Sister chromatid exchanges and aging. III. The effect of donor age on mutagen-induced sister chromatid exchange in human diploid fibroblasts

Putative mechanisms responsible for the decline in cancer prevalence during organism senescence

Most scientific literature reports that aging favors the development of cancers. Each type of cancer, however, initiates and evolves differently, and their natural history can start much earlier in life before their clinical manifestations. The incidence of cancers is spread throughout human life span, and is the result of pre- and post-natal aggressions, individual susceptibility, developmental changes that evolve continuously throughout an individual's life, and time of exposure to carcinogens. Finally, during human senescence, the incidence declines for all cancers. Frequently, the progression of cancers is also slower in aged individuals. There are several possible explanations for this decline at the tissue, cell, and molecular levels, which are described here in. It is time to ask why some tumors are characteristic of either the young, the aged, or during the time of a decline in the reproductive period, and finally, why the incidence of cancers declines late during senescence of human beings. These questions need to be addressed before the origin of cancers can be understood.

Cell division and aging of the organism

The capacity to regenerate cell compartments through cell proliferation is an important characteristic of many developed metazoan tissues. Pre- and post-natal development proceeds through the modifications occurring during cell division. Experiments with cultivated cells showed that cell proliferation originates changes in cell functions and coordinations that contribute to aging and senescence. The implications of the finite cell proliferation to aging of the organism is not the accumulation of cells at the end of their life cycle, but rather the drift in cell function created by cell division. Comparative gerontology shows that the regulation of the length of telomeres has no implications for aging. On the other hand there are interspecies differences in regard to the somatic cell division potential that seem to be related with the "plasticity" of the genome and with longevity, which should be viewed independently of the aging phenomenon. Telomeres may play a role in this plasticity through the regulation of chromosome recombination, and via the latter also in development.

Tissue-specific differences in the accumulation of sequence rearrangements with age

Mitotic homologous recombination (HR) is a critical pathway for the accurate repair of DNA double strand breaks (DSBs) and broken replication forks. While generally error-free, HR can occur between misaligned sequences, resulting in deleterious sequence rearrangements that can contribute to cancer and aging. To learn more about the extent to which HR occurs in different tissues during the aging process, we used Fluorescent Yellow Direct Repeat (FYDR) mice in which an HR event in a transgene yields a fluorescent phenotype. Here, we show tissue-specific differences in the accumulation of recombinant cells with age. Unlike pancreas, which shows a dramatic 23-fold increase in recombinant cell frequency with age, skin shows no increase in vivo. In vitro studies indicate that juvenile and aged primary fibroblasts are similarly able to undergo HR in response to endogenous and exogenous DNA damage. Therefore, the lack of recombinant cell accumulation in the skin is most likely not due to an inability to undergo de novo HR events. We propose that tissue-specific differences in the accumulation of recombinant cells with age result from differences in the ability of recombinant cells to persist and clonally expand within tissues.

Proliferative cell senescence, transformation, and the recombination potential of the genome

Fibroblast cell populations maintained in vitro display a different probability of escaping proliferative senescence and to transform. Observations at the cytogenetic level suggest that the long-term doubling potential of these cells is directly related to the potential for continuous chromosome rearrangements.

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Factors that influence formation of sister chromatid exchanges in human blood lymphocytes

Sister chromatid exchange (SCE) reflects an interchange of DNA sequences between helices in a replicating chromosome. This was initially accomplished by Taylor and colleagues (1957) using tritiated thymidine incorporation followed by autoradiography. The development of an elegant technique for differential staining of sister chromatids by incorporating a thymidine analog, 5-bromodeoxyuridine (BrdU) has greatly simplified the detection of SCEs in metaphase chromosomes. In recent years, the analysis of SCE has been considered to be a highly sensitive and additional (i.e., with chromosome aberrations) end point for measuring mutagenic/carcinogenic potential of various environmental agents and is increasingly being used to detect and differentiate among chromosome fragility human diseases that predispose to neoplasia. Attention has been focused to see if the induction of SCEs in lymphocyte cultures can be used as a reliable "biological dosimeter" for genetic risk assessment and to monitor the exposed populations. Several physical or preparatory as well as biological factors that modify the response and formation of SCEs make the monitoring difficult. The purpose of this article is to review and analyze these factors to facilitate an effective development of a standard protocol for SCE testing and for appropriate evaluation of test results. This may also provide clues to understand the yet unknown molecular mechanism(s) and biological significance of SCE formation.

Mitomycin C and ethyl methanesulphonate-induced sister-chromatid exchanges in lymphocytes from individuals with Alzheimer's pre-senile dementia

Baseline and mutagen-induced sister-chromatid exchange frequencies were compared in peripheral blood lymphocytes from patients with Alzheimer's pre-senile dementia and control individuals. No significant differences were found between the two groups.

Cancer and aging

It is widely accepted that the incidence of cancer increases with aging and many hypotheses have been put forward to explain this association. A review of the literature, however, shows that the relationship between cancer and aging has to be reappraised. Although the incidence of most cancers increases during the second half of the animal life span, only with a few does it increase progressively with senescence in humans as well as in animals. As a matter of fact in many cases the incidence does not vary, levels off or even decreases in old individuals. Late developing tumors also seem in many instances to progress more slowly. Thus it seems that the incidence of cancer is related to age rather than to aging and that some conditions may be created during senescence which oppose the development of some cancers.

Baseline frequency of sister-chromatid exchanges (SCE) in newborn lymphocytes and its relationship to in vivo aging in humans

Heparinised cord blood from newborns and peripheral venous blood from three other age groups of individuals (1-75 years) have been cultured in vitro to obtain baseline frequencies of SCE and to see if the frequency of baseline SCE in vitro varies as a function of aging in vivo. The results demonstrate an age-dependent variation in the frequency of SCEs. Although the SCE frequency was lowest (5.10/cell) in 1-5-year-old infants, a significantly higher (P less than 0.001) frequency (8.97/cell) was observed in the cord blood of newborns. In old age, the level of SCE also increased. The plausible reason(s) for such observations is discussed.

Serum- and plasma-dependent variations of benzo(a)pyrene-induced sister chromatid exchange in human lymphocytes

Sister chromatid exchange (SCE) is frequently used to assess the potential mutagenicity of chemical agents to human beings. We demonstrate here that levels of SCE induced by benzo(a)pyrene (BP) in the widely used blood lymphocyte assay are influenced by serum and plasma supplements. Sister chromatid exchange induction by BP was greatest when using fetal calf serum (FCS), intermediate with newborn calf serum (NCS), and lowest with autologous human plasma (AHP). This new finding adds to a growing list of factors capable of modulating the SCE response and underscores the need for researchers to consider serum and plasma supplements in the standardization of the SCE approach in human mutagen assessments. The data also demonstrate the potential of SCE to aid in the study of serum factors which modify the mutagen sensitivity of human cells towards environmental carcinogens.

The effect of age and cell proliferation on the frequency of sister chromatid exchange in human lymphocytes cultured in vitro

The impact of ageing on the frequencies of sister chromatid exchange of humans was determined in lymphocytes of 43 healthy male non-smokers. The frequencies of sister chromatid exchange in individuals under 70 years were the same, but were significantly lower than in men of 70 years and older. Since we did not see a difference in the frequencies of sister chromatid exchange between fast- and slow-cycling lymphocytes, this difference probably would still hold if the lymphocyte subpopulations of these people were all examined.

Chromosome pattern and survival in acute non-lymphocytic leukaemia in relation to age and occupational exposure to potential mutagenic/carcinogenic agents

The bone marrow karyotype was investigated in 98 patients with acute non-lymphocytic leukaemia (ANLL). The patients were divided into two groups according to age. (1) 47 patients were 20-54 (median 40) years old. 21 had a history of occupational exposure to chemical solvents, insecticides, or petrol products, and 26 were considered occupationally not having been exposed to such agents. In 4 exposed patients (19%) all bone marrow cells had clonal chromosomal aberrations (designated AA), while also 4 of the non-exposed patients (15%) were AA. Thus in young ANLL patients, there was no significant association between occupational exposure to potential mutagenic/carcinogenic agents and the AA constitution of the leukaemic cells. (2) 51 patients were 55 years of age or older (median 65 years). 16 were exposed and 8 of these (50%) had the AA constitution. 35 patients were non-exposed and only 4 (11%) were AA. It is known from previous studies that the survival of ANLL patients with AA is extraordinarily short. Accordingly the overrepresentation of AA in exposed patients 55 years or older, was associated with a shorter survival than that of the non-exposed elderly patients. The results suggest that etiologic factors may influence the clinical course of ANLL, especially in elderly patients.

Variation in the sensitivity of human lymphocytes to DNA-damaging agents measured by sister chromatid exchange frequency

Peripheral blood lymphocytes from normal donors were exposed to mitomycin C 0.01 microgram/ml, proflavine 0.4 microgram/ml, and 3H-uridine 3.7 Bq/ml and analyzed for the incidence of sister chromatid exchanges (SCEs). Cellular proliferation in the presence of these three agents was also measured by determining the number of first, second, and third divisions present. All donors showed a significant increase in SCE following treatment with mitomycin C, whereas only two-thirds showed an increase following treatment with proflavine and only half, with 3H-uridine. The variation in response was not related to either the age or sex of the donor. Cellular proliferation was likewise affected by all three agents. 3H-uridine caused the greatest reduction in the number of metaphases. There was no correlation between the sensitivity to one agent and another, nor was there any correlation the reduction in the number of metaphases and the increase in the number of exchanges.

Sister chromatid exchanges and structural chromosome aberrations in relation to age and sex

Sister chromatid exchanges (SCE) and structural chromosome aberrations were analyzed in peripheral blood lymphocytes of 100 individuals, and correlated to age and sex. No correlation was found between the frequency of SCE and age, but older individuals had significantly more structural aberrations than younger. Females had significantly more SCE as well as structural chromosome aberrations than males. The positive correlations of SCE and structural aberrations to age and sex were also significant when these factors, as well as smoking habits, were taken into consideration in an analysis of covariance.

Cell culture aging

Cellular research in aging has been stimulated by the observation that human diploid cells have a limited number of cell divisions in culture. This loss of cellular proliferation (in vitro senescence) has been extensively studied by biochemical, clonal, and genetic analysis. Studies of human skin fibroblast cultures have revealed that in vitro senescense is related to in vivo human cellular aging. Recently differentiated cells have been proposed for aging studies. These cells may provide additional information on aging since alterations of in vitro cellular functions may be related to the in vivo behavior of specific differented cell types.

Sister chromatid exchange in aged human lymphocytes. A brief note

A study of the sister chromatid exchange (SCE) frequency in peripheral lymphocytes from 26 human subjects, with in vivo age of the cells as the variable, demonstrated a significant increase in SCE frequency with increasing age of the subject (analysis of variance: p less than 0.01). The cells were cultured in Eagles' basal medium with 5-bromodeoxyuridine added to a final concentration of 10 microM. These results are contradictive to the brunt of the literature which indicates that background SCE frequency does not change with age.

Aging and cultured human skin fibroblasts

We have studied aging at the cellular level by examining skin fibroblast cultures derived from skin biopsy samples obtained from old and young members of a longitudinal study on human aging. Results from a number of studies already completed indicate that cell cultures from old human donors, unlike cultures from young donors, have impaired cell proliferation and reduced induction of sister chromatid exchanges. A number of other parameters, however, are not affected by the age of the human fibroblast donor. These include cell protein and RNA content and the rate of cellular macromolecular synthesis. Studying cell cultures derived from members of an ongoing longitudinal study permits examination of correlations between in vivo and in vitro measurements on the same people. In addition, it allows for future longitudinal studies on human cellular aging.

The effect of aging on sister chromatid exchange

The advent of the bromodeoxyuridine(BrdU)-differential staining techniques has greatly facilitated the detection of sister chromatid exchanges (SCE). These SCE have been demonstrated to be an accurate reflection of DNA damage both in vitro in cultured cells and in vivo in mouse and rate bone marrow and spleen cells. In this review, we examine the effect of cellular aging on both baseline and mutagen-induced SCE levels. In all systems examined, aging did not appear to significantly affect the baseline levels of SCE. However, in human fibroblast cultures we have found a significant decrease in the levels of mutagen-induced SCE as a function of both in vitro passage level (in vitro aging) and the age of the cell culture donor (in vivo aging). In addition we have found a similar decrease in mutagen-induced SCE levels in both mouse and rat bone marrow cells and mouse spleen cells where examinations were performed entirely in vivo. Diminished mutagen-induced SCE levels were obtained with a wide variety of agents including mitomycin-C, cyclophosphamide, adriamycin, ethyl methanesulfonate and N-acetyl-2-acetoxyamino-fluorene. These decreased SCE levels were accompanied by increased frequencies of chromosomal aberrations in the older cell populations. If SCE represents a form of DNA repair as has been suggested by several investigators, our finding would indicate impaired DNA repair occurring in old cells.