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Biomedical subjects

R Holliday

Publications and source records attributed to R Holliday.

At least 127 records · Page 7Linked to original sources
PubMed1982

Autofluorescence as an index of ageing in human fibroblasts in culture.

An exponential increase in autofluorescence (AF) during serial passaging of human diploid fibroblasts, strain MRC-5, was observed using a fluorescence-activated cell sorter. Skin fibroblasts from patients with premature-ageing syndromes had high levels of AF, whereas virus-transformed cell lines had negligible amounts of AF.

Adult↗
PubMed1982

Human lymphocytes resistant to 6-thioguanine increase with age.

Using an autoradiographic technique we determined the number of circulating lymphocytes that were resistant to 6-thioguanine and which were presumably mutants at the hypoxanthine--guanine phosphoribosyl transferase locus. The number in normal individuals was found to increase exponentially with age. The data suggest a relationship between mutagenesis and ageing, perhaps by way of a decline with age in the fidelity of DNA replication or repair.

Adolescent↗
PubMed1981

Cellular aging: further evidence for the commitment theory.

A large, transient reduction in the population size of human fibroblasts in early passages significantly increases the variability of the life-spans of cultures in comparison to control cultures, as predicted by the commitment theory of cellular aging. The theory also predicts that a constant population of noncycling cells will appear in the later part of the culture life-span. This was confirmed by labeling the cells in culture with tritiated thymidine.

Cell Division↗
PubMed1980

Error propagation in Escherichia coli and its relation to cellular ageing.

The mistranslation of alkaline phosphatase may not provide a definitive measure of errors in Escherichia coli protein synthesis. beta-Galactosidase which, unlike alkaline phosphatase, is an intracellular enzyme exhibits different mistranslation kinetics. Previous conclusions based on alkaline phosphatase data and showing no relation between error propagation and ageing may require re-evaluation.

Alkaline Phosphatase↗
PubMed1979

The evolution of ageing and longevity.

Ageing is not adaptive since it reduces reproductive potential, and the argument that it evolved to provide offspring with living space is hard to sustain for most species. An alternative theory is based on the recognition that the force of natural selection declines with age, since in most environments individuals die from predation, disease or starvation. Ageing could therefore be the combined result of late-expressed deleterious genes which are beyond the reach of effective negative selection. However, this argument is circular, since the concept of 'late expression' itself implies the prior existence of adult age-related physiological processes. Organisms that do not age are essentially in a steady state in which chronologically young and old individuals are physiologically the same. In this situation the synthesis of macromolecules must be sufficiently accurate to prevent error feedback and the development of lethal 'error catastrophes'. This involves the expenditure of energy, which is required for both kinetic proof-reading and other accuracy promoting devices. It may be selectively advantageous for higher organisms to adopt an energy saving strategy of reduced accuracy in somatic cells to accelerate development and reproduction, but the consequence will be eventual deterioration and death. This 'disposable soma' theory of the evolution of ageing also proposes that a high level of accuracy is maintained in immortal germ line cells, or alternatively, that any defective germ cells are eliminated. The evolution of an increase in longevity in mammals may be due to a concomitant reduction in the rates of growth and reproduction and an increase in the accuracy of synthesis of macromolecules. The theory can be tested by measuring accuracy in germ line and somatic cells and also by comparing somatic cells from mammals with different longevities.

Adaptation, Biological↗
PubMed1979

Human cells and the finite lifespan theory.

Cultures of human diploid fibroblasts are characterized by: i) finite lifespan, ii) marked heterogeneity in the growth potential of individual cells within the culture, iii) considerable variation in lifespans of parallel cultures of the same cell strain. To explain these properties, we have proposed a commitment theory of cellular aging. Cells are assumed initially to be uncommitted (potentially immortal) but, at each cell division, each daughter cell is assumed to have some fixed probability of becoming irreversibly committed to senesce and die after a specific number of cell generations. During the period between commitment and senescence, cells are assumed to multiply normally, so the uncommitted cells are diluted by committed ones and may be lost in subculturing. The theory explains features i) - iii) above and, in particular, suggests why diploid cultures have finite lifespans while transformed or permanent lines grow indefinitely. It also validly predicts the behaviour of mixed cultures of distinguishable but otherwise similar cell types, and that culture lifespan may be significantly decreased by drastic reduction of population size. The important converse prediction that culture lifespan may be extended indefinitely by growing sufficiently large cultures or by selectively isolating uncommitted cells remains to be tested.

Cell Division↗
PubMed1979

A new theory of carcinogenesis.

Although many carcinogens are mutagens, there is no direct evidence that the cancer-cell phenotype is the result of gene mutation. Transplantation experiments have strongly indicated that malignant cells can arise or revert to the normal phenotype in the absence of mutation. It is suggested that damage to DNA followed by repair triggers the epigenetic changes in gene expression which are responsible for malignancy. We previously proposed that methylation of specific DNA sequences adjacent to structural genes determines whether or not transcription will occur. Specific methylases are required for the switching on of genes and for the stable maintenance of the methylated state, which provides a basis for the control of gene expression in differentiated cells. It is now seen that damage to DNA followed by repair, just before or just after DNA replication, can lead to the loss of methyl groups. This can induce a switch in gene activity which is heritable, but potentially reversible. The known large difference in the probability of malignant transformation in cells of rodents and large mammals is hard to explain if mutation is responsible. On the other hand, this new theory provides an explanation for this difference, since the probability of epigenetic changes in gene activity will depend on the activity of methylating enzymes and the rate of excision repair. The theory is supported by the evidence that excision repair is more efficient in cultured fibroblasts from large long-lived animals than from small short-lived ones.

Animals↗
PubMed1977

A search for allelic recombination in Chinese hamster cell hybrids.

Mutants resistant to 6-thioguanine were selected from CHO cells which were either temperature sensitive or proline requiring. These mutants were stable and had low levels of hypoxanthine guanine phosphoribosyl transferase (HGPRT). Hybrids were selected which were heteroallelic at the hgprt locus and complementation between the mutants used was not observed. Interallelic recombination at this locus would generate hgprt+ cells which could be selected in Littlefield's HAT medium. Selection experiments with hybrids containing three different pairs of mutants yielded no recombinants among populations of 4 x 106 - 2 x 107 cells. After treatment with the recombinagen mitomycin C, 3 putative recombinants were detected amongst 1.4 x 107 surviving cells from one hybrid. One of these strains was examined and shown to have a normal level of HGPRT and its heterozygosity at this locus was demonstrated by the segregation of colonies resistant to 6-thioguanine. It cannot be excluded that the rare hgprt+ colonies seen arose by mutation rather than by recombination. Mitotic allelic recombination therefore appears to be a much less frequent event in CHO cells than it is in lower eukaryotes. It is possible that mitotic recombination is effectively suppressed in mammalian cells to prevent the expression of deleterious recessive mutants.

Alleles↗
PubMed1977

Testing the commitment theory of cellular aging.

The commitment theory may explain both the finite lifespan of diploid fibroblasts and the apparent immortality of transformed lines. Potentially immortal cells are assumed on division to generate with some fixed probability cells committed to senesce after a specific number of divisions. During the period between commitment and senescence, cells are assumed to maintain normal growth so that the uncommitted cells are diluted by committed ones and may ultimately be lost in subculturing. A number of predictions of this model are described and experiments strongly supporting the theory are reported. We conclude that the limited growth of diploid fibroblasts is, in effect, an artifact of normal culturing procedures.

Cell Cycle↗