecting a reduction in the threshold of depolarization required for activation. E1211K also caused the left shift of the voltage dependence of steady-state inactivation, suggesting that, at the physiologic resting membrane potential (−70 to −60 mV), more than 85% of Nav1.2 carrying E1211K would be in the inactivated state in comparison to ~50% wild-type Nav1.2. Furthermore, E1211K delayed the recovery from inactivated state possibly leading to a reduction in channel availability during repetitive firings. Taken altogether, E1211K produced mutant channels with mixed electrophysiologic properties indicating both augmented and reduced channel activities. Overall, the effects of E1211K on the excitability of neurons are currently difficult to predict. The consequences on the ultimate neuronal excitability might depend on other cellular conditions and the localization in specific neuronal types.
The other de novo mutation, I1473M, was identified in a patient with unusual neonatal epileptic encephalopathy.19
1.2 channel carrying I1473M showed a significant shift of the voltage dependence of activation to the hyperpolarized direction, suggesting that this mutation may increase the channel activity and thereby may cause hyperexcitation of neurons leading to epileptic seizures. Epileptic seizures responded to lidocaine, which might possibly reduce VGSC activities by affecting not only the mutant Nav
1.2 channel but also wild-type Nav
1.2 and other brain-type VGSCs.
We also identified an inherited mutation, A575V, in a patient with SMEB, which was not observed in our 311 healthy control individuals. Given that A575V was found in her asymptomatic father and that the functional effect of A575V on human Nav1.2 was insignificant, A575V is most likely a rare nonpathogenic variant. However, it could be also possible that A575V contributes to the susceptibility to develop epileptic seizures when combined with additional genetic or environmental modifiers, present in the patient but not in the father.
This and our previous studies have provided a total of 3 de novo SCN2A
mutations: E1211K, I1473M, and R102X.18
Notably, these mutations associated with distinct epileptic phenotypes and affected channel properties of Nav
1.2 differentially. There are 2 possible explanations for the phenotypic varieties. First, phenotypic variability could be due to the individual mutation or its location in the Nav
1.2 protein, which could differentially affect protein stability, subcellular trafficking, or channel functions. Alternatively, SCN2A
mutation mosaicism might be related to phenotypic variations. When de novo mutations occur during early development, the patients could be somatic mosaics for their mutations, and therefore brain regions in those patients could be affected differentially, leading to phenotypic variability. In support of this idea, SCN1A
mutation mosaicism occurs in some mildly affected or asymptomatic patients’ parents in several familial SMEI cases.20–23
mutations identified in BFNIS patients so far,14–17
4 were studied using patch-clamp recordings and had Nav
1.2 channels with altered channel properties predicting a gain of function24,25
or with reduced cell surface expression implicating a loss of function.26
Although there was a discrepancy among the results, the amounts of changes in half activation and half inactivation potentials by the BFNIS mutations were within ±6 mV. By contrast, the amounts of changes in half activation and half inactivation potentials in SCN2A
-E1211K were ~−18-mV and ~−22-mV, and that of the half activation potential in SCN2A
-I1473M was ~−14-mV. Thus, both E1211K and I1473M altered the channel properties of Nav
1.2 to a greater extent than the BFNIS mutations, suggesting a mechanism for more severe epileptic phenotypes. In order to confirm these correlations, the effects of the mutations on the functions of the neurons responsible for seizure development, which are currently unknown, should be examined further.
mutant mice with spontaneous epileptic seizures have reduced SCN1A
is prominently expressed in the parvalbumin-positive inhibitory interneurons and we proposed that SCN1A
mutations might cause functional defects in the parvalbumin-positive interneurons, which then fail to suppress overexcitation of neural circuits and result in epileptic seizures.28
In contrast to the case for SCN1A
mutations, the molecular mechanisms underlying seizures caused by SCN2A
mutations remain largely unknown. Since SCN2A
is highly expressed in both principal neurons and interneurons in rat hippocampus,29,30 SCN2A
mutations might alter the global function of the implicated neurons. Although loss or reduced SCN2A
expression did not generate spontaneous epileptic seizures in mice,31
our present study and others’ provide solid genetic evidence implicating SCN2A
in the etiology of human epilepsies.14–18
Further studies using mouse models with the SCN2A
mutations identified in human epilepsies may help to understand the impact of such mutations on the epileptic brain.