Genetic, pathogenetic, and phenotypic implications of the mitochondrial A3243G tRNALeu(UUR) mutation

Somatic alterations in mitochondrial DNA produce changes in cell growth and metabolism supporting a tumorigenic phenotype

There have been many reports of mitochondrial DNA (mtDNA) mutations associated with human malignancies. We have observed allelic instability in UV-induced cutaneous tumors at the mt-Tr locus encoding the mitochondrial tRNA for arginine. We examined the effects of somatic alterations at this locus by modeling the change in a uniform nuclear background by generating cybrids harboring allelic variation at mt-Tr. We utilized the naturally occurring mtDNA variation at mt-Tr within the BALB/cJ (BALB) and C57BL/6J (B6) strains of Mus musculus to transfer their mitochondria into a mouse ρ(0) cell line that lacked its own mtDNA. The BALB haplotype containing the mt-Tr 9821insA allele produced significant changes in cellular respiration (resulting in lowered ATP production), but increased rates of cellular proliferation in cybrid cells. Furthermore, the mtDNA genotype associated with UV-induced tumors endowed the cybrid cells with a phenotype of resistance to UV-induced apoptosis and enhanced migration and invasion capabilities. These studies support a role for mtDNA changes in cancer.

Large-scale screening of mitochondrial DNA mutations among Iranian patients with prelingual nonsyndromic hearing impairment

Hereditary hearing impairment (HI) is a genetically heterogeneous disorder caused by mutations either in nuclear DNA (nDNA) or in mitochondrial DNA (mtDNA). The nDNA mutations account for the majority of prelingual nonsyndromic HI (NSHI). The present survey was conducted to screen for known pathogenic mtDNA mutations including A1555G, A3243G, C1494T, and A7445G to provide an accurate estimate of their prevalence in prelingual NSHI for the first time in the Iranian subpopulations. One thousand unrelated probands with NSHI (including both GJB2-negative and GJB2 heterozygote cases) and 1000 healthy matched controls were investigated using the PCR/RFLP method followed by DNA sequencing to confirm the observed mtDNA mutations. Two of the studied mutations, namely A3243G and A7445G, were each found in a single family (a frequency of 0.1% for each). Mutation screening for A3243G followed by DNA sequencing led to the identification of G3316A substitution, with no prior link to HI. Surprisingly, screening for A3243G in the studied population identified 6 cases (0.6%) in probands and 10 (1%) in normal subjects. A1555G, the most common mtDNA mutation associated with deafness in other populations, was not found in the studied samples. To conclude, our findings indicate G3316A as a nonpathogenic variant in the prelingual NSHI subpopulations of Iran and suggest that mtDNA mutations do not play a major role in the etiology of NSHI in Iran.

Secondary coenzyme Q10 deficiency triggers mitochondria degradation by mitophagy in MELAS fibroblasts

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) is a mitochondrial disease most usually caused by point mutations in tRNA genes encoded by mtDNA. Here, we report on how this mutation affects mitochondrial function in primary fibroblast cultures established from 2 patients with MELAS who harbored the A3243G mutation. Both mitochondrial respiratory chain enzyme activities and coenzyme Q(10) (CoQ) levels were significantly decreased in MELAS fibroblasts. A similar decrease in mitochondrial membrane potential was found in intact MELAS fibroblasts. Mitochondrial dysfunction was associated with increased oxidative stress and the activation of mitochondrial permeability transition (MPT), which triggered the degradation of impaired mitochondria. Furthermore, we found defective autophagosome elimination in MELAS fibroblasts. Electron and fluorescence microscopy studies confirmed a massive degradation of mitochondria and accumulation of autophagosomes, suggesting mitophagy activation and deficient autophagic flux. Transmitochondrial cybrids harboring the A3243G mutation also showed CoQ deficiency and increased autophagy activity. All these abnormalities were partially restored by CoQ supplementation. Autophagy in MELAS fibroblasts was also abolished by treatment with antioxidants or cyclosporine, suggesting that both reactive oxygen species and MPT participate in this process. Furthermore, prevention of autophagy in MELAS fibroblasts resulted in apoptotic cell death, suggesting a protective role of autophagy in MELAS fibroblasts.

Bilateral Polymicrogyria and MELAS/A3243G Mutation. A Very Uncommon Association

A3243G mutation is associated with several mitochondrial disorders, MELAS syndrome being the most common. Polymicrogyrias constitute an extensive group of malformations of cortical development due to abnormal cortical organization. The association between MELAS/A3243G mutation and polymicrogyria is extremely rare; in 18 years of investigation regarding A3243G mutation and related disorders only one case has been reported. We describe a female patient with proven MELAS/A3243G mutation, developmental delay and mild left hemiparesis in whom MRI showed extensive bilateral polymicrogyria. The association of these two disorders and their possible relation are discussed.

Mutation of mitochondrial ATP8 gene improves hepatic energy status in a murine model of acute endotoxemic liver failure

AIMS

Mitochondria not only generate and modulate bioenergy but also serve as biosensors for oxidative stress, and eventually become effector organelles for cell viability. Therefore, the implications of mitochondrial (dys)function in the development of multiple organ failure are profound. We investigated whether a mutation in the ATPase subunit-8 gene affects the course of endotoxemic acute liver failure.

MAIN METHODS

C57BL/6J (ATP8 wild type) and C57BL/6J-mt(FVB/N) (ATP8 mutant) mice were challenged with d-galactosamine (GalN) and Escherichia coli lipopolysaccharide (LPS) for induction of acute liver failure, and studied 6 h thereafter. Control mice received physiological saline only. Analysis included in vivo fluorescence microscopy of hepatic microcirculation and levels of hepatocellular apoptosis, hepatic adenosine nucleotides and oxidative stress. Additionally, survival rates were assessed.

KEY FINDINGS

Induction of endotoxemic liver failure provoked marked liver damage, which was coexistent with a drop of total adenosine nucleotide levels and increased oxidative stress. Of interest, oxidative stress was higher in the GalN/LPS challenged ATP8 mutants compared to wild types. Concomitantly, adenosine triphosphate (ATP) levels in livers of mice carrying the ATP8 mutation remained higher than those in wild type mice. As net result, ATP8 mutants showed lower transaminase release and a tendency to better survival rate upon GalN/LPS exposure compared to wild types.

SIGNIFICANCE

Our findings demonstrate that mutation in the ATPase subunit-8 partially protects mice against endotoxemic stress, most probably due to better hepatic energy status despite elevated oxidative stress. Thus, modulating mitochondrial function to preserve bioenergetic status may be an effective strategy to protect against sepsis-induced multiorgan dysfunction.

Analysis of mtDNA A3243G mutation frequency in Hungary

The A3243G mutation in the mitochondrial tRNALeu (UUR) gene is one of the most common causes of mitochondrial DNA related disorders. Originally it was described in MELAS syndrome (Mitochondrial Encephalomyopathy, Lactic acidosis, Stroke-like episodes), later it had been found to be associated with various phenotypes. In our study the mutation frequency of the A3243G mtDNA mutation was investigated in patients with maternal sensoneural hearing loss, stroke-like episodes, ataxia and myopathy with undetermined etiology. We screened 631 Hungarian patients in North-East, South-West and Central Hungary between 1999 and 2008 for this mutation. The mtDNA analysis was performed from blood and/or muscle tissue. The A3243G substitution was present in 6 patients in heteroplasmic form. The segregation analysis detected 8 further cases. The frequency of the A3243G mutation was 2.22% in the investigated patients. The A3243G mutation frequency in Hungary does not differ significantly from other countries using similar patient selection criteria, however in Finland a higher mutation rate was found. In studies investigated the mutation frequency of this mutation in diabetes mellitus similarly wide variety was detected as well. We conclude that the study design has a huge impact on the result of the genetic epidemiological investigation analyzing the mutation frequency of the A3243G mutation due to the broad clinical phenotype and the different mutation load in different tissues.

Mitochondrial translation and beyond: processes implicated in combined oxidative phosphorylation deficiencies

Mitochondrial disorders are a heterogeneous group of often multisystemic and early fatal diseases, which are amongst the most common inherited human diseases. These disorders are caused by defects in the oxidative phosphorylation (OXPHOS) system, which comprises five multisubunit enzyme complexes encoded by both the nuclear and the mitochondrial genomes. Due to the multitude of proteins and intricacy of the processes required for a properly functioning OXPHOS system, identifying the genetic defect that underlies an OXPHOS deficiency is not an easy task, especially in the case of combined OXPHOS defects. In the present communication we give an extensive overview of the proteins and processes (in)directly involved in mitochondrial translation and the biogenesis of the OXPHOS system and their roles in combined OXPHOS deficiencies. This knowledge is important for further research into the genetic causes, with the ultimate goal to effectively prevent and cure these complex and often devastating disorders.

Clinical features of mitochondrial DNA m.3243A>G mutation in 47 Chinese families

m.3243A>G mutation in mitochondrial DNA is the most common pathogenic point mutation, causing a variety of phenotypes. To further elucidate its clinical characteristics, we recruited 47 Chinese families carrying m.3243A>G mutation and analyzed their symptoms, disease history, inheritance, and mitochondria-related complications. In the probands, lactic acidosis, myopathy, seizures, short stature, weight loss and hirsutism were the most common clinical features. In their mothers, lactic acidosis, exercise intolerance, short stature and weight loss were the frequent manifestations, and normal phenotype was found in 59.6% mothers. m.3243A>G mutation was detected in 47 probands and 42 mothers. In the probands, the mutation ratio in blood was threefold higher and the ratio in urine was twofold higher than those of their mothers. m.3243A>G mutation ratio in mothers' urine and in their probands' blood were weakly correlated. In conclusion, (a) stroke-like episode induced by m.3243A>G mutation may be the symptom predominantly found in older patients; (b) m.3243A>G mutation ratio correlates with the severity of the disease; (c) m.3243A>G mutation ratio in mothers' urine may correlate to the ratio in blood in their offspring.

Mitochondrial decay and impairment of antioxidant defenses in aging RPE cells

In the eye, the retinal pigment epithelium (RPE) is exposed to a highly oxidative environment, partly due to elevated oxygen partial pressure from the choriocapillaris and to digestion of polyunsaturated fatty acid laden photoreceptor outer segments. Here we examined the vulnerability of RPE cells to stress and changes in their mitochondria with increased chronological aging and showed that there is greater sensitivity of the cells to oxidative stress, alterations in their mitochondrial number, size, shape, matrix density, cristae architecture, and membrane integrity as a function of age. These features correlate with reduced cellular levels of ATP, ROS, and [Ca(2+)](c), lower Deltapsim, increased [Ca(2+)](m) sequestration and decreased expression of mtHsp70, UCP2, and SOD3. Mitochondrial decay, bioenergetic deficiencies, and weakened antioxidant defenses in RPE cells occur as early as age 62. With increased severity, these conditions may significantly reduce RPE function in the retina and contribute to age related retinal anomalies.

[Retinal pigment epithelium atrophy and hypacousia in monozygotic twin sisters]

Mitochondrial A3243G point mutations cause variable pathologic changes in different organs. Funduscopy revealed sharply demarcated central areas of atrophy of the retinal pigment epithelium (RPE) which corresponded to the visual field defects. Fundus autofluorescence was reduced in the areas of RPE atrophy but showed granular hyperfluorescence of the adjacent RPE. Heteroplasmic mitochondrial mutations may cause variable changes in different organ systems. However, the ocular phenotype in the described pair of twins was almost identical. Fundus autofluorescence showed little progression of the RPE atrophy.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.

MELAS: clinical features, muscle biopsy and molecular genetics

OBJECTIVE: The aim of the study was to analyze a series of Brazilian patients suffering from MELAS. METHOD: Ten patients with MELAS were studied with correlation between clinical findings, laboratorial data, electrophysiology, histochemical and molecular features. RESULTS: Blood lactate was increased in eight patients. Brain image studies revealed a stroke-like pattern in all patients. Muscle biopsy showed ralled-red fibers (RRF) in 90% of patients on modified Gomori-trichrome and in 100% on succinate dehydrogenase stains. Cytochrome c oxidase stain analysis indicated deficient activity in one patient and subsarcolemmal accumulation in seven patients. Strongly succinate dehydrogenase-reactive blood vessels (SSV) occurred in six patients. The molecular analysis of tRNA Leu(UUR) gene by PCR/RLFP and direct sequencing showed the A3243G mutation on mtDNA in 4 patients. CONCLUSION: The muscle biopsy often confirmed the MELAS diagnosis by presence of RRF and SSV. Molecular analysis of tRNA Leu(UUR) gene should not be the only diagnostic criteria for MELAS.

Heteroplasmy level of the mitochondrial tRNaLeu(UUR) A3243G mutation in a Chinese family is positively associated with earlier age-of-onset and increasing severity of diabetes

OBJECTIVE

To investigate the mutations of mitochondrial genome in a pedigree with suspected maternally inherited diabetes and deafness and to explore the correlations between the mutations and clinical features.

METHODS

Genomic DNA was isolated from blood leucocytes of each member of the pedigree. The mitochondrial genome was amplified with 24-pair primers that could cover the entire mitochondrial DNA. Direct sequencing of PCR products was used to identify any mitochondrial DNA mutations.

RESULTS

Family members on the maternal side all harbored the tRNALeu(UUR) A3243G mutation. The paternal side family members did not have the mutation. The age-of-onset of diabetes of the 4 maternal side family members was 15, 41, 44, and 65 years old, and their corresponding heteroplasmy level of the mutation was 34.5%, 14.9%, 14.6%, and 5.9%, respectively. The age-of-onset of diabetes and heteroplasmy level of A3243G mutation were negatively correlated with a correlation coefficient of -0.980 (P = 0.02). Meanwhile, patient with high heteroplasmy level of A3243G mutation had relatively low severity of disease. Moreover, 6 reported polymorphisms and 2 new variants were found.

CONCLUSIONS

The main cause of diabetes in this pedigree is the tRNALeu(UUR) A3243G mutation. However, other gene variants may contribute to its pathogenicity. The heteroplasmy level of the tRNALeu(UUR) A3243G mutation is positively associated with earlier age-of-onset and increasing severity of diabetes.

Mouse models of mitochondrial DNA defects and their relevance for human disease

Qualitative and quantitative changes in mitochondrial DNA (mtDNA) have been shown to be common causes of inherited neurodegenerative and muscular diseases, and have also been implicated in ageing. These diseases can be caused by primary mtDNA mutations, or by defects in nuclear-encoded mtDNA maintenance proteins that cause secondary mtDNA mutagenesis or instability. Furthermore, it has been proposed that mtDNA copy number affects cellular tolerance to environmental stress. However, the mechanisms that regulate mtDNA copy number and the tissue-specific consequences of mtDNA mutations are largely unknown. As post-mitotic tissues differ greatly from proliferating cultured cells in their need for mtDNA maintenance, and as most mitochondrial diseases affect post-mitotic cell types, the mouse is an important model in which to study mtDNA defects. Here, we review recently developed mouse models, and their contribution to our knowledge of mtDNA maintenance and its role in disease.

Detection of known base substitution mutations in human mitochondrial DNA of MERRF and MELAS by biochip technology

We developed a DNA biochip specialized for detection of known base substitution mutations in mitochondrial DNA causing mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) and myoclonic epilepsy associated with ragged-red fibers (MERRF). A set of probes sharing a given allele-specific sequence with a single base substitution near the middle of the sequence was covalently immobilized. Cy5-labeled DNA targets were amplified from sample DNAs containing 31 potential MELAS and/or MERRF mutations by a multiplex PCR method. Detection parameters for the DNA biochip-based assay were accordingly optimized. Seven clinically confirmed patients with MELAS, 5 patients with MERRF, 1 suspected MERRF case and 25 healthy controls were tested using the DNA biochip. For discriminating of homoplasmic and heteroplasmic point mutations in mtDNA, a diagnostic factor based on the ratio between the hybridization signals from the reference and test targets with each probe was used. The results showed that all the cases with MELAS had a causal heteroplasmic A3243G tRNA(Leu(UUR)) mutation. In the MERRF patients, four cases were found to be a homoplasmic A8344G tRNA(Lys) mutation and one case was a heteroplasmic T8356C tRNA(Lys) mutation. None of the healthy controls carried the potential mutations. The results of the DNA biochip were completely consistent with those by DNA sequencing. Thus, the DNA biochip would potentially become a valuable tool in clinical specific screening of the mtDNA point mutations associated with MELAS and/or MERRF syndrome.

Overexpressed mitochondrial leucyl-tRNA synthetase suppresses the A3243G mutation in the mitochondrial tRNA(Leu(UUR)) gene

The A3243G mutation in the human mitochondrial tRNA(Leu(UUR)) gene causes a number of human diseases. This mutation reduces the level and fraction of aminoacylated tRNA(Leu(UUR)) and eliminates nucleotide modification at the wobble position of the anticodon. These deficiencies are associated with mitochondrial translation defects that result in decreased levels of mitochondrial translation products and respiratory chain enzyme activities. We have suppressed the respiratory chain defects in A3243G mutant cells by overexpressing human mitochondrial leucyl-tRNA synthetase. The rates of oxygen consumption in suppressed cells were directly proportional to the levels of leucyl-tRNA synthetase. Fifteenfold higher levels of leucyl-tRNA synthetase resulted in wild-type respiratory chain function. The suppressed cells had increased steady-state levels of tRNA(Leu(UUR)) and up to threefold higher steady-state levels of mitochondrial translation products, but did not have rates of protein synthesis above those in parental mutant cells. These data suggest that suppression of the A3243G mutation occurred by increasing protein stability. This suppression of a tRNA gene mutation by increasing the steady-state levels of its cognate aminoacyl-tRNA synthetase is a model for potential therapies for human pathogenic tRNA mutations.

Mitochondrial DNA variant interactions modify breast cancer risk

Interactions between mitochondrial deoxyribonucleic acid (mtDNA) variants and the risk of developing breast cancer were investigated using DNA samples collected from non-Jewish European American breast cancer patients and ethnically age-matched female controls. Logistic regression was used to evaluate two-way interactions between 17 mtDNA variants. To control for multiple testing, empirical P values were calculated using permutation. Odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were calculated to measure the contribution of variants in modifying the risk of developing breast cancer. A highly significant interaction was identified between variants 12308G and 10398G (empirical P value = 0.0028), with results suggesting these variants increase the risk of a woman developing breast cancer (OR = 3.03; 95% CI 1.53-6.11). Nominal significant P values were also observed for interactions between mtDNA variants 709A and 16189C; 4216C and 10398G; 4216C and 16189C; 10398G and 16159C; 13368A and 16189C; and 14766T and 16519C. However, after adjusting for multiple testing, the P values did not remain significant. Although it is important to elucidate the main effect of mtDNA variants on the risk of developing breast cancer, understanding gene x gene interactions will give a greater knowledge of disease etiology and aid in interpreting a woman's risk of developing breast cancer.

Genetic, pathogenetic, and phenotypic implications of the mitochondrial A3243G tRNALeu(UUR) mutation

Mitochondrial disorders are frequently caused by mutations in mitochondrial genes and usually present as multisystem disease. One of the most frequent mitochondrial mutations is the A3,243G transition in the tRNALeu(UUR) gene. The phenotypic expression of the mutation is variable and comprises syndromic or non-syndromic mitochondrial disorders. Among the syndromic manifestations the mitochondrial encephalopathy, lactacidosis, and stroke-like episode (MELAS) syndrome is the most frequent. In single cases the A3,243G mutation may be associated with maternally inherited diabetes and deafness syndrome, myoclonic epilepsy and ragged-red fibers (MERRF) syndrome, MELAS/MERRF overlap syndrome, maternally inherited Leigh syndrome, chronic external ophthalmoplegia, or Kearns-Sayre syndrome. The wide phenotypic variability of the mutation is explained by the peculiarities of the mitochondrial DNA, such as heteroplasmy and mitotic segregation, resulting in different mutation loads in different tissues and family members. Moreover, there is some evidence that additional mtDNA sequence variations (polymorphisms, haplotypes) influence the phenotype of the A3,243G mutation. This review aims to give an overview on the actual knowledge about the genetic, pathogenetic, and phenotypic implications of the A3,243G mtDNA mutation.