Barth syndrome is an X-linked genetic disorder caused by mutations in the tafazzin (taz) gene and characterized by dilated cardiomyopathy, exercise intolerance, chronic fatigue, delayed growth, and neutropenia. Tafazzin is a mitochondrial transacylase required for cardiolipin remodeling. Although tafazzin function has been studied in non-mammalian model organisms, mammalian genetic loss of function approaches have not been used. We examined the consequences of tafazzin knockdown on sarcomeric mitochondria and cardiac function in mice. Tafazzin knockdown resulted in a dramatic decrease of tetralinoleoyl cardiolipin in cardiac and skeletal muscles and accumulation of monolysocardiolipins and cardiolipin molecular species with aberrant acyl groups. Electron microscopy revealed pathological changes in mitochondria, myofibrils, and mitochondrion-associated membranes in skeletal and cardiac muscles. Echocardiography and magnetic resonance imaging revealed severe cardiac abnormalities, including left ventricular dilation, left ventricular mass reduction, and depression of fractional shortening and ejection fraction in tafazzin-deficient mice. Tafazzin knockdown mice provide the first mammalian model system for Barth syndrome in which the pathophysiological relationships between altered content of mitochondrial phospholipids, ultrastructural abnormalities, myocardial and mitochondrial dysfunction, and clinical outcome can be completely investigated.

Deficiency of very-long-chain acyl-CoA dehydrogenase (VLCAD) is the most common long-chain fatty acid oxidation defect and presents with heterogeneous clinical manifestations. Accumulation of long-chain acylcarnitines and deficiency of free carnitine have often been proposed to play an important role in disease pathogenesis. The VLCAD-deficient mouse exhibits similar clinical and biochemical phenotypes to those observed in humans and, therefore, represents an excellent model to study VLCAD deficiency. We measured carnitine and acylcarnitine profiles in liver, skeletal muscle (SkM), bile, and blood from VLCAD knock-out mice and controls under nonstressed and various stress conditions. Carnitine and acylcarnitines were extracted from body fluids with methanol and from tissues with acetonitrile, respectively, and were analyzed as their butyl esters using electrospray ionization tandem mass spectrometry. Fasting combined with a cold challenge for 8 h significantly induced liver long-chain acylcarnitine and free carnitine production. Acylcarnitines in SkM predominantly accumulated during exercise with a concomitant decrease of free carnitine. Changes in blood free carnitine did not correlate with carnitine homeostasis in liver and SkM. Our results demonstrate different tissue-specific long-chain acylcarnitine profiles in response to various stressors, which may be of importance with respect to the heterogeneous clinical manifestations of VLCAD deficiency in humans. Furthermore, we conclude that carnitine biosynthesis in the liver seems sufficiently active to maintain liver carnitine levels during increased demand. Our data suggest that carnitine supplementation in long-chain ␤-oxidation defects may not be required, and blood carnitine concentrations do not reflect tissue carnitine homeostasis. Deficiency of very-long-chain acyl-CoA dehydrogenase (VLCAD) is the most common mitochondrial ␤-oxidation defect of long-chain fatty acids, with an incidence of~1:50,000 to 1:100,000 births (1). In humans, three distinct clinical phenotypes of different severities occur: a life-threatening, early-onset presentation with cardiomyopathy and hepatopathy; a hepatic phenotype with recurrent hypoketotic hypoglycemia and onset in infancy; and a milder, myopathic form with episodic muscle weakness, myalgia, and rhabdomyolysis presenting in adolescence or adulthood (2). Treatment interventions such as avoidance of fasting, reduced long-chain fat intake, and supplementation with medium-chain triglycerides may avoid metabolic decompensation, and long-term rehabilitation can be achieved (3,4).Characteristically, long-chain acyl-CoAs accumulate in mitochondria. To leave the mitochondria, they are converted into acylcarnitine esters, which can be assayed in blood (5). As a result of an increased production of acylcarnitines, blood free carnitine concentrations may decrease (5), a condition termed secondary carnitine deficiency. Nevertheless, supplementation of exogenous carnitine to restore intracellular carnitine pools has been controversia...

Long-chain acylcarnitines in blood increase in knockout mice in response to different stressors and concentrations correlate with the clinical condition. A decrease in blood free carnitine in response to severe stress is observed in knockout mice but also in wild-type littermates. Monitoring blood acylcarnitine profiles in response to different stressors may allow systematic analysis of therapeutic interventions in VLCAD knockout mice.