Clinically Relevant Variants Causing KCNQ1-KCNE2 Gain-of-Function Affect the Ca Sensitivity of the Channel.

Dominant variants are well-known for underlying cardiac arrhythmia syndromes. The two heterozygous missense variants, R116L and P369L, cause an allelic disorder characterized by pituitary hormone deficiency and maternally inherited gingival fibromatosis. Increased K conductance upon co-expression of KCNQ1 mutant channels with the beta subunit KCNE2 is suggested to underlie the phenotype; however, the reason for KCNQ1-KCNE2 (Q1E2) channel gain-of-function is unknown. We aimed to discover the genetic defect in a single individual and three family members with gingival overgrowth and identified the variants P369L and V185M, respectively. Patch-clamp experiments demonstrated increased constitutive K conductance of V185M-Q1E2 channels, confirming the pathogenicity of the novel variant. To gain insight into the pathomechanism, we examined all three disease-causing KCNQ1 mutants. Manipulation of the intracellular Ca concentration prior to and during whole-cell recordings identified an impaired Ca sensitivity of the mutant KCNQ1 channels. With low Ca, wild-type KCNQ1 currents were efficiently reduced and exhibited a pre-pulse-dependent cross-over of current traces and a high-voltage-activated component. These features were absent in mutant KCNQ1 channels and in wild-type channels co-expressed with calmodulin and exposed to high intracellular Ca. Moreover, co-expression of calmodulin with wild-type Q1E2 channels and loading the cells with high Ca drastically increased Q1E2 current amplitudes, suggesting that KCNE2 normally limits the resting Q1E2 conductance by an increased demand for calcified calmodulin to achieve effective channel opening. Our data link impaired Ca sensitivity of the KCNQ1 mutants R116L, V185M and P369L to Q1E2 gain-of-function that is associated with a particular KCNQ1 channelopathy.