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Lichter-Konecki U, Sanz JH, McCarter R. Relationship between longitudinal changes in neuropsychological outcome and disease biomarkers in urea cycle disorders. Pediatr Res 2023; 94:2005-2015. [PMID: 37454183 DOI: 10.1038/s41390-023-02722-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/12/2022] [Revised: 05/29/2023] [Accepted: 06/15/2023] [Indexed: 07/18/2023]
Abstract
BACKGROUND Urea cycle disorders (UCDs) cause impaired conversion of waste nitrogen to urea leading to rise in glutamine and ammonia. Elevated ammonia and glutamine have been implicated in brain injury. This study assessed relationships between biomarkers of metabolic control and long-term changes in neuropsychological test scores in participants of the longitudinal study of UCDs. The hypothesis was that elevated ammonia and glutamine are associated with neuropsychological impairment. METHODS Data from 146 participants who completed 2 neuropsychological assessments were analyzed. Neuropsychological tests that showed significant changes in scores over time were identified and associations between score change and interim metabolic biomarker levels were investigated. RESULTS Participants showed a significant decrease in performance on visual motor integration (VMI) and verbal learning immediate-recall. A decrease in scores was associated with experiencing interim hyperammonemic events (HAE) and frequency of HAE. Outside of HAE there was a significant association between median ammonia levels ≥50µmol/L and impaired VMI. CONCLUSION VMI and memory encoding are specifically affected in UCDs longitudinally, indicating that patients experience difficulties when required to integrate motor and visual functions and learn new information. Only ammonia biomarkers showed a significant association with impairment. Preventing HAE and controlling ammonia levels is key in UCD management. IMPACT The Beery-Buktenica Developmental Test of Visual-Motor Integration (Beery VMI) and List A Trial 5 of the California Verbal Learning Test (CVLT) may be good longitudinal biomarkers of treatment outcome in urea cycle disorders (UCD). This is the first report of longitudinal biomarkers for treatment outcome in UCD. These two biomarkers of outcome may be useful for clinical trials assessing new treatments for UCD. These results will also inform educators how to design interventions directed at improving learning in individuals with UCDs.
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Affiliation(s)
- Uta Lichter-Konecki
- Department of Pediatrics, University of Pittsburgh, Pittsburgh, PA, USA.
- UPMC Children's Hospital of Pittsburgh, Pittsburgh, PA, USA.
| | - Jacqueline H Sanz
- Department of Pediatrics, George Washington University School of Medicine, Washington, DC, USA
- Department of Psychiatry and Behavioral Sciences, George Washington University School of Medicine, Washington, DC, USA
- Children's National Hospital, Washington, DC, USA
| | - Robert McCarter
- Department of Pediatrics, George Washington University School of Medicine, Washington, DC, USA
- Children's National Hospital, Washington, DC, USA
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Bazo A, Lantero A, Mauleón I, Neri L, Poms M, Häberle J, Ricobaraza A, Bénichou B, Combal JP, Gonzalez-Aseguinolaza G, Aldabe R. Gene Therapy in Combination with Nitrogen Scavenger Pretreatment Corrects Biochemical and Behavioral Abnormalities of Infant Citrullinemia Type 1 Mice. Int J Mol Sci 2022; 23:14940. [PMID: 36499263 PMCID: PMC9736988 DOI: 10.3390/ijms232314940] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2022] [Revised: 11/22/2022] [Accepted: 11/23/2022] [Indexed: 12/02/2022] Open
Abstract
Citrullinemia type I (CTLN1) is a rare autosomal recessive disorder caused by mutations in the gene encoding argininosuccinate synthetase 1 (ASS1) that catalyzes the third step of the urea cycle. CTLN1 patients suffer from impaired elimination of nitrogen, which leads to neurotoxic levels of circulating ammonia and urea cycle byproducts that may cause severe metabolic encephalopathy, death or irreversible brain damage. Standard of care (SOC) of CTLN1 consists of daily nitrogen-scavenger administration, but patients remain at risk of life-threatening decompensations. We evaluated the therapeutic efficacy of a recombinant adeno-associated viral vector carrying the ASS1 gene under the control of a liver-specific promoter (VTX-804). When administered to three-week-old CTLN1 mice, all the animals receiving VTX-804 in combination with SOC gained body weight normally, presented with a normalization of ammonia and reduction of citrulline levels in circulation, and 100% survived for 7 months. Similar to what has been observed in CTLN1 patients, CTLN1 mice showed several behavioral abnormalities such as anxiety, reduced welfare and impairment of innate behavior. Importantly, all clinical alterations were notably improved after treatment with VTX-804. This study demonstrates the potential of VTX-804 gene therapy for future clinical translation to CTLN1 patients.
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Affiliation(s)
- Andrea Bazo
- Division of Gene Therapy and Regulation of Gene Expression, CIMA, University of Navarra, 31008 Pamplona, Spain
| | | | - Itsaso Mauleón
- Division of Gene Therapy and Regulation of Gene Expression, CIMA, University of Navarra, 31008 Pamplona, Spain
| | - Leire Neri
- Vivet Therapeutics, S.L., 31008 Pamplona, Spain
| | - Martin Poms
- Department of Clinical Chemistry and Biochemistry, University Children’s Hospital Zurich, University of Zurich, 8091 Zurich, Switzerland
| | - Johannes Häberle
- Division of Metabolism, Children’s Research Centre (CRC), University Children’s Hospital Zurich, 8091 Zurich, Switzerland
| | - Ana Ricobaraza
- Division of Gene Therapy and Regulation of Gene Expression, CIMA, University of Navarra, 31008 Pamplona, Spain
| | | | | | - Gloria Gonzalez-Aseguinolaza
- Division of Gene Therapy and Regulation of Gene Expression, CIMA, University of Navarra, 31008 Pamplona, Spain
- Vivet Therapeutics, S.L., 31008 Pamplona, Spain
| | - Rafael Aldabe
- Division of Gene Therapy and Regulation of Gene Expression, CIMA, University of Navarra, 31008 Pamplona, Spain
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New Insight in Hyperinsulinism/Hyperammonemia Syndrome by Magnetic Resonance Imaging and Spectroscopy. Brain Sci 2022; 12:brainsci12030389. [PMID: 35326344 PMCID: PMC8946637 DOI: 10.3390/brainsci12030389] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2022] [Revised: 02/21/2022] [Accepted: 03/07/2022] [Indexed: 11/16/2022] Open
Abstract
Hyperinsulinism/hyperammonemia syndrome (HI/HA) is an autosomal dominant disorder caused by monoallelic activating mutations in the glutamate dehydrogenase 1 (GLUD1) gene. While hyperinsulinism may be explained by a reduction in the allosteric inhibition of GLUD1, the pathogenesis of HA in HI/HA remains uncertain; interestingly, HA in the HI/HA syndrome is not associated with acute hyperammonemic intoxication events. We obtained a brain magnetic resonance (MR) in a woman with HI/HA syndrome with chronic asymptomatic HA. On MR spectroscopy, choline and myoinositol were decreased as in other HA disorders. In contrast, distinct from other HA disorders, combined glutamate and glutamine levels were normal (not increased). This observation suggests that brain biochemistry in HI/HA may differ from that of other HA disorders. In HI/HA, ammonia overproduction may come to the expense of glutamate levels, and this seems to prevent the condensation of ammonia with glutamate to produce glutamine that is typical of the other HA disorders. The absence of combined glutamate and glutamine elevation might be correlated to the absence of acute cerebral ammonia toxicity.
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Anderson AA, Gropman A, Le Mons C, Stratakis CA, Gandjbakhche AH. Hemodynamics of Prefrontal Cortex in Ornithine Transcarbamylase Deficiency: A Twin Case Study. Front Neurol 2020; 11:809. [PMID: 32922350 PMCID: PMC7456944 DOI: 10.3389/fneur.2020.00809] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2020] [Accepted: 06/29/2020] [Indexed: 11/30/2022] Open
Abstract
Ornithine transcarbamylase deficiency (OTCD) is the most common form of urea cycle disorder characterized by the presence of hyperammonemia (HA). In patients with OTCD, HA is known to cause impairments in domains of executive function and working memory. Monitoring OTCD progression and investigating neurocognitive biomarkers can, therefore, become critical in understanding the underlying brain function in a population with OTCD. We used functional near infrared spectroscopy (fNIRS) to examine the hemodynamics of prefrontal cortex (PFC) in a fraternal twin with and without OTCD. fNIRS is a non-invasive and wearable optical technology that can be used to assess cortical hemodynamics in a realistic clinical setting. We quantified the hemodynamic variations in total-hemoglobin as assessed by fNIRS while subjects performed the N-back working memory (WM) task. Our preliminary results showed that the sibling with OTCD had higher variation in a very low frequency band (<0.03 Hz, related to mechanism of cerebral autoregulation) compared to the control sibling. The difference between these variations was not as prominent in the higher frequency band, indicating the possible role of impaired autoregulation and cognitive function due to presence of HA. We further examined the functional connectivity in PFC, where the OTCD sibling showed lower interhemispheric functional connectivity as the task load increased. Our pilot results are the first to show the utility of fNIRS in monitoring OTCD cortical hemodynamics, indicating the possibility of inefficient neurocognitive function. This study provides a novel insight into the monitoring of OTCD focusing on the contribution of physiological process and neurocognitive function in this population.
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Affiliation(s)
- Afrouz A. Anderson
- National Institutes of Health (NIH), National Institute of Child Health and Human Development, Bethesda, MD, United States
| | - Andrea Gropman
- Children's National Medical Center, Division of Neurogenetics and Neurodevelopmental Pediatrics, Washington, DC, United States
| | - Cynthia Le Mons
- National Urea Cycle Disorders Foundation, Pasadena, CA, United States
| | - Constantine A. Stratakis
- National Institutes of Health (NIH), National Institute of Child Health and Human Development, Bethesda, MD, United States
| | - Amir H. Gandjbakhche
- National Institutes of Health (NIH), National Institute of Child Health and Human Development, Bethesda, MD, United States
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Waisbren SE, Stefanatos AK, Kok TMY, Ozturk‐Hismi B. Neuropsychological attributes of urea cycle disorders: A systematic review of the literature. J Inherit Metab Dis 2019; 42:1176-1191. [PMID: 31268178 PMCID: PMC7250134 DOI: 10.1002/jimd.12146] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/16/2018] [Revised: 06/25/2019] [Accepted: 07/01/2019] [Indexed: 12/30/2022]
Abstract
Urea cycle disorders (UCDs) are rare inherited metabolic conditions that impair the effectiveness of the urea cycle responsible for removing excess ammonia from the body. The estimated incidence of UCDs is 1:35 000 births, or approximately 113 new patients with UCD per year. This review summarizes neuropsychological outcomes among patients with the eight UCDs in reports published since 1980. Rates of intellectual disabilities published before (and including) 2000 and after 2000 were pooled and compared for each UCD. Since diagnoses for UCDs tended to occur earlier and better treatments became more readily available after the turn of the century, this assessment will characterize the extent that current management strategies have improved neuropsychological outcomes. The pooled sample included data on cognitive abilities of 1649 individuals reported in 58 citations. A total of 556 patients (34%) functioned in the range of intellectual disabilities. The decline in the proportion of intellectual disabilities in six disorders, ranged from 7% to 41%. Results from various studies differed and the cohorts varied with respect to age at symptom onset, age at diagnosis and treatment initiation, current age, severity of the metabolic deficiency, management strategies, and ethnic origins. The proportion of cases with intellectual disabilities ranged from 9% to 65% after 2000 in the seven UCDs associated with cognitive deficits. Positive outcomes from some studies suggest that it is possible to prevent or reverse the adverse impact of UCDs on neuropsychological functioning. It is time to "raise the bar" in terms of expectations for treatment effectiveness.
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Affiliation(s)
- Susan E. Waisbren
- Division of Genetics and Genomics, Department of Pediatrics, Boston Children's HospitalBostonMassachusetts
- Department of Medicine, Harvard Medical SchoolBostonMassachusetts
| | - Arianna K. Stefanatos
- Department of Child & Adolescent Psychiatry and Behavioral Sciences, Children's Hospital of PhiladelphiaPhiladelphiaPennsylvania
| | | | - Burcu Ozturk‐Hismi
- Division of Genetics and Genomics, Department of Pediatrics, Boston Children's HospitalBostonMassachusetts
- Tepecik Education and Research HospitalIzmirTurkey
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Stepien KM, Geberhiwot T, Hendriksz CJ, Treacy EP. Challenges in diagnosing and managing adult patients with urea cycle disorders. J Inherit Metab Dis 2019; 42:1136-1146. [PMID: 30932189 DOI: 10.1002/jimd.12096] [Citation(s) in RCA: 36] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/23/2018] [Accepted: 03/28/2019] [Indexed: 12/15/2022]
Abstract
Urea cycle disorders (UCD) are a group of rare inherited metabolic conditions of amino acid catabolism caused by an enzyme deficiency within the hepatic ammonia detoxification pathway. The presentation of these disorders ranges from life-threatening intoxication in the neonate to asymptomatic status in adults. Late-onset UCDs can present for the first time in adulthood and may mimic other causes of acute confusion or psychiatric diseases, and are often associated with neurological symptoms. Late-onset UCDs may become apparent during periods of metabolic stress such as rapid weight loss, gastric bypass surgery, chronic starvation or the postpartum period. Early diagnosis is critical for effective treatment and to prevent long-term complications of hyperammonemia. The challenges of management of adults include for example: (a) poor compliance to dietary and medical treatment which can result in recurrent hospital admissions; (b) severe neurological dysfunction; (c) the management of pregnancy and the postpartum period; and (d) access to multidisciplinary care peri-operatively. In this review, we highlight a number of challenges in the diagnosis and management of adult patient with late-onset UCDs and suggest a systematic management approach.
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Affiliation(s)
- Karolina M Stepien
- Mark Holland Metabolic Unit, Adult Inherited Metabolic Diseases Department, Salford Royal NHS Foundation Trust, Salford, UK
| | - Tarekegn Geberhiwot
- Centre for Endocrinology, Diabetes and Metabolism, Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, UK
| | - Christian J Hendriksz
- Department of Paediatrics, Steve Biko Academic Hospital, University of Pretoria, Pretoria, South Africa
| | - Eileen P Treacy
- National Centre for Inherited Metabolic Diseases, The Mater Misericordiae University Hospital, Dublin, Ireland
- Department of Paediatrics, Trinity College, Dublin, Ireland
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Enns GM, Porter MH, Francis-Sedlak M, Burdett A, Vockley J. Perspectives on urea cycle disorder management: Results of a clinician survey. Mol Genet Metab 2019; 128:102-108. [PMID: 31377149 DOI: 10.1016/j.ymgme.2019.07.009] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/15/2019] [Revised: 06/12/2019] [Accepted: 07/16/2019] [Indexed: 12/31/2022]
Abstract
BACKGROUND/AIMS Urea cycle disorders (UCDs) are rare inborn errors of urea synthesis. US and European consensus statements on the diagnosis and treatment of UCDs were last published in 2001 and 2019, respectively. Recommendations are based primarily on case reports and expert opinion and there is limited agreement or consistency related to long-term management approaches. A clinician survey was conducted to assess current real-world practices and perspectives on challenges and unmet needs. METHODS A 14-item multiple-choice survey was administered to physicians in 2017. Clinicians who reported actively managing at least 1 patient with UCD were eligible to participate. Descriptive statistics were calculated for each survey item (frequencies for categorical variables; means, standard deviations, medians, and ranges for continuous variables). RESULTS Sixty-six US clinicians completed the survey (65 geneticists; 1 pediatric neurologist). Over 90% of responders agreed or strongly agreed that even modest elevations in ammonia could cause physiological and functional brain damage; >80% of respondents agreed that asymptomatic UCD patients are at risk of brain damage over time due to mild/subclinical elevations in ammonia. Eighty-six percent of clinicians agreed or strongly agreed with recommending genetic testing for female relatives when a patient is diagnosed with ornithine transcarbamylase deficiency. Ninety-four percent of respondents agreed that patients have better disease control when they are more adherent to their UCD therapy. Nearly 90% indicated that clinicians and patients would benefit from updated UCD management guidance. More than half (53%) of respondents rated the symptoms of UCDs as extremely or very burdensome to the everyday lives of patients and their families; only 8% rated UCD symptoms as slightly or not at all burdensome. The majority of clinicians agreed (48%) or strongly agreed (32%) that caring for a child or family member with a UCD has a negative impact on the quality of life and/or health of family members/guardians (e.g. stress, relationships, ability to work). CONCLUSIONS This self-reported survey suggests a need for updated and expanded clinical guidance on the long-term treatment and management of UCD patients.
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Affiliation(s)
- Gregory M Enns
- Stanford University, School of Medicine, Stanford, CA, USA.
| | | | | | | | - Jerry Vockley
- University of Pittsburgh, School of Medicine, Graduate School of Public Health, Pittsburgh, PA, USA
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Buerger C, Garbade SF, Alber FD, Waisbren SE, McCarter R, Kölker S, Burgard P. Impairment of cognitive function in ornithine transcarbamylase deficiency is global rather than domain-specific and is associated with disease onset, sex, maximum ammonium, and number of hyperammonemic events. J Inherit Metab Dis 2019; 42:243-253. [PMID: 30671983 PMCID: PMC7439789 DOI: 10.1002/jimd.12013] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/22/2018] [Revised: 10/18/2018] [Accepted: 11/12/2018] [Indexed: 12/15/2022]
Abstract
Beginning in 2006, the Urea Cycle Disorders Consortium (UCDC) has conducted a longitudinal study of eight inherited deficiencies of enzymes and transporters of the urea cycle, including 444 individuals with ornithine transcarbamylase deficiency (OTCD), of whom 300 (67 males, 233 females) received psychological evaluation. In a cross-sectional study (age range, 3-71 years), analysis of covariance (ANCOVA) determined the association between outcomes in five cognitive domains (global intelligence, executive functions, memory, visuomotor integration, visual perception) and sex, age at testing and timing of disease onset defined as early onset (≤28 days; EO), late onset (LO), or asymptomatic (AS). The dataset of 183 subjects with complete datasets (31 males, 152 females) revealed underrepresentation of EO subjects (2 males, 4 females), who were excluded from the ANCOVA. Although mean scores of LO and AS individuals were within 1 SD of the population norm, AS subjects attained significantly higher scores than LO subjects and males higher scores than females. Correlations between cognitive domains were high, particularly intelligence proved to be a distinguished indicator for cognitive functioning. Maximum plasma ammonium concentration and intelligence correlated significantly higher in EO (r = -0.47) than in LO subjects (r = 0.04). Correlation between the number of hyperammonemic events and intelligence scores were similar for EO (r = -0.30) and LO (r = -0.26) individuals. The number of clinical symptoms was significantly associated with intelligence (r = -0.28) but not with scores in other domains. Results suggest that OTCD has a global impact on cognitive functioning rather than a specific effect on distinct cognitive domains.
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Affiliation(s)
- Corinna Buerger
- Division of Neuropaediatrics and Inherited Metabolic Diseases, Centre for Child and Adolescent Medicine, University Hospital Heidelberg, Heidelberg, Germany
| | - Sven F. Garbade
- Division of Neuropaediatrics and Inherited Metabolic Diseases, Centre for Child and Adolescent Medicine, University Hospital Heidelberg, Heidelberg, Germany
| | - Fabienne Dietrich Alber
- Division of Metabolism and Children’s Research Centre, University Children’s Hospital Zurich, Zurich, Switzerland
| | - Susan E. Waisbren
- Division of Genetics and Genomics, Children’s Hospital and Harvard Medical School, Boston, Massachusetts
| | - Robert McCarter
- Center for Translational Sciences, Children’s National Health System, The George Washington University, Washington, District of Columbia
| | - Stefan Kölker
- Division of Neuropaediatrics and Inherited Metabolic Diseases, Centre for Child and Adolescent Medicine, University Hospital Heidelberg, Heidelberg, Germany
| | - Peter Burgard
- Division of Neuropaediatrics and Inherited Metabolic Diseases, Centre for Child and Adolescent Medicine, University Hospital Heidelberg, Heidelberg, Germany
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Longo N, Holt RJ. Glycerol phenylbutyrate for the maintenance treatment of patients with deficiencies in enzymes of the urea cycle. Expert Opin Orphan Drugs 2017. [DOI: 10.1080/21678707.2017.1405807] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Affiliation(s)
- Nicola Longo
- Division of Medical Genetics, University of Utah, Salt Lake City, UT, USA
| | - Robert J. Holt
- Medical Affairs, Horizon Pharma, Lake Forest, IL, USA
- Department of Pharmacy Practice, University of Illinois, Chicago, IL, USA
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Tarasenko TN, Rosas OR, Singh LN, Kristaponis K, Vernon H, McGuire PJ. A new mouse model of mild ornithine transcarbamylase deficiency (spf-j) displays cerebral amino acid perturbations at baseline and upon systemic immune activation. PLoS One 2015; 10:e0116594. [PMID: 25647322 PMCID: PMC4315515 DOI: 10.1371/journal.pone.0116594] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2014] [Accepted: 12/12/2014] [Indexed: 01/09/2023] Open
Abstract
Ornithine transcarbamylase deficiency (OTCD, OMIM# 311250) is an inherited X-linked urea cycle disorder that is characterized by hyperammonemia and orotic aciduria. In this report, we describe a new animal model of OTCD caused by a spontaneous mutation in the mouse Otc gene (c.240T>A, p.K80N). This transversion in exon 3 of ornithine transcarbamylase leads to normal levels of mRNA with low levels of mature protein and is homologous to a mutation that has also been described in a single patient affected with late-onset OTCD. With higher residual enzyme activity, spf-J were found to have normal plasma ammonia and orotate. Baseline plasma amino acid profiles were consistent with mild OTCD: elevated glutamine, and lower citrulline and arginine. In contrast to WT, spf-J displayed baseline elevations in cerebral amino acids with depletion following immune challenge with polyinosinic:polycytidylic acid. Our results indicate that the mild spf-J mutation constitutes a new mouse model that is suitable for mechanistic studies of mild OTCD and the exploration of cerebral pathophysiology during acute decompensation that characterizes proximal urea cycle dysfunction in humans.
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Affiliation(s)
- Tatyana N Tarasenko
- National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland, United States of America
| | - Odrick R Rosas
- Universidad Central de Caribe, Bayamon, Puerto Rico, United States of America
| | - Larry N Singh
- National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland, United States of America
| | - Kara Kristaponis
- Department of Neurogenetics, Kennedy Krieger Institute, Johns Hopkins University, Baltimore, Maryland, United States of America
| | - Hilary Vernon
- Kennedy Krieger Institute, Johns Hopkins University, Baltimore, Maryland, United States of America
| | - Peter J McGuire
- National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland, United States of America
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