Pathophysiology of KCNQ Channels: Neonatal Epilepsy and Progressive Deafness

Over the past 10 years, an increasing number of inherited diseases were shown to be due to mutations in ion channels. Given the importance of ion channels for the signal transduction in nerve and muscle, many of these “channelopathies” affect the neuromuscular system. For instance, mutations in several ion channels cause the long-QT syndrome (LQTS), which is characterized by a prolongation of the QT interval in ECGs and leads to potentially fatal cardiac arrhythmias. One of these channels is the KCNQl (or KvLQTl) potassium channel (1). It is expressed in several organs, including the heart, cochlea, and intestine, but is virtually absent from brain. Mutations in the classic autosomal dominant form of the LQTS have a dominant negative effect on wild-type subunits, leading to a predicted 90% loss of function of channel activity. interestingly, recessive mutations in KCNQl cause the Jervell and Lange-Nielsen syndromes, whose symptoms include congenital deafness in addition to cardiac arrhythmias. Thus the total loss of function with mutations on both alleles leads to additional symptoms. By homology screening with KCNQ1, we isolated the novel potassium channels KCNQ2 and KCNQ3 (2). We localized KCNQ3 to human chromosome 8q24, and our collaborator Ortrud Steinlein (Bonn) had already localized KCNQ2 to 20q13. intriguingly, this coincided with two known loci (3,4) for benign familial neonatal convulsions (BFNCs). This autosomal dominant neonatal epilepsy is characterized by generalized seizures that start around day 3 of life and generally disappear after 3-4 weeks, although -15% of patients experience seizures later in life as well. The localization of these potassium channels to both BFNC loci made them excellent candidate genes for that disease. Together with 0. Steinlein, we indeed identified a mutation in a large Australian BFNC pedigree we obtained from Samuel Berkovic (Melbourne). The mutation caused a frame-shift, which led to a carboxy-terminal deletion of the predicted channel protein (2). Functional expression of KCNQ2 in Xenopus oocytes gave rather slowly activating, outwardly rectifying potassium currents that were abolished by the mutation. To mimic the situation of a heterozygous patient of that dominant disease, we co-expressed WT and mutant KCNQ2 cRNAs in Xeizr~pus oocytes. To our surprise, we did not find a dominant negative effect, suggesting that haploinsufficiency explains the dominant mode of inheritance (2). i n parallel to our work, Mark Leppert et ail. have found other mutations in KCNQ2 ( 5 ) and in KCNQ3 in the single available family (6). Interestingly, in several cases, the KCNQ2 gene showed large deletions, consistent with our conclusion that haploinsufficiency is sufficient to explain the dominant pattern of inheritance. We next showed that both KCNQ2 and KCNQ3 are broadly and specifically expressed in brain, where their expression is largely overlapping (7). Functional expression showed that KCNQ2 currents are strikingly enhanced by coexpressing KCNQ3, indicating the formation of functional KCNQ2/KCNQ3 heteromeric channels. Functional analysis of KCNQ2 and KCNQ3 mutations that cause epilepsy indicated only a slight loss of function of that heteromeric channel (on the order of 20-3076) (7), indicating that the activity of this channel is very close to the pathologic threshold during this period of life. Interestingly, KCNQ2/KCNQ3 channel activity could be stimulated by raising cyclic adenosine monophosphate (CAMP), which probably phosphorylates an aminoterminal consensus site for PKA. Because this stimulation is on the order of 50%, this suggests new strategies to treat epilepsy (7). By using a detailed kinetic analysis and specific inhibitors, David McKinnon et al. (8) could show in parallel studies that KCNQ2/KCNQ3 channels might be the molecular correlate of the M-current. This makes perfect sense, as this current, which is regulated by several receptor-ligand systems, is an important determinant of neuronal excitability. The slight loss of KCNQ2/3 channel activity observed in BFNC will lead to an hyperexcitability of certain neurons and will thereby cause epilepsy. We subsequently cloned another novel potassium channel, KCNQ4, by homology to KCNQ3, and localized it to human chromosome lp34 (9). An autosomal