Profound neurological phenotype in a patient presenting with disordered isoleucine and energy metabolism

Evidence that 2-methylacetoacetate induces oxidative stress in rat brain

In the present study we investigated the effects of 2-methylacetoacetate (MAA) and 2-methyl-3-hydroxybutyrate (MHB), the major metabolites accumulating in mitochondrial 2-methylacetoacetyl-CoA thiolase (KT) and 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiencies, on important parameters of oxidative stress in cerebral cortex from young rats. We verified that MAA induced lipid peroxidation (increase of thiobarbituric acid-reactive substances (TBA-RS) and chemiluminescence values), whereas MHB did not alter these parameters. MAA-induced increase of TBA-RS levels was fully prevented by free radical scavengers, indicating that free radicals were involved in this effect. Furthermore, MAA, but not MHB, significantly induced sulfhydryl oxidation, implying that this organic acid provokes protein oxidative damage. It was also observed that MAA reduced GSH, a naturally-occurring brain antioxidant, whereas MHB did not change this parameter. Furthermore, the decrease of GSH levels caused by MAA was not due to a direct oxidative action, since this organic acid did not alter the sulfhydryl content of a commercial solution of GSH in a cell free medium. Finally, MAA and MHB did not raise nitric oxide production. The data indicate that MAA induces oxidative stress in vitro in cerebral cortex. It is presumed that this pathomechanism may be involved in the brain damage found in patients affected by KT deficiency.

Amyloid-beta-induced mitochondrial dysfunction

As an important molecule in the pathogenesis of Alzheimer's disease (AD), amyloid-beta (Abeta) interferes with multiple aspects of mitochondrial function, including energy metabolism failure, production of reactive oxygen species (ROS) and permeability transition pore formation. Recent studies have demonstrated that Abeta progressively accumulates within mitochondrial matrix, providing a direct link to mitochondrial toxicity. Abeta-binding alcohol dehydrogenase (ABAD) is localized to the mitochondrial matrix and binds to mitochondrial Abeta. Interaction of ABAD with Abeta exaggerates Abeta-mediated mitochondrial and neuronal perturbation, leading to impaired synaptic function, and dysfunctional spatial learning/memory. Thus, blockade of ABAD/Abeta interaction may be a potential therapeutic strategy for AD.

Inborn errors of isoleucine degradation: a review

Three inborn errors have been identified in the pathway of isoleucine degradation. Deficiency of beta-ketothiolase (beta-KT, also known as T2, mitochondrial acetoacetyl-CoA thiolase and acetyl-CoA acetyltransferase 1) is a well-described disorder which presents with acute episodic ketoacidosis. In contrast, short/branched-chain acyl-CoA dehydrogenase (SBCAD) and 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiencies are recently described and relatively rare defects which present with predominantly neurological manifestations, although acute metabolic decompensation may occur in the early newborn period. Careful examination of urine organic acids is required for identification and differential diagnosis of these disorders, with awareness that the abnormalities may be subtle and variable. Tandem MS analysis of acylcarnitines may reveal elevated C5 (SBCAD) or C5:1 and/or OH-C5 species (MHBD and beta-KT deficiencies) but the abnormalities are non-diagnostic and may be intermittent or absent. Confirmation of diagnosis is therefore advisable by specific enzyme assay and/or mutation analysis of the ACAT1 (beta-KT), ACADSB (SBCAD) or HADH2 (MHBD) genes. The latter is located on the X chromosome, accounting for the milder clinical phenotype in females. If beta-KT deficiency is diagnosed early and treated by fasting avoidance and modest protein restriction, ketoacidosis episodes can be prevented and the prognosis is excellent. The role of treatment in SBCAD deficiency remains unclear pending further delineation of its clinical phenotype and pathogenicity, particularly regarding asymptomatic individuals detected by expanded newborn screening. The ineffectiveness of isoleucine restriction in MHBD deficiency is consistent with the additional roles of this multifunctional enzyme in sex steroid and neurosteroid metabolism and its interaction with amyloid-beta peptide.

2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency in a 23-year-old man

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (EC 1.1.1.178) deficiency is a recently described defect of isoleucine catabolism. The disorder is characterized by normal early development followed by a progressive loss of mental and motor skills. Deterioration may be rapid or may follow a slower decline with a possible stabilization of the disorder on a low-protein diet and appropriate medication. We report a 23-year-old man with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency with a very mild clinical course. He had apparently normal early development and remained relatively well until the age of 6 years, when he contracted measles. Following this illness, his motor skills and school progress deteriorated. At 15 years he had significant dysarthria, and generalized rigidity with some dystonic and unusual posturing. He was then treated with a low-protein high-carbohydrate diet with a good response in terms of balance and gait. At 18 years he was given benzhexol (Artane), increased slowly from 2 mg to 6 mg daily, resulting in improvement in tremor and dystonia. At 23 years he can dress himself and works in sheltered employment but remains severely dysarthric.