A tRNA(Lys) mutation in the mtDNA is the causal genetic lesion underlying myoclonic epilepsy and ragged-red fiber (MERRF) syndrome.

Skeletal muscle mtDNA of three patients with mitochondrial encephalomyopathy, characterized clinically by myoclonic epilepsy and ragged-red fiber (MERRF) syndrome, has been sequenced to determine the underlying molecular defect(s). An A-to-G substitution of nt 8344 in the tRNA(Lys) gene, a substitution suggested to be associated with MERRF encephalomyopathy, was detected in these patients. Abnormal patterns of mitochondrial translation products were observed in the skeletal muscle of patients, consistent with the expected consequential defect in protein synthesis. The genealogical studies of the three patients, as well as mtDNA from one published MERRF patient and from nine other normal and disease controls, revealed that the tRNA(Lys) mutations in the MERRF patients have arisen independently. These observations provided evidence that the base substitution is a causal mutation for MERRF.     

In vitro phosphorylation of bovine cardiac muscle high molecular weight calmodulin binding protein by cyclic AMP-dependent protein kinase and dephosphorylation by calmodulin-dependent phosphatase

MERRF (myoclonic epilepsy with ragged-red fibers) is a severe, multisystem disorder characterized by myoclonus, seizures, progressive cerebellar syndrome, muscle weakness, and the presence of ragged-red fibers in the muscle biopsy. MERRF is associated with heteroplasmic point mutations, either A8344G or T8356C, in the gene encoding the mitochondrial tRNALys. The human ro cell system was utilized to examine the phenotypic consequences of these mutations, and to investigate their molecular genetic causes. Wild-type and mutant transmitochondrial cell lines harboring a pathogenic point mutation at either A8344G or T8356C in the human mitochondrial tRNALys gene were isolated and examined. Mitochondrial transformants containing 100% mutated mitochondrial DNAs (mtDNAs) exhibited severe defects in respiratory chain activity, in the rates of protein synthesis, and in the steady-state levels of mitochondrial translation products as compared with mitochondrial transformants containing 100% wild-type mtDNAs. In addition, both mutant cell lines exhibited the presence of aberrant mitochondrial translation products. These results demonstrate that two different mtDNA point mutations in tRNALys result in fundamentally identical defects at the cellular level, and that these specific protein synthesis abnormalities contribute to the pathogenesis of MERRF. (Mol Cell Biochem 174: 215–219, 1997)     

A calcium signaling defect in the pathogenesis of a mitochondrial DNA inherited oxidative phosphorylation deficiency.

In recent years, genetic defects of the mitochondrial genome (mtDNA) were shown to be associated with a heterogeneous group of disorders, known as mitochondrial diseases, but the cellular events deriving from the molecular lesions and the mechanistic basis of the specificity of the syndromes are still incompletely understood. Mitochondrial calcium (Ca2+) homeostasis depends on close contacts with the endoplasmic reticulum and is essential in modulating organelle function. Given the strong dependence of mitochondrial Ca2+ uptake on the membrane potential and the intracellular distribution of the organelle, both of which may be altered in mitochondrial diseases, we investigated the occurrence of defects in mitochondrial Ca2+ handling in living cells with either the tRNALys mutation of MERRF (myoclonic epilepsy with ragged-red fibers) or the ATPase mutation of NARP (neurogenic muscle weakness, ataxia and retinitis pigmentosa). There was a derangement of mitochondrial Ca2+ homeostasis in MERRF, but not in NARP cells, whereas cytosolic Ca2+ responses were normal in both cell types. Treatment of MERRF cells with drugs affecting organellar Ca2+ transport mostly restored both the agonist-dependent mitochondrial Ca2+ uptake and the ensuing stimulation of ATP production. These results emphasize the differences in the cellular pathogenesis of the various mtDNA defects and indicate specific pharmacological approaches to the treatment of some mitochondrial diseases.     

Treatment of human cells derived from MERRF syndrome by peptide-mediated mitochondrial delivery

Background aimsThe feasibility of delivering mitochondria using the cell-penetrating peptide Pep-1 for the treatment of MERRF (myoclonic epilepsy with ragged red fibers) syndrome, which is caused by point mutations in the transfer RNA genes of mitochondrial DNA, is examined further using cellular models derived from patients with MERRF syndrome.MethodsHomogenesis of mitochondria (wild-type mitochondria) isolated from normal donor cells with about 83.5% preserved activity were delivered into MERRF fibroblasts by Pep-1 conjugation (Pep-1-Mito).ResultsDelivered doses of 52.5 μg and 105 μg Pep-1-Mito had better delivered efficiency and mitochondrial biogenesis after 15 days of treatment. The recovery of mitochondrial function in deficient cells receiving 3 days of treatment with peptide-mediated mitochondrial delivery was comprehensively demonstrated by restoration of oxidative phosphorylation subunits (complex I, III and IV), mitochondrial membrane potential, adenosine triphosphate synthesis and reduction of reactive oxygen species production. The benefits of enhanced mitochondrial regulation depended on the function of foreign mitochondria and not the existence of mitochondrial DNA and can be maintained for at least 21 days with dramatically elongated mitochondrial morphology. In contrast to delivery of wild-type mitochondria, the specific regulation of Pep-1-Mito during MERRF syndrome progression in cells treated with mutant mitochondria was reflected by the opposite performance, with increase in reactive oxygen species production and matrix metalloproteinase activity.ConclusionsThe present study further illustrates the feasibility of mitochondrial intervention therapy using the novel approach of peptide-mediated mitochondrial delivery and the benefit resulting from mitochondria-organelle manipulation.     

MERRF: Clinical features, muscle biopsy and molecular genetics in Brazilian patients

Myoclonic epilepsy with ragged red fibers (MERRF) is a mitochondrial disease that is characterized by myoclonic epilepsy with ragged red fibers (RRF) in muscle biopsies. The aim of this study was to analyze Brazilian patients with MERRF. Six patients with MERRF were studied and correlations between clinical findings, laboratory data, electrophysiology, histology and molecular features were examined. We found that blood lactate was increased in four patients. Electroencephalogram studies revealed generalized epileptiform discharges in five patients and generalized photoparoxysmal responses during intermittent photic stimulation in two patients. Muscle biopsies showed RRF in all patients using modified Gomori-trichrome and succinate dehydrogenase stains. Cytochrome c oxidase (COX) stain analysis indicated deficient activity in five patients and subsarcolemmal accumulation in one patient. Molecular analysis of the tRNA(Lys) gene with PCR/RFLP and direct sequencing showed the A8344G mutation of mtDNA in five patients. The presence of RRFs and COX deficiencies in muscle biopsies often confirmed the MERRF diagnosis. We conclude that molecular analysis of the tRNA(Lys) gene is an important criterion to help confirm the MERRF diagnosis. Furthermore, based on the findings of this study, we suggest a revision of the main characteristics of this disease.     

Genetic system for maintaining the mitochondrial human genome in yeast <Emphasis Type="Italic">Yarrowia lipolytica</Emphasis>

For the first time, the possibility of maintaining an intact human mitochondrial genome in a heterologous system in the mitochondria of yeast Yarrowia lipolytica is shown. A method for introducing directional changes into the structure of the mitochondrial human genome replicating in Y. lipolytica by an artificially induced ability of yeast mitochondria for homologous recombination is proposed. A method of introducing and using phenotypic selection markers for the presence or absence of defects in genes tRNA-Lys and tRNA-Leu of the mitochondrial genome is developed. The proposed system can be used to correct harmful mutations of the human mitochondrial genome associated with mitochondrial diseases and for preparative amplification of intact mitochondrial DNA with an adjusted sequence in yeast cells. The applicability of the new system for the correction of mutations in the genes of Lys- and Leu-specific tRNAs of the human mitochondrial genome associated with serious and widespread human mitochondrial diseases such as myoclonic epilepsy with lactic acidosis (MELAS) and myoclonic epilepsy with ragged-red fibers (MERRF) is shown.     

Identification of point mutations by mispairing PCR as exemplified in MERRF disease

The point mutation in the tRNA(Lys) gene of mitochondrial DNA (mtDNA) from patients with myoclonic epilepsy and ragged red fibers (MERRF) was quantitatively analyzed after digestion with the restriction endonuclease Nae I of the PCR amplified DNA. Since the point mutation is not part of a restriction site for a commonly available restriction endonuclease, the Nae I restriction site was introduced by PCR using a mispairing primer. The percentage of mutated mtDNA was determined in a few hairs of five members of an affected family by counting the radioactivity of the fragments after PCR amplification with labelled dATP.     

Rapid detection of the A----G(8344) mutation of mtDNA in Italian families with myoclonus epilepsy and ragged-red fibers (MERRF).

We devised a rapid PCR-based method to screen for an A----G transition at nucleotide 8344 of the human mitochondrial tRNA(Lys) gene, which was recently reported, by Shoffner and co-workers, to be associated with myoclonus epilepsy and ragged-red fibers (MERRF), a maternally transmitted mitochondrial encephalomyopathy (Shoffner et al. 1990). We confirmed this association in five of seven Italian MERRF pedigrees. The mutation was specific for the MERRF trait, because it was never found in mtDNA of non-MERRF individuals, including 14 normal and 110 diseased controls. Our study corroborates the idea that the A----G(8344) mutation is the most frequent and widespread genetic cause of MERRF.     

Diseases caused by mutations in mitochondrial DNA

Mitochondrial diseases associated with mutations within mitochondrial genome are a subgroup of metabolic disorders since their common consequence is reduced metabolic efficiency caused by impaired oxidative phophorylation and shortage of ATP. Although the vast majority of mitochondrial proteins (approximately 1500) is encoded by nuclear genome, mtDNA encodes 11 subunits of respiratory chain complexes, 2 subunits of ATP synthase, 22 tRNAs and 2 rRNAs. Up to now, more than 250 pathogenic mutations have been described within mtDNA. The most common are point mutations in genes encoding mitochondrial tRNAs such as 3243A-->G and 8344T-->G that cause, respectively, MELAS (mitochondrial encephalopathy, lactic acidosis and stroke-like episodes) or MIDD (maternally-inherited diabetes and deafness) and MERRF (myoclonic epilepsy with ragged red fibres) syndromes. There have been also found mutations in genes encoding subunits of ATP synthase such as 8993T-->G substitution associated with NARP (neuropathy, ataxia and retinitis pigmentosa) syndrome. It is worth to note that mitochondrial dysfunction can also be caused by mutations within nuclear genes coding for mitochondrial proteins.     

Role of taurine in the pathologies of MELAS and MERRF

Taurine is an abundant β-amino acid that concentrates in the mitochondria, where it participates in the conjugation of tRNAs for leucine, lysine, glutamate and glutamine. The formation of 5-taurinomethyluridine-tRNA strengthens the interaction of the anticodon with the codon, thereby promoting the decoding of several codons, including those for AAG, UUG, CAG and GAG. By preventing these series of events, taurine deficiency appears to diminish the formation of 5-taurinomethyluridine and causes inefficient decoding for the mitochondrial codons of leucine, lysine, glutamate and glutamine. The resulting reduction in the biosynthesis of mitochondria-encoded proteins deprives the respiratory chain of subunits required for the assembly of respiratory chain complexes. Hence, taurine deficiency is associated with a reduction in oxygen consumption, an elevation in glycolysis and lactate production and a decline in ATP production. A similar sequence of events takes place in mitochondrial diseases MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) and MERRF (myoclonic epilepsy and ragged-red fiber syndrome). In both diseases, mutations in their respective tRNAs interfere with the formation of 5-taurinomethyluridine in the wobble position. Hence, the taurine-deficient phenotype resembles the phenotypes of MELAS and MERRF.     

A common mitochondrial DNA mutation in the t-RNA(Lys) of patients with myoclonus epilepsy associated with ragged-red fibers

Nucleotide sequence analyses of muscle mitochondrial DNA (mtDNA) from a patient with myoclonus epilepsy associated with ragged-red fibers (MERRF) revealed 33 single base substitutions, including 23 in coding regions for mitochondrial polypeptides and 10 in non-coding regions, as compared with the normal human mtDNA sequence. Three substitutions, in COI, ND4, and Cytb, would result in amino acid substitutions, which are conserved among species. Of three patients with MERRF, all had an identical A to G base substitution only at nucleotide position 8344 in the t-RNA(Lys) region. The substitution was not found in 15 controls. Various degrees of the combined enzymic defects in the oxidative phosphorylation system of mitochondria were found in the MERRF patients. The defects could be explained by altered function or processing of the mutant t-RNA(Lys). This mutation in the t-RNA(Lys) is the most probable cause of MERRF.     

Myoclonic epilepsy and ragged-red fiber disease (MERRF) is associated with a mitochondrial DNA tRNA(Lys) mutation

An A to G transition mutation at nucleotide pair 8344 in human mitochondrial DNA (mtDNA) has been identified as the cause of MERRF. The mutation alters the T psi C loop of the tRNA(Lys) gene and creates a CviJI restriction site, providing a simple molecular diagnostic test for the disease. This mutation was present in three independent MERRF pedigrees and absent in 75 controls, altered a conserved nucleotide, and was heteroplasmic. All MERRF patients and their less-affected maternal relatives had between 2% and 27% wild-type mtDNAs and showed an age-related association between genotype and phenotype. This suggests that a small percentage of normal mtDNAs has a large protective effect on phenotype. This mutation provides molecular confirmation that some forms of epilepsy are the result of deficiencies in mitochondrial energy production.     

Tissue segregation of a heteroplasmic mtDNA mutation in MERRF (Myoclonic epilepsy with ragged red fibers) encephalomyopathy

The distributrion of the causal 8344AG mtDNA mutation has been examined in six tissues of a patient with myoclonic epilepsy with ragged red fibers (MERRF), to study the developmental genetics of this type of mitochondrial disorder, and to determine the pathophysiological importance of the mtDNA heteroplasmy generally observed in such patients. Heteroplasmy of the mtDNA was observed in all six tissues (cerebellum, cerebrum, pancreas, liver, muscle, and heart) suggesting that, whereas the mtDNA mutation is relatively new, the mutated population must have existed before the formation of the three primary embryonic layers. The tissue distribution reveals significant variations in the ratio between the mutated and the normal mtDNA species, indicating the randomness of mtDNA segregation during developmental cell division and differentiation events. The result suggests the existence of tissue-specific nuclear factor(s) that determines the expression of the 8344AG mutation in various tissues; in MERRF syndrome, expression is mainly in the central nervous system.     

The protective roles of phosphorylated heat shock protein 27 in human cells harboring myoclonus epilepsy with ragged-red fibers A8344G mtDNA mutation

Mitochondrial DNA (mtDNA) mutations are associated with a large number of neuromuscular diseases. Myoclonus epilepsy with ragged-red fibers (MERRF) syndrome is a mitochondrial disease inherited through the maternal lineage. The most common mutation in MERRF syndrome, the A8344G mutation of mtDNA, is associated with severe defects in mitochondrial protein synthesis, which impair the assembly and function of the respiratory chain. We have previously shown that there is a decreased level of heat shock protein 27 (HSP27) in lymphoblastoid cells derived from a MERRF patient and in cytoplasmic hybrids (cybrids) harboring the A8344G mutation of mtDNA. In the present study, we found a dramatic decrease in the level of phosphorylated HSP27 (p-HSP27) in the mutant cybrids. Even though the steady-state level of p-HSP27 was reduced in the mutant cybrids, normal phosphorylation and dephosphorylation were observed upon exposure to stress, indicating normal kinase and phosphatase activities. To explore the roles that p-HSP27 may play, transfection experiments with HSP27 mutants, in which three specific serines were replaced with alanine or aspartic acid, showed that the phosphomimicking HSP27 desensitized mutant cybrids to apoptotic stress induced by staurosporine (STS). After heat shock stress, p-HSP27 was found to enter the nucleus immediately, and with a prolonged interval of recovery, p-HSP27 returned to the cytoplasm in wild-type cybrids but not in mutant cybrids. The translocation of p-HSP27 was correlated with cell viability, as shown by the increased number of apoptotic cells after p-HSP27 returned to the cytoplasm. In summary, our results demonstrate that p-HSP27 provides significant protection when cells are exposed to different stresses in the cell model of MERRF syndrome. Therapeutic agents targeting anomalous HSP27 phosphorylation might represent a potential treatment for mitochondrial diseases.     

A new mtDNA mutation in the tRNA(Lys) gene associated with myoclonic epilepsy and ragged-red fibers (MERRF).

Myoclonic epilepsy with ragged-red fibers (MERRF) has been associated with an A--G transition at mtDNA nt 8344, within a conserved region of the tRNA(Lys) gene. Although the 8344 mutation is highly prevalent in patients with MERRF, it is not observed in 10%-20% of the cases, suggesting genetic heterogeneity. We have sequenced the tRNA(Lys) gene of five MERRF patients lacking the common 8344 mutation. One of these showed a novel T-->C transition at nucleotide position 8356, disrupting a highly conserved base pair in the T psi C stem. The mutant mtDNA population was essentially homoplasmic in muscle but was heteroplasmic in blood (47%). Neither 20 patients with other mitochondrial diseases nor 25 controls carried this mutation. These findings suggest that tRNA(Lys) alterations may play a specific role in the pathogenesis of MERRF syndrome.     

Targeting peptide nucleic acid (PNA) oligomers to mitochondria within cells by conjugation to lipophilic cations: implications for mitochondrial DNA replication, expression and disease

The selective manipulation of mitochondrial DNA (mtDNA) replication and expression within mammalian cells has proven difficult. One promising approach is to use peptide nucleic acid (PNA) oligomers, nucleic acid analogues that bind selectively to complementary DNA or RNA sequences inhibiting replication and translation. However, the potential of PNAs is restricted by the difficulties of delivering them to mitochondria within cells. To overcome this problem we conjugated a PNA 11mer to a lipophilic phosphonium cation. Such cations are taken up by mitochondria through the lipid bilayer driven by the membrane potential across the inner membrane. As anticipated, phosphonium–PNA (ph–PNA) conjugates of 3.4–4 kDa were imported into both isolated mitochondria and mitochondria within human cells in culture. This was confirmed by using an ion-selective electrode to measure uptake of the ph–PNA conjugates; by cell fractionation in conjunction with immunoblotting; by confocal microscopy; by immunogold-electron microscopy; and by crosslinking ph–PNA conjugates to mitochondrial matrix proteins. In all cases dissipating the mitochondrial membrane potential with an uncoupler prevented ph–PNA uptake. The ph–PNA conjugate selectively inhibited the in vitro replication of DNA containing the A8344G point mutation that causes the human mtDNA disease ‘myoclonic epilepsy and ragged red fibres’ (MERRF) but not the wild-type sequence that differs at a single nucleotide position. Therefore these modified PNA oligomers retain their selective binding to DNA and the lipophilic cation delivers them to mitochondria within cells. When MERRF cells were incubated with the ph–PNA conjugate the ratio of MERRF to wild-type mtDNA was unaffected, even though the ph–PNA content of the mitochondria was sufficient to inhibit MERRF mtDNA replication in a cell-free system. This unexpected finding suggests that nucleic acid derivatives cannot bind their complementary sequences during mtDNA replication. In summary, we have developed a new strategy for targeting PNA oligomers to mitochondria and used it to determine the effects of PNA on mutated mtDNA replication in cells. This work presents new approaches for the manipulation of mtDNA replication and expression, and will assist in the development of therapies for mtDNA diseases.     

Very rare complementation between mitochondria carrying different mitochondrial DNA mutations points to intrinsic genetic autonomy of the organelles in cultured human cells

In the present work, a large scale investigation was done regarding the capacity of cultured human cell lines (carrying in homoplasmic form either the mitochondrial tRNA(Lys) A8344G mutation associated with the myoclonic epilepsy and ragged red fiber (MERRF) encephalomyopathy or a frameshift mutation, isolated in vitro, in the gene for the ND4 subunit of NADH dehydrogenase) to undergo transcomplementation of their recessive mitochondrial DNA (mtDNA) mutations after cell fusion. The presence of appropriate nuclear drug resistance markers in the two cell lines allowed measurements of the frequency of cell fusion in glucose-containing medium, non-selective for respiratory capacity, whereas the frequency of transcomplementation of the two mtDNA mutations was determined by growing the same cell fusion mixture in galactose-containing medium, selective for respiratory competence. Transcomplementation of the two mutations was revealed by the re-establishment of normal mitochondrial protein synthesis and respiratory activity and by the relative rates synthesis of two isoforms of the ND3 subunit of NADH dehydrogenase. The results of several experiments showed a cell fusion frequency between 1.4 and 3.4% and an absolute transcomplementation frequency that varied between 1.2 x 10(-5) and 5.5 x 10(-4). Thus, only 0.3-1.6% of the fusion products exhibited transcomplementation of the two mutations. These rare transcomplementing clones were very sluggish in developing, grew very slowly thereafter, and showed a substantial rate of cell death (22-28%). The present results strongly support the conclusion that the capacity of mitochondria to fuse and mix their contents is not a general intrinsic property of these organelles in mammalian cells, although it may become activated in some developmental or physiological situations.