NORD gratefully acknowledges Stephanie Phan, NORD Editorial Intern from the Keck Graduate Institute, and Steven M. Willi, MD, Director, Diabetes Center for Children, Children’s Hospital of Philadelphia; Associate Professor of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, Buerger Center for Advanced Pediatric Care, for assistance in the preparation of this report.
Carnosinemia is a very rare inherited metabolic disorder characterized by developmental delays and seizures. Symptoms can begin during infancy and may include drowsiness, seizures that may be accompanied by involuntary jerking muscle movements of the arms, legs, or head (myoclonic seizures), and intellectual disability.
The symptoms of carnosinemia include extreme drowsiness and seizures that can occur in children under the age of one year. Slow growth, low muscle tone, motor delays, and delayed intellectual development also occur in children with this disorder. Seizures may be accompanied by myoclonic seizures. By approximately 2 years of age, affected children show variable degrees of intellectual deficit leading to intellectual disabilities and developmental regression. Some affected children also have muscle weakness (congenital myopathy). Electroencephalogram (EEG), a test that detects electrical activity in the brain, may be abnormal. A few patients reported with this condition have few or no symptoms.
The exact nature of the biochemical abnormality that causes carnosinemia is not clear, but carnosinase, the enzyme responsible for carnosine breakdown, is known to be present in the brain, as well as the blood. Studies of muscle tissue from affected individuals suggest that the metabolism of two dipeptides present in meats, carnosine and anserine, by the enzyme carnosinase, is abnormal. 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 how the neurological signs of this disorder are related to the low level of carnosinase and/or high level of the carnosine in the body.
Carnosinase, the enzyme responsible for degrading histidine-containing dipeptides, such as carnosine, anserine and homocarnosine, is encoded by the CNDP1 gene. Loss of carnosinase function has been reported in a small number of patients with highly elevated blood carnosine concentrations, but it is unclear whether the variety of clinical symptoms in these individuals is causally related to carnosinase deficiency.
Carnosinemia is thought to be inherited in an autosomal recessive inheritance pattern.
Recessive genetic disorders occur when an individual inherits an abnormal gene from each parent. If an individual receives one normal gene and one abnormal 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 abnormal 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 is 25%. The risk is the same for males and females.
Carnosinemia is a very rare disorder that affects males and females in equal numbers. Approximately 30 individuals with carnosinemia have been reported in the medical literature world-wide.
The diagnosis of carnosinemia may be made by testing the levels of amino acids in blood and/or urine, which reveals abnormally high levels of carnosine and anserine in the serum and urine. Very specialized testing of the blood will detect very low activity of the enzyme carnosinase in the blood. Diagnosis is based on amino acid analysis of serum and/or urine after exclusion of meat from the diet.
The treatment of carnosinemia is symptomatic and supportive as there has not been an effective drug treatment thus far. A vegetarian diet will reduce the amount of carnosine and anserine presented to the body, and may lower serum carnosine levels. However, it remains unclear whether this will have any effect of symptoms or progression of the disease.
Genetic counseling is recommended for people with carnosinemia and their families.
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Scriver CR, Gibson KM. Disorders of ß- and ?-Amino Acids in Free and Peptide-Linked Forms. In: Scriver CR, Beaudet AL, Sly WS, et al. Eds. The Metabolic Molecular Basis of Inherited Disease. 7th ed. McGraw-Hill Companies. New York, NY; 1995:1349-68.
Kramarenko GG, Markova ED, Ivanova-Smolenskaya IA, et al. Peculiarities of carnosine metabolism in a patient with pronounced homocarnosinemia. Bull Exp Biol Med. 2001;132:996-99.
Willi SM, Zhang Y, Hill JB, et al. A deletion in the long arm of chromosome 18 in a child with serum carnosinase deficiency. Pediatr Res.1997;41:210-13.
Wassif WS, Sherwood RA, Amir A, et al. Serum carnosinase activity in central nervous system disorders. Clin Chim Acta. 1994;225:57-64.
Gjessing LR, Lunde HA, Morkrid L, et al. Inborn errors of carnosine and homocarnosine metabolism. J Neural Transm. 1990;29(Suppl):91-106.
Jaeken J. Carnosinemia. Orphanet. Last update: March 2006. http://www.orpha.net/consor/cgi-bin/Disease_Search.php?lng=EN&data_id=1569&Disease_Disease_Search_diseaseGroup=carnosinemia&Disease_Disease_Search_diseaseType=Pat&Disease(s)/group%20of%20diseases=Carnosinemia&title=Carnosinemia&search=Disease_Search_Simple Accessed November 6, 2018.
McKusick VA, Ed. Online Mendelian Inheritance in Man (OMIM). The Johns Hopkins University. Entry Number: 212200: Last Edit Date 12/11/2009. https://www.omim.org/entry/212200?search=212200&highlight=212200 Accessed November 6, 2018.
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