Microheterogeneity detected in circular dimer mitochondrial DNA

Studies of sequence heterogeneity of mitochondrial DNA from rat and mouse tissues: evidence for an increased frequency of deletions/additions with aging

To obtain information on the extent of random nucleotide changes in mitochondrial DNA (mtDNA) from different organs of young adult and senescent Fischer 344 rats, the temperature of thermal denaturation (tm) was measured in (1) the native mtDNA cut at a single SstI site and (2) the reannealed duplexes formed after the initial melting of the mtDNA sample. No change was found between the two tm values in either young or senescent mtDNA, suggesting that the overall mismatch in nucleotide sequence in these samples was below the resolution of the method estimated at about 0.2%. In another experiment, mtDNA samples from young adult or senescent BALB/c mouse liver were digested with EcoRI, denatured and allowed to reanneal. The duplexes formed by the 14-kb EcoRI fragment were analyzed in randomly taken electron micrographs for the occurrence of mismatched segments. About 1.8% of reconstituted duplexes in adult mtDNA and 11% of those in senescent mtDNA contained small loops or knobs suggestive of deletions/additions of about 400 +/- 150 nucleotides. These data correspond to about 1% of the native mtDNA population in adult liver and about 5% in senescent liver having deleted/inserted segments. Although deletions/insertions may occur at variable sites, their distribution appears to be non-random. These findings suggest that small sequence rearrangements, which have been observed previously in unicircular dimers of mouse and human mtDNA, occur also in monomeric mtDNA from normal tissues and accumulate with aging.

Structural and replicative forms of mitochondrial DNA in tissues from adult and senescent BALB/c mice and Fischer 344 rats

Age-related changes in the structure and replication of mitochondrial DNA (mtDNA) were investigated in different organs from young adult (9-10 months' old) and senescent (28-29 months' old) BALB/c mice and Fischer 344 rats. Total mtDNA from brain, heart, kidney and liver was isolated by centrifugation in ethidium bromide-CsCl gradients and examined for the occurrence of complex forms and replicative intermediates by electron microscopy. The frequency of catenated mtDNA (interlinked molecules containing two or more circular units) varied from about 2.5% to 5% in adult tissues and showed a small increase in the majority of senescent organs. The frequency of double-sized circular molecules, or circular dimers, was very low in adult tissues, with an average of about 0.04% in mice and 0.1% in rats. The frequency of circular dimers increased with aging to 1.9% in mouse brain and 1.5% in rat kidney, with smaller increases (0.4% and 0.7%) in heart mtDNA from both species; there was no significant increase in the other organs. It is suggested that the increase in the frequency of circular dimer mtDNA reflects an overall deterioration of tissue physiology rather than intrinsic senescent changes in the mitochondria. The frequencies and types of the various replicative forms of mtDNA varied significantly according to tissue but not according to species or donor age. The only exception was a significant increase in the frequency of larger replicative forms in senescent mouse liver, to about 20% compared with 12% in adult liver, suggesting an age-related change in the rate of mtDNA replication and/or turnover in this organ.

Variation in the frequency of complex forms of mitochondrial DNA in different brain regions of senescent mice

It was shown previously that the frequency of an aberrant form of mitochondrial DNA (mtDNA), double-sized circular molecules or circular dimers, increased significantly in the brain of senescent mice, to about 2% versus less than 0.1% in the brain of adult mice. To follow up these observations, we isolated total mtDNA from 6 different brain regions of 29-month-old male BALB/c mice and examined it for the occurrence of circular dimers and other complex forms by electron microscopy. There was a statistically highly significant variability in the occurrence of circular dimer mtDNA among the 6 brain regions. The frequencies of circular dimers were: medulla, 3.3%; cortex, 1.7%; midbrain, 1.1%; cerebellum, 0.9%; hippocampus, 0.5%; and striatum, 0.2%. The frequency of catenated (topologically interlinked) molecules varied only slightly, from 4 to 6%. On the basis of the available literature, a correlation appears to exist between age-related tissue pathology of the mouse brain and the increased incidence of circular dimer mtDNA. Although no cause-effect relationships can be established, it is suggested that the frequency of circular dimer mtDNA may be a useful marker in assessing the general physiological condition of the aging brain.