Heteromeric Assembly of Truncated Neuronal Kv7 Channels: Implications for Neurologic 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 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 heteromerization are controlled by C-terminal helices. We identified 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 or stabilize KCNQ2/KCNQ3 heteromeric channel expression. Therefore, the frequent assumption that pathologies linked to KCNQ3 truncations arise from haplo-insufficiency should be reconsidered in some cases. Subtype-specific channel activators like ICA-069673 are a reliable tool to identify heteromeric assembly of KCNQ2 and KCNQ3. SIGNIFICANCE STATEMENT: Mutations that truncate the C terminus of neuronal Kv7/KCNQ channels are linked to a spectrum of seizure disorders. One role of the multifunctional KCNQ C terminus is to mediate subtype-specific assembly of heteromeric KCNQ channels. This study describes the use of a subtype-specific Kv7 activator to assess assembly of heteromeric KCNQ2/KCNQ3 (Kv7.2/Kv7.3) channels and demonstrates that large disease-linked and experimentally generated C-terminal truncated KCNQ3 mutants retain the ability to assemble with KCNQ2.