Non-ventricular, Clinical, and Functional Features of the RyR2(R420Q) Mutation Causing Catecholaminergic Polymorphic Ventricular Tachycardia

The childhood migraine syndrome

Migraine is a complex genetic brain disorder with an intricate pathogenesis and polymorphous clinical presentations, particularly in children. In this Perspective, we describe the different phenotypes of migraine in children, including conditions that have been referred to in the International Classification of Headache Disorders as "syndromes that may be related to migraine''. Evidence is presented for the integration of abdominal migraine, cyclical vomiting syndrome, benign paroxysmal vertigo, benign paroxysmal torticollis and infantile colic into the unified diagnosis of 'childhood migraine syndrome' on the basis of clinical and epidemiological characteristics, and shared inheritance. In our opinion, such integration will guide clinicians from specialities other than neurology to consider migraine in the assessment of children with these disorders, as well as stimulate research into the genetics, pathophysiology and clinical features of all disorders within the syndrome. A diagnosis of childhood migraine syndrome would also enable patients to benefit from inclusion in clinical trials of old and new migraine treatments, thus potentially increasing the number of treatment options available.

Impaired Binding to Junctophilin-2 and Nanostructural Alteration in CPVT Mutation

[Figure: see text].

Genetic Complexity of Sinoatrial Node Dysfunction

The pacemaker cells of the cardiac sinoatrial node (SAN) are essential for normal cardiac automaticity. Dysfunction in cardiac pacemaking results in human sinoatrial node dysfunction (SND). SND more generally occurs in the elderly population and is associated with impaired pacemaker function causing abnormal heart rhythm. Individuals with SND have a variety of symptoms including sinus bradycardia, sinus arrest, SAN block, bradycardia/tachycardia syndrome, and syncope. Importantly, individuals with SND report chronotropic incompetence in response to stress and/or exercise. SND may be genetic or secondary to systemic or cardiovascular conditions. Current management of patients with SND is limited to the relief of arrhythmia symptoms and pacemaker implantation if indicated. Lack of effective therapeutic measures that target the underlying causes of SND renders management of these patients challenging due to its progressive nature and has highlighted a critical need to improve our understanding of its underlying mechanistic basis of SND. This review focuses on current information on the genetics underlying SND, followed by future implications of this knowledge in the management of individuals with SND.

A Recurrent Exertional Syncope and Sudden Cardiac Arrest in a Young Athlete with Known Pathogenic p.Arg420Gln Variant in the RYR2 Gene

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is one of causes of sudden cardiac death in the young, especially in athletes. Diagnosis of CPVT may be difficult since all cardiological examinations performed at rest are usually normal, and exercise stress test-induced ventricular tachycardia is not commonly present. The identification of a pathogenic mutation in RYR2 or CASQ2 is diagnostic in CPVT. We report on a 20-year-old athlete who survived two sudden cardiac arrests during swimming. Moreover, he suffered repeated syncopal spells on exercise. The diagnosis was made only following genetic testing using a multi-gene panel, and the p.Arg420Gln RYR2 variant was identified. We present diagnostic and therapeutic issues in this young athlete with CPVT.

Reclassification of Variants of Uncertain Significance in Children with Inherited Arrhythmia Syndromes is Predicted by Clinical Factors

Genetic testing is important to augment clinical diagnosis and inform management of inherited arrhythmias syndromes (IAS), but variants of uncertain significance (VUS) are common and remain a challenge in clinical practice. In 2015, American College of Medical Genetics (ACMG) published updated guidelines for interpretation of genetic results. Despite increasing understanding of human genomic variation, there are no guidelines for reinterpretation of prior genetic test results. Patients at a single tertiary children's hospital with genetic testing for an IAS that demonstrated a VUS were re-evaluated using 2015 ACMG guidelines, clinical information, and publically available databases. Search of the electronic medical record identified 116 patients with genetic testing results available, and 24/116 (21%) harbored a VUS for an IAS. 23 unique VUS were evaluated from 12 genes. Over half of the VUS (12/23 (52%)) were reclassified using 2015 criteria, and 8 (35%) changed to pathogenic and 4 (17%) to benign. Relative risk of reclassification of VUS to a pathogenic variant in a patient with confirmed clinical diagnosis was 4.1 (95% CI 1.23-15.4). Reclassification was not associated with initial testing year. These data demonstrate 52% of VUS in children with IAS are reclassified with application of 2015 ACMG guidelines. Strength of phenotyping is associated with eventual pathogenic classification of genetic variants and periodic re-evaluation of VUS identified on genetic testing for IAS is warranted.

The N-Terminal Region of the Ryanodine Receptor Affects Channel Activation

Mutations in the cardiac ryanodine receptor (RyR2), the ion channel responsible for release of calcium ions from intracellular stores into cytoplasm, are the cause of several inherited cardiac arrhythmias. At the molecular level, disease symptoms can be mimicked by domain peptides from mutation-prone regions of RyR2 that bind to RyR2 and activate it. Here we show that the domain peptide DP_cpvtN2, corresponding to the central helix of the N-terminal region of RyR2, activates the RyR2 channel. Structural modeling of interaction between DP_cpvtN2 and the N-terminal region of RyR2 in the closed and open conformation provided three plausible structures of the complex. Only one of them could explain the dependence of RyR2 activity on concentration of DP_cpvtN2. The structure of the complex was at odds with the previously proposed “domain switch” mechanism of competition between domain peptides and ryanodine receptor domains. Likewise, in structural models of the N-terminal region, the conformational changes induced by DP_cpvtN2 binding were different from those induced by mutation of central helix amino acids. The activating effect of DP_cpvtN2 binding and of mutations in the central helix could be explained by their similar effect on the transition energy between the closed and open conformation of RyR2.

RyR2R420Q catecholaminergic polymorphic ventricular tachycardia mutation induces bradycardia by disturbing the coupled clock pacemaker mechanism

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a lethal genetic arrhythmia that manifests syncope or sudden death in children and young adults under stress conditions. CPVT patients often present bradycardia and sino-atrial node (SAN) dysfunction. However, the mechanism remains unclear. We analyzed SAN function in two CPVT families and in a novel knock-in (KI) mouse model carrying the RyR2R420Q mutation. Humans and KI mice presented slower resting heart rate. Accordingly, the rate of spontaneous intracellular Ca2+ ([Ca2+]i) transients was slower in KI mouse SAN preparations than in WT, without any significant alteration in the "funny" current (If ). The L-type Ca2+ current was reduced in KI SAN cells in a [Ca2+]i-dependent way, suggesting that bradycardia was due to disrupted crosstalk between the "voltage" and "Ca2+" clock, and the mechanisms of pacemaking was induced by aberrant spontaneous RyR2- dependent Ca2+ release. This finding was consistent with a higher Ca2+ leak during diastolic periods produced by long-lasting Ca2+ sparks in KI SAN cells. Our results uncover a mechanism for the CPVT-causing RyR2 N-terminal mutation R420Q, and they highlight the fact that enhancing the Ca2+ clock may slow the heart rhythm by disturbing the coupling between Ca2+ and voltage clocks.