Age-dependent excision repair of damaged thymine from gamma-irradiated DNA by isolated nuclei from human fibroblasts

The repair rate of radiation-induced DNA damage: A stochastic interpretation based on the Gamma function

There is a large body of evidence that stress-induced DNA damage may be responsible for cell lethality, cancer proneness and/or immune reaction. However, statistical features of their repair rate remain poorly documented. In order to interpret the shape of the radiation-induced DNA damage repair curves with a minimum of biological assumptions, we introduced the concept of repair probability, specific to any individual radiation-induced DNA damage, whatever its biochemical type. We strengthened the apparent paradox that the repair rate of a population of DNA damage is time-dependent even if the repair rate of the individual DNA damage is constant. Hence, the existing models, based on a dual approach of the DNA repair may be insufficient for describing the DNA repair rate over a large range of repair times. Since the repair probability of DNA damage cannot be assessed individually, the measurement of the DNA repair rate is assumed to consist in determining the instantaneous mean of all repair probabilities. The relevance of this model was examined with different endpoints: cell species, genotypes, radiation type and chromatin condensation. The Euler's Gamma function was shown to provide the distribution the most consistent with such hypotheses. Furthermore, formulas, deduced from the Gamma distribution, were found to be compatible with our previous model, empirically defined but based on a variable repair half-time.

Analysis of UV-induced damage and repair in young and senescent human dermal fibroblasts using the comet assay

A major cause of ageing is thought to be the accumulation of damage to macromolecules. Accumulation to DNA damage in cells therefore presupposes that aged cells are unable to repair this damage. We have used the in vitro model of cellular ageing to test the idea that senescent cells are deficient in some aspect of DNA repair. Using the alkaline single cell gel electrophoresis assay (comet assay), we have determined the responses of young and senescent human dermal fibroblasts to DNA damage caused by exposure to UVC light. At low doses of UVC, senescent cells generate smaller comets than young cells whilst at medium doses the situation is reversed. At high doses, young and senescent cells respond similarly to one another. Time course experiments revealing repair of DNA damage show that senescent cells generate larger comets than young cells at early stages of repair suggesting that either senescent cells bear more damage per genome than do young cells or that senescent cells are more efficient at excising bulky adducts from DNA. Cells maintained in low levels of serum irrespective of age are less able to repair DNA damage compared with cells maintained in high levels of serum, and furthermore young and senescent cells maintained in high levels of serum are equally able to repair DNA damage. Our data, therefore, reveal both age-dependent and age-independent responses to UV-induced DNA damage. Use of the comet assay highlights the heterogeneity of cellular responses to genotoxic stress.

Reduced 5-hydroxymethyluracil-DNA glycosylase activity in Werner's syndrome cells

Werner's syndrome (WS) is an autosomal recessive disease marked by early symptoms of accelerated aging. There is evidence indicating accumulation of oxidized DNA bases to be a major factor in cellular aging. The first step of excision repair of such bases in human cells is their removal from DNA by glycosylases. 5-Hydroxymethyluracil (HMU)-DNA glycosylase excises HMU from DNA; another glycosylase removes many non-aromatic pyrimidine derivatives. Levels of glycosylases that excise oxidized pyrimidines from DNA were compared between confluent and proliferating populations of WS cells, age-matched controls, and young control cells. They were assayed by measurements of direct release of free bases from their respective DNA substrates. Specific activities of the glycosylase that releases various modified pyrimidines and of uracil-DNA glycosylase (which removes uracil from DNA) were essentially the same in all cell lines. Cell cycle variations of these enzymes also did not differ between WS and control cells. HMU-DNA glycosylase specific activity was reduced in WS cells. Reduction of HMU-DNA glycosylase has been described in senescent human WI-38 cells. Therefore, while neither WS nor senescent cells have overall deficiencies of DNA glycosylase activities, they both might have reduced excision of HMU from DNA. This indicates a possible role of HMU accumulation in the aging process.

Glycosylases that excise modified DNA pyrimidines in young and senescent human WI-38 fibroblasts

Cellular DNA is continuously subject to damages by both endogenous and exogenous oxidizing agents. Excision repair in human cells is initiated by DNA glycosylases which remove oxidized bases from DNA. 5-Hydroxymethyluracil-DNA glycosylase excises 5-hydroxymethyluracil from DNA. A different enzyme has glycosylic activity against many ring-saturated DNA pyrimidines. Levels of these enzymes were examined in WI-38 fibroblasts of different culture ages. All glycosylases were assayed by measurements of direct release of modified free bases from their respective DNA substrates. Levels of 5-hydroxymethyluracil-DNA glycosylase were reduced in aging cells. Specific activities of the glycosylase that releases ring-saturated pyrimidines and of uracil-DNA glycosylase were not substantially altered in senescent cells. Therefore, although aging cells might have reduced excision of DNA 5-hydroxymethyluracil, there is no overall age-dependent decrease of DNA glycosylase activities.

Repair of H2O2-induced DNA damage in bovine lens epithelial cell cultures

H2O2 concentrations only slightly higher than normal physiological levels found in the lens and aqueous fluid produce a significant number of DNA single-strand breaks in lens epithelial cell cultures. In this investigation, the repair of DNA damaged by short-term, H2O2-induced oxidation was examined in bovine lens epithelial cell cultures. Repair was rapidly initiated and was almost completed in 30 min. A drop in NAD concentration was associated with the DNA damage. 3-Aminobenzamide inhibition of poly(ADP-ribose) polymerase, an enzyme believed to be stimulated by DNA oxidation and involved in DNA repair, prevented the loss of NAD. In contrast, a similar drop in ATP concentration was only slightly lessened by the presence of this inhibitor. Inhibition of the polymerase by 3-aminobenzamide primarily affected only the early recovery period. Overall, recovery occurred almost as effectively in the presence of the inhibitor as in its absence. Preincubation of lens cultures with o-phenanthroline, an iron chelator, prevented the drop in NAD levels associated with DNA damage. Since a hydroxyl radical is produced from H2O2 by a Fenton type reaction, this result supports the concept that the H2O2-induced oxidation of DNA is caused by hydroxyl radical. In contrast, peroxide-induced loss of activity of a cytosolic enzyme, glyceraldehyde-3-phosphate dehydrogenase, was unaffected by the presence of o-phenanthroline, suggesting direct H2O2 oxidation of this enzyme. The results of these experiments suggest that lens epithelium contains enzymes that rapidly repair single-strand DNA breaks induced by H2O2 insult.

Increase in abundance of a transcript hybridizing to elongation factor I alpha during cellular senescence and quiescence

We have isolated a senescence-specific clone (pSEN) from a cDNA library constructed from late passage WI-38 human diploid fibroblast that accounts for approximately 1% of the recombinants. Nucleotide sequence analysis of the partial cDNA clone has led to the identification of pSEN as elongation factor I alpha. Northern analysis of poly(A)+ RNA from various intermediate population doubling levels shows that a 2.2 kb transcript hybridizes to pSEN but is expressed prior to PDL-40 at very low levels. This transcript begins to accumulate at PDL-40 and is induced approximately 50-fold just prior to senescence. Furthermore, this transcript was shown to be specific to Go of the cell cycle whereas a second, lower molecular weight transcript (1.6 kb) was observed during S phase (Giordano and Foster, unpublished data). The 2.2 kb transcript is also detected in neonatal foreskin cells but very little increase in abundance is observed between early and late passage cells. Sucrose gradient fractionation of RNA from late passage WI-38 cells suggests that the lower molecular weight transcript is associated with the polysome fraction while the 2.2 kb transcript sediments with the nonpolysomal fraction. Thus, the possibility exists that the 1.6 kb transcript is derived from the 2.2 kb transcript.

Disparate effects of 5-bromodeoxyuridine on sister-chromatid exchanges and chromosomal aberrations in Bloom syndrome fibroblasts

The existence of a high frequency of spontaneous sister-chromatid exchanges (SCEs) in Bloom syndrome (BS) has thus far been supported by data on a small number of BS cell lines. To examine the cause of baseline SCEs more broadly, the frequencies of SCEs, as well as chromosomal aberrations (CAs) in 4 additional BS fibroblast strains were compared, under different assay and cell culture conditions, with those of normal cells in the range of approximately 0.9-90% 5-bromodeoxyuridine (BrdUrd) substitution into template DNA. SCEs at low levels of BrdUrd substitution were detected by an extremely sensitive immunofluorescent technique. From approximately 0.9% to 4.5% BrdUrd substitution, the SCE frequency in BS cells remained constant, at a level (40/cell) 8 times higher than that of normal cells. As BrdUrd substitution increased further, the SCE frequency in BS cells increased almost linearly, reaching 70-100 per cell at approximately 90% substitution, while the SCE increment in control fibroblasts was less than 5 per cell. Analysis of SCEs in 3 successive replication cycles similarly revealed that the SCE increment in BS cells depended on BrdUrd only at a high BrdUrd substitution level. In contrast to data on SCEs, CA induction by incorporated BrdUrd in BS cells was only slightly higher than that in normal cells. Thus, BS cells are extremely sensitive to BrdUrd for SCE induction, but much less so for CA induction.

DNA repair in human promyelocytic cell line, HL-60

The human promyelocytic cell line, HL-60, shows large changes in endogenous poly(ADP-ribose) and in nuclear ADP-ribosyl transferase activity (ADPRT) during its induced myelocytic differentiation. DNA strand-breaks are an essential activator for this enzyme; and transient DNA strand breaks occur during the myelocytic differentiation of HL-60 cells. We have tested the hypothesis that these post-mitotic, terminally differentiating cells are less efficient in DNA repair, and specifically in DNA strand rejoining, than their proliferating precursor cells. We have found that this hypothesis is not tenable. We observe that there is no detectable reduction in the efficiency of DNA excision repair after exposure to either dimethyl sulphate or gamma-irradiation in HL-60 cells induced to differentiate by dimethyl sulphoxide. Moreover, the efficient excision repair of either dimethyl sulphate or gamma-irradiation induced lesions, both in the differentiated and undifferentiated HL-60 cells, is blocked by the inhibition of ADPRT activity.

Sensitivity of cultured skin fibroblasts from patients with neurofibromatosis to DNA-damaging agents

Neurofibromatosis (NF) is an autosomal dominant disorder associated with various constitutional abnormalities as well as a striking predisposition for malignant and nonmalignant neoplasms, both in cells originating in and not originating in the neural crest. We have examined the sensitivity of cultured skin fibroblasts from patients with neurofibromatosis to several types of DNA damage. Fibroblasts in Dulbecco's modified Eagle's medium were plated at 10(2) to 2 X 10(4) cells per 75 cm2 tissue culture plates, and exposed to various doses of gamma radiation (leads to DNA scission), actinomycin D (a DNA intercalating agent), or mitomycin C (a bifunctional alkylating agent leading to DNA cross-links). Cells were reincubated for 15 to 40 days until surviving colonies exhibited greater than 30-50 cells. Plates were then stained with 1% methylene blue and the colonies counted, with surviving fraction determined relative to plating efficiency. Nine skin fibroblast cell strains from normal individuals were studied as controls. One neurofibromatosis (NF) cell strain, SB23, exhibited normal sensitivity to all three DNA-damaging agents studied in early (7-8) and middle (12-13) in vitro passage. Strain GM0622, on the other hand, exhibited normal sensitivity to the three DNA-damaging agents studied at early passage, but showed a significant decrease in survival after exposure to both gamma radiation (D0 = 106 rad) and actinomycin D (D0 = 0.024 mcg/ml) with increasing passage. Strain GM1639 exhibited decreased survival after actinomycin D exposure at early passage (D0 = 0.017 mcg/ml), with normal survival after exposure to gamma radiation and mitomycin C at the same passage. Cell strains exhibited decreasing low density plating efficiencies and growth rates with increasing passage such that study of cytotoxicity was not feasible after middle passage in strains SB23 and GM0622, and after early passage in strain GM1639. The results suggest that cultured fibroblast cell strains from patients with NF exhibit early in vitro senescence which sometimes is associated with an inability to handle certain DNA-damaging agents.

Three classes of DNA strand breaks induced by X-irradiation and internal beta-rays

Repair kinetics of DNA strand breaks were investigated after exposing exponentially growing CHO cells to X-radiation or to internal beta-rays from incorporated tritium, respectively. DNA strand breaks were analysed by the alkaline unwinding technique followed by chromatography on hydroxyapatite. For either type of radiation, the repair kinetics are statistically best described by a sum of three exponential components. The half-times determined are tau I approximately 2 min, tau II approximately 20 min and tau III approximately 170 min; they are identical for both types of radiation. But the initial fractions of the components are different for X- and internal beta-rays; X-rays; fI = 0.70, fII = 0.25, fIII = 0.05; internal beta-rays: fI = 0.40, fII = 0.40, fIII = 0.20. Components I and II are considered to represent the repair of two different classes of single-strand breaks and component III the repair of double-strand breaks. Two alternative interpretations for the occurrence of the two classes of single-strand breaks are discussed.

No change in DNA damage or repair of single- and double-strand breaks as human diploid fibroblasts age in vitro

Using the in vitro human diploid fibroblast model, we tested theories of aging which hypothesize that either accumulation of DNA damage or decreased DNA repair capacity is causally related to cellular senescence. Between population doubling level (PDL) 32 and 71, fetal lung-derived normal diploid human fibroblasts (IMR 90) were assayed for both DNA single-strand breaks (SSBs, spontaneous and induced by 6 Gy) and DNA double-strand breaks (DSBs, spontaneous and induced by 100 Gy). After gamma-irradiation cells were kept on ice unless undergoing repair incubation at 37 degrees C for 7.5-120 min or 18-24 h. To assay DNA strand breaks we used the filter elution technique in conjunction with a fluorometric determination of DNA which is not biased in favor of proliferating aging cells as are radioactive labelling methods. We found no change with in vitro age in the accumulation of spontaneous SSBs or DSBs, nor in the kinetics or completeness of DNA strand rejoining after gamma-irradiation. Cells at varying PDLs rejoined approx. 90% of SSBs and DSBs after 60 min repair incubation and 100% after 18-24 h repair incubation. We conclude that aging and senescence as measured by proliferative lifespan in IMR 90 cells are neither accompanied nor caused by accumulation of DNA strand breaks or by diminished capacity to rejoin gamma-radiation-induced SSBs or DSBs in DNA.

Absence of deoxyuridine and 5-hydroxymethyldeoxyuridine in the DNA from three tissues of mice of various ages

Mutational damage to DNA may modulate the aging process as well as contribute to the high incidence of cancer in older animals. Uracil (Ura) is the deamination product of cytosine and hydroxymethyluracil (HMU) is an oxidation product of thymine. Ura, when generated from cytosine, induces mutations by mispairing with adenine. Both HMU and Ura are known to be excised from DNA by glycosylases that cleave the respective N-glycosidic bonds. This hydrolysis leaves apyrimidinic sites which are subsequently repaired by excision repair. In this report a sensitive method to detect these altered bases of HPLC separation of the components of DNA hydrolysates is described. Neither deoxyuridine (dU) nor 5-hydroxymethyldeoxyuridine (dHMU) were found in hydrolysates of DNA samples from brain, liver or small intestinal mucosa of mice of different ages.

Levels of uracil DNA glycosylase and AP endonuclease in murine B- and T-lymphocytes do not change with age

Two DNA repair enzyme activities, uracil DNA glycosylase and AP endonuclease, were measured in extracts of T- and B-lymphocytes isolated from mice ranging in age from 3 to 24 months. T- and B-lymphocytes had roughly equal levels of AP endonuclease which did not change appreciably with age. T-lymphocytes had roughly twice as high a level of uracil DNA glycosylase as B-lymphocytes; these levels were not affected by age either. This constancy with age contrasts dramatically with increases in both enzymes--roughly 3-fold on a protein basis or 50-fold on a per cell basis--in a transformed line (MPC-11) derived from a carcinogen-induced lymphocytoma. These results are similar to those obtained with cultured murine fibroblasts, wherein a relative constancy was noted with passage of non-transformed cells, followed by dramatic changes upon transformation (La Belle, M & Linn, S, Mutat res 132 (1984) 51). Hence these enzyme assays do not support the notion of a drop in base excision DNA repair capacity as being a causative factor in aging, but suggest instead that DNA repair properties might differ dramatically in transformed vs non-transformed cells.

Age related change in the frequency of ara C-induced chromosome aberrations in human peripheral blood lymphocytes

1-beta-D-Arabinofuranosylcytosine (ara C) has been known to inhibit repair replication in the G1 phase of cell cycle and to convert certain types of DNA damage into chromosome-type exchanges: e.g., dicentric or ring chromosomes. It is then considered to be a useful cytogenetic method to investigate the frequency of ara C-induced dicentric and ring chromosomes (dic and ring) for estimating cellular DNA damage or capacity to repair it. Peripheral blood lymphocytes from donors ranging from newborn to 91 years old were treated with 10 microM ara C in their G1 phase and the resulting disc and ring were observed to investigate whether age-related change in peripheral lymphocytes would be present in the amount of spontaneous DNA damage or in the capacity to repair it. Chromatid-type (Ct) aberrations and sister chromatid exchanges (SCEs) were also scored for additional cytogenetic indices of DNA damage in the lymphocytes. The results showed that the frequency of dic and ring had a negatively linear correlation with the logarithm of the age of the blood donors, but that the frequencies of Ct aberrations and SCEs were not influenced by the age. The present study suggests the presence of age-related change in the amount or in the capacity to repair certain types of DNA damage in the G1 phase of human peripheral lymphocytes. Other possible explanations for the present results are also discussed.

Radiation survival of vascular smooth muscle cells as a function of age

Late damage to normal tissues is an important consideration in determining the dose of radiation which can be delivered to a given target volume in clinical radiation therapy. The response of large blood vessels to radiation injury is undoubtedly complex and is influenced by (1) the cellular composition of the vessel wall, (2) the slow turnover of vascular cells, and (3) vascular repair mechanisms. As a first order model for radiation effects in large vessels, we have studied the radiobiologic properties of cultured vascular smooth muscle cells. We have measured survival curves and repair of sublethal radiation damage in exponentially growing cultures of rat aortic smooth muscle cells as a function of animal age and site of origin (thoracic versus abdominal aorta). Radiation survival parameters (utilizing two different mathematical models for the survival curve) and repair of sublethal damage did not appear to vary significantly as a function of animal age (3-23 months) or site or origin.

DNA repair deficiencies and cellular senescence are unrelated in xeroderma pigmentosum cell lines

Senescence of skin fibroblast cultures from normal individuals occurred after 23.9 +/- 6.3 (S.D.) passages; senescence in DNA repair-deficient cell lines from xeroderma pigmentosum patients occurred at 22.9 +/- 5.5 passages. Cells from xeroderma pigmentosum variant and Cockayne syndrome patients reached senescence at similar passage numbers. Xeroderma pigmentosum patients contract skin cancer as a consequence of their repair deficiencies but show no symptoms of premature ageing; neither do their cells age prematurely in vitro. The clinical spectrum and the life-span of fibroblasts in culture therefore lend no support for a correlation between ageing and the DNA repair or DNA replication deficiencies found in xeroderma pigmentosum and Cockayne syndrome cells.

Increased DNA topoisomerase I activity in aging human cell chromatin

Chromatin-associated DNA topoisomerase I activity was measured in human diploid fibroblasts during in vitro aging. No difference was detected as a function of cell age in the nicking and the closing activities of the DNA-unwinding enzyme. The capacity of type-I topoisomerase to relax superhelical DNA molecules was, however, increased in aged cells. An age-related increase in nucleoprotein content was also observed.

Correlation of DNA repair synthesis with ageing in mice, evidenced by quantitative autoradiography

Fibroblasts from C57B1/6 female mice of different age have been treated with short-wave ultraviolet light. The amount of unscheduled DNA synthesis in the fibroblasts, determined by quantitative histoautoradiography, decreases with the donor's age. This result is discussed with regard to an increase in nuclear area and modifications of cell population dynamics with age. Moreover, differences between individuals of the same age group with regards to unscheduled DNA synthesis have been observed.

DNA repair in cultured mouse cells of increasing population doubling level

Cultures of mouse cells of various population doubling levels (PDL) were examined for DNA-repair capabilities as estimated by (i) the excision of pyrimidine dimers; (ii) unscheduled DNA synthesis (UDS) in response to UV-irradiation or N-methyl-N'-nitrosoguanidine (MNNG) treatment; (iii) the levels of two DNA-repair enzyme activities, uracil DNA glycosylase and AP endonuclease. The responses to ultraviolet light and MNNG decreased rapidly within the first two PDL and more slowly thereafter until essentially no repair was detected by PDL 12. A continuous cell line which emerged from the cultured cells after a crises period had some restoration of repair capability. The amount of uracil DNA glycosylase activity decreased by approximately 40% before the crises period then decreased by 90% in the continuous cell line. In contrast, the amount of AP endonuclease activity present in the precrises cells showed no significant change until PDL 12, then increased 6-7-fold in the continuous cell line.

Unscheduled DNA synthesis and elimination of DNA damage in liver cells of gamma-irradiated senescent mice

The level of 'spontaneous' and gamma-radiation-induced DNA synthesis which is not inhibited with hydroxyurea (unscheduled synthesis) is considerably lower in hepatocytes of 18-22-month-old mice than that of 1.5-2-month-old mice. The dose-dependent increase (10-300 Gy) of unscheduled DNA synthesis (UDS) in hepatocytes of senescent mice is higher than in young animals. The elimination of damage in DNA of gamma-irradiated hepatocytes (100 Gy) was examined by using an enzyme system (M. luteus extract and DNA-polymerase I of E. coli). It was found that the rate of elimination of the DNA damage in hepatocytes of 20-month-old mice is lower than that of 2-month-old mice although the activities of DNA-polymerase beta and apurinic endonuclease remain equal in the liver of both senescent and young mice. However, the nucleoids from gamma-irradiated liver nuclei of 2-month-old mice are relaxed to a greater extent (as judged by the criterion of ethidium-binding capacity) than those of 20-month-old mice. The results suggest that there are limitations in the functioning of repair enzymes and in their access to damaged DNA sites in the chromatin of senescent mouse liver cells.

Age dependency of DNA repair in rats after DNA damage by carcinogens

Damage to DNA seems to be an important cause of cancer and to play a role in aging. Much of this damage results from the action of chemical agents in the environment. These chemicals provide a chance to study DNA repair mechanisms and to construct a model for the investigation of changes in repair with aging. To damage the DNA of male Sprague-Dawley rats aged 6, 22-24 and 24-26 months, three carcinogens were used: N-methyl-N-nitrosourea (MNU), methyl methane sulfonate (MMS) and N,N-dimethyl-nitrosamine (DMN). DNA repair was measured as unscheduled DNA synthesis (UDS) in ten (MNU and DMN) and five (MMS) different organs. MNU and MMS react with DNA without being first metabolized and show a higher UDS in lower concentration than DMN which is metabolized enzymatically prior to the reaction. This result suggests that MNU and MMS produce more damage in the DNA. There are distinct differences in the spleen, lung, liver, kidney and heart in young animals as well as in the tissues of the kidney and the duodenum in old rats. Clearly we can see a reduction of UDS in the old as compared to the young animals after damage by MNU in the skin, lung, brain and heart, by MMS in the heart and liver, and by DMN in the kidney, duodenum, lung and liver, and by all three mutagens in the spleen and testes. These results confirm those obtained after damaging DNA by means of gamma- and UV-irradiation.

Newly synthesised DNA in ageing human cells in culture treated with 4-nitroquinoline-1-oxide

Two lines of normal human embryonic lung fibroblasts, MRC-5 and F2002, were serially subcultured until senescence was attained. When cells were exposed to varying concentrations of the chemical carcinogen 4-nitroquinoline-1-oxide (4-NQO), the rate of DNA synthesis (as measured by thymidine incorporation) was reduced in a dose-dependent fashion in cells from both early and late passages. While the overall amount of incorporation was considerably lower in old cells, the extent of inhibition caused by 4-NQO treatment (relative to appropriate controls) was not related to culture age. Alkaline sucrose density gradient analysis of newly synthesised DNA from cells pre-treated with 4-NQO failed to detect any significant variation in the size of labelled DNA from cells examined immediately after incubation with radioactive thymidine. The shift of this labelled material to high molecular weight in 4-NQO-treated cells also showed no age-related difference.

Changes in DNA polymerases alpha, beta, and gamma during the replicative life span of cultured human fibroblasts

DNA polymerases from IMR-90 human diploid fibroblasts at various passage levels and from HeLa cells were purified and fractionated into alpha 1, alpha 2, alpha 3, beta, and gamma species and subspecies, and than the accuracy with which each one copied synthetic template-primers was measured in the presence of Mn2+ or Mg2+. All activities from fibroblasts of later population doubling levels incorporated noncomplementary triphosphates more frequently than did the same polymerase type from earlier population doubling levels. HeLa polymerase activities copied several different templates in the presence of Mn2+ with greater fidelity than enzymes from fibroblasts of population doubling level 27 or greater. The total DNA polymerase activity extracted from IMR-90 cells decreased with increasing population doubling levels. The alpha-polymerase activity generally declined with increasing population doubling levels, while beta-polymerase activity remained relatively constant, except at the very end of the cellular replicative life span. In addition, the amounts of alpha 2 and alpha 3 became progressively lower relative to alpha 1, and a new alpha-type polymerase activity, alpha 0, appeared upon diethylaminoethylcellulose chromatography. HeLa cells also contained three alpha species, though two of them eluted from diethylaminoethylcellulose at higher phosphate concentrations than alpha species from fibroblasts. Postconfluent IMR-90 cells of population doubling level 21 had a decreased level of alpha-polymerase relative to that recovered from rapidly growing cells. This polymerase activity had some chromatographic properties similar to enzyme from late-passage cells. In addition, the alpha-, beta-, and gamma-polymerases from these cells had decreased fidelities relative to those isolated from subconfluent cells.

On the nature of susceptibility to cancer. The presidential address

The most prominent factor determining susceptibility to cancer is age. However, there is little evidence that the aging process per se increases susceptibility to cancer. Rather, age provides the time necessary for the accumulation of cellular events required for the development of neoplasia. The variations in the patterns of cancer incidence rates seen with age can be explained by alterations in conditions of exposure to carcinogenic stimuli. There is no evidence that the pool of susceptible individuals in a population is limited. Cancer occurs as a random event in a population with greater or lesser frequency according to the presence of risk factors. In populations with an increased frequency of cancer such as those with genetic abnormalities, immune deficiency syndromes, or altered hormonal states, the risk of developing cancer is never generalized to all tissues but is characteristic of particular tissues at risk. Any circumstance of internal or external origin that disturbs homeostasis of particular tissues at risk. Any circumstance of internal or external origin that disturbs homeostasis over a prolonged period of time increases the susceptibility to cancer for the tissue concerned. Susceptibility to cancer does not mean that cancer is inevitable. Only a small number of those susceptible to cancer by virtue of a special risk factor develop the disease. Furthermore, most patients who develop cancer have no determinable risk factors. Although all evidence points to a multifactorial, multistage process, the rate of somatic mutation appears to be the key determinant factor in susceptibility to cancer. This concept is supported by research studies showing that the onset of atherosclerosis is initiated by somatic mutations, and the finding that the same chemical mutagens can advance the development of both diseases.

Recent advances in the cell biology of aging

Cultured normal human and animal cells are predestined to undergo irreversible functional decrements that mimic age changes in the whole organism. When normal human embryonic fibroblasts are cultured in vitro, 50 +/- 10 population doublings occur. This maximum potential is diminished in cells derived from older donors and appears to be inversely proportional to their age. The 50 population doubling limit can account for all cells produced during a lifetime. The limitation on doubling potential of cultured normal cells is also expressed in vivo when serial transplants are made. There may be a direct correlation between the mean maximum life spans of several species and the population doubling potential of their cultured cells. A plethora of functional decrements occurs in cultured normal cells as they approach their maximum division capability. Many of these decrements are similar to those occurring in intact animals as they age. We have concluded that these functional decrements expressed in vitro, rather than cessation of cell division, are the essential contributors to age changes in intact animals. Thus, the study of events leading to functional losses in cultured normal cells may provide useful insights into the biology of aging.

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

Although baseline levels of SCE appear to be similar in metaphase cells derived from young and old human skin fibroblast cultures, a significant decline in both MMC- and AAAF-induced sister chromatid exchanges was observed in the older cell populations. Considerable variation exists in the levels of MMC-induced SCE between individual cell cultures. However, parallel examination of several cell strains from young and old donors confirmed the decrease in mutagen-induced SCE frequencies in older cell populations.

Normal level of unscheduled DNA synthesis in Werner's syndrome fibroblasts in culture

We investigated UV-induced unscheduled DNA synthesis (UDS) in skin fibroblasts from seven unrelated patients with clinically apparent Werner's syndrome (WS). WS cells exhibited greatly abbreviated in vitro lifespans, the extents of which ranged from about 20 to 50% of the normal. However, WS cells in early and senescent phases of growth showed the same quantity of DNA repair following UV exposure as did normal fibroblasts.

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.

A retarded rate of DNA replication and normal level of DNA repair in Werner's syndrome fibroblasts in culture

We investigated the cloning efficiency, DNA repair, and the rate of DNA replication in the skin fibroblasts from patients with Werner's syndrome (WS) of an autosomal recessive premature aging disease. Five WS strains exhibited normal levels of sensitivity toward X-ray and UV killings and repair of X-ray induced single strand breaks of DNA (rejoining) and UV damage to DNA (unscheduled DNA synthesis). The sedimentation of newly synthesizing DNA in alkaline sucrose gradients demonstrated a characteristic feature that only the elongation rate of DNA chains, estimated by the molecular weight increase, was significantly slower during early passages in WS cells than in normal Hayflick Phase II fibroblasts. In addition, plating efficiencies as well as the replicative potentials of five WS strains were more limited than those of normal cells under the identical culture conditions. It seems therefore that at least in the WS cells tested, the slow rate of DNA replication may be more related to the shortened lifespan and enhanced cell death, as manifestation of premature senescence at the cellular level, than be the DNA repair ability.

Normal DNA strand rejoining and absence of DNA crosslinking in progeroid and aging human cells

We have measured by alkaline elution and alkaline sedimentation the rate of rejoining of X-ray induced DNA single-strand breaks in terminally senescent cultured WI-38 cells. Using the alkaline elution method, we have also measured the rate of ligation in cultured progeroid cells. In both cells and by both methods of measurement the rates of strand rejoining were normal. Alkaline elution failed to disclose any DNA crosslinking in these cells.

Excision of ultraviolet and gamma ray products of the 5,6-dihydroxy-dihydrothymine type by nuclear preperations of xeroderma pigmentosum cells

Monomeric products of the 5,6-dihydroxy-dihydrothymine type are produced in the DNA by both ultraviolet and ionizing radiations. The capacity of nuclear preparations from normal and Xeroderma pigmentosum (XP) cells (Complementation groups A, B, C and D) to excise such products from ultraviolet or gamma-irradiated T7DNA was comparable and was independent of radiation induced strand breaks.

Deficiency of gamma-ray excision repair in skin fibroblasts from patients with Fanconi's anemia

The capacity of preparations of skin fibroblasts from normal individuals and patients with Fanconi's anemia to excise gamma-ray products of the 5,6-dihydroxydihydrothymine type from exogenous DNA was investigated. The excision capacity of whole-cell homogenates of fibroblasts from two of four patients with Fanconi's anemia was substantially below normal. This repair deficiency was further pronounced in nuclear preparations from cells of the same two patients.

Decreased unscheduled DNA synthesis in nondividing aged WI38 cells

The ability of cells from young and old cultures of WI38 human fibroblasts to undergo unscheduled DNA synthesis was studied. Following inhibition of semi-conservative synthesis in media lacking arginine and containing a hydroxyurea block, cells were irradiated with u.v. light and repair replication was measured by determination of specific activity of DNA and by autoradiography. Unscheduled DNA synthetic capacity was significantly reduced in old cultures. Quantitation of silver grains revealed that this reduction was a result of decreased repair activity by individual cells, which correlated well with a similar decrease in the proportion of dividing cells in the population. This observation that unscheduled DNA synthesis is reduced in older cells is in apparent contradiction to some earlier reports, and the failure of other investigators to identify aging changes in repair capacity is discussed in relation to differences in experimental procedure.

High blood pressure related to carcinogen-induced unscheduled DNA synthesis, DNA carcinogen binding, and chromosomal aberrations in human lymphocytes

Unscheduled DNA synthesis (excision-repair) of N-acetoxy-2-acetylaminofluorene (NA-AAF) damage to the DNA of human lymphocytes was determined quantitatively for 92 individuals with diastolic blood pressures ranging from 65 to 120 mm of Hg (8,7 to 16 kPa). Measurements of NA-AAF-induced repair synthesis (incorporation of[3H]thymidine in the presence of 10 mM hydroxyurea) showed linear increase with the blood pressure in the individuals under study. Concurrent determinations for the levels of 3H-labeled 7,12-dimethyl-benz[a]anthracene bound to the DNAs of lymphocytes after 18 hr of culturing have shown that increased amounts of DNA bound carcinogen were linearly proportional to increased NA-AAF-induced repair synthesis values, and therefore were correlated to high blood pressure. The number of NA-AAF-induced chromosomal abberations in lymphocytes increased linearly with the diastolic blood pressures of the individuals. High NA-AAF-induced repair synthesis values also tended to be associated with increased NA-AAF-induced chromosomal damage. Together, these results suggest that individuals with elevated blood pressures have a greater potential for accumulating DNA damage, because of an increased chemical reactivity of lymphocytes to carcinogen exposure, than do individuals with normal blood pressure.