Ronald J. Kanter

A Precision Medicine Approach to the Rescue of Function on Malignant Calmodulinopathic Long QT Syndrome

Rationale: Calmodulinopathies comprise a new category of potentially life-threatening genetic arrhythmia syndromes capable of producing severe long-QT syndrome (LQTS) with mutations involving CALM1, CALM2, or CALM3. The underlying basis of this form of LQTS is a disruption of Ca2+/calmodulin (CaM)-dependent inactivation of L-type Ca2+ channels. Objective: To gain insight into the mechanistic underpinnings of calmodulinopathies and devise new therapeutic strategies for the treatment of this form of LQTS. Methods and Results: We generated and characterized the functional properties of induced pluripotent stem cell–derived cardiomyocytes from a patient with D130G-CALM2–mediated LQTS, thus creating a platform with which to devise and test novel therapeutic strategies. The patient-derived induced pluripotent stem cell–derived cardiomyocytes display (1) significantly prolonged action potentials, (2) disrupted Ca2+ cycling properties, and (3) diminished Ca2+/CaM-dependent inactivation of L-type Ca2+ channels. Next, taking advantage of the fact that calmodulinopathy patients harbor a mutation in only 1 of 6 redundant CaM-encoding alleles, we devised a strategy using CRISPR interference to selectively suppress the mutant gene while sparing the wild-type counterparts. Indeed, suppression of CALM2 expression produced a functional rescue in induced pluripotent stem cell–derived cardiomyocytes with D130G-CALM2, as shown by the normalization of action potential duration and Ca2+/CaM-dependent inactivation after treatment. Moreover, CRISPR interference can be designed to achieve selective knockdown of any of the 3 CALM genes, making it a generalizable therapeutic strategy for any calmodulinopathy. Conclusions: Overall, this therapeutic strategy holds great promise for calmodulinopathy patients as it represents a generalizable intervention capable of specifically altering CaM expression and potentially attenuating LQTS-triggered cardiac events, thus initiating a path toward precision medicine.

LAMP2 microdeletions in patients with Danon disease.

Background—Danon disease is an X-linked dominant disorder characterized by the clinical triad of hypertrophic cardiomyopathy, skeletal myopathy, and variable mental retardation. Pathologically, autophagic vacuoles are noted in both skeletal and cardiac muscle. It exhibits an X-linked dominant mode of inheritance, and male carriers are severely affected, whereas female carriers develop milder and later-onset cardiac symptoms. Danon disease has been associated with mutations in the lysosome-associated membrane glycoprotein 2 (LAMP2) gene located at Xq24, typically resulting in splicing defects or protein truncation affecting the LAMP2. Because of its rarity, the full spectrum of genetic mutation resulting in Danon disease has not been elucidated. Methods and Results—We analyzed 3 male cases with clinical and pathological findings consistent with Danon disease. Comprehensive mutational analysis failed to yield detectable products for selected LAMP2 exons, and genomic DNA deletion was suspected. Genomic junction fragment polymerase chain reaction analysis in case 1 identified a novel Alu-mediated 34-kb microdeletion encompassing the entire 5′-untranslated region and exon 1 of LAMP2. In case 2 and 3, junctional polymerase chain reaction and Southern blot analyses mapped the breakpoint to an MIRb and (TA)n simple repeats present in intron 3, which determined a 64-kb and a 58-kb deletion, respectively, thereby ablating exons 4 to 10. Western blot analysis confirmed the absence of LAMP2 in protein extract from lymphocytes of index case 2. Conclusion—This article is the first report of Danon disease caused by microdeletions at Xq24, which functionally ablate LAMP2. The microdeletion mechanism appears to involve 1 Alu-mediated unequal recombination and 2 chromosomal breakage points involving TA-rich repeat sequences.

Spectrum and Prevalence of CALM1-, CALM2-, and CALM3-Encoded Calmodulin (CaM) Variants in Long QT Syndrome (LQTS) and Functional Characterization of a Novel LQTS-Associated CaM Missense Variant, E141G

Background—Calmodulin (CaM) is encoded by 3 genes, CALM1, CALM2, and CALM3, all of which harbor pathogenic variants linked to long QT syndrome (LQTS) with early and severe expressivity. These LQTS-causative variants reduce CaM affinity to Ca2+ and alter the properties of the cardiac L-type calcium channel (CaV1.2). CaM also modulates NaV1.5 and the ryanodine receptor, RyR2. All these interactions may play a role in disease pathogenesis. Here, we determine the spectrum and prevalence of pathogenic CaM variants in a cohort of genetically elusive LQTS, and functionally characterize the novel variants. Methods and Results—Thirty-eight genetically elusive LQTS cases underwent whole-exome sequencing to identify CaM variants. Nonsynonymous CaM variants were over-represented significantly in this heretofore LQTS cohort (13.2%) compared with exome aggregation consortium (0.04%; P<0.0001). When the clinical sequelae of these 5 CaM-positive cases were compared with the 33 CaM-negative cases, CaM-positive cases had a more severe phenotype with an average age of onset of 10 months, an average corrected QT interval of 676 ms, and a high prevalence of cardiac arrest. Functional characterization of 1 novel variant, E141G-CaM, revealed an 11-fold reduction in Ca2+-binding affinity and a functionally dominant loss of inactivation in CaV1.2, mild accentuation in NaV1.5 late current, but no effect on intracellular RyR2-mediated calcium release. Conclusions—Overall, 13% of our genetically elusive LQTS cohort harbored nonsynonymous variants in CaM. Genetic testing of CALM1-3 should be pursued for individuals with LQTS, especially those with early childhood cardiac arrest, extreme QT prolongation, and a negative family history.

The clinical and genetic spectrum of catecholaminergic polymorphic ventricular tachycardia: findings from an international multicentre registry

Aims Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an ion channelopathy characterized by ventricular arrhythmia during exertion or stress. Mutations in RYR2-coded Ryanodine Receptor-2 (RyR2) and CASQ2-coded Calsequestrin-2 (CASQ2) genes underlie CPVT1 and CPVT2, respectively. However, prognostic markers are scarce. We sought to better characterize the phenotypic and genotypic spectrum of CPVT, and utilize molecular modelling to help account for clinical phenotypes. Methods and results This is a Pediatric and Congenital Electrophysiology Society multicentre, retrospective cohort study of CPVT patients diagnosed at <19 years of age and their first-degree relatives. Genetic testing was undertaken in 194 of 236 subjects (82%) during 3.5 (1.4-5.3) years of follow-up. The majority (60%) had RyR2-associated CPVT1. Variant locations were predicted based on a 3D structural model of RyR2. Specific residues appear to have key structural importance, supported by an association between cardiac arrest and mutations in the intersubunit interface of the N-terminus, and the S4-S5 linker and helices S5 and S6 of the RyR2 C-terminus. In approximately one quarter of symptomatic patients, cardiac events were precipitated by only normal wakeful activities. Conclusion This large, multicentre study identifies contemporary challenges related to the diagnosis and prognostication of CPVT patients. Structural modelling of RyR2 can improve our understanding severe CPVT phenotypes. Wakeful rest, rather than exertion, often precipitated life-threatening cardiac events.

Wolff-Parkinson-White Syndrome A Stepwise Deterioration to Sudden Death

Wolff-Parkinson-White (WPW) syndrome can be associated with a malignant arrhythmia resulting in sudden death. Symptomatic patients have an estimated risk reported to be ≈0.25% per year, or 3% to 4% over a lifetime.1 In 1979, Klein et al2 reported 31 patients with WPW who had resuscitated ventricular fibrillation. During electrophysiological study, 25 of these patients had induction of atrial fibrillation that conducted rapidly via the accessory pathway. This finding supported the theory that rapid preexcited atrial fibrillation may result in ventricular fibrillation and sudden death. An …

What have we learned in the last 20 years? A comparison of a modern era pediatric and congenital catheter ablation registry to previous pediatric ablation registries

BACKGROUND Since the onset of pediatric catheter ablation, the pediatric electrophysiology community has reported outcomes via various registries (PAPCA [Prospective Assessment After Pediatric Cardiac Ablation], PCAR [Pediatric Catheter Ablation Registry]). Most recently, a modern era pediatric and congenital ablation registry (MAP-IT [Multicenter Pediatric and Congenital EP Quality Initiative]) was developed for eventual incorporation into the National Cardiovascular Data Registry (NCDR) IMPACT (Improving Pediatric and Adult Congenital Treatment) registry. OBJECTIVE The purpose of this study was to describe initial findings from the MAP-IT pilot registry and to compare these findings to earlier registries. METHODS Before entering the NCDR IMPACT registry, MAP-IT was active at 12 centers (11 in the United States) between October 2014 and April 2016. All electrophysiological studies for patients younger than 21 years and for patients of all ages with structural congenital heart disease were included. We compared the acute success, fluoroscopy and procedural times, and frequency of complications between MAP-IT and the earlier registries. RESULTS Acute success rates have improved from the initial PCAR registry for both accessory and slow pathway substrates. Both fluoroscopy and procedural times have significantly decreased across the time periods (fluoroscopy time 47.6 ± 40 minutes to 7.0 ± 9.2 minutes; P <.001; procedural time 257 ± 157 minutes to 166 ± 84 minutes; P <.001). CONCLUSION Acute success rates and fluoroscopy and procedural times in pediatric ablation all have improved over the last 25 years.

Arrhythmias Following the Mustard and Senning Operations for Dextro-Transposition of the Great Arteries: Clinical Aspects and Catheter Ablation

The atrial switch operations, the Mustard and Senning procedures, performed for dextro-transposition of the great arteries, have largely been supplanted by the arterial switch operation. As such, affected patients will only exist for approximately 30 more years. The main arrhythmias in these patients include sinoatrial node dysfunction, intraatrial reentry tachycardia, and sudden death. Device therapy for these patients is well-established, and catheter ablation for atrial tachycardias is highly efficacious. The application of meticulous procedural planning, customization of catheter courses, and electrophysiologic principles to this patient group may be extended to all postoperative complex congenital heart patients.

Ventricular Tachycardia Following Surgical Repair of Complex Congenital Heart Disease

A nine year old boy with complex congenital heart disease requiring right ventricular outflow tract surgery and palpitations had inducible monomorphic ventricular tachycardia at 300 bpm by programmed ventricular stimulation. He was treated with enteral phenytoin. With a therapeutic plasma level, repeat electrophysiological study was negative for inducible ventricular tachycardia using an aggressive pacing protocol. An insertable loop recorder was implanted, and the family was prescribed an automatic external defibrillator. The decision to not place an implantable cardioverter-defibrillator was based upon anticipated need for serial cardiac MRI scans to monitor the effect of progressive outflow tract stenosis and regurgitation.

Letter From Papagiannis and Kanter Regarding Article, “Variable Clinical Features and Ablation of Manifest Nodofascicular/Ventricular Pathways”

We have read with interest the article by Hoffmayer et al,1 titled Variable Clinical Features and Ablation of Manifest Nodofascicular/Ventricular Pathways. We agree with the authors’ conclusions that nodofascicular/ventricular (NFV) pathways can participate in tachycardia either as bystanders, as for example with atrioventricular node reentry tachycardia (AVNRT), which is a commonly associated arrhythmia, or as an integral part of the reentry circuit. We have previously published a case report of an 11-year-old patient with these characteristics in whom cryoablation in the right midseptal area eliminated both the slow atrioventricular nodal pathway and the NFV pathway, thus proving a close anatomic relationship between the 2.2 We have several comments to make on the 4 cases presented in this article. 1. Case 1: Among the 4, this case best documents the presence of a NFV pathway. There is evidence of antegrade conduction with decremental properties (Figure 2A); inducibility of tachycardia with the same morphology as with the pre-excited atrial extrasystoles; ventriculo-atrial decremental conduction; and continuation of the tachycardia despite ventriculo-atrial block (Figure 2C). Our interpretation of Figure 2D is as follows: the initial 3 beats of narrow complex tachycardia represent typical AVNRT with retrograde concealed conduction in the NFV pathway, thus preventing antegrade conduction. In the fourth beat, block in the distal atrioventricular node (or in structures downstream of it) develops, allowing antegrade conduction down the NFV pathway and resulting in antidromic tachycardia. The change from simultaneous retrograde conduction from atrioventricular …

Defibrillation Testing Strategies of Pediatric and Adult Congenital Electrophysiologists during ICD Implantation

Recently, there has been an increasing number of internal medicine‐trained electrophysiologists who choose not to test for acute defibrillation efficacy during implantable cardioverter defibrillator (ICD) implantation. It is not known whether this same trend is seen in pediatric electrophysiologists, who care primarily for patients with congenital heart disease or primary electrical problems.