Clinical, biochemical, and molecular spectrum of hyperargininemia due to arginase I deficiency

Clinical features and neurologic progression of hyperargininemia

Hyperargininemia is an autosomal recessive metabolic disorder caused by a deficiency of enzyme arginase I. It is a rare pan-ethnic disease with a clinical presentation distinct from that of other urea cycle disorders, and hyperammonemic encephalopathy is not usually observed. Hyperargininemia is one of the few treatable causes of pediatric spastic paraparesis, and can be confused with cerebral palsy. We retrospectively evaluated the clinical onset, neurologic manifestations, progression of abnormalities, electroencephalographic abnormalities, and laboratory findings of 16 Brazilian patients with hyperargininemia. Relevant data about the clinical spectrum and natural history of hyperargininemia are detailed. Progressive spastic diplegia constituted the key clinical abnormality in this group, but variability in clinical presentation and progression were evident in our series. Seizures in hyperargininemia may be more common than reported in previous studies. Features distinguishing hyperargininemia from cerebral palsy and hereditary spastic paraplegia are emphasized in this large series of patients.

Suggested guidelines for the diagnosis and management of urea cycle disorders

Urea cycle disorders (UCDs) are inborn errors of ammonia detoxification/arginine synthesis due to defects affecting the catalysts of the Krebs-Henseleit cycle (five core enzymes, one activating enzyme and one mitochondrial ornithine/citrulline antiporter) with an estimated incidence of 1:8.000. Patients present with hyperammonemia either shortly after birth (~50%) or, later at any age, leading to death or to severe neurological handicap in many survivors. Despite the existence of effective therapy with alternative pathway therapy and liver transplantation, outcomes remain poor. This may be related to underrecognition and delayed diagnosis due to the nonspecific clinical presentation and insufficient awareness of health care professionals because of disease rarity. These guidelines aim at providing a trans-European consensus to: guide practitioners, set standards of care and help awareness campaigns. To achieve these goals, the guidelines were developed using a Delphi methodology, by having professionals on UCDs across seven European countries to gather all the existing evidence, score it according to the SIGN evidence level system and draw a series of statements supported by an associated level of evidence. The guidelines were revised by external specialist consultants, unrelated authorities in the field of UCDs and practicing pediatricians in training. Although the evidence degree did hardly ever exceed level C (evidence from non-analytical studies like case reports and series), it was sufficient to guide practice on both acute and chronic presentations, address diagnosis, management, monitoring, outcomes, and psychosocial and ethical issues. Also, it identified knowledge voids that must be filled by future research. We believe these guidelines will help to: harmonise practice, set common standards and spread good practices with a positive impact on the outcomes of UCD patients.

Treatable inborn errors of metabolism causing intellectual disability: a systematic literature review

BACKGROUND

Intellectual disability ('developmental delay' at age<5 years) affects 2.5% of population worldwide. Recommendations to investigate genetic causes of intellectual disability are based on frequencies of single conditions and on the yield of diagnostic methods, rather than availability of causal therapy. Inborn errors of metabolism constitute a subgroup of rare genetic conditions for which an increasing number of treatments has become available. To identify all currently treatable inborn errors of metabolism presenting with predominantly intellectual disability, we performed a systematic literature review.

METHODS

We applied Cochrane Collaboration guidelines in formulation of PICO and definitions, and searched in Pubmed (1960-2011) and relevant (online) textbooks to identify 'all inborn errors of metabolism presenting with intellectual disability as major feature'. We assessed levels of evidence of treatments and characterised the effect of treatments on IQ/development and related outcomes.

RESULTS

We identified a total of 81 'treatable inborn errors of metabolism' presenting with intellectual disability as a major feature, including disorders of amino acids (n=12), cholesterol and bile acid (n=2), creatine (n=3), fatty aldehydes (n=1); glucose homeostasis and transport (n=2); hyperhomocysteinemia (n=7); lysosomes (n=12), metals (n=3), mitochondria (n=2), neurotransmission (n=7); organic acids (n=19), peroxisomes (n=1), pyrimidines (n=2), urea cycle (n=7), and vitamins/co-factors (n=8). 62% (n=50) of all disorders are identified by metabolic screening tests in blood (plasma amino acids, homocysteine) and urine (creatine metabolites, glycosaminoglycans, oligosaccharides, organic acids, pyrimidines). For the remaining disorders (n=31) a 'single test per single disease' approach including primary molecular analysis is required. Therapeutic modalities include: sick-day management, diet, co-factor/vitamin supplements, substrate inhibition, stemcell transplant, gene therapy. Therapeutic effects include improvement and/or stabilisation of psychomotor/cognitive development, behaviour/psychiatric disturbances, seizures, neurologic and systemic manifestations. The levels of available evidence for the various treatments range from Level 1b,c (n=5); Level 2a,b,c (n=14); Level 4 (n=45), Level 4-5 (n=27). In clinical practice more than 60% of treatments with evidence level 4-5 is internationally accepted as 'standard of care'.

CONCLUSION

This literature review generated the evidence to prioritise treatability in the diagnostic evaluation of intellectual disability. Our results were translated into digital information tools for the clinician (www.treatable-id.org), which are part of a diagnostic protocol, currently implemented for evaluation of effectiveness in our institution. Treatments for these disorders are relatively accessible, affordable and with acceptable side-effects. Evidence for the majority of the therapies is limited however; international collaborations, patient registries, and novel trial methodologies are key in turning the tide for rare diseases such as these.

Hyperargininemia: a family with a novel mutation in an unexpected site

Hyperargininemia is a rare autosomal recessive disorder of the last step of the urea cycle characterized by a deficiency in liver arginase1. Clinically, it differs from other urea cycle defects by a progressive paraparesis of the lower limbs (spasticity and contractures) with hyperreflexia, neurodevelopmental delay and regression in early childhood. Growth is affected as well. Hyperammonemia is episodic, if present at all. The disease is caused by mutations in the ARG1 gene; there are approximately 20 different known ARG1 mutations with considerable genetic heterogeneity. We describe two Arab siblings with a late diagnosis of hyperargininemia and present the genetic findings in their family. As ARG1 sequencing was unrevealing despite suggestive clinical and laboratory findings, molecular cDNA analysis was performed. The ARG1 expression pattern identified a 125-bp out-of-frame insertion between exons 3 and 4, leading to the addition of 41 amino acids and a premature termination codon TGA at the sixth codon downstream. The insertion originated at intron 3 and was attributable to a novel c.305 + 1323 t > c intronic base change that enabled an enhancement phenomenon. This is the first reported exon-splicing-enhancer mutation in patients with hyperargininemia. The clinical course and genetic findings emphasize the possibility that hyperargininemia causes neurological deterioration in children and the importance of analyzing the expression pattern of the candidate gene when sequencing at the DNA level is unrevealing.

Evaluation and long-term follow-up of infants with inborn errors of metabolism identified in an expanded screening programme

Newborn screening (NBS) by tandem mass spectrometry started in Galicia (Spain) in 2000. We analyse the results of screening and clinical follow-up of inborn errors of metabolism (IEM) detected during 10 years. Our programme basically includes the disorders recommended by the American College of Medical Genetics. Since 2002, blood and urine samples have been collected from every newborn on the 3rd day of life; before then, samples were collected between the 5th and 8th days. Newborns who show abnormal results are referred to the clinical unit for diagnosis and treatment. In these 10 years, NBS has led directly to the identification of 137 IEM cases (one per 2060 newborns, if 35 cases of benign hyperphenylalaninemia are excluded). In addition, 33 false positive results and 10 cases of transitory elevation of biomarkers were identified (making the positive predictive rate 76.11%), and 4 false negative results. The use of urine samples contributed significantly to IEM detection in 44% of cases. Clinical symptoms appeared before positive screening results in nine patients (6.6%), four of them screened between days 5 and 8. The death rate was 2.92%; of the survivors, 95.5% were asymptomatic after a mean observation period of 54 months, and only two had an intellectual/psychomotor development score less than 85. Compared to other studies, a high incidence of type I glutaric aciduria was detected, one in 35,027 newborns. This report highlights the benefits of urine sample collection during screening, and it is the first study on expanded newborn screening results in Spain.

Arginase I deficiency: severe infantile presentation with hyperammonemia: more common than reported?

Enzyme defects of the urea cycle typically present with significant hyperammonemia and its associated toxicity, in the first few months of life. However, arginase I (ARG1) deficiency, a rare autosomal recessive disorder, has classically been the exception. ARG1 deficiency usually presents later in life with spasticity, seizures, failure to thrive and developmental regression. Neonatal and early infantile presentation of ARG1 deficiency with severe hyperammonemia remains rare and only six such cases have been described. We report a severely affected infant with ARG1 deficiency who presented at 6 weeks of age with lethargy, poor feeding and severe encephalopathy caused by hyperammonemia. The clinical and biochemical features of the proband and six other previously reported cases with neonatal or infantile-onset presentation of ARG1 deficiency with hyperammonemia are reviewed. In addition, the clinical spectrum of seven previously unpublished patients with later onset ARG1 deficiency, who also experienced recurrent hyperammonemia, is presented. Several biochemical abnormalities have been postulated to play a role in the pathogenesis of the neurological changes in ARG1 deficiency including hyperargininemia, elevated guanidino compounds and elevated glutamine levels, as well as the hyperammonemia. The index case demonstrated many of these. The cases reviewed here suggest a genotype/phenotype correlation and advocate for the addition of arginine as a primary target in newborn screening programs.

Argininemia presenting with progressive spastic diplegia

Argininemia is caused by a deficiency of arginase 1, which catalyzes the final step in the urea cycle, i.e., the cytosolic hydrolysis of arginine to ornithine and urea. In contrast to other urea cycle disorders, hyperammonemic encephalopathy is rarely observed in patients with argininemia. Rather, most exhibit an insidious onset and progression of neurologic manifestations, including spastic diplegia. We describe the first Korean patient with argininemia, manifesting as slowly progressive spastic diplegia. Our patient carries c.[32T>C]+[913G>A] (p.[Ile11Thr]+[Gly305Arg]) mutations in the ARG1 gene. The latter mutation was not previously reported. Although argininemia is a very rare disease, it is recognized as a pan-ethnic disorder. We conclude that argininemia should be considered more frequently in the differential diagnosis of a patient with slowly progressive neurologic manifestations, especially progressive spastic diplegia, even in a population where argininemia was previously unknown.

Urinary 2-hydroxy-5-oxoproline, the lactam form of α-ketoglutaramate, is markedly increased in urea cycle disorders

α-Ketoglutaramate (KGM) is the α-keto acid analogue of glutamine, which exists mostly in equilibrium with a lactam form (2-hydroxy-5-oxoproline) under physiological conditions. KGM was identified in human urine and its concentration quantified by gas chromatography/mass spectrometry (GC/MS). The keto acid was shown to be markedly elevated in urine obtained from patients with primary hyperammonemia due to an inherited metabolic defect in any one of the five enzymes of the urea cycle. Increased urinary KGM was also noted in other patients with primary hyperammonemia, including three patients with a defect resulting in lysinuric protein intolerance and one of two patients with a defect in the ornithine transporter I. These findings indicate disturbances in nitrogen metabolism, most probably at the level of glutamine metabolism in primary hyperammonemia diseases. Urinary KGM levels, however, were not well correlated with secondary hyperammonemia in patients with propionic acidemia or methylmalonic acidemia, possibly as a result, in part, of decreased glutamine levels. In conclusion, the GC/MS procedure has the required lower limit of quantification for analysis of urinary KGM, which is markedly increased in urea cycle disorders and other primary hyperammonemic diseases.

Neonatal cholestasis: an uncommon presentation of hyperargininemia

Hyperargininemia is a rare inborn error of metabolism due to arginase deficiency, which is inherited in an autossomal recessive manner. Arginase is the final enzyme of the urea cycle and catalyzes the conversion of arginine to urea and ornithine. This condition typically presents in early childhood (between 2 and 4 years of age) with developmental delay associated with progressive spastic paraparesis. Neonatal presentation is very uncommon with a poorly described outcome. Here, we discuss two cases of neonatal cholestasis as initial clinical presentation of hyperargininemia. In case 1, diagnosis was established at 2 months of age upon investigation of the etiology of cholestatic injury pattern and hepatosplenomegaly, and treatment was then initiated at when the patient was 3 months old. Unfortunately, the patient had progressive biliary cirrhosis to end-stage liver disease complicated with portal hypertension for which she underwent successful orthotopic liver transplant at 7 years of age. In case 2, hyperargininemia was identified through newborn screening and treatment was started when patient was 21 days old. Cholestasis was only identified in the patient's further evaluation and it resolved 2 weeks into treatment. The patient is currently 18 months old and her development and neurological examination remain unremarkable. Neonatal cholestasis as first presentation of hyperargininemia is rare, but this disorder should be included in the differential diagnosis of unexplained cholestasis in the neonate. In fact, these two cases suggest that arginase deficiency may be the cause of cholestatic liver disease.

Four years of expanded newborn screening in Portugal with tandem mass spectrometry

INTRODUCTION

The Portuguese Neonatal Screening Programme (PNSP) was started in 1979 for phenylketonuria (2,590,700 newborns screened; prevalence 1:11,031) and, shortly after, for congenital hypothyroidism (2,558,455 newborns screened; prevalence 1:3,174). In 2004, expanded neonatal screening was implemented in the National Laboratory. The programme is not mandatory and has 99.8% coverage of the country (including Madeira and the Azores islands).

MATERIAL AND METHODS

In the past 4 years, 316,243 neonates were screened with the use of tandem mass spectrometry (MS/MS) to test for selected amino acids and acylcarnitines.

RESULTS

During this time, 132 patients were identified with 24 different inherited metabolic diseases (classic forms and variants). To date, the global frequency for all disorders integrated into the PNSP is estimated to be 1:1,380, with 1:2,396 for metabolic disorders. A total of 379 tests (0.12%) were classified as having false positive results, yielding an overall specificity of 99.9%. Despite the low frequency of several disorders, the positive predictive value of the overall MS/MS screening was found to be 26%, reflecting high diagnostic specificity of the method. Diagnostic sensitivity of extended screening for the different groups of disorders was 100%. Eight cases of maternal disorders [three glutaric aciduria type I, one carnitine transporter defect, and four 3-methylcrotonyl coenzyme A (CoA) carboxylase deficiency] were also detected through newborn screening.

CONCLUSIONS

Our data support the advantage of a centralised laboratory for screening an elevated number of samples and making decisions if relying on a clinical network able to provide fast treatment and a good outcome in the screened cases.

Diffusion tensor imaging in arginase deficiency reveals damage to corticospinal tracts

Individuals with a proximal urea cycle disorder, such as carbamoyl phosphate synthetase deficiency 1 or ornithine transcarbamylase deficiency, may present with encephalopathy resulting from hyperammonemia. The clinical presentation of arginase deficiency is considerably different, characterized by progressive spasticity involving the lower extremities and usually dementia. Diagnosis may be delayed, and patients are often thought to have cerebral palsy. The true etiology of brain injury in arginase deficiency is unknown, but is not thought to be due to hyperammonemia and brain swelling, the mechanism of injury recognized in ornithine transcarbamylase deficiency. Elevated arginine could augment nitric oxide synthesis, leading to oxidative damage. The hypothesis for the present study was that specific brain vulnerability in arginase deficiency would involve microstructural alterations in corticospinal tracts and that this finding, as measured by diffusion tensor imaging, would differ from age-matched control subjects and those with ornithine transcarbamylase deficiency. Diffusion tensor imaging data were compared for a 17-year-old male patient with arginase deficiency, age-matched normal control subjects, and age-matched individuals with ornithine transcarbamylase deficiency. Significant differences were found in suspected areas of interest, specifically in the corticospinal tracts. This finding confirms the hypothesis that the mechanism of injury in arginase deficiency, although still unknown, is unlikely to be similar to that causing ornithine transcarbamylase deficiency.

Creatine metabolism and the urea cycle

Because creatine and creatine phosphate are irreversibly converted to creatinine, there is a continuous need for their replacement. This occurs by means of diet and de novo synthesis. Dietary creatine is provided in animal products and can amount to about half of the required amount. Synthesis provides the remainder. Creatine synthesis is a major component of arginine metabolism, amounting to more than 20% of the dietary intake of this amino acid. Creatine metabolism is of importance to patients with urea cycle disorders in two ways, both related to arginine levels. In patients with arginase deficiency, markedly elevated arginine levels may result in higher concentrations of guanidinoacetate and higher rates of creatine synthesis. This is of concern because it is thought that elevated levels of guanidinoacetate may exert neurotoxic effects. In the case of the other urea cycle disorders, arginine levels are markedly decreased unless the patients are supplemented with this amino acid. Decreased levels of arginine may result in decreased rates of creatine synthesis. This may be compounded by the fact that such patients, maintained on low protein diets, will also have lower dietary creatine intakes. There is some evidence that this may decrease brain creatine levels which may contribute to the neurological symptoms exhibited by these patients. It is clear that patients with urea cycle disorders also have altered creatine metabolism. Whether this contributes in a significant way to their neurological symptoms remains an open question.

Ammonia toxicity to the brain: effects on creatine metabolism and transport and protective roles of creatine

Hyperammonemia can provoke irreversible damage to the developing brain, with the formation of cortical atrophy, ventricular enlargement, demyelination or gray and white matter hypodensities. Among the various pathogenic mechanisms involved, alterations in cerebral energy have been demonstrated. In particular, we could show that ammonia exposure generates a secondary deficiency in creatine in brain cells, by altering the brain expression and activity of the genes allowing creatine synthesis (AGAT and GAMT) and transport (SLC6A8). On the other hand, it is known that creatine administration can exert protective effects in various neurodegenerative processes. We could also show that creatine co-treatment under ammonia exposure can protect developing brain cells from some of the deleterious effects of ammonia, in particular axonal growth impairment. This article focuses on the effects of ammonia exposure on creatine metabolism and transport in developing brain cells, and on the potential neuroprotective properties of creatine in the brain exposed to ammonium.

Current concepts in the pathogenesis of urea cycle disorders

The common feature of urea cycle diseases (UCD) is a defect in ammonium elimination in liver, leading to hyperammonemia. This excess of circulating ammonium eventually reaches the central nervous system, where the main toxic effects of ammonium occur. These are reversible or irreversible, depending on the age of onset as well as the duration and the level of ammonium exposure. The brain is much more susceptible to the deleterious effects of ammonium during development than in adulthood, and surviving UCD patients may develop cortical and basal ganglia hypodensities, cortical atrophy, white matter atrophy or hypomyelination and ventricular dilatation. While for a long time, the mechanisms leading to these irreversible effects of ammonium exposure on the brain remained poorly understood, these last few years have brought new data showing in particular that ammonium exposure alters several amino acid pathways and neurotransmitter systems, cerebral energy, nitric oxide synthesis, axonal and dendritic growth, signal transduction pathways, as well as K(+) and water channels. All these effects of ammonium on CNS may eventually lead to energy deficit, oxidative stress and cell death. Recent work also proposed neuroprotective strategies, such as the use of NMDA receptor antagonists, nitric oxide inhibitors, creatine and acetyl-l-carnitine, to counteract the toxic effects of ammonium. Better understanding the pathophysiology of ammonium toxicity to the brain under UCD will allow the development of new strategies for neuroprotection.

Early-onset hyperargininaemia: a severe disorder?

Hyperargininaemia is a rare inborn error of metabolism due to a defect in the final step of the urea cycle. Infantile onset is the most common presentation with recurrent vomiting and psychomotor delay associated with spastic paraparesis; chronic hyperammonaemia is often overlooked. Neonatal and early-onset presentations are very uncommon and their clinical course not well-described. We report on a 3-week-old hyperargininaemic girl who presented with neurological deterioration associated with liver failure and 47-day ammonia intoxication before diagnosis could be made and treatment started. Despite appropriate but delayed treatment, our patient exhibited severe psychomotor delay at age 1 year.

CONCLUSION

Early identification and management of this rare but potentially treatable affection is crucial as delayed management may result in poor neurological outcome.

Inborn errors of metabolism and motor disturbances in children

Motor disturbances are very common in paediatric neurology. Often families can be reassured that these are just variants of normal development. However, abnormal movements can also be the hallmark of severe brain dysfunction of different and complex origins. This review concentrates on motor disturbances as frequent and important symptoms of inborn errors of metabolism. A structured diagnostic approach is developed taking into account age-dependent physiological developments and pathophysiological responses of gross and fine motor functions. A series of investigations are presented with the primary aim of early diagnosis of treatable conditions. The correct recognition and differentiation of movement disorders (ataxia, rigid akinetic syndrome (Fparkinsonism_), dystonia, athetosis, tremor,and others), spasticity, and neuromuscular disorders, requires profound neurological expertise. A high level of suspicion and close interaction between paediatric neurologists and specialists in inborn errors of metabolism are indispensable to effectively and timely identify patients in whom motor disturbances are the presenting and/or main symptom of an inborn error.

Amino acids in CSF and plasma in hyperammonaemic coma due to arginase1 deficiency

We report the CSF and plasma amino acid concentrations and their ratios in a male patient with arginase1 deficiency with an unusual early presentation at 34 days of age. He developed hyperammonaemic coma (ammonia >400 μmol/L; normal <90 μmol/L) on postnatal day 35. CSF and plasma concentrations were assayed by ion-exchange chromatography on day 36. Arginine was increased both in plasma (971 μmol/L; controls (mean ± 2SD) 50 ± 42) and in CSF (157 μmol/L; controls 19 ± 8.6), resulting in a normal CSF/plasma ratio of 0.16 (controls 0.41 ± 0.26). Interestingly, glutamine was disproportionately high in CSF (3114 μmol/L; controls 470 ± 236) but normal in plasma (420 μmol/L; controls 627 ± 246); the ratio exceeded unity (7.4; controls 0.76 ± 0.31). The CSF/plasma ratios of most neutral amino acids were elevated but not those of the imino- and of the dibasic amino acids lysine and ornithine. The mechanism leading to the increase of most neutral amino acids in brain is not known.

CONCLUSION

A normal glutamine in plasma does not exclude an increased concentration in CSF; it could be useful to ascertain by MRS that a high CSF glutamine concentration truly reflects a high concentration in brain tissue for better understanding its pathogenesis.

L-arginine and Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by progressive neurodegeneration and loss of cognitive and memory functions. Although the exact causes of AD are still unclear, evidence suggests that atherosclerosis, redox stress, inflammation, neurotransmitter dysregulation, and impaired brain energy metabolism may all be associated with AD pathogenesis. Herein, we explore a possible role for L-arginine (L-arg) in AD, taking into consideration known functions for L-arg in atherosclerosis, redox stress and the inflammatory process, regulation of synaptic plasticity and neurogenesis, and modulation of glucose metabolism and insulin activity. L-arg, a precursor of nitric oxide and polyamine, exhibits multiple functions in human health and may play a prominent role in age-related degenerative diseases such as AD.

Neurologic damage and neurocognitive dysfunction in urea cycle disorders

Although the survival of patients who have urea cycle disorders has improved with the use of modalities such as alternative pathway therapy and hemodialysis, neurologic outcome is suboptimal. Patients often manifest with a variety of neurologic abnormalities, including cerebral edema, seizures, cognitive impairment, and psychiatric illness. Current hypotheses of the pathogenesis underlying brain dysfunction in these patients have focused on several lines of investigation, including the role of glutamine in causing cerebral edema, mitochondrial dysfunction leading to energy failure and the production of free radicals, and altered neurotransmitter metabolism. Advances in understanding the pathogenetic mechanisms underlying brain impairment in urea cycle disorders may lead to the development of therapies designed to interfere with the molecular cascade that ultimately leads to cerebral edema and other brain pathological findings.

A review of newborn screening in the era of tandem mass spectrometry: what's new for the pediatric neurologist?

Newborn screening has evolved from a single test for a single metabolite to a test that detects more than 90 metabolites on a single blood spot. In the past decade, the panel of newborn-screening disorders has rapidly expanded and will continue to grow as more is discovered about the human genome. It continues to be a very sensitive population screening tool that is susceptible to the status of the infant and the timing of the specimen collection. This review discusses the disorders that should be detected on neonatal bloodspot screening and what pediatric neurologists may see in those that were detected on newborn screening and treated and those that have been untreated.

New indications and controversies in arginine therapy

Arginine is an important, versatile and a conditionally essential amino acid. Besides serving as a building block for tissue proteins, arginine plays a critical role in ammonia detoxification, and nitric oxide and creatine production. Arginine supplementation is an essential component for the treatment of urea cycle defects but recently some reservations have been raised with regards to the doses used in the treatment regimens of these disorders. In recent years, arginine supplementation or restriction has been proposed and trialled in several disorders, including vascular diseases and asthma, mitochondrial encephalopathy lactic acidosis and stroke-like episodes (MELAS), glutaric aciduria type I and disorders of creatine metabolism, both production and transportation into the central nervous system. Herein we present new therapeutic indications and controversies surrounding arginine supplementation or deprivation.

Increased plasma and tissue guanidino compounds in a mouse model of hyperargininemia

In humans, arginase I (AI)-deficiency results in hyperargininemia, a metabolic disorder with symptoms of progressive neurological and intellectual impairment, spasticity, persistent growth retardation, and episodic hyperammonemia. A deficiency of arginase II (AII) has never been detected and the clinical disorder, if any, associated with its deficiency has not been defined. Since the spasticity and paucity of hyperammonemic crises seen in human AI-deficient patients are not features of the other urea cycle disorders, the likelihood of ammonia as the main neuropathogenic agent becomes extremely low, and the modest elevations of arginine seen in the brains of our mouse model of hyperargininemia make it an unlikely candidate as well. Specific guanidino compounds, direct or indirect metabolites of arginine, are elevated in the blood of patients with uremia. Other guanidino compounds are also increased in plasma and cerebrospinal fluid of hyperargininemic patients making them plausible as neurotoxins in these disorders. We analyzed several guanidino compounds in our arginase single and double knockout animals and found that alpha-keto-delta-guanidinovaleric acid, alpha-N-acetylarginine, and argininic acid were increased in the brain tissue from the AI knockout and double knockout animals. Several compounds were also increased in the plasma, liver, and kidneys. This is the first time that several of the guanidino compounds have been shown to be elevated in the brain tissue of an arginase-deficient mammal, and it further supports their possible role as the neuropathogenic agents responsible for the complications seen in arginase deficiency.

Hyperammonemia-induced toxicity for the developing central nervous system

In pediatric patients, hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle deficiencies or organic acidemias. The brain is much more susceptible to the deleterious effects of ammonium during development than in adulthood. Hyperammonemia can provoke irreversible damages to the developing central nervous system that lead to cortical atrophy, ventricular enlargement and demyelination, responsible for cognitive impairment, seizures and cerebral palsy. Until recently, the mechanisms leading to these irreversible cerebral damages were poorly understood. Using experimental models allowing the analysis of the neurotoxic effects of ammonium on the developing brain, these last years have seen the emergence of new clues showing that ammonium exposure alters several amino acid pathways and neurotransmitter systems, as well as cerebral energy metabolism, nitric oxide synthesis, oxidative stress, mitochondrial permeability transition and signal transduction pathways. Those alterations may explain neuronal loss and impairment of axonal and dendritic growth observed in the different models of congenital hyperammonemia. Some neuroprotective strategies such as the potential use of NMDA receptor antagonists, nitric oxide inhibitors, creatine and acetyl-l-carnitine have been suggested to counteract these toxic effects. Unraveling the molecular mechanisms involved in the chain of events leading to neuronal dysfunction under hyperammonemia may be useful to develop new potential strategies for neuroprotection.