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At a glance

Inborn error of metabolism affecting the mitochondrial catabolism of valine and isoleucine untreated resulting in ketoacidosis, lethargy, coma, and, finally, death.

Synonyms

Propionic Aciduria; Ketotic Hyperglycinemia; Hyperglycinemia with Ketoacidosis and Leukopenia; Propionyl-CoA Carboxylase Deficiency.

History

First described in 1961 by B. Childs, an American pediatrician.

Incidence

Approximately 1:100,000 live births with both genders equally affected. Incidences as high as 1:2,000 to 1:5,000 live births have been reported from Saudi Arabia.

Genetic inheritance

Autosomal recessive.

Pathophysiology

A defect in the mitochondrial enzyme propionyl-CoA carboxylase causes propionic acidemia. This enzyme is responsible for the generation of methylmalonyl-CoA from propionyl-CoA, which derives from the catabolism of essential amino acids such as isoleucine, valine, methionine, and threonine, as well as from odd-chain fatty acids and cholesterol. The enzyme propionyl-CoA carboxylase is composed of two independently encoded enzyme subunits, an α subunit encoded by the PCCA gene on chromosome 136.32.3 and a β subunit encoded by the PCCB gene on chromosome 3q.22.3. Biotin is a cofactor and binds to the α subunit. Two distinct genotypes can be identified by cell complementation: PCCA (propionyl-CoA carboxylase A) resulting in a defect of the α subunit and PCCB (propionyl-CoA carboxylase B) in a defect of the β subunit. A defect in the synthesis or function of biotin (deficiency of biotinidase, which is responsible for the cleaving of biotin from biocytin), or a deficiency of holocarboxylase synthetase (biotinmethylcrotonyl-CoA-carboxylase ligase), which catalyzes the incorporation of biotin into apocarboxylases, can however, lead to a similar clinical picture. Accumulation of propionyl-CoA results in inhibitory effects on various pathways of intermediary mitochondrial metabolism and secondary carnitine deficiency. The appearance of increased levels of long, odd-chain fatty acids may be related to the increased concentration of propionyl-CoA, which is a primer for these compounds. Ketoacidosis develops secondary to inhibition of the citric acid cycle enzymes by propionic acid. Propionyl-CoA is split in coenzyme-A and propionic acid, which contributes to the metabolic acidosis. Decreased ureagenesis and hyperammonemia seem to be secondary to inhibition of mitochondrial carbamyl phosphate synthetase (CPS) by intramitochondrial accumulation of organic acids and CoA-esters caused by the defect in propionyl-CoA carboxylase. Furthermore, increased concentration of propionate results in decreased levels of hepatic N-acetyl-glutamate, which is an allosteric effector for CPS. In contrast, hyperglycinemia may be nonspecific because it can also be found in other conditions in children with negative nitrogen balance. Moreover, anaerobic fermentation of odd-chain fatty acids in the GI tract also yields propionic acid.

Diagnosis

Made by clinical presentation and organic aciduria with high levels of propionic acid, methylcitrate, and tiglylglycine. Tiglylglycine is an intermediate product of the catabolism of isoleucine. It is a potential marker of disorders of the respiratory chains. Cultured fibroblasts are used to ...

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