Cloning and characterization of human ORNT2: a second mitochondrial ornithine transporter that can rescue a defective ORNT1 in patients with the hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, a urea cycle disorder.

We recently characterized the mitochondrial ornithine transporter (ORNT1), the gene defective in the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, a urea cycle disorder. Despite the apparent functional ablation of ORNT1 in 10 French-Canadian probands with the ORNT1-F188 Delta allele, these patients are mildly affected when compared to patients with other urea cycle disorders such as deficiency of ornithine transcarbamylase. Given that the inner mitochondrial membrane is impermeable to solutes, we hypothesize that other unidentified carriers have some degree of functional redundancy with ORNT1. Using conserved sequences of mammalian and fungal mitochondrial ornithine transporters, we screened the Expressed Sequence Tag database for additional transporters belonging to the ORNT subfamily. Here we identify a new intronless gene, ORNT2, located on chromosome 5. The gene product of ORNT2 is 88% identical to ORNT1, targets to the mitochondria and is expressed in human liver, pancreas, kidney, and cultured fibroblasts from control and HHH patients. When ORNT2 is overexpressed transiently in cultured fibroblasts from HHH patients, it rescues the deficient ornithine metabolism in these cells. Our results suggest that ORNT2 may in part be responsible for the milder phenotype in HHH patients secondary to a gene redundancy effect. We believe ORNT2 arose from a retrotransposition event. To our knowledge, this is the first report of a functional retroposon (ORNT2) that can rescue the disease phenotype of the gene it arose from, ORNT1. As such, ORNT2 may eventually become a candidate for pharmacological-based approaches to correct a urea cycle disorder.