Respiratory chain deficiencies exhibit a wide variety of clinical phenotypes resulting from defective mitochondrial energy production through oxidative phosphorylation. These defects can be caused by either mutations in the mtDNA or mutations in nuclear genes coding for mitochondrial proteins. The underlying pathomechanisms can affect numerous pathways involved in mitochondrial physiology. By whole-exome and candidate gene sequencing, we identified 11 individuals from 9 families carrying compound heterozygous or homozygous mutations in GTPBP3, encoding the mitochondrial GTP-binding protein 3. Affected individuals from eight out of nine families presented with combined respiratory chain complex deficiencies in skeletal muscle. Mutations in GTPBP3 are associated with a severe mitochondrial translation defect, consistent with the predicted function of the protein in catalyzing the formation of 5-taurinomethyluridine (τm(5)U) in the anticodon wobble position of five mitochondrial tRNAs. All case subjects presented with lactic acidosis and nine developed hypertrophic cardiomyopathy. In contrast to individuals with mutations in MTO1, the protein product of which is predicted to participate in the generation of the same modification, most individuals with GTPBP3 mutations developed neurological symptoms and MRI involvement of thalamus, putamen, and brainstem resembling Leigh syndrome. Our study of a mitochondrial translation disorder points toward the importance of posttranscriptional modification of mitochondrial tRNAs for proper mitochondrial function.

Manifestations of brain abscess may be subtle. A high index of suspicion and early imaging are warranted, different predisposing factors may reflect early intervention for congenital heart diseases. Mortality is rare in the modern medicine era.

BACKGROUND Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a lethal arrhythmia provoked by physical or emotional stress and mediated by spontaneous Ca2+ release and delayed after-depolarizations. Beta-adrenergic blockers are the therapy of choice but fail to control arrhythmia in up to 50% of patients. OBJECTIVE To optimize antiarrhythmic therapy in recessively inherited CPVT caused by calsequestrin (CASQ2) mutations. METHODS Murine heart rhythm telemetry was obtained at rest, during treadmill exercise, and after injection of epinephrine. The protocol was repeated after injection of different antiarrhythmic drugs. Results were then validated in human patients. RESULTS Adult CASQ2 mutant mice had complex ventricular arrhythmia at rest and developed bidirectional and polymorphic ventricular tachycardia on exertion. Class I antiarrhythmic agents (procainamide, lidocaine, flecainide) were ineffective in controlling arrhythmia. Propranolol and sotalol attenuated arrhythmia at rest but failed to prevent VT during sympathetic stimulation. The calcium channel blocker verapamil showed a dose-dependent protection against CPVT. Verapamil was more effective than the dihydropyridine L-type Ca2+ channel blocker nifedipine, and its activity was markedly enhanced when combined with propranolol. Human patients homozygous for CASQ2D307H mutation, remaining symptomatic despite chronic β-blocker therapy, underwent exercise testing according to the Bruce protocol with continuous electrocardiogram recording. Verapamil was combined with propranolol at maximum tolerated doses. Adding verapamil attenuated ventricular arrhythmia and prolonged exercise duration in five of 11 patients. CONCLUSION Verapamil is highly effective against catecholamine-induced arrhythmia in mice with CASQ2 mutations and may potentiate the antiarrhythmic activity of β-blockers in humans with CPVT2.