Complex mitochondrial DNA in animal thyroids. A comparative study

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.

Intraspecific nucleotide sequence variability surrounding the origin of replication in human mitochondrial DNA

We have cloned the major noncoding region of human mitochondrial DNA (mtDNA) from 11 human placentas. Partial nucleotide sequences of five of these clones have been determined and they share a maximum of 900 bp around the origin of H-strand replication. Alignment of these sequences with others previously determined has revealed a striking pattern of nucleotide substitutions and insertion/deletion events. The level of sequence divergence significantly exceeds the reported estimates of divergence in coding regions. Two particularly hypervariable regions have also been defined. More than 96% of the base changes are transitions, and length alterations have occurred exclusively by addition or deletion of mono-or dinucleotide segments within serially repeating stretches. This region of the mitochondrial genome, which contains the initiation sites for replication and transcription, is the least conserved among species with respect to both sequence and length (Anderson et al., 1981; Walberg and Clayton, 1981). Despite this overall lack of primary sequence conservation, several consistencies appear among the available mammalian mtDNA sequences within this region. Between species, a conserved linear array of characteristic stretches exists which nonetheless differ in primary sequence. Among humans, several conserved blocks of nucleotides appear within domains deleted from the mtDNA of other species. These observations are consistent with both a species-specificity of nucleotide sequence, and a preservation of the necessary genetic functions among species. This provides a model for the evolution of protein-nucleic acid interactions in mammalian mitochondria.

Changes in mitochondrial DNA in cardiac hypertrophy in the rat

We studied DNA (mtDNA) replication in adult female rat hearts undergoing hypertrophy secondary to constriction of the ascending aorta. MtDNA was measured in isolated mitochondria by a fluorometric method adapted for that purpose. The conditions for removal of contaminating nuclear DNA were developed, and the purity of the mtDNA was assessed from its molecular conformation (open and closed circles) and by renaturation-kinetic analysis. The mtDNA concentration in mitochondria, expressed as micrograms of DNA per milligram of mitochondrial protein, increased 2, 4, and 7 days postoperatively by 21, 73, and 98%, respectively. Similar results were obtained when mtDNA was expressed per nonomole of cytochrome a. The population of replicative intermediates of mtDNA was analyzed by electron microscopy. In normal hearts, we observed molecular forms characteristic of animal mtDNA, such as circular monomers and dimers, catenated molecules, D-loops, expanded D-loops, and gapped molecules. D-loop frequency, which was near 50% in the mtDNA of control hearts, was markedly reduced to 5-7% in hypertrophying hearts. This result indicates that the increase in replicative flux of mtDNA is associated with the removal of a block in the conversion of D-loops to other intermediates.

Structural and replicative forms of mitochondrial DNA from human leukocytes in relation to age

The structure and replication of human leukocyte mitochondrial DNA (mtDNA) was investigated in healthy young adult males (23--37 years old), middle-aged males (42--52 years old) with secondary polycythemia, and elderly males (80--89 years old) who exhibited different degrees of age-related disease syndromes. The distribution of the various cell types within the white cell population was within normal limits in all samples. Total mtDNA was isolated in ethidium bromide--CsCl gradients and examined by electron microscopy after spreading by the aqueous and formamide techniques. The individual frequencies of catenated forms ranged from 2 to 6% but showed relatively little change (declining slightly) with age. The individual frequencies of circular dimers varied from 0 to 0.1% in the young adult and polycythemic groups and in 10 out of 12 elderly individuals. One elderly individual had a circular dimer frequency of 0.3% (including a circular molecular of tetramer size) and another had 4.5%. This finding suggest that agerelated cellular pathology may exist in the blood-forming system in some cases. The mode of replication of leukocyte mtDNA agrees well with that described for mouse L cells. There was no evidence of aberrant mtDNA replication as a result of aging.

Complex mitochondrial DNA in Drosophila

The larval mtDNA isolated from D. virilis, D. simulans and D. melanogaster exists in complex molecular forms in addition to the simple monomeric circular form. The frequency of circular dimers and oligomers is highly elevated in apparently normal larval tissues. These complex forms of mtDNA are separable on agarose gels. Hind III restriction endonuclease and electron microscopic analyses used in the present study have revealed that circular dimers are simply the circular concatemers of two monomeric circles which are arranged in a head-to-tail structure with no detectable heterologous regions such as insertions or deletions. The electrophoretic patterns of Hind III digested mtDNAs of D. simulans and D. melanogaster (sibling species) are identical and distinguishable from that of distantly related species, D. virilis.

Complex forms and replicative intermediates of mitochondrial DNA in tissues from adult and senescent mice

The occurrence and types of complex forms and replicative intermediates of mitochondrial DNA (mtDNA) were investigated in tissues from C57BL/6J mice aged 10-11 months or 29-30 months. Total mtDNA from brain, heart, kidney and liver was isolated in ethidium bromide-CsCl gradients and examined by electron microscopy after aqueous or formamide spreading. Contour length measurements indicated no difference in the monomer size of mtDNA according to either tissue or donor age. The frequencies of catenated mtDNA, ranging from 4 to 8%, varied significantly according to tissue but changed relatively little as a result of donor age. The main age-related effect observed in this study was a significant increase in the frequency of circular dimers, from about 0.05% in adult tissues to 0.3% in kidney, 0.5% in liver, 0.6% in heart and 1.9% in brain of senescent mice. The frequency of D-loop DNA varied from 30 to 60% and that of larger replicative intermediates from 1 to 10%, suggesting differences in the rate of mtDNA replication according to tissue. The frequencies and types of the various replicative intermediates were unaffected by donor age.