Histidinemia

Deficiency of the enzyme histidase

Histidinemia is an inherited metabolic disorder in which there is a deficiency of the enzyme histidase, which is necessary for the breakdown of the amino acid histidine. Without histidase, histidine builds up in the blood and urine.

Histidinemia affects both males and females. It is inherited as an autosomal recessive disorder. It affects anywhere from 1 in 8,000 people (in Japan) to 1 in 37,000 (in Sweden). The gene for histidase is on chromosome 12.

Symptoms

Many individuals with histidinemia have no symptoms and normal intelligence. However, some individuals with the disorder may have:

• speech delay
• mental retardation
• autistic symptoms

Since histidinemia often produces no symptoms, there is some disagreement among scientists about whether it is actually a disease or not.

Diagnosis

Histidinemia is diagnosed by blood test and urine test, both of which will show high levels of histidine. In blood, the normal levels of histidine are 70 to 120 microM. In histidinemia, the levels range from 290 to 1420 microM.

Treatment

Because many people with histidinemia have no symptoms, there is disagreement among researchers about whether people with high histidine levels should be treated with a low-histidine diet. For example, one study in Austria of children with histidinemia who were treated with a histidine-restricted diet found no benefit from dietary treatment. In general, a low-histidine diet reduces histidine levels in the blood but does not improve any symptoms present.

_Sources:

Widhalm, K., & K. Virmani. "Long-term follow-up of 58 patients with histidinemia treated with a histidine-restricted diet: no effect of therapy." Pediatrics 94(1994): 861-866.

Page, T. "Metabolic approaches to the treatment of autism spectrum disorders." J Autism Dev Disord 30(2000): 463-469.

Kawai, Y., A. Moriyama, K. Asai, C.M. Coleman-Campbell, S. Sumi, H. Morishita, & M. Suchi. "Molecular characterization of histidinemia: identification of four missense mutations in the histidase gene." Human Genetics 116(2005): 340-346.

Virmani, K., & K. Widhalm. "Histidinemia: a biochemical variant or a disease?." J Am Coll Nutr 12(1993): 115-124.

"Histidinemia." Index of Rare Diseases. National Organization for Rare Disorders. 20 Feb 2008