A number sign (#) is used with this entry because of evidence that cardiac arrhythmia syndrome with or without skeletal muscle weakness (CARDAR) is caused by homozygous or compound heterozygous mutation in the triadin gene (TRDN; 603283) on chromosome 6q22.

For a general phenotypic description and a discussion of genetic heterogeneity of CPVT, see 604772.

Cardiac arrhythmia syndrome with or without skeletal muscle weakness (CARDAR) is characterized by onset of exercise- or emotion-induced cardiac arrhythmias in infancy or early childhood, associated with syncope or cardiac arrest. Electrocardiography shows variable abnormalities, including polymorphic or bidirectional ventricular extrasystoles and/or transient or persistent prolonged QT intervals, as well as inverted T-waves across the precordial leads. Cardiac events are refractory to both beta-blockers and left cardiac sympathetic denervation. Skeletal muscle weakness has been reported in some patients (Roux-Buisson et al., 2012; Altmann et al., 2015).

Reviews

Giudicessi and Ackerman (2016) reviewed the role of Ca(2+) cycling in cardiac repolarization and in the pathogenesis of long QT-associated cardiac arrhythmias. They noted that TRDN-null mouse models show remodeling of the calcium release unit molecular architecture, implicating either early or delayed after-depolarization as the mechanism predominantly responsible for the observed ventricular arrhythmias.

Clemens et al. (2019) established an International Triadin Knockout Syndrome Registry and reviewed 14 previously published patients with TRDN-associated cardiac arrhythmias, as well as 7 additional patients. Affected individuals presented with either cardiac arrest or syncope at an average age of 3 years. The most common trigger was physical exertion, although a large number of events were not associated with a specific trigger. Mild skeletal myopathy or slight proximal muscle weakness was observed in 6 (29%) of the patients. Two patients died after cardiac events. Of the 19 surviving patients, 16 (84%) showed T-wave inversions across precordial leads, extending to V3 or V4, and 10 (53%) had transient QT prolongation greater than 480 ms. In addition, 8 (89%) of 9 patients who underwent exercise stress testing exhibited ventricular ectopy. All 16 patients tested had normal echocardiograms. The 19 surviving patients were treated with beta-blockers, and 13 (68%) also received implantable defibrillators; however, despite treatment, 14 (74%) of the patients experienced recurrent breakthrough cardiac events.

Roux-Buisson et al. (2012) studied 2 families with cardiac arrhythmias. In the first family, which originated from the French West Indies, the 2-year-old proband experienced syncope followed by cardiac arrest after a shock while playing with his 7-year-old brother. Resting electrocardiogram (ECG) after resuscitation showed numerous polymorphic or bidirectional ventricular extra beats and runs of polymorphic ventricular tachycardia. The proband died in the hospital 3 weeks after the initial cardiac arrest, following a severe postanoxic coma. His parents and brother were unaffected, with normal ECGs, Holter recordings, and exercise stress tests. The proband of the second family, which originated from western France, was a 26-year-old man who had recurrent exercise-induced syncope since infancy. Resting ECG was normal with no prolongation of the QT interval, but exercise testing showed numerous bidirectional ventricular extra beats. In addition, he had proximal muscle weakness. Examination of his dizygotic twin brother revealed that he also had catecholaminergic polymorphic ventricular tachycardia (CPVT; see 604772). Other family members had no abnormalities on clinical evaluation, Holter recording, and exercise testing.

Altmann et al. (2015) reported a 10-year-old girl (family 1) who experienced exercise-associated syncope at age 1 year and again at age 2 years, and experienced cardiac arrest at age 3 years. After placement of an implantable cardioverter-defibrillator (ICD), she experienced ventricular tachycardia (VT)- or ventricular fibrillation (VF)-terminating ICD shocks during exercise, emotional stimulation, and sleep. Electrocardiography (ECG) showed a prolonged corrected QT interval (QTc) of 500 ms, as well as extensive T-wave inversion in the precordial leads V1 through V4. Her stress test was atypical for long QT syndrome (LQTS; see 192500), showing ventricular ectopy during stress and recovery, until her heart rate was less than 85 bpm. Her parents were unaffected, with normal ECGs and negative personal and family histories of cardiac-related events. The authors studied 4 more similarly affected children (families 2 to 5) who experienced early-onset syncope or cardiac arrest within the first few years of life. All had a prolonged QTc interval on ECG, with inverted T waves in precordial leads V1 through V4. One patient, a 6-year-old Indian girl (family 4), also had brief runs of polymorphic VT, including bidirectional VT, on telemetry. Two of the children (families 2 and 5) showed mild to moderate skeletal muscle weakness. Noting that these patients exhibited features that were atypical for LQTS, the authors proposed the designation 'triaden knockout syndrome' for the phenotype.

Rooryck et al. (2015) reported 2 French sisters with CPVT and mutations in the TRDN gene. The older (II.1) experienced syncope at age 5.5 years while playing; cardiac evaluation showed normal QT interval on resting ECG, and 24-hour monitoring showed only a few isolated premature ventricular contractions (PVCs) and auricular contractions with a normal QT interval. Echocardiogram was normal, and exercise test showed isolated monomorphic PVCs. A few months later, her younger 4.5-year-old sister (II.2) experienced cardiac arrest while excited, but was resuscitated. Resting ECG showed many monomorphic PVCs with normal QT interval, and exercise testing was not informative. However, isoproterenol infusion induced polymorphic PVCs in both girls. Neither had proximal or distal muscle weakness. Extensive cardiac assessment in both parents, including isoproterenol infusion, was normal.

Walsh et al. (2016) reported a brother and sister who both had an out-of-hospital cardiac arrest at age 2 years, while running and while playing, respectively. ECG showed a QTc of 480 in the brother, which the authors suggested was attributable to neurologic injury sustained during cardiac arrest. The sister had a pre-arrest ECG showing borderline QT prolongation (460 ms); during resuscitation, a prolonged QTc (500 ms) was observed, which again was suggested to be the result of resuscitation or neurologic injury.

Rossi et al. (2020) studied an Afghan family in which a brother and sister had died suddenly at 2 years and 3 years of age, respectively, while playing at home. A younger brother experienced cardiac arrest at age 14 months and was resuscitated. Follow-up ECGs showed intermittent prolongation of the QT interval (QTc max, 580 ms) as well as giant inverted T-waves on leads V1 to V3. Isoproteronol testing resulted in complex ventricular arrhythmias and isorhythmic atrioventricular dissociation. The proband did not show signs of skeletal myopathy.

The transmission pattern of CARDAR in the families reported by Roux-Buisson et al. (2012) and Altmann et al. (2015) was consistent with autosomal recessive inheritance.

In a cohort of 97 patients with CPVT in whom mutations in the RYR2 (180902) and CASQ2 (114251) genes had been excluded, Roux-Buisson et al. (2012) analyzed the candidate genes TRDN and ASPH (600582) and identified homozygosity for a frameshift mutation in the TRDN gene (603283.0001) in a family from the French West Indies, and compound heterozygosity for a missense (T59R; 603283.0002) and a nonsense (Q205X; 603283.0003) TRDN mutation in a French family. No mutations were identified in the ASPH gene, and the TRDN mutations segregated with disease in each family. Roux-Buisson et al. (2012) noted that all 3 TRDN mutations were located in a region of the gene common to all triadin isoforms, including skeletal muscle isoforms.

By whole-exome sequencing (WES) in a 10-year-old girl (family 1) with severe cardiac arrhythmias and a prolonged QT interval, who was negative for mutation in known LQTS-associated genes, Altmann et al. (2015) identified homozygosity for a 4-bp deletion in the TRDN gene (603283.0001) for which her unaffected parents were heterozygous. Homozygosity for the same deletion had previously been reported by Roux-Buisson et al. (2012) in a 2-year-old boy from the French West Indies with cardiac arrest due to CPVT. Analysis of the TRDN gene in 33 unrelated patients with LQTS revealed 4 more children with mutations in TRDN: 3 were homozygous for a 5-bp deletion (603283.0004) and 1 was compound heterozygous for the 5-bp deletion and a splicing mutation (603283.0005).

In 2 French sisters with CPVT and syncope or cardiac arrest at 5 years and 4 years of age, Rooryck et al. (2015) screened 45 cardiac arrhythmia-associated genes and identified compound heterozygosity for 2 previously reported mutations in the TRDN gene: Q205X (603283.0003) and a splicing mutation (603283.0005). Their unaffected parents were each heterozygous for 1 of the mutations, and an asymptomatic 3-year-old sister was also compound heterozygous for the mutations. All 3 sisters were treated with beta-blockers.

In a brother and sister with cardiac arrest at age 2 years and possible prolongation of the QT interval on ECG, Walsh et al. (2016) identified compound heterozygosity for the previously reported 4-bp deletion (603283.0001) and a nonsense mutation in the TRDN gene (E168X; 603283.0006). The authors stated that the sibs did not show T-wave inversion like that observed in previously reported patients with mutations in TRDN.

O'Callaghan et al. (2018) studied an Omani male infant who experienced cardiac arrest at age 16 months and showed prolonged QTc and T-wave inversion in the anterior precordial leads on ECG. Subsequent ECGs documented torsades de pointes and ventricular fibrillation. Next-generation sequencing targeting 54 cardiac arrhythmia-associated genes revealed mutations in 3 genes: an apparently homozygous deletion of exon 2 of the TRDN gene; a previously reported missense mutation in the KCNE2 gene (I57T; 603796.0003), associated with long QT syndrome (LQT6; 613693); and a E3783Q substitution in the RYR2 gene of uncertain significance.

In an Afghan family in which 2 sibs had died suddenly and a third child was resuscitated from cardiac arrest, Rossi et al. (2020) identified homozygosity for a missense mutation in the TRDN gene (L56P; 603283.0007) that segregated with disease in the family.