Hyperornithinemia, hyperammonemia and homocitrullinuria--a case report and study of ornithine metabolism using in vivo deuterium labelling

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clinical, biochemical and genetic profile of 111 HHH syndrome patients, 109 reported in 61 published articles, and two unpublished cases.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.

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.

Retinal risks of high-dose ornithine supplements: a review

We reviewed the literature on ornithine supplementation and related topics. Nutritionists and physicians have reported that ornithine supplementation is useful. Paediatricians and biochemists have reported that ornithine is supplemented for NH3detoxification in the hyperornithinaemia–hyperammonaemia–homocitrullinuria (HHH) syndrome. In contrast, ophthalmic researchers have reported retinotoxicity associated with high-dose ornithine.In vivoandin vitroexperiments have shown that high concentrations of ornithine or its metabolites are toxic to the retinal pigment epithelial (RPE) cells. Long-term (exceeding a few years) and high concentrations (exceeding 600 μmol/l) of ornithine in the blood induce retinal toxicity in gyrate atrophy of the choroid and retina (GA). Intermittent high levels of ornithine do not lead to retinal lesions. Constant blood ornithine levels between 250 and 600 μmol/l do not induce retinal lesions or cause a very slowly progressive retinal degeneration. Blood ornithine levels below 250 μmol/l do not produce retinal alteration. We concluded that short-term, low-dose or transient high-dose ornithine intake is safe for the retina; its nutritional usefulness and effect on NH3detoxification are supported by many researchers, but the effect may be limited; and long-term, high-dose ornithine intake may be risky for the retina. Patients with GA should avoid taking ornithine; amino acid supplementation should be administered carefully for patients with the HHH syndrome, relatives of patients with GA (heterozygotes) and subjects with RPE lesions; and blood ornithine levels and retinal conditions should be evaluated in individuals taking long-term, high-dose ornithine.

Effect of L-ornithine hydrochloride ingestion on intermittent maximal anaerobic cycle ergometer performance and fatigue recovery after exercise

L-Ornithine plays an important role in ammonia metabolism via the urea cycle. This study aimed to examine the effect of L-ornithine hydrochloride ingestion on ammonia metabolism and performance after intermittent maximal anaerobic cycle ergometer exercise. Ten healthy young adults (age, 23.8 ± 3.9 year; height, 172.3 ± 5.5 cm; body mass, 67.7 ± 6.1 kg) with regular training experience ingested L-ornithine hydrochloride (0.1 g/kg, body mass) or placebo after 30 s of maximal cycling exercise. Five sets of the same maximal cycling exercise were conducted 60 min after ingestion, and maximal cycling exercise was conducted after a 15 min rest. The intensity of cycling exercise was based on each subject's body mass (0.74 N kg(-1)). Work volume (watt), peak rpm (rpm) before and after intermittent maximal ergometer exercise and the following serum parameters were measured before ingestion, immediately after exercise and 15 min after exercise: ornithine, ammonia, urea, lactic acid and glutamate. Peak rpm was significantly greater with L-ornithine hydrochloride ingestion than with placebo ingestion. Serum ornithine level was significantly greater with L-ornithine hydrochloride ingestion than with placebo ingestion immediately and 15 min after intermittent maximal cycle ergometer exercise. In conclusion, although maximal anaerobic performance may be improved by L-ornithine hydrochloride ingestion before intermittent maximal anaerobic cycle ergometer exercise, the above may not depend on increase of ammonia metabolism with L-ornithine hydrochloride.

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.

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH)-syndrome. Ultrastructural changes of mitochondria in cultured dermal fibroblasts of three patients

Mitochondria of fibroblasts cultured from the skin obtained at biopsy from three patients with the hyperornithinemia-hyperammonemia-homocitrullinuria (HHH)-syndrome, one of the autosomal recessive, heritable urea cycle disorders, were studied with appropriate controls ultrastructurally. The patients were two severely retarded 10- and 12-year-old boys, and a 22-year-old sister of the former whose mental status was at the low normal range; she never had motor impairments or seizures. The mitochondria, similar in all three patients, were increased in number, very long, branching and/or "looping," and tortuous. "Spurs" or "buddings" extended from their lateral surfaces and the terminal segments were often bulbous. Other unusual configurations were also present. In addition, giant forms with large diameter contained innumerable closely-packed and parallel cristae which traversed the entire width of these mitochondria; at times they assumed a "whirled" pattern. The mitochondrial matrix was usually of high electron density. These changes were not a feature of fibroblastic mitochondria of controls. Several changes resembled those of hepatic mitochondria in this disorder. All features are interpreted as an attempt at expanding the mitochondrial volume (via structural substratum) to compensate for the metabolic incompetence of these organelles (a block in transmembranous transfer of ornithine from hyaloplasm into mitochondria for conversion to citrulline).