Ultrastructure of hepatic mitochondria in a child with hyperornithinemia, hyperammonemia, and homocitrullinuria

Liver involvement in urea cycle disorders: a review of the literature

Urea cycle disorders (UCDs) are inborn errors of metabolism of the nitrogen detoxification pathway and encompass six principal enzymatic deficiencies. The aging of UCD patients leads to a better knowledge of the long-term natural history of the condition and to the reporting of previously unnoticed manifestations. Despite historical evidence of liver involvement in UCDs, little attention has been paid to this organ until recently. Hence, we reviewed the available scientific evidence on acute and chronic liver dysfunction and liver carcinogenesis in UCDs and discuss their pathophysiology. Overall, liver involvement, such as acute liver failure or steatotic-like disease, which may evolve toward cirrhosis, has been reported in all six main UCDs. Excessive glycogen storage is also a prominent histologic feature, and hypoglycemia has been reported in citrin deficiency. Hepatocarcinomas seem frequent in some UCDs, such as in citrin deficiency, and can sometimes occur in non-cirrhotic patients. UCDs may differ in liver involvement according to the enzymatic deficiency. Ornithine transcarbamylase deficiency may be associated more with acute liver failure and argininosuccinic aciduria with chronic liver failure and cirrhosis. Direct toxicity of metabolites, downstream metabolic deficiencies, impaired tricarboxylic acid cycle, oxidative stress, mitochondrial dysfunction, energy deficit, and putative toxicity of therapies combine in various ways to cause the different liver diseases reported.

Ornithine and Homocitrulline Impair Mitochondrial Function, Decrease Antioxidant Defenses and Induce Cell Death in Menadione-Stressed Rat Cortical Astrocytes: Potential Mechanisms of Neurological Dysfunction in HHH Syndrome

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is caused by deficiency of ornithine translocase leading to predominant tissue accumulation and high urinary excretion of ornithine (Orn), homocitrulline (Hcit) and ammonia. Although affected patients commonly present neurological dysfunction manifested by cognitive deficit, spastic paraplegia, pyramidal and extrapyramidal signs, stroke-like episodes, hypotonia and ataxia, its pathogenesis is still poorly known. Although astrocytes are necessary for neuronal protection. Therefore, in the present study we investigated the effects of Orn and Hcit on cell viability (propidium iodide incorporation), mitochondrial function (thiazolyl blue tetrazolium bromide-MTT-reduction and mitochondrial membrane potential-ΔΨm), antioxidant defenses (GSH) and pro-inflammatory response (NFkB, IL-1β, IL-6 and TNF-α) in unstimulated and menadione-stressed cortical astrocytes that were previously shown to be susceptible to damage by neurotoxins. We first observed that Orn decreased MTT reduction, whereas both amino acids decreased GSH levels, without altering cell viability and the pro-inflammatory factors in unstimulated astrocytes. Furthermore, Orn and Hcit decreased cell viability and ΔΨm in menadione-treated astrocytes. The present data indicate that the major compounds accumulating in HHH syndrome impair mitochondrial function and reduce cell viability and the antioxidant defenses in cultured astrocytes especially when stressed by menadione. It is presumed that these mechanisms may be involved in the neuropathology of this disease.

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.

Impairment of brain redox homeostasis caused by the major metabolites accumulating in hyperornithinemia-hyperammonemia-homocitrullinuria syndrome in vivo

Ornithine, ammonia and homocitrulline are the major metabolites accumulating in hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, a genetic disorder characterized by neurological regression whose pathogenesis is still not understood. The present work investigated the in vivo effects of intracerebroventricular administration of ornithine and homocitrulline in the presence or absence of hyperammonemia induced by intraperitoneal urease treatment on a large spectrum of oxidative stress parameters in cerebral cortex from young rats in order to better understand the role of these metabolites on brain damage. Ornithine increased thiobarbituric acid-reactive substances (TBA-RS) levels and carbonyl formation and decreased total antioxidant status (TAS) levels. We also observed that the combination of hyperammonemia with ornithine resulted in significant decreases of sulfhydryl levels, reduced glutathione (GSH) concentrations and the activities of catalase (CAT) and glutathione peroxidase (GPx), highlighting a synergistic effect of ornithine and ammonia. Furthermore, homocitrulline caused increases of TBA-RS values and carbonyl formation, as well as decreases of GSH concentrations and GPx activity. Hcit with hyperammonemia (urease treatment) decreased TAS and CAT activity. We also showed that urease treatment per se was able to enhance TBA-RS levels. Finally, nitric oxide production was not altered by Orn and Hcit alone or in combination with hyperammonemia. Our data indicate that the major metabolites accumulating in hyperornithinemia-hyperammonemia-homocitrullinuria syndrome provoke lipid and protein oxidative damage and a reduction of the antioxidant defenses in the brain. Therefore, it is presumed that oxidative stress may represent a relevant pathomechanism involved in the brain damage found in patients affected by this disease.

Experimental evidence that ornithine and homocitrulline disrupt energy metabolism in brain of young rats

Tissue accumulation of ornithine (Orn), homocitrulline (Hcit), ammonia and orotic acid (Oro) is the biochemical hallmark of patients affected by hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, a disorder clinically characterized by neurological symptoms, whose pathophysiology is practically unknown. In the present study, we investigated the in vitro effect of Orn, Hcit and Oro on important parameters of energy metabolism in brain of 30-day-old Wistar rats since mitochondrial abnormalities have been observed in the affected patients. We first verified that Orn and Hcit significantly inhibited the citric acid cycle (inhibition of CO(2) synthesis from [1-(14)C] acetate, as well as aconitase and alpha-ketoglutarate dehydrogenase activities), the aerobic glycolytic pathway (reduced CO(2) production from [U-(14)C] glucose) and moderately the electron transfer flow (inhibitory effect on complex I-III). Hcit, but not Orn, was also able to significantly inhibit the mitochondrial creatine kinase activity. Furthermore, this inhibition was prevented by GSH, suggesting a possible role of reactive species oxidizing critical thiol groups of the enzyme. In contrast, the other enzyme activities of the citric acid cycle and of the electron transfer chain, as well as synaptic Na(+),K(+)-ATPase were not altered by either Orn or Hcit at concentrations as high as 5.0 mM. Similarly, Oro did not interfere with any of the tested parameters. Taken together, these data strongly indicate that Orn and Hcit compromise brain energy metabolism homeostasis and Hcit also interferes with cellular ATP transfer and buffering. It is therefore suggested that Orn and especially Hcit may be involved in the neurological damage found in patients affected by HHH syndrome.

HHH syndrome (hyperornithinaemia, hyperammonaemia, homocitrullinuria), with fulminant hepatitis-like presentation

We report a 3-year-old Italian patient with the hyperornithinaemia, hyperammonaemia, homocitrullinuria (HHH) syndrome who presented with neurological deterioration after an intercurrent infection. Hyperammonaemia, coagulopathy and moderate hypertransaminasaemia were detected on hospital admission. Severe hepatocellular necrosis with hypertransaminasaemia (aspartate aminotransferase 20,000 UI/L, alanine aminotransferase 18,400 UI/L) and coagulopathy (PT < 5%) rapidly developed within few days, prompting evaluation for liver transplantation. A protein-restricted diet and arginine supplementation were immediately started, with a rapid improvement of the patient's neurological conditions and normalization of liver function tests and blood ammonia. The diagnosis of HHH syndrome was based on the presence of the typical metabolic abnormalities. Molecular analysis of the SLC25A15 gene showed that the patient was heterozygous for two novel mutations (G113C and M273K). The diagnosis of HHH syndrome should be considered in patients with fulminant hepatitis-like presentations. Early identification and treatment of these patients can be life-saving and can avoid liver transplantation.

Hyperornithinemia, hyperammonemia, and homocitrullinuria syndrome with evidence of mitochondrial dysfunction due to a novel SLC25A15 (ORNT1) gene mutation in a Palestinian family

Hyperornithinemia, hyperammonemia, and homocitrullinuria (HHH) syndrome is caused by mutations in the SLC25A15 (ORNT1) gene encoding the mitochondrial ornithine transporter, but the mechanism of pathogenesis of the encephalopathy, spastic paraparesis and hepatopathy remains undetermined. HHH syndrome was diagnosed in a 2-year-old Palestinian boy with developmental delay and seizures, and subsequently in his 13-year-old brother with developmental delay. Direct sequencing of the PCR products of SLC25A15 exon amplifications revealed that both brothers were homozygous for a novel 446G deletion in exon 3 as well as for a 760A>T (I254L) polymorphism in exon 5, which is downstream of a premature termination codon produced by the frameshift resulting from the 446G deletion. The index patient had elevated liver enzymes as well as hyperalaninemia, lactic acidemia with an elevated lactate to pyruvate ratio, and increased urinary excretion of lactate, glutarate and Krebs cycle intermediates. These findings are indicative of mitochondrial dysfunction and are in accordance with ultrastructural studies showing increased numbers of large and bizarre mitochondria in liver, muscle, leukocytes and fibroblasts of some HHH patients. Neurologic and hepatic manifestations are characteristic of some primary mitochondrial disorders. Secondary mitochondrial dysfunction may contribute to the pathogenesis of these same features in HHH syndrome.

Electron tomography of neuronal mitochondria: three-dimensional structure and organization of cristae and membrane contacts

The structure of neuronal mitochondria from chick and rat was examined using electron microscope tomography of chemically fixed tissue embedded in plastic and sliced in approximately 500 nm-thick sections. Three-dimensional reconstructions of representative mitochondria were made from single-axis tilt series acquired with an intermediate voltage electron microscope (400 kV). The tilt increment was either 1 degree or 2 degrees ranging from -60 degrees to +60 degrees. The mitochondrial ultrastructure was similar across species and neuronal regions. The outer and inner membranes were each approximately 7 nm thick. The inner boundary membrane was found to lie close to the outer membrane, with a total thickness across both membranes of approximately 22 nm. We discovered that the inner membrane invaginates to form cristae only through narrow, tubular openings, which we call crista junctions. Sometimes the cristae remain tubular throughout their length, but often multiple tubular cristae merge to form lamellar compartments. Punctate regions, approximately 14 nm in diameter, were observed in which the inner and outer membranes appeared in contact (total thickness of both membranes approximately 14 nm). These contact sites are known to a play a key role in the transport of proteins into the mitochondrion. It has been hypothesized that contact sites may be proximal to crista junctions to facilitate transport of proteins destined for the cristae. However, our statistical analyses indicated that contact sites are randomly located with respect to these junctions. In addition, a close association was observed between endoplasmic reticulum membranes and the outer mitochondrial membrane, consistent with the reported mechanism of transport of certain lipids into the mitochondrion.

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).

Ciliated cultured dermal fibroblasts in a patient with hyperornithinemia, hyperammonemia and homocitrullinuria (HHH) syndrome

Hyperornithinemia, hyperammonemia and homocitrullinuria (HHH)-syndrome is a rare autosomal recessive disorder of the urea cycle, probably caused by a defect in ornithine transport across the hepatic inner mitochondrial membrane. Single rudimentary cilia were present in approximately ten percent of post-divisional or dividing fibroblasts cultured from the skin of a patient with the HHH-syndrome, whereas no such organelles were observed in dermal fibroblasts cultured from normal controls. Since single rudimentary ("primary," "oligo," "solitary") cilia have been observed in a variety of cells in animals and men but the stimuli for their formation and their significance remain controversial, a brief report on their presence in the as yet unreported condition (HHH-syndrome) was considered of interest; hopefully, it might contribute to the ultimate unravelling of some of the unresolved problems. It is of note that unlike the author's previous findings of these unusual organelles in cells affected by a pathological process (atherosclerosis), the rudimentary cilia were observed in the present instance in dividing or postdivisional cells. The implications of these (and other) observations must await further work.

The pathologist's approach to the diagnosis of metabolic disease

Many inborn errors of metabolism have tissue changes that suggest or indicate a diagnosis on morphologic bases. This emphasizes the involvement of the pathologist in the study of tissues obtained by biopsy as part of the diagnostic workup of cases with clinical evidence suggesting a metabolic disorder. Some of these diseases involve proteins with enzymatic activity with a consequent backlog of precursor metabolites. If these metabolites are insoluble or compartmentalized in cells, they accumulate in tissues. The diagnosis in those cases rests on establishing the nature of the accumulated materials and their topographic distribution. In some storage diseases, the abnormal substance can be identified in the affected tissues by histochemistry or ultrastructure. Another group of inborn disorders of metabolism produce changes in subcellular organelles such as peroxisomes and mitochondria. A third group is expressed by rather unique tissue changes which strongly suggest the diagnosis of metabolic disease.

Variations in mitochondrial ultrastructure and dynamics observed by high resolution scanning electron microscopy (HRSEM)

Rat adrenal cortex was processed for high resolution scanning electron microscopy (HRSEM) to confirm tubular cristae, reported by transmission electron microscopy to be present in cortex mitochondria. Mitochondria in several other tissue and cell types were also observed and their ultrastructure confirmed by using three-dimensional, stereo, high resolution scanning electron microscopy. The mitochondria in rat and human hepatocytes as well as human skin fibroblasts grown in culture contained tubular cristae approximately 30 nanometers in diameter. The fibroblast mitochondria proved to be long, up to 46 micrometers and branching, as compared to those in liver which were spherical in shape. Cold adapted brown fat cells were packed with mitochondria, these containing plate or shelf-like cristae. Branched, rat striated muscle mitochondria were observed to curve around contractile protein filament bundles. The muscle mitochondrial cristae were found to be both tubular and plate-like, within the same mitochondrion. The ratio of tubular cristae to plate-like cristae varied considerably between muscle mitochondria. In order to use ultrastructural changes in mitochondria for differential diagnosis, and because 3D reconstruction of mitochondria based on transmission electron microscopy serial sections is severely limited in resolution, it is imperative to first develop a correct understanding of tissue specific, normal mitochondrial ultrastructure based on three-dimensional, HRSEM methods.

Ultrastructural findings in citrullinaemia in Holstein-Friesian calves

Ultrastructural findings in the brains and livers of four calves affected with citrullinaemia due to a hereditary deficiency of the urea cycle enzyme arginosuccinate synthetase are described. The calves were affected in the 1st week of life with severe neurological disease characterised by depression, head pressing, stupor, convulsions and coma, accompanied by marked elevation of plasma citrulline and increasing plasma ammonia levels. Lesions included mild to moderate diffuse astroglial oedema in the cerebrocortical grey matter, and mild to severe hepatocellular hydropic change. The onset of the severe neurological signs was correlated with increasing levels of plasma ammonia, and the cerebral lesions were considered consistent with a bovine hyperammonaemic encephalopathy.

Argininosuccinic aciduria: long-term treatment with arginine

The presentation and 2 year treatment of a patient with argininosuccinic aciduria is reported. Erythrocyte argininosuccinate lyase activity was less than 2% of normal. Long-term management included protein restriction and arginine dietary supplementation. The child experienced three episodes of hyperammonaemia (greater than 100 microns), the first at birth, the second at 6.5 months and the third at 16 months. Neurological development deteriorated between 14 and 24 months. Hepatomegaly and biochemical hepatitis, a feature of this condition, was accompanied by enlarged mitochondria with tubular paracrystalline inclusions not previously recognized in this disorder.

Liver fibrosis in carbamoylphosphate synthetase deficiency

Structural sequelae of inherited defects of the urea cycle in general, and their liver pathology in particular, are still not well understood. This holds true especially for the possible late effects in involved organs of patients now surviving longer because of more effective therapy. Some urea cycle defects may result in chronic and progressive liver damage, as has been reported. A peculiar type of liver fibrosis was observed in a girl with carbamoylphosphate synthetase deficiency, who survived for 1 year and 7 months. Hepatic fibrosis, or even cirrhosis, has been observed in argininosuccinic aciduria. Long-term survivors with urea cycle disorders may form a group at risk for the development of chronic fibrosing liver disease.

Ultrastructural changes in fibroblast mitochondria of a patient with HHH-syndrome

Electron micrographs of fibroblasts of an HHH-syndrome patient showed abnormal structures, similar, but not identical, to those observed in the liver of such patients. It is suggested that incorporation of a mutated protein into the inner mitochondrial membrane gives rise to a rearrangement of that membrane, resulting in unusual structures.

The pH of mitochondria of fibroblasts from a hyperornithinaemia, hyperammonaemia, homocitrullinuria-syndrome patient

The intracellular pH of control fibroblasts and of fibroblasts of a HHH -syndrome patient have been determined. Values of 6.94 +/- 0.15 and 7.05 +/- 0.14 for control and patient fibroblasts, respectively, were found. By means of analyses of malate in the cytosolic and particulate fractions of the fibroblasts the differences in pH between these two cellular compartments were estimated to be 1.34 +/- 0.12 and 1.38 +/- 0.18 for control and patient, respectively. Neither difference was statistically significant. The decrease in the rate of ornithine uptake by mitochondria of the patient's fibroblast is therefore not due to an increased intramitochondrial pH.