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

The SLC6A15-SLC6A20 Neutral Amino Acid Transporter Subfamily: Functions, Diseases, and Their Therapeutic Relevance

The neutral amino acid transporter subfamily that consists of six members, consecutively SLC6A15-SLC620, also called orphan transporters, represents membrane, sodium-dependent symporter proteins that belong to the family of solute carrier 6 (SLC6). Primarily, they mediate the transport of neutral amino acids from the extracellular milieu toward cell or storage vesicles utilizing an electric membrane potential as the driving force. Orphan transporters are widely distributed throughout the body, covering many systems; for instance, the central nervous, renal, or intestinal system, supplying cells into molecules used in biochemical, signaling, and building pathways afterward. They are responsible for intestinal absorption and renal reabsorption of amino acids. In the central nervous system, orphan transporters constitute a significant medium for the provision of neurotransmitter precursors. Diseases related with aforementioned transporters highlight their significance; SLC6A19 mutations are associated with metabolic Hartnup disorder, whereas altered expression of SLC6A15 has been associated with a depression/stress-related disorders. Mutations of SLC6A18-SLCA20 cause iminoglycinuria and/or hyperglycinuria. SLC6A18-SLC6A20 to reach the cellular membrane require an ancillary unit ACE2 that is a molecular target for the spike protein of the SARS-CoV-2 virus. SLC6A19 has been proposed as a molecular target for the treatment of metabolic disorders resembling gastric surgery bypass. Inhibition of SLC6A15 appears to have a promising outcome in the treatment of psychiatric disorders. SLC6A19 and SLC6A20 have been suggested as potential targets in the treatment of COVID-19. In this review, we gathered recent advances on orphan transporters, their structure, functions, related disorders, and diseases, and in particular their relevance as therapeutic targets. SIGNIFICANCE STATEMENT: The following review systematizes current knowledge about the SLC6A15-SLCA20 neutral amino acid transporter subfamily and their therapeutic relevance in the treatment of different diseases.

Sleep deprivation and aging are metabolically linked across tissues

STUDY OBJECTIVES

Insufficient sleep is a concerning hallmark of modern society because sleep deprivation (SD) is a risk factor for neurodegenerative and cardiometabolic disorders. SD imparts an aging-like effect on learning and memory, although little is known about possible common molecular underpinnings of SD and aging. Here, we examine this question by profiling metabolic features across different tissues after acute SD in young adult and aged mice.

METHODS

Young adult and aged mice were subjected to acute SD for 5 hours. Blood plasma, hippocampus, and liver samples were subjected to UPLC-MS/MS-based metabolic profiling.

RESULTS

SD preferentially impacts peripheral plasma and liver profiles (e.g. ketone body metabolism) whereas the hippocampus is more impacted by aging. We further demonstrate that aged animals exhibit SD-like metabolic features at baseline. Hepatic alterations include parallel changes in nicotinamide metabolism between aging and SD in young animals. Overall, metabolism in young adult animals is more impacted by SD, which in turn induces aging-like features. A set of nine metabolites was classified (79% correct) based on age and sleep status across all four groups.

CONCLUSIONS

Our metabolic observations demonstrate striking parallels to previous observations in studies of learning and memory and define a molecular metabolic signature of sleep loss and aging.

Physiological and biochemical responses of Limonium tetragonum to NaCl concentrations in hydroponic solution

Introduction

Limonium (L.) tetragonum (Thunb.) A. A. Bullock, a halophyte that grows all over the southwest coast of Korea, is a medicinal plant with various pharmacological effects. The salt defense mechanism stimulates the biosynthesis of various secondary metabolites and improves functional substances. In this study, we investigated the optimal NaCl concentration for the growth and enhancement of secondary metabolites in hydroponically grown L. tetragonum.

Methods

The seedlings grown for 3 weeks in a hydroponic cultivation system were treated with 0-, 25-, 50-, 75-, and 100-mM NaCl in Hoagland's nutrient solution for 8 weeks. No significant effect on the growth and chlorophyll fluorescence was observed for the NaCl concentrations below 100-mM.

Results and discussions

The increase in the NaCl concentration resulted in the decrease in the water potential of the L. tetragonum leaves. The Na⁺ content accumulated in the aerial part increased rapidly and the content of K⁺, which acts as an antagonist, decreased with the increase in NaCl concentrations in hydroponics. The total amino acid content of L. tetragonum decreased compared to the 0-mM NaCl, and most of the amino acid content decreased as the NaCl concentration increased. In contrast, the content of urea, proline (Pro), β-alanine, ornithine, and arginine was increased with an increase in NaCl concentration. The Pro content at 100-mM NaCl accounted for 60% of the total amino acids and was found to be a major osmoregulator as an important component of the salt defense mechanisms. The top five compounds identified in the L. tetragonum were classified as flavonoids while the flavanone compound was detected only in the NaCl treatments. A total of four myricetin glycosides were increased in comparison to the 0-mM NaCl. Among the differentially expressed genes, a significantly large change in Gene ontology was seen in the circadian rhythm. NaCl treatment enhanced the flavonoid-based substances of L. tetragonum. The optimum NaCl concentration for the enhancement of secondary metabolites of the L. tetragonum in the vertical farm-hydroponic cultivation system was 75-mM NaCl.

Metabolic differences among newborns born to mothers with a history of leukemia or lymphoma

BACKGROUND

Leukemia and lymphoma are cancers affecting children, adolescents, and young adults and may affect reproductive outcomes and maternal metabolism. We evaluated for metabolic changes in newborns of mothers with a history of these cancers.

METHODS

A cross-sectional study was conducted on California births from 2007 to 2011 with linked maternal hospital discharge records, birth certificate, and newborn screening metabolites. History of leukemia or lymphoma was determined using ICD-9-CM codes from hospital discharge data and newborn metabolite data from the newborn screening program.

RESULTS

A total of 2,068,038 women without cancer history and 906 with history of leukemia or lymphoma were included. After adjusting for differences in maternal age, infant sex, age at metabolite collection, gestational age, and birthweight, among newborns born to women with history of leukemia/lymphoma, several acylcarnitines were significantly (p < .001 - based on Bonferroni correction for multiple testing) higher compared to newborns of mothers without cancer history: C3-DC (mean difference (MD) = 0.006), C5-DC (MD = 0.009), C8:1 (MD = 0.008), C14 (MD = 0.010), and C16:1 (MD = 0.011), whereas citrulline levels were significantly lower (MD = -0.581) among newborns born to mothers with history of leukemia or lymphoma compared to newborns of mothers without a history of cancer.

CONCLUSION

The varied metabolite levels suggest history of leukemia or lymphoma has metabolic impact on newborn offspring, which may have implications for future metabolic consequences such as necrotizing enterocolitis and urea cycle enzyme disorders in children born to mothers with a history of leukemia or lymphoma.

Guidelines for the diagnosis and treatment of acute encephalopathy in childhood

The cardinal symptom of acute encephalopathy is impairment of consciousness of acute onset during the course of an infectious disease, with duration and severity meeting defined criteria. Acute encephalopathy consists of multiple syndromes such as acute necrotizing encephalopathy, acute encephalopathy with biphasic seizures and late reduced diffusion and clinically mild encephalitis/encephalopathy with reversible splenial lesion. Among these syndromes, there are both similarities and differences. In 2016, the Japanese Society of Child Neurology published 'Guidelines for the Diagnosis and Treatment of Acute Encephalopathy in Childhood', which made recommendations and comments on the general aspects of acute encephalopathy in the first half, and on individual syndromes in the latter half. Since the guidelines were written in Japanese, this review article describes extracts from the recommendations and comments in English, in order to introduce the essence of the guidelines to international clinicians and researchers.

Occurrence of Inborn Errors of Metabolism in Newborns, Diagnosis and Prophylaxis

Inborn errors of metabolism (IEM) are a heterogeneous group of rare genetic disorders that are generally transmitted as autosomal or X-linked recessive disorders. These defects arise due to mutations associated with specific gene(s), especially the ones associated with key metabolic enzymes. These enzymes or their product(s) are involved in various metabolic pathways, leading to the accumulation of intermediary metabolite(s), reflecting their toxic effects upon mutations. The diagnosis of these metabolic disorders is based on the biochemical analysis of the clinical manifestations produced and their molecular mechanism. Therefore, it is imperative to devise diagnostic tests with high sensitivity and specificity for early detection of IEM. Recent advances in biochemical and polymerase chain reaction-based genetic analysis along with pedigree and prenatal diagnosis can be life-saving in nature. The latest development in exome sequencing for rapid diagnosis and enzyme replacement therapy would facilitate the successful treatment of these metabolic disorders in the future. However, the longterm clinical implications of these genetic manipulations is still a matter of debate among intellectuals and requires further research.

Irritability, Poor Feeding and Respiratory Alkalosis in Newborns: Think about Metabolic Emergencies. A Brief Summary of Hyperammonemia Management

The urea cycle is a series of metabolic reactions that convert ammonia into urea in order to eliminate it from the body. Urea cycle disorders are characterized by hyperammonemia, which can cause irreversible damages in central nervous system. We report a series of three newborns presenting irritability, poor feeding and tachypnea. Their first gas analysis revealed respiratory alkalosis. Hyperammonemia was confirmed, and three different enzymatic blocks in the urea cycle were diagnosed. Immediate treatment consisted in the removal of ammonia by reduction of the catabolic state, dietary adjustments, use of nitrogen scavenging agents and ultimately hemodiafiltration. Hyperammonemia is a medical emergency whose treatment should not be delayed. This report aims to highlight the importance of suspecting urea cycle disorders in newborns with aspecific signs of hyperammonemia and respiratory alkalosis, and to sum up the broad lines of hyperammonemia management.

Detection of homozygous genotypes for a putatively lethal recessive mutation in the porcine argininosuccinate synthase 1 (ASS1) gene

The sequencing of the pig genome revealed the existence of homozygous individuals for a nonsense mutation in the argininosuccinate synthase 1 (ASS1) gene (rs81212146, c.944T>A, L315X). Paradoxically, an AA homozygous genotype for this polymorphism is expected to abolish the function of the ASS1 enzyme that participates in the urea cycle, leading to citrullinemia, hyperammonemia, coma and death. Sequencing of five Duroc boars that sired a population of 350 Duroc barrows revealed the segregation of the c.944T>A polymorphism, so we aimed to investigate its phenotypic consequences. Genotyping of this mutation in the 350 Duroc barrows revealed the existence of seven individuals homozygous (AA) for the nonsense mutation. These AA pigs had a normal weight despite the fact that mild citrullinemia often involves impaired growth. Sequencing of the region surrounding the mutation in TT, TA and AA individuals revealed that the A substitution in the second position of the codon (c.944T>A) is in complete linkage disequilibrium with a C replacement (c.943T>C) in the first position of the codon. This second mutation would compensate for the potentially damaging effect of the c.944T>A replacement. In fact, this is the most probable reason why pigs with homozygous AA genotypes at the 944 site of the ASS1 coding region are alive. Our results illustrate the complexities of predicting the consequences of nonsense mutations on gene function and phenotypes, not only because of annotation issues but also owing to the existence of genetic mechanisms that sometimes limit the penetrance of highly harmful mutations.

Anesthetic management of a pediatric patient with arginase-1 deficiency undergoing strabismus operation: a case report

Background

Urea cycle disorders are rare; arginase-1 deficiency is one of those extremely rare autosomal recessive metabolic disorders. Arginase-1 is one among the enzymes involved in the production of urea from ammonia in the liver, and its deficiency produces the characteristic feature, hyperargininemia.

Case presentation

We report a case of a girl, aged 5 years and 10 months presenting with arginase-1 deficiency. The patient was scheduled to undergo strabismus surgery for intermittent exotropia under general anesthesia. Preoperative blood tests showed high serum arginine levels, but ammonia levels were within the normal range. Anesthesia was induced with sevoflurane and nitrous oxide via face mask and maintained with sevoflurane, fentanyl, and rocuronium. Vital signs were stable throughout the surgery. There was an intraoperative decrease in blood glucose levels (from 82 mg/dL to 42 mg/dL) that was treated with intravenous glucose. Arginine levels remained high after surgery; however, hyperammonemia did not develop. There were no complications and the patient was discharged on the following day.

Conclusions

We successfully performed general anesthesia in a child with hyperargininemia. Only a few cases of arginase-1 deficiency had been reported and much remains unknown about its pathology. Therefore, information sharing among medical professionals is essential to customize the plan for the management of this disorder in patients.

Influence of glutamine synthetase gene polymorphisms on the development of hyperammonemia during valproic acid-based therapy

PURPOSE

Valproic acid (VPA), which is widely used to treat epilepsy, migraine, and bipolar disorder, can causes severe hyperammonemia. However, the mechanism responsible for this adverse effect is not readily apparent. We previously reported that phenytoin coadministration is a strong risk factor for the development of hyperammonemia during VPA-based therapy. In this study, we focused on glutamine synthetase, which catalyzes the synthesis of glutamine from glutamate and ammonia and examined the association with the development of hyperammonemia during VPA-based therapy.

METHODS

For this study, we recruited 202 Japanese pediatric patients having epilepsy. We selected three polymorphisms (rs10911070, rs10797771, and rs10911021) in the glutamine synthetase (GLUL) gene. Hyperammonemia was defined as a plasma ammonia level exceeding 200 or 170 μg/dL. We evaluated the association between the development of hyperammonemia during VPA-based therapy and the patient characteristics, including three GLUL polymorphisms.

RESULTS

The number of patients who developed hyperammonemia during VPA-based therapy was 20 (9.9%) using the 200 μg/dL cutoff value and 30 (14.9%) using the 170 μg/dL cutoff value. Using a multivariate logistic regression analysis, the GLUL rs10797771 polymorphism and phenytoin coadministration in the 200 μg/dL cutoff value, and female in addition to two factors in the 170 μg/dL cutoff value, had significant associations with a plasma ammonia level elevation during VPA-based therapy.

CONCLUSION

Phenytoin coadministration, GLUL rs10797771 polymorphism in the 200μg/dL cutoff value, and female in addition to two factors in the 170μg/dL cutoff value, are independent risk factors for elevated plasma ammonia levels during VPA-based therapy.

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.

Diagnosis and treatment of urea cycle disorder in Japan

Urea cycle disorder (UCD) is an inborn error of the metabolic pathway producing urea from ammonia, which occurs primarily in the liver. Decreased excretion of nitrogen in the urea cycle due to deficiency of carbamoyl phosphate synthase I (CPSI), ornithine transcarbamylase (OTC), argininosuccinate synthase (ASS), argininosuccinate lyase (ASL), and N-acetyl glutamate synthase (NAGS) causes hyperammonemia. We examined the clinical manifestations, treatment, and prognosis of 177 patients with UCD from January 1999 to March 2009 in Japan. Compared with a previous study conducted in Japan, a larger number of patients survived without mental retardation, even when the peak blood ammonia was >360 μmol/L. In those with peak blood ammonia >360 μmol/L, an indicator of poor prognosis, the frequency of convulsions, mental retardation, brain abnormality on magnetic resonance imaging, hemodialysis, liver transplantation, and intake of non-protein formulas was significantly higher than in those with peak blood ammonia <360 μmol/L. In this article, we have reported the current state of UCD to evaluate prognosis and its relationship with peak blood ammonia and hemodialysis.

Monogenic diseases that can be cured by liver transplantation

While the prevalence of most diseases caused by single-gene mutations is low and defines them as rare conditions, all together, monogenic diseases account for approximately 10 in every 1000 births according to the World Health Organisation. Orthotopic liver transplantation (LT) could offer a therapeutic option in monogenic diseases in two ways: by substituting for an injured liver or by supplying a tissue that can replace a mutant protein. In this respect, LT may be regarded as the correction of a disease at the level of the dysfunctional protein. Monogenic diseases that involve the liver represent a heterogeneous group of disorders. In conditions associated with predominant liver parenchymal damage (i.e., genetic cholestatic disorders, Wilson's disease, hereditary hemochromatosis, tyrosinemia, α1 antitrypsin deficiency), hepatic complications are the major source of morbidity and LT not only replaces a dysfunctional liver but also corrects the genetic defect and effectively cures the disease. A second group includes liver-based genetic disorders characterised by an architecturally near-normal liver (urea cycle disorders, Crigler-Najjar syndrome, familial amyloid polyneuropathy, primary hyperoxaluria type 1, atypical haemolytic uremic syndrome-1). In these defects, extrahepatic complications are the main source of morbidity and mortality while liver function is relatively preserved. Combined transplantation of other organs may be required, and other surgical techniques, such as domino and auxiliary liver transplantation, have been attempted. In a third group of monogenic diseases, the underlying genetic defect is expressed at a systemic level and liver involvement is just one of the clinical manifestations. In these conditions, LT might only be partially curative since the abnormal phenotype is maintained by extrahepatic synthesis of the toxic metabolites (i.e., methylmalonic acidemia, propionic acidemia). This review focuses on principles of diagnosis, management and LT results in both paediatric and adult populations of selected liver-based monogenic diseases, which represent examples of different transplantation strategies, driven by the understanding of the expression of the underlying genetic defect.

Urea cycle disorder--argininosuccinic lyase deficiency

An increased level of ammonia in the bloodstream, or hyperammonemia, is a symptom associated with metabolic disorders referred to as inborn errors of metabolism. Urea cycle disorder is a congenital abnormality or absence of one of the six enzymes involved in the elimination of ammonia. Administration of certain medications, high protein diet, excessive exercise, surgical procedures, or trauma can precipitate symptoms of mental confusion, seizure-like activity, and ataxia. This paper reviews the literature with insight into current treatment and management options of the disorder and modification of treatment for the dental patient.

Paediatric liver transplantation for metabolic disorders. Part 1: Liver-based metabolic disorders without liver lesions

Liver-based metabolic disorders account for 10 to 15% of the indications for paediatric liver transplantation. In the last three decades, important progress has been made in the understanding of these diseases, and new therapies have emerged. Concomitantly, medical and surgical innovations have lead to improved results of paediatric liver transplantation, patient survival nowadays exceeding 80% 10-year after surgery with close to normal quality of life in most survivors. This review is a practical update on medical therapy, indications and results of liver transplantation, and potential future therapies, for the main liver-based metabolic disorders in which paediatric liver transplantation may be considered. Part 1 focuses on metabolic based liver disorders without liver lesions, and part 2 on metabolic liver diseases with liver lesions.

Metabolic investigations prevent liver transplantation in two young children with citrullinemia type I

Acute liver failure may be caused by a variety of disorders including inborn errors of metabolism. In those cases, rapid metabolic investigations and adequate treatment may avoid the need for liver transplantation. We report two patients who presented with acute liver failure and were referred to our center for liver transplantation work-up. Urgent metabolic investigations revealed citrullinemia type I. Treatment for citrullinemia type I avoided the need for liver transplantation. Acute liver failure as a presentation of citrullinemia type I has not previously been reported in young children. Although acute liver failure has occasionally been described in other urea cycle disorders, these disorders may be underestimated as a cause. Timely diagnosis and treatment of these disorders may avoid liver transplantation and improve clinical outcome. Therefore, urea cycle disorders should be included in the differential diagnosis in young children presenting with acute liver failure.

Hyperammonemia in the pediatric emergency care setting

Hyperammonemia is encountered frequently in acutely ill children presenting for emergency care with altered levels of consciousness (ALOC). Ammonia production, metabolism, and excretion are affected by different variables. Hyperammonemia may be a transient state or may signify more grave etiologies as inborn errors of metabolism. Levels of ammonia are also affected by proper sampling technique, transport, and analysis.

OBJECTIVES

To determine the level of ammonia in acutely ill children with ALOC, identify causes of hyperammonemia, and correlate levels with illness severity and morbidity.

DESIGN

Observational study.

SETTING

Emergency department at Cairo University Specialized Paediatric Hospital.

METHODS

Fifty cases of acutely ill pediatric patients with ALOC who presented to the emergency department were included in the study from 2008 through 2009. Emergency department patients (n = 20) with known diseases that may induce hyperammonemia were excluded. Patients were subjected to detailed history taking with emphasis on factors affecting ammonia levels and thorough clinical examination. A cohort group of age- and sex-matched children acted as a control group.

RESULTS

The measured blood ammonia level ranged between 13 and 265 μmol/L, with a mean level of 95 μmol/L. Sixty percent of the children with ALOC had ammonia levels of greater than 75 μmol/L, with levels greater than 200 μmol/L seen in 6% of the studied sample. The study demonstrated a highly significant statistical difference between children with ALOC and control groups.There was no correlation between blood ammonia level and age. Correlations of ammonia levels were also conducted in comparison with etiological diagnoses and laboratory parameters with no statistical significance.There was no statistical significance between ammonia level and duration of illness, Sequential Organ Failure Assessment score, or Glasgow Coma Scale score/Morray Scale score.

CONCLUSIONS

Clinicians should consider testing children with ALOC for hyperammonemia, provided that a clear understanding of its metabolism and factors controlling it are understood. Proper sampling must be ensured. Mild elevations of ammonia levels are fairly common, but exceedingly high levels should raise concern and may require further evaluation.

Sequence capture and next-generation resequencing of multiple tagged nucleic acid samples for mutation screening of urea cycle disorders

BACKGROUND

Molecular genetic testing is commonly used to confirm clinical diagnoses of inherited urea cycle disorders (UCDs); however, conventional mutation screenings encompassing only the coding regions of genes may not detect disease-causing mutations occurring in regulatory elements and introns. Microarray-based target enrichment and next-generation sequencing now allow more-comprehensive genetic screening. We applied this approach to UCDs and combined it with the use of DNA bar codes for more cost-effective, parallel analyses of multiple samples.

METHODS

We used sectored 2240-feature medium-density oligonucleotide arrays to capture and enrich a 199-kb genomic target encompassing the complete genomic regions of 3 urea cycle genes, OTC (ornithine carbamoyltransferase), CPS1 (carbamoyl-phosphate synthetase 1, mitochondrial), and NAGS (N-acetylglutamate synthase). We used the Genome Sequencer FLX System (454 Life Sciences) to jointly analyze 4 samples individually tagged with a 6-bp DNA bar code and compared the results with those for an individually sequenced sample.

RESULTS

Using a low tiling density of only 1 probe per 91 bp, we obtained strong enrichment of the targeted loci to achieve ≥90% coverage with up to 64% of the sequences covered at a sequencing depth ≥10-fold. We observed a very homogeneous sequence representation of the bar-coded samples, which yielded a >30% increase in the sequence data generated per sample, compared with an individually processed sample. Heterozygous and homozygous disease-associated mutations were correctly detected in all samples.

CONCLUSIONS

The use of DNA bar codes and the use of sectored oligonucleotide arrays for target enrichment enable parallel, large-scale analysis of complete genomic regions for multiple genes of a disease pathway and for multiple samples simultaneously. This approach thus may provide an efficient tool for comprehensive diagnostic screening of mutations.

Thiol-independent action of mitochondrial thioredoxin to support the urea cycle of arginine biosynthesis in Schizosaccharomyces pombe

Thioredoxins usually perform a role as a thiol-disulfide oxidoreductase using their active-site cysteines. The fission yeast Schizosaccharomyces pombe contains two thioredoxins: Trx1 for general stress protection and Trx2 for mitochondrial functions. The Deltatrx2 mutant grows as well as the wild type on complex media containing glucose. However, on nonfermentable carbon source such as glycerol, the mutant did not grow, indicating a defect in mitochondrial function. The mutant also exhibited auxotrophy for arginine and cysteine on minimal medium. In order to find the reason for the unexpected arginine auxotrophy, we searched for multicopy suppressors and found that the arg3(+) gene encoding ornithine carbamoyltransferase (OCTase) in the urea cycle of the arginine biosynthetic pathway rescued the arginine auxotrophy. The levels of arg3(+) transcript, Arg3 protein, and OCTase activity were all decreased in Deltatrx2. Through immunocoprecipitation, we observed a direct interaction between Trx2 and Arg3 in cell extracts. The mutant forms of Trx2 lacking either one or both of the active site cysteines through substitution to serines also rescued the arginine auxotrophy and restored the decreased OCTase activity. They also rescued the growth defect of Deltatrx2 on glycerol medium. This contrasts with the thiol-dependent action of overproduced Trx2 in complementing glutathione reductase. Therefore, Trx2 serves multiple functions in mitochondria, protecting mitochondrial components against thiol-oxidative damage as a thiol-disulfide oxidoreductase, and supporting urea cycle and respiration in mitochondria in a manner independent of active site thiols.

Dysregulation of C/EBPalpha by mutant Huntingtin causes the urea cycle deficiency in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a CAG trinucleotide expansion in the Huntingtin (Htt) gene. Using two mouse models of HD, we demonstrate that the urea cycle deficiency characterized by hyperammonemia, high blood citrulline and suppression of urea cycle enzymes is a prominent feature of HD. The resultant ammonia toxicity might exacerbate the neurological deficits of HD. Suppression of C/EBPalpha, a crucial transcription factor for the transcription of urea cycle enzymes, appears to mediate the urea cycle deficiency in HD. We found that in the presence of mutant Htt, C/EBPalpha loses its ability to interact with an important cofactor (CREB-binding protein). Moreover, mutant Htt recruited C/EBPalpha into aggregates, as well as suppressed expression of the C/EBPalpha gene. Consumption of protein-restricted diets not only led to the restoration of C/EBPalpha's activity, and repair of the urea cycle deficiency and hyperammonemia, but also ameliorated the formation of Htt aggregates, the motor deterioration, the suppression of striatal brain-derived neurotrophic factor and the normalization of three protein chaperones (Hsp27, Hsp70 and Hsp90). Treatments aimed at repairing the urea cycle deficiency may provide a new strategy for dealing with HD.

Expanded newborn screening of inherited metabolic disorders by tandem mass spectrometry: Clinical and laboratory aspects

Newborn screening started in the 1960s for the purpose of identifying phenylketonuric patients to begin early intervention and to prevent mental retardation in these patients. Soon thereafter, screening programs expanded to include additional genetic disorders added individually one at a time. In the 1980s, tandem mass spectrometry (MS/MS) was introduced in clinical laboratories, and in the 1990s, the technique was used for newborn screening. Unlike measuring one analyte at a time, MS/MS allows measurement of >40 analytes, in a few minutes with the use of a single assay. Currently, MS/MS is being used for the identification of several amino acid, organic acid and fatty acid disorders. Several states in the United States and many other countries are using MS/MS in newborn screening. However, there is a significant disparity among different newborn screening programs for disorders being screened by MS/MS and many other challenges are faced by the expanded newborn screening. It is anticipated that in the future the use of MS/MS in newborn screening will expand both at the analyte and geographic levels. Clinicians and laboratory scientists should become familiar with MS/MS, disorders being screened in their patients' population and the future of this emerging technology.