Developmental and epileptic encephalopathies (DEE) are a heterogeneous group of 
disorders characterized by epilepsy with comorbid intellectual disability. 
Recently, two de novo heterozygous mutations in the gene encoding TRPM3, a 
calcium permeable ion channel, were identified as the cause of DEE in eight 
probands, but the functional consequences of the mutations remained elusive. 
Here we demonstrate that both mutations (V990M and P1090Q) have distinct effects 
on TRPM3 gating, including increased basal activity, higher sensitivity to 
stimulation by the endogenous neurosteroid pregnenolone sulfate (PS) and heat, 
and altered response to ligand modulation. Most strikingly, the V990M mutation 
affected the gating of the non-canonical pore of TRPM3, resulting in large 
inward cation currents via the voltage sensor domain in response to PS 
stimulation. Taken together, these data indicate that the two DEE mutations in 
TRPM3 result in a profound gain of channel function, which may lie at the basis 
of epileptic activity and neurodevelopmental symptoms in the patients.