Conservation of genes coding for proteins synthesized in human mitochondria

The deafness-associated mitochondrial DNA mutation at position 7445, which affects tRNASer(UCN) precursor processing, has long-range effects on NADH dehydrogenase subunit ND6 gene expression

The pathogenetic mechanism of the deafness-associated mitochondrial DNA (mtDNA) T7445C mutation has been investigated in several lymphoblastoid cell lines from members of a New Zealand pedigree exhibiting the mutation in homoplasmic form and from control individuals. We show here that the mutation flanks the 3' end of the tRNASer(UCN) gene sequence and affects the rate but not the sites of processing of the tRNA precursor. This causes an average reduction of approximately 70% in the tRNASer(UCN) level and a decrease of approximately 45% in protein synthesis rate in the cell lines analyzed. The data show a sharp threshold in the capacity of tRNASer(UCN) to support the wild-type protein synthesis rate, which corresponds to approximately 40% of the control level of this tRNA. Strikingly, a 7445 mutation-associated marked reduction has been observed in the level of the mRNA for the NADH dehydrogenase (complex I) ND6 subunit gene, which is located approximately 7 kbp upstream and is cotranscribed with the tRNASer(UCN) gene, with strong evidence pointing to a mechanistic link with the tRNA precursor processing defect. Such reduction significantly affects the rate of synthesis of the ND6 subunit and plays a determinant role in the deafness-associated respiratory phenotype of the mutant cell lines. In particular, it accounts for their specific, very significant decrease in glutamate- or malate-dependent O2 consumption. Furthermore, several homoplasmic mtDNA mutations affecting subunits of NADH dehydrogenase may play a synergistic role in the establishment of the respiratory phenotype of the mutant cells.

The interorganellar interaction between distinct human mitochondria with deletion mutant mtDNA from a patient with mitochondrial disease and with HeLa mtDNA

For the examination of possible intermitochondrial interaction of human mitochondria from different cells, cybrids were constructed by introducing HeLa mitochondria into cells with respiration-deficient (rho-) mitochondria. Respiration deficiency was due to the predominance of mutant mtDNA with a 5,196-base pair deletion including five tRNA genes (DeltamtDNA5196). The HeLa mtDNA and DeltamtDNA5196 encoded chloramphenicol-resistant (CAPr) and chloramphenicol-sensitive (CAPs) 16 S rRNA, respectively. The first evidence for the interaction was that polypeptides exclusively encoded by DeltamtDNA5196 were translated on the introduction of HeLa mitochondria, suggesting supplementation of the missing tRNAs by rho- mitochondria from HeLa mitochondria. Second, the exchange of mitochondrial rRNAs was observed; even in the presence of CAP, CAPs DeltamtDNA5196-specific polypeptides as well as those encoded by CAPr HeLa mtDNA were translated in the cybrids. These phenomena can be explained assuming that the translation in rho- mitochondria was restored by tRNAs and CAPr 16 S rRNA supplied from HeLa mitochondria, unambiguously indicating interorganellar interaction. These observations introduce a new concept of the dynamics of the mitochondrial genetic system and help in understanding the relationship among mtDNA mutations and expression of human mitochondrial diseases and aging.

Mitochondrial myopathies: clinical and biochemical features of 30 patients with major deletions of muscle mitochondrial DNA

Analysis of mitochondrial DNA (mtDNA) in muscle and blood from 72 patients with mitochondrial myopathy showed that 30 had major deletions of a variable proportion of muscle mtDNA. All of these 30 patients presented with progressive external ophthalmoplegia and limb weakness, and 8 had the additional features of the Kearns-Sayre syndrome. Of the 42 patients without detectable muscle mtDNA deletions, 10 had progressive external ophthalmoplegia and limb weakness, 2 had the Kearns-Sayre syndrome, 11 had limb weakness without extraocular involvement, and 19 had multisystem disorders predominantly affecting the central nervous system. Only 2 patients with mtDNA deletions had clinically affected relatives, compared with 10 of those without deletions. In the 4 patients with polarographic defects exclusively involving complex I (NADH coenzyme Q reductase), the deleted protein-coding genes were confined to those for complex I subunits. Thirteen other patients with apparently identical deletions had variable clinical and biochemical features. Immunoblots of complex I polypeptides from patients with deletions were either indistinguishable from controls or showed only a mild generalized decrease in all identifiable subunits.

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.

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.

Mitochondrial DNA polymorphism in mitochondrial myopathy

In order to test the hypothesis that mitochondrial myopathy may be caused by mutation of the mitochondrial (mt) genome, restriction fragment length polymorphism in leucocyte mt DNA has been studied in 38 patients with mitochondrial myopathy, 44 of their unaffected matrilineal relatives, and 35 normal control subjects. Previously unreported mt DNA polymorphisms were identified in both patients and controls. 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 mitochondrial myopathy, but this has not been excluded.

Inherent resistance of HeLa cell derivatives to paromomycin

The human tumor-derived cell line HeLa S3 and nuclear and mitochondrial gene mutants derived from it are resistant to the aminoglycoside antibiotic, paromomycin (PAR). Other carcinoma-derived cells, SV40-transformed cells, and four human diploid fibroblast cell lines are all sensitive to PAR. Sensitivity is dependent on cell density, and at cell numbers greater than 400/cm2 sensitive cells will proliferate in PAR. The resistance to PAR is inherited in a dominant manner in cell-to-cell fusion hybrids, but is not transferred in cytoplast-to-cell fusions. PAR resistance is therefore encoded by a nuclear gene(s). Resistance to PAR is not caused by changes in the response of mitochondrial or cytoplasmic protein synthesis to PAR in vitro. The uptake of PAR is similar in resistant and sensitive cells, and dimethyl sulfoxide does not render resistant cells more sensitive. Thus, HeLa cell PAR resistance is unlike previously reported ribosomal mutations and may derive from differences in the intracellular metabolism of PAR.

Mitotic segregation of mitochondrial DNAs in human cell hybrids and expression of chloramphenicol resistance

The relationship between the chloramphenicol (CAP)-resistant phenotype and the mtDNA genotype was investigated in segregating human, HeLa X HT1080, somatic cell hybrids. The parental mtDNAs were quantitated in heteroplasmic cells by using restriction fragment length polymorphisms (RFLPs) detected in Southern blots. CAP-resistant (R) X CAP-sensitive (S) hybrids selected and grown in CAP for brief periods had as little as 25% CAP-R mtDNA. With prolonged selection, the CAP-R mtDNA increased to 90-95%. Hybrids selected and passaged without CAP either retained both mtDNAs or progressively lost one mtDNA (mitotic segregation). The CAP-resistance phenotype of these hybrids changed abruptly when the proportion of CAP-R mtDNAs fluctuated around approximately 10% (threshold effect). Hybrids with greater than 25% HT1080 mtDNA had an additional characteristic. They cloned better with CAP than without. The cloning efficiency in CAP of hybrids having 90% HT1080 mtDNA was more than fivefold greater than the control.

Comparison of mitochondrially synthesized polypeptides of human, mouse, and monkey cell lines by a two-dimensional protease gel system

Mitochondrially synthesized polypeptides of human, monkey, and mouse cells were compared using SDS-polyacrylamide gel electrophoresis (SDS-PAGE). A single molecular weight variant, the major interspecific variant (MIV), was identified in human cells as compared to monkey and mouse. The peptide maps of MIV were compared between the three species using a two-dimensional proteolytic digest (2D-PD) gel system. A number of conserved peptides were found, indicating that the MIVs have a common function. Other MIV peptides were species specific. These results confirm the conserved nature of mitochondrial polypeptides and demonstrate the utility of 2D-PD gels in testing for protein alleles and detecting subtle protein variants.

Assignment of two mitochondrially synthesized polypeptides to human mitochondrial DNA and their use in the study of intracellular mitochondrial interaction

Two mitochondrially synthesized marker polypeptides, MV-1 and MV-2, were found in human HeLa and HT1080 cells. These were assigned to the mitochondrial DNA in HeLa-HT1080 cybrids and hybrids by demonstrating their linkage to cytoplasmic genetic markers. These markers include mitochondrial DNA restriction site polymorphisms and resistance to chloramphenicol, an inhibitor of mitochondrial protein synthesis. In the absence of chloramphenicol, the expression of MV-1 and MV-2 in cybrids and hybrids was found to be directly proportional to the ratio of the parental mitochondrial DNAs. In the presence of chloramphenicol, the marker polypeptide linked to the chloramphenicol-sensitive mitochondrial DNA continued to be expressed. This demonstrated that resistant and sensitive mitochondrial DNAs can cooperate within a cell for gene expression and that the CAP-resistant allele was dominant or codominant to sensitive. Such cooperation suggests that mitochondrial DNAs can be exchanged between mitochondria.

Energy metabolism in cultured human fibroblasts during aging in vitro

To explore the relationship between energy metabolism and the limited replicative life span of cultured human fibroblasts, we studied several bioenergetic parameters in normal fibroblasts at early passage (young cells) and at late passage (old cells) and early passage cells from a subject with the Hutchinson-Gilford (progeria) syndrome. Old cells consumed more glucose and produced more lactate during growth, but O2 consumption, both basal and following maximum uncoupling of oxidative phosphorylation by SF-6847, was the same as in young cells. Progeria cells produced the most lactate but did not consume more glucose, while their basal and uncoupled O2 consumption was similar to that of young and old cells during both log and confluent states. Consumption of glutamine, a source of both oxidative energy and lactate, was approximately the same in all three cell types as was 14CO2 production from 2- 14C-pyruvate and 5- 14C-glutamate. ATP and ADP concentrations were similar in all cell types with a rise in the ATP/ADP ratio during growth from log to confluent state. Thus, old and progeria cells, in contrast to young cells, produce more lactate during growth consistent with a rise in energy demand and/or inefficiency of oxidative phosphorylation. Although limitations in total energy output do not appear to be causal to the loss of replicative capacity in normal cells after serial passage, they could play a role in the curtailed replicative capacity of progeria cells.

Assignment of two mitochondrially synthesized polypeptides to human mitochondrial DNA and their use in the study of intracellular mitochondrial interaction

Two mitochondrially synthesized marker polypeptides, MV-1 and MV-2, were found in human HeLa and HT1080 cells. These were assigned to the mitochondrial DNA in HeLa-HT1080 cybrids and hybrids by demonstrating their linkage to cytoplasmic genetic markers. These markers include mitochondrial DNA restriction site polymorphisms and resistance to chloramphenicol, an inhibitor of mitochondrial protein synthesis. In the absence of chloramphenicol, the expression of MV-1 and MV-2 in cybrids and hybrids was found to be directly proportional to the ratio of the parental mitochondrial DNAs. In the presence of chloramphenicol, the marker polypeptide linked to the chloramphenicol-sensitive mitochondrial DNA continued to be expressed. This demonstrated that resistant and sensitive mitochondrial DNAs can cooperate within a cell for gene expression and that the CAP-resistant allele was dominant or codominant to sensitive. Such cooperation suggests that mitochondrial DNAs can be exchanged between mitochondria.

Werner's syndrome: a review of recent research with an analysis of connective tissue metabolism, growth control of cultured cells, and chromosomal aberrations

Werner's syndrome is a rare, autosomal recessive condition with multiple progeroid features, but it is an imitation of aging rather than accelerated or premature senescence. Somatic chromosome aberrations occur in multiple tissues in vivo and in vitro, and there is an increased incidence of neoplasia. Thus. Werner's syndrome can be classified in the group of chromosome instability syndromes. Recent findings provide additional support for the concept that there is an aberration of connective tissue metabolism in Werner's syndrome, but it is unclear whether this is a primary or secondary manifestation of the underlying genetic defect. Abnormal growth characteristics are observed in cultured skin fibroblast-like cells and this provides another avenue for current research. Identification of the basic genetic defect in Werner's syndrome might clarify our understanding of the normal aging process in general, or might elucidate specific aspects such as the development of neoplasia, atherosclerosis, diabetes, or osteoporosis.

Fidelity of protein synthesis does not decline during aging of cultured human fibroblasts

To ascertain whether the fidelity of protein synthesis declines during cellular aging in vitro, we have developed a cell-free protein synthesizing system from cultured human fibroblasts which actively incorporates phenylalanine into acid-insoluble material upon addition of poly (U). The accuracy of poly (U)-directed protein synthesis was determined by comparing the ratio of leucine to phenylalanine incorporation in extracts of early- and late-passage fibroblasts derived from normal persons and from subjects with two genetic disorders of premature aging, progeria, and Werner syndrome. The results show no decline in translational fidelity at late passage or in prematurely aging cells, and thus fail to support the error catastrophe theory of cellular aging.

Protein synthetic errors do not increase during aging of cultured human fibroblasts

To test the error catastrophe theory of aging we determined the error frequency of protein synthesis in several strains of cultured human fibroblasts at early and late passage. Error rates were calculated from analysis of native and substituted actins on two-dimensional gels of cellular proteins after induction of mistranslation by histidine starvation in the presence of histidinol. Early-passage cells from fetal, young, and old donors and cells from subjects with the Hutchinson-Gilford and Werner syndromes of accelerated aging had similar error frequencies. Late-passage cells from fetal, young, and old normal donors had similar or lower error frequencies than corresponding early-passage cells. No correlation was observed between error frequency, donor age, or maximal life span in vitro. We also examined an immortal cell line, simian virus 40-transformed W138 fibroblasts. These cells had a significantly elevated rate of mistranslation (2.8 +/- 0.2 x 10(-4))(+/- SEM) compared to their untransformed counterpart WI38 (0.6 +/- 0.1 X 10(-4)) or all diploid cells combined (1.1 +/- 0.1 x 10(-4)). Taken together, the data fail to support the error catastrophe theory of aging.