Human C4orf14 interacts with the mitochondrial nucleoid and is involved in the biogenesis of the small mitochondrial ribosomal subunit.

The bacterial homologue of C4orf14, YqeH, has been linked to assembly of the small ribosomal subunit. Here, recombinant C4orf14 isolated from human cells, co-purified with the small, 28S subunit of the mitochondrial ribosome and the endogenous protein co-fractionated with the 28S subunit in sucrose gradients. Gene silencing of C4orf14 specifically affected components of the small subunit, leading to decreased protein synthesis in the organelle. The GTPase of C4orf14 was critical to its interaction with the 28S subunit, as was GTP. Therefore, we propose that C4orf14, with bound GTP, binds to components of the 28S subunit facilitating its assembly, and GTP hydrolysis acts as the release mechanism. C4orf14 was also found to be associated with human mitochondrial nucleoids, and C4orf14 gene silencing caused mitochondrial DNA depletion. In vitro C4orf14 is capable of binding to DNA. The association of C4orf14 with mitochondrial translation factors and the mitochondrial nucleoid suggests that the 28S subunit is assembled at the mitochondrial nucleoid, enabling the direct transfer of messenger RNA from the nucleoid to the ribosome in the organelle.

Mitochondrial nucleoid interacting proteins support mitochondrial protein synthesis.

Mitochondrial ribosomes and translation factors co-purify with mitochondrial nucleoids of human cells, based on affinity protein purification of tagged mitochondrial DNA binding proteins. Among the most frequently identified proteins were ATAD3 and prohibitin, which have been identified previously as nucleoid components, using a variety of methods. Both proteins are demonstrated to be required for mitochondrial protein synthesis in human cultured cells, and the major binding partner of ATAD3 is the mitochondrial ribosome. Altered ATAD3 expression also perturbs mtDNA maintenance and replication. These findings suggest an intimate association between nucleoids and the machinery of protein synthesis in mitochondria. ATAD3 and prohibitin are tightly associated with the mitochondrial membranes and so we propose that they support nucleic acid complexes at the inner membrane of the mitochondrion.

Actin and myosin contribute to mammalian mitochondrial DNA maintenance.

Mitochondrial DNA maintenance and segregation are dependent on the actin cytoskeleton in budding yeast. We found two cytoskeletal proteins among six proteins tightly associated with rat liver mitochondrial DNA: non-muscle myosin heavy chain IIA and ß-actin. In human cells, transient gene silencing of MYH9 (encoding non-muscle myosin heavy chain IIA), or the closely related MYH10 gene (encoding non-muscle myosin heavy chain IIB), altered the topology and increased the copy number of mitochondrial DNA; and the latter effect was enhanced when both genes were targeted simultaneously. In contrast, genetic ablation of non-muscle myosin IIB was associated with a 60% decrease in mitochondrial DNA copy number in mouse embryonic fibroblasts, compared to control cells. Gene silencing of ß-actin also affected mitochondrial DNA copy number and organization. Protease-protection experiments and iodixanol gradient analysis suggest some ß-actin and non-muscle myosin heavy chain IIA reside within human mitochondria and confirm that they are associated with mitochondrial DNA. Collectively, these results strongly implicate the actomyosin cytoskeleton in mammalian mitochondrial DNA maintenance.

A tRNA suppressor mutation in human mitochondria.

Mitochondrial mutations are associated with a wide spectrum of human diseases. A common class of point mutations affects tRNA genes, and mutations in the tRNA-leu(UUR) gene (MTTL1) are the most frequently detected. In earlier studies, we showed that lung carcinoma cybrid cells containing high levels (greater than 95%) of mutated mtDNA from a patient with the pathological nucleotide pair (np) 3243 tRNA-leu(UUR) mutation can remain genotypically stable over time, and exhibit severe defects in mitochondrial respiratory metabolism. From such a cybrid containing 99% mutated mtDNA, we have isolated a spontaneous derivative that retains mutant mtDNA at this level but which has nevertheless reverted to the wild-type phenotype, based on studies of respiration, growth in selective media, mitochondrial protein synthesis and biogenesis of mitochondrial membrane complexes. The cells are heteroplasmic for a novel anticodon mutation in tRNA-leu(CUN) at np 12300, predicted to generate a suppressor tRNA capable of decoding UUR leucine codons. The suppressor mutation represents approximately 10% of the total mtDNA, but was undetectable in a muscle biopsy sample taken from the original patient or in the parental cybrid. These results indicate that the primary biochemical defect in cells with high levels of np 3243 mutated mtDNA is the inability to translate UUR leucine codons.

The accessory subunit of mitochondrial DNA polymerase gamma determines the DNA content of mitochondrial nucleoids in human cultured cells.

The accessory subunit of mitochondrial DNA polymerase gamma, POLGbeta, functions as a processivity factor in vitro. Here we show POLGbeta has additional roles in mitochondrial DNA metabolism. Mitochondrial DNA is arranged in nucleoprotein complexes, or nucleoids, which often contain multiple copies of the mitochondrial genome. Gene-silencing of POLGbeta increased nucleoid numbers, whereas over-expression of POLGbeta reduced the number and increased the size of mitochondrial nucleoids. Both increased and decreased expression of POLGbeta altered nucleoid structure and precipitated a marked decrease in 7S DNA molecules, which form short displacement-loops on mitochondrial DNA. Recombinant POLGbeta preferentially bound to plasmids with a short displacement-loop, in contrast to POLGalpha. These findings support the view that the mitochondrial D-loop acts as a protein recruitment centre, and suggest POLGbeta is a key factor in the organization of mitochondrial DNA in multigenomic nucleoprotein complexes.

The molecular pathology of respiratory-chain dysfunction in human mitochondrial myopathies.

Some of the different molecular pathologies of respiratory-chain dysfunction in human mitochondrial myopathies will be reviewed in relation to the findings in 58 cases. Deletions of mitochondrial DNA were identified in 21 cases [36%]. There was some correlation between the sites of the deletion and the mitochondrial biochemistry in patients with defects of Complex I but not in cases with more extensive loss of respiratory chain activity. Complex I and Complex IV polypeptides were usually normal in deleted cases. Non-deleted cases, however, often showed specific subunit deficiencies which involved the products of both nuclear and mitochondrial genes. Immunoblots of respiratory-chain polypeptides in one case pointed to defective translocation of the Rieske precursor from the cytosol into the mitochondria. The pathogenic role of circulating autoantibodies to specific matrix proteins and the nature of the target antigens in two patients with mitochondrial encephalomyopathies and respiratory-chain dysfunction will also be discussed.

Genotypic stability, segregation and selection in heteroplasmic human cell lines containing np 3243 mutant mtDNA.

The mitochondrial genotype of heteroplasmic human cell lines containing the pathological np 3243 mtDNA mutation, plus or minus its suppressor at np 12300, has been followed over long periods in culture. Cell lines containing various different proportions of mutant mtDNA remained generally at a consistent, average heteroplasmy value over at least 30 wk of culture in nonselective media and exhibited minimal mitotic segregation, with a segregation number comparable with mtDNA copy number (>/=1000). Growth in selective medium of cells at 99% np 3243 mutant mtDNA did, however, allow the isolation of clones with lower levels of the mutation, against a background of massive cell death. As a rare event, cell lines exhibited a sudden and dramatic diversification of heteroplasmy levels, accompanied by a shift in the average heteroplasmy level over a short period (<8 wk), indicating selection. One such episode was associated with a gain of chromosome 9. Analysis of respiratory phenotype and mitochondrial genotype of cell clones from such cultures revealed that stable heteroplasmy values were generally reestablished within a few weeks, in a reproducible but clone-specific fashion. This occurred independently of any straightforward phenotypic selection at the individual cell-clone level. Our findings are consistent with several alternate views of mtDNA organization in mammalian cells. One model that is supported by our data is that mtDNA is found in nucleoids containing many copies of the genome, which can themselves be heteroplasmic, and which are faithfully replicated. We interpret diversification and shifts of heteroplasmy level as resulting from a reorganization of such nucleoids, under nuclear genetic control. Abrupt remodeling of nucleoids in vivo would have major implications for understanding the developmental consequences of heteroplasmy, including mitochondrial disease phenotype and progression.

Rhabdomyosarcoma rho(0) cells: isolation and characterization of a mitochondrial DNA depleted cell line with 'muscle-like' properties.

Mutations of mitochondrial DNA are a significant cause of neuromuscular disease. Pathological mutant mitochondrial DNA has been studied in control nuclear backgrounds. These experiments entailed transfer of patient-derived mitochondria to rho(0) cells that lack mtDNA. A limitation of these studies has been the fact that the control nuclear backgrounds were unrelated to the affected tissues of patients. Therefore a rhabdomyosarcoma cell line that has 'muscle-like' properties was tested to determine whether it could be depleted of mtDNA. A human rhabdomyosarcoma cell line was treated with the DNA intercalating dye ethidium bromide (3, 8-diamino-5-ethyl-6-phenylphenanthridinium bromide) for 45 days. The treatment induced complete and permanent loss of mitochondrial DNA (rho(0)) in the rhabdomyosarcoma cells, as mtDNA remained undetectable after 8 months of growth in medium without drug. Crucially, the rhabdomyosarcoma rho(0) cells retained the ability to differentiate into myotubes with expression of muscle specific isoenzymes. The rhabdomyosarcoma rho(0) cell line provides a model system for studying pathological mutant mtDNA in cells that more closely resemble human muscle than the hitherto available human rho(0) cell lines.

Evidence for intramitochondrial complementation between deleted and normal mitochondrial DNA in some patients with mitochondrial myopathy.

Twenty-three patients with mitochondrial myopathies and mitochondrial DNA deletions in muscle were studied by means of deletion mapping and sequencing, histochemistry and polarography. Histochemistry showed significantly less focal cytochrome oxidase deficiency relative to number of ragged red fibres when the deletion did not involve reading frames for cytochrome oxidase subunits. Polarography in such patients showed defects exclusively involving complex I, in contrast to the others with larger deletions who generally had more diffuse respiratory chain defects. Analysis of other published histochemical data showed similar findings to our own. It is concluded that translation of a proportion of deleted mitochondrial DNAs occurs in at least some patients with mitochondrial DNA deletions, implying that deleted and normal mitochondrial genomes share transfer RNAs within mitochondria in such cases.

The molecular pathology of human respiratory chain defects.

Deletions of the mitochondrial genome were identified in 21 out of 58 patients (36 percent) with mitochondrial myopathies, 47 of whom had defects in the mitochondrial respiratory chain. In cases with Complex I defects, the deleted regions of mtDNA, were confined to structural genes encoding Complex I subunits but additionally involved the intervening tRNA genes and in one case included the large and small rRNA genes. In cases with more extensive loss of respiratory chain function, the deletions eliminated genes encoding subunits of Complexes I, IV and V, as well as several tRNAs. Complex I and Complex IV polypeptides were usually normal in deleted cases. This was in contrast to 7 out of 22 patients without detectable mtDNA deletions, who showed specific deficiencies of subunits encoded by nuclear genes. Further studies in one of these cases pointed to defective translocation of the Rieske precursor from the cytosol into the mitochondria. The genetic basis of the disease in 15 cases without detectable deletions or specific subunit deficiencies, remains unknown. The multiple biochemical abnormalities encountered in these cases would be consistent with more subtle alterations of the mitochondrial genome.

Mitochondrial myopathies.

The mitochondrial myopathies give rise to a diverse group of clinical syndromes, variably involving skeletal muscle and the central nervous system, with onset in childhood or adult life. In vitro studies of mitochondrial metabolism have identified a variety of functional defects of the respiratory chain, predominantly affecting complex I or complex III in adults, and complex IV in children. The increased incidence of maternal, as opposed to paternal, transmission in familial mitochondrial myopathy has led to the suggestion that these disorders may be caused by mutations of mitochondrial (mt) DNA. This hypothesis is derived from observations that mtDNA encodes subunits of the respiratory chain proteins and is exclusively maternally transmitted. Analysis of muscle mtDNA shows two populations, one normal and the other deleted by up to nearly half its length, in about 40% of cases of mitochondrial myopathy. Only a single normal length population of mtDNA is seen in blood from these patients, and in blood and muscle from control subjects. Patients with muscle mtDNA deletions reported to date have all presented with progressive external ophthalmoplegia, including some with the Kearns-Sayre syndrome. They rarely have affected relatives. Deletions are not detected in cases of proximal myopathy alone, or those with adult onset syndromes predominantly affecting the central nervous system. There is no clear correlation between the deleted coding regions and the biochemical defects; even patients with seemingly identical muscle mtDNA deletions may be clinically and biochemically heterogeneous.

The np 3243 MELAS mutation: damned if you aminoacylate, damned if you don't.

The np 3243 MELAS mtDNA mutation in tRNA(leu(UUR))has been variously proposed as a loss-of-function or as a gain-of-function mutation, based on apparently contradictory studies in cultured cell lines. A new report describing the molecular effects of the mutation in vivo now mirrors this variability. This should prompt a more systematic re-investigation of cells carrying the mutation, in order to separate primary from secondary and pathogenic from compensatory effects, all of which may contribute to disease phenotype. Nuclear genetic and developmental background, mitochondrial haplotype, and epigenetic effects may all influence the pathological outcome. Defects in both base-modification and aminoacylation of the mutant tRNA could play critical roles.

Restriction endonuclease analysis of leukocyte mitochondrial DNA in Leber's optic atrophy.

In order to test the hypothesis that Leber's optic atrophy may be caused by mutation of the mitochondrial (mt) genome, restriction fragment length polymorphism in leukocyte mt DNA was studied in 16 patients with Leber's optic atrophy, 28 of their unaffected matrilineal relatives, and 35 normal control subjects. No differences in restriction fragment patterns were observed between affected and unaffected individuals in the same maternal line, and there was no evidence of major deletion of mt DNA in patients. This study provides no positive evidence of mitochondrial inheritance in Leber's optic atrophy but does not exclude it.

Genetic heterogeneity and mitochondrial DNA heteroplasmy in Leber's hereditary optic neuropathy.

Analysis of mitochondrial DNA from patients with Leber's hereditary optic neuropathy and their relatives showed that the previously reported mutation at base pair (bp) 11778, shown by loss of a recognition site for the restriction endonuclease SfaNI, was present in only four out of eight families. This mutation was associated with a poor prognosis for visual recovery, whereas four of five affected males without the 11778 bp mutation followed for four years or more had regained useful vision. All but one of the subjects showing the SfaNI site loss had a variable mixture of mutant and normal mitochondrial DNA in peripheral blood, and the relative proportions appeared to be correlated with the risk of developing or transmitting Leber's hereditary optic neuropathy.

Coupled leading- and lagging-strand synthesis of mammalian mitochondrial DNA.

Analysis of mammalian mtDNA by two-dimensional agarose gel electrophoresis revealed two classes of replication intermediate. One was resistant to single-strand nuclease digestion and displayed the mobility properties of coupled leading- and lagging- strand replication products. Intermediates of coupled, unidirectional mtDNA replication were found in mouse liver and human placenta and were the predominant species in cultured cells recovering from transient mtDNA replication. Replication intermediates sensitive to single-strand nuclease were most abundant in untreated cultured cells. These are presumed to derive from the orthodox, strand-asynchronous mode of mtDNA replication. These findings indicate that two modes of mtDNA replication operate in mammalian cells and that changes in mtDNA copy number involve an alteration in the mode of mtDNA replication.

Comprehensive care of the child with a chronic condition. Part 1. Understanding chronic conditions in childhood.

Understanding the care of children with chronic illnesses and disabilities is an important part of the practice of pediatrics. Children with special health care needs and their families benefit from the support which comprehensive, coordinated, accessible, and responsive services provide. Although the primary care pediatrician is often most appropriate to serve as the overall coordinator of such services, many challenges to providing such care exist. Part 2, "Primary Care Management," will suggest guiding principles and management structures that allow the pediatrician to respond most effectively over time to the needs of children with chronic conditions and their families.

Behaviour of a population of partially duplicated mitochondrial DNA molecules in cell culture: segregation, maintenance and recombination dependent upon nuclear background.

We have studied the dynamics of mitochondrial DNA maintenance and segregation in human cells using serial cybrid transfer of partially duplicated mitochondrial DNA, from a mitochondrial myopathy patient, to two distinct recipient cell types. The results indicate two radically different outcomes dependent upon nuclear background. In one case (lung carcinoma) there is systematic loss of the partial duplication by an implied recombinational mechanism. In another nuclear background (osteosarcoma) the duplicated molecules can survive, having only a marginal effect on mitochondrial respiratory function. Moreover, in the osteosarcoma nuclear background further disturbances of mtDNA maintenance frequently follow from cybrid transfer. These are progressive, catastrophic loss of mtDNA and further rearrangement to generate partially triplicated molecules. The results imply differential expression of nuclear genes regulating mtDNA copy number, replication and recombination in different human cell types.

Impaired ATP synthase assembly associated with a mutation in the human ATP synthase subunit 6 gene.

Mutations in human mitochondrial DNA are a well recognized cause of disease. A mutation at nucleotide position 8993 of human mitochondrial DNA, located within the gene for ATP synthase subunit 6, is associated with the neurological muscle weakness, ataxia, and retinitis pigmentosa (NARP) syndrome. To enable analysis of this mutation in control nuclear backgrounds, two different cell lines were transformed with mitochondria carrying NARP mutant mitochondrial DNA. Transformant cell lines had decreased ATP synthesis capacity, and many also had abnormally high levels of two ATP synthase sub-complexes, one of which was F(1)-ATPase. A combination of metabolic labeling and immunoblotting experiments indicated that assembly of ATP synthase was slowed and that the assembled holoenzyme was unstable in cells carrying NARP mutant mitochondrial DNA compared with control cells. These findings indicate that altered assembly and stability of ATP synthase are underlying molecular defects associated with the NARP mutation in subunit 6 of ATP synthase, yet intrinsic enzyme activity is also compromised.

Prenatal diagnosis of mitochondrial DNA8993 T----G disease.

We have previously described a family with a neurological syndrome comprising neurogenic muscle weakness, ataxia, retinitis pigmentosa, and variable sensory neuropathy, seizures, and mental retardation or dementia. This is associated with a heteroplasmic point mutation of mtDNA at bp 8993. The mother of a severely affected child underwent prenatal diagnosis in two further pregnancies. Analysis of chorionic villus samples showed a higher proportion of mutant mtDNA on both occasions, and this was reflected in the majority of fetal tissues, including brain and muscle. Prenatal diagnosis is a rational approach to the prevention of severe diseases caused by point mutations of mtDNA but is currently hampered by incomplete knowledge concerning the proportion of mutant mtDNA: its relationship to disease severity, how it may change during fetal and postnatal development, and its tissue distribution.