Crystal structure of the human HCN4 channel C-terminus carrying the S672R mutation
[HCN4_HUMAN] Sick sinus syndrome;Brugada syndrome. Sick sinus syndrome 2 (SSS2) [MIM:163800]: The term 'sick sinus syndrome' encompasses a variety of conditions caused by sinus node dysfunction. The most common clinical manifestations are syncope, presyncope, dizziness, and fatigue. Electrocardiogram typically shows sinus bradycardia, sinus arrest, and/or sinoatrial block. Episodes of atrial tachycardias coexisting with sinus bradycardia ('tachycardia-bradycardia syndrome') are also common in this disorder. SSS occurs most often in the elderly associated with underlying heart disease or previous cardiac surgery, but can also occur in the fetus, infant, or child without heart disease or other contributing factors. SSS2 onset is in utero or at birth. Note=The disease is caused by mutations affecting the gene represented in this entry.  Brugada syndrome 8 (BRGDA8) [MIM:613123]: A tachyarrhythmia characterized by right bundle branch block and ST segment elevation on an electrocardiogram (ECG). It can cause the ventricles to beat so fast that the blood is prevented from circulating efficiently in the body. When this situation occurs, the individual will faint and may die in a few minutes if the heart is not reset. Note=The disease is caused by mutations affecting the gene represented in this entry.
[HCN4_HUMAN] Hyperpolarization-activated ion channel with very slow activation and inactivation exhibiting weak selectivity for potassium over sodium ions. May contribute to the native pacemaker currents in heart (If) and in neurons (Ih). Activated by cAMP. May mediate responses to sour stimuli. 
Publication Abstract from PubMed
Hyperpolarization-activated, cAMP-gated (HCN) channels sense membrane potential and intracellular cAMP levels. A mutation identified in the cAMP binding domain (CNBD) of the human HCN4 channel, S672R, severely reduces the heart rate, but the molecular mechanism has been unclear. Our biochemical binding assays on isolated CNBD and patch-clamp recordings on the functional channel show that S672R reduces cAMP binding. The crystal structure of the mutant CNBD revealed no global changes except a disordered loop on the cAMP entry path. To address this localized structural perturbation at a whole protein level, we studied the activity-dependent dynamic interaction between cAMP and the functional channel using the patch-clamp fluorometry technique. S672R reduces the binding of cAMP to the channels in the resting state and significantly increases the unbinding rate during channel deactivation. This study on a disease-causing mutation illustrates the important roles played by the structural elements on the ligand entry-exit path in stabilizing the bound ligand in the binding pocket.
Local and Global Interpretations of a Disease-Causing Mutation near the Ligand Entry Path in Hyperpolarization-Activated cAMP-Gated Channel.,Xu X, Marni F, Wu S, Su Z, Musayev F, Shrestha S, Xie C, Gao W, Liu Q, Zhou L Structure. 2012 Dec 5;20(12):2116-23. doi: 10.1016/j.str.2012.09.017. Epub 2012, Oct 25. PMID:23103389
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.