Abstracts

Rationale: Dravet Syndrome (DS) is a severe form of epilepsy with a high rate of sudden unexpected death in epilepsy (SUDEP). Respiratory failure is a leading cause of SUDEP, despite this it is not clear how DS-associated genetic variants or seizure activity in general disrupts respiratory control. Most DS cases are caused by mutations in the Scn1a gene, which encodes a voltage-gated Na+ channel that preferentially regulates activity of inhibitory neurons early in development. Consistent with this, we showed that loss of Scn1a function in inhibitory neurons disrupted CO2/H+-dependent regulation of breathing by the retrotrapezoid nucleus (i.e., RTN chemoreception) and caused seizures and premature death. However, Scn1a is also expressed by excitatory neurons particularly later in development, thus indicating loss of Scn1a function in excitatory neurons contributes to progression of DS symptoms. Therefore, the goal of this work is to determine whether and how respiratory function is perturbed across development in a global Scn1a haploinsuficient (Scn1a+/-) mouse model of DS.

Methods: Scn1a+/- and litter mate controls were maintained on 50:50 background of 129S1/SvlmJ::C57BL6/J. Respiratory activity was measured in the same animals at 13 and 21-22 days of age by whole-body plethysmography during exposure to room air, high CO2 (3-7% CO2) or low O2 (10% O2). At the cellular level, baseline activity and CO2/H+-sensitivity of RTN neurons in brain slices obtained from each genotype was characterized in cell-attached voltage clamp mode.

Results: Neonatal Scn1a+/- mice breath normally in room air but show a blunted ventilatory response to graded increases in CO2. Note all mice < 2 weeks of age do not increase breathing in response to hypoxia. At the cellular level, RTN neurons in slices from Scn1a+/- mice show increased baseline activity and lower delta CO2 compared to RTN neurons from control tissue. We also found the ventilatory response to CO2 improved in these same mice at a later developmental time point. However, breathing problems persisted in juvenile Scn1a+/- as a blunted ventilatory response to hypoxia and importantly this respiratory deficit positively correlated with mortality; mice that died by 24 days of age had a diminished hypoxic ventilatory response compared to Scn1a+/- that survived.

Conclusions: Loss of Scn1a preferentially disrupts the ventilatory response to CO2 at the cellular and whole animal levels early in development. These results are similar to the phenotype of mice with inhibitory neuron specific Scn1a disruption, suggesting a dominate role of Scn1a in inhibitory neurons at this developmental time point. We also show that a blunted hypoxic ventilatory response correlates with premature death in juvenile Scn1a+/-, suggesting multiple aspects of respiratory control are compromised in DS. These results identify diminished ventilatory responses to CO2 and O2 and early biomarkers of SUDEP in DS, suggesting that mechanisms contributing to CO2 and O2 chemotransduction can be targeted to improve breathing and mortality in DS.

Funding: Please list any funding that was received in support of this abstract.: NIH/NINDS F31 NS120467.