Abstracts

Rationale: Sudden unexpected death in epilepsy (SUDEP) is the leading cause of epilepsy-related mortality, but its genetic etiology is largely unknown and likely complex involving multiple genes. The Kcna1 gene encodes Kv1.1 potassium channels that act to dampen neuronal excitability, whereas the Scn2a gene encodes Nav1.2 sodium channels important for action potential conduction. In the Kcna1-null mouse model of SUDEP, we investigated the ability of subclinical Scn2a-null heterozygosity to act as a protective genetic modifier of epilepsy and premature death. In particular, we tested the hypothesis that subclinical Scn2a heterozygosity reduces SUDEP incidence in Kcna1-null mice by suppressing neurocardiac dysfunction associated with the absence of Kv1.1 channels. Methods: Age-matched mice (n=4/genotype) with varying Scn2a and Kcna1 null allele combinations were implanted with thoracic ECG electrodes and bilateral EEG electrodes overlying temporal cortex. Simultaneous EEG and ECG activity was recorded for a continuous 18-24 hour period and neurocardiac abnormalities were quantified. Each recording was divided into segments of 10 s and the following variables were estimated: 1) brain connectivity across channels at traditional EEG bands (coherence C(band)); 2) complexity of EEG and ECG signals (Shannon's Entropy - ENTR); and 3) heart rate variability (the median (M) and Inter Quartile Range (IQR) of the R peak amplitudes and length of RR intervals from QRS complexes in ECG). The degree of association of the extracted features from the EEG and ECG over the whole record was then evaluated using the φ coefficient. Statistically significant differences between the genotypes were assessed with a t-test on the φ coefficient values. Results: SUDEP-prone mice lacking Kv1.1 channels exhibited increased survival when heterozygous for the Scn2a-null allele, that is Scn2a^+/-, Kcna1^-/-. Heterozygosity for the Scn2a mutation did not eliminate seizures but did exhibit a trend towards decreased seizure burden (Fig. 1A). Analysis of EEG and ECG revealed a statistically significant (p<0.05) higher degree of association between neural and cardiac activity in Scn2a^+/-, Kcna1^-/- mice (SUDEP low-risk) compared to Kcna1^-/- (SUDEP high-risk) animals (Fig. 1B). This result was very robust and it was shown by several measures employed (Table I). Conclusions: The subclinical Scn2a-null allele acts as a protective genetic modifier of epilepsy and SUDEP in Kcna1-null mice. The increased EEG-ECG association in Scn2a^+/-, Kcna1^-/- mice suggests that Scn2a-null heterozygosity alters neural control of the heart and reduces SUDEP risk. These findings are significant because they expand our understanding of the complex genetic interactions underlying SUDEP and identify EEG-ECG association as a potential new biomarker of SUDEP susceptibility. This work is supported by National Institute of Neurological Disorders and Stroke grant NS089397 (E.G.).