Arginases I and II: do their functions overlap?

Arginase, often perceived solely as the last of the now six enzymes of the urea cycle, exists in two forms and has a broad tissue distribution. A cytosolic form, AI, is highly expressed in the liver and is thought to be primarily involved in ureagenesis. A mitochondrial form, AII, has been thought to be more widely expressed and to be involved in the biosynthesis of polyamines, the amino acids ornithine, proline, and glutamate and in the inflammatory process, among others. This paper will address recent experiments that cast some doubt on the validity of these distinctions. Studies have now suggested that macrophages may express AI or AII in different experimental conditions, both in vivo and in vitro. In contrast, most studies, at least in cell culture, suggest that AII may be most highly expressed in cancers of a number of different types. Inhibition of arginase activity in vivo and in vitro has implicated this activity in maintaining ornithine levels for polyamine synthesis. In situ and "quantitative" PCR studies in mouse have demonstrated that AI and not AII is the predominant isoform expressed during development and in the majority of organs. Mouse knockout models for both AI and AII have been produced and are available to address their functions. Surprisingly, the AII knockout animal has no apparent phenotype except for some diminished fertility in homozygous males, consistent with the belief that AII, highly expressed in prostate, is important for sperm function in semen. The AI knockout animal has a more dramatic phenotype and dies at 10-12 days of life of hyperammonemia. The reason for the prolonged survival, as compared to other urea cycle knockout animals, may be due to the later occurrence of hypo-ornithinemia, a contention not yet proven. Transgenic manipulation of the AI knockout animal and breeding the AI and AII knockouts into single animals may address the ability of AII to rescue animals from some of the metabolic consequences of AI deficiency, as appears to happen in man. Newborn screening has given particular hope to patients affected by arginase (AI) deficiency. Increased arginine appears to be detectable by newborn screening with tandem mass spectrometry and the past years continue to demonstrate the therapeutic effectiveness of dietary management of the disorder, with patients treated from birth remaining normal and those treated late, ceasing to deteriorate and even improving in cognitive and physical functioning. Finally, prenatal diagnosis appears to be possible as was predicted, but never proven, some years ago.

Hereditary urea cycle diseases in Finland

AIM

To estimate the incidence of urea cycle diseases (UCDs) in Finland and determine the course of the various disorders as well as the outcome.

METHODS

The original data were collected in the years 1998-2001. The diagnoses made after 2001, as well as the current status of the patients, were updated by surveys in the spring of 2007.

RESULTS

We found a total of 55 cases of UCDs in Finland by 2007: 30 cases of ornithine transcarbamylase (OTC) deficiency, 20 of argininosuccinate lyase (ASL) deficiency, 3 of carbamyl phosphate synthetase (CPS-I) deficiency, 1 of type 1 citrullinaemia and 1 of argininaemia. The estimated total incidence of UCDs was 1:39 000. The incidences of individual disorders were: OTC deficiency 1:62 000, ASL deficiency 1:144 000, CPS deficiency 1:539 000 and citrullinaemia 1:1 616 000. Eighteen (33%) of the patients with a diagnosis of UCD have died, most during their first hyperammonaemic crisis. One patient with OTC deficiency has had a liver transplant. Neurological symptoms of varying severity are common among these patients, particularly those with ASL deficiency.

CONCLUSION

The first survey on the incidence of UCDs in Finland shows some differences in the occurrence rates compared to other countries. Hyperammonaemia, and the neurological symptoms caused by it, can be avoided in most patients with late-onset UCDs with a standard treatment. However, in patients with ASL deficiency, the development of neurological symptoms seems to be inevitable in spite of careful treatment and avoidance of hyperammonaemia.

Newborn screening for hyperargininemia due to arginase 1 deficiency

Hyperargininemia caused by Arginase 1 deficiency is a rare disorder of the urea cycle that can be diagnosed by elevation of arginine in newborn screening blood spots when analyzed by tandem mass spectrometry. Hyperargininemia is currently included as a secondary target on the U.S. Recommended Uniform Screening Panel, which directly influences state-based newborn screening. Because of the apparent low disease frequency and lack of case detection and treatment data, detailed attention has not been given to a model newborn screening algorithm including appropriate analytical cutoff values for disease indicators. In this paper we assess the frequency of hyperargininemia in the U.S. identified by newborn screening to date and document the current status and variability of hyperargininemia newborn screening across U.S. newborn screening programs. We also review other data that support improved screening efficacy by utilizing the arginine/ornithine ratio and other amino acid ratios as discriminators in the screening algorithm. Analysis of archived California screening data showed that an arginine cutoff of 50μM combined with an arginine/ornithine ratio of 1.4 would have resulted in a recall rate of 0.01%. Using an arginine cutoff of 60μM and an arginine/(phenylalanine x leucine) ratio of 1.4, reportedly used in one screening program, or the R4S Tool Runner, would have resulted in a recall rate of <0.005%. All 9 diagnosed patients would have been found for either protocol. Thus, use of appropriate ratios as part of the screening algorithm has the potential to increase both screening sensitivity and specificity. Improved newborn screening effectiveness should lead to better case detection and more rapid treatment to lower plasma arginine levels hence improving long term outcome of individuals with hyperargininemia.

Long-term neurodevelopmental effects of early detection and treatment in a 6-year-old patient with argininaemia diagnosed by newborn screening

Newborn screening makes possible the early identification and treatment of asymptomatic ARG1-deficient patients; however, it is unknown whether early intervention prevents neurological insults. We identified a full-term Hispanic male infant with argininaemia by newborn screening with a serum arginine of 327 µmol/L (reference values 0-140); ARG1 was undetectable on enzyme assay. Sequence analysis of ARG1 revealed a heterozygous nonsense mutation, c.223A>T (p.K75X), and a novel heterozygous missense variant, c.425G>A (p.G142E). Dietary protein restriction began from age 3 months, with addition of sodium benzoate at 4 months, and carnitine from 14 months. For the past 6 years, his serum arginine concentrations were maintained between 268 and 763 µmol/L (reference values 10-140). He has normal development without spastic paraplegia, but with mild hepatomegaly and stable hepatic dysfunction. A full neurodevelopmental assessment was conducted at age 5 years. The BASC-2 rated the patient's behaviours as age-appropriate. The Leiter-R assessed his 'Fundamental Visualization', 'Sequential Order', and 'Picture Concept' at 'Average', 'Form Completion' and 'Matching' at 'Low Average', and 'Figure Ground' and 'Repeated Patterns' in the 'Deficit' range. The full-scale IQ and the functioning ability presented in the 'Borderline' range and in the 'Low Average' range, respectively. The VABS/Survey - Spanish Version showed difficulty in receptive and written language and fine and gross motor skills, and his performance to be at younger than his chronological age. The Short Sensory Profile showed some difficulty with taste and smell sensitivity. Long-term observation over 6 years in a patient with early treated argininaemia shows promising neurodevelopmental results.

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.

Utilization of targeted array comparative genomic hybridization, MitoMet, in prenatal diagnosis of metabolic disorders

Metabolic disorders are inborn errors that often present in the neonatal period with a devastating clinical course. If not treated promptly, these diseases can result in severe, irreversible disease or death. Determining the molecular defects in metabolic diseases is important in providing a definitive diagnosis for patient management. Therefore, prenatal diagnosis for families with known mutations causing metabolic disorders is crucial for timely intervention. Here we present three families in which standard Sanger sequencing failed to provide a definitive diagnosis, but the detection of genomic deletions by array comparative genomic hybridization (CGH) specifically targeted to mitochondrial and metabolic disease genes, MitoMet®, was fundamental in providing accurate prenatal diagnosis. In addition, to our knowledge, two deletions are the smallest detected by oligonucleotide array CGH reported for their respective genes, OTC and ARG1. These data highlight the importance of targeted array CGH in patients with suspected metabolic disorders and incomplete or negative sequencing results, as well as its emerging role in prenatal diagnosis.

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.

Arginase deficiency

Hyperargininemia due to arginase deficiency is a rare, inherited, urea cycle disorder. We report a case of arginase deficiency in a 5-year old boy presenting with mild hyperammonemia, hyperargininemia, and dibasic aminoaciduria.

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.

Coagulation Disturbances in Patients with Argininemia

BACKGROUND

Argininemia is an autosomal recessive urea cycle disorder (UCD). Unlike other UCD, hyperammonemia is rarely seen. Patients usually present in childhood with neurological symptoms. Uncommon presentations like neonatal cholestasis or cirrhosis have been reported. Although transient elevations of liver transaminases and coagulopathy have been reported during hyperammonemia episodes, a permanent coagulopathy has never been reported.

METHODS

In this retrospective study, coagulation disturbances are examined in 6 argininemia patients. All of the patients were routinely followed up for hepatic involvement due to argininemia. Laboratory results, including liver transaminases, albumin, prothrombin time (PT), international normalized ratio (INR), activated partial thromboplastin time (aPTT), and clotting factor levels, were assessed in all of the patients.

RESULTS

All of the patients had a prolonged PT and an increased INR, while none of the patients had a prolonged aPTT. Five patients had slightly elevated liver transaminases. A liver biopsy was performed in 1 patient but neither cirrhosis nor cholestasis was documented. Five of the 6 patients had low factor VII and factor IX levels, while other clotting factors were normal.

CONCLUSIONS

Argininemia patients should be investigated for coagulation disorders even if there is no apparent liver dysfunction or major bleeding symptoms.

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.

A case report of neurological complications owing to lately diagnosed hyperargininemia emphasizing the role of national neonatal screening policies in the kingdom of Bahrain

INTRODUCTION

Arginine is an essential amino acid that plays an important role in various body functions including cell division, wound healing, removal of ammonia, immune function, and release of hormones. Hyperargininemia, an autosomal recessive genetic disorder, is considered one of the least common urea cycle disorders. It rarely presents in the neonatal period but rather appears in children at the age between 2 and 4 years.

CASE PRESENTATION

Herein, we demonstrate a case of a 14-year-old female who presented to the neurology clinic with several neurological complications, which were found to be a consequence of high levels of arginine discovered after performing a metabolic screening test. The hyperargininemia was because of a point mutation of A1 gene on 6q23 resulting in deficiency in arginase enzyme. The complications of this lately diagnosed case of hyperargininemia would have been avoided if a newborn screen were done as a part of a national program.

CONCLUSION

This study presented certain neurological complications in a 14-year-old female who was lately diagnosed with hyperargininemia. Out case report strongly emphasizes the importance of establishing a national neonatal screening policy to ensure early detection of inherited metabolic disorders, in particular those which can be easily treated, in the Kingdom of Bahrain.

Development of a Protein Scaffold for Arginine Sensing Generated through the Dissection of the Arginine-Binding Protein from Thermotoga maritima

Arginine is one of the most important nutrients of living organisms as it plays a major role in important biological pathways. However, the accumulation of arginine as consequence of metabolic defects causes hyperargininemia, an autosomal recessive disorder. Therefore, the efficient detection of the arginine is a field of relevant biomedical/biotechnological interest. Here, we developed protein variants suitable for arginine sensing by mutating and dissecting the multimeric and multidomain structure of Thermotoga maritima arginine-binding protein (TmArgBP). Indeed, previous studies have shown that TmArgBP domain-swapped structure can be manipulated to generate simplified monomeric and single domain scaffolds. On both these stable scaffolds, to measure tryptophan fluorescence variations associated with the arginine binding, a Phe residue of the ligand binding pocket was mutated to Trp. Upon arginine binding, both mutants displayed a clear variation of the Trp fluorescence. Notably, the single domain scaffold variant exhibited a good affinity (~3 µM) for the ligand. Moreover, the arginine binding to this variant could be easily reverted under very mild conditions. Atomic-level data on the recognition process between the scaffold and the arginine were obtained through the determination of the crystal structure of the adduct. Collectively, present data indicate that TmArgBP scaffolds represent promising candidates for developing arginine biosensors.

[Advances in clinical and molecular genetics studies on argininemia]

Argininemia is a rare, autosomal recessive, metabolic disorder caused by an hereditary deficiency of hepatocytes arginase due to ARG1 gene defect. Arginase is the final enzyme in the urea cycle, catalyzing the hydrolysis of arginine to ornithine and urea. Research advances in the clinical manifestations, diagnosis, treatment, prenatal diagnosis and genetics of argininemia were reviewed in this paper. The clinical manifestations of patients with argininemia are complicated and nonspecific so that clinical diagnosis is usually difficult and delayed. Progressive spastic tetraplegia, seizures and cerebella atrophy are common clinical features of the disease. Blood amino acids analysis, arginase assay and ARG1 gene analysis are important to the diagnosis of argininemia. Early diagnosis and a protein-restricted diet with citrulline and benzoate supplements can contribute a lot to improve patient prognosis. With the application of liquid chromatography-tandem mass spectrometry in selective screening and newborn screening for inborn errors of metabolism, an ever-increasing number of patients with argininemia are detected at the asymptomatic or early stages.

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.

[Seven patients of argininemia with spastic tetraplegia as the first and major symptom and prenatal diagnosis of two fetuses with high risk]

OBJECTIVE

Argininemia is a rare disorder of urea cycle defect. The clinical manifestations of this disorder are similar to those of cerebral palsy so that the diagnosis is usually much delayed. This study aimed to investigate the phenotypes and genotypes of seven Chinese patients suffering from argininemia.

METHOD

Three boys and four girls with spastic tetraplegia were diagnosed as argininemia by blood aminoacids analysis and ARG1 gene study. Patients were given a protein-restricted diet, citrulline, sodium benzoate, and other treatment intervention. The mother of Patient 5 and 6 accepted genetic counseling and underwent prenatal diagnosis by amniocentesis.

RESULT

Seven patients presented with progressive spastic tetraplegia and poor physical growth from the age of 1 month to 4 years. Argininemia was found at the age of 1 year and 10 months to 12 years. Five patients had mental retardations. Three had seizures. Their blood arginine elevated (86.66 to 349.83 µmol/L, normal controls 5 to 25 µmol/L). Liver dysfunction was found in six patients. Five patients had elevated blood ammonia levels. In four patients, cerebral atrophy was observed by cranial magnetic resonance imaging. Nine mutations in the ARG1 gene were identified from 7 patients. Only two mutations, c.703G > A in exon 7 and c.32T > C in exon 1 had been reported. c.34G > T, c.53G > A, c.67delG, c.232dupG, c.374C > T, c.539G > C and c.646-649delCTCA, were novel mutations of ARG1. A homozygous mutation c.703G > A was found in the amniocytes of Patient 5's mother, indicating that the fetus was affected by argininemia. Induced abortion was performed. c.53G > A from Patient 6 was not found in the amniocytes of her mother, indicating that the fetus was not affected by hepatocyte arginase deficiency. The result was confirmed by postnatal mutation analysis of cord blood and the normal blood arginine of the newborn.

CONCLUSION

Argininemia is one of the few treatable causes of pediatric spastic paralysis. In this study, seven Chinese patients with spastic tetraplegia were detected by blood aminoacids analysis and confirmed by molecular analysis. Seven novel mutations on ARG1 gene were identified. Prenatal diagnosis of the fetus of a family was performed by amniocytes ARG1 gene analysis.