Carnosinemia is a very rare inherited metabolic disorder characterized by impaired neurological function and developmental delays. Symptoms that begin during infancy may include drowsiness, seizures that may be accompanied by involuntary jerking muscle movements of the arms, legs, or head (myoclonic seizures), and mental retardation.
The symptoms of Carnosinemia during infancy include extreme drowsiness and seizures that can occur in children under the age of one year. Slow growth, motor delays, and delayed mental development also occur in children with this disorder. As damage to the nervous system progresses, seizures may be accompanied by involuntary jerking muscle movements involving the head, arms, and/or legs (myoclonic seizures). At approximately 2 years of age, mental retardation may become apparent.
Carnosinemia is inherited as an autosomal recessive genetic trait. The defective gene has been traced to Gene Map Locus 18q21.3
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%. The risk is the same for males and females.
All individuals carry 4-5 abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.
The exact nature of the biochemical defect that causes Carnosinemia is not clear. Studies of muscle tissue from affected individuals suggest that the metabolism of carnosine by the enzyme carnosinase is defective. The role of the enzyme carnosinase is to break down carnosine into two basic elements. Affected individuals usually have abnormally high levels of carnosine in their urine (carnosinuria) and abnormally low levels of the enzyme carnosinase in their blood.
It remains unclear just how the neurological signs of this disorder are related to the presence or absence of the carnosine, its constituents or carnosinase.
Carnosinemia is a very rare disorder that affects males and females in equal numbers. The symptoms typically begin during the first few months of life. Approximately 30 cases of Carnosinemia have been reported in the medical literature.
The diagnosis of Carnosinemia may be made by specialized testing that reveals abnormally high levels of carnosine in the urine and abnormally low amounts of the enzyme carnosinase in the blood. The treatment of Carnosinemia is symptomatic and supportive. Genetic counseling may be of benefit for people with Carnosinemia and their families.
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FROM THE INTERNET
Jaeken J; Carnosinemia. Orphanet encyclopedia, August 2001.
McKusick VA, Ed. Online Mendelian Inheritance In Man (OMIM). The Johns Hopkins University. Entry Number;212200: Last Edit Date; 6/10/1997.
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