Argininemia: a treatable genetic cause of progressive spastic diplegia simulating cerebral palsy: case reports and literature review

Arginase deficiency-An unheralded cause of developmental epileptic encephalopathy

Arginase deficiency, which leads to hyperargininaemia is a rare urea cycle disorder caused by a mutation in the ARG1 gene. It is an under-recognized cause of pediatric developmental epileptic encephalopathy, with the key coexistent clinical features being developmental delay or regression and spasticity. Detection of ARG1 gene mutation on genetic testing is the confirmatory diagnostic test. However, elevated levels of plasma arginine and low plasma arginase level can be considered as biochemical markers for diagnosis. We present two cases of arginase deficiency with genetically confirmed ARG1 mutation in one and biochemical confirmation in both. As the spectrum of epilepsy in arginase deficiency has been less explored, we attempted to elucidate the novel electroclinical features and syndromic presentations in these patients. Informed consent was obtained from families of patients. Electroclinical diagnosis was consistent with Lennox Gastaut syndrome (LGS) in the first patient while the second patient had refractory atonic seizures with electrophysiological features consistent with developmental and epileptic encephalopathy. Though primary hyperammonaemia is not a consistent feature, secondary hyperammonaemia in the setting of infectious triggers and drugs like valproate (valproate sensitivity) has been well described as also observed in our patient. In the absence of an overt antecedent in a child with spasticity and seizure disorder, with a progressive course consistent with a developmental epileptic encephalopathy, arginase deficiency merits consideration. Diagnosis often has important therapeutic implications with respect to dietary management and choice of the appropriate antiseizure medications.

L-arginine homeostasis governs adult neural stem cell activation by modulating energy metabolism in vivo

Neurogenesis in the developing and adult brain is intimately linked to remodeling of cellular metabolism. However, it is still unclear how distinct metabolic programs and energy sources govern neural stem cell (NSC) behavior and subsequent neuronal differentiation. Here, we found that adult mice lacking the mitochondrial urea metabolism enzyme, Arginase-II (Arg-II), exhibited NSC overactivation, thereby leading to accelerated NSC pool depletion and decreased hippocampal neurogenesis over time. Mechanistically, Arg-II deficiency resulted in elevated L-arginine levels and induction of a metabolic shift from glycolysis to oxidative phosphorylation (OXPHOS) caused by impaired attachment of hexokinase-I to mitochondria. Notably, selective inhibition of OXPHOS ameliorated NSC overactivation and restored abnormal neurogenesis in Arg-II deficient mice. Therefore, Arg-II-mediated intracellular L-arginine homeostasis directly influences the metabolic fitness of neural stem cells that is essential to maintain neurogenesis with age.

The role and control of arginine levels in arginase 1 deficiency

Arginase 1 Deficiency (ARG1-D) is a rare urea cycle disorder that results in persistent hyperargininemia and a distinct, progressive neurologic phenotype involving developmental delay, intellectual disability, and spasticity, predominantly affecting the lower limbs and leading to mobility impairment. Unlike the typical presentation of other urea cycle disorders, individuals with ARG1-D usually appear healthy at birth and hyperammonemia is comparatively less severe and less common. Clinical manifestations typically begin to develop in early childhood in association with high plasma arginine levels, with hyperargininemia (and not hyperammonemia) considered to be the primary driver of disease sequelae. Nearly five decades of clinical experience with ARG1-D and empirical studies in genetically manipulated models have generated a large body of evidence that, when considered in aggregate, implicates arginine directly in disease pathophysiology. Severe dietary protein restriction to minimize arginine intake and diversion of ammonia from the urea cycle are the mainstay of care. Although this approach does reduce plasma arginine and improve patients' cognitive and motor/mobility manifestations, it is inadequate to achieve and maintain sufficiently low arginine levels and prevent progression in the long term. This review presents a comprehensive discussion of the clinical and scientific literature, the effects and limitations of the current standard of care, and the authors' perspectives regarding the past, current, and future management of ARG1-D.

Arginase 1 Deficiency in Patients Initially Diagnosed with Hereditary Spastic Paraplegia

Background

Arginase 1 Deficiency (ARG1-D) is a rare autosomal recessive urea cycle disorder (UCD) characterized by pathologic elevation of plasma arginine and debilitating manifestations. Based on clinical commonalities and low disease awareness, ARG1-D can be diagnosed as hereditary spastic paraplegia (HSP), leading to treatment delays.

Cases

A Hispanic woman with unremarkable medical history experienced progressive lower-limb spasticity in her 20s and received a diagnosis of HSP. She developed significant gait abnormalities and is unable to walk without assistance. More recently, two Hispanic brothers with childhood-onset manifestations including lower-limb spasticity, developmental delays, and seizures presented with suspected HSP. All three patients were ultimately diagnosed with ARG1-D based on plasma arginine several-fold above normal levels and loss-of-function ARG1 variants. Disease progression occurred before ARG1-D was correctly diagnosed.

Literature Review

Retrospective analyses demonstrate that diagnostic delays in ARG1-D are common and can be lengthy. Because of clinical similarities between ARG1-D and HSP, such as insidious onset and progressive spasticity, accurate diagnosis of ARG1-D is challenging. Timely ARG1-D diagnosis is critical because this UCD is a treatable genetic cause of progressive lower-limb spasticity.

Conclusions

Arginase 1 Deficiency should be considered in HSP differential diagnosis until biochemically/genetically excluded, and should be routinely included in HSP gene panels.

Epidemiology, methods of diagnosis, and clinical management of patients with arginase 1 deficiency (ARG1-D): A systematic review

BACKGROUND

Arginase 1 Deficiency (ARG1-D) is a rare, progressive, metabolic disorder that is characterized by devastating manifestations driven by elevated plasma arginine levels. It typically presents in early childhood with spasticity (predominately affecting the lower limbs), mobility impairment, seizures, developmental delay, and intellectual disability. This systematic review aims to identify and describe the published evidence outlining the epidemiology, diagnosis methods, measures of disease progression, clinical management, and outcomes for ARG1-D patients.

METHODS

A comprehensive literature search across multiple databases such as MEDLINE, Embase, and a review of clinical studies in ClinicalTrials.gov (with results reported) was carried out per PRISMA guidelines on 20 April 2020 with no date restriction. Pre-defined eligibility criteria were used to identify studies with data specific to patients with ARG1-D. Two independent reviewers screened records and extracted data from included studies. Quality was assessed using the modified Newcastle-Ottawa Scale for non-comparative studies.

RESULTS

Overall, 55 records reporting 40 completed studies and 3 ongoing studies were included. Ten studies reported the prevalence of ARG1-D in the general population, with a median of 1 in 1,000,000. Frequently reported diagnostic methods included genetic testing, plasma arginine levels, and red blood cell arginase activity. However, routine newborn screening is not universally available, and lack of disease awareness may prevent early diagnosis or lead to misdiagnosis, as the disease has overlapping symptomology with other diseases, such as cerebral palsy. Common manifestations reported at time of diagnosis and assessed for disease progression included spasticity (predominately affecting the lower limbs), mobility impairment, developmental delay, intellectual disability, and seizures. Severe dietary protein restriction, essential amino acid supplementation, and nitrogen scavenger administration were the most commonly reported treatments among patients with ARG1-D. Only a few studies reported meaningful clinical outcomes of these interventions on intellectual disability, motor function and adaptive behavior assessment, hospitalization, or death. The overall quality of included studies was assessed as good according to the Newcastle-Ottawa Scale.

CONCLUSIONS

Although ARG1-D is a rare disease, published evidence demonstrates a high burden of disease for patients. The current standard of care is ineffective at preventing disease progression. There remains a clear need for new treatment options as well as improved access to diagnostics and disease awareness to detect and initiate treatment before the onset of clinical manifestations to potentially enable more normal development, improve symptomatology, or prevent disease progression.

Intermittent lipid nanoparticle mRNA administration prevents cortical dysmyelination associated with arginase deficiency

Arginase deficiency is associated with prominent neuromotor features, including spastic diplegia, clonus, and hyperreflexia; intellectual disability and progressive neurological decline are other signs. In a constitutive murine model, we recently described leukodystrophy as a significant component of the central nervous system features of arginase deficiency. In the present studies, we sought to examine if the administration of a lipid nanoparticle carrying human ARG1 mRNA to constitutive knockout mice could prevent abnormalities in myelination associated with arginase deficiency. Imaging of the cingulum, striatum, and cervical segments of the corticospinal tract revealed a drastic reduction of myelinated axons; signs of degenerating axons were also present with thin myelin layers. Lipid nanoparticle/ARG1 mRNA administration resulted in both light and electron microscopic evidence of a dramatic recovery of myelin density compared with age-matched controls; oligodendrocytes were seen to be extending processes to wrap many axons. Abnormally thin myelin layers, when myelination was present, were resolved with intermittent mRNA administration, indicative of not only a greater density of myelinated axons but also an increase in the thickness of the myelin sheath. In conclusion, lipid nanoparticle/ARG1 mRNA administration in arginase deficiency prevents the associated leukodystrophy and restores normal oligodendrocyte function.

Fifteen years of urea cycle disorders brain research: Looking back, looking forward

Urea cycle disorders (UCD) are inherited diseases resulting from deficiency in one of six enzymes or two carriers that are required to remove ammonia from the body. UCD may be associated with neurological damage encompassing a spectrum from asymptomatic/mild to severe encephalopathy, which results in most cases from Hyperammonemia (HA) and elevation of other neurotoxic intermediates of metabolism. Electroencephalography (EEG), Magnetic resonance imaging (MRI) and Proton Magnetic resonance spectroscopy (MRS) are noninvasive measures of brain function and structure that can be used during HA to guide management and provide prognostic information, in addition to being research tools to understand the pathophysiology of UCD associated brain injury. The Urea Cycle Rare disorders Consortium (UCDC) has been invested in research to understand the immediate and downstream effects of hyperammonemia (HA) on brain using electroencephalogram (EEG) and multimodal brain MRI to establish early patterns of brain injury and to track recovery and prognosis. This review highlights the evolving knowledge about the impact of UCD and HA in particular on neurological injury and recovery and use of EEG and MRI to study and evaluate prognostic factors for risk and recovery. It recognizes the work of others and discusses the UCDC's prior work and future research priorities.

Review of Multi-Modal Imaging in Urea Cycle Disorders: The Old, the New, the Borrowed, and the Blue

The urea cycle disorders (UCD) are rare genetic disorder due to a deficiency of one of six enzymes or two transport proteins that act to remove waste nitrogen in form of ammonia from the body. In this review, we focus on neuroimaging studies in OTCD and Arginase deficiency, two of the UCD we have extensively studied. Ornithine transcarbamylase deficiency (OTCD) is the most common of these, and X-linked. Hyperammonemia (HA) in OTCD is due to deficient protein handling. Cognitive impairments and neurobehavioral disorders have emerged as the major sequelae in Arginase deficiency and OTCD, especially in relation to executive function and working memory, impacting pre-frontal cortex (PFC). Clinical management focuses on neuroprotection from HA, as well as neurotoxicity from other known and yet unclassified metabolites. Prevention and mitigation of neurological injury is a major challenge and research focus. Given the impact of HA on neurocognitive function of UCD, neuroimaging modalities, especially multi-modality imaging platforms, can bring a wealth of information to understand the neurocognitive function and biomarkers. Such information can further improve clinical decision making, and result in better therapeutic interventions. In vivo investigations of the affected brain using multimodal neuroimaging combined with clinical and behavioral phenotyping hold promise. MR Spectroscopy has already proven as a tool to study biochemical aberrations such as elevated glutamine surrounding HA as well as to diagnose partial UCD. Functional Near Infrared Spectroscopy (fNIRS), which assesses local changes in cerebral hemodynamic levels of cortical regions, is emerging as a non-invasive technique and will serve as a surrogate to fMRI with better portability. Here we review two decades of our research using non-invasive imaging and how it has contributed to an understanding of the cognitive effects of this group of genetic conditions.

Neurological Deterioration in Three Siblings: Exploring the Spectrum of Argininemia

Argininemia or hyperargininemia is a urea cycle disorder caused by deficiency of the enzyme arginase 1. It is inherited in an autosomal recessive fashion. It commonly leads to spastic diplegia in childhood, but other important features include cognitive deterioration and epilepsy. Unlike other disorders of the urea cycle, hyperammonemia is not prominent. The authors report three siblings with genetically proven argininemia who presented with diverse phenotypes but with spasticity being a common feature. Sibling 1 developed motor regression in early childhood, sibling 2 developed delayed motor milestones from early infancy, whereas sibling 3 had global developmental delay in late infancy after a period of normal development. All siblings had mild hyperammonemia only. Early recognition is imperative, not only to initiate ammonia scavenging therapy which may lead to definite clinical improvement, but also to provide genetic counselling.

Arginase-1 deficiency in neural cells does not contribute to neurodevelopment or functional outcomes after sciatic nerve injury

Arginase-1 (Arg1) is an enzyme controlling the final step of the urea cycle, with highest expression in the liver and lower expression in the lungs, pancreas, kidney, and some blood cells. Arg1 deficiency is an inherited urea cycle disorder presenting with neurological dysfunction including spastic diplegia, intellectual and growth retardation, and encephalopathy. The contribution of Arg1 expression in the central and peripheral nervous system to the development of neurological phenotypes remains largely unknown. Previous studies have shown prominent arginase-1 expression in the nervous system and post-peripheral nerve injury in mice, but very low levels in the naïve state. To investigate neurobiological roles of Arg1, we created a conditional neural (n)Arg1 knockout (KO) mouse strain, with expression eliminated in neuronal and glial precursors, and compared them to littermate controls. Long-term analysis did not reveal any major differences in blood amino acid levels, body weight, or stride gait cycle from 8 to 26-weeks of age. Brain structure measured by magnetic resonance imaging at 16-weeks of age observed only a significant decrease in the volume of the mammillary bodies. We also assessed whether nArg1, which is expressed by sensory neurons after injury, may play a role in regeneration following sciatic nerve crush. Only subtle differences were observed in locomotor and sensory recovery between nArg1 KO and control mice. These results suggest that arginase-1 expression in central and peripheral neural cells does not contribute substantially to the phenotypes of this urea cycle disorder, nor is it likely crucial for post-injury regeneration in this mouse model.

Novel ARG1 variants identified in a patient with arginase 1 deficiency

We report a case of a 13-year-old boy with arginase 1 deficiency carrying a new variant in ARG1. Sanger sequencing identified the compound heterozygous variants: NM_000045.4: c.365G>A (p.Trp122*)/c.820G>A (p.Asp274Asn). Although not previously reported, the p.Asp274Asn variant is predicted to have strong pathogenicity because it is located in a highly conserved domain in the protein core and arginase activity in the patient was below measurement sensitivity.

A novel compound heterozygous mutation in the arginase-1 gene identified in a Chinese patient with argininemia: A case report

INTRODUCTION

Arginineemia, also known as arginase deficiency, is a rare autosomal recessive metabolic disease. The diagnosis sometimes may be delayed due to atypical clinical manifestations. Confirmation of arginineemia depends on genetic testing.

PATIENT CONCERNS

We reported a Chinese male child presenting with hyperargininemia and progressive spastic diplegia, who has a novel compound heterozygous mutation in the arginase-1 (ARG1) gene (c.263-266delAGAA, p.K88Rfs45;c.674T>C,p.L216P), respectively, coming from his mother and father.

DIAGNOSIS

The patient was diagnosed with argininemia with a novel compound homozygous mutation of the ARG1 gene at the age of 12 years.

INTERVENTIONS

The patient had a low-protein diet (0.8 g/kg/day). Baclofen, eperisone hydrochloride, botulinum toxin, and rehabilitation training were used to improve his spastic diplegia symptoms for 3 months.

OUTCOMES

The patient's blood arginine was still high after 3 months' low-protein diet. His spastic diplegia symptoms had not aggravated after 3 months' treatment.

CONCLUSIONS

Argininemia should be considered in a patient with slowly progressive neurologic manifestations, especially spastic diplegia. This case also suggests that tandem mass spectrometry should be used as an effective tool in the validity of neonatal screening for early diagnosis.

Hepatic arginase deficiency fosters dysmyelination during postnatal CNS development

Deficiency of arginase is associated with hyperargininemia, and prominent features include spastic diplegia/tetraplegia, clonus, and hyperreflexia; loss of ambulation, intellectual disability and progressive neurological decline are other signs. To gain greater insight into the unique neuromotor features, we performed gene expression profiling of the motor cortex of a murine model of the disorder. Coexpression network analysis suggested an abnormality with myelination, which was supported by limited existing human data. Utilizing electron microscopy, marked dysmyelination was detected in 2-week-old homozygous Arg1-KO mice. The corticospinal tract was found to be adversely affected, supporting dysmyelination as the cause of the unique neuromotor features and implicating oligodendrocyte impairment in a deficiency of hepatic Arg1. Following neonatal hepatic gene therapy to express Arg1, the subcortical white matter, pyramidal tract, and corticospinal tract all showed a remarkable recovery in terms of myelinated axon density and ultrastructural integrity with active wrapping of axons by nearby oligodendrocyte processes. These findings support the following conclusions: arginase deficiency is a leukodystrophy affecting the brain and spinal cord while sparing the peripheral nervous system, and neonatal AAV hepatic gene therapy can rescue the defects associated with myelinated axons, strongly implicating the functional recovery of oligodendrocytes after restoration of hepatic arginase activity.

Genetic mimics of cerebral palsy

The term "cerebral palsy mimic" is used to describe a number of neurogenetic disorders that may present with motor symptoms in early childhood, resulting in a misdiagnosis of cerebral palsy. Cerebral palsy describes a heterogeneous group of neurodevelopmental disorders characterized by onset in infancy or early childhood of motor symptoms (including hypotonia, spasticity, dystonia, and chorea), often accompanied by developmental delay. The primary etiology of a cerebral palsy syndrome should always be identified if possible. This is particularly important in the case of genetic or metabolic disorders that have specific disease-modifying treatment. In this article, we discuss clinical features that should alert the clinician to the possibility of a cerebral palsy mimic, provide a practical framework for selecting and interpreting neuroimaging, biochemical, and genetic investigations, and highlight selected conditions that may present with predominant spasticity, dystonia/chorea, and ataxia. Making a precise diagnosis of a genetic disorder has important implications for treatment, and for advising the family regarding prognosis and genetic counseling. © 2019 International Parkinson and Movement Disorder Society.

Arginase I mRNA therapy - a novel approach to rescue arginase 1 enzyme deficiency

Arginase I (ARG1) deficiency is an autosomal recessive urea cycle disorder, caused by deficiency of the enzyme Arginase I, resulting in accumulation of arginine in blood. Current Standard of Care (SOC) for ARG1 deficiency in patients or those having detrimental mutations of ARG1 gene is diet control. Despite diet and drug therapy with nitrogen scavengers, ~25% of patients suffer from severe mental deficits and loss of ambulation. 75% of patients whose symptoms can be managed through diet therapy continue to suffer neuro-cognitive deficits. In our research, we demonstrate in vitro and in vivo that administration of ARG1 mRNA increased ARG1 protein expression and specific activity in relevant cell types, including ARG1-deficient patient cell lines, as well as in wild type mice for up to 4 days. These studies demonstrate that ARG1 mRNA treatment led to increased functional protein expression of ARG1 and subsequently an increase in urea. Hence, ARG1 mRNA therapy could be a potential treatment option to develop for patients.

Human hepatocyte transplantation corrects the inherited metabolic liver disorder arginase deficiency in mice

The transplantation, engraftment, and expansion of primary hepatocytes have the potential to be an effective therapy for metabolic disorders of the liver including those of nitrogen metabolism. To date, such methods for the treatment of urea cycle disorders in murine models has only been minimally explored. Arginase deficiency, an inherited disorder of nitrogen metabolism that presents in the first two years of life, has the potential to be treated by such methods. To explore the potential of this approach, we mated the conditional arginase deficient mouse with a mouse model deficient in fumarylacetoacetate hydrolase (FAH) and with Rag2 and IL2-Rγ mutations to give a selective advantage to transplanted (normal) human hepatocytes. On day -1, a uroplasminogen-expressing adenoviral vector was administered intravenously followed the next day with the transplantation of 1 × 106 human hepatocytes (or vehicle alone) by intrasplenic injection. As the initial number of administered hepatocytes would be too low to prevent hepatotoxicity-induced mortality, NTBC cycling was performed to allow for hepatocyte expansion and repopulation. While all control mice died, all except one human hepatocyte transplanted mice survived. Four months after hepatocyte transplantation, 2 × 1011 genome copies of AAV-TBG-Cre recombinase was administered IV to disrupt endogenous hepatic arginase expression. While all control mice died within the first month, human hepatocyte transplanted mice did well. Ammonia and amino acids, analyzed in both groups before and after disruption of endogenous arginase expression, while well-controlled in the transplanted group, were markedly abnormal in the controls. Ammonium challenging further demonstrated the durability and functionality of the human repopulated liver. In conclusion, these studies demonstrate that human hepatocyte repopulation in the murine liver can result in effective treatment of arginase deficiency.

Argininemia as a cause of severe chronic stunting and partial growth hormone deficiency (PGHD): A case report

RATIONALE

Argininemia is an autosomal recessive inherited disorder of the urea cycle. Because of its atypical symptoms in early age, diagnosis can be delayed until the typical chronic manifestations - including spastic diplegia, deterioration in cognitive function, and epilepsy - appear in later childhood.

PATIENT CONCERNS

A Chinese boy initially presented with severe stunting and partial growth hormone deficiency (PGHD) at 3 years old and was initially treated with growth hormone replacement therapy. Seven years later (at 10 years old), he presented with spastic diplegia, cognitive function lesions, epilepsy, and peripheral neuropathy.

DIAGNOSES

Ultimately, the patient was diagnosed with argininemia with homozygous mutation (c.32T>C) of the ARG1 gene at 10 years old. Blood tests showed mildly elevated blood ammonia and creatine kinase, and persistently elevated bilirubin.

INTERVENTIONS

Protein intake was limited to 0.8 g/kg/day, citrulline (150-200 mg [kg d]) was prescribed.

OUTCOMES

The patient's mental state and vomiting had improved after 3 months treatment. At 10 years and 9 month old, his height and weight had reached 121cm and 22kg, respectively, but his spastic diplegia symptoms had not improved.

LESSONS

This case demonstrates that stunting and PGHD that does not respond to growth hormone replacement therapy might hint at inborn errors of metabolism (IEM). IEM should also be considered in patients with persistently elevated bilirubin with or without abnormal liver transaminase, as well as elevated blood ammonia and creatine kinase, in the absence of hepatic disease.

Rescue of the Functional Alterations of Motor Cortical Circuits in Arginase Deficiency by Neonatal Gene Therapy

Arginase 1 deficiency is a urea cycle disorder associated with hyperargininemia, spastic diplegia, loss of ambulation, intellectual disability, and seizures. To gain insight on how loss of arginase expression affects the excitability and synaptic connectivity of the cortical neurons in the developing brain, we used anatomical, ultrastructural, and electrophysiological techniques to determine how single-copy and double-copy arginase deletion affects cortical circuits in mice. We find that the loss of arginase 1 expression results in decreased dendritic complexity, decreased excitatory and inhibitory synapse numbers, decreased intrinsic excitability, and altered synaptic transmission in layer 5 motor cortical neurons. Hepatic arginase 1 gene therapy using adeno-associated virus rescued nearly all these abnormalities when administered to neonatal homozygous knock-out animals. Therefore, gene therapeutic strategies can reverse physiological and anatomical markers of arginase 1 deficiency and therefore may be of therapeutic benefit for the neurological disabilities in this syndrome.

SIGNIFICANCE STATEMENT

These studies are one of the few investigations to try to understand the underlying neurological dysfunction that occurs in urea cycle disorders and the only to examine arginase deficiency. We have demonstrated by multiple modalities that, in murine layer 5 cortical neurons, a gradation of abnormalities exists based on the functional copy number of arginase: intrinsic excitability is altered, there is decreased density in asymmetrical and perisomatic synapses, and analysis of the dendritic complexity is lowest in the homozygous knock-out. With neonatal administration of adeno-associated virus expressing arginase, there is near-total recovery of the abnormalities in neurons and cortical circuits, supporting the concept that neonatal gene therapy may prevent the functional abnormalities that occur in arginase deficiency.

The Measurement of Ammonia in Human Breath and its Potential in Clinical Diagnostics

Ammonia is an important component of metabolism and is involved in many physiological processes. During normal physiology, levels of blood ammonia are between 11 and 50 µM. Elevated blood ammonia levels are associated with a variety of pathological conditions such as liver and kidney dysfunction, Reye's syndrome and a variety of inborn errors of metabolism including urea cycle disorders (UCD), organic acidaemias and hyperinsulinism/hyperammonaemia syndrome in which ammonia may reach levels in excess of 1 mM. It is highly neurotoxic and so effective measurement is critical for assessing and monitoring disease severity and treatment. Ammonia is also a potential biomarker in exercise physiology and studies of drug metabolism. Current ammonia testing is based on blood sampling, which is inconvenient and can be subject to significant analytical errors due to the quality of the sample draw, its handling and preparation for analysis. Blood ammonia is in gaseous equilibrium with the lungs. Recent research has demonstrated the potential use of breath ammonia as a non-invasive means of measuring systemic ammonia. This requires measurement of ammonia in real breath samples with associated temperature, humidity and gas characteristics at concentrations between 50 and several thousand parts per billion. This review explores the diagnostic applications of ammonia measurement and the impact that the move from blood to breath analysis could have on how these processes and diseases are studied and managed.

Untargeted metabolomic analysis for the clinical screening of inborn errors of metabolism

Global metabolic profiling currently achievable by untargeted mass spectrometry-based metabolomic platforms has great potential to advance our understanding of human disease states, including potential utility in the detection of novel and known inborn errors of metabolism (IEMs). There are few studies of the technical reproducibility, data analysis methods, and overall diagnostic capabilities when this technology is applied to clinical specimens for the diagnosis of IEMs. We explored the clinical utility of a metabolomic workflow capable of routinely generating semi-quantitative z-score values for ~900 unique compounds, including ~500 named human analytes, in a single analysis of human plasma. We tested the technical reproducibility of this platform and applied it to the retrospective diagnosis of 190 individual plasma samples, 120 of which were collected from patients with a confirmed IEM. Our results demonstrate high intra-assay precision and linear detection for the majority compounds tested. Individual metabolomic profiles provided excellent sensitivity and specificity for the detection of a wide range of metabolic disorders and identified novel biomarkers for some diseases. With this platform, it is possible to use one test to screen for dozens of IEMs that might otherwise require ordering multiple unique biochemical tests. However, this test may yield false negative results for certain disorders that would be detected by a more well-established quantitative test and in its current state should be considered a supplementary test. Our findings describe a novel approach to metabolomic analysis of clinical specimens and demonstrate the clinical utility of this technology for prospective screening of IEMs.

Arginase-1 deficiency

Arginase-1 (ARG1) deficiency is a rare autosomal recessive disorder that affects the liver-based urea cycle, leading to impaired ureagenesis. This genetic disorder is caused by 40+ mutations found fairly uniformly spread throughout the ARG1 gene, resulting in partial or complete loss of enzyme function, which catalyzes the hydrolysis of arginine to ornithine and urea. ARG1-deficient patients exhibit hyperargininemia with spastic paraparesis, progressive neurological and intellectual impairment, persistent growth retardation, and infrequent episodes of hyperammonemia, a clinical pattern that differs strikingly from other urea cycle disorders. This review briefly highlights the current understanding of the etiology and pathophysiology of ARG1 deficiency derived from clinical case reports and therapeutic strategies stretching over several decades and reports on several exciting new developments regarding the pathophysiology of the disorder using ARG1 global and inducible knockout mouse models. Gene transfer studies in these mice are revealing potential therapeutic options that can be exploited in the future. However, caution is advised in extrapolating results since the lethal disease phenotype in mice is much more severe than in humans indicating that the mouse models may not precisely recapitulate human disease etiology. Finally, some of the functions and implications of ARG1 in non-urea cycle activities are considered. Lingering questions and future areas to be addressed relating to the clinical manifestations of ARG1 deficiency in liver and brain are also presented. Hopefully, this review will spark invigorated research efforts that lead to treatments with better clinical outcomes.

The differential diagnosis of spastic diplegia

Spastic diplegia is the most common form of cerebral palsy worldwide. Many disorders mimic spastic diplegia, which can result in misdiagnosis for the child with resultant negative treatment and family counselling implications. In this paper, the authors provide a brief review of spastic diplegia and the various disorders in the differential diagnosis. We also provide a diagnostic algorithm to assist physicians in making the correct diagnosis.

Minimal ureagenesis is necessary for survival in the murine model of hyperargininemia treated by AAV-based gene therapy

Hyperammonemia is less severe in arginase 1 deficiency compared with other urea cycle defects. Affected patients manifest hyperargininemia and infrequent episodes of hyperammonemia. Patients typically suffer from neurological impairment with cortical and pyramidal tract deterioration, spasticity, loss of ambulation, seizures and intellectual disability; death is less common than with other urea cycle disorders. In a mouse model of arginase I deficiency, the onset of symptoms begins with weight loss and gait instability, which progresses toward development of tail tremor with seizure-like activity; death typically occurs at about 2 weeks of life. Adeno-associated viral vector gene replacement strategies result in long-term survival of mice with this disorder. With neonatal administration of vector, the viral copy number in the liver greatly declines with hepatocyte proliferation in the first 5 weeks of life. Although the animals do survive, it is not known from a functional standpoint how well the urea cycle is functioning in the adult animals that receive adeno-associated virus. In these studies, we administered [1-13C] acetate to both littermate controls and adeno-associated virus-treated arginase 1 knockout animals and examined flux through the urea cycle. Circulating ammonia levels were mildly elevated in treated animals. Arginine and glutamine also had perturbations. Assessment 30 min after acetate administration demonstrated that ureagenesis was present in the treated knockout liver at levels as low at 3.3% of control animals. These studies demonstrate that only minimal levels of hepatic arginase activity are necessary for survival and ureagenesis in arginase-deficient mice and that this level of activity results in control of circulating ammonia. These results may have implications for potential therapy in humans with arginase deficiency.

Differential diagnosis of Mendelian and mitochondrial disorders in patients with suspected multiple sclerosis

Several single gene disorders share clinical and radiologic characteristics with multiple sclerosis and have the potential to be overlooked in the differential diagnostic evaluation of both adult and paediatric patients with multiple sclerosis. This group includes lysosomal storage disorders, various mitochondrial diseases, other neurometabolic disorders, and several other miscellaneous disorders. Recognition of a single-gene disorder as causal for a patient's 'multiple sclerosis-like' phenotype is critically important for accurate direction of patient management, and evokes broader genetic counselling implications for affected families. Here we review single gene disorders that have the potential to mimic multiple sclerosis, provide an overview of clinical and investigational characteristics of each disorder, and present guidelines for when clinicians should suspect an underlying heritable disorder that requires diagnostic confirmation in a patient with a definite or probable diagnosis of multiple sclerosis.

Myocyte-mediated arginase expression controls hyperargininemia but not hyperammonemia in arginase-deficient mice

Human arginase deficiency is characterized by hyperargininemia and infrequent episodes of hyperammonemia that cause neurological impairment and growth retardation. We previously developed a neonatal mouse adeno-associated viral vector (AAV) rh10-mediated therapeutic approach with arginase expressed by a chicken β-actin promoter that controlled plasma ammonia and arginine, but hepatic arginase declined rapidly. This study tested a codon-optimized arginase cDNA and compared the chicken β-actin promoter to liver- and muscle-specific promoters. ARG1(-/-) mice treated with AAVrh10 carrying the liver-specific promoter also exhibited long-term survival and declining hepatic arginase accompanied by the loss of AAV episomes during subsequent liver growth. Although arginase expression in striated muscle was not expected to counteract hyperammonemia, due to muscle's lack of other urea cycle enzymes, we hypothesized that the postmitotic phenotype in muscle would allow vector genomes to persist, and hence contribute to decreased plasma arginine. As anticipated, ARG1(-/-) neonatal mice treated with AAVrh10 carrying a modified creatine kinase-based muscle-specific promoter did not survive longer than controls; however, their plasma arginine levels remained normal when animals were hyperammonemic. These data imply that plasma arginine can be controlled in arginase deficiency by muscle-specific expression, thus suggesting an alternative approach to utilizing the liver for treating hyperargininemia.

Amand T, Kyriakopoulou L, Schulze A, Funk CD. Inducible arginase 1 deficiency in mice leads to hyperargininemia and altered amino acid metabolism

Arginase deficiency is a rare autosomal recessive disorder resulting from a loss of the liver arginase isoform, arginase 1 (ARG1), which is the final step in the urea cycle for detoxifying ammonia. ARG1 deficiency leads to hyperargininemia, characterized by progressive neurological impairment, persistent growth retardation and infrequent episodes of hyperammonemia. Using the Cre/loxP-directed conditional gene knockout system, we generated an inducible Arg1-deficient mouse model by crossing “floxed” Arg1 mice with CreER^T2 mice. The resulting mice (Arg-Cre) die about two weeks after tamoxifen administration regardless of the starting age of inducing the knockout. These treated mice were nearly devoid of Arg1 mRNA, protein and liver arginase activity, and exhibited symptoms of hyperammonemia. Plasma amino acid analysis revealed pronounced hyperargininemia and significant alterations in amino acid and guanidino compound metabolism, including increased citrulline and guanidinoacetic acid. Despite no alteration in ornithine levels, concentrations of other amino acids such as proline and the branched-chain amino acids were reduced. In summary, we have generated and characterized an inducible Arg1-deficient mouse model exhibiting several pathologic manifestations of hyperargininemia. This model should prove useful for exploring potential treatment options of ARG1 deficiency.

Lethal phenotype in conditional late-onset arginase 1 deficiency in the mouse

Human arginase deficiency is characterized by hyperargininemia and infrequent episodes of hyperammonemia, which lead to neurological impairment with spasticity, loss of ambulation, seizures, and severe mental and growth retardation; uncommonly, patients suffer early death from this disorder. In a murine targeted knockout model, onset of the phenotypic abnormality is heralded by weight loss at around day 15, and death occurs typically by postnatal day 17 with hyperargininemia and markedly elevated ammonia. This discrepancy between the more attenuated juvenile-onset human disease and the lethal neonatal murine model has remained suboptimal for studying and developing therapy for the more common presentation of arginase deficiency. These investigations aimed to address this issue by creating an adult conditional knockout mouse to determine whether later onset of arginase deficiency also resulted in lethality. Animal survival and ammonia levels, body weight, circulating amino acids, and tissue arginase levels were examined as outcome parameters after widespread Cre-recombinase activation in a conditional knockout model of arginase 1 deficiency. One hundred percent of adult female and 70% of adult male mice died an average of 21.0 and 21.6 days, respectively, after the initiation of tamoxifen administration. Animals demonstrated elevated circulating ammonia and arginine at the onset of phenotypic abnormalities. In addition, brain and liver amino acids demonstrated abnormalities. These studies demonstrate that (a) the absence of arginase in adult animals results in a disease profile (leading to death) similar to that of the targeted knockout and (b) the phenotypic abnormalities seen in the juvenile-onset model are not exclusive to the age of the animal but instead to the biochemistry of the disorder. This adult model will be useful for developing gene- and cell-based therapies for this disorder that will not be limited by the small animal size of neonatal therapy and for developing a better understanding of the characteristics of hyperargininemia.

AAV-based gene therapy prevents neuropathology and results in normal cognitive development in the hyperargininemic mouse

Complete arginase I deficiency is the least severe urea cycle disorder, characterized by hyperargininemia and infrequent episodes of hyperammonemia. Patients suffer from neurological impairment with cortical and pyramidal tract deterioration, spasticity, loss of ambulation, and seizures, and is associated with intellectual disability. In mice, onset is heralded by weight loss beginning around day 15; gait instability follows progressing to inability to stand and development of tail tremor with seizure-like activity and death. Here we report that hyperargininemic mice treated neonatally with an adeno-associated virus expressing arginase and followed long-term lack any presentation consistent with brain dysfunction. Behavioral and histopathological evaluation demonstrated that treated mice are indistinguishable from littermates and that putative compounds associated with neurotoxicity are diminished. In addition, treatment results in near complete resolution of metabolic abnormalities early in life; however there is the development of some derangement later with decline in transgene expression. Ammonium challenging revealed that treated mice are affected by exogenous loading much greater than littermates. These results demonstrate that AAV-based therapy for hyperargininemia is effective and prevents development of neurological abnormalities and cognitive dysfunction in a mouse model of hyperargininemia; however nitrogen challenging reveals that these mice remain impaired in the handling of waste nitrogen.

Ammonium metabolism in humans

Free ammonium ions are produced and consumed during cell metabolism. Glutamine synthetase utilizes free ammonium ions to produce glutamine in the cytosol whereas glutaminase and glutamate dehydrogenase generate free ammonium ions in the mitochondria from glutamine and glutamate, respectively. Ammonia and bicarbonate are condensed in the liver mitochondria to yield carbamoylphosphate initiating the urea cycle, the major mechanism of ammonium removal in humans. Healthy kidney produces ammonium which may be released into the systemic circulation or excreted into the urine depending predominantly on acid-base status, so that metabolic acidosis increases urinary ammonium excretion while metabolic alkalosis induces the opposite effect. Brain and skeletal muscle neither remove nor produce ammonium in normal conditions, but they are able to seize ammonium during hyperammonemia, releasing glutamine. Ammonia in gas phase has been detected in exhaled breath and skin, denoting that these organs may participate in nitrogen elimination. Ammonium homeostasis is profoundly altered in liver failure resulting in hyperammonemia due to the deficient ammonium clearance by the diseased liver and to the development of portal collateral circulation that diverts portal blood with high ammonium content to the systemic blood stream. Although blood ammonium concentration is usually elevated in liver disease, a substantial role of ammonium causing hepatic encephalopathy has not been demonstrated in human clinical studies. Hyperammonemia is also produced in urea cycle disorders and other situations leading to either defective ammonium removal or overproduction of ammonium that overcomes liver clearance capacity. Most diseases resulting in hyperammonemia and cerebral edema are preceded by hyperventilation and respiratory alkalosis of unclear origin that may be caused by the intracellular acidosis occurring in these conditions.

Arginase deficiency with new phenotype and a novel mutation: contemporary summary

In areas without expanded newborn screening, instead of presenting neonatally, patients with arginase deficiency typically present with spastic paraplegia in early childhood. Diagnosis of this rare neurometabolic disease poses the first challenge because it is often misdiagnosed as cerebral palsy during initial stages. We describe arginase deficiency in a 20-year-old woman with spastic paraplegia, progressive dystonia, dementia, peripheral neuropathy, epilepsy, liver cirrhosis, and non-B/non-C hepatocellular carcinoma. A novel homozygous mutation NM_000045.2 (ARG1):c.673del (p.Arg225GlyfsX5) was detected. We suggest that all children presenting with progressive neurodegeneration or spastic paraplegia in the absence of risk factors for cerebral palsy should be screened for inborn errors of metabolism, including arginase deficiency. For monitoring urea cycle defects, noninvasive imaging screening for liver fibrosis and hepatocellular carcinoma can help ensure early detection, with potential treatment implications.

Analysis of novel ARG1 mutations causing hyperargininemia and correlation with arginase I activity in erythrocytes

Hyperargininemia (HA) is an autosomal recessive disease that typically has a clinical presentation that is distinct from other urea cycle disorders. It is caused by the deficient activity of the enzyme arginase I, encoded by the gene ARG1. We screened for ARG1 mutations and measured erythrocyte enzyme activity in a series of 16 Brazilian HA patients. Novel mutations, in addition to previously described missense mutations, were analysed for their effect on the structure, stability and/or function of arginase I (ARG1) using bioinformatics tools. Three previously reported mutations were found (p.R21X; p.I11T and p.W122X), and five novel mutations were identified (p.G27D; p.G74V; p.T134I; p.R308Q; p.I174fs179). The p.T134I mutation was the most frequent in the Brazilian population. Patients carrying the p.R308Q mutation had higher residual ARG1 decreased activity, but presented no distinguishable phenotype compared to the other patients. Bioinformatics analyses revealed that missense mutations (1) affect the ARG1 active site, (2) interfere with the stability of the ARG1 folded conformation or (3) alter the quaternary structure of the ARG1. Our study reinforced the role of Arg308 residue for assembly of the ARG1 homotrimer. The panel of heterogeneous ARG1 mutations that cause HA was expanded, nevertheless a clear genotype-phenotype correlation was not observed in our series.

Long-term survival of the juvenile lethal arginase-deficient mouse with AAV gene therapy

Arginase deficiency is characterized by hyperargininemia and infrequent episodes of hyperammonemia. Human patients suffer from neurological impairment with spasticity, loss of ambulation, seizures, and severe mental and growth retardation. In a murine model, onset of the phenotypic abnormality is heralded by weight loss beginning around day 15 with death occurring typically by postnatal day 17 with hyperargininemia and markedly elevated ammonia. The goal of this study was to address the development of a gene therapy approach for arginase deficiency beginning in the neonatal period. Lifespan extension, body weight, circulating amino acids and ammonia levels were examined as outcome parameters after gene therapy with an adeno-associated viral vector expressing arginase was administered to mice on the second day of life (DOL). One-hundred percent of untreated arginase-deficient mice died by DOL 24, whereas 89% of the adeno-associated virus (AAV)-treated arginase deficient mice have survived for >8 months. While animals at 8 months demonstrate elevated glutamine levels, ammonia is less than three times that of controls and arginine levels are normal. These studies are the first to demonstrate that AAV-based therapy for arginase deficiency is effective and supports the development of gene therapy for this and the other urea cycle disorders.

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.

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.

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.

Pediatric liver transplantation for inherited metabolic liver disease: a single-center experience

OBJECTIVE

We performed single-center outcome comparison of pediatric recipients who underwent liver transplantation for either genetic or metabolic disease including the clinical impact of using heterozygote parents as living donors.

MATERIALS AND METHODS

Pediatric liver transplant recipients from September 2007 to December 2010 were included. Patients were separated into 2 categories by etiology of liver disease: (1) genetic or metabolic liver disease (G/M) and (2) nongenetic or metabolic liver disease (non-G/M), which included all other remaining etiologies combined. Patient demographics, recipient and donor characteristics, graft type, operative data, recipient complications, allograft and patient survival were analyzed.

RESULTS

Forty liver transplants were performed on 40 patients; 18 were transplanted for G/M; mean waiting time was 101 days for G/M group and 57 days for non-G/M group; 9 patients were listed as status 1, 5 were granted PELD/MELD exceptions; the overall mean PELD/MELD score was 21. Four G/M patients had hepatocellular carcinoma in the explant without microvascular invasion. Overall complications requiring either surgery or interventional radiology occurred in 14 patients-G/M (n = 5); CMV viremia was seen in 11 patients (G/M, n = 1); detectable EBV DNA was detectable in 8 patients (G/M, n = 4), acute cellular rejection was seen in 10 (G/M, n = 5), postransplant lymphoproliferative disease occurred in 2 G/M patients; and 1 G/M patient showed significantly improved posttransplant neurologic motor function. Children with G/M who received a living donor liver transplant from heterozygote parents did well without any signs of expressing underlying metabolic disease. Posttransplant graft and patient overall survival at 12 months for G/M and non-G/M was 100%, and at 36 months, 83% and 100%, respectively.

CONCLUSION

The majority of children transplanted for either genetic or metabolic disease were status 1 or awarded UNOS exception points. Cadaveric split livers and live donors including obligate heterozygotes resulted in excellent allograft and patient survival outcomes. In metabolic and genetic liver diseases, close follow-up and timely transplantation can preclude malignant spread and prevent disease progression and consequences, as well as reverse neurologic sequelae.

A long-term survival case of arginase deficiency with severe multicystic white matter and compound mutations

Neuropathology and neuroimaging of long-term survival cases of arginase deficiency are rarely reported. The magnetic resonance imaging (MRI) of our case showed severe multicystic white matter lesions with cortical atrophy, which were more severe compared with previous reports. In this patient, low-protein diet successfully reduced hyperammonemia, but hyperargininemia persisted. These severe neurological and MRI findings may be explained by a compound heterozygote, inheriting both of severe mutant alleles from her parents.

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.

Neurological implications of urea cycle disorders

The urea cycle disorders constitute a group of rare congenital disorders caused by a deficiency of the enzymes or transport proteins required to remove ammonia from the body. Via a series of biochemical steps, nitrogen, the waste product of protein metabolism, is removed from the blood and converted into urea. A consequence of these disorders is hyperammonaemia, resulting in central nervous system dysfunction with mental status changes, brain oedema, seizures, coma, and potentially death. Both acute and chronic hyperammonaemia result in alterations of neurotransmitter systems. In acute hyperammonaemia, activation of the NMDA receptor leads to excitotoxic cell death, changes in energy metabolism and alterations in protein expression of the astrocyte that affect volume regulation and contribute to oedema. Neuropathological evaluation demonstrates alterations in the astrocyte morphology. Imaging studies, in particular (1)H MRS, can reveal markers of impaired metabolism such as elevations of glutamine and reduction of myoinositol. In contrast, chronic hyperammonaemia leads to adaptive responses in the NMDA receptor and impairments in the glutamate-nitric oxide-cGMP pathway, leading to alterations in cognition and learning. Therapy of acute hyperammonaemia has relied on ammonia-lowering agents but in recent years there has been considerable interest in neuroprotective strategies. Recent studies have suggested restoration of learning abilities by pharmacological manipulation of brain cGMP with phosphodiesterase inhibitors. Thus, both strategies are intriguing areas for potential investigation in human urea cycle disorders.

The cost-effectiveness of expanding newborn screening for up to 21 inherited metabolic disorders using tandem mass spectrometry: results from a decision-analytic model

OBJECTIVES

In 2005, in Ontario, Canada, newborns were only screened for phenylketonuria (PKU) and hypothyroidism. Tandem mass spectrometry (MS/MS) has since been implemented as a new screening technology because it can screen for PKU and many other diseases simultaneously. We estimated the cost-effectiveness of using this technology to expand the Ontario newborn screening program to screen for each disease independently and for hypothetical bundles of up to 21 metabolic diseases.

METHODS

We constructed a decision-analytic model to estimate the incremental costs and life-years of survival that can be gained by screening or changing screening technologies. Costs and health benefits were estimated for a cohort of babies born in Ontario in 1 year. Secondary sources and expert opinion were used to estimate the test characteristics, disease prevalence, treatment effectiveness, disease progression rates, and mortality. The London Health Sciences Centre Case Costing Initiative, the Ontario Health Insurance Plan Schedule, and the Ontario Drug Benefits plan formulary were used to estimate costs.

RESULTS

Changing screening technologies, from the Guthrie test to MS/MS, for PKU detection had an incremental cost of $5,500,000 per life-year (LY) gained. We identified no diseases for which the incremental cost of screening for just that disease was less than $100,000 per LY gained. The incremental costs of screening ranged from $222,000 (HMG-CoA lyase deficiency) to $142,500,000 (glutaric acidemia type II) per LY gained. Screening for a bundle of diseases including PKU and the 14 most cost-effective diseases to screen for cost less than $70,000 per LY gained, and the incremental cost-effectiveness of adding each of the 14 diseases to the bundle was less than $100,000 per LY gained. The incremental cost of adding the 15th most cost-effective disease was $309,400 per LY gained.

CONCLUSIONS

Early diagnosis and treatment of metabolic disease is important to reduce disease severity and delay or prevent the onset of the disease. Screening at birth reduces the morbidity, mortality, and social burden associated with the irreversible effects of disease on the population. Our analysis suggests that the cost-efficiencies gained by using MS/MS to screen for bundles of diseases rather than just one disease are sufficient to warrant consideration of an expanded screening program. It is, however, not cost-effective to screen for all diseases that can be screened for using this technology.

Inborn errors of metabolism: the flux from Mendelian to complex diseases

Inborn errors of metabolism are characterized by dysregulation of the metabolic networks that underlie development and homeostasis, and constitute an important and expanding group of genetic disorders in humans. Diagnostic methods that are based on molecular genetic tools have a limited ability to correlate phenotypes with subtle changes in metabolic fluxes. We argue that the direct and dynamic measurement of metabolite flux will facilitate the integration of environmental, genetic and biochemical factors with phenotypic information. Ultimately, this integration will lead to new diagnostic and therapeutic approaches that are focused on the manipulation of these pathways.

Nutritional therapy improves growth and protein status of children with a urea cycle enzyme defect

BACKGROUND

Poor growth has been described in patients with urea cycle enzyme defects treated with protein-restricted diets, while protein status is seldom reported.

OBJECTIVE

To assess the effects of nutritional therapy with a medical food on growth and protein status of patients with a urea cycle enzyme defect.

METHODS

A 6-mo multicenter outpatient study was conducted with infants and toddlers managed by nutrition therapy with Cyclinex-1 Amino Acid-Modified Medical Food with Iron (Ross Products Division, Abbott Laboratories, Columbus, OH). Main outcome variables were anthropometrics and plasma amino acids (selected), albumin, and transthyretin concentrations.

RESULTS

Seventeen patients completed the study. Mean (+/-SE) baseline age was 11.30+/-3.20 months (median 4.40 months; range 0.22-38.84 months). Length and weight z-scores increased significantly during the 6-month study. Head circumference increased, but not significantly. Three patients were stunted and two were wasted (-2.0 z-score) at baseline while at study end, only one patient was both stunted and wasted. The majority of patients increased in length, head circumference, and weight z-scores during study. Mean (+/-SE) plasma albumin concentration increased from 34+/-2g/L at baseline to 38+/-1g/L at study end. Plasma transthyretin increased from a mean (+/-SE) of 177+/-13 mg/L at baseline to 231+/-15 mg/L at study end. No correlation was found between plasma NH(3) concentrations and medical food intake. Plasma NH(3) concentration was positively correlated with the percentage of Food and Agriculture Organization/World Health Organization/United Nations recommended protein ingested.

CONCLUSIONS

Intakes of adequate protein and energy for age result in anabolism and linear growth without increasing plasma NH(3) concentrations. Medical food intakes did not correlate with plasma NH(3) concentrations.

Clinical manifestations of inborn errors of the urea cycle and related metabolic disorders during childhood

Various disorders cause hyperammonemia during childhood. Among them are those caused by inherited defects in urea synthesis and related metabolic pathways. These disorders can be grouped into two types: disorders of the enzymes that comprise the urea cycle, and disorders of the transporters or metabolites of the amino acids related to the urea cycle. Principal clinical features of these disorders are caused by elevated levels of blood ammonium. Additional disease-specific symptoms are related to the particular metabolic defect. These specific clinical manifestations are often due to an excess or lack of specific amino acids. Treatment of urea cycle disorders and related metabolic diseases consists of nutritional restriction of proteins, administration of specific amino acids, and use of alternative pathways for discarding excess nitrogen. Although combinations of these treatments are extensively employed, the prognosis of severe cases remains unsatisfactory. Liver transplantation is one alternative for which a better prognosis is reported.

Evaluation of the child with cerebral palsy

Cerebral palsy (CP) is a common problem, occurring in about 2 to 2.5 per 1000 live births. The diagnosis of CP is based upon a history of abnormal motor development that is not progressive coupled with an examination (e.g. hypertonicity, increased reflexes, clonus) "placing" the lesion in the brain. In order to establish that a brain abnormality exists in children with CP that may, in turn, suggest an etiology and prognosis, neuroimaging is recommended with magnetic resonance imaging preferred to computed tomography. Metabolic and genetic studies should be obtained if there are atypical features in the history or on the examination. Detection of a brain malformation in a child with CP might suggest an underlying genetic or metabolic etiology. As cerebral infarction is high in children with hemiplegic CP, diagnostic testing for coagulation disorders should be considered. However, there is insufficient evidence at present to be precise as to what studies should be ordered. An electroencephalogram is not recommended unless there are features suggestive of epilepsy or a specific epileptic syndrome. As children with CP may have associated deficits of mental retardation, ophthalmologic and hearing impairments, speech and language disorders and oral-motor dysfunction, screening for these conditions should be part of the initial assessment.

Prenatal diagnosis for arginase deficiency by second-trimester fetal erythrocyte arginase assay and first-trimesterARG1 mutation analysis

Hyperargininemia is a progressive neurometabolic disorder caused by deficiency of hepatic cytosolic arginase I, resulting from mutations in the ARG1 gene. We diagnosed arginase deficiency in a three-year-old male child of first-cousin Palestinian Arab parents. Prenatal diagnosis of an unaffected fetus was achieved in the second trimester of a subsequent pregnancy by cordocentesis and analysis of arginase activity in fetal erythrocytes. ARG1 mutation analysis in the proband revealed homozygosity for a deletion of 10,753 bp extending from the first intron to beyond the poly (A) site of the gene. This is the first gross deletion in the ARG1 gene to be identified and the first mutation to be described in an arginase-deficient patient of this ethnic origin. The identification of the ARG1 deletion in this family enabled first-trimester prenatal diagnosis in a subsequent pregnancy by multiplex PCR analysis performed on chorionic villous DNA.

Neuroimaging in spasticity and movement disorders

Advances in neuroimaging provide unique opportunities to evaluate brain structure, biochemistry, and function. Although a number of imaging techniques have been used in newborns, cranial ultrasonography in premature infants and nuclear magnetic resonance modalities, including magnetic resonance imaging and diffusion-weighted imaging, in high-risk term infants are of foremost benefit. Interpretation is based on knowledge of characteristic imaging findings in specific childhood neurologic disorders and an understanding of differential diagnosis in cerebral palsy syndromes, such as spastic diplegia and various subtypes of extrapyramidal cerebral palsy. This review focuses on imaging studies that can be effectively used in at-risk infants and in children with spasticity and movement disorders to refine diagnosis and guide therapeutic interventions.