Diagnosis of Japanese patients with HHH syndrome by molecular genetic analysis: a common mutation, R179X

Reversible Leukoencephalopathy in a Man with Childhood-onset Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome

A 49-year-old Japanese man had shown developmental delay, learning difficulties, epilepsy, and slowly progressive gait disturbance in elementary school. At 46 years old, he experienced repeated drowsiness with or without generalized convulsions, and hyperammonemia was detected. Brain magnetic resonance imaging detected multiple cerebral white matter lesions. An electroencephalogram showed diffuse slow basic activities with 2- to 3-Hz δ waves. Genetic tests confirmed a diagnosis of hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome. Leukoencephalopathy was resolved following the administration of L-arginine and lactulose with a decrease in plasma ammonia levels and glutamine-glutamate peak on magnetic resonance spectroscopy. Leukoencephalopathy in HHH syndrome may be reversible with the resolution of hyperammonemia-induced glutamine toxicity.

Computational studies of the mitochondrial carrier family SLC25. Present status and future perspectives

The members of the mitochondrial carrier family, also known as solute carrier family 25 (SLC25), are transmembrane proteins involved in the translocation of a plethora of small molecules between the mitochondrial intermembrane space and the matrix. These transporters are characterized by three homologous domains structure and a transport mechanism that involves the transition between different conformations. Mutations in regions critical for these transporters’ function often cause several diseases, given the crucial role of these proteins in the mitochondrial homeostasis. Experimental studies can be problematic in the case of membrane proteins, in particular concerning the characterization of the structure–function relationships. For this reason, computational methods are often applied in order to develop new hypotheses or to support/explain experimental evidence. Here the computational analyses carried out on the SLC25 members are reviewed, describing the main techniques used and the outcome in terms of improved knowledge of the transport mechanism. Potential future applications on this protein family of more recent and advanced in silico methods are also suggested.

Amino Acid Transport Defects in Human Inherited Metabolic Disorders

Amino acid transporters play very important roles in nutrient uptake, neurotransmitter recycling, protein synthesis, gene expression, cell redox balance, cell signaling, and regulation of cell volume. With regard to transporters that are closely connected to metabolism, amino acid transporter-associated diseases are linked to metabolic disorders, particularly when they involve different organs, cell types, or cell compartments. To date, 65 different human solute carrier (SLC) families and more than 400 transporter genes have been identified, including 11 that are known to include amino acid transporters. This review intends to summarize and update all the conditions in which a strong association has been found between an amino acid transporter and an inherited metabolic disorder. Many of these inherited disorders have been identified in recent years. In this work, the physiological functions of amino acid transporters will be described by the inherited diseases that arise from transporter impairment. The pathogenesis, clinical phenotype, laboratory findings, diagnosis, genetics, and treatment of these disorders are also briefly described. Appropriate clinical and diagnostic characterization of the underlying molecular defect may give patients the opportunity to avail themselves of appropriate therapeutic options in the future.

Corticospinal tract damage in HHH syndrome: a metabolic cause of hereditary spastic paraplegia

BACKGROUND

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare disorder of urea cycle characterized by progressive pyramidal and cerebellar dysfunction, whose pathophysiology is not yet fully understood. Here we describe the spectrum of the long fibers involvement in HHH syndrome, attempting a correlation between clinical, electrophysiological and neuro-radiological data.

METHODS

Nine HHH patients were longitudinally evaluated by clinical examination, neurophysiological assessment including motor (MEPs), somato-sensory evoked potentials (PESS) and nerve conduction velocity (NCV), brain and spinal cord MRI RESULTS: All patients had pyramidal dysfunction and 3/9 an overt spastic paraplegia. Mild to moderate cerebellar signs were found in 7/9, intellectual disability in 8/9. At lower limbs, MEPs resulted abnormal in 7/8 patients and PESS in 2/8; peripheral sensory-motor neuropathy was found in 1/9. MRI documented atrophic changes in supra-tentorial brain regions in 6/9 patients, cerebellum in 6/9, spinal cord in 3/7.

CONCLUSIONS

A predominant corticospinal dysfunction is evident in HHH syndrome, along with milder cerebellar signs, intellectual disability of variable degree and rare peripheral neuropathy. Phenotypical similarities with other disorders affecting the urea cycle (argininemia and pyrroline-5-carboxylate synthetase deficiency) suggest possible common mechanisms contributing in the maintenance of the corticospinal tract integrity. HHH syndrome phenotype largely overlaps with complex Hereditary Spastic Paraplegias (HSPs), in the list of which it should be included, emphasizing the importance to screen all the unsolved cases of HSPs for metabolic biomarkers.

Emerging Roles in the Biogenesis of Cytochrome c Oxidase for Members of the Mitochondrial Carrier Family

The mitochondrial carrier family (MCF) is a group of transport proteins that are mostly localized to the inner mitochondrial membrane where they facilitate the movement of various solutes across the membrane. Although these carriers represent potential targets for therapeutic application and are repeatedly associated with human disease, research on the MCF has not progressed commensurate to their physiologic and pathophysiologic importance. Many of the 53 MCF members in humans are orphans and lack known transport substrates. Even for the relatively well-studied members of this family, such as the ADP/ATP carrier and the uncoupling protein, there exist fundamental gaps in our understanding of their biological roles including a clear rationale for the existence of multiple isoforms. Here, we briefly review this important family of mitochondrial carriers, provide a few salient examples of their diverse metabolic roles and disease associations, and then focus on an emerging link between several distinct MCF members, including the ADP/ATP carrier, and cytochrome c oxidase biogenesis. As the ADP/ATP carrier is regarded as the paradigm of the entire MCF, its newly established role in regulating translation of the mitochondrial genome highlights that we still have a lot to learn about these metabolite transporters.

The genetic nomenclature of recessive cerebellar ataxias

The recessive cerebellar ataxias are a large group of degenerative and metabolic disorders, the diagnostic management of which is difficult because of the enormous clinical and genetic heterogeneity. Because of several limitations, the current classification systems provide insufficient guidance for clinicians and researchers. Here, we propose a new nomenclature for the genetically confirmed recessive cerebellar ataxias according to the principles and criteria laid down by the International Parkinson and Movement Disorder Society Task Force on Classification and Nomenclature of Genetic Movement Disorders. We apply stringent criteria for considering an association between gene and phenotype to be established. The newly proposed list of recessively inherited cerebellar ataxias includes 62 disorders that were assigned an ATX prefix, followed by the gene name, because these typically present with ataxia as a predominant and/or consistent feature. An additional 30 disorders that often combine ataxia with a predominant or consistent other movement disorder received a double prefix (e.g., ATX/HSP). We also identified a group of 89 entities that usually present with complex nonataxia phenotypes, but may occasionally present with cerebellar ataxia. These are listed separately without the ATX prefix. This new, transparent and adaptable nomenclature of the recessive cerebellar ataxias will facilitate the clinical recognition of recessive ataxias, guide diagnostic testing in ataxia patients, and help in interpreting genetic findings. © 2018 International Parkinson and Movement Disorder Society.

Mitochondrial transporters for ornithine and related amino acids: a review

Among the members of the mitochondrial carrier family, there are transporters that catalyze the translocation of ornithine and related substrates, such as arginine, homoarginine, lysine, histidine, and citrulline, across the inner mitochondrial membrane. The mitochondrial carriers ORC1, ORC2, and SLC25A29 from Homo sapiens, BAC1 and BAC2 from Arabidopsis thaliana, and Ort1p from Saccharomyces cerevisiae have been biochemically characterized by transport assays in liposomes. All of them transport ornithine and amino acids with side chains terminating at least with one amine. There are, however, marked differences in their substrate specificities including their affinity for ornithine (KM values in the mM to μM range). These differences are most likely reflected by minor differences in the substrate binding sites of these carriers. The physiological role of the above-mentioned mitochondrial carriers is to link several metabolic pathways that take place partly in the cytosol and partly in the mitochondrial matrix and to provide basic amino acids for mitochondrial translation. In the liver, human ORC1 catalyzes the citrulline/ornithine exchange across the mitochondrial inner membrane, which is required for the urea cycle. Human ORC1, ORC2, and SLC25A29 are likely to be involved in the biosynthesis and transport of arginine, which can be used as a precursor for the synthesis of NO, agmatine, polyamines, creatine, glutamine, glutamate, and proline, as well as in the degradation of basic amino acids. BAC1 and BAC2 are implicated in some processes similar to those of their human counterparts and in nitrogen and amino acid metabolism linked to stress conditions and the development of plants. Ort1p is involved in the biosynthesis of arginine and polyamines in yeast.

The hyperornithinemia-hyperammonemia-homocitrullinuria syndrome

Background

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive disorder of the urea cycle. HHH has a panethnic distribution, with a major prevalence in Canada, Italy and Japan. Acute clinical signs include intermittent episodes of vomiting, confusion or coma and hepatitis-like attacks. Alternatively, patients show a chronic course with aversion for protein rich foods, developmental delay/intellectual disability, myoclonic seizures, ataxia and pyramidal dysfunction. HHH syndrome is caused by impaired ornithine transport across the inner mitochondrial membrane due to mutations in SLC25A15 gene, which encodes for the mitochondrial ornithine carrier ORC1. The diagnosis relies on clinical signs and the peculiar metabolic triad of hyperammonemia, hyperornithinemia, and urinary excretion of homocitrulline. HHH syndrome enters in the differential diagnosis with other inherited or acquired conditions presenting with hyperammonemia.

Methods

A systematic review of publications reporting patients with HHH syndrome was performed.

Results

We retrospectively evaluated the clinical, biochemical and genetic profile of 111 HHH syndrome patients, 109 reported in 61 published articles, and two unpublished cases. Lethargy and coma are frequent at disease onset, whereas pyramidal dysfunction and cognitive/behavioural abnormalities represent the most common clinical features in late-onset cases or during the disease course. Two common mutations, F188del and R179* account respectively for about 30% and 15% of patients with the HHH syndrome. Interestingly, the majority of mutations are located in residues that have side chains protruding into the internal pore of ORC1, suggesting their possible interference with substrate translocation. Acute and chronic management consists in the control of hyperammonemia with protein-restricted diet supplemented with citrulline/arginine and ammonia scavengers. Prognosis of HHH syndrome is variable, ranging from a severe course with disabling manifestations to milder variants compatible with an almost normal life.

Conclusions

This paper provides detailed information on the clinical, metabolic and genetic profiles of all HHH syndrome patients published to date. The clinical phenotype is extremely variable and its severity does not correlate with the genotype or with recorded ammonium/ornithine plasma levels. Early intervention allows almost normal life span but the prognosis is variable, suggesting the need for a better understanding of the still unsolved pathophysiology of the disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13023-015-0242-9) contains supplementary material, which is available to authorized users.

A novel mutation in the SLC25A15 gene in a Turkish patient with HHH syndrome: functional analysis of the mutant protein

The hyperornithinemia-hyperammonemia-homocitrullinuria syndrome is a rare autosomal recessive disorder caused by the functional deficiency of the mitochondrial ornithine transporter 1 (ORC1). ORC1 is encoded by the SLC25A15 gene and catalyzes the transport of cytosolic ornithine into mitochondria in exchange for citrulline. Although the age of onset and the severity of the symptoms vary widely, the disease usually manifests in early infancy. The typical clinical features include protein intolerance, lethargy, episodic confusion, cerebellar ataxia, seizures and mental retardation. In this study, we identified a novel p.Ala15Val (c.44C>T) mutation by genomic DNA sequencing in a Turkish child presenting severe tantrum, confusion, gait disturbances and loss of speech abilities in addition to hyperornithinemia, hyperammonemia and homocitrullinuria. One hundred Turkish control chromosomes did not possess this variant. The functional effect of the novel mutation was assessed by both complementation of the yeast ORT1 null mutant and transport assays. Our study demonstrates that the A15V mutation dramatically interferes with the transport properties of ORC1 since it was shown to inhibit ornithine transport nearly completely.

Long-term follow-up of four patients affected by HHH syndrome

BACKGROUND

In hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome, impaired ornithine transport across the mitochondrial membrane causes ornithine accumulation in cytoplasm. The resulting mitochondrial ornithine deficiency leads to reduced clearance of ammonia through the urea cycle. First described in 1969, no long-term follow-up has been reported.

METHODS

Four patients were followed up for 11 to 38y. Diagnosis was made by plasma amino acid analysis using ion exchange chromatography, HPLC orotic acid measurement, and (14)C-ornithine incorporation study using cultured fibroblasts. DNA from fibroblasts was amplified and sequenced. Blood ammonia was controlled by restriction of protein intake.

RESULTS

All patients had reduced (14)C-ornithine incorporation. Mutation analysis revealed two novel mutations in the ORNT1 gene. Neurologic outcome included memory loss, low IQ, tremor, spasticity of extremities, bladder incontinence, and abnormal gait. Neuroimaging revealed subcortical, cerebral and cerebellar atrophy, sparing the basal ganglia. Individual examination showed pyramidal signs, cerebellar signs, paraplegia, movement disorder, dystonia, and epilepsy. One patient had 3 pregnancies, one of which resulted in intrauterine growth retardation.

CONCLUSIONS

Our patients expand the clinical phenotype of adults with HHH. Long-term follow-up showed serious neurologic outcomes in all patients; three patients clearly exhibited progression of neurologic dysfunction despite control of hyperammonemia. Intracellular ornithine deficiency may adversely affect brain functions.

The role of amino acid transporters in inherited and acquired diseases

Amino acids are essential building blocks of all mammalian cells. In addition to their role in protein synthesis, amino acids play an important role as energy fuels, precursors for a variety of metabolites and as signalling molecules. Disorders associated with the malfunction of amino acid transporters reflect the variety of roles that they fulfil in human physiology. Mutations of brain amino acid transporters affect neuronal excitability. Mutations of renal and intestinal amino acid transporters affect whole-body homoeostasis, resulting in malabsorption and renal problems. Amino acid transporters that are integral parts of metabolic pathways reduce the function of these pathways. Finally, amino acid uptake is essential for cell growth, thereby explaining their role in tumour progression. The present review summarizes the involvement of amino acid transporters in these roles as illustrated by diseases resulting from transporter malfunction.

Diagnosis and high incidence of hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome in northern Saskatchewan

Mutations in the SLC25A15 gene, encoding the human inner mitochondrial membrane ornithine transporter, are thought to be responsible for hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome, a rare autosomal recessive condition. HHH syndrome has been detected in several small, isolated communities in northern Saskatchewan (SK). To determine the incidence of HHH syndrome in these communities, a PCR method was set up to detect F188Δ, the common French-Canadian mutation. Neonatal blood spots collected from all newborns from the high risk area were genotyped for the F188Δ mutation for seven consecutive years. Using DNA analysis, we estimated that the heterozygote frequency for the mutant allele for HHH syndrome to be about 1 in 19 individuals, predicting one affected child with HHH syndrome for approximately every 1,500 individuals (1 in 1,550 live births; 1 child every 12 years) in this isolated population. The frequency for the mutant allele for HHH syndrome in this isolated community is probably the highest in the world for this rare disorder. We determined that ornithine levels, by tandem mass spectrometry, were not abnormal in newborns with F188Δ mutation, carriers and normals. Ornithine rises to abnormally high levels at some time after birth well past the time that the newborn screening blood spot is collected. The timing or the reasons for the delayed rise of ornithine in affected children with HHH syndrome have not been determined. Newborn screening for HHH Syndrome in this high risk population is only possible by detection of the mutant allele using DNA analysis.

Hyperornithinemia–hyperammonemia–homocitrullinuria syndrome with stroke-like imaging presentation: Clinical, biochemical and molecular analysis

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is an autosomal recessive disorder caused by mutations in ORNT1 gene that encodes a mitochondrial ornithine transporter. It has variable clinical presentations with episodic hyperammonemia, liver dysfunction, and chronic neurological manifestations. In this work, we report the findings of HHH syndrome in 3 Saudi siblings. The 4-year-old proband presented with recurrent Reye-like episodes, hypotonia, and multiple stroke-like lesions on brain MRI. Biochemical and molecular analysis confirmed that she had HHH syndrome. She significantly improved on protein restriction and sodium benzoate. Her two older siblings have milder phenotypes with protein intolerance and learning problems. In comparison to their sister, their homocitrulline and orotic acid were only mildly elevated even before treatment. The three patients were homozygous for a novel mutation in ORNT1 with a Gly220Arg change. In view of the CNS lesions, which initially were felt to be suggestive of MELAS, we sequenced the entire mtDNA genome and no potential pathogenic mutations were detected. Analysis of ORNT2 did not provide explanation of the clinical and biochemical variability. This work presents a yet unreported CNS involvement pattern, notably multiple supratentorial stroke-like lesions in association with HHH syndrome. Moreover, it illustrates considerable clinical/biochemical correlation, and describes a novel mutation. We suggest including HHH syndrome in the differential diagnosis of patients found to have stroke-like lesions on brain MRI.

Hyperammonemia in the ICU

Patients experiencing acute elevations of ammonia present to the ICU with encephalopathy, which may progress quickly to cerebral herniation. Patient survival requires immediate treatment of intracerebral hypertension and the reduction of ammonia levels. When hyperammonemia is not thought to be the result of liver failure, treatment for an occult disorder of metabolism must begin prior to the confirmation of an etiology. This article reviews ammonia metabolism, the effects of ammonia on the brain, the causes of hyperammonemia, and the diagnosis of inborn errors of metabolism in adult patients.

A novel R275X mutation of the SLC25A15 gene in a Japanese patient with the HHH syndrome

The hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome (MIM 238970) is an autosomal recessive metabolic disorder caused by a deficiency of the mitochondrial ornithine transporter, one of the urea cycle components. Mutations in the SLC25A15 gene have been coupled to the HHH syndrome. We describe a Japanese female patient with the HHH syndrome due to a novel homozygous R275X SLC25A15 mutation and male sibling who presumably carried the same mutation. He exhibited slowly progressive deterioration with seizures, a gait disturbance due to polyneuropathy, episodic confusion, and died of acute encephalopathy at 34 years of age while the proband exhibited moderate mental retardation, seizures, mild spastic paraplegia, and deafness without neurological deterioration for more than 20 years. The clinical features of previously documented patients with the homozygous SLC25A15 mutation demonstrated that genotype did not simply correlate with clinical severity. The phenotypic variability might depend on other factors, such as dietary and other genetic ones.

The human mitochondrial transport/carrier protein family. Nonsynonymous single nucleotide polymorphisms (nsSNPs) and mutations that lead to human diseases

There are 67 proteins in the human mitochondrial transport protein family. They have been identified from among the proteins of the RefSeq database on the basis of sequence similarity to proteins that have been functionally identified as mitochondrial transport proteins. They have also been identified by matching their predicted structure to the high resolution structure of the bovine ADP/ATP T1 transporter subunit/carboxyatractyloside complex. 74 nonsynonymous single nucleotide polymorphisms (nsSNP) have been identified in their gene sequences. These nsSNPs are present in genes of 30 of the proteins. No nsSNP has been found in 24 of the protein genes and no search has as yet been carried out on the rest (13) of them. The largest number of nsSNPs are in the ADP/ATP T3 transporter, the uncoupling protein 3 L, and the phosphate transporter genes with 7, 6, and 6, respectively. nsSNPs are located in groups along the protein sequence suggesting that certain protein domains are too critical for transport function to tolerate mutations. This interpretation has been validated with mutation and function studies of the phosphate transporter. Human diseases have been identified with replacement mutations in seven of these proteins. Their genes are not abnormally susceptible to mutations since they have the smallest number of nsSNPs. Disease causing mutations have also been observed as: substitution, silent (may affect stability of messages), frameshift (protein truncation or elongation), splicing (exon skipping), residue deletion. Disease causing mutations have only been identified in few transporter genes because others do not yield dramatic symptoms or are essential and thus lethal. Mutations in other transporter genes may also only have a major impact through their combination with other genes and their nsSNPs.

The use of yeast mitochondria to study the properties of wild-type and mutant human mitochondrial ornithine transporter

Yeast deletion models have general utility for the study of a variety of inherited metabolic disorders. Mutations in the mitochondrial ornithine transporter result in hyperammonemia, hyperornithinemia, homocitrullinuria syndrome, a disorder of the urea cycle. To study the effects of mutations in a model system that more closely resembles the in vivo environment, we have developed an expression system based on a yeast strain lacking its endogenous ornithine transporter homologue. Wild-type human ornithine transporter and a recurrent mutation, DeltaF 188, were expressed and characterized with this system. The wild-type transporter appeared to insert into yeast mitochondria in the same orientation as in mammalian mitochondria. It showed stereospecificity, strong antiport activity and ornithine transport was competed by citrulline and arginine. The DeltaF 188 mutant was not incorporated into the membrane to the same extent as wild type, but retained significant residual activity and lost stereospecificity. In these isolated mitochondria, samarium chloride was found to be a potent blocker of transport compared to previously reported sulfhydryl-based inhibitors. A low-affinity background transport activity that promoted the exchange of ornithine for either acidic or basic amino acids was observed. This yeast model can readily be extended to the study of protein:protein interactions. In this manner, the use of yeast deletion strains can serve as a general framework to perform metabolic pathway analysis.

A novel mutation, P126R, in a Japanese patient with HHH syndrome

Mitochondrial ornithine transporter deficiency, or HHH syndrome, is a metabolic disorder resulting in various neurologic symptoms, including mental retardation, spastic paraparesis with pyramidal signs, cerebellar ataxia, and episodic disturbance of consciousness or coma caused by hyperammonemia. Several mutations have been reported in the ORNT1 gene encoding mitochondrial ornithine transporter of patients with this disorder. In this article, we report a new patient, a male 15 years of age, who had typical clinical features of HHH syndrome. Because the patient did not have any of the three mutations previously described in other Japanese patients with HHH syndrome, and the only material available from the patient was peripheral leukocytes, we established a genomic polymerase chain reaction method using intronic primers to amplify every exon of the ORNT1 gene, and we directly sequenced the polymerase chain reaction products. Using this method, we documented a novel mutation in this patient, P126R, and demonstrated that HHH syndrome is genetically heterogeneous, even in the Japanese population.