Sarcoplasmic Reticulum Ca²⁺ Release Threshold and the Adrenergic Signaling

Activation of the adrenergic nervous system, mainly through β-adrenergic receptors, has profound effects on calcium handling, and it is often the initiator of calcium-mediated arrhythmogenesis.¹² Adrenergic activation has two major effects on calcium handling: the enhancement of the amplitude of the L-type calcium current (ICa) and the increase of SR Ca²⁺ levels induced through the activation of SERCA¹³. The latter is responsible for an increase of Ca²⁺-transient amplitude. The effects of adrenergic activation can take place because of the phosphorylation target proteins¹⁴ induced by the activation of two enzymes with kinase activity: protein kinase A¹⁴ and Ca²⁺ calmodulin kinase II (CAMKII).¹⁵ Among the several target proteins, the adrenergic-dependent changes of Ca²⁺ handling are mainly due to the phosphorylation of L-type Ca²⁺ channel (increased current amplitude) phospholamban (removal of SERCA inhibition and increase of SR Ca reuptake) and RyR2 (increased open probability and increased transients).¹⁶ Thus, protein phosphorylation is an important mechanism that enables adrenergic activation to enhance the SR calcium release. In physiological conditions, this response is useful to react to environmental stressors by improving myocardial contractility.

The importance of the adrenergic nervous system in the modulation of the function of the CRUs is highlighted by the evidence that mutations in the genes encoding for three of its major components, the ryanodine receptor (RyR2), cardiac calsequestrin (CASQ2), triadin (TRDN) lead to a severe inherited arrhythmogenic syndrome characterized by life-threatening arrhythmias induced by adrenergic stimulation, namely catecholaminergic polymorphic ventricular tachycardia (CPVT). Recently, some authors have provided data supporting a causal link between RyR2 mutations and the onset of structural cardiomyopathy. This chapter reviews the current evidences and pathophysiological knowledge on phenotypes associated with intracellular calcium handling proteins.

Phenotypes Associated With Mutation in Calcium Handling Proteins

CPVT is the most frequent phenotype associated with mutations involved in intracellular calcium handling. Three genes have been implicated in its pathogenesis (Table 53-1): ryanodine receptor (RyR2 [CPVT1]),¹⁷ cardiac calsequestrin (CASQ2 [CPVT2]),¹⁸ and cardiac triadin (TRDN [CPVT3]).¹⁹ CPVT1 is an autosomal dominant trait, whereas CPVT2 and CPVT3 are rare autosomal recessive disorders. In 2001, Priori et al¹⁷ identified RyR2 as the gene responsible for the most frequent variant of CPVT. The prevalence of RyR2 mutations in patients with a clearly diagnostic phenotype is high (≈60% to 70%).^20,21 Mutations concentrate in three specific areas of the RyR2 protein (amino acids 77-466, 2246-2534, and 3778-4967); however, 14% of CPVT patients harbor RyR2 mutations located outside these areas.²¹ Recent data suggest that there is no significant difference of outcome of CPVT according to mutation site, including the outcome of the subgroup with mutations outside the canonical clusters.²² N, RyR2 mutations have been reported also in patients referred for idiopathic ventricular fibrillation and in relatives of subjects who died suddenly.^23,24 Autosomal recessive CPVT is rarely identified in the clinical setting. Combined estimated prevalence of the two recessive variants is approximately 5%.

Clinical Manifestations and Management of CPVT

CPVT is a severe inherited arrhythmogenic disease manifesting with adrenergically mediated arrhythmias, often leading to syncope or cardiac arrest.17,23,25–27 Although the resting electrocardiogram is unremarkable, the reproducible inducibility of ventricular tachycardia (VT) during exercise stress test is the hallmark of the disease. Most patients with CPVT show a bidirectional VT pattern characterized by beat-to-beat 180-degree rotation of the QRS axis (Figure 53-2)^17,23,26; however, polymorphic tachycardia or ventricular fibrillation may also be part of the picture.^23,27 This unstable, catecholamine-sensitive, substrate can lead to sudden death as the first manifestation of the disease in up to 30% of cases.²³ Symptoms suggesting the presence of arrhythmias tend to manifest early in life (median age, 12 years), although later onset is possible. The development of palpitations and syncopal events during adrenergic stress is a critical element to suspect the diagnosis of CPVT, which is confirmed by the induction of bidirectional VT on an exercise stress test. In untreated patients, the occurrence of severe arrhythmias is approximately 60% to 70%, and approximately 30% experience a cardiac arrest or sudden death upon first manifestation (see Figure 53-2).^23,28 CPVT patients typically present with structurally normal heart. However, some RyR2 mutations have been associated anecdotally with structural cardiomyopathies. One of the first reports on RyR2 gene mutations suggested an association with arrhythmogenic right ventricular cardiomyopathy (ARVC).²⁹ More recently, other authors have reported preliminary data linking RyR2 with hypertrophic cardiomyopathy (HCM).³⁰ Although careful scrutiny of these clinical reports does not establish a causal link, recent experimental findings suggest a mechanistic explanation as to why some RyR2 variants could in principle make the heart more prone to develop structural abnormalities (discussed later).

β-Adrenergic receptor block represents the first-choice therapeutic approach. The use of β-blockers is based on the evidence of the direct link between adrenergic activation and cardiac events in CPVT. Clinical data show that β-blockers have an effect on the natural history of CPVT by achieving a significant reduction of cardiac events. In this context, exercise stress testing is important for dosage titration because the threshold for arrhythmias in CPVT is reproducible. β-Blockers can prevent the onset of arrhythmia in 70% to 80% of cases.23,26 In addition, nonselective β-blockers (e.g., nadolol, propranolol) confer the highest degree of protection against the onset of VT and SCD. However, the nontrivial incidence of recurrent cardiac on optimal β-blocker dosage (approximately 30%)²² calls for the identification of additional therapeutic strategies. Implantable cardioverter defibrillator (ICD) implant, left cardiac sympathetic denervation, and other pharmacologic approaches have been proposed, and flecainide appears to be the most promising strategy. Although the mechanisms for its effectiveness are debatable (discussed later), clinical data and personal experience suggest that flecainide affords additional protection when added to β-blockers in CPVT.^31,32

Abnormalities of Calcium Handling in CPVT

From a mechanistic standpoint, similarities and significant differences exist among CPVT1-3 genetic variants. In all three forms of CPVT, arrhythmogenesis is a consequence of the occurrence of spontaneous calcium release (SCR; Figure 53-3). This term is used to refer to SR Ca²⁺ release events that are not driven by a stimulated action potential. Whenever an SCR occurs and the levels of Ca²⁺ in the sarcolemma increase, the NCX activates to extrude the excess of ions. As mentioned earlier, NCX activation generates the Iti current, which is the cause of the development of delayed afterdepolarizations (DADs)³³ and transient membrane depolarizations occurring during phase 4 of the action potential (see Figure 53-3). When DAD amplitude reaches the voltage threshold for sodium channel activation, triggered beats occur.

The effects of CPVT mutations have been analyzed in a variety of experimental settings, including lipid bilayer, heterologous expression systems, murine transgenic models, and in myocytes derived from patient-specific induced pluripotent stem cells.³⁴ The following sections outline the key concepts about arrhythmogenesis in each form of CPVT.