We addressed the question of whether both mitochondrial and cytoplasmic translation activities decreased simultaneously in human skin fibroblasts with the age of the donors and found that the age-related reduction was limited to mitochondrial translation. Then, to determine which genome, mitochondrial or nuclear, was responsible for this age-related, mitochondria-specific reduction, pure nuclear transfer was carried out from mitochondrial DNA (mtDNA)-less HeLa cells to four fibroblast lines, two from aged subjects, one from a fetus, and one from a patient with cardiomyopathy, and their nuclear hybrid clones were isolated. A normal fibroblast line from the fetus and a respiration-deficient fibroblast line from the patient were used as a positive and a negative control, respectively. Subsequently, the mitochondrial translation and respiration properties of the nuclear hybrid clones were compared. A negative control experiment showed that this procedure could be used to isolate even nuclear hybrids expressing overall mitochondrial respiration deficiency, whereas no respiration deficiencies were observed in any nuclear hybrids irrespective of whether their mtDNAs were exclusively derived from aged or fetal donors. These observations suggest that nuclear-recessive mutations of factors involved in mitochondrial translation but not mtDNA mutations are responsible for age-related respiration deficiency of human fibroblasts.It has been presumed that somatic mutations accumulate in mitochondrial DNA (mtDNA) much faster than in nuclear DNA because mitochondria are highly oxygenic organelles due to their function in producing energy, mtDNA lacks histones protecting it from mutagenic damage, and its repair systems are limited (1). Therefore, it has been proposed that the accumulation of various somatic mutations in mtDNA and the resultant decrease in mitochondrial respiratory function could be involved in aging processes in mammals (2-4). There have been many reports that the respiration capacity of mitochondria in highly oxidative tissues decreases during aging (4). Moreover, the accumulation of somatic and pathogenic mtDNA mutations, which have been shown to cause various kinds of mitochondrial encephalomyopathies (5-8), was also shown to increase with age in normal subjects (9, 10). However, as the nuclear genome encodes most mitochondrial proteins including factors necessary for replication and expression of the mitochondrial genome, it is possible that only mutations in the nuclear genome contribute to the age-related decline of mitochondrial respiratory function. In fact, there is no convincing evidence that mtDNA somatic mutations are responsible for this age-related phenotype.Previously, we observed age-related reduction of cytochrome c oxidase (COX) 1 activity and mitochondrial translation in cultured human skin fibroblasts isolated from donors of various ages (0 -97 years), and in studies on their mtDNA transfer to mtDNA-less ( 0 ) HeLa cells, we showed that mtDNA mutations were not responsible for the observed age...

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We examined whether X radiation induces a particular deletion in the mitochondrial DNA (mtDNA) of the cells of two human squamous cell carcinoma lines with different sensitivity to radiation and in a radiosensitive ataxia telangiectasia (AT) cell line. We used polymerase chain reaction (PCR) to quantify the accumulation of a particular 4977-bp deletion (delta mtDNA4977). PCR products of delta mtDNA4977 were detectable after exposure to 10 Gy in the radioresistant squamous cell carcinoma cells, 2 Gy in the radiosensitive squamous cell carcinoma cells and 1 Gy in the radiosensitive AT cells. These observations suggest that ionizing radiation induces the delta mtDNA4977 in human cells and that the radiation doses required to induce this deletion reflect the sensitivity of cells to radiation.

The present study was conducted to determine the effects of acute overload exercise on mitochondrial DNA and the structure of skeletal muscles. Rats were forced to run for 20 min until reaching complete exhaustion. We detected the large‐scale deletion (7052 bp) of mitochondrial DNA by the nested polymerase chain reaction, and also observed mitochondrial ultrastructural changes in the soleus muscle. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 258–261, 1999

Three clones of non-repetitive sequences and six clones containing repetitive sequences were obtained from micronuclear DNA of Tetrahymena thermophila. All the non-repetitive and three repetitive sequences had the same organization in micro- and macronuclear DNAs as revealed by blot hybridization. On the other hand, the remaining three clones with repetitive sequences had apparently different organization in the two nuclear DNAs. All these repetitive sequences showed a smear on the blot in addition to a number of discrete bands when micronuclear DNA was digested with EcoR I. In macronuclear DNAs, these sequences invariably became one or two bands and the smear disappeared. We conclude that, when a macronucleus develops from a micronucleus, the non-repetitive sequences amplify by more than 20 times with relatively few rearrangement, whereas some selected portions of repeated and/or repeat-contiguous sequences are amplified with rather extensive reorganization.