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Pyruvate Carboxylase Deficiency: Mechanisms, Mimics and Anaplerosis

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Pyruvate Carboxylase Deficiency: Mechanisms, Mimics and Anaplerosis Molecular Genetics and Metabolism 101 (2010) 9–17 Contents lists available at ScienceDirect Molecular Genetics and Metabolism journal homepage: www.elsevier.com/locate/ymgme Minireview Pyruvate carboxylase deficiency: Mechanisms, mimics and anaplerosis Isaac Marin-Valencia a, Charles R. Roe a, Juan M. Pascual a,b,* a Department of Neurology, UT Southwestern Medical Center, USA b Departments of Physiology and Pediatrics and Division of Pediatric Neurology, UT Southwestern Medical Center, USA article info abstract Article history: Pyruvate carboxylase (PC) is a regulated mitochondrial enzyme that catalyzes the conversion of pyruvate Received 26 May 2010 to oxaloacetate, a critical transition that replenishes citric acid cycle intermediates and facilitates other Accepted 26 May 2010 biosynthetic reactions that drive anabolism. Its deficiency causes multiorgan metabolic imbalance that Available online 9 June 2010 predominantly manifests with lactic acidemia and neurological dysfunction at an early age. Three clinical forms of PC deficiency have been identified: an infantile form (Type A), a severe neonatal form (Type B), Keywords: and a benign form (Type C), all of which exhibit clinical or biochemical correlates of impaired anaplerosis. Pyruvate carboxylase There is no effective treatment for these patients and most, except those affected by the benign form, die Lactic acidosis in early life. We review the physiology of this enzyme and dissect the major clinical, biochemical, and Anaplerosis Citric acid cycle genetic aspects of its dysfunction, emphasizing features that distinguish PC deficiency from other causes of lactic acidemia that render PC deficiency potentially treatable using novel interventions capable of enhancing anaplerosis. Ó 2010 Elsevier Inc. All rights reserved. Introduction or infantile period. Several dozen patients afflicted by the disorder have been described in detail, allowing for the formulation of over- Pyruvate carboxylase (PC; EC 6.4.1.1) is a biotin-containing nu- lapping disease-associated clinical and biochemical phenotypes. PC clear genome-encoded mitochondrial enzyme discovered in 1959 deficiency has been reported more often in particular ethnic by Utter and Keech [1]. The enzymatic activity facilitates flux groups such as Algonquian-speaking Amerindians and Arabs, but through a key intermediary metabolism reaction: PC is responsible in most populations the general incidence of PC deficiency is rela- for the ATP-dependent carboxylation of pyruvate, yielding oxaloac- tively low (1:250,000) [2]. However, it is plausible that ascertain- etate. This reaction constitutes the best recognized interconversion ment biases limit the recognition of hypomorphic or altogether required for the replenishment of pools of intermediates of the cit- divergent clinical syndromes. In practice, biochemical abnormali- ric acid cycle (CAC), a process named anaplerosis that restores ties assist in the differential diagnosis of PC deficiency, while enzy- losses that CAC derivative products are subject to during normal matic assay is still often required to establish the definitive metabolism. By extension, PC also participates in numerous meta- diagnosis. Current symptomatic or supportive treatments prove bolic pathways that depend on the availability of oxaloacetate such largely ineffective. Therefore, understanding alternative metabolic as gluconeogenesis, glycogen synthesis, lipogenesis, glycerogene- pathways that can be enhanced and associated metabolic dysregu- sis, the synthesis of amino acids and neurotransmitters, and glu- lation potentially amenable to compensatory intervention in light cose-dependent insulin secretion. Because PC is essential for of recent biochemical information is a prerequisite for the develop- these interrelated aspects of anabolism, inherited deficiency of this ment of new therapies. enzyme can a priori be expected to cause metabolic disturbances in numerous tissues, with a predilection for organs whose metabo- PC as an anabolic carbon flux regulator lism depends upon high CAC flux such as liver and brain. PC defi- ciency (OMIM 266150) is an uncommon autosomal recessive PC is localized within the mitochondrial matrix of the cells of disorder that predominantly causes lactic acidemia (serum lactic many organs and tissues. Expression levels are highest in liver, adi- acid >5 mmol/L and bicarbonate <18 mmol/L) and neurological pose tissue, kidney, lactating mammary gland, and pancreatic dysfunction – encephalopathy – first manifested in the neonatal islets; modest in heart, brain, and adrenal gland; lowest in white blood cells and skin fibroblasts [3–5]. PC normally fulfills an ana- plerotic function by converting pyruvate into oxaloacetate in the * Corresponding author. Address: Rare Disorders Clinic, The University of Texas presence of elevated acetyl-CoA levels, a reaction that places PC Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8813, USA. Fax: +1 214 645 6238. at the gateway of multiple synthetic mechanisms. It commits pyru- E-mail address: [email protected] (J.M. Pascual). vate to gluconeogenesis by providing oxaloacetate for subsequent 1096-7192/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.ymgme.2010.05.004 10 I. Marin-Valencia et al. / Molecular Genetics and Metabolism 101 (2010) 9–17 conversion into phosphoenolpyruvate, a precursor of glycerol relevant because new therapeutic approaches for PC deficiency dis- (Fig. 1). Glycerol can also support the re-esterification of fatty acids cussed below aim to enhance ATP production, decreasing AMP/ATP (glycerogenesis). On the other hand, oxaloacetate can join acetyl- ratio and inactivating AMPK, re-establishing flux via the synthetic CoA in the mitochondrion to yield citrate by the action of citrate pathways. synthase (EC 2.3.3.1). Citrate can be exported from mitochondria to the cytoplasm and cleaved to form acetyl-CoA and oxaloacetate. Impact of PC dysfunction Cytoplasmic acetyl-CoA serves a building block for de novo fatty acid synthesis (carried out mainly in liver and adipose tissue) A salient feature of PC deficiency is the conjunction of central and also contributes to the down-regulation of fatty acid b-oxida- nervous system (CNS) maldevelopment (hypotrophy) and degener- tion through the formation of malonyl-CoA, an intermediate of ation, which often dominate the phenotype. Neurological manifes- fatty acid synthesis that inhibits the transport of fatty acids into tations are prominent in PC deficiency as the result of primary mitochondria by carnitine palmitoyltransferase-I (CPT-I). glial dysfunction. In the CNS, PC activity is robust in glia but absent During catabolic states, PC also sustains intra-mitochondrial from neurons [7]. The two major glial cell types include astrocytes production of oxaloacetate. The integrity of both pyruvate dehy- (abundant wherever neurons are located) and oligodendrocytes drogenase (which supplies acetyl-CoA derived from pyruvate) (present in white matter myelin). In astrocytes, the anaplerotic and PC is critical to maintain flux through citrate synthase aimed function of PC is required for gluconeogenesis and glycogen synthe- to fuel the CAC and leading to energy production via the respira- sis and to replenish a-ketoglutarate removed from the CAC for the tory chain. In fact, the most immediate role of the oxidation of synthesis of glutamine, the main neuronal precursor of both gluta- glucose, branched-chain and other amino acids, as well as of the mate and c-aminobutyric acid (GABA) [7]. PC is also involved in b-oxidation of fatty acids is to provide carbon to the CAC. Impair- myelin lipid synthesis in oligodendrocytes, an underexplored pro- ment of these catabolic pathways by disease-related enzymatic cess in the context of developmental disorders that may underlie defects compromises the efficiency of the CAC in terms of ATP pro- the paucity of myelin and abundance of white matter lesions ob- duction and re-activation of synthetic pathways. Parallel covalent served in these patients. Additionally, PC is indirectly involved in modification mediated by nutrient sensors can also suppress syn- maintaining the highly active glutathione system by supplying oxa- thetic pathways [6]: AMP-mediated protein kinase (AMPK) is a ma- loacetate to counteract the loss of malate that may be required for jor regulator of catabolic flux. AMPK is activated under conditions the generation of NADPH both in microglia (a ubiquitous neural cell associated with elevated AMP/ATP ratio such as those that cause a type) and oligodendrocytes [7]. Lastly, in ependymal cells, the ana- decrease in energy. When activated, AMPK phosphorylates serine plerotic function of PC may be required for the oxidation of sub- and threonine residues present in many cytosolic enzymes. The strates such as branched-chain amino acids used for energy immediate result of these modifications is that sets of enzymes generation and needed to sustain kinocilliary activity [7,8]. Impair- that participate in synthetic pathways become inactivated, while ment of these critical roles of PC justifies the prominent brain lesions other enzymes that promote catabolism (by providing substrates that PC deficiency patients manifest, such as spongiform degenera- for the CAC) undergo stimulation such that the net result is the tion, neuronal loss, gliosis, delayed myelination, ventricular enhancement of catabolism. These considerations are particularly enlargement, and hypodevelopment of the corpus callosum [9–12]. Glucose
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