A very rare inherited metabolic disorder where ammonia builds up in the body due to a defect in the transport of ornithine which prevents ammonia being converted to urea and being excreted through the urine. The severity of the condition is variable.


* Vomiting * Lethargy * Mental retardation * Hyperornithinemia * Homocitrullinuria


Hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome is a genetic/metabolic disorder caused by a defect in the mitochondrial ornithine transporter ORNT1. * The ORNT1 gene has been mapped to band 13q14. This gene is also identified as SLC25A15 because of its membership in the solute mitochondrial carrier protein family. Its expression is similar to that of other urea-cycle enzyme genes; it is expressed at high levels in hepatocytes, and an increase in dietary protein can promote its expression. * Three ORNT1 mutant alleles were identified in a survey of 11 hyperornithinemia-hyperammonemia-homocitrullinemia probands; these mutant alleles accounted for 21 of 22 possible mutant ORNT1 genes in the population.