Differentiation of transient hyperammonemia of the newborn and urea cycle enzyme defects by clinical presentation

Continuous renal replacement therapy for severe transient hyperammonemia in a preterm infant weighing 1120 g: A case report

Transient hyperammonemia of the newborn is a rare form of hyperammonemia with an unclear, likely nongenetic etiology, primarily affecting larger preterm infants. However, lower birth weight and gestational age are associated with higher ammonia levels, increasing the risk of neurotoxicity and hepatotoxicity. Transient hyperammonemia of the newborn typically manifests as respiratory distress within the first 24 h post-birth, progressing to seizures and coma within 48 h. Continuous renal replacement therapy has demonstrated considerable efficacy in managing severe transient hyperammonemia of the newborn due to its high ammonia clearance rate; however, its application remains limited in very low birth weight preterm infants. Herein, we report the case of a male infant born at 28+2 weeks gestation, weighing 1120 g, who developed transient hyperammonemia of the newborn 22 h post-birth. Despite initial pharmacotherapy and peritoneal dialysis, his ammonia levels continued to rise, necessitating continuous renal replacement therapy. After 42 h of continuous renal replacement therapy, his ammonia levels decreased significantly and he recovered fully, eventually being discharged in good health. This case highlights continuous renal replacement therapy as a viable, life-saving intervention for severe transient hyperammonemia of the newborn, even in very low birth weight preterm infants.

Targeted Sequencing Approach and Its Clinical Applications for the Molecular Diagnosis of Human Diseases

The outbreak of COVID-19 has positively impacted the NGS market recently. Targeted sequencing (TS) has become an important routine technique in both clinical and research settings, with advantages including high confidence and accuracy, a reasonable turnaround time, relatively low cost, and fewer data burdens with the level of bioinformatics or computational demand. Since there are no clear consensus guidelines on the wide range of next-generation sequencing (NGS) platforms and techniques, there is a vital need for researchers and clinicians to develop efficient approaches, especially for the molecular diagnosis of diseases in the emergency of the disease and the global pandemic outbreak of COVID-19. In this review, we aim to summarize different methods of TS, demonstrate parameters for TS assay designs, illustrate different TS panels, discuss their limitations, and present the challenges of TS concerning their clinical application for the molecular diagnosis of human diseases.

A glance at transient hyperammonemia of the newborn: Pathophysiology, diagnosis, and treatment: A review

Hyperammonemia is the excessive accumulation of ammonia in the blood, and is usually defined as a plasma level above 100 µmol/L in neonates or above 50 µmol/L in term infants, children, and adolescents. Patients with hyperammonemia usually experience life-threatening neuropsychiatric symptoms, especially newborns. It is routinely caused by inherited metabolic diseases and also by acquired disorders, such as liver failure, portosystemic shunting, gastrointestinal hemorrhage, ureterosigmoidostomy, renal tubular acidosis, hypoxic ischemic encephalopathy, infections with urea-metabolizing organisms, and some drugs. Transient hyperammonemia of the newborn (THAN) is a special type of hyperammonemia acknowledged in the field of metabolic disease as an inwell-defined or well-understood entity, which can be diagnosed only after the exclusion of genetic and acquired causes of hyperammonemia. Although the prognosis for THAN is good, timely identification and treatment are essential. Currently, THAN is underdiagnosed and much less is mentioned for early diagnosis and vigorous treatment. Herein, we present common themes that emerge from the pathogenesis, diagnosis, and management of THAN, based on current evidence. When a newborn presents with sepsis, intracranial hemorrhage, or asphyxia that cannot explain coma and seizures, doctors should always keep this disease in mind.

Ornithine Transcarbamylase - From Structure to Metabolism: An Update

Ornithine transcarbamylase (OTC; EC 2.1.3.3) is a ubiquitous enzyme found in almost all organisms, including vertebrates, microorganisms, and plants. Anabolic, mostly trimeric OTCs catalyze the production of L-citrulline from L-ornithine which is a part of the urea cycle. In eukaryotes, such OTC localizes to the mitochondrial matrix, partially bound to the mitochondrial inner membrane and part of channeling multi-enzyme assemblies. In mammals, mainly two organs express OTC: the liver, where it is an integral part of the urea cycle, and the intestine, where it synthesizes citrulline for export and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine. Here, we give an overview on OTC genes and proteins, their tissue distribution, regulation, and physiological function, emphasizing the importance of OTC and urea cycle enzymes for metabolic regulation in human health and disease. Finally, we summarize the current knowledge of OTC deficiency, a rare X-linked human genetic disorder, and its emerging role in various chronic pathologies.

Suggested guidelines for the diagnosis and management of urea cycle disorders: First revision

In 2012, we published guidelines summarizing and evaluating late 2011 evidence for diagnosis and therapy of urea cycle disorders (UCDs). With 1:35 000 estimated incidence, UCDs cause hyperammonemia of neonatal (~50%) or late onset that can lead to intellectual disability or death, even while effective therapies do exist. In the 7 years that have elapsed since the first guideline was published, abundant novel information has accumulated, experience on newborn screening for some UCDs has widened, a novel hyperammonemia-causing genetic disorder has been reported, glycerol phenylbutyrate has been introduced as a treatment, and novel promising therapeutic avenues (including gene therapy) have been opened. Several factors including the impact of the first edition of these guidelines (frequently read and quoted) may have increased awareness among health professionals and patient families. However, under-recognition and delayed diagnosis of UCDs still appear widespread. It was therefore necessary to revise the original guidelines to ensure an up-to-date frame of reference for professionals and patients as well as for awareness campaigns. This was accomplished by keeping the original spirit of providing a trans-European consensus based on robust evidence (scored with GRADE methodology), involving professionals on UCDs from nine countries in preparing this consensus. We believe this revised guideline, which has been reviewed by several societies that are involved in the management of UCDs, will have a positive impact on the outcomes of patients by establishing common standards, and spreading and harmonizing good practices. It may also promote the identification of knowledge voids to be filled by future research.

Impairment of cognitive function in ornithine transcarbamylase deficiency is global rather than domain-specific and is associated with disease onset, sex, maximum ammonium, and number of hyperammonemic events

Beginning in 2006, the Urea Cycle Disorders Consortium (UCDC) has conducted a longitudinal study of eight inherited deficiencies of enzymes and transporters of the urea cycle, including 444 individuals with ornithine transcarbamylase deficiency (OTCD), of whom 300 (67 males, 233 females) received psychological evaluation. In a cross-sectional study (age range, 3-71 years), analysis of covariance (ANCOVA) determined the association between outcomes in five cognitive domains (global intelligence, executive functions, memory, visuomotor integration, visual perception) and sex, age at testing and timing of disease onset defined as early onset (≤28 days; EO), late onset (LO), or asymptomatic (AS). The dataset of 183 subjects with complete datasets (31 males, 152 females) revealed underrepresentation of EO subjects (2 males, 4 females), who were excluded from the ANCOVA. Although mean scores of LO and AS individuals were within 1 SD of the population norm, AS subjects attained significantly higher scores than LO subjects and males higher scores than females. Correlations between cognitive domains were high, particularly intelligence proved to be a distinguished indicator for cognitive functioning. Maximum plasma ammonium concentration and intelligence correlated significantly higher in EO (r = -0.47) than in LO subjects (r = 0.04). Correlation between the number of hyperammonemic events and intelligence scores were similar for EO (r = -0.30) and LO (r = -0.26) individuals. The number of clinical symptoms was significantly associated with intelligence (r = -0.28) but not with scores in other domains. Results suggest that OTCD has a global impact on cognitive functioning rather than a specific effect on distinct cognitive domains.

Late-onset ornithine transcarbamylase deficiency caused by a somatic mosaic mutation

An 18-month-old boy was diagnosed with late-onset ornithine transcarbamylase deficiency. Genetic analysis revealed a mosaic frameshift mutation (p.Q279fs) in the OTC gene. Despite the presence of a null mutation, he exhibited a milder phenotype, suggesting that the wild-type allele could rescue the function of OTC. The presence of mosaicism has great effects on the clinical phenotype and recurrence-risk assessment, which should be taken into consideration for genetic counseling.

Alternative Start Sites Downstream of Non-Sense Mutations Drive Antigen Presentation and Tolerance Induction to C-Terminal Epitopes

CTL responses to the transgene product remain an active area of concern for the gene therapy field. A patient's underlying genetic mutation may influence the qualitative nature of these potentially destructive T cell responses. Individuals with a mutation that introduces a premature termination codon (PTC) that prevents synthesis of the full-length peptide are considered more likely to mount a transgene-specific T cell response because of a lack of immune tolerance to C-terminal epitopes as a consequence of absent endogenous Ag presentation. In this article, we demonstrate that a human ornithine transcarbamylase gene containing various PTC-inducing non-sense mutations is able to generate and present epitopes downstream of the termination codon. Generation of these epitopes occurs primarily from alternative translation start sites downstream of the stop codon. Furthermore, we show that expression of these genes from adeno-associated virus vectors in C57BL/6 mice is able to induce peripheral tolerance to epitopes downstream of the PTC. These results suggest that, despite the lack of full-length endogenous protein, patients with PTC-inducing non-sense mutations may still present T cell epitopes downstream of the premature termination site that may render the subject tolerant to wild-type transgene products.

Genotype-Phenotype Correlations in Ornithine Transcarbamylase Deficiency: A Mutation Update

Ornithine transcarbamylase (OTC) deficiency is an X-linked trait that accounts for nearly half of all inherited disorders of the urea cycle. OTC is one of the enzymes common to both the urea cycle and the bacterial arginine biosynthesis pathway; however, the role of OTC has changed over evolution. For animals with a urea cycle, defects in OTC can trigger hyperammonemic episodes that can lead to brain damage and death. This is the fifth mutation update for human OTC with previous updates reported in 1993, 1995, 2002, and 2006. In the 2006 update, 341 mutations were reported. This current update contains 417 disease-causing mutations, and also is the first report of this series to incorporate information about natural variation of the OTC gene in the general population through examination of publicly available genomic data and examination of phenotype/genotype correlations from patients participating in the Urea Cycle Disorders Consortium Longitudinal Study and the first to evaluate the suitability of systematic computational approaches to predict severity of disease associated with different types of OTC mutations.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Clinical and biochemical aspects of primary and secondary hyperammonemic disorders

An increased concentration of ammonia is a non-specific laboratory sign indicating the presence of potentially toxic free ammonia that is not normally removed. This does occur in many different conditions for which hyperammonemia is a surrogate marker. Hyperammonemia can occur due to increased production or impaired detoxification of ammonia and should, if associated with clinical symptoms, be regarded as an emergency. The conditions can be classified into primary or secondary hyperammonemias depending on the underlying pathophysiology. If the urea cycle is directly affected by a defect of any of the involved enzymes or transporters, this results in primary hyperammonemia. If however the function of the urea cycle is inhibited by toxic metabolites or by substrate deficiencies, the situation is described as secondary hyperammonemia. For removal of ammonia, mammals require the action of glutamine synthetase in addition to the urea cycle, in order to ensure lowering of plasma ammonia concentrations to the normal range. Independent of its etiology, hyperammonemia may result in irreversible brain damage if not treated early and thoroughly. Thus, early recognition of a hyperammonemic state and immediate initiation of the specific management are of utmost importance. The main prognostic factors are, irrespective of the underlying cause, the duration of the hyperammonemic coma and the extent of ammonia accumulation. This paper will discuss the biochemical background of primary and secondary hyperammonemia and will give an overview of the various underlying conditions including a brief clinical outline and information on the genetic backgrounds.

Suggested guidelines for the diagnosis and management of urea cycle disorders

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

Preimplantation genetic diagnosis for ornithine transcarbamylase deficiency by simultaneous analysis of duplex-nested PCR and fluorescence in situ hybridization: a case report

Ornithine transcarbamylase (OTC) deficiency is an X-linked co-dominant disorder. A couple, with a previous history of a neonatal death and a therapeutical termination due to OTC deficiency, was referred to our center for preimplantation genetic diagnosis (PGD). The female partner has a nonsense mutation in the exon 9 of the OTC gene (R320X). We carried out nested polymerase chain reaction (PCR) for R320X mutation and fluorescence in situ hybridization (FISH) for aneuploidy screening. Among a total of 11 embryos, two blastomeres per embryo from 9 embryos were biopsied and analyzed by duplex-nested PCR and FISH, and one blastomere per embryo from 2 embryos by only duplex-nested PCR. As a result of PCR and restriction fragment length polymorphism analysis, four embryos were diagnosed as unaffected embryos having the normal OTC gene. Among these embryos, only one embryo was confirmed as euploidy for chromosome X, Y and 18 by FISH analysis. A single normal embryo was transferred to the mother, yielding an unaffected pregnancy and birth of a healthy boy. Based on our results, PCR for mutation loci and FISH for aneuploidy screening with two blastomeres from an embryo could provide higher accuracy for the selection of genetically and chromosomally normal embryos in the PGD for single gene defects.

Mutations and polymorphisms in the human ornithine transcarbamylase (OTC) gene

Ornithine transcarbamylase (OTC) deficiency is the most common inherited disorder of the urea cycle and is transmitted as an X-linked trait. Defects in the OTC gene cause a block in ureagenesis resulting in hyperammonemia, which can lead to brain damage and death. Three previous mutation updates for the OTC gene have been published, in 1993, 1995, and 2002. The most recent comprehensive update, in 2002, contained 244 mutations including 13 nondisease-causing mutations and polymorphisms. This current update reports 341 mutations, of which 93 have not been previously reported, and an additional 29 nondisease-causing mutations and polymorphisms. Out of the 341 mutations, 149 were associated with neonatal onset of hyperammonemia (within the first week of life), 70 were seen in male patients with later onset of hyperammonemia, and 121 were found in heterozygous females (one unknown). Along with the reported mutations, residual enzyme activities and other pertinent clinical information are included whenever available. Most mutations in the OTC gene are specific to a particular family ("private" mutations). They are distributed throughout the gene, with a significant paucity of mutations in the 32 first codons encoding the "leader" peptide (exon 1 and the beginning of exon 2). Almost all mutations in consensus splice sites confer a neonatal onset phenotype. Using the current molecular screening methods, mutations are found in about 80% of the patients. The remaining patients may have mutations in regulatory domains or mutations deep in the introns, which constitute 98.5% of the genomic sequence. In addition, a phenocopy of OTC deficiency caused by mutations in another unknown gene cannot be excluded.

Diagnosis and early management of inborn errors of metabolism presenting around the time of birth

Inherited metabolic diseases often present around the time of birth. They are responsible for some cases of hydrops fetalis and a number of dysmorphic syndromes. Patients with inborn errors may also present at (or shortly after) birth with seizures or severe hypotonia. Most affected babies, however, appear normal at birth and subsequently deteriorate, with hypoglycaemia, acidosis, neurological or cardiac problems, or liver disease. Treatment often involves measures to reduce catabolism and to remove toxic metabolites. It should not be delayed for a definitive diagnosis.

CONCLUSION

In the newborn period, inborn errors can easily be misdiagnosed as sepsis or birth asphyxia; prompt detection requires vigilance and the early measurement of biochemical markers, such as plasma ammonia.

Mutations and polymorphisms in the human ornithine transcarbamylase gene

Ornithine transcarbamylase (OTC) deficiency, an X-linked, semidominant disorder, is the most common inherited defect in ureagenesis resulting in hyperammonemia. The previous two mutation updates for the OTC gene were published in 1993 and 1995 and included 36 and 30 mutations respectively. This comprehensive update contains a compilation of 244 mutations including 13 polymorphisms. Twenty-four of the mutations are reported here for the first time. Forty-two percent of the disease-causing mutations are associated with acute neonatal hyperammonemia; 21% were found in patients with late onset disease and approximately 37% were found in manifesting heterozygous females, most of which are presumed to confer a neonatal phenotype in hemizygous males. Also included are residual enzyme activities and residual incorporation of ammonium nitrogen into urea and results of expression studies for a small proportion of the mutations. Most mutations in the OTC gene are "private" and are distributed throughout the gene with paucity of mutation in the sequence encoding the leader peptide (exon 1 and beginning of exon 2) and in exon 7. Almost all mutations in consensus splicing sites confer a neonatal phenotype. Thirteen polymorphisms have been found, several of which are useful for allele tracking in patients in whom the mutation can't be found. Even with sequencing of the entire reading frame and exon/intron boundaries, only about 80% of the mutations are detected in patients with proven OTC deficiency. The remaining probably occur within the introns or in regulatory domains.

Urea cycle disorders

Most patients with urea cycle disorders who present as neonates, do so with deteriorating feeding, drowsiness and tachypnoea, following a short initial period when they appear well. The plasma ammonia should be measured at the same time as the septic screen in such patients. Ammonia levels above 200 micromol/l are usually caused by inherited metabolic diseases and it is essential to make a diagnosis for genetic counselling, even if the patients die. The aim of treatment is to lower the ammonia concentrations as fast as possible. Sodium benzoate, sodium phenylbutyrate and arginine can exploit alternative pathways for the elimination of nitrogen but haemodialysis or haemofiltration should be instituted if ammonia concentrations are >500 micromol/l or if they do not fall promptly. Long-term management involves drugs, dietary protein restriction and use of an emergency regimen during illness. Severe hyperammonaemia is usually associated with irreversible neurological damage, particularly if levels have been above 800 micromol/l for >24 hours, and the option of withdrawing treatment should be discussed with the family.

Urea cycle disorders

Deficiency of any of the five enzymes in the urea cycle results in the accumulation of ammonia and leads to encephalopathy. Episodes of encephalopathy and associated symptoms are unpredictable and, if untreated, are lethal or produce devastating neurologic sequelae in long-term survivors. Although these disorders do not produce liver disease, the consequences of hyperammonemia resemble those seen in patients with hepatic failure or in a transient interference with the urea cycle, as seen in some forms of organic acidemias. Therefore, investigation for hyperammonemia in any infant or child with altered mental status (in the absence of obvious causes, such as trauma, infection, or poisoning) is essential for prompt diagnosis of urea cycle disorders and institution of treatment to avoid brain damage and death. This article addresses the function of the urea cycle and the diagnosis and management of urea cycle disorders.

Inborn errors of metabolism in infancy: a guide to diagnosis

Recent advances in the diagnosis and treatment of inborn errors of metabolism have improved substantially the prognosis for many of these conditions. This makes it essential that the practicing pediatrician be familiar with the clinical presentation of these disorders. A practical clinical approach to the recognition of inborn errors of metabolism in the young infant is presented in this review. Indications for specific laboratory studies are discussed. Guidelines are provided for the stabilization and emergency treatment of critically ill infants. This approach will identify those infants who will benefit from additional evaluation and specific treatment. Many of the inborn errors of metabolism, including urea cycle defects, organic acidemias, and certain disorders of amino acid metabolism, present in the young infant with symptoms of an acute or chronic metabolic encephalopathy. Typical symptoms include lethargy, poor feeding, apnea or tachypnea, and recurrent vomiting. Metabolic acidosis and/or hyperammonemia are observed in many of these conditions, but there are notable exceptions, including nonketotic hyperglycinemia and molybdenum co-factor deficiency. Therefore, appropriate laboratory testing for metabolic disorders should be performed in any infant who exhibits these findings. Although sepsis may be the initial consideration in a neonate with these symptoms, inborn errors of metabolism should always be in the differential diagnosis, particularly in a full-term infant with no specific risk factors. Hypoglycemia may be the predominant finding in a number of inborn errors of metabolism, including glycogen storage disorders, defects in gluconeogenesis, and fatty acid oxidation defects. The latter disorders, among the most common encountered, exhibit marked clinical variability and also may present as a sudden death, a Reye's-like episode, or a cardiomyopathy. Jaundice or other evidence of hepatic dysfunction is the mode of presentation of another important group of inborn errors of metabolism including galactosemia, hereditary tyrosinemia, neonatal hemochromatosis, and a number of other conditions. A subset of lysosomal storage disorders may present very early with coarse facial features, organomegaly, or even hydrops fetalis. Specific patterns of dysmorphic features and congenital anomalies characterize yet another group of inherited metabolic disorders, such as Zellweger syndrome and the Smith-Lemli-Opitz syndrome. Each of these symptom complexes, and the appropriate evaluation of the affected infants, is discussed in more detail in this review.

Hyperammonaemia with distal renal tubular acidosis

The case is reported of an infant with hyperammonaemia secondary to severe distal renal tubular acidosis. A clinical association between increased concentrations of ammonia in serum and renal tubular acidosis has not previously been described. In response to acidosis the infant's kidneys presumably increased ammonia synthesis but did not excrete ammonia, resulting in hyperammonaemia. The patient showed poor feeding, frequent vomiting, and failure to thrive, but did not have an inborn error of metabolism. This case report should alert doctors to consider renal tubular acidosis in the differential diagnosis of severely ill infants with metabolic acidosis and hyperammonaemia.

Inborn errors of urea synthesis

Inborn errors of urea synthesis can present in the newborn period as a catastrophic illness or later in childhood or adulthood with an indolent course punctuated by hyperammonemic episodes. Because symptoms mimic other neuropsychiatric disorders, it is common for there to be a delay in diagnosis, often with dire consequences. Diagnosis relies on the combination of clinical suspicion and the measurement of ammonium, lactate, and amino acids in plasma and organic acids and orotic acid in urine. Treatment involves nitrogen restriction combined with the stimulation of alternate pathways of waste nitrogen excretion. More recently liver transplantation has been performed as enzyme replacement therapy. The outcome is poor in children who survive prolonged neonatal hyperammonemic coma, with most manifesting developmental disabilities. The etiology of neuronal injury in this disorder is unclear but may involve some combination of ammonia/amino acid accumulation, neurotransmitter alterations, and excitotoxic injury. Gene therapy holds the promise of improved treatment in the future.

Menstrual cycle and gonadal steroid effects on symptomatic hyperammonaemia of urea-cycle-based and idiopathic aetiologies

We report two female patients, one with a known inborn error of ureagenesis and the other of unknown cause, in whom recurrent, transient episodes of severe hyperammonaemia increased in frequency and severity with sexual maturity and parturition. Both responded to ovarian steroids administered continuously to suppress ovulation and menstruation, and ultimately to simple hysterectomy. These studies suggest a new therapeutic approach to defective ureagenesis in female patients and a relationship between ammonia production or disposal and the menstrual cycle.

Mutations and polymorphisms in the human ornithine transcarbamylase gene

Deletions of variable size involving one or more exons, 29 different missense, nonsense, or frameshift mutations, and three polymorphisms have been found in patients with ornithine transcarbamylase (OTC) deficiency. Most of the deletions and mutations were found in patients with severe disease manifested clinically as acute neonatal hyperammonemia. A small number of mutations or somatic mosaicism for deletions were found in males with "late onset" disease and heterozygous females who were symptomatic. Approximately 10-15% of all molecular alterations associated with OTC deficiency are large deletions involving all or part of the OTC gene with or without contiguous genes on the short arm of the X chromosome. Approximately 10% of all point mutations involve the CpG dinucleotide of codon 141 with a CGA-->CAA transition producing a deleterious Arg-->Gln substitution in position 109 of the mature enzyme and causing the elimination of a TaqI recognition site. The majority of the remaining mutations in the OTC gene are unique to the affected family and are usually not found in unrelated patients. To date, two mutations have been described in the sequence of the "leader" peptide, 23 mutations have been found in the coding sequence of the "mature" enzyme, and four mutations have been discovered in splicing recognition sites. Approximately 20 single base polymorphisms have been postulated to exist by comparing two reported OTC gene sequences; six of these substitutions cause amino acid changes of which three have been confirmed in patients. Of the known point mutations, 27 are single base substitutions: 17 missense, 6 nonsense, 4 splice site, and the remaining 2 are single base deletions.

Prospective versus clinical diagnosis and therapy of acute neonatal hyperammonaemia in two sisters with carbamyl phosphate synthetase deficiency

Two female siblings were treated for acute neonatal hyperammonaemia due to complete carbamyl phosphate synthetase I deficiency. The first child was detected clinically at 65 hours of age and therapy started at 79 hours. The second child was followed from birth and therapy started at 5 hours of age. The extrapolated rate of increase of blood ammonia, in the first hours of life before therapy started, was 19 mumol L-1 h-1 in both babies. Peak blood ammonia level was 2235 mumol/L in the first (clinically detected) child and 271 mumol/L in the second (prospectively followed) child. The second child became symptomatic at 3 hours of age when blood ammonia level was as low as 90 mumol/L, whereas blood ammonia levels above 100 mumol/L caused no symptoms during recovery. The child detected clinically required haemodialysis and peritoneal dialysis to treat the hyperammonaemia. In the prospectively treated child, early therapy with intravenous sodium benzoate and sodium phenylacetate slowed the rate of increase in blood ammonia level, but this therapy did not prevent the need for peritoneal dialysis.

Ornithine carbamoyl transferase deficiency: findings, models and problems

The initial clinical symptoms of ornithine carbamoyl transferase deficiency depend on the age of onset. Respiratory distress on the first day of life does not allow exclusion of OCT deficiency in the individual patient. The acid-base status is not useful as a discriminant between urea-cycle disorders and organic acidurias. Beyond the neonatal age, a second period of increased risk for often lethal hyperammonaemic crises is found between 12 and 15 years of age. For definite diagnosis (pre- and postnatal) of heterozygotes the quantity of tissue obtained should be sufficient to obtain a representative sample for a mosaic structure. Experimental work gives some clues for the interpretation of reversible symptoms of hyperammonaemia. The increased transport of tryptophan at the blood-brain barrier in presence of increased glutamine concentration in tissue appears to depend on intact gammaglutamyl transpeptidase in brain microvessels and involves at least in part the L-carrier. Animal research on the mechanisms leading to irreversible damage in hyperammonaemia should be encouraged in order to define reliable predictive criteria for clinical decisions.

Computerized tomography in primary hyperammonemia

Urea cycle enzymopathies are rare in the literature. They are accompanied by neurological disorders somehow related to the increase in blood ammonia. Reviewing the possible physiopathological course we present a case in which during an eight month period severe cerebral atrophy developed, more intense in the occipital region but totally sparing the posterior cranial fossa structures.

When and how should we measure plasma ammonia?

Hyperammonaemia is associated with a high morbidity and mortality. It is important to diagnose as it is often treatable and perhaps, most importantly, may be genetically determined. Measurement of plasma ammonia is rarely necessary in adult medicine. In paediatrics, and particularly in the neonatal period, it is an important investigation in the diagnosis and often in the subsequent management of several inherited metabolic disorders. Patients with these disorders, particularly neonates, will deteriorate over a period of hours and investigation cannot wait. In these situations a plasma ammonia together with other investigations for metabolic disorders must be available urgently and are sometimes necessary out of normal laboratory hours. Interpretation must take into account the age and maturity of the child as well as the clinical state and results of other investigations. If treatment is initiated, frequent monitoring of plasma ammonia may be required. Plasma ammonia can be reliably and conveniently measured using a specific ion-selective electrode system or an automated enzyme method. Screening using a microdiffusion method is not a satisfactory alternative to a quantitative assay. Plasma ammonia is no longer solely the province of the specialised paediatric laboratory, but should be part of the repertoire of every laboratory supporting neonatal or paediatric units. The threshold for accepting requests should be lower than at present if we are to prevent misdiagnoses.

Neurologic syndromes in hyperammonemic disorders

This article reviews the neurologic manifestations of hyperammonemic disorders. A variety of inborn errors of metabolism, as well as transient neonatal or acquired hepatic dysfunctions, are identified as causes of hyperammonemia. The pathogenesis of hyperammonemia in these disorders is outlined. Catastrophic hyperammonemia and hyperammonemic coma are described; algorithms to establish specific diagnosis are suggested. Symptoms of less severe ammonia intoxication, which also occur in more common diseases, are described. The need to consider hyperammonemia as a potential etiology for these symptoms is emphasized. Finally, the neurotoxicity of ammonia is discussed.