Mitochondrial DNA Point Mutations Research Articles

P4.63 MELAS presenting marked brain atrophy and hypertrophic cardiomyopathy: report of an autopsy case

MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) is a maternally inherited multisystem mitochondrial disorder. The common cause of MELAS is a point mutation of mitochondrial DNA. The brain shows various profiles of infarct-like lesions, whose pathogenesis is still unclear, although the involvement of defective mitochondria in neurons and/or in vascular wall cells (mitochondrial angiopathy) is inferred.

Autonomic Symptoms in Carriers of the m.3243A>G Mitochondrial DNA Mutation

The m.3243A>G mutation can cause multisystem medical problems and can affect the autonomic nervous system. To study the frequency and spectrum of autonomic symptoms associated with the m.3243A>G mitochondrial DNA point mutation. Design, Setting, and Patients We studied a cohort of 88 matrilineal relatives from 40 families, including 35 fully symptomatic patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS), 53 carrier relatives, and 16 controls using a questionnaire based on existing standard instruments for the evaluation of autonomic dysfunction. We compared the questionnaire with an expert evaluation. We compared data among the 3 groups using the Mantel-Haenszel chi(2) test to determine the statistical significance of differences between groups. Mutation carriers frequently had symptoms of autonomic dysfunction, specifically gastrointestinal and orthostatic intolerance. Carriers of the m.3243A>G mutation have frequent autonomic symptoms. The m.3243A>G mutation should be considered as an etiological factor in patients with autonomic dysfunction and a medical or family history suggestive of mitochondrial disease. Because some autonomic symptoms are treatable, early detection and proactive management may mitigate the burden of morbidity.

Disruption of Endothelial Tight Junctions in a Patient with Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-Like Episodes (MELAS)

An electron microscopic study revealed disruption of capillary endothelial tight junctions (TJs) in both biopsied muscle, taken at 5 years and 1 month of age, and the autopsied brain, taken at 13 years and 6 months of age, in a patient with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) and mitochondrial DNA (mtDNA) point mutation A3243G. This endothelial barrier disruption might result in vasogenic edema and systemic lactic acidosis, possibly the critical pathology of MELAS.

Successful Amelioration of Mitochondrial Optic Neuropathy Using the Yeast NDI1 Gene in a Rat Animal Model

BackgroundLeber's hereditary optic neuropathy (LHON) is a maternally inherited disorder with point mutations in mitochondrial DNA which result in loss of vision in young adults. The majority of mutations reported to date are within the genes encoding the subunits of the mitochondrial NADH-quinone oxidoreductase, complex I. Establishment of animal models of LHON should help elucidate mechanism of the disease and could be utilized for possible development of therapeutic strategies.Methodology/Principal FindingsWe established a rat model which involves injection of rotenone-loaded microspheres into the optic layer of the rat superior colliculus. The animals exhibited the most common features of LHON. Visual loss was observed within 2 weeks of rotenone administration with no apparent effect on retinal ganglion cells. Death of retinal ganglion cells occurred at a later stage. Using our rat model, we investigated the effect of the yeast alternative NADH dehydrogenase, Ndi1. We were able to achieve efficient expression of the Ndi1 protein in the mitochondria of all regions of retinal ganglion cells and axons by delivering the NDI1 gene into the optical layer of the superior colliculus. Remarkably, even after the vision of the rats was severely impaired, treatment of the animals with the NDI1 gene led to a complete restoration of the vision to the normal level. Control groups that received either empty vector or the GFP gene had no effects.Conclusions/SignificanceThe present study reports successful manifestation of LHON-like symptoms in rats and demonstrates the potential of the NDI1 gene therapy on mitochondrial optic neuropathies. Our results indicate a window of opportunity for the gene therapy to be applied successfully after the onset of the disease symptoms.

ALS spinal neurons show varied and reduced mtDNA gene copy numbers and increased mtDNA gene deletions

BackgroundSpinal cord neurons of ALS patients demonstrate reduced cytochrome oxidase histochemical activity, and ALS spinal cord tissues have increased mitochondrial DNA (mtDNA) point mutations and depleted mtDNA levels. It is presently unknown whether mtDNA abnormalities are present in single human ALS neurons.ResultsUsing laser capture microdissection (LCM) we isolated several hundred individual anterior spinal neurons from unfixed, frozen sections of 10 ALS and 7 age-matched CTL cervical spinal cords. DNA from each individual neuron was analyzed with multiplex qPCR for ND2, CO3, and ND4, three mitochondrial DNA genes encoding respiratory proteins. Scatterplots of individual spinal neuron results showed extensive heterogeneity of mtDNA gene levels across 4-5 orders of magnitude that were much more clustered in single Purkinje neurons isolated from CTL cerebella. Plots of ratios of ND4/ND2 and CO3/ND2 showed that many but not all ALS neurons from individuals contained low ratios of these mtDNA genes, implying greater abundances of mtDNA deletions in the major arc. Single CTL cerebellar Purkinje neurons did not contain high levels of apparent mtDNA deletions observed in anterior spinal neurons.ConclusionsAt the time of ALS subjects' deaths, many but not all surviving anterior neurons in their cervical spinal cords have reduced mtDNA gene levels and increased mtDNA deletion abundances that arise for unclear reasons. If these anterior spinal neuron mtDNA gene deficiencies contribute to bioenergetic impairments, reduced synaptic function and increased risk of degeneration, then introduction into mitochondria and expression of intact mtDNA, now available through use of recently developed recombinant human TFAM, may reverse the course of ALS.

A second MNGIE patient without typical mitochondrial skeletal muscle involvement

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disease caused by mutations in the gene encoding thymidine phosphorylase (TYMP). Clinically, MNGIE is characterized by gastrointestinal dysmotility, cachexia, ptosis, ophthalmoparesis, peripheral neuropathy and leukoencephalopathy. Most MNGIE patients have signs of mitochondrial dysfunction in skeletal muscle at morphological and enzyme level, as well as mitochondrial DNA depletion, multiple deletions and point mutations. A case without mitochondrial skeletal muscle involvement and with a TYMP splice-acceptor site mutation (c. 215-1 G>C) has been reported. Here, we describe an Italian patient with the same mutation and without mitochondrial skeletal muscle involvement, suggesting a possible genotype-phenotype correlation.

Detection of eight frequently encountered point mutations in mitochondria in Chinese patients suggestive of mitochondrial encephalomyopathies

To evaluate eight frequently encountered mitochondrial DNA (mtDNA) point mutations (A3243G, T8993G/C, A8344G, A1555G, G11778A, G3460A and T14484C) in Chinese, we recruited 1559 sporadic patients suspected of mitochondrial diseases and 206 family members. In suspected patients, 158 cases were detected with one of these eight mtDNA mutations (10.1%). A3243G was the most common mtDNA mutation both in suspected patients (9.4%) and in the relatives (34.2%). In addition, the ratios of A3243G (mutant/wild-type) and A8344G were significantly correlated with the patients’ age of examination. Moreover, in 76 unrelated probands, the ratio of A3243G was correlated well with their seizures and myopathies.

Extensive and rapid screening for major mitochondrial DNA point mutations in patients with hereditary hearing loss

Sensorineural hearing loss (HL) is one of the most frequent clinical features in patients with mitochondrial diseases caused by mitochondrial DNA (mtDNA) mutations, and hearing is impaired in over half of all cases with mitochondrial disorders. This study analyzed 373 patients with suspected hereditary HL using an extensive and rapid suspension-array screening system for 29 major mtDNA mutations, including the m.1555A>G homoplasmic mutation in the MT-RNR1 gene, which causes non-syndromic sensorineural HL and aminoglycoside-induced HL, and the m.3243A>G heteroplasmic mutation in the MT-TL1 gene. This method is rapid and suitable for large-scale screening because universal 96-well plates are available for use, and because an analysis of each plate can be completed within 1 h. This system detected five different mtDNA mutations in 24 of the 373 (6.4%) patients. The m.1555A>G and m.3243A>G mutations were detected in 11 (2.9%) and 9 (2.7%) patients, respectively. In addition, three mutations, that is, m.8348A>G in the MT-TK gene, m.11778G>A in the MT-ND4 gene and 15498G>A in the MT-CYB gene were detected in one patient for each. This screening system is useful for the genetic diagnosis and epidemiological study of both syndromic and non-syndromic HL.

Extensive screening system using suspension array technology to detect mitochondrial DNA point mutations

We established an extensive and rapid system using suspension array to detect 61 representative mitochondrial DNA (mtDNA) heteroplasmic or homoplasmic point mutations (29 for Series A and 32 for Series B) in 22 genes: 1 each in MT-RNR1, -TV, -ND1, -TQ, -TW, -TC, and -TH genes; 2 each in MT-TN, -TG, -ND4, -TL2, -TE, and -CYB genes; 3 each in MT-ATP6, -ND3, and -ND5 genes; 4 each in MT-CO1 and -TK genes; 5 each in MT-TI, -TS1, and -ND6 genes; and 10 in the MT-TL1 gene. We carefully selected 5′-biotinylated primers and pooled primers for use in two sets of multiplex-PCR amplifications. To detect both mutant and wild-type mtDNA, even when polymorphisms were present near the target mutation sites, we designed specific oligonucleotide probes. By using the mtDNA point mutation detection system of Series A (29 mutations) and Series B (32 mutations), we screened a total of 3103 mutant sites in 107 DNA samples for Series A and 13,101 mutant sites in 397 DNA samples for Series B. We succeeded in determining 99.4% (Series A) and 99.6% (Series B) of the targeted mutant sites by use of the system. The 22 samples with the m.3243A>G heteroplasmic mutation revealed positive signals with both mutant- and wild-type-specific probes in this detection system with a detection limit of approximately 2%. This genetic screening platform is useful to reach a definitive diagnosis for mitochondrial diseases.

Stochastic Drift in Mitochondrial DNA Point Mutations: A Novel Perspective Ex Silico

The mitochondrial free radical theory of aging (mFRTA) implicates Reactive Oxygen Species (ROS)-induced mutations of mitochondrial DNA (mtDNA) as a major cause of aging. However, fifty years after its inception, several of its premises are intensely debated. Much of this uncertainty is due to the large range of values in the reported experimental data, for example on oxidative damage and mutational burden in mtDNA. This is in part due to limitations with available measurement technologies. Here we show that sample preparations in some assays necessitating high dilution of DNA (single molecule level) may introduce significant statistical variability. Adding to this complexity is the intrinsically stochastic nature of cellular processes, which manifests in cells from the same tissue harboring varying mutation load. In conjunction, these random elements make the determination of the underlying mutation dynamics extremely challenging. Our in silico stochastic study reveals the effect of coupling the experimental variability and the intrinsic stochasticity of aging process in some of the reported experimental data. We also show that the stochastic nature of a de novo point mutation generated during embryonic development is a major contributor of different mutation burdens in the individuals of mouse population. Analysis of simulation results leads to several new insights on the relevance of mutation stochasticity in the context of dividing tissues and the plausibility of ROS ”vicious cycle” hypothesis.

The Background of Mitochondrial DNA Haplogroup J Increases the Sensitivity of Leber's Hereditary Optic Neuropathy Cells to 2,5-Hexanedione Toxicity

Leber's hereditary optic neuropathy (LHON) is a maternally inherited blinding disease due to mitochondrial DNA (mtDNA) point mutations in complex I subunit genes, whose incomplete penetrance has been attributed to both genetic and environmental factors. Indeed, the mtDNA background defined as haplogroup J is known to increase the penetrance of the 11778/ND4 and 14484/ND6 mutations. Recently it was also documented that the professional exposure to n-hexane might act as an exogenous trigger for LHON. Therefore, we here investigate the effect of the n-hexane neurotoxic metabolite 2,5-hexanedione (2,5-HD) on cell viability and mitochondrial function of different cell models (cybrids and fibroblasts) carrying the LHON mutations on different mtDNA haplogroups. The viability of control and LHON cybrids and fibroblasts, whose mtDNAs were completely sequenced, was assessed using the MTT assay. Mitochondrial ATP synthesis rate driven by complex I substrates was determined with the luciferine/luciferase method. Incubation with 2,5-HD caused the maximal loss of viability in control and LHON cells. The toxic effect of this compound was similar in control cells irrespective of the mtDNA background. On the contrary, sensitivity to 2,5-HD induced cell death was greatly increased in LHON cells carrying the 11778/ND4 or the 14484/ND6 mutation on haplogroup J, whereas the 11778/ND4 mutation in association with haplogroups U and H significantly improved cell survival. The 11778/ND4 mutation on haplogroup U was also more resistant to inhibition of complex I dependent ATP synthesis by 2,5-HD. In conclusion, this study shows that mtDNA haplogroups modulate the response of LHON cells to 2,5-HD. In particular, haplogroup J makes cells more sensitive to its toxic effect. This is the first evidence that an mtDNA background plays a role by interacting with an environmental factor and that 2,5-HD may be a risk element for visual loss in LHON. This proof of principle has broad implications for other neurodegenerative disorders such as Parkinson's disease.

Leber hereditary optic neuropathy: bad habits, bad vision?

The puzzle that Leber hereditary optic neuropathy (LHON) represents has not been completely solved for over 250 years, despite 20 recent years of rapid advances in mitochondrial genetics (Newman, 2005; Yu-Wai-Man et al ., 2009). Although the majority of the underlying causative point mutations in mitochondrial DNA (mtDNA) have been identified, we still cannot answer the most fundamental questions about this disease (Newman, 2002). Why does not everyone who carries a LHON mitochondrial DNA point mutation have visual loss in their lifetime? Why are males affected more often than females? Why does visual loss occur so preferentially during the second and third decades of life, and so infrequently past the age of 50? What accounts for such an abrupt and catastrophic loss of vision, either simultaneously or sequentially, within weeks to months? And, finally, what is so special about the optic nerve, and presumably the retinal ganglion cells, that makes these structures so exclusively sensitive to an abnormality in mitochondrial DNA, when this is present in every cell of the body? Researchers have proposed multiple theories to account for these unusual features of LHON, only very few of which have been proven. Regarding possible genetic/epigenetic factors, the presence of a primary mitochondrial DNA mutation, primarily those at nucleotide positions 11 778, 14 484 or 3460, is necessary but not sufficient for the phenotypic expression of the disorder (Yu-Wai-Man et al ., 2009). Heteroplasmy, presumably in the retinal ganglion cells, may diminish the chances of visual loss, but even homoplasmy cannot of itself account for most cases of LHON (Chinnery et al ., 2001). Certain mtDNA background haplotypes may influence expression (Hudson et al ., 2007), in particular haplotype J for the 11 778 and 14 484 mutations and haplotype K for the 3460 mutation; yet still, the majority of …

The low abundance of clonally expanded mitochondrial DNA point mutations in aged substantia nigra neurons

Clonally expanded mitochondrial DNA (mtDNA) deletions accumulate with age in human substantia nigra (SN) and high levels cause respiratory chain deficiency. In other human tissues, mtDNA point mutations clonally expand with age. Here, the abundance of mtDNA point mutations within single SN neurons from aged controls was investigated. From 31 single cytochrome c oxidase normal SN neurons, only one clonally expanded mtDNA point mutation was identified, suggesting in these neurons mtDNA point mutations occur rarely, whereas mtDNA deletions are frequently observed. This contrasts observations in mitotic tissues and suggests that different forms of mtDNA maintenance may exist in these two cell types.

Mitochondrial Encephalomyopathy Lactic Acidosis and Strokelike Episodes Mimicking Occipital Idiopathic Epilepsy

We report on a 19-year-old man with a 9-year history of occipital seizures characterized by deviation of the eyes and tonic ipsilateral turning of the head during sleep, initially diagnosed as idiopathic childhood occipital epilepsy, Gastaut type. The eventual development of status epilepticus, associated with a T(1) hypointense as well as T(2), fluid-attenuated inversion recovery, and diffusion-weighted hyperintense brain lesion led to pathologic and genetic testing that identified a A3243G mitochondrial DNA point mutation associated with mitochondrial, encephalomyopathy, lactic acidosis, and strokelike episodes. This case emphasizes that occipital epileptic seizures can be the only presenting and long-lasting sign in patients with mitochondrial, encephalomyopathy, lactic acidosis, and strokelike episodes.

Madelung′s disease with myopathy

Madelung′s disease with myopathy

Protean phenotypic features of the A3243G mitochondrial DNA mutation.

To describe the spectrum of clinical symptoms, signs, and laboratory features associated with A3243G, a mitochondrial DNA point mutation that affects multiple organs with varying severity, making the diagnosis and treatment of these patients complex. Cohort study. Columbia University Medical Center. A cohort of 123 matrilineal relatives from 45 families, including 45 fully symptomatic patients with mitochondrial myopathy, encephalomyopathy, lactic acidosis, and stroke-like episodes (syndrome), 78 carrier relatives, and 30 controls. Data gathered from standardized medical history questionnaires, neurological and ophthalmological examination forms, and laboratory tests. We compared data between 3 groups. Mutation carriers' clinical and laboratory results frequently had many abnormalities. In addition to neurological symptoms, they often had cardiac, endocrine, gastrointestinal, and psychiatric symptoms. The A3243G mutation carriers have multiple medical problems, suggesting that the A3243G mutation should be considered as an etiological factor in patients with multisystem clinical presentations or a family history compatible with matrilineal inheritance. Because some medical problems affecting A3243G mutation carriers are treatable, early detection and proactive management may mitigate the burden of morbidity.

Effects of the S288c genetic background and common auxotrophic markers on mitochondrial DNA function in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a valuable model organism for the study of eukaryotic processes. Throughout its development as a research tool, several strain backgrounds have been utilized and different combinations of auxotrophic marker genes have been introduced into them, creating a useful but non-homogeneous set of strains. The ade2 allele was used as an auxotrophic marker, and for 'red-white' screening for respiratory competence. his3 alleles that influence the expression of MRM1 have been used as selectable markers, and the MIP1[S] allele, found in the commonly used S228c strain, is associated with mitochondrial DNA defects. The focus of the current work was to examine the effects of these alleles, singly and in combination, on the maintenance of mitochondrial function. The combination of the ade2 and MIP1[S] alleles is associated with a slight increase in point mutations in mitochondrial DNA. The deletion in the his3Delta200 allele, which removes the promoter for MRM1, is associated with loss of respiratory competence at 37 degrees C in the presence of either MIP1 allele. Thus, multiple factors can contribute to the maintenance of mitochondrial function, reinforcing the concept that strain background is an important consideration in both designing experiments and comparing results obtained by different research groups.

Etiological profile of presumptive optic neuritis in China

To determine the diagnoses in a population of patients with presumed optic neuritis (ON) in China and compare the spectrum of etiologies with those in western countries, we studied a group of 204 patients referred to a teaching hospital with a presumptive diagnosis of ON. We conducted detailed medical and family history, thorough ophthalmologic and neurological examinations, and laboratory tests including neuroimaging. Patients were diagnosed using international diagnostic criteria. One hundred and thirteen patients (55.4%) met ON criteria, among whom 83 patients (73.5%) were diagnosed with idiopathic demyelinating optic neuritis (IDON). Brain MRI white matter lesions were found in 11 of 78 (14.1%) clinically “isolated” IDON patients. Tuberculosis meningitis was found in 2 patients and syphilis in 1 patient. The cause of ON in 26 patients (23.0%) remained uncertain, but it was still likely to be IDON accompanied by secondary axonal degeneration. Leber’s hereditary optic neuropathy (LHON) was found in 38 patients (18.6%), confirmed by mitochondrial DNA point mutation screening. Seventeen patients (8.3%) had ischemic optic neuropathy. A spectrum of other entities mimicked ON. Therefore, idiopathic demyelinating ON was the most common type of optic neuropathy in our group. Despite minor differences regarding the causes and prognosis, the etiology of ON in our population is similar to that reported in Western countries.

Identification of novel mutations in five patients with mitochondrial encephalomyopathy

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 tRNATrp and tRNAPhe, respectively. Altogether, these data indicate that mtDNA analysis must become part of the routine screening for mitochondrial disorders.

Mitochondrial DNA Haplogroups and Highly Active Antiretroviral Therapy–Related Lipodystrophy

The combination of different point mutations in mitochondrial DNA (mtDNA), which are defined as haplogroups, may cause modification in organelle function and may be involved in several pathologies. We analyzed the distribution of mtDNA polymorphisms in human immunodeficiency virus (HIV)-infected patients with lipodystrophy, a relevant adverse event caused by highly active antiretroviral therapy, and their correlation with metabolic and viroimmunologic parameters. The frequency of the 9 most common European haplogroups was investigated in 346 white, HIV-infected patients with lipodystrophy. Haplogroups were identified on the basis of classic methods. Statistical analysis was performed with use of 1-way analysis of variance, the chi(2) test, and principal-components analysis. The distribution of mtDNA haplogroups among patients with lipodystrophy was similar to that among the general European population. We found no differences between patients with different haplogroups with regard to viroimmunologic results (plasma HIV load, CD4(+) T cell count, and nadir CD4(+) T cell count), glucose data (glucose, insulin, C-peptide, and glycosylated hemoglobin concentrations and insulin resistance), lipid data (levels of triglycerides, total cholesterol, high- and low-density lipoproteins, and apolipoprotein A1 and B), acid-base balance parameters (lactate level and anion gap), or anthropometric measures (weight, body mass index, and waist-to-hip ratio). No differences were observed in trunk fat levels, leg-fat ratio (which was determined by dual-energy x-ray absorptiometry), or exposure to different drug classes. Principal-components analysis confirmed that the spatial distribution of patients belonging to a given haplogroup was not influenced by different clinical parameters. Our study indicates that, in HIV-infected patients with lipodystrophy, mtDNA haplogroups are not related to major metabolic changes or to particular viroimmunologic features.