Novel gain-of-function mechanism in K+ channel-related Long-QT syndrome: Altered gating and selectivity in the HERG1 N629D mutant

The N629D mutation, adjacent to the GFG signature sequence of the HERG1 A K + channel, causes long-QT syndrome (LQTS). Expression of N629D in Xenopus o ocytes produces a rapidly activating, noninactivating current. N629D is non selective among monovalent cations; permeation of K+ was similar to that of Na+ or Cs+. During repolarization to potentials between -30 and -70 mV, N6 29D manifested an inward tail current, which was abolished by replacement o f extracellular Na+ (Na-e(+)) with extracellular N-methyl-D-glucamine (NMG( e)). Because LQTS occurs in heterozygous patients, we coexpressed N629D and wild type (WT) at equimolar concentrations. Heteromultimer formation was d emonstrated by analyzing the response to 0 [K+](e). The outward time-depend ent current was nearly eliminated for WT at 0 [K+](e), whereas no reduction was observed for homomultimeric N629D or for the equimolar coexpressed cur rent. To assess physiological significance, dofetilide-sensitive currents w ere recorded during application of simulated action potential clamps. Durin g phase 3 repolarization, WT manifested outward currents, whereas homomulti meric N629D manifested inward depolarizing currents. During coexpression st udies, variable phenotypes were observed ranging from a reduction in outwar d repolarizing current to net inward depolarizing current during phase 3. I n summary, N629D replaces the WT outward repolarizing tail current with an inward depolarizing sodium current, which is expected to delay later stages of repolarization and contribute to arrhythmogenesis. Thus, the consequenc es of N629D resemble the pathophysiology seen in LQT3 Na+ channel mutations and may be considered the first LQTS K+ channel mutation that exhibits gai n of function.