Histidinemia is considered a benign condition. For years, mental retardation and speech defects were associated with histidinemia. However, these findings are now considered coincidental and not due to the metabolic defect of histidinemia because reports of follow-up from newborn screening have demonstrated that the majority of infants with histidinemia do not develop clinical symptoms (asymptomatic). Nevertheless, clinical symptoms have been reported in some patients with histidinemia. To reconcile this with the benign findings from newborn screening, it has been suggested that histidinemia might be a risk factor for the development of CNS problems, and that such problems might develop only in an unfavorable circumstance such as an abnormal perinatal event.
Individuals with histidinemia have elevated levels of the amino acid histidine in the blood and excessive amounts of histidine, imidazole pyruvic acid, and other imidazole metabolism products in the urine. Most persons with histidinemia adapt to the presence of excessive histidine in the blood and do not suffer any ill effects.
According to the medical literature, infants born to mothers with histidinemia (maternal histinemia) have exhibited no symptoms.
Histidinemia 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 an 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 for 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%.
Researchers believe that histidinemia is due to mutations of the human histidase gene (HAL) located on the long arm (q) of chromosome 12 (12q22-q24.1). Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Pairs of human chromosomes are numbered from 1 through 22, and an additional 23rd pair of sex chromosomes which include one X and one Y chromosome in males and two X chromosomes in females. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 12q22″ refers to region 2, band 2 on the long arm of chromosome 12. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Histidinemia is one of the most common inborn errors of metabolism. Based upon newborn screening of more than 20 million infants in several countries, histidinemia is estimated to occur in about one in 11,500 births overall. The disorder appears to be most prevalent among people of French Canadian or Japanese descent. Based upon newborn screening reports, approximately one in 8,600 infants in Quebec and one in 9,500 infants in Japan are affected by the disorder. The abnormality begins at birth and affects males and females in equal numbers. Histidinemia is now thought to be a primarily benign disorder.
In some states in the United States (e.g., New York and Massachusetts) routine screening of newborns for histidinemia was conducted through blood or urine tests. However, newborn screening for histidinemia has been discontinued. A diagnosis of histidinemia may be made based upon increased levels of histidine in the blood or urine.
Histidinemia is considered a benign, asymptomatic disorder that does not require treatment. According to the medical literature, therapy consisting of a carefully monitored histidine-restricted diet was once recommended, but is no longer called for.
Genetic counseling may also be of benefit to affected individuals and their families. Treatment is symptomatic and supportive for coincidental problems.
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