Ornithine transcarbamylase deficiency: a urea cycle defect

Wireless Detection of Trace Ammonia: A Chronic Kidney Disease Biomarker

Elevated levels of ammonia in breath can be linked to medical complications, such as chronic kidney disease (CKD), that disturb the urea balance in the body. However, early stage CKD is usually asymptomatic, and mass screening is hindered by high instrumentation and operation requirements and accessible and reliable detection methods for CKD biomarkers, such as trace ammonia in breath. Enabling methods would have significance in population screening for early stage CKD patients. We herein report a method to effectively immobilize transition metal selectors in close proximity to a single-walled carbon nanotube (SWCNT) surface using pentiptycene polymers containing metal-chelating backbone structures. The robust and modular nature of the pentiptycene metallopolymer/SWCNT complexes creates a platform that accelerates sensor discovery and optimization. Using these methods, we have identified sensitive, selective, and robust copper-based chemiresistive ammonia sensors that display low parts per billion detection limits. We have added these hybrid materials to the resonant radio frequency circuits of commercial near-field communication (NFC) tags to achieve robust wireless detection of ammonia at physiologically relevant levels. The integrated devices offer a noninvasive and cost-effective approach for early detection and monitoring of CKD.

Clinical trials and promising preclinical applications of CRISPR/Cas gene editing

Treatment of genetic disorders by genomic manipulation has been the unreachable goal of researchers for many decades. Although our understanding of the genetic basis of genetic diseases has advanced tremendously in the last few decades, the tools developed for genomic editing were not efficient and practical for their use in the clinical setting until now. The recent advancements in the research of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-associated protein (Cas) systems offered an easy and efficient way to edit the genome and accelerated the research on their potential use in the treatment of genetic disorders. In this review, we summarize the clinical trials that evaluate the CRISPR/Cas systems for treating different genetic diseases and highlight promising preclinical research on CRISPR/Cas mediated treatment of a great diversity of genetic disorders. Ultimately, we discuss the future of CRISPR/Cas mediated genome editing in genetic diseases.

Inherited Ataxias in Children

The purpose of this review is to describe the current diagnostic approach to inherited ataxias during childhood. With the expanding use and availability of gene testing technologies including large sequencing panels, the ability to arrive at a precise genetic diagnosis in this group of disorders has been improving. We have reviewed all the gene sequencing studies of ataxias available by a comprehensive literature search and summarize their results. We provide a logical algorithm for a diagnostic approach in the context of this evolving information. We stress the fact that both autosomal recessive and autosomal dominant mutations can occur in children with ataxias and the need for keeping in mind nucleotide repeat expansions, which cannot be detected by sequencing technologies, as a possible cause of progressive ataxias in children. We discuss the traditional phenotype-based diagnostic approach in the context of gene testing technologies. Finally, we summarize those disorders in which a specific therapy may be indicated.

Hemizygous deletion in the OTC gene results in ornithine transcarbamylase deficiency: A case report

BACKGROUND

Ornithine transcarbamylase deficiency (OTCD) is a common ornithine cycle disorder, and OTC gene variation is the main pathogenic factor of this disease. This study explored and validated a variant in the OTC gene.

CASE SUMMARY

The neonate exhibited high blood ammonia, lactic acid, and homocysteine levels on the fifth day after birth. A novel deletion variant in the OTC gene [NM_000531.5, c.970_979delTTCCCAGAGG, p.Phe324GlnfsTer16] was uncovered by exome sequencing. The variant caused a protein-coding frameshift and resulted in early translation termination at the 16^th amino acid after the variant site.

CONCLUSION

Our results provide a novel pathogenic variant in OTC and related clinical features for further OTCD screening and clinical consultation.

Prenatal Diagnosis of Two Common Inborn Errors of Metabolism by Genetic and Mass Spectrometric Analysis of Amniotic Fluid

Methylmalonic acidaemia (MMA) and ornithine transcarbamylase deficiency (OTCD) are both intoxication-type inborn errors of metabolism (IEM). Presently, genetic testing is the primary method for prenatally diagnosing these diseases. However, some reports have demonstrated that mass spectrometry approaches can prenatally diagnose some forms of inborn errors of metabolism using amniotic fluid. Therefore, in this study, genetic and mass spectrometry approaches were used for prenatally diagnosing MMA and OTCD. We collected amniotic fluid samples from 19 foetuses referred, 15 cases were referred for MMA and 4 for OTCD. Of the 15 MMA cases, seven were affected, as determined by genetic testing and the metabolite levels; the characteristic metabolites propionylcarnitine (C3), C3/acetylcarnitine (C2) ratio, methylmalonic acid and methylcitrate levels were significantly higher than the reference range. Eight foetuses were unaffected, and the C3, C3/C2 ratio, methylmalonic acid and methylcitrate levels were within the reference range. The C3, C3/C2, methylmalonic acid, and methylcitrate levels in the amniotic fluid significantly differed between the affected and unaffected foetuses (P = 0.0014, P = 0.0014, P = 0.0003, P = 0.0014, respectively). Moreover, the homocysteine level increased in the amniotic fluid of affected foetuses with MMACHC gene mutations. Of the four OTCD cases, genetic testing confirmed that two foetuses were affected and two were unaffected. However, the characteristic metabolite levels were within the reference range for all foetuses, including citrulline, orotic acid, and uracil. The genetic testing results were confirmed to be correct through the abortion tissue of the foetus and the postnatal follow-up. Our results suggest that mass spectrometry approaches are convenient method for improving the prenatal diagnosis of MMA. The characteristic metabolites C3, C3/C2, methylmalonic acid, and methylcitrate levels in amniotic fluid were reliable biochemical markers for the prenatal diagnosis of MMA.

Long-term correction of ornithine transcarbamylase deficiency in Spf-Ash mice with a translationally optimized AAV vector

Ornithine transcarbamylase deficiency (OTCD) is an X-linked liver disorder caused by partial or total loss of OTC enzyme activity. It is characterized by elevated plasma ammonia, leading to neurological impairments, coma, and death in the most severe cases. OTCD is managed by combining dietary restrictions, essential amino acids, and ammonia scavengers. However, to date, liver transplantation provides the best therapeutic outcome. AAV-mediated gene-replacement therapy represents a promising curative strategy. Here, we generated an AAV2/8 vector expressing a codon-optimized human OTC cDNA by the α1-AAT liver-specific promoter. Unlike standard codon-optimization approaches, we performed multiple codon-optimization rounds via common algorithms and ortholog sequence analysis that significantly improved mRNA translatability and therapeutic efficacy. AAV8-hOTC-CO (codon optimized) vector injection into adult OTC^Spf-Ash mice (5.0E11 vg/kg) mediated long-term complete correction of the phenotype. Adeno-Associated viral (AAV) vector treatment restored the physiological ammonia detoxification liver function, as indicated by urinary orotic acid normalization and by conferring full protection against an ammonia challenge. Removal of liver-specific transcription factor binding sites from the AAV backbone did not affect gene expression levels, with a potential improvement in safety. These results demonstrate that AAV8-hOTC-CO gene transfer is safe and results in sustained correction of OTCD in mice, supporting the translation of this approach to the clinic.

Progress and challenges in CRISPR-mediated therapeutic genome editing for monogenic diseases

There are an estimated 10 000 monogenic diseases affecting tens of millions of individuals worldwide. The application of CRISPR/Cas genome editing tools to treat monogenic diseases is an emerging strategy with the potential to generate personalized treatment approaches for these patients. CRISPR/Cas-based systems are programmable and sequence-specific genome editing tools with the capacity to generate base pair resolution manipulations to DNA or RNA. The complexity of genomic insults resulting in heritable disease requires patient-specific genome editing strategies with consideration of DNA repair pathways, and CRISPR/Cas systems of different types, species, and those with additional enzymatic capacity and/or delivery methods. In this review we aim to discuss broad and multifaceted therapeutic applications of CRISPR/Cas gene editing systems including in harnessing of homology directed repair, non-homologous end joining, microhomology-mediated end joining, and base editing to permanently correct diverse monogenic diseases.

Induction of Hepatic Metabolic Functions by a Novel Variant of Hepatocyte Nuclear Factor 4γ

Hepatocyte nuclear factor 4α (HNF4α) is a critical factor for hepatocyte differentiation. HNF4α expression is decreased in hepatocellular carcinoma (HCC), which suggests a role in repression of hepatocyte dedifferentiation. In the present study, hepatic expression of HNF4γ was increased in liver-specific Hnf4a-null mice. The HNF4γ whose expression was increased contained two variants, a known short variant, designated HNF4γ1, and a novel long variant, designated HNF4γ2. HNF4G2 mRNA was highly expressed in small intestine, and the transactivation potential of HNF4γ2 was the strongest among these variants, but the potential of HNF4γ1 was the lowest. Cotransfection experiments revealed that HNF4γ1 repressed HNF4α- and HNF4γ2-dependent transactivation, while HNF4γ2 promoted HNF4α-dependent transactivation. HNF4γ1 and HNF4γ2 were able to bind to the HNF4α binding sites with an affinity similar to that of HNF4α. Furthermore, HNF4γ2, but not HNF4γ1, robustly induced the expression of typical HNF4α target genes to a greater degree than HNF4α. Additionally, HNF4γ2 suppressed proliferation of hepatoma cells as well as HNF4α and HNF4γ1 did, and HNF4γ2 induced critical hepatic functions, such as glucose and urea production, and cytochrome P450 1A2 activity more strongly than HNF4α and HNF4γ1 did. These results indicate that HNF4γ2 has potential for redifferentiation of HCC and thus may be explored as a target for HCC therapy.

The phenotypic and mutational spectrum of Thai female patients with ornithine transcarbamylase deficiency

Ornithine transcarbamylase deficiency (OTCD) is an X-linked urea cycle disorder affecting both males and females. Hemizygous males commonly present with severe hyperammonemic encephalopathy during the neonatal period. Heterozygous females have great phenotypic variability. The majority of female patients can manifest later in life or have unrecognized symptoms, making the diagnosis of OTCD in females very challenging. Here we report on three unrelated Thai female cases with OTCD presenting with different manifestations including aggressive behavior, acute liver failure and severe encephalopathy. Whole exome sequencing successfully identified disease-causing mutations in all three cases including two novel ones: the c.209_210delAA (p.Lys70Argfs*17) and the c.850T>A (p.Tyr284Asn). This study affirms variable symptoms in female patients with OTCD and emphasizes the importance of early recognition and prompt management for favorable outcomes. In addition, identification of two novel causative variants expands the genotypic spectrum of OTC.

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.

Hiding in Plain Sight: A Case of Ornithine Transcarbamylase Deficiency Unmasked Post-Liver Transplantation

Ornithine transcarbamylase deficiency represents the most common inherited defect of the urea cycle. This enzyme, predominantly found in the liver, plays a crucial role in recycling free ammonia, with deficiencies often leading to fatal complications. Here, we present the case of a 63-year-old man with alcoholic cirrhosis who underwent orthotopic liver transplantation, gradual worsening of his mental status, and progressive elevation of ammonia levels. Liver allograft function was deemed normal, raising concern for a donor-derived metabolic disorder of the urea cycle. Evaluation of the donor patient's blood revealed that the donor was heterozygous for the OTC gene. Posttransplantation changes in mental status should prompt a clinician to consider the most likely causes; however, once these have been ruled out, it is important to consider the less common causes of metabolic derangements. The rarity of these disorders makes expertise of diagnosis, standardization of evaluation, and treatment strategies challenging.

Involvement of Ornithine Carbamoyltransferase in the Progression of Chronic Hepatitis C and Liver Cirrhosis

Background: The involvement of serum ornithine carbamoyltransferase (OCT) in the progression of chronic hepatitis and liver cirrhosis is unclear. Methods: A total 256 patients with chronic hepatitis C and 5 healthy controls were examined. Serum OCT concentrations were measured by enzyme-linked immunosorbent assay. Serum OCT concentrations were compared with serum cytokine and chemokine levels, and with disease severity and development of hepatocellular carcinoma (HCC). Results: The median OCT concentrations were 21.8 ng/ml for healthy controls, 36.7 ng/ml for F0 stage disease, 48.7 ng/ml for F1 stage, 77.9 ng/ml for F2 stage, 104.8 ng/ml for F3 stage, and 121.4 ng/ml for F4 stage. OCT concentrations were correlated with aspartate aminotransferase, alanine aminotransferase, γ-glutamyl transpeptidase, platelet counts, indocyanine green retention rate at 15 min, prothrombin times, the molar ratio of branched chain amino acids to tyrosine, and tyrosine. Furthermore, there were significant correlations among OCT concentrations and IP10 and IL18 levels. There were weak correlations between serum OCT concentrations and liver histology. The cumulative incidence of HCC in the high-OCT concentration group (≥75.3 ng/ml) was higher than that in the low-OCT concentration group. Conclusion: The measurement of serum OCT concentration may provide a useful marker of disease severity, and thus could be a useful marker for a high risk of HCC occurrence.

Clinical aspects of urea cycle dysfunction and altered brain energy metabolism on modulation of glutamate receptors and transporters in acute and chronic hyperammonemia

In living organisms, nitrogen arise primarily as ammonia (NH3) and ammonium (NH4(+)), which is a main component of the nucleic acid pool and proteins. Although nitrogen is essential for growth and maintenance in animals, but when the nitrogenous compounds exceeds the normal range which can quickly lead to toxicity and death. Urea cycle is the common pathway for the disposal of excess nitrogen through urea biosynthesis. Hyperammonemia is a consistent finding in many neurological disorders including congenital urea cycle disorders, reye's syndrome and acute liver failure leads to deleterious effects. Hyperammonemia and liver failure results in glutamatergic neurotransmission which contributes to the alteration in the function of the glutamate-nitric oxide-cGMP pathway, modulates the important cerebral process. Even though ammonia is essential for normal functioning of the central nervous system (CNS), in particular high concentrations of ammonia exposure to the brain leads to the alterations of glutamate transport by the transporters. Several glutamate transporters have been recognized in the central nervous system and each has a unique physiological property and distribution. The loss of glutamate transporter activity in brain during acute liver failure and hyperammonemia is allied with increased extracellular brain glutamate concentrations which may be conscientious for the cerebral edema and ultimately cell death.

Pride and protein

OBJECTIVE

In Jane Austen's Pride and Prejudice, members of the Bennet family are either sensible or silly, and males are under-represented. This study searches for an underlying medical diagnosis that explains these features.

DESIGN

Very retrospective literature review.

PARTICIPANTS

Mrs Bennet, her five daughters (Jane, Elizabeth, Mary, Kitty and Lydia), her brother (Mr Gardiner) and her sister (Mrs Phillips).

MAIN OUTCOME MEASURES

Family tree and associated phenotypes

METHODS

The author read Pride and Prejudice. A Bennet family tree was constructed. The number of male and female descendants was analysed using a binomial model. For each character, evidence of behaviour was collected, and members of the Bennet family were categorised as either sensible or silly.

RESULTS

Males are under-represented in Mrs Bennet's family. Assuming an equal probability of male or female offspring, this reaches statistical significance (binomial model, P = 0.03). Approximately 50% of females in the family are silly. Silly behaviour is more prevalent during social gatherings.

CONCLUSIONS

The family tree suggests an X-linked genetic disorder, fatal in utero or in early life to affected males, explaining the paucity of male offspring. Female carriers survive, but with cognitive difficulties, explaining the approximate 50-50 distribution of sensible and silly females in the family. The exacerbation of silliness during social gatherings may suggest an effect of protein intake, raising suspicions of a disorder of protein metabolism. Ornithine transcarbamylase deficiency is one such condition. Unfortunately, there remain significant challenges in performing genetic testing on fictional characters, so definitive evidence remains elusive. Jane and Elizabeth Bennet do not show signs of the disorder. However, carriers may be asymptomatic; they should be offered genetic counselling before Bingley or Darcy offspring are considered.

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

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

Nonhepatic hyperammonemic encephalopathy due to undiagnosed urea cycle disorder

Ornithine transcarbamoylase deficiency is the most common inherited urea cycle disorder. In adults, its phenotypes are diverse. In asymptomatic patients with late presentations, symptom onset is often associated with a precipitating factor. We present a case of a woman with urea cycle disorder diagnosed after an acute peptic ulcer bleed and fasting.

High Protein Diet and Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by the huntingtin (HTT) gene with expanded CAG repeats. In addition to the apparent brain abnormalities, impairments also occur in peripheral tissues. We previously reported that mutant Huntingtin (mHTT) exists in the liver and causes urea cycle deficiency. A low protein diet (17%) restores urea cycle activity and ameliorates symptoms in HD model mice. It remains unknown whether the dietary protein content should be monitored closely in HD patients because the normal protein consumption is lower in humans (~15% of total calories) than in mice (~22%). We assessed whether dietary protein content affects the urea cycle in HD patients. Thirty HD patients were hospitalized and received a standard protein diet (13.7% protein) for 5 days, followed by a high protein diet (HPD, 26.3% protein) for another 5 days. Urea cycle deficiency was monitored by the blood levels of citrulline and ammonia. HD progression was determined by the Unified Huntington's Disease Rating Scale (UHDRS). The HPD increased blood citrulline concentration from 15.19 μmol/l to 16.30 μmol/l (p = 0.0378) in HD patients but did not change blood ammonia concentration. A 2-year pilot study of 14 HD patients found no significant correlation between blood citrulline concentration and HD progression. Our results indicated a short period of the HPD did not markedly compromise urea cycle function. Blood citrulline concentration is not a reliable biomarker of HD progression.

Long-term outcomes in Ornithine Transcarbamylase deficiency: a series of 90 patients

Background

The principal aim of this study was to investigate the long-term outcomes of a large cohort of patients with ornithine transcarbamylase deficiency (OTCD) who were followed up at a single medical center.

Methods

We analyzed clinical, biochemical and genetic parameters of 90 patients (84 families, 48 males and 42 females) with OTCD between 1971 and 2011.

Results

Twenty-seven patients (22 boys, 5 girls) had a neonatal presentation; 52 patients had an “intermediate” late-onset form of the disease (21 boys, 31 girls) that was revealed between 1 month and 16 years; and 11 patients (5 boys, 6 girls) presented in adulthood (16 to 55 years). Patients with a neonatal presentation had increased mortality (90% versus 13% in late-onset forms) and peak plasma ammonium (mean value: 960 μmol/L versus 500 μmol/L) and glutamine (mean value: 4110 μmol/L versus 1000 μmol/L) levels at diagnosis. All of the neonatal forms displayed a greater number of acute decompensations (mean value: 6.2/patient versus 2.5 and 1.4 in infants and adults, respectively). In the adult group, some patients even recently died at the time of presentation during their first episode of coma. Molecular analyses identified a deleterious mutation in 59/68 patients investigated. Single base substitutions were detected more frequently than deletions (69% and 12%, respectively), with a recurrent mutation identified in the late-onset groups (pArg40 His; 13% in infants, 57% in adults); inherited mutations represented half of the cases. The neurological score did not differ significantly between the patients who were alive in the neonatal or late-onset groups and did not correlate with the peak ammonia and plasma glutamine concentrations at diagnosis. However, in late-onset forms of the disease, ammonia levels adjusted according to the glutamine/citrulline ratio at diagnosis were borderline predictors of low IQ (p = 0.12 by logistic regression; area under the receiver operating characteristic curve of 76%, p <0.05).

Conclusions

OTCD remains a severe disease, even in adult-onset patients for whom the prevention of metabolic decompensations is crucial. The combination of biochemical markers warrants further investigations to provide additional prognostic information regarding the neurological outcomes of patients with OTCD.

Posterior fossa syndrome in a patient with an ornithine transcarbamylase deficiency

The posterior fossa syndrome (PFS) is a well-known clinical entity and mainly occurs in children. Ornithine transcarbamylase deficiency (OTC) is the most common urea cycle disorder, which occurs in an estimated 1 per 50.000 live births in Japan. Symptoms are mostly due to hyperammonemia and include nausea, vomiting, lethargia and even convulsions and coma. Common neurological symptoms at presentation of a hyperammonemia are a decreased level of consciousness, abnormal motor function or seizures. In this case we describe a girl with late onset OCT deficiency presenting with transient mutism and subsequent dysarthria, ataxia and behavioural changes. This is an exceptional report of a not yet described neurologic syndrome in OTC.

SYNOPSIS

Neurologic symptoms in ornithine transcarbamylase deficiency do not only occur during an episode of hyperammonemia and may present as a transient neurologic symptoms compatible with the posterior fossa syndrome.

Antepartum ornithine transcarbamylase deficiency

Ornithine transcarbamylase deficiency (OTCD) is the most common type urea cycle enzyme deficiencies. This syndrome results from a deficiency of the mitochondrial enzyme ornithine transcarbamylase, which catalyzes the conversion of ornithine and carbamoyl phosphate to citrullin. Our case was a 28-year-old female diagnosed with OTCD following neurocognitive deficit during her first pregnancy. Although hyperammonemia was suspected as the cause of the patient's mental changes, there was no evidence of chronic liver disease. Plasma amino acid and urine organic acid analysis revealed OTCD. After combined modality treatment with arginine, sodium benzoate and hemodialysis, the patient's plasma ammonia level stabilized and her mental status returned to normal. At last she recovered without any damage left.

OTC gene in ornithine transcarbamylase deficiency: clinical course and mutational spectrum in seven Korean patients

BACKGROUND

Ornithine transcarbamylase deficiency, an inborn error of metabolism, is the most common urea cycle disorder and is caused by mutations in the OTC gene located on Xp21. In this study, the clinical and genetic characteristics of seven Korean patients with ornithine transcarbamylase deficiency were analyzed.

METHODS

During 2009-2012, a total of seven patients (three male and four female patients) from six unrelated families were diagnosed with ornithine transcarbamylase deficiency by biochemical or molecular analysis. OTC gene sequencing analysis was performed in six of these patients. Clinical manifestations, clinical courses, and the results of genetic studies were reviewed retrospectively.

RESULTS

The median follow-up period for the seven patients with ornithine transcarbamylase deficiency was 44 months (11.9-150 months). Clinical manifestations of ornithine transcarbamylase deficiency included vomiting and seizure, which were the most frequent signs at admission. Two of the four heterozygous female patients (50%) experienced severe neurological sequelae. The early onset male patient characterized severe neurological deficits. The late-onset male patient recovered completely from acute encephalopathy and coma without any neurological deficits. Direct sequencing and multiplex ligation-dependent probe amplification analysis of OTC gene revealed five different mutations. Of these mutations, two were novel (c.867-3T>C and c.664_667delinsAC).

CONCLUSION

Ornithine transcarbamylase deficiency was genetically heterogeneous in the seven Korean patients with confirmed ornithine transcarbamylase deficiency diagnosis by biochemical findings and/or genetic analysis, together with two novel mutations in the OTC gene. We hope that these data will contribute to a better understanding of the clinical course and distinct molecular genetic characteristics of Korean patients with ornithine transcarbamylase deficiency.

Late-onset ornithine transcarbamylase deficiency: An under recognized cause of metabolic encephalopathy

INTRODUCTION

Ornithine transcarbamylase deficiency is the most common inherited disorder of the urea cycle, has a variable phenotype, and is caused by mutations in the OTC gene. We report three cases of ornithine transcarbamylase deficiency to illustrate the late-onset presentation of this disorder and provide strategies for diagnosis and treatment. The patients were maternal first cousins, presenting with hyperammonemia and obtundation. Urea cycle disorder was not initially suspected in the first patient, delaying diagnosis.

RESULTS

Sequencing of the OTC gene showed a novel missense mutation, c.563G > C (p.G188A). Numerous family members were found to carry this mutation, which shows a trend toward later onset. Each urea cycle disorder has its own unique pattern of biochemical abnormalities, which differ from non-metabolic causes of critical illness.

CONCLUSION

Regardless of age, clinical suspicion of a urea cycle disorder is important in encephalopathic patients to ensure quick diagnosis and definitive treatment of the underlying inborn error of metabolism.

The importance of the ionic product for water to understand the physiology of the acid-base balance in humans

Human plasma is an aqueous solution that has to abide by chemical rules such as the principle of electrical neutrality and the constancy of the ionic product for water. These rules define the acid-base balance in the human body. According to the electroneutrality principle, plasma has to be electrically neutral and the sum of its cations equals the sum of its anions. In addition, the ionic product for water has to be constant. Therefore, the plasma concentration of hydrogen ions depends on the plasma ionic composition. Variations in the concentration of plasma ions that alter the relative proportion of anions and cations predictably lead to a change in the plasma concentration of hydrogen ions by driving adaptive adjustments in water ionization that allow plasma electroneutrality while maintaining constant the ionic product for water. The accumulation of plasma anions out of proportion of cations induces an electrical imbalance compensated by a fall of hydroxide ions that brings about a rise in hydrogen ions (acidosis). By contrast, the deficiency of chloride relative to sodium generates plasma alkalosis by increasing hydroxide ions. The adjustment of plasma bicarbonate concentration to these changes is an important compensatory mechanism that protects plasma pH from severe deviations.

The long-term safety of antiepileptic drugs

Antiepileptic drugs (AEDs) are used by millions of people worldwide for the treatment of epilepsy, as well as in many other neurological and psychiatric conditions. They are frequently associated with adverse effects (AEs), which have an impact on the tolerability and success of treatment. Half the people who develop intolerable AEs discontinue treatment early on after initiation, while the majority of people will continue to be exposed to their effects for long periods of time. The long-term safety of AEDs reflects their potential for chronic, cumulative dose effects; rare, but potentially serious late idiosyncratic effects; late, dose-related effects; and delayed, teratogenic or neurodevelopmental effects. These AEs can affect every body system and are usually insidious. With the exception of delayed effects, most other late or chronic AEs are reversible. To date, there is no clear evidence of a carcinogenic effect of AEDs in humans. While physicians are aware of the long-term AEs of old AEDs (the traditional liver enzyme-inducing AEDs and valproate), information about AEs of new AEDs (such as lamotrigine, levetiracetam, oxcarbazepine, topiramate or zonisamide), particularly of their teratogenic effects, has emerged over the years. Sporadic publications have raised issues about AEs of the newer AEDs eslicarbazepine, retigabine, rufinamide, lacosamide and perampanel but their long-term safety profiles may take years to be fully appreciated. Physicians should not only be aware of the late and chronic AEs of AEDs but should systematically enquire and screen for these according to the individual AED AE profile. Care should be taken for individuals with comorbid conditions that may render them more susceptible to specific AEs. Prevention and appropriate management of long-term AED AEs is expected to improve adherence to treatment, quality of life and control of epilepsy.

Recurrent somnolence in a 17-month-old infant: late-onset ornithine transcarbamylase (OTC) deficiency due to the novel hemizygous mutation c.535C > T (p.Leu179Phe)

Herein, we describe a case of a now 28-month-old boy who presented at the age of 17 months with four episodes of recurrent vomiting and somnolence during a period of four months with increasing severity. A comprehensive clinical and metabolic evaluation revealed normal blood pH and blood glucose, normal cerebral computed tomography and electroencephalogram but an elevated plasma ammonia concentration, which raised the suspicion of a urea cycle disorder. The combination of elevated urinary orotic acid and plasma glutamine with normal citrulline suggested the diagnosis of ornithine transcarbamylase (OTC) deficiency, which was confirmed by molecular genetic testing revealing the novel hemizygous mutation c.535C > T (p.Leu179Phe) of the OTC gene. After restitution of anabolism by administration of parenteral glucose, substitution of citrulline and detoxification of ammonia with sodium benzoate, the patient recovered rapidly and is in a stable metabolic and neurological state since then. This case underlines that the diagnosis of a urea cycle defect should be considered in the differential diagnosis of recurrent idiopathic vomiting in combination with unexplained neurological symptoms also beyond the neonatal period due to the possibility of mild or atypical late-onset presentation (e.g. OTC deficiency in hemizygous males).

Ammonium metabolism in humans

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

Simultaneous detection of diagnostic biomarkers of alkaptonuria, ornithine carbamoyltransferase deficiency, and neuroblastoma disease by high-performance liquid chromatography/tandem mass spectrometry

BACKGROUND

Urinary homovanillic acid (HVA)/vanillylmandelic acid (VMA), orotic acid (OA), and homogentisic acid (HGA) are diagnostic biomarkers of neuroblastoma, ornithine carbamoyl transferase deficiency (OCTD), and alkaptonuria (AKU), respectively. In this study, a high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for simultaneous quantification of HVA, VMA, OA, and HGA in urine.

METHODS

After sample preparation, which involved only the dilution procedure, samples were quantified by LC-MS/MS. Full-scan MS/MS mode enabled the urinary markers to be quantified with a high degree of specificity and sensitivity. Rather than using a separate enzymatic method to normalize the concentration of creatinine in urine, we quantified the level of creatinine in urine in one LC-MS run.

RESULTS

The limits of detection were 10 μg/l for HGA, 25 μg/l for HVA/VMA, and 50 μg/l for OA with a single-to-noise ratio of 3; the limits of quantification were 50 μg/l for HVA and HGA, 100 μg/l for VMA, and 250 μg/l for OA. The linear dynamic range for quantification of the analytes covered 2 to 3 orders of magnitude, depending on the analyte. The relative standard deviation of the developed LC-MS/MS method was less than 4% for the intra-day validation and 10% for the inter-day validation.

CONCLUSIONS

The results show that our LC-MS/MS technique is a highly sensitive and rapid method for screening for biomarkers that are diagnostic of three metabolic diseases.

Hyperammonemia in a patient with late-onset ornithine carbamoyltransferase deficiency

Ornithine carbamoyltransferase (OTC) deficiency is a urea cycle disorder that causes the accumulation of ammonia, which can lead to encephalopathy. Adults presenting with hyperammonemia who are subsequently diagnosed with urea cycle disorders are rare. Herein, we report a case of a late-onset OTC deficient patient who was successfully treated with arginine, benzoate and hemodialysis. A 59-yr-old man was admitted to our hospital with progressive lethargy and confusion. Although hyperammonemia was suspected as the cause of the patient's mental changes, there was no evidence of chronic liver disease. A plasma amino acid and urine organic acid analysis revealed OTC deficiency. Despite the administration of a lactulose enema, the patient's serum ammonia level increased and he remained confused, leading us to initiate acute hemodialysis. After treatment with arginine, sodium benzoate and hemodialysis, the patient's serum ammonia level stabilized and his mental status returned to normal.

Clinical and experimental applications of sodium phenylbutyrate

Histone acetyltransferase and histone deacetylase are enzymes responsible for histone acetylation and deacetylation, respectively, in which the histones are acetylated and deacetylated on lysine residues in the N-terminal tail and on the surface of the nucleosome core. These processes are considered the most important epigenetic mechanisms for remodeling the chromatin structure and controlling the gene expression. Histone acetylation is associated with gene activation. Sodium phenylbutyrate is a histone deacetylase inhibitor that has been approved for treatement of urea cycle disorders and is under investigation in cancer, hemoglobinopathies, motor neuron diseases, and cystic fibrosis clinical trials. Due to its characteristics, not only of histone deacetylase inhibitor, but also of ammonia sink and chemical chaperone, the interest towards this molecule is growing worldwide. This review aims to update the current literature, involving the use of sodium phenylbutyrate in experimental studies and clinical trials.

Peripartum management of two parturients with ornithine transcarbamylase deficiency

Ornithine transcarbamylase deficiency is a rare X-linked disorder in which female carriers are usually heterozygous for the ornithine transcarbamylase deficiency gene. In pregnancy it has been associated with altered mental status, seizures, coma and death, especially in the postpartum period. We report the management of labor and delivery in two parturients with known ornithine transcarbamylase deficiency. Both patients were maintained on arginine, citrulline and sodium phenylacetate therapy with restricted protein intake during pregnancy. Neuraxial techniques were used for pain relief in labor and anesthesia for operative delivery. A dextrose infusion provided caloric intake during labor and perioperatively.

MR imaging workup of inborn errors of metabolism of early postnatal onset

Immediate or early postnatal onset forms of neurometabolic disorders represent a clinically important subgroup because these often present as a life-threatening episode of metabolic decompensation shortly after birth. This article focuses on this group of diseases, often referred to as "devastating neurometabolic diseases" of the newborn. Awareness of the most common entities and their clinical, biochemical, and diagnostic imaging manifestations is important because if undiagnosed and untreated, the diseases may have catastrophic consequences. Although formal diagnosis relies on laboratory tests, diagnostic imaging is often pivotal in both reaching the correct diagnosis and/or orienting further targeted investigative efforts.

Hyperammonemic coma in an ornithine transcarbamylase mutation carrier following antepartum corticosteroids

Women who are carriers of the ornithine transcarbamylase (OTC) mutation are at risk for developing hyperammonemia during the postpartum period and at times of metabolic stress. We present a unique case of hyperammonemic coma occurring in an OTC mutation carrier during the antepartum period. Multiple factors, including the administration of antenatal corticosteroids, likely precipitated this critical condition. Clinicians should be aware of this life-threatening clinical presentation and be prepared to identify, treat, and prevent hyperammonemia in affected individuals.

Sirt3 promotes the urea cycle and fatty acid oxidation during dietary restriction

Emerging evidence suggests that protein acetylation is a broad-ranging regulatory mechanism. Here we utilize acetyl-peptide arrays and metabolomic analyses to identify substrates of mitochondrial deacetylase Sirt3. We identified ornithine transcarbamoylase (OTC) from the urea cycle, and enzymes involved in β-oxidation. Metabolomic analyses of fasted mice lacking Sirt3 (sirt3(-/-)) revealed alterations in β-oxidation and the urea cycle. Biochemical analysis demonstrated that Sirt3 directly deacetylates OTC and stimulates its activity. Mice under caloric restriction (CR) increased Sirt3 protein levels, leading to deacetylation and stimulation of OTC activity. In contrast, sirt3(-/-) mice failed to deacetylate OTC in response to CR. Inability to stimulate OTC under CR led to a failure to reduce orotic acid levels, a known outcome of OTC deficiency. Thus, Sirt3 directly regulates OTC activity and promotes the urea cycle during CR, and the results suggest that under low energy input, Sirt3 modulates mitochondria by promoting amino acid catabolism and β-oxidation.

Transient hyperammonemia due to L-asparaginase therapy in children with acute lymphoblastic leukemia or non-Hodgkin lymphoma

The standard treatment protocol for acute lymphoblastic leukemia (ALL) and non-Hodgkin lymphoma (NHL) in childhood includes intravenous therapy with asparaginase (Asp), which may cause hyperammonemia. In this study, all patients receiving asparaginase therapy at the Hospital for Children and Adolescents of the University of Leipzig between January 2002 and December 2007 were reviewed for the occurrence of hyperammonemia. Fifty-four patients were identified (22 girls, 32 boys; mean age 5.8 years). Blood ammonia concentrations were determined in 4 patients due to suspicious clinical signs. All showed hyperammonemia with NH(3) concentrations between 260 and 700 μmol/L. They received specific acute detoxification therapy consisting in protein restriction, administration of benzoic acid, glucose/insulin. All 4 recovered completely. All patients receiving therapeutic regimes that include asparaginase (Asp) should be monitored for the development of transient hyperammonemia.

Assessment of the perimortem protocol in neonates for the diagnosis of inborn errors of metabolism

AIM

To assess the efficacy of the perimortem protocol in neonates with suspected inborn errors of metabolism (IEM).

METHODS

Retrospective analysis of medical records from January 2000 through December 2007 was performed. Only neonates (</=1 month of life) in whom the perimorterm protocol was applied were included in the study. The samples were collected following the instructions of our exitus kit, which contains the perimortem protocol and the material for the extraction of biological specimens.

RESULTS

Among the 42 neonates studied, in 28 an IEM was suspected during hospitalization and 15 (36%) were diagnosed with IEM. Mitochondrial disorders were the most frequent diagnosis (8 patients), followed by urea cycle disorders (3 patients), organic acidemias (2 patients), one patient with congenital disorder of glycosylation (CDG type Ia), and one patient with molybdenum cofactor deficiency. Sepsis and other life-threatening conditions appeared to have a biochemical profile very similar to IEM.

CONCLUSION

This protocol was especially useful for collecting all biological samples in patients with rapidly fatal evolution with a non-specific diagnostic suspicion, and to collect special tissues in previously diagnosed patients. However, only the combination of clinical and biochemical data could lead to a diagnosis which would be confirmed by enzymatic/genetic studies.

Contiguous gene deletion syndrome in a female with ornithine transcarbamylase deficiency

OTC deficiency, a partially dominant X-linked trait, is the most frequent inborn error of the urea cycle. We describe a female patient with a contiguous gene deletion syndrome encompassing the OTC, DMD, RPGR, CYBB and XK genes, amongst others, only manifesting features of OTC deficiency. Molecular characterization was ascertained by MLPA and confirmed by CGH microarray, which revealed an 8.7 Mb deletion of the X-chromosome. Complete de novo deletion of the OTC gene led to a severe clinical phenotype in the proband. The application of high resolution molecular genetic techniques such as MLPA and array CGH, in mutation negative OTC cases allows the identification of chromosomal rearrangements, such as large deletions and provides information for accurate genetic counseling and prenatal diagnosis.

[Ornithine transcarbamylase deficiency in adolescence and adulthood: first manifestation with life-threatening decompensation]

BACKGROUND

Ornithine transcarbamylase (OTC) deficiency is the most frequent innate disorder of the urea cycle and is X-chromosome linked. The disease normally manifests itself shortly after birth and is fatal when untreated. Due to the different expression and X-chromosomal inheritance the manifestation of symptoms can appear later particularly in girls and young women. The first symptoms are non-specific signs of elevated cerebral pressure as a result of a hyperammonemia, which range from nausea and headache up to cerebral herniation with fatal outcome. Measurement of plasma ammonia levels is a simple yet important screening test for patients with unexpected stupor or delirium.

CASE REPORTS

The two case reports show the clinical range from acute decompensation with acute cerebral herniation followed by fatal outcome to recovery under emergency therapy without substantial neurological deficits.

THERAPY

Emergency treatment consists of symptomatic securing of vital parameters and an immediate reduction in the ammonia level using high calorie, protein-free nutrition to avoid catabolism together with administration of arginine, benzoate or phenyl butyrate. In cases of coma with severe cerebral edema and the threat of a herniation reaction or excessive ammonia levels, emergency hemodialysis must be immediately carried out.

CONCLUSIONS

In the clinical routine it is extremely important to consider a metabolic defect at an early phase and among others to determine the ammonia level so that the appropriate treatment can be instigated in time.

Inherited metabolic disorders and cerebral infarction

The association of genetic factors and cerebral infarction (CI) has long been established. A positive family history alone is a recognized risk factor for CI and vascular events in general. However, there are certain inherited conditions that further increase the risk of stroke. These conditions are generally metabolic and mitochondrial genetic defects that have variable modes of inheritance. This article reviews major inherited metabolic disorders that predispose an individual to CI. Ten main conditions will be discussed: Fabry's disease, cerebrotendinous xanthomatosis, tangier disease, familial hypercholesterolemia, homocystinuria, methylmalonic acidemia, glutaric aciduria type I, propionic acidemia, ornithine transcarbamylase deficiency and mitochondrial encephalopathy, lactic acidosis and stroke-like phenomenon.

Acute fatal presentation of ornithine transcarbamylase deficiency in a previously healthy male

Ornithine transcarbamylase (OTC) deficiency is an X-linked urea cycle defect. While hemizygous males typically present with hyperammonemic coma in infancy, reports of rare late-onset presentations exist, with poor outcomes in males up to 58 years old. Relatives with mutations identical to affected patients often remain asymptomatic, and it is likely that environmental and genetic factors influence disease penetrance and expression. Here, we present our investigation of a patient with late-onset presentation, and we emphasize the potential role of environmental and genetic factors on disease expression. The patient was a previously healthy 62-year-old man who developed mental slowing, refractory seizures, and coma over an 8-day period. Interestingly, the patient had recently used home gardening fertilizers and pesticides. Evaluations for drug and alcohol use, infections, and liver disease were negative. Despite aggressive therapy, blood NH₃ concentration peaked at 2,050 μM and the patient died from cerebral edema and cerebellar herniation. Analysis of the OTC gene showed a Pro-225-Thr (P225T) change in exon 7, a mutation that has been previously implicated in OTC deficiency. This case illustrates that OTC deficiency can cause acute, severe hyperammonemia in a previously healthy adult and that the P225T mutation can be associated with late-onset OTC deficiency. We speculate that exposure to organic chemicals might have contributed to the onset of symptoms in this patient. This case also emphasizes that persistent hyperammonemia may cause irreversible neurologic damage and that after the diagnosis of hyperammonemia is established in an acutely ill patient, certain diagnostic tests should be performed to differentiate between urea cycle disorders and other causes of hyperammonemic encephalopathy.