Histidinemia
Histidinemia is a rare autosomal recessive metabolic disorder caused by a deficiency of the enzyme histidase. Histidase is needed for the metabolism of the amino acid histidine. Although originally thought to be linked to multiple developmental disorders histidinemia is now accepted as a relatively benign disorder, leading to a reduction in the prevalence of neonatal screening procedures.
Presentation
Histidinemia is considered benign as most patients remain asymptomatic, early correlational evidence from the first decade of histidinemia research lead to the theory that histidinemia was associated with multiple developmental symptoms including hyperactivity, speech impediment, developmental delay, learning difficulties, and sometimes intellectual disability. However, these claims were later deemed coincidental as a large subpopulation of infants that tested positive for histidinemia were found to have normal IQ and speech characteristics; as such histidinemia has since been reclassified as a benign inborn error of metabolism.Molecular mechanism
[Image:autorecessive.svg|thumb|right|Histidinemia has an autosomal recessive pattern of inheritance.]Histidinemia occurs as the result of an inborn error of metabolism that may result in either an inactive or a severely reduced histidine ammonia-lyase enzyme activity. The gene that encodes for HAL spans a roughly 25 kb and consists of 21 exons located at the 12q22-q24.1 position of human chromosome 12. There are eight mutations currently associated with autosomal recessive histidinemia, that include: four missense mutations, two exonic polymorphisms and two intronic polymorphisms.