Mutant DNAJC19 causes altered mitochondrial respiration and structural abnormalities

K. Walz (Würzburg)¹, A. Janz (Würzburg)², A. Cirnu (Würzburg)², J. Surjanto (Würzburg)¹, D. Urlaub (Würzburg)¹, A. Nickel (Würzburg)², T. Brand (Würzburg)³, C. Maack (Würzburg)², J. Dudek (Würzburg)², K. Lorenz (Würzburg)³, B. Gerull (Würzburg)^2
1Universitätsklinikum Würzburg Deutsches Zentrum für Herzinsuffizienz Würzburg, Deutschland; ²Universitätsklinikum Würzburg Deutsches Zentrum für Herzinsuffizienz/DZHI Würzburg, Deutschland; ³Institut für Pharmakologie und Toxikologie Würzburg, Deutschland

Background:
Dilated cardiomyopathy with ataxia (DCMA) is an autosomal recessive mitochondrial disorder, clinically characterized by early onset cardiomyopathy associated with metabolic features. The occurrence of DCMA has been attributed to homozygous truncation mutations in DnaJ Heat Shock Protein Family (Hsp40) Member C19 (DNAJC19tv), which encodes the inner mitochondrial membrane protein DNAJC19. Previous data of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from two affected siblings with DCMA and gene-edited truncation variants of DNAJC19 displayed altered mitochondria function and morphology besides an altered metabolic state.

Methods and Results:
We further characterized mutant iPSC-CMs and analyzed the mitochondrial function in live cell imaging. DNAJC19tv iPSC-CMs showed a disturbed mitochondrial network, elevated reactive oxygen species levels and an increased mitochondrial membrane potential compared to the isogenic control, matching elevated oxidative phosphorylation and glycolysis rates. To validate these and previous findings in a second cell system, we generated DNAJC19tv HeLa cells lacking the DnaJ domain to further analyze the respiratory chain (RC). DNAJC19tv HeLa cells showed similar mitochondrial contents while the RC protein levels were increased compared to the isogenic control in western blot analysis. To assess the RC complex formation and composition, we applied solubilized mitochondrial membrane proteins to blue native gel electrophoresis and subsequent western blotting. We could show a more frequent occurrence of the individual complexes (III2/IV and III2-IV), a consistent occurrence of supercomplexes while complex V levels were elevated in the monomeric and dimeric form in DNAJC19tv HeLas. We then investigated if the elevated RC protein expression induces elevated mitochondrial respiration by O2 flux measurements. DNAJC19tv HeLas presented with increased respiration in basal conditions and with carbohydrate substrates which was not evident with fatty acids as substrates. As intracellular ATP levels were equal in both HeLa lines, the increased respiration might be required to maintain ATP levels to a physiological amount as a compensatory measure to bypass the DNACJ19-based alterations in mitochondrial metabolism.

Conclusion:
Loss of DNAJC19 prevents physiological mitochondrial structure and function, resulting in compensatory increase of cellular respiration and glycolysis to meet to cover energy demands.