Acyl-CoA dehydrogenase, very long chain, deficiency of

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Definition

Very-Long-Chain Acyl-CoA Dehydrogenase Deficiency (VLCADD) is a rare autosomal recessive condition in which the body fails to oxidize fatty acids because an enzyme is either missing or not functioning correctly. Long-chain 3-hydroxy acyl-coenzyme A dehydrogenase (LCHAD) is 1 of 3 enzymatic activities that comprise the trifunctional protein of the inner mitochondrial membrane. The other 2 activities of the protein are long-chain 3-ketoacyl CoA thiolase (LCKT) and 2-enoyl coenzyme A (CoA) hydratase (LCEH). The protein is an octamer made up of 4 alpha subunits that contain the LCEH and LCHAD activities, and 4 beta subunits that contain the LCKT activity. This enzyme complex helps metabolize long-chain fatty acids, and the LCHAD activity is specific for compounds of C12-C16 chain length. The genes for the alpha and beta subunits have been traced to chromosome 2. Affected infants with LCHAD deficiency, which is inherited as an autosomal recessive trait, arise in infancy with acute hypoketotic hypoglycemia. These episodes usually appear for the first time after a fast, which usually occurs in the context of intercurrent illness with vomiting.

Prevalence

Occurrence frequency of either isolated LCHAD deficiency or trifunctional protein deficiency is not known in the United States.

Pathophysiology

The molecular defect appears in the mitochondrial trifunctional protein (MTP). Some patients who are deficient in all 3 enzymatic activities of the protein have been described, though most have an isolated LCHAD deficiency, which leads to the inability to metabolize long-chain fatty acids. Thus, the clinical features may arise from either toxicity due to long-chain acyl-CoA esters that cause cardiomyopathy and cardiac arrhythmias or from a block in long-chain fatty acid oxidation that leads to an inability to synthesize ketone bodies and/or adenosine triphosphate from long-chain fatty acids. The gene for the protein has been copied and a common mutation, G1528C, has been identified in 87% of mutant alleles. The fatty acid oxidation defect arises in adverse effects on a number of organ systems, including the CNS, secondary to the hypoketotic hypoglycemia. Hypotonia and cardiomyopathy also are typically present, reflecting the underlying energy deficiency. In addition, hepatomegaly usually is evident, and biopsy of the liver shows fat accumulation and fibrosis.

Treatment

Treatment of VLCADD typically consists of avoidance of fasting (by frequent meals) and use of IV glucose required when food cannot be tolerated (such as with a virus, cold, flu, or other common illness). Intake of long-chain fatty acids is best avoided. Supplemental carnitine is recommended for some affected children with VLCADD.

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