Urea Cycle Deficiency Research Articles

Identification of Clinical Variants beyond the Exome in Inborn Errors of Metabolism.

Inborn errors of metabolism (IEM) constitute a huge group of rare diseases affecting 1 in every 1000 newborns. Next-generation sequencing has transformed the diagnosis of IEM, leading to its proposed use as a second-tier technology for confirming cases detected by clinical/biochemical studies or newborn screening. The diagnosis rate is, however, still not 100%. This paper reports the use of a personalized multi-omics (metabolomic, genomic and transcriptomic) pipeline plus functional genomics to aid in the genetic diagnosis of six unsolved cases, with a clinical and/or biochemical diagnosis of galactosemia, mucopolysaccharidosis type I (MPS I), maple syrup urine disease (MSUD), hyperphenylalaninemia (HPA), citrullinemia, or urea cycle deficiency. Eight novel variants in six genes were identified: six (four of them deep intronic) located in GALE, IDUA, PTS, ASS1 and OTC, all affecting the splicing process, and two located in the promoters of IDUA and PTS, thus affecting these genes' expression. All the new variants were subjected to functional analysis to verify their pathogenic effects. This work underscores how the combination of different omics technologies and functional analysis can solve elusive cases in clinical practice.

Late-onset Urea Cycle Deficiency is an Under-recognised Cause of Metabolic Childhood Encephalopathy: a Case Report

Late-onset Urea Cycle Deficiency is an Under-recognised Cause of Metabolic Childhood Encephalopathy: a Case Report

In SilicoModeling for the Prediction of Dose and Pathway-Related Adverse Effects in Humans FromIn VitroRepeated-Dose Studies

Long-term repeated-dose toxicity is mainly assessed in animals despite poor concordance of animal data with human toxicity. Nowadays advanced human in vitro systems, eg, metabolically competent HepaRG cells, are used for toxicity screening. Extrapolation of in vitro toxicity to in vivo effects is possible by reverse dosimetry using pharmacokinetic modeling. We assessed long-term repeated-dose toxicity of bosentan and valproic acid (VPA) in HepaRG cells under serum-free conditions. Upon 28-day exposure, the EC50 values for bosentan and VPA decreased by 21- and 33-fold, respectively. Using EC(10) as lowest threshold of toxicity in vitro, we estimated the oral equivalent doses for both test compounds using a simplified pharmacokinetic model for the extrapolation of in vitro toxicity to in vivo effect. The model predicts that bosentan is safe at the considered dose under the assumed conditions upon 4 weeks exposure. For VPA, hepatotoxicity is predicted for 4% and 47% of the virtual population at the maximum recommended daily dose after 3 and 4 weeks of exposure, respectively. We also investigated the changes in the central carbon metabolism of HepaRG cells exposed to orally bioavailable concentrations of both drugs. These concentrations are below the 28-day EC(10) and induce significant changes especially in glucose metabolism and urea production. These metabolic changes may have a pronounced impact in susceptible patients such as those with compromised liver function and urea cycle deficiency leading to idiosyncratic toxicity. We show that the combination of modeling based on in vitro repeated-dose data and metabolic changes allows the prediction of human relevant in vivo toxicity with mechanistic insights.

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.

Living Donor Liver Transplantation for Pediatric Patients With Metabolic Disorders: The Japanese Multicenter Registry.

Living donor liver transplantation (LDLT) is indicated for a variety of metabolic disorders, primarily in Asian countries due to the absolute scarcity of deceased donor liver transplantation (LT). We analyzed data for all pediatric LDLTs performed between November 1989 and December 2010, during which 2,224 pediatric patients underwent LDLT in Japan. Of these patients, 194 (8.7%) underwent LDLT for metabolic disorders. Wilson’s disease (n=59; 30.4%) was the most common indication in the patients with metabolic disorders, followed by ornithine transcarbamylase deficiency (OTCD; n=40; 20.6%), methylmalonic acidemia (MMA; n=20, 10.3%) and glycogen storage disease (GSD; n=15, 7.7%).Figure: No Caption available.The 1-, 5-, 10- and 15-year patient and graft survival rates were 91.2%, 87.9%, 87.0% and 79.3% and 91.2%, 87.9%, 86.1% and 74.4%, respectively.Figure: No Caption available.Wilson’s disease and urea cycle deficiency were associated with better patient survival. The use of heterozygous donors demonstrated no negative impact on either the donors or recipients. With regard to X-linked OTCD, symptomatic heterozygote maternal donors should not be considered potential donor candidates. Improving the understanding of the long-term suitability of this treatment modality will require the registration and ongoing evaluation of all patients with inherited metabolic disease considered for LT.

Living donor liver transplantation for pediatric patients with metabolic disorders: The Japanese multicenter registry

LDLT is indicated for a variety of metabolic disorders, primarily in Asian countries due to the absolute scarcity of deceased donor LT. We analyzed data for all pediatric LDLTs performed between November 1989 and December 2010, during which 2224 pediatric patients underwent LDLT in Japan. Of these patients, 194 (8.7%) underwent LDLT for metabolic disorders. Wilson's disease (n = 59; 30.4%) was the most common indication in the patients with metabolic disorders, followed by OTCD (n = 40; 20.6%), MMA (n = 20; 10.3%), and GSD (n = 15; 7.7%). The one-, five-, 10-, and 15-yr patient and graft survival rates were 91.2%, 87.9%, 87.0%, and 79.3%, and 91.2%, 87.9%, 86.1%, and 74.4%, respectively. Wilson's disease and urea cycle deficiency were associated with better patient survival. The use of heterozygous donors demonstrated no negative impact on either the donors or recipients. With regard to X-linked OTCD, symptomatic heterozygote maternal donors should not be considered potential donor candidates. Improving the understanding of the long-term suitability of this treatment modality will require the registration and ongoing evaluation of all patients with inherited metabolic disease considered for LT.

4-Phenylbutyric Acid Reduces Endoplasmic Reticulum Stress, Trypsin Activation, and Acinar Cell Apoptosis While Increasing Secretion in Rat Pancreatic Acini

Endoplasmic reticulum (ER) stress leads to misfolded proteins inside the ER and initiates unfolded protein response (UPR). Unfolded protein response components are involved in pancreatic function and activated during pancreatitis. However, the exact role of ER stress in the exocrine pancreas is unclear. The present study examined the effects of 4-phenylbutyric acid (4-PBA), an ER chaperone, on acini and UPR components. Rat acini were stimulated with cholecystokinin (10 pmol/L to 10 nmol/L) with or without preincubation of 4-PBA. The UPR components were analyzed, including chaperone-binding protein, protein kinaselike ER kinase, X-box-binding protein 1, c-Jun NH(2)-terminal kinase, CCAAT/enhancer-binding protein homologous protein, caspase 3, and apoptosis. Effects of 4-PBA were measured on secretion, calcium, and trypsin activation. 4-Phenylbutyric acid led to an increase of secretion, whereas trypsin activation with supraphysiological cholecystokinin was significantly reduced. 4-Phenylbutyric acid prevented chaperone-binding protein up-regulation, diminished protein kinaselike ER kinase, and c-Jun NH2-terminal kinase phosphorylation, prohibited X-box-binding protein 1 splicing and CCAAT/enhancer-binding protein homologous protein expression, caspase 3 activation, and apoptosis caused by supraphysiological cholecystokinin. By incubation with 4-PBA, beneficial in urea cycle deficiency, it was possible to enhance enzyme secretion to suppress trypsin activation, UPR activation, and proapoptotic pathways. The data hint new perspectives for the use of chemical chaperones in pancreatic diseases.

Threshold for toxicity from hyperammonemia in critically ill children

Hyperammonemia results from reduction of hepatocyte function or enzyme of urea cycle deficiency. Hyperammonemia contributes to cerebral edema that may lead to cerebral herniation. The threshold of toxicity of ammonemia is unknown. We conducted a retrospective observational study in our pediatric intensive care unit. All children who developed hyperammonemia from January 2000 to April 2009 were included. Clinical and laboratory data at admission, specific treatments implemented, and ammonemias the first 7 days after inclusion were collected. The outcome assessed was 28 day mortality. Risk of mortality was estimated by a logistic regression model. Ninety patients with liver failure (63.3%) and primary or secondary urea cycle defect (23.3%) were included. Patients with urea cycle defects were more likely to receive ammonia scavengers than patients with liver failure (47.6% versus 3.5%). The 28 day mortality rate was 31.1%. Risk of mortality increased according to the ammonemia within 48 h: odds ratio 1.5, 1.9, 3.3, 2.4 for ammonemia above 100, 150, 200, and 300 μmol/L, respectively. Peak ammonemia ≥200 μmol/L within the first 48 h was an independent risk factor for mortality, with greater risk found in liver failure than in urea cycle defect. Our study identifies a threshold of exposure to ammonia (≥200 μmol/L) above which mortality increases significantly, especially in liver failure. Specific treatments of hyperammonemia are rarely used in liver failure when compared with urea cycle defect even though use of ammonia scavengers may help to decrease ammonemia.

Fatal Hyperammonemia After Renal Transplant due to Late-Onset Urea Cycle Deficiency: A Case Report

We present a case of severe hyperammonemia with subsequent brain herniation in an adult man after renal transplantation. After successful surgery and an initially uneventful postoperative course, the patient developed significant mental status changes associated with seizure activity. His condition rapidly deteriorated, requiring mechanical ventilation and cardiovascular support. Laboratory studies at that time demonstrated an increased serum ammonia level without evidence of liver or kidney dysfunction. Further investigation revealed an increased orotic acid level in the urine, suggesting a urea cycle disorder (UCD). Despite aggressive therapy, the patient's condition continued to deteriorate. Magnetic resonance imaging demonstrated severe brain edema with no cerebral perfusion; after consultation with the family, care was withdrawn. The combination of hyperammonemia and elevated urine orotic acid with normal liver and kidney function suggested a UCD. It is important to note that patients with a UCD may be free of symptoms for many years. Several factors are able to trigger the disease in adulthood, leading to encephalopathy and death. In this case, the patient's seizures were initially assumed to be a side effect of immunosuppressive therapy. Further diagnostic measures were only performed late in the course of the disease, which delayed the diagnosis of UCD.

The A2A adenosine receptor rescues the urea cycle deficiency of Huntington's disease by enhancing the activity of the ubiquitin–proteasome system

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a CAG trinucleotide expansion in the Huntingtin (Htt) gene. The resultant mutant Htt protein (mHtt) forms aggregates in the brain and several peripheral tissues (e.g. the liver) and causes devastating neuronal degeneration. Metabolic defects resulting from Htt aggregates in peripheral tissues also contribute to HD pathogenesis. Simultaneous improvement of defects in both the CNS and peripheral tissues is thus the most effective therapeutic strategy and is highly desirable. We earlier showed that an agonist of the A(2A) adenosine receptor (A(2A) receptor), CGS21680 (CGS), attenuates neuronal symptoms of HD. We found herein that the A(2A) receptor also exists in the liver, and that CGS ameliorated the urea cycle deficiency by reducing mHtt aggregates in the liver. By suppressing aggregate formation, CGS slowed the hijacking of a crucial transcription factor (HSF1) and two protein chaperons (Hsp27 and Hsp70) into hepatic Htt aggregates. Moreover, the abnormally high levels of high-molecular-mass ubiquitin conjugates in the liver of an HD mouse model (R6/2) were also ameliorated by CGS. The protective effect of CGS against mHtt-induced aggregate formation was reproduced in two cells lines and was prevented by an antagonist of the A(2A) receptor and a protein kinase A (PKA) inhibitor. Most importantly, the mHtt-induced suppression of proteasome activity was also normalized by CGS through PKA. Our findings reveal a novel therapeutic pathway of A(2A) receptors in HD and further strengthen the concept that the A(2A) receptor can be a drug target in treating HD.

Hyperammonemia from a Urea Cycle Disorder Presenting in Adulthood

OBJECTIVE: The aim of this report is to describe a patient with late presentation of carbamyl phosphate syn- thetase I (CPS-I, EC 6.3.4.16) deficiency, a rare urea cycle deficiency, and to facilitate recognition and treatment of pa- tients presenting with encephalopathy and hyperammonemia in a critical care setting. DESIGN: Case Report. SETTING: Intensive care unit of Saint Mary's Hospital, Mayo Clinic, Rochester, Minnesota. PATIENT: A 65-year-old woman ad- mitted with progressive encephalopathy. INTERVENTIONS: Intubation and mechanical ventilation, protein-restricted parenteral nutrition, intravenous arginine, hemodiafiltration, and intravenous antibiotic therapy. MEASUREMENTS AND MAIN RESULTS: Serum ammonia and glutamine levels were elevated, but other laboratory and imaging investigations were unremarkable. Despite the above interventions, her mental status deteriorated. She developed ventilator associated pneumonia, which worsened despite antibiotic treatment. The family decided to withdraw care and the patient expired on hospital day 10. A postmortem enzyme assay on fresh-frozen liver tissue showed severely diminished CPS-I activity. CONCLUSIONS: To our knowledge, this is the oldest reported age at presentation of CPS-I deficiency. Urea cycle disor- ders should be part of the differential diagnosis of hyperammonemia regardless of age as early treatment may ameliorate mortality and morbidity in these patients.

Dysregulation of C/EBPα 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.