Cardiomyopathies due to defective energy metabolism: morphological and functional features

Cardiomyopathies are defined as diseases of the myocardium associated with cardiac dysfunction and are classified by morphological characteristics as hypertrophic (HCM), dilated (DCM) arrhithmogenic right ventricular (ARVC) and restrictive cardiomyopathy. These were once considered as specific diagnoses but there is now considerable evidence that many different gene mutations can cause these pathologies. In recent years, big emphasis has been given to the possibility that deregulation of cardiac metabolism may play a role in the mechanisms that lead to cardiac maladaptive remodelling. Cardiac energy metabolism is tightly controlled in mammalian organisms during development and in response to diverse dietary, physiologic, and pathologic conditions. The cardiac phenotype of many genetic diseases caused by mutations in proteins involved in mitochondrial energy production and/or homeostasis, underscores the importance of energetic pathway on cardiac function. For example, inborn errors in nuclear-encoded mitochondrial fatty acid oxidation (FAO) pathway enzymes and defects in fatty acid uptake are an important cause of childhood HCM and sudden death. Abnormalities in mitochondrial respiratory chain function, particularly those caused by mitochondrial DNA (mtDNA) mutations, are responsible for a heterogeneous group of clinical disorders, including isolated HCM. Mitochondrial cardiomyopathies (MCM) are characterized by an adverse clinical course with biventricular dilation and failure, even at a young age. Mutations in genes encoding the gamma2 subunit of AMP-activated protein kinase (PRKAG2), alpha-galactosidase A (GLA) and lysosome-associated membrane proteine-2 (LAMP2) can cause profound myocardial hypertrophy in association with electrophysiological defects. Unlike HCM due to sarcomere gene mutations, which is characterized by myofiber disarray and fibrosis, large cytosolic vacuoles characterize cardiomyopathy due to defect in energy metabolism. Ultrastructural analysis revealed massive mitochondrial proliferation in MCM and glycogen in complexes with protein and/or lipids in cardiomyopathy due to PRKAG2, GLA and LAMP2 mutations.