ryanodine and Arrhythmogenic-Right-Ventricular-Dysplasia

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a form of cardiomyopathy characterized by ventricular tachyarrhythmias and a fibrofatty infiltrate that is believed to preferentially affect the right ventricle. Mutations in the cardiac ryanodine receptor (RyR2) gene have been identified in some human families with a unique form of ARVC, ARVC2. Although the RyR2 has significant importance in excitation-contraction coupling across the ventricles, mutations in the gene encoding for it appear to have the greatest impact on the right ventricle in ARVC2. Using a canine model (boxer), the RyR2 protein and message RNA in the right ventricle, left ventricle and interventricular septum from normal dogs and dogs with ARVC were investigated by immunoblotting and real time PCR. The cardiac RyR2 message and protein expression were differentially expressed across the cardiac walls in the normal heart, with the lowest concentration expressed in the right ventricle (P < 0.05). The message and protein expression of the RyR2 were reduced in all chambers in the canine model of ARVC. We propose that the increased susceptibility of the right ventricle to ARVC may be associated with the lower baseline protein concentration of RyR2 in the normal right ventricle compared to the left ventricle and interventricular septum and that all three areas are equally affected in this canine model of ARVC. Using this naturally occurring model of canine ARVC, we may have provided new insights into the pathogenesis of this cardiomyopathy.

Topics: Animals; Arrhythmogenic Right Ventricular Dysplasia; Blotting, Western; Dogs; Electrocardiography; Female; Gene Expression Profiling; Genetic Linkage; Genotype; Heart; Male; Myocardium; Myosins; Pedigree; RNA, Messenger; Ryanodine; Ryanodine Receptor Calcium Release Channel

Ventricular tachycardia (VT) is the leading cause of sudden death, and the cardiac ryanodine receptor (RyR2) is emerging as an important focus in its pathogenesis. RyR2 mutations have been linked to VT and sudden death, but their precise impacts on channel function remain largely undefined and controversial. We have previously shown that several disease-linked RyR2 mutations in the C-terminal region enhance the sensitivity of the channel to activation by luminal Ca2+. Cells expressing these RyR2 mutants display an increased propensity for spontaneous Ca2+ release under conditions of store Ca2+ overload, a process we referred to as store overload-induced Ca2+ release (SOICR). To determine whether common defects exist in disease-linked RyR2 mutations, we characterized 6 more RyR2 mutations from different regions of the channel. Stable inducible HEK293 cell lines expressing Q4201R and I4867M from the C-terminal region, S2246L and R2474S from the central region, and R176Q(T2504M) and L433P from the N-terminal region were generated. All of these cell lines display an enhanced propensity for SOICR. HL-1 cardiac cells transfected with disease-linked RyR2 mutations also exhibit increased SOICR activity. Single channel analyses reveal that disease-linked RyR2 mutations primarily increase the channel sensitivity to luminal, but not to cytosolic, Ca2+ activation. Moreover, the Ca2+ dependence of [3H]ryanodine binding to RyR2 wild type and mutants is similar. In contrast to previous reports, we found no evidence that disease-linked RyR2 mutations alter the FKBP12.6-RyR2 interaction. Our data indicate that enhanced SOICR activity and luminal Ca2+ activation represent common defects of RyR2 mutations associated with VT and sudden death. A mechanistic model for CPVT/ARVD2 is proposed.

Topics: Animals; Arrhythmogenic Right Ventricular Dysplasia; Calcium; Calsequestrin; Cell Line; Cytosol; Death, Sudden; Humans; Mice; Mutation; Myocardium; Ryanodine; Ryanodine Receptor Calcium Release Channel; Tachycardia, Ventricular; Tacrolimus Binding Proteins