Abstract-Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited disease characterized by life threatening arrhythmias and mutations in the gene encoding the ryanodine receptor (RyR2). Disagreement exists on whether (1) RyR2 mutations induce abnormal calcium transients in the absence of adrenergic stimulation; (2) decreased affinity of mutant RyR2 for FKBP12.6 causes CPVT; (3) K201 prevent arrhythmias by normalizing the FKBP12.6-RyR2 binding. We studied ventricular myocytes isolated from wild-type (WT) and knock-in mice harboring the R4496C mutation (RyR2 R4496Cϩ/Ϫ ). Pacing protocols did not elicit delayed afterdepolarizations (DADs) (nϭ20) in WT but induced DADs in 21 of 33 (63%) RyR2R4496Cϩ/Ϫ myocytes (Pϭ0.001). Superfusion with isoproterenol (30 nmol/L) induced small DADs (45%) and no triggered activity in WT myocytes, whereas it elicited DADs in 87% and triggered activity in 60% of RyR2R4496Cϩ/Ϫ myocytes (Pϭ0.001). DADs and triggered activity were abolished by ryanodine (10 mol/L) but not by K201 (1 mol/L or 10 mol/L). In vivo administration of K201 failed to prevent induction of polymorphic ventricular tachycardia (VT) in RyR2R4496Cϩ/Ϫ mice. Measurement of the FKBP12.6/RyR2 ratio in the heavy sarcoplasmic reticulum membrane showed normal RyR2-FKBP12.6 interaction both in WT and RyR2R4496Cϩ/Ϫ either before and after treatment with caffeine and epinephrine. We suggest that (1) triggered activity is the likely arrhythmogenic mechanism of CPVT; (2) K201 fails to prevent DADs in RyR2R4496Cϩ/Ϫ myocytes and ventricular arrhythmias in RyR2R4496Cϩ/Ϫ mice; and (3) RyR2-FKBP12.6 interaction in RyR2 R4496Cϩ/Ϫ is identical to that of WT both before and after epinephrine and caffeine, thus suggesting that it is unlikely that the R4496C mutation interferes with the RyR2/FKBP12.6 complex. Key Words: cardiac electrophysiology Ⅲ ryanodine receptor Ⅲ sudden death Ⅲ transgenic mice Ⅲ ventricular tachycardia C atecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disease characterized by adrenergically mediated bidirectional or polymorphic ventricular tachycardia leading to syncope and/or sudden cardiac death in individuals without structural heart disease. 1,2 In 2001, we reported that the autosomal dominant form of CPVT is caused by mutations in the ryanodine receptor gene (RyR2). 3 Based on the evidence that the morphology of ventricular tachycardia observed in CPVT resembles that of digitalis induced ventricular tachycardia (VT), it had been suggested that arrhythmogenesis in CPVT could be mediated by delayed afterdepolarizations (DADs) and triggered activity. Although the discovery that CPVT is caused by mutations in the ryanodine receptor has substantiated this hypothesis, up to now no conclusive demonstration that DADs cause CPVT is available.Furthermore, although several authors have characterized in vitro the functional consequences of RyR2 mutations, 4 -6 the molecular and electrophysiological derangements leading to arrhythmias in CPVT patients are still uncl...

Here we demonstrate biallelic mutations in sorbitol dehydrogenase (SORD) as the most frequent recessive form of hereditary neuropathies. We identified 45 cases from 38 families across multiple ethnicities, carrying a particular nonsense mutation in SORD, c.753delG; p.Ala253GlnfsTer27, either in homozygous or compound heterozygous state with a second variant. With an allele frequency of 0.004 in healthy controls, the p.Ala253GlnfsTer27 variant represents one of the most common pathogenic alleles in humans. SORD is an enzyme that converts sorbitol into fructose, in the two-step polyol pathway that has been implicated in diabetic neuropathy. In patient-derived fibroblasts, we find a complete loss of SORD protein as well as increased intracellular sorbitol. Also, serum fasting sorbitol level was over 100 times higher in patients homozygous for the p.Ala253GlnfsTer27 mutation compared to healthy individuals. In Drosophila, we show that loss of SORD orthologues causes synaptic degeneration and progressive motor impairment. Reducing the polyol influx by treatment with aldose reductase inhibitors normalized intracellular sorbitol levels in patient fibroblasts and in Drosophila, and also dramatically ameliorated motor and eye phenotypes. Together, these findings establish a potentially treatable cause in a significant fraction of patients with inherited neuropathies and may contribute to a better understanding of the pathophysiology of diabetic neuropathy.

Methods and Results: Flecainide prevented catecholamine-induced sustained ventricular tachycardia in RyR2R4496C؉/؊ mice. Cellular studies were performed with isolated RyR2 R4496C؉/؊ myocytes. Isoproterenol caused the appearance of spontaneous Ca 2؉ transients, which were unaffected by flecainide (6 mol/L). Flecainide did not affect Ca 2؉ transient amplitude, decay, or sarcoplasmic reticulum Ca 2؉ content. Moreover, it did not affect the frequency of spontaneous Ca 2؉ sparks in permeabilized myocytes. In contrast, flecainide effectively prevented triggered activity induced by isoproterenol. The threshold for action potential induction was increased significantly (P<0.01), which suggests a primary extracellular antiarrhythmic effect mediated by Na ؉ channel blockade. Conclusions: Flecainide prevents catecholaminergic polymorphic ventricular tachycardia in RyR2R4496C؉/؊ mice; however, at variance with previous reports, we observed minimal effects on intracellular Ca 2؉ homeostasis. Our data suggest that the antiarrhythmic activity of the drug is caused by reduction of Na