Characterization of COX17, a yeast gene involved in copper metabolism and assembly of cytochrome oxidase

Mutations in the COX17 gene of Saccharomyces cerevisiae cause a respiratory deficiency due to a block in the production of a functional cytochrome oxidase complex. Because cox17 mutants are able to express both the mitochondrially and nuclearly encoded subunits of cytochrome oxidase, the Cox17p most likely affects some late posttranslational step of the assembly pathway. A fragment of yeast nuclear DNA capable of complementing the mutation has been cloned by transformation of the cox17 mutant with a library of genomic DNA. Subcloning and sequencing of the COX17 gene revealed that it codes for a cysteine-rich protein with a molecular weight of 8,057. Unlike other previously described accessory factors involved in cytochrome oxidase assembly, all of which are components of mitochondria, Cox17p is a cytoplasmic protein. The cytoplasmic location of Cox17p suggested that it might have a function in delivery of a prosthetic group to the holoenzyme. A requirement of Cox17p in providing the copper prosthetic group of cytochrome oxidase is supported by the finding that a cox17 null mutant is rescued by the addition of copper to the growth medium. Evidence is presented indicating that Cox17p is not involved in general copper metabolism in yeast but rather has a more specific function in the delivery of copper to mitochondria.     

A double mutation (A8296G and G8363A) in the mitochondrial DNA tRNA (Lys) gene associated with myoclonus epilepsy with ragged-red fibers

OBJECTIVE: To define potential pathogenic mitochondrial DNA (mtDNA) point mutations in a patient with myoclonus epilepsy with ragged-red fibers (MERRF) syndrome. BACKGROUND: MERRF syndrome is typically associated with point mutations in the mtDNA tRNALys gene. METHODS: We performed morphologic, biochemical, and genetic analysis of muscle samples from the patient and four relatives. Molecular genetic studies included sequencing, PCR, and restriction enzyme analysis on whole muscle, blood, and single muscle fibers. RESULTS: Muscle biopsy showed cytochrome c oxidase (COX), negative ragged-red fibers (RRF), and a defect of complex I of the mitochondrial respiratory chain. We found an A8296G transition and a G8363A mutation in the mtDNA tRNALYs gene. The A8296G was almost homoplasmic in muscle and blood from the propositus and his oligosymptomatic maternal relatives. The G8363A mutation was heteroplasmic and more abundant in muscle than in blood, and its proportion correlated with clinical severity. Single muscle fiber analysis showed significantly higher levels of G8363A genomes in COX-negative than in normal fibers, and almost homoplasmic levels of mutant A8296G mtDNA in both COX-negative and normal fibers. The two mutations affect highly conserved nucleotides and were not found in controls. CONCLUSIONS: The G8363A mutation is pathogenic; the co-occurrence of the A8296G mutation is of unclear significance and is likely to be a rare polymorphism.     

Intratympanic gentamicin in Meniere''s disease

In Chlamydomonas reinhardtii, mutants defective in the cytochrome pathway of respiration lack the capacity to grow under heterotrophic conditions (in darkness on acetate). In the dark- strain duM18, a + 1 T addition in a run of four Ts, located at codon 145 of the mitochondrial cox1 gene encoding subunit I of cytochrome c oxidase, is responsible for the mutant phenotype. A leaky revertant (su11) that grows heterotrophically at a lower rate than wild-type cells was isolated from dum18. Its respiration sensitivity to cyanide was low and its cytochrome c oxidase activity was only 4% of that of the wild-type enzyme. Meiotic progeny obtained from crosses between revertant and wild-type cells inherited the phenotype of the mt- parent, showing that the suppressor mutation, like dum18 itself, is located in the mitochondrial genome. In order to map the su11 mutation relative to dum18, a recombinational analysis was performed on the diploid progeny. It demonstrated that su11 was very closely linked to the dum18 mutation less than 20-30 bp away. The cox1 gene of the su11 revertant was then sequenced. In addition to the + 1 T frameshift mutation still present at codon 145, an A-->C substitution was found at codon 146, leading to the replacement of a glutamic acid by an alanine in the polypeptide chain. No other mutations were detected in the cox1 coding sequence. As the new GCG codon (Ala) created at position 146 is very seldom used in the mitochondrial genome of C. reinhardtii, we suggest that the partial frameshift suppression by the nearby substitution is due to an occasional abnormal translocation of the ribosome (+ 1 base shift) facilitated both by the run of Ts and the low level of weak interaction of alanyl-tRNA.     

A novel point mutation in the mitochondrial tRNA(Ser(UCN)) gene detected in a family with MERRF/MELAS overlap syndrome

We found a new point mutation in the mitochondrial tRNA(Ser(UCN)) gene in a family with MERRF/MELAS overlap syndrome by screening for heteroplasmy by means of chemical cleavage of mismatch (CCM). Our strategy was based on the previous observations that most pathogenic mtDNA mutations in mitochondrial encephalomyopathies are heteroplasmic, whereas almost all neutral mitochondrial polymorphisms are homoplasmic. CCM followed by nucleotide sequencing of the corresponding region of the mitochondrial genome revealed a heteroplasmic mutation at nt 7512 in the tRNA(Ser(UCN)) gene. The 7512 (T to C) mutation disrupts a highly conserved base pair in the acceptor stem, and this mutation was not found in any of 120 normal controls, or in 43 patients with mitochondrial diseases. The proportion of the mutant mtDNA was 93% in muscle, 76 and 87% in the blood of the patients. A family member without apparent neuromuscular symptoms carried less mutant mtDNA. These findings support the view that this mutation is pathogenic in this family. Detection of heteroplasmy by CCM is an efficient means of screening pathogenic mtDNA point mutations.     

Peak oxygen uptake, muscle volume, and the growth hormone-insulin-like growth factor-I axis in adolescent males

OBJECTIVES: To define the molecular genetic basis of the MELAS phenotype in five patients without any known mutation of mitochondrial DNA. METHODS: Systematic automated mitochondrial DNA sequencing of all mitochondrial transfer RNA and cytochrome c oxidase genes was undertaken in five patients who had the MELAS phenotype. RESULTS: A novel heteroplasmic mitochondrial DNA mutation was identified in the transfer RNA gene for phenylalanine in one case (patient 3). This mutation was not detected in the patient's blood or in her mother's blood. No pathogenic mutations were identified in the other four patients. CONCLUSIONS: This is the first point mutation in the transfer RNA gene for phenylalanine to be associated with MELAS. The absence of mutations in the remaining four patients suggests that there is further genetic heterogeneity associated with this mitochondrial phenotype.     

Ulnocarpal abutment. Treatment options

Rapid progress has been made in the identification of mitochondrial DNA mutations which are typically associated with diseases of the nervous system and muscle. The well established mitochondrial disorders are maternally inherited and males and females are equally affected. An exception is Leber's hereditary optic atrophy (LHON) which is observed much more frequently in males than in females. There are three common point mutations in LHON which can be homoplasmic or heteroplasmic. In mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) most mutations are single base changes and lie within the tRNA-Leu gene. Point mutations in myoclonic epilepsy with ragged red fibres (MERRF) usually occur within the tRNA-Lys gene but mutations of the tRNA-Leu gene are also observed. MELAS and MERRF mutations are heteroplasmic and there is considerable clinical overlap between these diseases. Point mutations within the ATPase6 gene result in either neuropathy, ataxia and retinitis pigmentosa (NARP) or in Leigh's syndrome. The latter occurs if the mutation is present in the majority of mitochondria (extreme heteroplasmy). Finally, mitochondrial DNA deletions are the cause underlying Kearns-Sayre syndrome (KSS). Apart from the well-established mitochondrial diseases, there is increasing evidence that mitochondrial mutations may also play a role in the neurodegenerative disorders Parkinson, Alzheimer and Huntington disease. The complex I defect found in Parkinson disease is especially interesting in this respect. However, no causative mitochondrial mutation has as yet been established in any of these three common disorders.     

Model multiple antigenic and homopolymeric peptides from non-repetitive sequences of malaria merozoite proteins elicit biologically irrelevant antibodies

We describe here a new type of mitochondrial mutation (dum24; for dark uniparental minus inheritance) of the unicellular photosynthetic alga Chlamydomonas reinhardtii. The mutant fails to grow under heterotrophic conditions and displays reduced growth under both photoautotrophic and mixotrophic conditions. In reciprocal crosses between mutant and wild-type cells, the meiotic progeny only inherit the phenotype of the mating-type minus parent, indicating that the dum24 mutation exclusively affects the mitochondrial genome. Digestion with various restriction enzymes followed by DNA gel blot hybridizations with specific probes demonstrated that dum24 cells contain four types of altered mitochondrial genomes: deleted monomers lacking cob, nd4, and the 3' end of the nd5 gene; deleted monomers deprived of cob, nd4, nd5, and the 5' end of the cox1 coding sequence; and two types of dimers produced by end-to-end fusions between monomers similarly or differently deleted. Due to these mitochondrial DNA alterations, complex I activity, the cytochrome pathway of respiration, and presumably, the three phosphorylation sites associated with these enzyme activities are lacking in the mutant. The low respiratory rate of the dum24 cells results from the activities of rotenone-resistant NADH dehydrogenase, complex II, and alternative oxidase, with none of these enzymes being coupled to ATP production. To our knowledge, this type of mitochondrial mutation has never been described for photosynthetic organisms or more generally for obligate aerobes.     

Novel function of the regulatory subunit of protein kinase A: regulation of cytochrome c oxidase activity and cytochrome c release

There have been speculations that the regulatory (R) subunit of the cAMP-dependent protein kinase (PKA) may have other functions. A recent study has shown that the catalytic (C) subunit of PKA may be regulated in a cAMP- and R subunit-independent manner. However, evidence linking a function to the R subunit apart from inhibiting the C subunit has been elusive. In this report, interaction cloning experiments showed that the RIalpha subunit association with the cytochrome c oxidase subunit Vb (CoxVb) is cAMP-sensitive. Interaction was detected with a GST-RIalpha fusion protein as well as by coimmunoprecipitation. Transient treatment with cAMP-elevating agents inhibited cytochrome c oxidase in Chinese hamster ovary (CHO) cells with a concomitant decrease in cytochrome c levels in the mitochondria and an increase in its release into the cytosol. Furthermore, mutant cells harboring a defective RIalpha show increased cytochrome c oxidase activity and also constitutively lower levels of cytochrome c in comparison to either the wild-type cells or the C subunit mutant. These results suggest a novel mechanism of cAMP signaling through the interaction of RIalpha with CoxVb thereby regulating cytochrome c oxidase activity as well as the cytochrome c levels.     

c-type cytochromes and manganese oxidation in Pseudomonas putida MnB1

Pseudomonas putida MnB1 is an isolate from an Mn oxide-encrusted pipeline that can oxidize Mn(II) to Mn oxides. We used transposon mutagenesis to construct mutants of strain MnB1 that are unable to oxidize manganese, and we characterized some of these mutants. The mutants were divided into three groups: mutants defective in the biogenesis of c-type cytochromes, mutants defective in genes that encode key enzymes of the tricarboxylic acid cycle, and mutants defective in the biosynthesis of tryptophan. The mutants in the first two groups were cytochrome c oxidase negative and did not contain c-type cytochromes. Mn(II) oxidation capability could be recovered in a c-type cytochrome biogenesis-defective mutant by complementation of the mutation.     

Loss of function of cytochrome c in Jurkat cells undergoing fas-mediated apoptosis

Mitochondrial function was examined in Jurkat cells undergoing Fas-mediated apoptosis. With succinate or ascorbate/tetramethylphenylenediamine as substrate, oxygen uptake by digitonin-permeabilized apoptotic mitochondria was greatly decreased as compared with control. Assessment of the function of the cytochrome c-cytochrome oxidase segment of the electron transport chain of apoptotic mitochondria showed that the activity of cytochrome oxidase appeared to be normal, but that of cytochrome c was greatly diminished. A death protease was found to participate in the events leading to the loss of cytochrome c activity, but the cytochrome did not seem to be extensively degraded during the course of apoptosis. Our results suggest that a rapid loss in mitochondrial function due at least in part to the inhibition or inactivation of cytochrome c is a potentially fatal component of the apoptosis program of Jurkat cells.     

Paramagnetic effects on nuclear relaxation in enzyme-bound Co(II)-adenine nucleotide complexes: relative contributions of dipolar and scalar interactions

Five patients with diminished activity of complex III of the mitochondrial respiratory chain have been screened for mutations in the mitochondrial cytochrome b (cyt b) gene. In 1 patient, a young boy with an akinetic rigid syndrome and a mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS), a novel 4-base pair deletion was identified. This mutation in this highly conserved gene is considered to be pathogenic since it is a heteroplasmic frame shift mutation predicted to lead to a truncated protein.     

Mitochondrial fixation for the detection of cytochrome oxidase activity using microwave irradiation

We examined cell fixation with microwave irradiation (MWI) used in cytochemistry. MWI was applied to blocks of about 1 mm3 of mouse parotid glands at 500 W for about 5 sec in a fixative at 37 degrees C. The activities of endogenous peroxidase and mitochondrial cytochrome oxidase were demonstrated by using the DAB method with 3,3'-diaminobenzidine (DAB) and 0.01% H2O2. Under electron microscopy, peroxidase activity was localized in the nuclear envelope, endoplasmic reticulum and secretory granules. However, mitochondria cytochrome oxidase activity seemed to be rather weak against the MWI at 37 degrees C. Moreover, suspension of isolated hamster liver mitochondria was fixed by MWI and also demonstrated cytochrome oxidase activity by using the cytochemical methods with DAB, cytochrome c, catalase and sucrose. Such mitochondrial fractions were subjected to 6-second MWI given 10 or 18 times with an interval of 10 seconds with and without a chilled water bath. The final temperature of each fixative was kept at about 10 degrees C or rose to about 37 and 55 degrees C. When we took care to keep the temperature below 10 degrees C, the DAB reaction products accumulated in the mitochondrial intermembrane-intracristal space. No mitochondrial deposits were observed when the temperatures of the fixatives rose to 37 and 55 degrees C. These results indicated that peroxidase was very resistant to the heat with MWI fixation. Cytochrome oxidase is sensitive to the heat with MWI, so, a chilled water bath had to be used.     

Electron transport chain defects in Alzheimer's disease brain

Previous work suggested a deficiency in the terminal complex of the mitochondrial electron transport chain, cytochrome c oxidase (COX), in platelet mitochondria of Alzheimer's disease (AD) patients. The present study extends this observation to AD brain mitochondria through assay of electron transport chain activities in mitochondria isolated from autopsied brain samples from AD patients (n = 9) and from controls with and without known neurologic disease (n = 8). AD brain mitochondria demonstrated a generalized depression of activity of all electron transport chain complexes. This depression was most marked in COX activity (p < 0.001). Concentrations of cytochromes b, c1, and aa3 were similar in AD and controls. The electron transport chain is defective in AD brain, and the defect centers about COX.     

Content of mutant mitochondrial DNA and organ dysfunction in a patient with a MELAS subgroup of mitochondrial encephalomyopathies

A point mutation of mitochondrial tRNALeu(UUR) gene is responsible for a MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) subgroup of mitochondrial encephalomyopathies. In most cases, the mutant mitochondrial DNA (mtDNA) coexists with normal mtDNA in a heteroplasmic manner. In order to quantify the content of mutant mtDNA, we developed a quantitative method of PCR. Using this method, the distribution of the mutant mtDNA was examined in 32 different tissues among 18 autopsied organs from a patient with MELAS, who had shown hypophyseal dysfunction. The percentage of the mutant mtDNA at nucleotide number 3243 in each tissue was ranged between 22% and 95%. The content of the mutant mtDNA was at the highest (95%) in the hypophysis and higher in the cerebral cortex than in the white matter. This study shows a possible correlation of tissue dysfunction with accumulation of the mutant mtDNA within the brain.     

Retransplantation of patients with severe posttransplant hepatitis B in the first allograft

The concentration and submitochondrial distribution of the subunit polypeptides of cytochrome oxidase have been studied in wild type yeast and in different mutants impaired in assembly of this respiratory complex. All the subunit polypeptides of the enzyme are associated with mitochondrial membranes of wild type cells, except for a small fraction of subunits 4 and 6 that is recovered in the soluble protein fraction of mitochondria. Cytochrome oxidase mutants consistently display a severe reduction in the steady-state concentration of subunit 1 due to its increased turnover. As a consequence, most of subunit 4, which normally is associated with subunit 1, is found in the soluble fraction. A similar shift from membrane-bound to soluble subunit 6 is seen in mutants blocked in expression of subunit 5a. In contrast, null mutations in COX6 coding for subunit 6 promote loss of subunit 5a. The absence of subunit 5a in the cox6 mutant is the result of proteolytic degradation rather than regulation of its expression by subunit 6. The possible role of the ATP-dependent proteases Rca1p and Afg3p in proteolysis of subunits 1 and 5a has been assessed in strains with combined mutations in COX6, RCA1, and/or AFG3. Immunochemical assays indicate that another protease(s) must be responsible for most of the proteolytic loss of these proteins.     

Noncontact Er:YAG laser ablation: clinical evaluation

The A3243G mutation of mitochondrial DNA is associated to the MELAS syndrome and to transmitted forms of diabetes mellitus. This mutation exists in a heteroplasmic state and can be present at a minor and hardly detectable level. The aim was to design a method which could be applied to large series of samples and could provide rapid, sensitive and quantitative detection of this mutation in the wild-type mitochondrial DNA background. The ability of ligation detection reaction (LDR) to satisfy these objectives was evaluated. Ligation detection reaction was performed on a model template composed of mixtures of various proportions of plasmids bearing the wild-type or mutant mitochondrial DNA sequence. Radiolabelled or fluorescent primers and the wild-type and mutant LDR products were separated by electrophoresis on conventional denaturating gel or on an Applied Biosystem 373. The ratios of mutant/wild-type products were consistent with the initial ratios of the plasmids in the template. The sensitivity and accuracy of the fluorescence and isotopic detection methods were similar. The detection limit of mutant DNA was 10% of total mitochondrial DNA. The percentage of mutant DNA in DNA samples extracted from leukocytes of 19 patients having the mutation at different levels, was evaluated by fluorescent or isotopic LDR.     

Association of myopathy with large-scale mitochondrial DNA duplications and deletions: which is pathogenic?

We identified large-scale heteroplasmic mitochondrial DNA (mtDNA) rearrangements in a 50-year-old woman with an adult-onset progressive myopathy. The predominant mtDNA abnormality was a 21.2-kb duplicated molecule. In addition, a small population of the corresponding partially deleted 4.6-kb molecule was detected. Skeletal muscle histology revealed fibers that were negative for cytochrome c oxidase (COX) activity and had reduced mtDNA-encoded COX subunits. By single-fiber polymerase chain reaction analysis, COX-negative fibers contained a low number of wild-type or duplicated mtDNA molecules (ie, nondeleted). In situ hybridization demonstrated that the abnormal fibers contained increased amounts of mtDNA compared with normal fibers and that most of the genomes were deleted. We concluded that deleted mtDNA molecules were primarily responsible for the phenotype in this patient.