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Propionic acidemia

Propionic acidemia, also known as propionic aciduria, propionyl-CoA carboxylase deficiency and ketotic glycinemia, is an autosomal recessive metabolic disorder, classified as a branched-chain organic acidemia.

The disorder presents in the early neonatal period with progressive encephalopathy. Death can occur quickly, due to secondary hyperammonemia, infection, cardiomyopathy, or basal ganglial stroke.

Propionic Acidemia is a rare disorder that is inherited from both parents. Being autosomal recessive, neither parent shows symptoms, but both carry a defective gene responsible for this disease. It takes two faulty genes to cause PA, so there is a 1 in 4 chance for these parents to have a child with PA.

Diagnosis

Individuals with PA can not break down parts of protein and some types of fat due to a non-functioning enzyme called PCC. Without the enzyme propionyl CoA carboxylase, four essential amino acids in protein (isoleucine, valine, threonine, and methionine) are only partially processed. Too much protein causes propionic acid to build-up in the bloodstream. This in turn causes a build-up of dangerous acids and toxins, which can cause damage to the organs. In many cases, PA can damage the brain, heart, and liver, cause seizures, and delays to normal development like walking and talking. During times of illness the affected person may need to be hospitalized to prevent breakdown of proteins within the body. Each meal presents a challenge to those with PA. If not constantly monitored, the effects would be devastating. Dietary needs must be closely managed by a metabolic geneticist or metabolic dietician.

Pathophysiology

Mutations in both copies of the PCCA or PCCB genes cause propionic acidemia. These genes are responsible for the formation of the enzyme propionyl-CoA carboxylase referred to as PCC.

PCC is required for the normal breakdown of the essential amino acids valine, isoleucine, threonine, and methionine, as well as certain odd-chained fatty-acids. Mutations in the PCCA or PCCB genes disrupt the function of the enzyme, preventing these acids from being metabolized. As a result, propionyl-CoA, propionic acid, ketones and other toxic compounds accumulate in the blood, causing the signs and symptoms of propionic acidemia.

Symptoms

Propionic acidemia is characterized almost immediately in newborns. Symptoms include poor feeding, vomiting, dehydration, acidosis, low muscle tone (hypotonia), seizures, and lethargy. The effects of propionic acidemia quickly become life-threatening.

Genetic prevalence

Propionic acidemia is inherited in an autosomal recessive pattern and is found in about 1 in 35,000 live births in the United States. The condition appears to be more common in Saudi Arabia, with a frequency of about 1 in 3,000. The condition also appears to be common in Amish and Mennonite populations.

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