Surprisingly, the number of energy metabolism disorders compatible with life is still expanding, and their manifestations are reaching truly pleomorphic proportions. Collectively, these disorders spare no organ or tissue and can mimic many of the diseases routinely encountered by primary care clinicians. In addition to the well-known role of energy metabolism enzymes in balancing the flux of high-energy bonds inside cells and the supply of fuels to them, some also seem to serve multiple roles. For example, mutations in some pyruvate metabolism enzymes impair axonal migration and can also alter craniofacial configuration. Many mutations are linked to selective neuronal necrosis and apoptosis and to edema (spongiosis) of the cerebral white matter; paradoxically, most cause enhanced excitation and epilepsy and result in increased neuronal energy demands. These unexpected manifestations probably occur because flux through energy metabolism pathways sustains the synthesis and recycling of neurotransmitters and other signaling molecules by groups of neural cells. Consequently, the brain usually bears the full burden of these diseases, but cardiac and skeletal muscles, liver, and kidney are also frequently involved.
PYRUVATE DEHYDROGENASE DEFICIENCY
Defects in the pyruvate dehydrogenase (PDH) complex are a frequent cause of lactic acidosis. PDH is a large mitochondrial matrix enzyme complex that catalyzes the oxidative decarboxylation of pyruvate to form acetyl-CoA, nicotinamide adenine dinucleotide (NADH), and carbon dioxide (CO2). Symptoms vary considerably in patients with PDH complex deficiency, and almost equal numbers of males and females are affected, despite the location of the PDH E1 alpha subunit gene (PDHA) in the X chromosome, a paradox explained by selective female X-inactivation. Thus, the phenotype of PDH deficiency is dictated by mutation severity (especially in males) and by the pattern of X-inactivation in females. Dozens of PDHA1 mutations have been identified. In addition, there are patients harboring mutations in the E1 beta subunit, the E2 dihydrolipoyl transacetylase segment of the complex, the E3 (dihydrolipoamide dehydrogenase) subunit, the E3-binding protein, the lipoyl-containing protein X, lipoyltransferase, and the PDH phosphatase (Table 154-1).
TABLE 154-1PYRUVATE METABOLISM DISORDERS ||Download (.pdf) TABLE 154-1PYRUVATE METABOLISM DISORDERS
|Disease ||Biochemical Variants ||Inheritance ||Manifestations |
|Pyruvate dehydrogenase deficiency ||E1a ||X-linked ||Lactic acidosis |
|E1b ||AR ||Episodic ataxia |
|E2 ||AR ||Cerebral dysgenesis |
| ||E3 ||AR ||Infantile epilepsy |
| ||Pyruvate dehydrogenase phosphatase ||AR ||Leigh syndrome |
| ||X protein ||AR ||Alternating hemiplegia |
| ||Lipoyltransferase ||AR ||Lactic acidosis |
|Pyruvate carboxylase deficiency ||A (infantile) ||AR ||Lactic acidosis |
|B (neonatal) ||AR ||Hyperammonemia |
|C (benign) ||AR ||Hypercitrullinemia |
| || || ||Basal ganglia necrosis |
| || || ||Infantile epilepsy |
Neonates with pyruvate dehydrogenase complex (PDC) defects may present with severe acidosis caused by progressive lactate and pyruvate accumulation, hypotonia, microcephaly, partial, or total agenesis of the corpus callosum (Fig. 154-1), and dysmorphic features similar to those seen in fetal alcohol syndrome. The acidosis is refractory to treatment, but thiamine ...