Generalized epilepsy with febrile seizures plus–associated sodium channel β1 subunit mutations severely reduce beta subunit–mediated modulation of sodium channel function

Two novel mutations (R85C and R85H) on the extracellular immunoglobulin-like domain of the sodium channel β1 subunit have been identified in individuals from two families with generalized epilepsy with febrile seizures plus (GEFS+). The functional consequences of these two mutations were determined by co-expression of the human brain NaV1.2 α subunit with wild type or mutant β1 subunits in human embryonic kidney (HEK)-293T cells. Patch clamp studies confirmed the regulatory role of β1 in that relative to NaV1.2 alone the NaV1.2+β1 currents had right-shifted voltage dependence of activation, fast and slow inactivation and reduced use dependence. In addition, the NaV1.2+β1 current entered fast inactivation slightly faster than NaV1.2 channels alone. The β1(R85C) subunit appears to be a complete loss of function in that none of the modulating effects of the wild type β1 were observed when it was co-expressed with NaV1.2. Interestingly, the β1(R85H) subunit also failed to modulate fast kinetics, however, it shifted the voltage dependence of steady state slow inactivation in the same way as the wild type β1 subunit. Immunohistochemical studies revealed cell surface expression of the wild type β1 subunit and undetectable levels of cell surface expression for both mutants. The functional studies suggest association of the β1(R85H) subunit with the α subunit where its influence is limited to modulating steady state slow inactivation. In summary, the mutant β1 subunits essentially fail to modulate α subunits which could increase neuronal excitability and underlie GEFS+ pathogenesis.