Two KCNQ2 Encephalopathy Variants in the Calmodulin-Binding Helix A Exhibit Dominant-Negative Effects and Altered PIP2 Interaction

Abstract

Heterozygous missense variants in KCNQ2 , which encodes the potassium channel subunit Kv7.2, are among the most common genetic causes of severe neonatal-onset epileptic encephalopathy. Because about 20% of known severe Kv7.2 missense changes lie within the intracellular C-terminal region, improving understanding of the underlying pathogenic mechanisms is important. We analyzed the basis for the severe phenotypes of Kv7.2 A337T and A337G, variants in the C-terminal’s calmodulin (CaM)-binding Helix A. When expressed heterologously in mammalian cells, alone or in combination with wild type Kv7.2 or with wild type Kv7.2 and Kv7.3, both variants strongly suppressed channel currents. A337T channels expressed alone exhibited significantly reduced protein half-life and surface trafficking and co-immunoprecipitated less CaM. For both variants, increasing cellular phosphatidylinositol 4,5-bisphosphate (PIP 2 ) by overexpression of PI(4)P5-kinase restored current densities. For both variants, the fraction of current suppressed by activation of M1 muscarinic receptors with 10 μM oxotremorine methiodide, which depletes PIP 2 , was less than for controls. During voltage-sensitive phosphatase-induced transient PIP 2 depletion and resynthesize, potassium current inhibition and recovery kinetics were both markedly slowed. These results suggest that these variants may reduce currents by a mechanism not previously described: slowing of PIP 2 migration between the bulk membrane and binding sites mediating channel electromechanical coupling. A novel Kv7.2/3-selective opener, SF0034, rescued current amplitudes. Our findings show that these two Helix A variants suppress channel current density strongly, consistent with their severe heterozygous phenotypes, implicate impairment of CaM and PIP 2 regulation in KCNQ2 encephalopathy pathogenesis, and highlight the potential usefulness of selective Kv7 openers for this distinctive pathogenic mechanism and patient subgroup.

High-throughput evaluation of epilepsy-associated KCNQ2 variants reveals functional and pharmacological heterogeneity

“…KCNQ2 loss-of-function has many potential underlying mechanisms, including impaired trafficking to the plasma membrane, impaired subcellular localization, impaired ion conduction, and impaired activation gating ( 12 , 21 , 39 , 40 , 49 , 50 ), which are not mutually exclusive. Investigating surface expression and subcellular targeting of the variants in our study may help distinguish among these possibilities.…”

“…Electroporated cells were thawed and plated onto glass cover slips, then maintained initially at 37°C and subsequently at 28°C as described for automated patch clamp experiments. Recordings were obtained using the same internal solution used for automated patch clamp and with internal solutions used previously for manual recordings ( 50 ). There were no differences in channel behavior between the 2 internal solutions.…”

“…Manual patch recordings were obtained using an Axon 200B amplifier and pClamp 9 software (Molecular Devices, Sunnyvale, CA) as previously described (50). Electroporated cells were thawed and plated onto glass cover slips, then maintained initially at 37°C and subsequently 28°C as described for automated patch clamp experiments.…”

High-throughput Evaluation of Epilepsy-associated KCNQ2 Variants Reveals Functional and Pharmacological Heterogeneity

“…Recordings were obtained using the same internal solution used for automated patch clamp and with internal solutions used previously for manual recordings. 30 There were no differences in channel behavior between the two internal solutions.…”

“…Manual patch recordings were obtained using an Axon 200B amplifier and pClamp 9 software (Molecular Devices, Sunnyvale, CA) as previously described. 30 Electroporated cells were thawed and plated onto glass cover slips, then maintained initially at 37°C and subsequently 28°C as described for automated patch clamp experiments. Recordings were obtained using the same internal solution used for automated patch clamp and with internal solutions used previously for manual recordings.…”