Background —Catecholaminergic polymorphic ventricular tachycardia is a genetic arrhythmogenic disorder characterized by stress-induced, bidirectional ventricular tachycardia that may degenerate into cardiac arrest and cause sudden death. The electrocardiographic pattern of this ventricular tachycardia closely resembles the arrhythmias associated with calcium overload and the delayed afterdepolarizations observed during digitalis toxicity. We speculated that a genetically determined abnormality of intracellular calcium handling might be the substrate of the disease; therefore, we considered the human cardiac ryanodine receptor gene ( hRyR2 ) a likely candidate for this genetically transmitted arrhythmic disorder. Methods and Results —Twelve patients presenting with typical catecholaminergic polymorphic ventricular tachycardia in the absence of structural heart abnormalities were identified. DNA was extracted from peripheral blood lymphocytes, and single-strand conformation polymorphism analysis was performed on polymerase chain reaction–amplified exons of the hRyR2 gene. Four single nucleotide substitutions leading to missense mutations were identified in 4 probands affected by the disease. Genetic analysis of the asymptomatic parents revealed that 3 probands carried de novo mutations. In 1 case, the identical twin of the proband died suddenly after having suffered syncopal episodes. The fourth mutation was identified in the proband, in 4 clinically affected family members, and in none of 3 nonaffected family members in a kindred with 2 sudden deaths that occurred at 16 and 14 years, respectively, in the sisters of the proband. Conclusions —We demonstrated that, in agreement with our hypothesis, hRyR2 is a gene responsible for catecholaminergic polymorphic ventricular tachycardia.

Ryanodine receptors (RyRs) are intracellular calcium release channels that participate in controlling cytosolic calcium levels. At variance with the probably ubiquitous inositol 1,4,5-trisphosphate-operated calcium channels (1,4,5-trisphosphate receptors), RyRs have been mainly regarded as the calcium release channels controlling skeletal and cardiac muscle contraction. Increasing evidence has recently suggested that RyRs may be more widely expressed, but this has never been extensively examined. Therefore, we cloned three cDNAs corresponding to murine RyR homologues to carry a comprehensive analysis of their expression in murine tissues. Here, we report that the three genes are expressed in almost all tissues analyzed, where tissue-specific patterns of expression were observed. In the uterus and vas deferens, expression of RyR3 was localized to the smooth muscle component of these organs. In the testis, expression of RyR1 and RyR3 was detected in germ cells. RyR mRNAs were also detected in in vitro-cultured cell lines. RyR1, RyR2, and RyR3 mRNA were detected in the cerebrum and in the cerebellum. In situ analysis revealed a cell type-specific pattern of expression in the different regions of the central nervous system. The differential expression of the three ryanodine receptor genes in the central nervous system was also confirmed using specific antibodies against the respective proteins. This widespread pattern of expression suggests that RyRs may participate in the regulation of intracellular calcium homeostasis in a range of cells wider than previously recognized.

Ryanodine receptors (RyRs) are intracellular channels that release calcium ions from the sarcoplasmic reticulum (SR) in response to either plasma membrane depolarization (in skeletal muscle) or increases in the concentration of intracellular free Ca ²⁺ (in the heart). A gene (β4) encoding a ryanodine receptor (similar to, but distinct from, the muscle RyRs) was identified. The β4 gene was expressed in all tissues investigated, with the exception of heart. Treatment of mink lung epithelial cells (Mv1Lu) with transforming growth factor beta (TGF-β) induced expression of the β4 gene together with the release of Ca ²⁺ in response to ryanodine (but not in response to caffeine, the other drug active on muscle RyRs). This ryanodine receptor may be important in the regulation of intracellular Ca ²⁺ homeostasis.

CELL growth and differentiation are usually mutually exclusive¹. Transformation of myoblasts by retroviruses containing the myc oncogene inhibits differentiation, preventing cells from withdrawing from the cell cycle^4,5. If cell-cycle withdrawal is a prerequisite for myoblast differentiation, it is probably an early event in terminal cell differentiation, but this has not yet been established⁶. MyoD1 regulates myogenesis⁷. It is expressed only in skeletal muscle⁸, but can convert other cells to muscle cells^9,10. The MyoD1 protein, a nuclear phosphoprotein in part similar to the myc family of proteins, is a DNA-binding protein binding to the enhancer sequences of the muscle-specific creatine phosphokinase gene¹¹. Thus, introduction of MyoD1 into cells provides a simple approach to study the effect of induction of differentiation on cell growth. In cultured NIH 3T3 cells, inhibition of cell proliferation occurs within 18 hours, and expression of myosin starts after 72 hours. Furthermore, injection of MyoD1 into quiescent NIH3T3 cells inhibits the serum-induced G0–S phase transition of the cell cycle. In CV1 cells, which are not induced to differentiate by MyoD1, DNA synthesis is also inhibited. Expression of MyoD1 can thus inhibit cell proliferation independently of induction of differentiation. Deletion of the myc-like domain in the MyoD1 gene¹² eliminates the inhibition of DNA synthesis, but substitution of the basic domain with the analogous domain from the E12 transcription factor¹³ inhibits growth yet fails to induce differentiation. Inhibition of DNA synthesis, therefore, seems to be controlled separately from myogenic differentiation.