Arginase deficiency with lethal neonatal expression: evidence for the glutamine hypothesis of cerebral edema

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.

Current status of surviving patients with arginase 1 deficiency in Japan

Arginase 1 (ARG1) deficiency is a rare urea cycle disorder (UCD), with an estimated frequency of 1 per 2,200,000 births in Japan. Patients with ARG1 deficiency develop symptoms in late infancy or pre-school age with progressive neurological manifestations and sometimes present with severe hepatic disease. We previously investigated the status of UCDs in Japan; however, only one patient was identified as having ARG1 deficiency. Therefore, we aimed to investigate the current status of patients with ARG1 deficiency in 2018–2021 because almost 10 years have passed since the previous study. We present the disease history, clinical outcome, and treatment of five surviving patients with ARG1 deficiency and discuss the features of ARG1 deficiency in Japan. We found that clinicians often face difficulty in diagnosing ARG1 deficiency at the early stage of onset because of interpatient variability in onset time and clinical manifestations. Blood L-arginine and guanidino compounds were considered to be the major factors causing adverse neurodevelopmental outcomes. Therefore, early detection and intervention of ARG1 deficiency is essential for improved neurodevelopmental outcomes. Liver transplantation has been considered an effective treatment option that can dramatically improve the quality of life of patients, prior to the neurological manifestation of symptoms caused by ARG1 deficiency.

Hyperargininemia Experiences over Last 7 Years from a Tertiary Care Center

Context

Several Enzymes carry out chemical reactions for the production of energy and carrying out normal functioning of the organism. Disorders of these functions can result in permanent damage to the child affecting multiple systems. Most metabolic disorders are at least controllable and therefore it is important to recognize them early for ensuring optimum growth and development. This involves proper pattern recognition by the clinician.

Aims

In this study we are discussing a rare treatable metabolic disorder namely Hyperargininemia seen by the authors in the last seven years.

Settings and Design

Various parameters of confirmed hyperargininemia patients were analysed.

Methods and Material

It is a descriptive study where all patients were confirmed cases with red blood cell arginase levels <10.

Statistical Analysis used

Descriptive statistical analysis, Mann-whitney test, spearman's rho.

Results

In this study we found consanguinity in 30 % of patients. At least one sibling was affected in 13 % of patients. Females were more in this group though the pattern remains AR. Symptom onset showed variability from less than 1 year to up to 17 years. Commonest clinical feature was cognitive dysfunction, spasticity, seizures, microcephaly and lesser number with extrapyramidal and cerebellar features. Failure to thrive and dysmorphic features were also seen.

Conclusion

Hyperargininemia commonly manifests as regression, failure to thrive, spasticity, seizures with or without microcephaly. When the above phenotype is seen, it is mandatory to screen for urea cycle disorders.

The impact of ammonia levels and dialysis on outcome in 202 patients with neonatal onset urea cycle disorders

Neonatal onset hyperammonemia in patients with urea cycle disorders (UCDs) is still associated with high morbidity and mortality. Current protocols consistently recommend emergency medical and dietary management. In case of increasing or persistent hyperammonemia, with continuous or progressive neurological signs, dialysis is performed, mostly as ultima ratio. It is presently unknown whether the currently defined ammonia threshold (e.g., at 500 μmol/L) to start dialysis is useful to improve clinical outcome. A systematic review of clinical and biochemical data from published neonatal onset UCD patients was performed to identify factors determining clinical outcome and to investigate in which clinical and biochemical setting dialysis was most effective. A total of 202 patients (118 proximal and 84 distal UCDs) described in 90 case reports or case series were included according to predefined inclusion/exclusion criteria. Median age at onset was three days and mean ammonia that triggered start of dialysis was 1199 μmol/L. Seventy-one percent of all patients received any form of dialysis. Total mortality was 25% and only 20% of all patients had a "normal" outcome. In general, patients with higher ammonia levels were more likely to receive dialysis, but this had for most patients no influence on outcome. In conclusion, in severe neonatal onset hyperammonemia, the current practice of dialysis, which effectively clears ammonia, had no impact on outcome. It may be essential for improving outcome to initiate all available treatment options, including dialysis, as early as possible.

Argininic acid alters markers of cellular oxidative damage in vitro: Protective role of antioxidants

We, herein, investigated the in vitro effects of argininic acid on thiobarbituric acid-reactive substances (TBA-RS), total sulfhydryl content and on the activities of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in the blood, kidney and liver of 60-day-old rats. We also verified the influence of the antioxidants (each at 1.0mM) trolox and ascorbic acid, as well as of N^G-nitro-l-arginine methyl ester (L-NAME) at 1.0mM, a nitric oxide synthase inhibitor, on the effects elicited by argininic acid on the parameters tested. The liver, renal cortex and renal medulla were homogenized in 10vol (1:10w/v) of 20mM sodium phosphate buffer, pH 7.4, containing 140mM KCl; and erythrocytes and plasma were prepared from whole blood samples obtained from rats. For in vitro experiments, the samples were pre-incubated for 1h at 37°C in the presence of argininic acid at final concentrations of 0.1, 1.0 and 5.0μM. Control experiments were performed without the addition of argininic acid. Results showed that argininic acid (5.0μM) enhanced CAT and SOD activities and decreased GSH-Px activity in the erythrocytes, increased CAT and decreased GSH-Px activities in the renal cortex and decreased CAT and SOD activities in the renal medulla of 60-day-old rats, as compared to the control group. Antioxidants and/or L-NAME prevented most of the alterations caused by argininic acid on the oxidative stress parameters evaluated. Data suggest that argininic acid alters antioxidant defenses in the blood and kidney of rats; however, in the presence of antioxidants and L-NAME, most of these alterations in oxidative stress were prevented. These findings suggest that oxidative stress may be make an important contribution to the damage caused by argininic acid in hyperargininemic patients and that treatment with antioxidants may be beneficial in this pathology.

Metabolic profiling of presymptomatic Huntington's disease sheep reveals novel biomarkers

The pronounced cachexia (unexplained wasting) seen in Huntington’s disease (HD) patients suggests that metabolic dysregulation plays a role in HD pathogenesis, although evidence of metabolic abnormalities in HD patients is inconsistent. We performed metabolic profiling of plasma from presymptomatic HD transgenic and control sheep. Metabolites were quantified in sequential plasma samples taken over a 25 h period using a targeted LC/MS metabolomics approach. Significant changes with respect to genotype were observed in 89/130 identified metabolites, including sphingolipids, biogenic amines, amino acids and urea. Citrulline and arginine increased significantly in HD compared to control sheep. Ten other amino acids decreased in presymptomatic HD sheep, including branched chain amino acids (isoleucine, leucine and valine) that have been identified previously as potential biomarkers of HD. Significant increases in urea, arginine, citrulline, asymmetric and symmetric dimethylarginine, alongside decreases in sphingolipids, indicate that both the urea cycle and nitric oxide pathways are dysregulated at early stages in HD. Logistic prediction modelling identified a set of 8 biomarkers that can identify 80% of the presymptomatic HD sheep as transgenic, with 90% confidence. This level of sensitivity, using minimally invasive methods, offers novel opportunities for monitoring disease progression in HD patients.

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.

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.

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.

Argocytes containing enzyme nanoparticles reduce toxic concentrations of arginine in the blood

A method for incorporation of arginase nanoparticles into mouse erythrocytes has been developed and the possibility of reducing blood arginine concentration in animals with experimental hyperargininemia with arginase-loaded erythrocytes (argocytes) has been studied. Argocyte infusion to animals with hyperargininemia led to a rapid decrease in blood arginine concentration within 1 h and this effect of argocytes persisted for at least 4 h. This was paralleled by an increase in plasma concentrations of urea and ornithine. Hence, plasma arginine is hydrolyzed by arginase incorporated into argocytes; argocytes are functionally active and can serve as a defense system in pathological hyperargininemia, while the method developed by us can be regarded as a new nanobiotechnology for medicine and veterinary.

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.

Argininemia presenting with progressive spastic diplegia

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

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.

Disruption of peripheral circadian timekeeping in a mouse model of Huntington's disease and its restoration by temporally scheduled feeding

Behavioral circadian rhythms disintegrate progressively in the R6/2 mouse model of Huntington's disease (HD), recapitulating the sleep-wake disturbance seen in HD patients. Here we show that disturbances in circadian pacemaking are not restricted to the brain, but also encompass peripheral metabolic pathways in R6/2 mice. Notably, circadian rhythms of clock-driven genes that are key metabolic outputs in the liver are abolished in vivo. This deficiency is accompanied by arrhythmic expression of the clock genes Cry1 and Dbp, and a phase-advanced Per2 cycle. Compromised circadian metabolic cycles are not, however, a consequence of deficient pacemaking intrinsic to the liver, because when cultured in vitro, R6/2 liver slices exhibit self-sustained circadian bioluminescence rhythms. We therefore propose that compromised metabolic cycles arise from an internal desynchronization secondary to altered feeding patterns and impaired circadian signaling from the central pacemaker of the suprachiasmatic nucleus (SCN). Importantly, the SCN-independent food-entrainable oscillator remains intact in R6/2 mice and, when invoked, can restore daily behavioral cycles and reverse some of the metabolic abnormalities seen in the liver. Disturbances of metabolism have long been thought to be an important feature of HD. Uncoupling liver metabolism from circadian drives will reduce metabolic efficiency and cause imbalances in metabolites known to be deleterious to brain function. Thus, even subtle imbalances in liver function may exacerbate symptoms of HD, where neurological function is already compromised.

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.

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

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

CONCLUSION

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

Neurologic damage and neurocognitive dysfunction in urea cycle disorders

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

Cranial ultrasound in metabolic disorders presenting in the neonatal period: characteristic features and comparison with MR imaging

BACKGROUND AND PURPOSE

Brain imaging is an integral part of the diagnostic work-up for metabolic disorders, and the bedside availability of cranial ultrasonography (cUS) allows very early brain imaging in symptomatic neonates. Our aim was to investigate the role and range of abnormalities seen on cUS in neonates presenting with metabolic disorders. A secondary aim, when possible, was to address the question of whether brain MR imaging is more informative by comparing cUS to MR imaging findings.

MATERIALS AND METHODS

Neonates with a metabolic disorder who had at least 1 cUS scan were eligible. cUS images were reviewed for anatomic and maturation features, cysts, calcium, and other abnormalities. When an MR imaging scan had been obtained, both sets of images were compared.

RESULTS

Fifty-five infants (35 also had MR imaging) were studied. The most frequent findings were in oxidative phosphorylation disorders (21 cUS and 12 MR imaging): ventricular dilation (11 cUS and 6 MR imaging), germinolytic cysts (GLCs; 7 cUS and 5 MR imaging), and abnormal white matter (7 cUS and 6 MR imaging); in peroxisomal biogenesis disorders (13 cUS and 9 MR imaging): GLCs (10 cUS and 6 MR imaging), ventricular dilation (10 cUS and 5 MR imaging), abnormal cortical folding (8 cUS and 7 MR imaging), and lenticulostriate vasculopathy (8 cUS); in amino acid metabolism and urea cycle disorders (14 cUS and 11 MR imaging): abnormal cortical folding (9 cUS and 4 MR imaging), abnormal white matter (8 cUS and 8 MR imaging), and hypoplasia of the corpus callosum (7 cUS and 6 MR imaging); in organic acid disorders (4 cUS and 2 MR imaging): periventricular white matter echogenicity (2 cUS and 1 MR imaging); and in other disorders (3 cUS and 1 MR imaging): ventricular dilation (2 cUS and 1 MR imaging). cUS findings were consistent with MR imaging findings. cUS was better for visualizing GLCs and calcification. MR imaging was more sensitive for subtle tissue signal intensity changes in the white matter and abnormality in areas difficult to visualize with cUS, though abnormalities of cortical folding suggestive of polymicrogyria were seen on cUS.

CONCLUSION

A wide range of abnormalities is seen using cUS in neonatal metabolic disorders. cUS is a reliable bedside tool for early detection of cysts, calcium, structural brain abnormalities, and white matter echogenicity, all suggestive of metabolic disorders.

NTPDase and 5'-nucleotidase activities of synaptosomes from hippocampus of rats subjected to hyperargininemia

ATP is an important excitatory neurotransmitter and adenosine acts as a neuromodulatory structure inhibiting neurotransmitters release in the central nervous system. Since the ecto-nucleotidase cascade that hydrolyzes ATP to adenosine is involved in the control of brain functions and previous studies realized in our laboratory have recently reported that acute administration of Arg decreases the NTPDase and 5'-nucleotidase activities of rat blood serum, in the present study we investigated the effect of arginine administration on NTPDase and 5'-nucleotidase activities by synaptosomes from hippocampus of rats. First, sixty-days-old rats were treated with a single or a triple intraperitoneal injection of arginine (0.8 g/Kg) or an equivalent volume of 0.9% saline solution (control) and were killed 1 h later. Second, rats received an intracerebroventricular injection of 1.5 mM arginine solution or saline (5 microL) and were killed 1 h later. We also tested the in vitro effect of arginine (0.1-1.5 mM) on nucleotide hydrolysis in synaptosomes from rat hippocampus. Results showed that intraperitoneal arginine administration did not alter nucleotide hydrolysis. On the other hand, arginine administered intracerebroventricularly reduced ATP (32%), ADP (30%) and AMP (21%) hydrolysis, respectively. In addition, arginine added to the incubation medium, provoked a decrease on ATP (19%), ADP (17%) and AMP (23%) hydrolysis, respectively. Furthermore, kinetic studies showed that the inhibitory effect of arginine was uncompetitive in relation to ATP, ADP and AMP. In conclusion, according to our results it seems reasonable to postulate that arginine alters the cascade involved in the extracellular degradation of ATP to adenosine.

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

The urea cycle consists of six consecutive enzymatic reactions that convert waste nitrogen into urea. Urea cycle disorders are a group of inborn errors of hepatic metabolism that often result in life threatening hyperammonemia and hyperglutaminemia. Deficiencies of all of the enzymes of the cycle have been described and although each specific disorder results in the accumulation of different precursors, hyperammonemia and hyperglutaminemia are common biochemical hallmarks of these disorders. Arginase is the enzyme involved in the last step of the urea cycle. It catalyzes the conversion of arginine to urea and ornithine. The latter reenters the mitochondrion to continue the cycle. Hyperargininemia is an autosomal recessive disorder caused by a defect in the arginase I enzyme. Unlike other urea cycle disorders, this condition is not generally associated with a hyperammonemic encephalopathy in the neonatal period. It typically presents later in childhood between 2 and 4 years of age with predominantly neurological features. If untreated, it progresses with gradual developmental regression. A favorable outcome can be achieved if dietary treatment and alternative pathway therapy are instituted early in the disease course. With this approach, further neurological deterioration is prevented and partial recovery of skills ensues. Early diagnosis of this disorder through newborn screening programs may lead to a better outcome. This review article summarizes the clinical characterization of this disorder; as well as its biochemical, enzymatic, and molecular features. Treatment, prenatal diagnosis and diagnosis through newborn screening are also discussed.

Hyperargininemia due to liver arginase deficiency

The urea cycle is a series of six reactions necessary to rid the body of the nitrogen generated by the metabolism, primarily of amino acids, from the diet or released as the result of endogenous protein catabolism. Arginase is the sixth and final enzyme of this cycle. Arginase catalyzes the conversion of arginine to urea and ornithine, the latter recycled to continue the cycle. Hyperargininemia due to arginase deficiency is inherited in an autosomal recessive manner and gene for arginase, designated AI, has been cloned. Unlike the other urea cycle enzymes, a second gene encoding arginase, with similar structural properties and enzyme characteristics, exists and has been named Arginase II (AII). Comprehensive histories and physical examinations confirm a strikingly uniform clinical picture and one notably different from patients with other urea cycle disorders. This condition rarely presents in the neonatal period and first symptoms typically present in children between 2 and 4 years of age. First symptoms are often neurologically based. If untreated, symptoms are progressive with a gradual loss of developmental milestones. With adherence to a dietary and drug regimen, a favorable outcome can be expected, with cessation of further neurological deterioration and in some instances, of improvement. This article summarizes the clinical course of selected patients who represent the full spectrum of presentations of arginase deficiency. In addition to the clinical characterization of this disorder; the biochemical, enzymatic, and molecular evidence of disease is summarized. Treatment and prenatal diagnosis are also discussed.

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.