RESUMEN
To date, only few mutations in the mitochondrial DNA (mtDNA)-encoded ND2 subunit of Complex I have been reported, usually presenting a severe phenotype characterized by early onset encephalomyopathy and early death. In this report, we describe a new mutation in the MTND2 gene in a 21-year-old man with a mild myopathic phenotype characterized by exercise intolerance and increased plasma lactate at rest. Electromyography and brain NMR were normal, and no cardiac involvement was present. Muscle biopsy showed a massive presence of ragged red - COX-positive fibres, with enlarged mitochondria containing osmiophilic inclusions. Biochemical assays revealed a severe isolated complex I deficiency. We identified a novel, heteroplasmic mutation m.4831G > A in the MTND2 gene, causing the p.Gly121Asp substitution in the ND2 protein. The mutation was present in the 95% of mitochondrial genomes from patient's muscle tissue, at a lower level in cells from the urinary tract and at a lowest level in lymphocytes from patient's blood; the base substitution was absent in fibroblasts and in the tissues from proband's healthy mother and brother. The specific skeletal muscle tissue involvement can explain the childhood-onset and the relatively benign, exclusively myopathic course of the disease.
RESUMEN
Chronic progressive external ophthalmoplegia (CPEO) is common in mitochondrial disorders and is frequently associated with multiple mtDNA deletions. The onset is typically in adulthood, and affected subjects can also present with general muscle weakness. The underlying genetic defects comprise autosomal-dominant or recessive mutations in several nuclear genes, most of which play a role in mtDNA replication. Next-generation sequencing led to the identification of compound-heterozygous RNASEH1 mutations in two singleton subjects and a homozygous mutation in four siblings. RNASEH1, encoding ribonuclease H1 (RNase H1), is an endonuclease that is present in both the nucleus and mitochondria and digests the RNA component of RNA-DNA hybrids. Unlike mitochondria, the nucleus harbors a second ribonuclease (RNase H2). All affected individuals first presented with CPEO and exercise intolerance in their twenties, and these were followed by muscle weakness, dysphagia, and spino-cerebellar signs with impaired gait coordination, dysmetria, and dysarthria. Ragged-red and cytochrome c oxidase (COX)-negative fibers, together with impaired activity of various mitochondrial respiratory chain complexes, were observed in muscle biopsies of affected subjects. Western blot analysis showed the virtual absence of RNase H1 in total lysate from mutant fibroblasts. By an in vitro assay, we demonstrated that altered RNase H1 has a reduced capability to remove the RNA from RNA-DNA hybrids, confirming their pathogenic role. Given that an increasing amount of evidence indicates the presence of RNA primers during mtDNA replication, this result might also explain the accumulation of mtDNA deletions and underscores the importance of RNase H1 for mtDNA maintenance.
Asunto(s)
Replicación del ADN/genética , ADN Mitocondrial/fisiología , Encefalomiopatías Mitocondriales/genética , Oftalmoplejía Externa Progresiva Crónica/genética , ARN/metabolismo , Ribonucleasa H/genética , Adulto , Secuencia de Aminoácidos , Secuencia de Bases , Southern Blotting , Western Blotting , ADN Mitocondrial/genética , Femenino , Secuenciación de Nucleótidos de Alto Rendimiento , Humanos , Masculino , Persona de Mediana Edad , Encefalomiopatías Mitocondriales/patología , Datos de Secuencia Molecular , Mutación/genética , Oftalmoplejía Externa Progresiva Crónica/patología , LinajeRESUMEN
YARS2 encodes the mitochondrial tyrosyl-tRNA synthetase that catalyzes the covalent binding of tyrosine to its cognate mt-tRNA. Mutations in YARS2 have been identified in patients with myopathy, lactic acidosis, and sideroblastic anemia type 2 (MLASA2). We report here on two siblings with a novel mutation and a review of literature. The older patient presented at 2 months with marked anemia and lactic acidemia. He required periodic blood transfusions until 14 months of age. Cognitive and motor development was normal. His younger sister was diagnosed at birth, presenting with anemia and lactic acidosis at 1 month of age requiring periodical transfusions. She is now 14 months old and doing well. For both our patients, there was no clinical evidence of muscle involvement. We found a new homozygous mutation in YARS2, located in the α-anticodon-binding (αACB) domain, involved in the interaction with the anticodon of the cognate mt-tRNA(Tyr).Our study confirms that MLASA must be considered in patients with congenital sideroblastic anemia and underlines the importance of early diagnosis and supportive therapy in order to prevent severe complications. Clinical severity is variable among YARS2-reported patients: our review of the literature suggests a possible phenotype-genotype correlation, although this should be confirmed in a larger population.
RESUMEN
Respiratory chain disorders (RCDs) have been included in the differential diagnosis of adult-onset leukodystrophies. Here, we first report a 32-year-old female with an atypical, adult-onset, non-syndromic RCD due to a mitochondrial DNA deletion and manifesting as complicated ataxia. A 'leukodystrophic' pattern was found on brain MRI, but it was neither isolated nor predominant because of the presence of overt basal ganglia and infratentorial lesions, which led us to the proper diagnosis. Subsequently, we evaluated our series of patients with RCDs in order to verify whether a 'leukodystrophic' pattern with little or no involvement of deep grey structures and brainstem may be found in adult-onset RCDs, as reported in children. Among 52 patients with adult-onset RCDs, no case with a 'leukodystrophic' pattern was found, apart from three cases with a classical phenotype of mitochondrial neurogastrointestinal encephalopathy. In addition, no case of RCDs was found among six cases of adult-onset leukodystrophy of unknown origin and at least one feature suggestive of mitochondrial disease. The review of the literature was in agreement with these findings. Thus, we provide evidence that, unlike in children, RCDs should not be included in the differential diagnosis of adult-onset leukodystrophies, except when there are additional MRI findings or clinical features which unequivocally point towards a mitochondrial disorder.
Asunto(s)
Encéfalo/patología , Leucodistrofia Metacromática/diagnóstico , Enfermedades Mitocondriales/diagnóstico , Adulto , Edad de Inicio , Diagnóstico Diferencial , Femenino , Humanos , Imagen por Resonancia MagnéticaRESUMEN
Mitochondrial disorders with multiple mitochondrial respiratory chain (MRC) enzyme deficiency and depletion of mitochondrial DNA (mtDNA) are autosomal recessive conditions due to mutations in several nuclear genes necessary for proper mtDNA maintenance. In this report, we describe two Italian siblings presenting with encephalomyopathy and mtDNA depletion in muscle. By whole exome-sequencing and prioritization of candidate genes, we identified a novel homozygous missense mutation in the SUCLA2 gene in a highly conserved aminoacid residue. Although a recurrent mutation in the SUCLA2 gene is relatively frequent in the Faroe Islands, mutations in other populations are extremely rare. In contrast with what has been reported in other patients, methyl-malonic aciduria, a biomarker for this genetic defect, was absent in our proband and very mildly elevated in her affected sister. This report demonstrates that next-generation technologies, particularly exome-sequencing, are user friendly, powerful means for the identification of disease genes in genetically and clinically heterogeneous inherited conditions, such as mitochondrial disorders.
Asunto(s)
ADN Mitocondrial/genética , Exoma , Encefalomiopatías Mitocondriales/genética , Músculo Esquelético/enzimología , Mutación Missense , Succinato-CoA Ligasas/genética , Adolescente , Secuencia de Aminoácidos , Niño , Femenino , Homocigoto , Humanos , Encefalomiopatías Mitocondriales/enzimología , Encefalomiopatías Mitocondriales/patología , Datos de Secuencia Molecular , Músculo Esquelético/patología , Linaje , Alineación de Secuencia , Análisis de Secuencia de ADN , HermanosRESUMEN
We report a 35-year-old woman presenting a stroke-like episode with transitory aphasia followed by generalized tonic-clonic seizures. She had severe hearing loss and suffered from frequent episodes of migraine. Although a brain MRI disclosed a T2-hyperintense lesion in the left parietal lobe, she had hardly any long-term sequela. Exercise intolerance, myalgias and limb-girdle muscle weakness indicated a slowly progressive myopathy. Extra-neurological features included short stature, and secondary amenorrhea with low gonadotropin levels, indicating secondary hypogonadism. However, she had three mutation-free, healthy children by ovarian stimulation. A muscle biopsy showed ragged-red, cytochrome c oxidase-negative fibers, and an isolated defect of cytochrome c oxidase activity in muscle mitochondria. Sequence analysis of muscle mtDNA revealed a previously unreported heteroplasmic m.6597C>A transversion in the MTCOI gene, encoding subunit I of cytochrome c oxidase, corresponding to p.Q232K aminoacid change. Analysis on transmitochondrial cybrids demonstrated that the mutation is indeed associated with COX deficiency, i.e. pathogenic.
Asunto(s)
ADN Mitocondrial/genética , Complejo IV de Transporte de Electrones/genética , Síndrome MELAS/genética , Enfermedades Musculares/genética , Mutación , Mutación Puntual/genética , Deficiencia de Citocromo-c Oxidasa/genética , Progresión de la Enfermedad , Femenino , Humanos , Síndrome MELAS/diagnóstico , Síndrome MELAS/patología , Imagen por Resonancia Magnética/métodos , Enfermedades Musculares/patología , Linaje , Subunidades de Proteína/genética , Convulsiones/genética , Convulsiones/patologíaRESUMEN
MELAS, MERRF, LHON and NARP, are well-established mitochondrial syndromes associated with specific point mutations of mitochondrial DNA (mtDNA). However, these recurrent mtDNA mutations account for only a minority of mitochondrial disease cases. To evaluate the impact of novel mtDNA mutations, we performed mtDNA sequence analysis in muscle and other tissues of 240 patients with different mitochondrial neuromuscular syndromes. We identified a total of 33 subjects with novel, private or uncommon mutations. Among these, five novel mutations were found in both paediatric and adult cases. We here report on the clinical description of these patients, as well as the biochemical and molecular genetic characterization of the corresponding mutations. Patients 1 and 2 showed changes in ND genes, patient 3 carried a heteroplasmic deletion in the COI gene, patients 4 and 5 carried heteroplasmic mutations in tRNA(Trp) and tRNA(Phe), respectively. Altogether, these data indicate that mtDNA analysis must become part of the routine screening for mitochondrial disorders.
Asunto(s)
ADN Mitocondrial/genética , Encefalomiopatías Mitocondriales/genética , Mutación , Adulto , Encéfalo/patología , Niño , ADN/genética , ADN/aislamiento & purificación , Cartilla de ADN , Complejo IV de Transporte de Electrones/genética , Humanos , Imagen por Resonancia Magnética , Mitocondrias Musculares/genética , Mitocondrias Musculares/patología , Encefalomiopatías Mitocondriales/enzimología , Encefalomiopatías Mitocondriales/patología , Músculo Esquelético/patología , NADH Deshidrogenasa/genética , Reacción en Cadena de la Polimerasa , Polimorfismo de Longitud del Fragmento de Restricción , Succinato Deshidrogenasa/genéticaRESUMEN
Autosomal dominant optic atrophy (ADOA), the commonest cause of inherited optic atrophy, is caused by mutations in the ubiquitously expressed gene optic atrophy 1 (OPA1), involved in fusion and biogenesis of the inner membrane of mitochondria. Bioenergetic failure, mitochondrial network abnormalities and increased apoptosis have all been proposed as possible causal factors. However, their relative contribution to pathogenesis as well as the prominent susceptibility of the retinal ganglion cell (RGC) in this disease remains uncertain. Here we identify a novel deletion of OPA1 gene in the GTPase domain in three patients affected by ADOA. Muscle biopsy of the patients showed neurogenic atrophy and abnormal morphology and distribution of mitochondria. Confocal microscopy revealed increased mitochondrial fragmentation in fibroblasts as well as in myotubes, where mitochondria were also unevenly distributed, with clustered organelles alternating with areas where mitochondria were sparse. These abnormalities were not associated with altered bioenergetics or increased susceptibility to pro-apoptotic stimuli. Therefore, changes in mitochondrial shape and distribution can be independent of other reported effects of OPA1 mutations, and therefore may be the primary cause of the disease. The arrangement of mitochondria in RGCs, which degenerate in ADOA, may be exquisitely sensitive to disturbance, and this may lead to bioenergetic crisis and/or induction of apoptosis. Our results highlight the importance of mitochondrial dynamics in the disease per se, and point to the loss of the fine positioning of mitochondria in the axons of RGCs as a possible explanation for their predominant degeneration in ADOA.
Asunto(s)
GTP Fosfohidrolasas/genética , Mitocondrias/metabolismo , Atrofia Óptica Autosómica Dominante/genética , Adolescente , Adulto , Apoptosis , Células Cultivadas , Niño , Metabolismo Energético , Femenino , GTP Fosfohidrolasas/metabolismo , Regulación Enzimológica de la Expresión Génica , Humanos , Masculino , Persona de Mediana Edad , Mitocondrias/patología , Músculo Esquelético/anomalías , Músculo Esquelético/enzimología , Atrofia Óptica Autosómica Dominante/fisiopatología , Linaje , Especies Reactivas de Oxígeno/metabolismo , Retina/patología , Eliminación de Secuencia , Adulto JovenRESUMEN
Different mutations, or combinations of mutations, in POLG1, the gene encoding pol gammaA, the catalytic subunit of mitochondrial DNA polymerase, are associated with a spectrum of clinical presentations including autosomal dominant or recessive progressive external ophthalmoplegia (PEO), juvenile-onset ataxia and epilepsy, and Alpers-Huttenlocher syndrome. Parkinsonian features have been reported as a late complication of POLG1-associated dominant PEO. Good response to levodopa or dopamine agonists, reduced dopamine uptake in the corpus striatum and neuronal loss of the Substantia Nigra pars compacta have been documented in a few cases. Here we report two novel mutations in POLG1 in a compound heterozygous patient with autosomal recessive PEO, followed by pseudo-orthostatic tremor evolving into levodopa-responsive parkinsonism. These observations support the hypothesis that mtDNA dysfunction is engaged in the pathogenesis of idiopathic Parkinson's disease.
Asunto(s)
ADN Polimerasa Dirigida por ADN/genética , Mutación/genética , Oftalmoplejía Externa Progresiva Crónica/genética , Trastornos Parkinsonianos/genética , Temblor/genética , Análisis Mutacional de ADN , ADN Polimerasa gamma , ADN Mitocondrial/genética , Humanos , Masculino , Persona de Mediana Edad , Oftalmoplejía Externa Progresiva Crónica/complicaciones , Trastornos Parkinsonianos/complicaciones , Análisis de Secuencia de Proteína , Temblor/complicacionesRESUMEN
The mitochondrial DNA A3243G mutation causes neuromuscular disease. To investigate the muscle-specific pathophysiology of mitochondrial disease, rhabdomyosarcoma transmitochondrial hybrid cells (cybrids) were generated that retain the capacity to differentiate to myotubes. In some cases, striated muscle-like fibres were formed after innervation with rat embryonic spinal cord. Myotubes carrying A3243G mtDNA produced more reactive oxygen species than controls, and had altered glutathione homeostasis. Moreover, A3243G mutant myotubes showed evidence of abnormal mitochondrial distribution, which was associated with down-regulation of three genes involved in mitochondrial morphology, Mfn1, Mfn2 and DRP1. Electron microscopy revealed mitochondria with ultrastructural abnormalities and paracrystalline inclusions. All these features were ameliorated by anti-oxidant treatment, with the exception of the paracrystalline inclusions. These data suggest that rhabdomyosarcoma cybrids are a valid cellular model for studying muscle-specific features of mitochondrial disease and that excess reactive oxygen species production is a significant contributor to mitochondrial dysfunction, which is amenable to anti-oxidant therapy.
Asunto(s)
Antioxidantes/farmacología , Miopatías Mitocondriales/metabolismo , Células Musculares/ultraestructura , Adulto , Animales , Diferenciación Celular , Células Cultivadas , ADN Mitocondrial/genética , Regulación de la Expresión Génica , Humanos , Masculino , Microscopía Electrónica , Mitocondrias Musculares/efectos de los fármacos , Mitocondrias Musculares/ultraestructura , Miopatías Mitocondriales/genética , Miopatías Mitocondriales/patología , Células Musculares/efectos de los fármacos , Fibras Musculares Esqueléticas/metabolismo , Oxidación-Reducción , Ratas , Ratas Sprague-Dawley , Especies Reactivas de Oxígeno/metabolismo , Células Tumorales CultivadasRESUMEN
Isolated Complex I (CI) deficiency, the most frequent cause of mitochondrial disease, is a clinically and genetically heterogeneous condition. Complex I is a giant multiheteromeric enzyme composed of seven ND subunits encoded by mitochondrial DNA (mtDNA) genes, and at least 38 subunits encoded by nuclear genes. To establish the contribution to human mitochondrial encephalopathy of ND versus nuclear gene mutations, we have been undertaking a systematic analysis of CI genes in a cohort of 46 adult and paediatric patients with biochemically defined CI defect. Sequence analysis of the entire mtDNA let us identify six patients with mutations in ND genes. The clinical presentations varied, from infantile Leigh syndrome, to childhood MELAS, to adult-onset encephalopathic syndromes of variable severity. Three of the mutations were not previously reported (3481G > A, 14600G > A and 13063G > A, in ND1, ND6 and ND5 genes, respectively) and were further investigated in mutant transmitochondrial cybrids. Tight correlation between mutation load and decrease in CI activity was observed in each of the three mutant cybrid lines, supporting the pathogenic role of the novel mutations. Structural studies on mutant cybrids showed impaired assembly or reduced stability of the holoenzyme complex. In our experience ND gene mutations are relatively common in CI-defective mitochondrial encephalopathy of both children and adults.
Asunto(s)
Complejo I de Transporte de Electrón/genética , Encefalomiopatías Mitocondriales/genética , Mutación , NADH Deshidrogenasa/genética , Adolescente , Adulto , Animales , Secuencia de Bases , Niño , Análisis Mutacional de ADN/métodos , ADN Mitocondrial/genética , Complejo I de Transporte de Electrón/deficiencia , Electroforesis en Gel Bidimensional/métodos , Femenino , Humanos , Lactante , Imagen por Resonancia Magnética/métodos , Masculino , Encefalomiopatías Mitocondriales/enzimología , Encefalomiopatías Mitocondriales/patología , Linaje , Polimorfismo de Longitud del Fragmento de Restricción , Especificidad de la EspecieRESUMEN
We investigated two unrelated children with an isolated defect of mitochondrial complex III activity. The clinical picture was characterized by a progressive encephalopathy featuring early-onset developmental delay, spasticity, seizures, lactic acidosis, brain atrophy and MRI signal changes in the basal ganglia. Both children were compound heterozygotes for novel mutations in the human bc1 synthesis like (BCS1L) gene, which encodes an AAA mitochondrial protein putatively involved in both iron homeostasis and complex III assembly. The pathogenic role of the mutations was confirmed by complementation assays, using a DeltaBcs1 strain of Saccharomyces cerevisiae. By investigating complex III assembly and the structural features of the BCS1L gene product in skeletal muscle, cultured fibroblasts and lymphoblastoid cell lines from our patients, we have demonstrated, for the first time in a mammalian system, that a major function of BCS1L is to promote the maturation of complex III and, more specifically, the incorporation of the Rieske iron-sulfur protein into the nascent complex. Defective BCS1L leads to the formation of a catalytically inactive, structurally unstable complex III. We have also shown that BCS1L is contained within a high-molecular-weight supramolecular complex which is clearly distinct from complex III intermediates.
Asunto(s)
Encefalopatías Metabólicas Innatas/genética , Encefalopatías Metabólicas Innatas/metabolismo , Complejo III de Transporte de Electrones/metabolismo , Enfermedades Mitocondriales/genética , Enfermedades Mitocondriales/metabolismo , Mutación , ATPasas Asociadas con Actividades Celulares Diversas , Secuencia de Aminoácidos , Secuencia de Bases , Encéfalo/patología , Encefalopatías Metabólicas Innatas/patología , Preescolar , ADN Complementario/genética , Complejo III de Transporte de Electrones/química , Complejo III de Transporte de Electrones/genética , Femenino , Prueba de Complementación Genética , Heterocigoto , Humanos , Proteínas Hierro-Azufre/química , Proteínas Hierro-Azufre/metabolismo , Imagen por Resonancia Magnética , Enfermedades Mitocondriales/patología , Datos de Secuencia Molecular , Complejos Multiproteicos , Mutagénesis Sitio-Dirigida , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Homología de Secuencia de AminoácidoRESUMEN
The mitochondrial (mt) DNA depletion syndromes (MDDS) are genetic disorders characterized by a severe, tissue-specific decrease of mtDNA copy number, leading to organ failure. There are two main clinical presentations: myopathic (OMIM 609560) and hepatocerebral (OMIM 251880). Known mutant genes, including TK2, SUCLA2, DGUOK and POLG, account for only a fraction of MDDS cases. We found a new locus for hepatocerebral MDDS on chromosome 2p21-23 and prioritized the genes on this locus using a new integrative genomics strategy. One of the top-scoring candidates was the human ortholog of the mouse kidney disease gene Mpv17. We found disease-segregating mutations in three families with hepatocerebral MDDS and demonstrated that, contrary to the alleged peroxisomal localization of the MPV17 gene product, MPV17 is a mitochondrial inner membrane protein, and its absence or malfunction causes oxidative phosphorylation (OXPHOS) failure and mtDNA depletion, not only in affected individuals but also in Mpv17-/- mice.
Asunto(s)
ADN Mitocondrial/genética , Membranas Intracelulares/metabolismo , Hepatopatías/genética , Proteínas de la Membrana/genética , Mitocondrias/metabolismo , Mutación , Secuencia de Aminoácidos , Animales , Células Cultivadas , Cromosomas Humanos Par 2 , Clonación Molecular , Femenino , Técnica del Anticuerpo Fluorescente , Humanos , Masculino , Proteínas de la Membrana/química , Ratones , Datos de Secuencia Molecular , Linaje , SíndromeRESUMEN
The mitochondrial DNA (mtDNA) of 87 index cases with Leber hereditary optic neuropathy (LHON) sequentially diagnosed in Italy, including an extremely large Brazilian family of Italian maternal ancestry, was evaluated in detail. Only seven pairs and three triplets of identical haplotypes were observed, attesting that the large majority of the LHON mutations were due to independent mutational events. Assignment of the mutational events into haplogroups confirmed that J1 and J2 play a role in LHON expression but narrowed the association to the subclades J1c and J2b, thus suggesting that two specific combinations of amino acid changes in the cytochrome b are the cause of the mtDNA background effect and that this may occur at the level of the supercomplex formed by respiratory-chain complexes I and III. The families with identical haplotypes were genealogically reinvestigated, which led to the reconnection into extended pedigrees of three pairs of families, including the Brazilian family with its Italian counterpart. The sequencing of entire mtDNA samples from the reconnected families confirmed the genealogical reconstruction but showed that the Brazilian family was heteroplasmic at two control-region positions. The survey of the two sites in 12 of the Brazilian subjects revealed triplasmy in most cases, but there was no evidence of the tetraplasmy that would be expected in the case of mtDNA recombination.
Asunto(s)
ADN Mitocondrial/genética , Haplotipos , Atrofia Óptica Hereditaria de Leber/genética , Recombinación Genética , Femenino , Humanos , Masculino , Datos de Secuencia Molecular , LinajeRESUMEN
Myoclonus epilepsy with ragged red fibers (MERRF) is one of the major mitochondrial encephalomyopathies. Its main clinical features are myoclonus epilepsy, ataxia, and myopathy with ragged red fibers. Whereas there is a close correlation between MERRF syndrome and the A8344G mutation of mitochondrial DNA, the reverse is not true. In fact, this mutation is also responsible for various other syndromes, such as Leigh syndrome, spinocerebellar degeneration, atypical Charcot-Marie-Tooth disease, and multiple truncal lipomas. We describe a child with the A8344G mutation of mitochondrial DNA and an unusual clinical, neuroradiologic, and biochemical phenotype, characterized by early-onset, nonprogressive cerebellar ataxia, and subclinical myoclonias in association with bilateral putaminal necrosis on magnetic resonance imaging and a reduction in complex V activity. Our case confirms the existence of a relationship between alteration in adenosine triphosphatase activity and basal ganglia involvement. We recommend that the possibility of a mitochondrial pathology should always be taken into consideration in the presence of bilateral symmetric lesions of the basal ganglia, even when the typical clinical picture is lacking. (J Child Neurol 2006;21:79-82).
Asunto(s)
ADN Mitocondrial , Síndrome MERRF/complicaciones , Síndrome MERRF/genética , Mutación Puntual/genética , Putamen/patología , Niño , Análisis Mutacional de ADN , Diagnóstico Diferencial , Electroencefalografía , Lateralidad Funcional , Humanos , Síndrome MERRF/diagnóstico , Imagen por Resonancia Magnética , Masculino , Necrosis/etiologíaRESUMEN
Mitochondrial DNA (mtDNA) mutations are a major cause of human disease. A large number of different molecular defects ultimately compromise oxidative phosphorylation, but it is not clear why the same biochemical defect can cause diverse clinical phenotypes. There is emerging evidence that nuclear genes modulate the phenotype of primary mtDNA disorders. Here, we define an X-chromosomal haplotype that interacts with specific MTND mutations to cause visual failure in the most common mtDNA disease, Leber hereditary optic neuropathy. This effect is independent of the mtDNA genetic background and explains the variable penetrance and sex bias that characterizes this disorder.
Asunto(s)
Cromosomas Humanos X , ADN Mitocondrial/genética , Haplotipos , Enfermedades Mitocondriales/epidemiología , Enfermedades Mitocondriales/genética , Fenotipo , Alelos , Mapeo Cromosómico , Estudios de Cohortes , Femenino , Frecuencia de los Genes , Marcadores Genéticos , Heterocigoto , Homocigoto , Humanos , Modelos Lineales , Desequilibrio de Ligamiento , Masculino , Repeticiones de Microsatélite , Atrofia Óptica Hereditaria de Leber/epidemiología , Atrofia Óptica Hereditaria de Leber/genética , Linaje , Penetrancia , Mutación Puntual , Polimorfismo de Longitud del Fragmento de Restricción , Factores Sexuales , Estadísticas no ParamétricasRESUMEN
Mutations and instability of mitochondrial DNA (mtDNA) are frequent in tumors but their pathogenic relevance is not established. To assess their role in the clinical management of malignant gliomas we have studied the D loop of mtDNA in 42 such tumors. Alterations were found in 36% of the cases. The MRI and the clinical follow-up of these patients suggest that these mutations are not associated with increased aggressiveness. mtDNA could be amplified from post-surgical tumor cavities in patients undergoing a loco-regional treatment. These results imply that mtDNA mutations are unlikely to play a role in diagnostic or prognostic evaluations of gliomas: their detection, however, could be of use for the clinical follow-up of malignant gliomas.
Asunto(s)
Biomarcadores de Tumor/genética , Neoplasias Encefálicas/genética , ADN Mitocondrial/genética , Glioma/genética , Secuencia de Bases , Neoplasias Encefálicas/mortalidad , Neoplasias Encefálicas/patología , Inestabilidad Genómica , Glioma/mortalidad , Glioma/patología , Humanos , Datos de Secuencia Molecular , Reacción en Cadena de la Polimerasa , Polimorfismo Genético , Pronóstico , Análisis de SupervivenciaRESUMEN
We studied nine infant patients with a combination of progressive neurological and hepatic failure. Eight children, including two sibling pairs and four singletons, were affected by Alpers' hepatopathic poliodystrophy. A ninth baby patient suffered of a severe floppy infant syndrome associated with liver failure. Analysis of POLG1, the gene encoding the catalytic subunit of mitochondrial DNA polymerase, revealed that all the patients carried different allelic mutations in this gene. POLG1 is a major disease gene in mitochondrial disorders. Mutations in this gene can be associated with multiple deletions, depletion or point mutations of mitochondrial DNA (mtDNA). In turn, these different molecular phenotypes dictate an extremely heterogeneous spectrum of clinical outcomes, ranging from adult-onset progressive ophthalmoplegia to juvenile ataxic syndromes with epilepsy, to rapidly fatal hepatocerebral presentations, including Alpers' syndrome.
Asunto(s)
ADN Mitocondrial/genética , ADN Polimerasa Dirigida por ADN/genética , Esclerosis Cerebral Difusa de Schilder/genética , Fallo Hepático/genética , Mutación , Encéfalo/patología , ADN Polimerasa gamma , Esclerosis Cerebral Difusa de Schilder/patología , Progresión de la Enfermedad , Resultado Fatal , Femenino , Humanos , Lactante , Imagen por Resonancia Magnética , MasculinoRESUMEN
Thymidine kinase 2 (TK2) and deoxyguanosine kinase (dGK) are the two key enzymes in mitochondrial DNA (mtDNA) precursor synthesis. Deficiencies in TK2 or dGK activity, due to genetic alteration, have been shown to cause tissue-specific depletion of mtDNA. In the case of TK2 deficiency, affected individuals suffer severe myopathy and, in the case of dGK deficiency, devastating liver or multi-systemic disease. Here, we report clinical and biochemical findings from two patients with mtDNA depletion syndrome. Patient A was a compound heterozygote carrying the previously reported T77M mutation and a novel mutation (R161K) in the TK2 gene. Patient B carried a novel mutation (L250S) in the dGK gene. The clinical symptoms of patient A included muscular weakness and exercise intolerance due to a severe mitochondrial myopathy associated with a 92% reduction in mtDNA. There was minimal involvement of other organs. Patient B suffered from rapidly progressive, early onset fatal liver failure associated with profoundly decreased mtDNA levels in liver and, to a lesser extent, in skeletal muscle. Site-directed mutagenesis was used to introduce the mutations detected in patients A and B into the TK2 and dGK cDNAs, respectively. We then characterized each of these recombinant enzymes. Catalytic activities of the three mutant enzymes were reduced to about 2-4% for TK2 and 0.5% for dGK as compared to the wild-type enzymes. Altered competition between dCyd and dThd was observed for the T77M mutant. The residual activities of the two mitochondrial enzymes correlated directly with disease development.
Asunto(s)
ADN Mitocondrial/metabolismo , Miopatías Mitocondriales/genética , Músculo Esquelético/patología , Mutación/genética , Fosfotransferasas (Aceptor de Grupo Alcohol)/genética , Timidina Quinasa/genética , Preescolar , Cromatografía en Gel , Cartilla de ADN , ADN Complementario/genética , ADN Mitocondrial/genética , Resultado Fatal , Femenino , Humanos , Recién Nacido , Hígado/patología , Masculino , Miopatías Mitocondriales/patología , Mutagénesis Sitio-Dirigida , Fosfotransferasas (Aceptor de Grupo Alcohol)/metabolismo , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Timidina Quinasa/metabolismoRESUMEN
We describe a second patient carrying the 5698G-->A transition in the mitochondrial DNA gene encoding tRNA(Asn), who has an apparently isolated mitochondrial myopathy with chronic progressive external ophthalmoplegia. A muscle biopsy showed the presence of ragged-red and COX-negative fibres. Analysis of the mutation load on single muscle fibres showed significant segregation of the 5698G-->A with COX-depleted fibres. These results indicate that the 5698G-->A is pathogenic.