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ABSTRACT: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. T he HHH syndrome (OMIM #238970) is an autosomal recessive disorder of the urea cycle and ornithine degradation pathway caused by the deficient transport of ornithine across the inner mitochondrial membrane (1,2). The gene defective in HHH syndrome is the mitochondrial ornithine transporter (ORNT1) that is localized in the q14.1 region of Ch13 and is a member of the MCF of proteins that includes the uncoupling protein, carnitine/acyl-carnitine translocase, and the ADP/ATP transporter (3,4). The human ORNT1 gene is expressed in the periportal hepatocytes, which contain the urea cycle pathway, and in the pericentral hepatocytes and skin fibroblasts that express the ornithine degradation pathway (1,2). Physiologically, ORNT1 allows ornithine to serve as a substrate for the ornithine transcarbamylase (OTC) and ornithine amino transferase (OAT) reactions that produce citrulline and the two amino acids, glutamate and proline. In vitro studies have demonstrated that ORNT1 transports ornithine, lysine, and arginine across the inner mitochondrial membrane in exchange for a hydrogen ion and citrulline (5).Biochemically, HHH syndrome is characterized by persistent elevation of plasma ornithine, episodic or postprandial hyperammonemia, and the urinary excretion of homocitrulline and orotic acid (1). The homocitrulline is believed to be the product of transcarbamoylation of lysine whereas the orotic aciduria occurs secondary to decreased OTC activity. Numerous studies have clearly demonstrated a link between hyperammonemia and CNS pathology in urea cycle disorders. However, little is known ...

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.

The Hyperornithinemia-Hyperammonemia-