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Mitochondrial neurogastrointestinal encephalopathy due to mutations in RRM2B.

ABSTRACT: Mitochondrial neurogastrointestinal encephalopathy (MNGIE) is a progressive neurodegenerative disorder associated with thymidine phosphorylase deficiency resulting in high levels of plasma thymidine and a characteristic clinical phenotype.To investigate the molecular basis of MNGIE in a patient with a normal plasma thymidine level.Clinical, neurophysiological, and histopathological examinations as well as molecular and genetic analyses.Nerve and muscle center and genetic clinic. Patient A 42-year-old woman with clinical findings strongly suggestive for MNGIE.Clinical description of the disease and its novel genetic cause.Identification of mitochondrial DNA depletion in muscle samples (approximately 12% of the control mean content) prompted us to look for other causes of our patient's condition. Sequencing of genes associated with mitochondrial DNA depletion-POLG, PEO1, ANT1, SUCLG1, and SUCLA2-did not reveal deleterious mutations. Results of sequencing and array comparative genomic hybridization of the mitochondrial DNA for point mutations and deletions in blood and muscle were negative. Sequencing of RRM2B, a gene encoding cytosolic p53-inducible ribonucleoside reductase small subunit (RIR2B), revealed 2 pathogenic mutations, c.329G>A (p.R110H) and c.362G>A (p.R121H). These mutations are predicted to affect the docking interface of the RIR2B homodimer and likely result in impaired enzyme activity.This study expands the clinical spectrum of impaired RIR2B function, challenges the notion of locus homogeneity of MNGIE, and sheds light on the pathogenesis of conditions involved in the homeostasis of the mitochondrial nucleotide pool. Our findings suggest that patients with MNGIE who have normal thymidine levels should be tested for RRM2B mutations.

SUBMITTER: Shaibani A

PROVIDER: S-EPMC2747647 | biostudies-literature | 2009 Aug

REPOSITORIES: biostudies-literature

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Mitochondrial neurogastrointestinal encephalopathy due to mutations in RRM2B.

Shaibani Aziz A Shchelochkov Oleg A OA Zhang Shulin S Katsonis Panagiotis P Lichtarge Olivier O Wong Lee-Jun LJ Shinawi Marwan M

Archives of neurology 20090801 8

<h4>Background</h4>Mitochondrial neurogastrointestinal encephalopathy (MNGIE) is a progressive neurodegenerative disorder associated with thymidine phosphorylase deficiency resulting in high levels of plasma thymidine and a characteristic clinical phenotype.<h4>Objective</h4>To investigate the molecular basis of MNGIE in a patient with a normal plasma thymidine level.<h4>Design</h4>Clinical, neurophysiological, and histopathological examinations as well as molecular and genetic analyses.<h4>Sett ...[more]

PMID: 19667227

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Project description:BACKGROUND AND PURPOSE:Mitochondrial neurogastrointestinal encephalopathy is a rare disorder due to recessive mutations in the thymidine phosphorylase gene, encoding thymidine phosphorylase protein required for mitochondrial DNA replication. Clinical manifestations include gastrointestinal dysmotility and diffuse asymptomatic leukoencephalopathy. This study aimed to elucidate the mechanisms underlying brain leukoencephalopathy in patients with mitochondrial neurogastrointestinal encephalopathy by correlating multimodal neuroradiologic features to postmortem pathology. MATERIALS AND METHODS:Seven patients underwent brain MR imaging, including single-voxel proton MR spectroscopy and diffusion imaging. Absolute concentrations of metabolites calculated by acquiring unsuppressed water spectra at multiple TEs, along with diffusion metrics based on the tensor model, were compared with those of healthy controls using unpaired t tests in multiple white matters regions. Brain postmortem histologic, immunohistochemical, and molecular analyses were performed in 1 patient. RESULTS:All patients showed bilateral and nearly symmetric cerebral white matter hyperintensities on T2-weighted images, extending to the cerebellar white matter and brain stem in 4. White matter, N-acetylaspartate, creatine, and choline concentrations were significantly reduced compared with those in controls, with a prominent increase in the radial water diffusivity component. At postmortem examination, severe fibrosis of brain vessel smooth muscle was evident, along with mitochondrial DNA replication depletion in brain and vascular smooth-muscle and endothelial cells, without neuronal loss, myelin damage, or gliosis. Prominent periependymal cytochrome C oxidase deficiency was also observed. CONCLUSIONS:Vascular functional and histologic alterations account for leukoencephalopathy in mitochondrial neurogastrointestinal encephalopathy. Thymidine toxicity and mitochondrial DNA replication depletion may induce microangiopathy and blood-brain-barrier dysfunction, leading to increased water content in the white matter. Periependymal cytochrome C oxidase deficiency could explain prominent periventricular impairment.

Project description:Balanced pools of deoxyribonucleoside triphosphate precursors are required for DNA replication, and alterations of this balance are relevant to human mitochondrial diseases including mitochondrial neurogastrointestinal encephalopathy. In this disease, autosomal recessive TYMP mutations cause severe reductions of thymidine phosphorylase activity; marked elevations of the pyrimidine nucleosides thymidine and deoxyuridine in plasma and tissues, and somatic multiple deletions, depletion and site-specific point mutations of mitochondrial DNA. Thymidine phosphorylase and uridine phosphorylase double knockout mice recapitulated several features of these patients including thymidine phosphorylase activity deficiency, elevated thymidine and deoxyuridine in tissues, mitochondrial DNA depletion, respiratory chain defects and white matter changes. However, in contrast to patients with this disease, mutant mice showed mitochondrial alterations only in the brain. To test the hypothesis that elevated levels of nucleotides cause unbalanced deoxyribonucleoside triphosphate pools and, in turn, pathogenic mitochondrial DNA instability, we have stressed double knockout mice with exogenous thymidine and deoxyuridine, and assessed clinical, neuroradiological, histological, molecular, and biochemical consequences. Mutant mice treated with exogenous thymidine and deoxyuridine showed reduced survival, body weight, and muscle strength, relative to untreated animals. Moreover, in treated mutants, leukoencephalopathy, a hallmark of the disease, was enhanced and the small intestine showed a reduction of smooth muscle cells and increased fibrosis. Levels of mitochondrial DNA were depleted not only in the brain but also in the small intestine, and deoxyribonucleoside triphosphate imbalance was observed in the brain. The relative proportion, rather than the absolute amount of deoxyribonucleoside triphosphate, was critical for mitochondrial DNA maintenance. Thus, our results demonstrate that stress of exogenous pyrimidine nucleosides enhances the mitochondrial phenotype of our knockout mice. Our mouse studies provide insights into the pathogenic role of thymidine and deoxyuridine imbalance in mitochondrial neurogastrointestinal encephalopathy and an excellent model to study new therapeutic approaches.

Project description:Mitochondrial neurogastrointestinal encephalomyopathy is a rare multisystemic autosomic recessive disorder characterized by: onset typically before the age of 30 years; ptosis; progressive external ophthalmoplegia; gastrointestinal dysmotility; cachexia; peripheral neuropathy; and leucoencephalopathy. The disease is caused by mutations in the TYMP gene encoding thymidine phosphorylasethymine phosphorylase. Anecdotal reports suggest that allogeneic haematopoetic stem cell transplantation may be beneficial for mitochondrial neurogastrointestinal encephalomyopathy, but is associated with a high mortality. After selecting patients who fulfilled the clinical criteria for mitochondrial neurogastrointestinal encephalomyopathy and had severe thymidine phosphorylase deficiency in the buffy coat (<10% of normal activity), we reviewed their medical records and laboratory studies. We identified 102 patients (50 females) with mitochondrial neurogastrointestinal encephalomyopathy and an average age of 32.4 years (range 11-59 years). We found 20 novel TYMP mutations. The average age-at-onset was 17.9 years (range 5 months to 35 years); however, the majority of patients reported the first symptoms before the age of 12 years. The patient distribution suggests a relatively high prevalence in Europeans, while the mutation distribution suggests founder effects for a few mutations, such as c.866A>G in Europe and c.518T>G in the Dominican Republic, that could guide genetic screening in each location. Although the sequence of clinical manifestations in the disease varied, half of the patients initially had gastrointestinal symptoms. We confirmed anecdotal reports of intra- and inter-familial clinical variability and absence of genotype-phenotype correlation in the disease, suggesting genetic modifiers, environmental factors or both contribute to disease manifestations. Acute medical events such as infections often provoked worsening of symptoms, suggesting that careful monitoring and early treatment of intercurrent illnesses may be beneficial. We observed endocrine/exocrine pancreatic insufficiency, which had not previously been reported. Kaplan-Meier analysis revealed significant mortality between the ages of 20 and 40 years due to infectious or metabolic complications. Despite increasing awareness of this illness, a high proportion of patients had been misdiagnosed. Early and accurate diagnosis of mitochondrial neurogastrointestinal encephalomyopathy, together with timely treatment of acute intercurrent illnesses, may retard disease progression and increase the number of patients eligible for allogeneic haematopoetic stem cell transplantation.

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Mitochondrial neurogastrointestinal encephalopathy due to mutations in RRM2B.

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Mitochondrial neurogastrointestinal encephalopathy due to mutations in RRM2B.

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