Mutation specific impairment of fatty acid and pyruvate oxidation in hypertrophic cardiomyopathy mouse models

Hypertrophic cardiomyopathy (HCM) is considered a metabolic disease, but it is unknown whether all sarcomeric protein gene mutations lead to similar derangements of cardiac energy metabolism. Our prior studies showing differences in mitochondrial function in mouse models led us to hypothesize that HCM mutations lead to allele-specific metabolic pathway remodeling. In order to test this hypothesis, we examined cardiac substrate utilization using a triple tracer approach that simultaneously assays fatty acid (FA), ketone, and glucose oxidation, in R403Q-MyHC and R92W-TnT perfused hearts at established disease stage.

Mouse hearts (14-16 weeks old) were Langendorff-perfused to steady state ¹³C enrichment (∼30 min) while placed in a 14.1 T NMR magnet for in situ ³¹P spectroscopy. The perfusate contained substrates which each produce distinctively labeled acetyl-CoA: [1,6-¹³C₂]glucose, [U¹³C]mixed fatty acids, and [1,3-¹³C₂]β-hydroxybutyrate. At the end of perfusion, hearts were freeze clamped and extracted for ¹³C NMR isotopomer analysis. Oxygen concentration in afferent/efferent perfusate was measured using a Clark electrode, and used to compute myocardial oxygen consumption rate (MVO₂).

Using a model of citric acid cycle (CAC) metabolism that equates MVO₂ with production of reducing equivalents, we observed significantly lower fatty acid (FA) oxidation and pyruvate dehydrogenase (PDH) flux in MyHC mutants, but similar substrate utilization in TnT mutants, when compared to respective controls. ³¹P-spectroscopy revealed significantly lower energy reserves reflected by lower PCr/ATP ratios in MyHC mutants, but similar PCr/ATP ratios in TnT mutants compared to controls.

Cardiac metabolic flux measurements revealed an energy-deficient cardiac phenotype in R403Q-MyHC mutants, but not R92W-TnT mutants, reflecting allele-specific remodeling of cardiac energy metabolism. These results suggest that therapeutics aimed at downregulation of FA or carbohydrate metabolism may have mutation-specific effects.