Adult-onset presentation of a hyperornithinemia-hyperammonemia-homocitrullinuria patient without prior history of neurological complications.

The Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) syndrome is a disorder of the urea cycle and ornithine degradation pathway caused by mutations in the mitochondrial ornithine transporter, ORNT1 (SLC25A15). In general, the majority of patients with HHH syndrome come to medical attention during infancy or early school years with symptoms such as learning disabilities, changes in cognitive development, spasticity, or liver dysfunction. In this report, we describe a 35-year-old male of Indian descent who was diagnosed with HHH syndrome after he presented to the emergency room with gastroenteritis, disorientation, and slurred speech. Molecular analysis revealed that this patient was heterozygous for two ORNT1 mutations, p.[Gly220Arg(+)Arg275X] (c.[658G>A(+)823C>T]) that had been previously reported in homozygous probands who presented during the first year of life. Cellular studies revealed that the ORNT1 p.Gly220Arg mutation was nonfunctional but targeted to the mitochondria. Given that this patient was a successful college graduate on a vegetarian diet without a prior history of learning or neurological impairment, additional factors such as gene redundancy, environmental, and epigenetic factors may have contributed to the delay in onset of presentation and lack of any previous symptoms. To the best of our knowledge, this is the first reported case of an adult-onset HHH syndrome presentation without a prior history of neurological or cognitive deficiency.

The human and mouse SLC25A29 mitochondrial transporters rescue the deficient ornithine metabolism in fibroblasts of patients with the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome.

The hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle (UCD) and ornithine degradation pathway caused by mutations in the mitochondrial ornithine transporter (ORNT1). Unlike other UCDs, HHH syndrome is characterized by a less severe and variable phenotype that we believe may, in part, be due to genes with redundant function to ORNT1, such as the previously characterized ORNT2 gene. We reasoned that SLC25A29, a member of the same subfamily of mitochondrial carrier proteins as ORNT1 and ORNT2, might also have overlapping function with ORNT1. Here, we report that both the human and mouse SLC25A29, previously identified as mitochondrial carnitine/acyl-carnitine transporter-like, when overexpressed transiently also rescues the impaired ornithine transport in cultured HHH fibroblasts. Moreover, we observed that, in the mouse, the Slc25a29 message is more significantly expressed in the CNS and cultured astrocytes when compared with the liver and kidney. These results suggest a potential physiologic role for the SLC25A29 transporter in the oxidation of fatty acids, ornithine degradation pathway, and possibly the urea cycle. Our results show that SLC25A29 is the third human mitochondrial ornithine transporter, designated as ORNT3, which may contribute to the milder and variable phenotype seen in patients with HHH syndrome.

Clinical and functional characterization of a human ORNT1 mutation (T32R) in the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome.

We studied two related families (HHH013 and HHH015) with the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, a disorder of the urea cycle and ornithine degradation pathway, who have the same novel ornithine transporter (ORNT1) genotype (T32R) but a variable phenotype. Both HHH015 patients are doing well in school and are clinically stable; conversely, the three affected HHH013 siblings had academic difficulties and one suffered recurrent episodes of hyperammonemia and ultimately died. Overexpression studies revealed that the product of the ORNT1-T32R allele has residual function. Ornithine transport studies in HHH015 fibroblasts, however, showed basal activity similar to fibroblasts carrying nonfunctional ORNT1 alleles. We also examined two potential modifying factors, the ORNT2 gene and the mitochondrial DNA lineage (haplogroup). Haplogroups, associated with specific diseases, are hypothesized to influence mitochondrial function. Results demonstrated that both HHH015 patients are heterozygous for an ORNT2 gain of function polymorphism and belong to haplogroup A whereas the HHH013 siblings carry the wild-type ORNT2 and are haplogroup H. These observations suggest that the ORNT1 genotype cannot predict the phenotype of HHH patients. The reason for the phenotypic variability is unknown, but factors such as redundant transporters and mitochondrial lineage may contribute to the neuropathophysiology of HHH patients.

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.