Background: Dilated cardiomyopathy with ataxia (DCMA) is an autosomal recessive disorder arising from mutations in the DNAJC19 gene encoding an inner mitochondrial membrane (IMM) protein. The main clinical features are life threatening early onset cardiomyopathy associated with a metabolic syndrome. DNAJC19 is constituent of the IMM TIM23 import machinery and its direct interaction with prohibitin 2 (PHB2) suggests an additional key role in cardiolipin remodeling.

Methods and Results: To elucidate the pathomechanisms underlying DCMA, we established a novel iPSC-based in vitro model system by the generation of two patient-derived iPSC lines of siblings with discordant cardiac phenotypes. A third isogenic mutant iPSC line (DNAJC19tv) was created using CRISPR/Cas9 technology in healthy control iPSCs. The mutation is predicted to cause a loss of the DnaJ interaction domain which was observed by a shared loss of full-length DNAJC19 protein in all mutant cell lines by western blot analysis. Subcellular examination of DNAJC19 via immunofluorescence revealed a nuclear restricted expression pattern in mutant iPSC-derived cardiomyocytes (iPSC-CMs). Additionally, the loss of DNAJC19 co-localization with mitochondrial structures (Mitotracker) was accompanied by abnormal fragmentation, an overall decrease of mitochondria mass and smaller cardiomyocytes. Ultrastructural analysis unraveled reduced mitochondria sizes and the presence of abnormal cristae suggesting a link to defects in mitochondrial biogenesis and cardiolipin remodeling. However, the assessment of mitochondrial function unexpectedly revealed an overall higher oxygen consumption rate (OCR) in all three mutant iPSC-CMs compared to controls indicating a higher electron transport chain activity to meet cellular ATP demands. Double radioactive tracer uptakes to measure glucose and fatty acid uptake simultaneously (¹⁸F-FDG, ¹²⁵I-BMIPP) displayed decreased fatty acid uptake that was in one patient cell line accompanied by enhanced glucose uptake. To assess physiological processes IonOptix measurements (Indo-1 AM) were performed to investigate Ca²⁺ kinetics, contractility and arrhythmic potential on >120 days mattress-matured iPSC-CMs. We observed significantly increased beating frequencies, elevated diastolic Ca²⁺ concentrations and a shared trend towards reduced sarcomere shortenings in all mutant cell lines basal and upon isoproterenol stimulation. Interestingly, the speed of recovery (sin exp tau) was extended in all mutant iPSC-CMs but especially in the male patient cells, which also showed significantly prolonged relaxation times (RT_90%, RT_50%). Detailed examination of Ca²⁺ transient shapes revealed overall raised arrhythmic features in mutant cells, comprised by both the occurrence of DADs/EADs and fibrillation-like events with discordant preferences.