Barth Syndrome: Connecting Cardiolipin to Cardiomyopathy

Abstract

The Barth syndrome (BTHS) is caused by an inborn error of metabolism that manifests characteristic phenotypic features including altered mitochondrial membrane phospholipids, lactic acidosis, organic acid-uria, skeletal muscle weakness and cardiomyopathy. The underlying cause of BTHS has been definitively traced to mutations in the tafazzin (TAZ) gene locus on chromosome X. TAZ encodes a phospholipid transacylase that promotes cardiolipin acyl chain remodeling. Absence of tafazzin activity results in cardiolipin molecular species heterogeneity, increased levels of monolysocardiolipin and lower cardiolipin abundance. In skeletal muscle and cardiac tissue mitochondria these alterations in cardiolipin perturb the inner membrane, compromising electron transport chain function and aerobic respiration. Decreased electron flow from fuel metabolism via NADH ubiquinone oxidoreductase activity leads to a buildup of NADH in the matrix space and product inhibition of key TCA cycle enzymes. As TCA cycle activity slows pyruvate generated by glycolysis is diverted to lactic acid. In turn, Cori cycle activity increases to supply muscle with glucose for continued ATP production. Acetyl CoA that is unable to enter the TCA cycle is diverted to organic acid waste products that are excreted in urine. Overall, reduced ATP production efficiency in BTHS is exacerbated under conditions of increased energy demand. Prolonged deficiency in ATP production capacity underlies cell and tissue pathology that ultimately is manifest as dilated cardiomyopathy.

Keywords: 3-Methylglutaconic acid; Barth syndrome; Cardiolipin; Cori cycle; Dilated cardiomyopathy; Electron transport chain; Inner mitochondrial membrane; Lactic acid; NADH oxidation; Organic aciduria; Tafazzin.

Figure 1. Tafazzin-mediated transacylation activity

The tafazzin enzyme has been shown to catalyze transfer of an acyl group (red) from phospatidylcholine (or other glycerophospholipids) to monolysocardiolipin. Through an iterative process that may involve additional enzymatic reactions, tafazzin functions in maturation of nascent cardiac and skeletal muscle cardiolipin, resulting in a predominantly uniform molecular species, tetralinoleoylcardiolipin.

Figure 2. Efficient ETC activity is required for sustained aerobic ATP production

Two key products of fuel metabolism in the matrix, NADH and FADH₂, are oxidized to NAD⁺ and FAD⁺ by Complex I and II of the ETC, respectively. The flow of electrons to Complexes III and IV via coenzyme Q and cytochrome c, respectively, is coupled to proton movement to the inner membrane space (Upper panel). The proton gradient thus formed drives oxidative phosphorylation via Complex V (F₁ ATPase). When the content and composition of cardiolipin in the IMM is perturbed by TAZ mutations (Lower panel) the orientation, stability and function of ETC complexes are affected in a manner that compromises electron flow (dashed lines). Likewise decreased membrane integrity can increase proton leakage (solid red line).

Figure 3. Effect of NADH/FADH₂ accumulation on TCA cycle activity

When TAZ mutation-dependent alterations in cardiolipin content and composition interfere with ETC activity, NADH and FADH₂ accumulate in the matrix space. Elevated levels of the reduced form of these coenzymes lead to product inhibition of TCA cycle enzymes, slowing/shutting down the cycle. Unable to enter the TCA cycle, acetyl CoA and propionyl CoA accumulate in the matrix space of skeletal muscle and cardiac tissue mitochondria.

Figure 4. Inhibition of oxidative metabolism induces Cori cycle activity

As TCA cycle activity slows in cardiac and skeletal muscle tissue due to compromised ETC function and buildup of NADH and FADH₂, a shift in metabolism occurs wherein anaerobic glycolysis increases and the pyruvate generated is converted to lactic acid. Lactic acid migrates to liver where gluconeogenesis converts lactate to glucose, at the expense of 6 ATP. Glucose generated in hepatocytes via this process transits back to muscle where anaerobic glycolysis yields 2 ATP for use in muscle contraction.

Figure 5. Diversion of acetyl CoA to organic acid waste products

The inability to oxidize acetyl CoA via the TCA cycle leads to transient accumulation of this metabolite. Above a threshold concentration the enzyme T2 thiolase reverses direction and condenses 2 acetyl CoA into acetoacetyl CoA plus CoASH. Further condensation of acetoacetyl CoA and acetyl CoA generates HMG CoA that is converted to 3-methylglutaconyl CoA (3MG CoA) by 3MG CoA hydratase. As the concentration of 3MG CoA increases it becomes a substrate for thioesterase-mediated hydrolysis (forming 3-methylglutaconic acid; 3MGA) or oxidoreductase-mediated conversion to 3-methylglutaryl CoA. This latter metabolite is subsequently hydrolyzed to 3-methylglutarate and excreted in urine along with 3MGA.