Heteromeric assembly of truncated neuronal Kv7 channels: implications for neurological disease and pharmacotherapy.

Neuronal voltage-gated potassium channels (Kv) are critical regulators of electrical activity in the central nervous system. Mutations in the KCNQ (Kv7) ion channel family are linked to epilepsy, seizures, and neurodevelopmental disorders. These channels underlie the neuronal 'M-current', and cluster in the axon initial segment to regulate the firing of action potentials. There is general consensus that KCNQ channel assembly and clustering is controlled by the C-terminus, specifically in the distal C-terminal helices, which are reported to influence channel assembly and subunit specificity. We investigated a pediatric patient with neurodevelopmental disability including autism traits, inattention and hyperactivity, and ataxia, who carries a de novo frameshift mutation in KCNQ3 (KCNQ3-FS534), leading to truncation of ~300 amino acids in the C-terminus. We investigated possible molecular mechanisms of channel dysfunction, including haplo-insufficiency, or a dominant-negative effect caused by the assembly of truncated KCNQ3 and functional KCNQ2 subunits. We also used a recently recognized property of the KCNQ2-specific activator ICA-069673 to identify assembly of heteromeric channels. ICA-069673 exhibits a functional signature that depends on the subunit composition of KCNQ2/3 channels, allowing us to determine whether truncated KCNQ3 subunits can assemble with KCNQ2. Our findings demonstrate that although the KCNQ3-FS534 mutant does not generate functional channels on its own, large C-terminal truncations of KCNQ3 (including the KCNQ3-FS534 mutation) assemble efficiently with KCNQ2, but fail to promote/stabilize KCNQ2/KCNQ3 heteromeric channel expression. Therefore, the frequent assumption that pathologies linked to KCNQ3 truncations arise from haploinsufficiency should be reconsidered in some cases. The use of subtype specific channel activators like ICA-069673 provides a reliable tool to identify heteromeric assembly of KCNQ3 truncation mutants. SIGNIFICANCE STATEMENT: Mutated of neuronal Kv7/KCNQ channels underlie a spectrum of neurological diseases. This study uses a subtype-specific Kv7 channel activator to investigate the assembly of disease-linked and synthetic KCNQ3 truncation mutants with KCNQ2. This study demonstrates that large C-terminal truncations influence expression, but not assembly, of KCNQ2/KCNQ3 heteromeric channels. The American Society for Pharmacology and Experimental Therapeutics.