Effect of an autism-associated KCNMB2 variant, G124R, on BK channel properties

BK K(+) channels are critical regulators of neuron and muscle excitability, comprised of a tetramer of pore-forming αsubunits from the KCNMA1 gene and cell- and tissue-selective β subunits (KCNMB1-4). Mutations in KCNMA1 are associated with neurological disorders, including autism. However, little is known about the role of neuronal BK channel β subunits in human neuropathology. The β2 subunit is expressed in central neurons and imparts inactivation to BK channels, as well as altering activation and deactivation gating. In this study, we report the functional effect of G124R, a novel KCNMB2 mutation obtained from whole-exome sequencing of a patient diagnosed with autism spectrum disorder. Residue G124, located in the extracellular loop between TM1 and TM2, is conserved across species, and the G124R missense mutation is predicted deleterious with computational tools. To investigate the pathogenicity potential, BK channels were co-expressed with β2(WT) and β2(G124R) subunits in HEK293T cells. BK/β2 currents were assessed from inside-out patches under physiological K(+) conditions (140/6 mM K(+) and 10 μM Ca(2+)) during activation and inactivation (voltage-dependence and kinetics). Using β2 subunits lacking inactivation (β2IR) revealed that currents from BK/β2IR(G124R) channels activated 2-fold faster and deactivated 2-fold slower compared with currents from BK/β2IR(WT) channels, with no change in the voltage-dependence of activation (V(1/2)). Despite the changes in the BK channel opening and closing, BK/β2(G124R) inactivation rates (τ(inact) and τ(recovery)), and the V(1/2) of inactivation, were unaltered compared with BK/β2(WT) channels under standard steady-state voltage protocols. Action potential-evoked current was also unchanged. Thus, the mutant phenotype suggests the β2(G124R) TM1-TM2 extracellular loop could regulate BK channel activation and deactivation kinetics. However, additional evidence is needed to validate pathogenicity for this patient-associated variant in KCNMB2.