Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is a rare genetic metabolic disorder characterized by a deficiency of the enzyme medium chain acyl-CoA dehydrogenase. This enzyme is found to be most active in the liver, certain white blood cells (leukocytes), and certain connective tissue cells (fibroblasts) and is necessary for the breakdown (oxidation) of certain fats (medium chain fatty acids). Failure to break down these fats can lead to the abnormal accumulation of fatty acids in the liver and the brain. Abnormally low levels of the MCAD enzyme may also hamper or interrupt other processes associated with the metabolism of fatty acids.
In infants with MCAD deficiency, symptoms may include recurrent episodes of unusually low levels of a certain sugar (glucose) in the blood (hypoglycemia), lack of energy (lethargy), vomiting, and/or liver malfunction. These symptoms are most frequently triggered when an affected infant does not eat for an extended period of time (fasting). In some cases, a viral illness (e.g., upper respiratory infection) that limits food intake may cause the symptoms to occur. MCAD deficiency is the most common disease in a group of disorders that involve abnormalities of fatty acid metabolism (fatty acid oxidation disorders [FODs]). MCAD deficiency is inherited as an autosomal recessive trait.
The symptoms of MCAD deficiency usually begin during infancy, most frequently between three to 15 months of age. However, in extremely rare cases, onset may not be until later in childhood. Infants with MCAD Deficiency usually appear normal until they go without eating for a period of 12 to 16 hours (fasting). The symptoms often follow mild infections such as middle ear infection (otitis media) or inflammation of the digestive tract (gastroenteritis).
Extended periods of time without food leads to an episode of low levels of a certain sugar (glucose) in the blood (hypoglycemia), lack of energy (lethargy), and vomiting. In severe episodes, coma, seizures, and life-threatening complications (e.g., severe metabolic acidosis) may ensue. In some cases, affected infants may not exhibit symptoms, such as lethargy or vomiting, between episodes of more severe symptoms (i.e., those brought on by fasting).
Infants with MCAD deficiency may also exhibit an abnormal accumulation of fat in the liver and brain, unusually high levels of ammonia in the blood (hyperammonemia), an abnormally large liver (hepatomegaly), and/or improper liver function (hepatic dysfunction). In addition, affected infants may exhibit low levels of a substance (carnitine) that carries fatty acids to the energy centers of muscles (mitochondria). Deficient levels of carnitine may lead to the accumulation of acidic wastes in the blood (organic acidemia).
Some severely affected infants may also have an abnormal accumulation of fluid around the brain (cerebral edema) that may lead to brain damage (encephalopathy). In some cases, as affected individuals age, other problems may develop. These may include delays in developmental milestones, such as learning to crawl, walk, and/or speak. In addition, some affected individuals may also experience behavioral problems (e.g., attention deficit hyperactivity disorder [ADHD]). (For more information on this disorder, choose “Attention Deficit Hyperactivity” as your search term in the Rare Disease Database.)
In very rare cases, individuals with MCAD deficiency may experience loss of speech (aphasia), muscle weakness (hypotonia), and/or failure to gain weight and grow at the expected rate (failure to thrive). The range and severity of symptoms may vary greatly from case to case. Some individuals may not exhibit symptoms (asymptomatic); others, even those within the same family, may develop serious, life-threatening complications.
MCAD deficiency is inherited as an autosomal recessive trait. Genetic diseases are determined by two genes, one received from the father and one from the mother.
Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%.
Most parents of children with MCAD deficiency have about half the normal level of the medium chain acyl-CoA dehydrogenase enzyme. However, these parents do not exhibit the symptoms of this disorder. Researchers believe that these parents are heterozygotes for the defective MCAD gene. Heterozygous is a condition in which a person has two different genes at the same place on matched chromosomes. An individual who is heterozygous for a particular trait has inherited a gene for that trait from one parent and the alternative gene from the other parent. An individual heterozygous for a hereditary disorder produced by a recessive gene will not show the disease or will have a milder form of it. The offspring of a heterozygous carrier of a genetic disorder will have a fifty percent chance of inheriting the gene dominant for the trait.
The gene that is responsible for regulating the production of the enzyme medium chain acyl-CoA dehydrogenase is thought to be located on the short arm (p) of chromosome 1 (1p). Chromosomes are found in the nucleus of all body cells. They carry the genetic characteristics of each individual. Pairs of human chromosomes are numbered from 1 through 22, with an unequal 23rd pair of X and Y chromosomes for males, and two X chromosomes for females. Each chromosome has a short arm designated as “p” and a long arm identified by the letter “q”.
Symptoms of MCAD deficiency develop due to deficient levels of the enzyme medium chain acyl-CoA dehydrogenase, which is needed for the breakdown of medium chain fatty acids. Low levels of this enzyme interrupt fatty acid metabolism in the liver. Symptoms of MCAD Deficiency are triggered when an affected individual fails to eat for an extended period of time. In some cases, a viral illness (e.g., upper respiratory infection) that limits the intake of food may result in an episode of symptoms. Improper fatty acid metabolism may also lead to the abnormal accumulation of fatty acids in the liver and brain, causing these organs to function improperly.
The abnormal abundance of medium chain fatty acids may inhibit or affect several chemical processes in the body including the production of substances used by muscle and brain tissue for fuel (ketone bodies) and the formation of the blood sugar glucose (gluconeogenesis). If gluconeogenesis is hampered, it may contribute to the development of hypoglycemia. MCAD deficiency may also result in abnormally low levels of carnitine (carnitine deficiency). Carnitine deficiency may cause the accumulation of acidic wastes in the blood (organic acidemia). MCAD deficiency may also interrupt or hinder other chemical processes in the body.
There are several different allelic variants of MCAD deficiency. An allele is any of a series of two or more genes that may occupy the same position (locus) on a specific chromosome. Symptoms of these allelic variants of MCAD deficiency are essentially the same.
MCAD deficiency is a rare metabolic disorder that affects males and females in equal numbers. More than 200 hundred cases have been reported in the medial literature. In the general population, it occurs in approximately 1 in 50,000 live births. In Americans of Northwestern European origin, it may occur in 1 in 6,400 to 1 in 46,000 individuals. In most cases, onset occurs during infancy, usually between the ages of 3 to 15 months. However, in some rare cases, symptoms may not become apparent until later in childhood.
MCAD deficiency may be diagnosed before birth (prenatally) by a specialized test called amniocentesis. During amniocentesis, a sample of the fluid around the fetus is removed and studied. Such studies can reveal dramatically reduced activity of the medium chain acyl-CoA dehydrogenase enzyme.
An acylcarnitine profile test is often used to diagnose infants with MCAD deficiency. During this test, blood or tissue samples are taken and studied to detect and analyze acylcarnitine, a substance that builds up in individuals with fatty acid oxidation disorders such as MCAD deficiency. Each FOD has a unique acylcarnitine profile.
Additional diagnostic tests include enzyme tests (assays) on cultured white blood cells (leukocytes), blood plasma, and certain connective tissue cells (fibroblasts) that may reveal reduced activity of the medium chain acyl-CoA dehydrogenase enzyme. Another diagnostic procedure is urinary analysis, which may reveal excess levels of certain compounds (e.g., suberylglycine and phenylpropionylglycine) in the urine as well as abnormally high levels of dicarboxylic acid, especially during a hypoglycemic episode. Amplication refractory mutation system (ARMS) along with organic acid analysis may also be used to diagnose MCAD Deficiency.
The earlier the diagnosis is made and treatment initiated, the lower the chances become that an infant with MCAD Deficiency will exhibit developmental disabilities. In addition, all siblings of children with MCAD deficiency should be tested immediately to determine whether they have the disorder.
Preventive measures should be taken to ensure that affected individuals do not go without food for extended periods of time (12 to 16 hours). These measures may include awakening a child at night for feeding (e.g., intravenous or parental). A low-fat diet may be of benefit to some people with MCAD deficiency. During periods of fasting, oral cornstarch may be used to prevent hypoglycemia. During episodes of hypoglycemia, intravenous fluids containing 10 percent dextrose should be administered promptly. Glycine or oral carnitine may also be administered to build up carnitine levels in the body.
Genetic counseling will be of benefit for affected individuals and their families. Other treatment is symptomatic and supportive. A team approach for infants with this disorder may be of benefit and may include special social support and other medical services.
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