Rate of DNA repair in progeric and normal human fibroblasts

Fibroblast clones from patients with Hutchinson-Gilford progeria can senesce despite the presence of telomerase

Hutchinson-Gilford progeria (HGP) is a genetic disorder in which individuals prematurely display features of ageing. Mutations in LMNA (lamin A) have recently been shown to underlie HGP, although how such mutations lead to the complex phenotype seen in the disease remains unclear. HGP is often associated with the premature replicative senescence of dermal fibroblasts. Normally dermal fibroblast senescence is initiated by erosion of chromosomal ends (telomeres) resulting from sustained cell division. Since ectopic expression of telomerase reproducibly immortalises human dermal fibroblasts, it is of interest to determine whether HGP fibroblasts immortalise via the same route, and at the same frequency. Three strains of HGP fibroblasts (AGO6917A, AGO6297B and AGO8466) were infected with a retroviral vector expressing the catalytic subunit of telomerase (hTERT). Here we report that fibroblast clones derived from HGP donors frequently fail to immortalise with telomerase. Of the 15 independently isolated clones from the three donors, five failed to immortalise despite the restoration of telomerase activity and the stabilisation of telomere length. In contrast, out of four clones isolated from a culture of hTERT transduced control fibroblasts, no failures to immortalise were detected. This suggests a novel cellular phenotype in HGP, one whereby the HGP mutation confers resistance to 'telomerisation'.

Richard B. Setlow, a commentary on seminal contributions and scientific controversies

Richard B. Setlow inspired the field of DNA repair. His demonstration that photoproducts could be quantified within cells and their excision examined experimentally pioneered the identification of nucleotide excision repair. His early work was associated with the discovery of many founding phenomena of photobiology and DNA repair: the concept of excision repair itself, correlations between DNA repair, life span and aging, variations in repair among mammalian species, caffeine sensitization to UV damage, and the xeroderma pigmentosum (XP) repair deficiencies. We may now have mapped thoroughly the landscape of DNA repair that Dick helped open to exploration, but questions persist of how comprehensively we have explored all its canyons and mesas. Research into nontraditional species and kingdoms may yet provide unexpected surprises. The signal transduction pathways and mechanisms of DNA replication arrest in damaged mammalian cells remain a challenge. The importance of repair in vivo also provides many difficult research questions. One problem of current interest is the role of endogenous DNA damage and repair in human pathology, especially neurodegeneration exemplified by many XP patients. Cancer and neurodegeneration may represent converse responses of dividing and nondividing cells to mutagenic and lethal effects of DNA damaging agents. Cell death from endogenous oxidative DNA damage (apoptosis) may be antagonistic to malignant transformation in dividing cells but may cause neurodegeneration in nondividing neural tissue. Small reductions in the efficiency of repair, especially transcription-coupled repair, may overemphasize carcinogenesis in mice, while minimizing neurodegeneration, as compared to human patients.

Reduction of unscheduled DNA synthesis and plasminogen activator activity in Hutchinson-Gilford fibroblasts during passaging in vitro: partial correction by interferon-beta

Two fibroblast cell lines (PG3KT and PG1NA) derived from Hutchinson-Gilford syndrome (progeria) cases were characterized, at various population doubling levels (PDL), with respect to the capacity of ultraviolet light (UV, mainly 254 nm wavelength)-induced unscheduled DNA synthesis (UDS) and plasminogen activator-like protease activity (PA). The UDS levels in PG3KT and PG1NA cells at PDL 2-3 were only slightly less than those in normal fibroblasts. With increasing PDL, both progeria cell lines exhibited reduction of the UDS levels and undetectable ones at PDL 9-11. Prompt and transient induction of PA was also detectable at less than PDL 5, whereas it was undectable at higher PDL. However, the levels of UDS and PA induction were increased about 3-7 times after pretreatment with 100 IU/ml human interferon (HuIFN)-beta preparations for more than 24 h prior to UV irradiation, although UDS and PA were undetectable at more than PDL 10. These results suggest that cytokines such as HuIFN-beta transiently compensate for the decreases in UDS and PA inducibility in progeria cells with aging.

Immortalization of Werner syndrome and progeria fibroblasts

Human fibroblast cells from two different progeroid syndromes, Werner syndrome (WS) and progeria, were established as immortalized cell lines by transfection with plasmid DNA containing the SV40 early region. The lineage of each immortalized cell line was confirmed by VNTR analysis. Each of the immortalized cell lines maintained its original phenotype of slow growth. DNA repair ability of these cells was also studied by measuring sensitivity to killing by uv or the DNA-damaging drugs methyl methansulfonate, bleomycin, and cis-dichlorodiamine platinum. The results showed that both WS and progeria cells have normal sensitivity to these agents.

Reduced DNA repair in progeria cells and effects of gamma-ray irradiation on UV-induced unscheduled DNA synthesis in normal and progeria cells

A reduction in the amount of UV-induced unscheduled DNA synthesis (UDS), and reduced cell survival and host-cell reactivation against UV exposure in Hutchinson-Gilford progeria syndrome cell strains were shown. UV-induced UDS in 4 progeria cell strains was 33-50% of the normal level. A similar reduction in the UV-induced UDS in normal cells was caused by gamma-ray irradiation to the cells before UV irradiation. The dose of gamma-rays required to cause a reduction in UDS of normal cells to the level of progeria cells was 40 Gy and the reduction was reversible after 2 days. In progeria cells, gamma-ray irradiation further reduced UDS with a lower gamma-ray dose required than in normal cells, and the reduction was also reversible but with less relative recovery than in normal cells. The presence of a 'built-in' defect in progeria cells responsible for the reduced DNA-repair capacity was suggested, and such defect may share a common mechanism with the reduction of UV-induced UDS in normal cells caused by gamma-ray irradiation.

Proliferation-dependent regulation of DNA glycosylases in progeroid cells

The regulation of the base excision repair enzymes uracil DNA glycosylase and hypoxanthine DNA glycosylase was examined in 2 different progeroid cell strains. The immunoreactivity of the uracil DNA glycosylase in progeroid cells was examined by enzyme linked immunosorbent assay (ELISA) and by immunoblot analysis. The enzyme was recognized in a quantitative manner by 2 different anti-human uracil DNA glycosylase monoclonal antibodies in the ELISA. Western blot analysis identified a glycosylase protein of Mr = 37,000. In randomly proliferating progeroid cells, the uracil DNA glycosylase was enhanced 3-fold during cell growth. In synchronous cells, uracil DNA glycosylase and hypoxanthine DNA glycosylase were induced with an extent of induction (5-6-fold) comparable to that observed for normal human cells. Further, the activity of each base excision repair enzyme was enhanced with a comparable temporal sequence prior to the induction of DNA synthesis and DNA polymerase activity. These results indicate a normal cell cycle regulation of base excision repair in progeroid cells.

Reduced DNA-repair capacity in cells originating from a progeria patient

A Chinese boy was identified to be suffering from progeria (Hutchinson-Gilford syndrome), the first case of the disease ever reported in China. Cells originating from the patient had a reduced amount of unscheduled DNA synthesis after irradiation with ultraviolet light (UV). The fractions of the progeria cells surviving against UV irradiation measured by colony-forming ability, and the host-cell reactivation capacity of the progeria cells, measured by the plaque formation of UV-irradiated herpes simplex virus were lower than those measured in normal cells. The progeria cells appear to have a reduced capacity to repair UV excision damage.

Sister chromatid exchange and proliferation pattern in lymphocytes from newborns, elderly subjects and in premature aging syndromes

Sister chromatid exchange (SCE) frequency and cell proliferation were examined in lymphocyte cultures from a group of newborns, a group of elderly subjects and from patients with syndromes who exhibit progeriform characteristics (progeria, Cockayne syndrome, Rothmund-Thomson syndrome and Christ-Siemens-Touraine syndrome) by using the bromodeoxyuridine incorporation differential staining technique. We observed a significantly increase in basal SCE frequency and a less intensive cell proliferation in cultures from elderly subjects than from newborns, as shown by the significant increase in percentage of cells in first generation simultaneous with a reduction of cells in more advanced generations. Lymphocyte cultures from each one of the patients studied also showed a decreased cell proliferation in relation to their respective control and to newborn cultures. Each of these syndromes showed higher baseline SCE levels than the control and than the newborn and elderly groups. Only the patient with progeria showed values similar to those for the elderly group. Thus, in addition to showing clinical characteristics similar to those observed during the normal aging process, these progeriform syndromes also show cytogenetic characteristics similar to those of older individuals.

Cell-cycle defect of DNA repair in progeria skin fibroblasts

We examined the temporal regulation of DNA repair during synchronous cell proliferation in normal and progeroid human fibroblasts. Ultraviolet light-induced (254 nm, 20 J/m2) unscheduled DNA synthesis was measured at 4-h intervals after serum stimulation, for up to 32 h. Normal cells regulated DNA repair in a defined temporal sequence, showing a peak in the induction of DNA repair just before DNA synthesis. Progeroid skin fibroblasts failed to show an increase in nucleotide excision repair before scheduled DNA synthesis, but the background level of DNA repair was not significantly different from that in controls. Regulation of repair in progeroid human fibroblasts appeared similar, but not identical to that previously reported by Gupta and Sirover (1984b) for xeroderma pigmentosum complementation group C. Our results suggest that patients with Hutchinson-Gilford progeria may have a defect in DNA repair; the results offer nominal evidence that the average level of UV-induced DNA is decreased, and that individuals with this disease lack both the normal enhancement of DNA repair before scheduled DNA synthesis and the temporal control of DNA repair.

Augmented nuclease activity during cellular senescence in vitro

The molecular correlates of the limited proliferative potential of normal human diploid fibroblasts and extensive single-strand breaks in the genomic DNA of these cells were examined by transfection analyses in which DNA replication could be uncoupled from DNA damage and repair. Both supercoiled (fmI), and restriction endonuclease-cleaved, linear (fmIII) molecules of a well-defined bacterial plasmid DNA, pBR322, were transfected into, and subsequently recovered from, early and late passage fibroblasts. Southern blot analysis revealed that fmI DNA was converted by random nicks into fmII DNA slightly more rapidly in late passage cells compared with cells at early passage. Similarly, fmII and fmIII DNAs also sustained multiple random nicks and no appreciable net religation of free ends of fmIII DNA could be detected at either passage. In addition, the efficiency of in vitro ligation of fmIII DNA recovered from late passage cells was also reduced, compared with that from early passage cells, as determined by Southern blotting. These data suggest that in the absence of DNA replication, a putative nuclease activity may contribute to DNA damage observed in senescent cells, which, in turn, may be causally related to their limited replicative potential.

Age-correlated DNA damage in human muscle tissue

This investigation represents the largest study so far published on human DNA damage and aging. The subject of this investigation is damage, determined as DNA alterations which give rise to complete molecular breaks in the course of treatment of purified DNA solutions with single-strand-specific nucleases. The DNA is derived from milligram samples of human muscle of individuals mostly undergoing surgical treatment. Care has been taken to bring the muscle samples, once shut off from blood circulation to liquid nitrogen temperatures within few seconds. The DNA is prepared by a procedure keeping breaks by handling and by DNAase attack as low as possible, however pushing DNA purity, especially with respect to protein as high as possible. Highly purified DNA treated in this way has some sites which are susceptible to single-strand (ss) specific DNAase splitting (ss-events). Three different deoxyribonucleases have been used: Nuclease S1, Nuclease BAL31 and Pea Endo-Nuclease. They give very similar results, i.e. splitting of the DNA so as to yield DNA pieces of given distribution. The lengths of these double-strand (ds) pieces have been determined from their electron microscopical pictures, either by following the image contours with a magnetostrictive stylo of the projected photo on a pad, by following the contours with a mileage ruler, or by integrating the silver grains on the photo. The molecular weight averages of the ds DNA threads between two ss-events for each individual have been determined from 20 to 200 molecules. The 470 individuals contributing their data were from age groups from 1 to 91 years. The molecular weights show a considerable scatter with an average molecular weight of the DNA ds pieces between two ss-events of 43.93 MDa and a standard deviation of 17.99 MDa. Among the single-strand breaks (ssb) that split the DNA into such pieces is a fraction, the number of which increases in a highly significant fashion with the age of the donor. From this derives the fact that the average molecular weight of the DNA strand pieces between two ssb decreases with age. It is remarkable that the standard deviation of the molecular weights of such pieces increases with age significantly, too. On the basis of additional information mainly supplied by the DNA donor himself or by his parents the 470 members of the main group M where grouped according to their life-style, into: (1) abstinent people, essentially non-smokers and refraining from use of licit or illicit drugs, sub-group N.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

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.

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.

Changes in endogenous DNA damage in aging mice in response to butylated hydroxyanisole and oltipraz

The extent of DNA damage (single strand breaks) was measured in the livers of female Swiss-Webster mice up to 24 months of age. The effects of the antioxidants butylated hydroxyanisole (BHA) and oltipraz on this DNA damage were also measured. Oltipraz is an antioxidant which is structurally related to compounds found in cruciferous vegetables. From 6 months on the extent of DNA damage increased, reaching a maximum about 18 months old. Limited administration of both BHA and oltipraz to mice significantly reduced the levels of hepatic DNA damage.

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.

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.

Measurement of the molecular weight distributions in human muscular deoxyribonucleic acid

Highly purified DNA has been prepared from human muscle tissue and treated with three different single-strand specific desoxyribonucleases. The molecular weight of treated individual DNA molecules was determined by length measurements. Average DNA molecular weights and their standard deviations were correlated to the age of donors. No significant correlation could be established using seven different mathematical functions. Factor analytical considerations suggested grouping according to extreme smoking habits. Two populations with significant double linear correlations of age versus molecular weight and standard deviation then emerge. From the data it may be speculated that in heavy smokers with increasing age (or time of smoking) high molecular weight DNA integrity is lost to an increasing extent in post-mitotic tissue.

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.

DNA repair in a congeneic pair of mice with different longevities

The rate of ultraviolet light (UV)-induced DNA excision repair was determined in embryonic cells derived from a congeneic pair of short-lived (C57BL/10.F) and long-lived (C57BL/10) mice. Excision repair was measured by both bromodeoxyuridine photolysis and arabinofuranosyl cytosine inhibition. No difference in rate of repair was observed between the two cell lines.

Spontaneous structural changes in DNA during fibroblast aging and the establishment process in vitro

The molecular weight of single-stranded DNA isolated from human fibroblasts decreased in phase III by comparison with phase II. Mouse fibroblast DNA isolated during the growth crisis had a decreased molecular weight compared to the initial DNA. Established mouse cells recovered this high molecular weight DNA. Cells treated with caffeine during the growth crisis did not survive while established cells were resistant to the same conditions of caffeine treatment. A DNA repair process may play a role in establishing a permanent cell line.

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.

Changes in DNA alkali-sensitive sites during senescence and establishment of fibroblasts in vitro

DNA molecular weight was studied in human embryonic and mouse newborn lung fibroblasts in vitro at different passages of the culture using alkaline and neutral sucrose gradient techniques. Reduction of molecular weight of single-stranded DNA due to alkaline-sensitive sites appeared spontaneously during the growth decline of the mouse cells. These changes disappeared when the mouse fibroblasts became a permanent cell line. At the end of phase II of the human fibroblasts, the molecular weight of single-stranded DNA also decreased, followed by the restitution of some high molecular weight DNA in the ultimate passages. When treated with 1 mM caffeine, the mouse fibroblasts during growth crisis did not survive, while cells of the established line resisted. Thus it might be possible that a DNA repair process was involved in the recovery of the mouse fibroblasts. Furthermore, results favor the hypothesis that the cells that become established are not present in the primary culture but originate in vitro.

Effect of low dose rate irradiation on the division potential of cells in vitro. VI. Changes in DNA and in radiosensitivity during aging of human fobroblasts

The sensitivity to low dose rate ionizing radiation increases progressively through the lifespan of human embryonic lung fibroblasts. There is also an increase in the number of alkali-sensitive sites leading to an increase in single-strand breaks and in DNA with low molecular weight during cell lysis. These DNA changes become pronounced at the very end of the lifespan. The correlation between aging, increased radiosensitivity and accumulation of DNA damage is discussed.

Human mutations affecting aging--a review

A review of human genetic mutations that affect aging and their potential contribution to help understand normal aging processes is presented. The lifespans of most animal species, including man, have a genetically determined maximum. The lifespan of man appears to have evolved exceedingly rapidly, which suggests that relatively few genes may determine longevity. Analysis of biochemical evolution suggests that the regulation of enzyme levels may underlie most evolutionary changes. There is a wide spectrum of human genetic mutations. Some, such as progeria and Werner's syndrome, produce a phenotype resembling premature aging and may involve genes related to the aging process. Certain human chromosomal abnormalities, such as Down's syndrome, produce an appearance of premature aging and may be due to abnormal gene regulatory mechanisms. Progress in understanding the genetic mechanisms underlying aging is likely to come from elucidation of the molecular defects that result in the premature aging syndromes and from insights gained regarding the regulatory mechanisms governing eukaryotic genetic expression.

Longevity, stability and DNA repair

The functional capacity of a cell, tissue, organ, or organism is dependent upon its ability to maintain the stability of its unit components. The higher the differentiated state of the system, the greater the amount of stability required to maintain that state as a function of time. Stability can be achieved via either redundancy or repair. Redundancy while easily achievable in biological systems is both costly and limited by thermodynamic considerations. Repair, in its general sense, has no such limitations. Repair at the cellular and macromolecular level is multiple in its forms and varies as a function of species, tissue, and stage of the cell cycle. The repair of DNA damage is a dynamic process with many components and subcomponents, each interacting with one another in order to achieve a balance between individual stability and evolutionary diversity. Thus, between internal and external factors which damage DNA and the subsequent expression of alterations in the functional stability of DNA lie the multi-functional pathways which attempt to maintain DNA fidelity. A strong correlation between ulta-violet light induced excision or pre-replication repair, as measured by autoradiogrphy and maximum species lifespan has been reported within different strains of the same species, between related species (e.g. Mus musculus and Peromyscus leucopus), between five orders of mammals, and most recently within members of the primate family. As has been demonstrated by the authors and others, differences in excision repair between species and tissues may relate to the turning off of portions of the repair processes during embryogenesis. Regardless of why such correlations exist or the nature of their mechanisms, it is naive to either assert or deny a causal role for DNA repair in longevity assurance systems. For example, while species-related differences in DNA repair may reflect the turning off of such repair processes during fetal development this does not mean that rates of accumulation of DNA damage are not altered by such changes. Indeed, such a phenomena might well explain the rapid evolution of lifespan within the primates without a concurrent input of new genes.

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.

Inhibition of postreplication repair and the enhancement of induction of SV40 virus from transformed hamster kidney cells

The induction by ultraviolet light of simian virus 40 (SV40) from two SV40--transformed hamster kidney cell lines is enhanced by caffeine. In order to investigate the mechanism responsible for this enhancement, the effect of caffeine on postreplication repair of DNA damaged by UV light was studied utilizing alkaline sucrose-gradient sedimentation. Caffeine at concentrations of 0.5, 1.0 or 2.0 mM inhibited the filling of gaps during postreplication repair. In addition, caffeine was found to potentiate cell killing by mitomycin C, an alkylating agent, and to enhance SV40 induction by mitomycin C. We postulate that the persistence of gaps in DNA, caused by the presence of caffeine, results in the enhancement of SV40 virus induction.

Unrepaired DNA strand breaks in irradiated ataxia telangiectasia lymphocytes suggested from cytogenetic observations

It is suggested here that the unusually high level of radiation-induced chromosome and chromatid-type aberrations in cells from patients with ataxia telangiectasia, compared with normals, is due to a significantly increased fraction of unrepaired double and single strand breaks. A hypothesis is proposed to explain how unrepaired and misrepaired DNA single or double strand breaks might be the basic lesion leading to the typical chromosome aberrations seen following X-irradiation of both normal or AT cells.

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.