DNA repair and mutagenesis in mammalian cells

Health effects of herbicides and its current removal strategies

The continually expanding global population has necessitated increased food supply production. Thus, agricultural intensification has been required to keep up with food supply demand, resulting in a sharp rise in pesticide use. The pesticide aids in the prevention of potential losses caused by pests, plant pathogens, and weeds, but excessive use over time has accumulated its occurrence in the environment and subsequently rendered it one of the emerging contaminants of concern. This review highlights the sources and classification of herbicides and their fate in the environment, with a special focus on the effects on human health and methods to remove herbicides. The human health impacts discussion was in relation to toxic effects, cell disruption, carcinogenic impacts, negative fertility effects, and neurological impacts. The removal treatments described herein include physicochemical, biological, and chemical treatment approaches, and advanced oxidation processes (AOPs). Also, alternative, green, and sustainable treatment options were discussed to shed insight into effective treatment technologies for herbicides. To conclude, this review serves as a stepping stone to a better environment with herbicides.

Organophosphate pesticide-induced toxicity through DNA damage and DNA repair mechanisms

Organophosphate pesticides (OPs) are widely used in agriculture, healthcare, and other industries due to their ability to kill pests. However, OPs can also have genotoxic effects on humans who are exposed to them. This review summarizes the research on DNA damage caused by OPs, the mechanisms behind this damage, and the resulting cellular effects. Even at low doses, OPs have been shown to damage DNA and cause cellular dysfunction. Common phenomena seen in cells that are exposed to OPs include the formation of DNA adducts and lesions, single-strand and double-strand DNA breaks, and DNA and protein inter and intra-cross-links. The present review will aid in comprehending the extent of genetic damage and the impact on DNA repair pathways caused by acute or chronic exposure to OPs. Additionally, understanding the mechanisms of the effects of OPs will aid in correlating them with various diseases, including cancer, Alzheimer's, and Parkinson's disease. Overall, knowledge of the potential adverse effects of different OPs will help in monitoring the health complications they may cause.

Unboxing the molecular modalities of mutagens in cancer

The etiology of the majority of human cancers is associated with a myriad of environmental causes, including physical, chemical, and biological factors. DNA damage induced by such mutagens is the initial step in the process of carcinogenesis resulting in the accumulation of mutations. Mutational events are considered the major triggers for introducing genetic and epigenetic insults such as DNA crosslinks, single- and double-strand DNA breaks, formation of DNA adducts, mismatched bases, modification in histones, DNA methylation, and microRNA alterations. However, DNA repair mechanisms are devoted to protect the DNA to ensure genetic stability, any aberrations in these calibrated mechanisms provoke cancer occurrence. Comprehensive knowledge of the type of mutagens and carcinogens and the influence of these agents in DNA damage and cancer induction is crucial to develop rational anticancer strategies. This review delineated the molecular mechanism of DNA damage and the repair pathways to provide a deep understanding of the molecular basis of mutagenicity and carcinogenicity. A relationship between DNA adduct formation and cancer incidence has also been summarized. The mechanistic basis of inflammatory response and oxidative damage triggered by mutagens in tumorigenesis has also been highlighted. We elucidated the interesting interplay between DNA damage response and immune system mechanisms. We addressed the current understanding of DNA repair targeted therapies and DNA damaging chemotherapeutic agents for cancer treatment and discussed how antiviral agents, anti-inflammatory drugs, and immunotherapeutic agents combined with traditional approaches lay the foundations for future cancer therapies.

Organopesticides and fertility: where does the link lead to?

Organopesticides (OPs) are a group of various synthetic chemicals prevalently used in agriculture and homestead plantations. OPs were originally developed to remove insects, weeds, and other pests from agricultural fields for improving crop yields. Modern pesticides including organochlorine pesticides, organophosphorus pesticides, and amido-formyl ester are closely related to our lives. Many people are exposed to various OPs during farming practice. OPs can cause adverse effects and provoke serious impacts on normal reproductive functions of humans, resulting in loss of fertility. The effects of OPs in the reproductive system include association with fluctuation in the levels of sex hormones, delayed menstrual cycle, ovarian dysfunction, alteration in ovary weight, changes of follicle growth, altered oocyte feasibility, and changed the quality of spermatogenesis. Current literature clearly states that exposure to various OPs can impair the fertility of women and cause a high risk of reproductive potential. However, investigations on OPs exposure to woman fertility remain scarce. This review highlights effects of exposure to OPs on the fertility of occupational women and mechanisms of action involved in such effects on the reproductive function of women along with their related impacts.

Environmentally relevant exposures of male mice to carbendazim and thiram cause persistent genotoxicity in male mice

Carbendazim and thiram are fungicides used in combination to prevent mold destruction of crops. Studies have demonstrated genotoxicity by these agents, but have not used concentrations below their water solubility limits in drinking water to test for persistence of genotoxicity due to chronic exposure. Ten 8-week old male Swiss-Webster mice were exposed to tap water, or nominal concentrations of 20 μM carbendazim, 20 μM thiram or 20 μM of both fungicides for 90 days (total of 40 mice). Five mice from tap water controls, carbendazim, thiram and combination-treated groups (20 mice total) had genotoxicity detected by comet assay of lymphocytes at the termination of the exposure period. The other 20 mice (4 treatment groups) were all switched to tap water and allowed a 45-day recovery period to check for persistence of DNA damage. The damage was compared with commercial control cells exposed to increasingly harsh treatment by etopside. Comet assay (mean % tail DNA + SE) of control mice (9.8 + 0.9) was similar to commercial control (CC0) cells (8.5 + 0.9). Carbendazim, thiram or the combination treatment caused similar mean % tail DNA with 33.0 + 2.9, 30.1 + 3.3 and 29.1 + 1.8, respectively, comparable with commercial cells slightly damaged by etopside (CC1 with 31.4 + 2.9) with no statistical change in water or food intake, body weight or liver or kidney weights. The key result was that a 45-day recovery period had no observable difference in the DNA damage as assessed by DNA % in comet tail with tap water controls and CCO control cells at 7.0 + 0.7 and 9.7 + 1.2 versus 27.5 + 1.9, 29.3 + 2.2 and 32.0 + 1.8, respectively, for carbendazim, thiram and combination treatments. It is of concern that the use of these agents in developing countries with little training or regulation results in water pollution that may cause significant persistent DNA damage in animal or human populations that may not be subject to repair.

Occupational Pesticide Exposure, Impaired DNA Repair, and Diseases

Pesticides are a mixture of chemical substances used to kill pests. Apart from their toxicity to pests, thy affect nontarget organisms. They also generate free radicals producing reactive oxygen species (ROS) which can disturb cellular pathways by inhibiting various enzymes or receptors. Pesticides also induce oxidative DNA damage, DNA adducts, and single or double strand DNA breaks. Various mechanisms of DNA repair deal with such damages and help to maintain cell integrity. Alteration in DNA repair genes modulates the individual's susceptibility towards DNA repair and various diseases. Biological monitoring provides a useful tool for the estimation of genetic risk in populations exposed to pesticides. Large numbers of evidences show that occupational exposure to pesticides in agricultural workers has been associated with an increased incidence of various diseases such as cancer, Parkinson's disease, Alzheimer's disease, reproductive disorders, and birth defects. In this review, we have discussed occupational pesticide exposure, various mechanisms of DNA damage caused by pesticides, DNA repair mechanisms, biomonitoring tools, and various diseases caused by pesticide exposure.

Formation of cytotoxic metabolites from phenytoin, imipramine, desipramine, amitriptyline and mianserin by mouse and human hepatic microsomes

The effects of enzyme induction on the generation of cytotoxic metabolites from phenytoin, mianserin, imipramine, desipramine and amitriptyline by mouse liver microsomes has been investigated and then compared with the bioactivation mediated by human hepatic microsomes. Cytotoxicity was assessed by co-incubation of drug and microsomes with human mononuclear leucocytes which served as target cells. Enzyme induction was assessed by measurement of hepatic cytochrome P-450 content, and determination of alkoxycoumarin O-dealkylase activity. None of the compounds investigated were metabolized to cytotoxic metabolites in the presence of control mouse microsomes. However, significant bioactivation could be observed for each drug when incubated with microsomes prepared from mice pretreated with either phenobarbitone (60 mg/kg) or beta-naphthoflavone (75 mg/kg). The rank order for metabolism-dependent cytotoxicity with phenobarbitone-induced mouse microsomes (expressed as % cell death) was phenytoin (14.6%) greater than desipramine (10.5%) greater than imipramine (7.5%) greater than mianserin (3.4%) greater than amitriptyline (3.1%). Expression of cytotoxicity with phenytoin required pre-exposure of the target cells to trichloropropane oxide, an opoxide hydrolase inhibitor. Only mianserin and desipramine were activated to cytotoxic metabolites by human liver microsomes. Analysis of stable metabolites revealed that mianserin underwent extensive (greater than 80%) metabolism by both control and induced mouse microsomes and that the principal metabolites, 8-hydroxymianserin, desmethylmianserin and mianserin N-oxide, were the same as those produced by human liver microsomes. These data suggest that mianserin is activated to a cytotoxic metabolite selectively by a constitutive form of human cytochrome P-450, whereas phenytoin, amitriptyline and imipramine are selectively activated by forms of mouse cytochrome P-450 which are induced by either phenobarbitone or beta-naphthoflavone.

Unscheduled DNA synthesis in human skin after in vitro ultraviolet-excimer laser ablation

DNA damage repaired by the excision repair system and measured as unscheduled DNA synthesis (UDS) was assessed in freshly excised human skin after 193 and 248 nm ultraviolet (UV)-excimer laser ablative incisions. Laser irradiation at 248 nm induced DNA damage throughout a zone of cells surrounding the ablated and heat-damaged area. In contrast, with 193 nm irradiation UDS was not detected in cells adjacent to the ablated area, even though DNA strongly absorbs this wavelength. Our results suggest that the lack of UDS after 193 nm irradiation is due to: "shielding" of DNA by the cellular interstitium, membrane, and cytoplasm, DNA damage that is not repaired by excision repair, or thermal effects that either temporarily or permanently inhibit the excision repair processes.

Longevity in the protozoa

In ciliates there are examples of cells which have different proliferation potential in the macronucleus. Those species with limited macronuclear proliferation potential require sex to activate the reserve nucleus. In terms of the capital investment theory, some ciliates invested in their spare nucleus without loss of their original potential, while others accumulated debts and needed the reserve account to maintain life. Other cells neglected maintenance of their reserve account and failed unless their venture capital account was not a self-sustaining venture. Sex provided access to the reserve account and had to occur before deterioration of the reserve account. The question is not when cellular immortality was lost, but rather when immortality was partitioned from a mortal segment. The separation provided the option both for senescence and evolution in multicellular organisms. In colonial flagellates, separation of cells with infinite and finite cell lifespan potential occurred in some species, while in others the separation did not involve loss of immortality. In colonial flagellates, sex did not become an obligate stage. The immortal cells are haploid and could not accumulate damage and live (in contrast with the diploids in the ciliated protozoans). The present theory predicts that differences between species or cells with infinite versus finite lifespan potential may reveal differences in the critical determinants of longevity. Senescence could arise as an accident, as well as a design of nuclear differentiation. Cells therefore may have a much greater reserve for totipotency than would be predicted if they were assumed to lose immortality simply by the act of differentiation.

Distress and DNA repair in human lymphocytes

This research assessed differences in DNA repair in lymphocytes from high- and low-distressed individuals. A median split on Minnesota Multiphasic Personality Inventory (MMPI) Scale 2 divided 28 newly admitted nonpsychotic psychiatric inpatients into high- and low-distress subgroups. The high-distress subgroup had significantly poorer DNA repair in lymphocytes exposed to X-irradiation than low-distress subjects. We also found that lymphocytes obtained from this psychiatric sample had significantly poorer DNA repair than lymphocytes from nonpsychiatric control subjects when compared 5 hr after X-irradiation. A high level of distress therefore appears to be associated with significant dysfunctional differences at the molecular level which may have important implications for health. These data provide evidence for a direct pathway through which distress could influence the incidence of cancer.

Mosaicism in female hybrid hares heterozygous for glucose-6-phosphate dehydrogenase. V. The recovery of DNA synthesis of hare fibroblasts after ultraviolet irradiation

The effect of uv irradiation on the recovery of DNA synthesis is examined in a population of hare fibroblasts exhibiting heterozygosity with reference to the X-linked enzyme, glucose-6-phosphate-dehydrogenase (G-6-PD). These cells have been grown from skin explants of a hybrid female cross between Lepus timidus (female) and L. europaeus (male), the former carrying the G-6-PD gene for the slow-moving "T" variant and the latter with the fast-moving "E" variant gene. The hybrid, therefore, exhibits genetic mosaicism due to random inactivation in each cell, of one of the two X chromosomes in the embryonic stage. Exponentially growing cells from 13 fibroblast strains, comprising a wide range of E to T ratios, were exposed to moderately low dose of uv irradiation (6 J/m2). The recovery in DNA synthesis during the 2- to 8-h postirradiation period was calculated as the mean percentage rates of [3H]thymidine incorporated during the time as compared to the unirradiated zero-time controls. The results show a statistically significant positive correlation as determined by linear regression analysis between the levels of E and the rate of recovery in DNA synthesis. This is valid also at the higher dose of uv (21 J/m2). These results strengthen our earlier observations with 25-hydroxycholesterol that in the in vitro system the cell expressing the E variant is perhaps more resistant to cytotoxic agents. This also indicates that various factors contribute to the development of monotypism which include cell growth, cell death, mutation, and selection, to name a few.

Cancer and normal ageing

Cancer has a monoclonal origin in a pre-mitotic cell and usually a multistep pathogenesis. The initiation of tumor development and most stages in tumor progression involve point mutations, chromosomal rearrangements, and often changes in gene dosage. Simultaneously, there is continuing selection of the cell clones most resistant to growth-regulating substances and/or lacking specific immunologic markers. Normal ageing is partly pre-programmed as demonstrated by the constancy of the maximum survival time for a species under various external conditions resulting in different mean survival times. Emerging evidence of characteristic DNA changes in post-mitotic cells of old individuals may turn out to be part of the programmed changes. However, random accumulation of mutational defects in both pre- and post-mitotic cells is an unavoidable consequence of physics and therefore contributes to normal ageing and the accompanying increase in cell diversity. Cancer incidence increases with age. Firstly, because extended exposure increases the risk of inflicting the DNA changes prerequisite to oncogenesis; secondly, because the progression from one malignant cell to detectable tumor is a matter of 10-30 per cent of a species maximum life span; and thirdly, because some alterations characteristic of normal ageing increase the susceptibility to carcinogens. There probably is an overlap of etiologic/accelerating factors for cancer and ageing. Such aspects of normal ageing such as decline in DNA repair capacity and decline in cellular immune reactivity should facilitate induction and early growth of neoplasia. Age changes that counteract cancer development include (hormonal) loss of proliferative stimulation and depletion of the pool of immature cells at greatest risk.

A correlation between aging and DNA repair in human epidermal cells

Ultraviolet-induced unscheduled DNA synthesis (i.e. repair synthesis) in human epidermal cells was measured as a function of age. Normal mammary skin specimens were obtained at surgery from 36 female patients, ranging in age from 17 to 77 years. The enzymatically isolated epidermal cells were analyzed for two parameters: (1) the number and percentage of cells carrying out repair synthesis, and (2) the rate of ultraviolet-induced unscheduled thymidine incorporation in individual cells. The results show that the percentage of epidermal cells capable of DNA excision repair synthesis does not decrease significantly with age, but that the rate of unscheduled DNA synthesis in individual cells decreases to a highly significant degree with advancing age.

Inhibitory effect of partial cystectomy on experimental carcinogenesis in the urinary bladder

It was our aim in the present animal experiments to study the influence of stimulation of proliferative activity on carcinogenesis in the urinary bladder. Stimulation of urothelial proliferation was achieved by a one-third resection of the bladder. N-butyl-N-(4-hydroxybutyl)- nitrosamine (BBN), which was used as a carcinogen, was administered by gavage in three fractionated doses when proliferative activity was highest at 30, 45, and 70 h postoperatively. Contrary to our working hypothesis, the incidence of urinary bladder tumors proved to be significantly reduced by partial cystectomy. After administration of a low total dose of BBN (300 mg/kg bodyweight) and an experimental period of 6, 12, and 18 months, only 2.6% of the rats with a partial cystectomy, but 12.6% of the control animals with an intact bladder had developed papillomas and noninvasive papillary transitional cell carcinomas. Following administration of BBN at a higher total dose (1,300 mg/kg bodyweight), bladder tumors occurred after an induction period of 4, 6, and 12 months in 27.4% of the partially cystectomized and 48.1% of the nonoperated rats. Multiple tumors were found more frequently in the controls than in the operated animals. The reduction in the tumor incidence following one-third resection of the bladder evidently does not depend on a prolongation of the latency period or induction time. From findings in analogous experimental models it is conceivable that the observed inhibition of experimental bladder carcinogenesis is brought about by an increased capacity of the proliferating urothelial cells to repair carcinogen-induced DNA damage. Further studies are required to elucidate the significance of a stimulated proliferation for the repair system and neoplastic transformation of the urothelium.

Unscheduled DNA synthesis in normal human skin after single and combined doses of UV-A, UV-B and UV-A with methoxsalen (PUVA)

The aim of this study was to measure unscheduled DNA synthesis (UDS) by autoradiography in normal human skin (I) after high dose UV-A, (2) after low dose UV-A applied before or after erythemogenic doses of UV-B, (3) after high dose PUVA and (4) after therapeutic doses of PUVA applied before and after erythemogenic doses of UV-B. Single high dose UV-A exposure induced roughly 60% of the amount of UDS induced by equally erythemogenic doses of UV-B. Single low dose UV-A exposure did not induce UDS, nor did it significantly alter the amount of UV-B induced UDS when combined with UV-B exposure. Single high dose PUVA did not lead to UDS and had no influence on UV-B induced UDS when combined with UV-B exposure. Our findings indicate: (I) erythemogenic doses of UV-A induce a considerable DNA excision repair; (2) low dose UV-A neither augments UV-B induced DNA repair nor does it inhibit the repair process; (3) no UDS was shown to occur after eight high or therapeutic doses of PUVA. This was unexpected since psoralen-DNA monoadducts have been shown to be repairable by a mechanism similar to excision repair of pyrimidine dimers. It is therefore assumed that PUVA as performed for therapeutic purposes either preferentially induced interstrand crosslinks not repairable via the classical repair mechanism or the repair of monoadducts was below resolution in this study; (4) therapeutic PUVA doses apparently do not interfere with excision repair of UV-B induced DNA lesions.

Evolution and longevity-assurance processes

The role of the development of information fidelity systems in evolution is explored, with evidence that the loss in the ability to maintain the integrity of homeostasis in organisms over time, aging, is correlated with the loss in the ability to maintain integrity at the molecular biological level. Evolutionary-comparative analysis places an upper limit on the number of these systems important to the evolution of longevity in the primates, and suggests the importance of insuring the stability of information bearing macromolecules in evolution and the role of modulators of damage to these moieties in the expression of the senescent state.

A UV-sensitive human clonal cell line, RSa, which has low repair activity

The repair activity of a human transformed cell line, RSa, which was found to be highly sensitive to the lethal effects of 254 nm far-ultraviolet radiation, was compared with that of HeLa cells by evaluating the range of UV-induced incorporation of [methyl-3H]thymidine ([3H]dThd) or 5-[6-3H]bromodeoxyuridine ([3H]BrdUrd) into deoxyribonucleic acid. Direct scintillation counting was used for measuring the extent of unscheduled DNA synthesis (UDS) in UV-irradiated cells, which were treated with hydroxyurea or with arginine deprivation. More quantitative measurements were made by using the density labeling and equilibrium centrifugation method for assaying repair replication. All the amounts of UDS and repair replication in RSa cells were markedly below those in HeLa cells. The possible relationships of the low repair activity to abnormally high UV sensitivity in RSa cells are discussed.

Excision repair of UV-induced pyrimidine dimers in human skin in vivo

The induction and loss of pyrimidine dimers in human skin in vivo was determined using UV endonuclease, alkaline sucrose sedimentations, and the fluorescent detection of nonradiolabeled DNA. The number of dimers induced following exposure of the skin to radiation emitted from a Burdick UV-800 sunlamp was quantitated by reacting the extracted DNA with Micrococcus luteus endonuclease specific for pyrimidine dimers. Exposure to 15 and 30 seconds of radiation emitted from this lamp produced the formation of 12.8 and 23.6 dimers per 10(8) daltons DNA, respectively. Approximately 50% of the dimers induced were lost 58 min after irradiation. Only a small percentage (less than 10) remained 24 hr postirradiation. These data partially characterize the process by which pyrimidine dimers are excised from human skin DNA in vivo.

DNA repair, H-2, and aging in NZB and CBA mice

Current evidence suggests that a correlation exists between the capacity to perform excision repair of UV-induced DNA damage and maximum lifespan in different species. Preliminary evidence has also indicated differences of DNA repair capacities in lymphocytes of several strains of mice congenic at the H-2 locus. It is known that the H-2 system influences maximum lifespan potential in mice. In the present studies excision repair of UV-induced DNA damage, but not gamma-induced damage, was found to correlate the mean survival in the adult inbred mouse strains NZB and CBA, using PHA stimulated splenic lymphocytes. Furthermore, in (NZB X CBA)F2 hybrid with adult progeny the level of DNA repair of UV-induced damage corresponded to the H-2 allele (H-2d/2d from NZB or H-2b/2b from CBA) inherited from the parental strain. These studies suggest the possibility of a tricornered relationship between the main histocompatibility complex, one form of DNA repair, and lifespan within the species.

DNA repair in lymphoblastoid cell lines established from human genetic disorders

Lymphoblastoid cell lines (LCLs) established from chromosomal breakage syndromes or related genetic disorders have been used to study the effects of mutagens on human lymphoid cells. The disorders studied include xeroderma pigmentosum, ataxia telangiectasia, Fanconi's anemia, Bloom's syndrome and Cockayne's syndrome. Three approaches were used to assess the cells' ability to cope with a particular mutagen: (1) assaying recovery of DNA synthetic capabilities as measured by [3H]thymidine (dT) incorporation; (2) measurements of classical excision DNA repair by isopyknic sedimentation of DNA density labeled with 5-bromo-2-deoxyuridine (BrdU); (3) determining cell survival by colony formation in microtiter plates. LCLs established from xeroderma pigmentosum showed increased sensitivities to ultraviolet (354 nm) light and N-acetoxy-2-acetylaminofluorene (AAAF) as determined by DNA synthesis or colony formation and had diminished levels of excision-repair. Cockayne's syndrome LCLs, on the other hand, had increased sensitivities to ultraviolet (UV) light, AAAF and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) while showing near normal levels of DNA-repair after treatment with each agent. An LCL established from ataxia telangiectasia had decreased DNA repair synthesis and defective colony-forming ability following treatment with MNNG. LCLs, in addition to ease of establishment, appear likely to provide useful material for the study of DNA repair replication and its relationship to carcinogenesis.

Quantitation of induced mutation in Dictyostelium discoideum: characterization and use of a methanol-resistance mutation assay

An assay based on forward mutation of Dictyostelium discoideum to 3% methanol resistance allows quantitation of induced mutation following treatment with physical and chemical agents. Properties of this assay are: uniform recovery of methanol-resistant (MeOHr) cells over a wide range of plated cell densities, equal growth rates of methanol-sensitive and methanol-resistant cells in the absence of methanol during the expression period and the attainment of plateau in the number of mutants as a function of expression time. When 4 mutagens were tested with this system and compared at the 10% survival level, N-methyl-N'-nitrosoguanidine was the most effective for mutation induction, followed by 60Co-gamma-rays, ultraviolet light, and methyl methanesulfonate.

Correlation of the colony-forming abilities of xeroderma pigmentosum fibroblasts with repair-specific DNA incision reactions catalyzed by cell-free extracts

Several normal and XP group A fiblast cell lines were exposed to the weakly carcinogenic and toxic agent methyl methanesulfonate, and the differences in their abilities to form colonies were determined. The XP group A cell lines investigated exhibited higher sensitivity towards methyl methanesulfonate than normal cell lines. Correspondingly, cell-free extracts of the same XP cell lines differed from normal ones in cleaving methyl methane-sulfonate-treated double-stranded DNA less rapidly. Since depurinated DNA was cleaved by XP and normal cell lines at equal rates, it was concluded that the differences observed with methylated DNA were due to a reaction preceding cleavage at apurinic sites. In control experiments using extracts from Chinese hamster ovary cells liberation of m3Ade was observed indicating the presence of 3-methyl-adenine DNA glycosylase activity. Furthermore, extracts from a normal fibroblast line liberated small amounts of m3Ade, whereas the one of a XP group A cell line was found to be less effective. The possible role of 3-methyl-adenine DNA glycosylase activity as a rate-limiting factor in the incision step has been discussed.

DNA repair in Syrian hamster embryo cells treated with 7,12-dimethylbenz[a]anthracene and its weakly carcinogenic 5-fluoro analog

The postreplication repair capacity of Syrian hamster embryo cells in culture was determined after treatment with the potent carcinogen 7,12-dimethylbenz[a]anthracene (DMBA) and its weakly carcinogenic analog 5-F-DMBA. The size and amount of daughter DNA sedimenting as high-molecular-weight DNA were found to be less in the DMBA treated cells than in the 5-F-DMBA-treated cells. This difference probably depends upon the types of adducts entering DNA replication.

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.