Relationship between O6-alkylguanine-DNA alkyltransferase activity and N-methyl-N'-nitro-N-nitrosoguanidine-induced mutation, transformation, and cytotoxicity in C3H/10T1/2 cells expressing exogenous alkyltransferase genes

Programming of Cell Resistance to Genotoxic and Oxidative Stress

Different organisms, cell types, and even similar cell lines can dramatically differ in resistance to genotoxic stress. This testifies to the wide opportunities for genetic and epigenetic regulation of stress resistance. These opportunities could be used to increase the effectiveness of cancer therapy, develop new varieties of plants and animals, and search for new pharmacological targets to enhance human radioresistance, which can be used for manned deep space expeditions. Based on the comparison of transcriptomic studies in cancer cells, in this review, we propose that there is a high diversity of genetic mechanisms of development of genotoxic stress resistance. This review focused on possibilities and limitations of the regulation of the resistance of normal cells and whole organisms to genotoxic and oxidative stress by the overexpressing of stress-response genes. Moreover, the existing experimental data on the effect of such overexpression on the resistance of cells and organisms to various genotoxic agents has been analyzed and systematized. We suggest that the recent advances in the development of multiplex and highly customizable gene overexpression technology that utilizes the mutant Cas9 protein and the abundance of available data on gene functions and their signal networks open new opportunities for research in this field.

Live and let die: in vivo selection of gene-modified hematopoietic stem cells via MGMT-mediated chemoprotection

Gene transfer into hematopoietic stem cells (HSC) provides a potential means of correcting monogenic defects and altering drug sensitivity of normal bone marrow to cytotoxic agents. These applications have significant therapeutic potential but the translation of successful murine studies into human therapies has been hindered by low gene transfer in large animals (including humans), and recent serious side effects in a human immunodeficiency trial related to insertional mutagenesis. The latter trial, along with other subsequent trials, while bringing into focus the potential risks of integrating vector systems, also clearly demonstrate the potential usefulness of in vivo selection as it relates to inefficient stem cell transduction. Developing from initial studies by our group and other investigators in which drug resistance was utilized to demonstrate the feasibility of using gene transfer to effect protection from myelotoxicity of chemotherapeutic agents, expression of mutant forms of O(6)-methyguanine-DNA-methytransferase (MGMT) coupled with the simultaneous use of pharmacologic inhibitors and chemotherapeutic agents has been shown to provide a powerful method to select HSC in vivo. While stem and progenitor cell protection and resulting selection in vivo has potential applications for the treatment of selected cancers (allowing dose escalation) and for correction of monogenic disease (allowing an iatrogenic survival advantage of transduced cells in vivo), such an in vivo selection may have untoward effects on stem cell behavior. These deleterious effects may include stem cell exhaustion; lineage skewing; accumulation of genotoxic lesions; and clonal dominance driven towards a pro-leukemic phenotype. Knowledge of the likelihood of such deleterious events occurring as well as their potential implications will be critical to future clinical applications and may also enhance our understanding of both normal stem cell behavior and the evolution of hematopoietic malignancies.

O6-pyridyloxobutylguanine adducts contribute to the mutagenic properties of pyridyloxobutylating agents

The tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) are potent carcinogens in animal models and likely human carcinogens. Both NNK and NNN can be activated to a pyridyloxobutylating agent. This alkylating agent contributes to the carcinogenic effects of NNK and NNN via the formation of miscoding DNA adducts. One of these adducts, O6-[4-oxo-4-(3-pyridyl)butyl]guanine (O6-pobG) has been characterized as a mutagenic adduct which is a substrate for the repair protein O6-alkylguanine-DNA alkyltransferase (AGT). Repair of O6-alkylguanine adducts by AGT protects cells from the mutagenic and carcinogenic effects of alkylating agents and is likely to play a similar role in shielding cells from the adverse effects of pyridyloxobutylating agents. Therefore, we examined the mutagenicity of the model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc), in Salmonella typhimurium YG7108 expressing hAGT. Expression of hAGT protected cells from NNKOAc-induced mutagenicity. Interestingly, hAGT did not shield cells from the toxicity of this agent. To confirm that the repair of O6-pobG was increased in the bacteria expressing hAGT, we measured levels of this adduct in NNKOAc-treated cultures. The levels of O6-pobG were lower in DNA from bacteria expressing hAGT. This work establishes an important role for O6-pobG in mediating the mutagenic, and possibly carcinogenic, effects of pyridyloxobutylating compounds.

DNA sequence context affects repair of the tobacco-specific adduct O(6)-[4-Oxo-4-(3-pyridyl)butyl]guanine by human O(6)-alkylguanine-DNA alkyltransferases

The repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) protects cells from the mutagenic and carcinogenic effects of alkylating agents by removing O(6)-alkylguanine adducts from DNA. Recently, we established that AGT protects against the mutagenic effects of pyridyloxobutylation resulting from the metabolic activation of the tobacco-specific nitrosamines (TSNA) 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and N-nitrosonornicotine by repairing O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine (O(6)-pobG). There have been several epidemiologic studies examining the association between the I143V/K178R AGT genotype and lung cancer risk. Two studies have found positive associations, suggesting that AGT proteins differ in their repair of DNA damage caused by TSNA. However, it is not known how this genotype alters the biochemical activity of AGT. We proposed that AGT proteins may differ in their ability to remove large O(6)-alkylguanine adducts, such as O(6)-pobG, from DNA. Therefore, we examined the repair of O(6)-pobG by wild-type (WT) human, I143V/K178R, and L84F AGT proteins when contained in multiple sequence contexts, including the twelfth codon of H-ras, a mutational hotspot within this oncogene. The AGT-mediated repair of O(6)-pobG was more profoundly influenced by sequence context than that of O(6)-methylguanine. These differences are not the result of secondary structure (hairpin) formation in DNA. In addition, the I143V/K178R variant seems less sensitive to the effects of sequence context than the WT or L84F proteins. These studies indicate that the sequence dependence of O(6)-pobG repair by human AGT (hAGT) varies with subtle changes in protein structure. These data establish a novel functional difference between the I143V/K178R protein and other hAGTs in the repair of a toxicologically relevant substrate, O(6)-pobG.

Overexpression of enzymes that repair endogenous damage to DNA

A significant contribution to human mutagenesis and carcinogenesis may come from DNA damage of endogenous, rather than exogenous, origin. Efficient repair mechanisms have evolved to cope with this. The main repair pathway involved in repair of endogenous damage is DNA base excision repair. In addition, an important contribution is given by O6-alkylguanine DNA alkyltranferase, that repairs specifically the miscoding base O6-alkylguanine. In recent years, several attempts have been carried out to enhance the efficiency of repair of endogenous damage by overexpressing in mammalian cells single enzymatic activities. In some cases (e.g. O6-alkylguanine DNA alkyltransferase or yeast AP endonuclease) this approach has been successful in improving cellular protection from endogenous and exogenous mutagens, while overexpression of other enzymatic activities (e.g. alkyl N-purine glycosylase or DNA polymerase beta) were detrimental and even produced a genome instability phenotype. The reasons for these different outcomes are analyzed and alternative enzymatic activities whose overexpression may improve the efficiency of repair of endogenous damage in human cells are proposed.

Influence of O6-benzylguanine on the anti-tumour activity and normal tissue toxicity of 1,3-bis(2-chloroethyl)-1-nitrosourea and molecular combinations of 5-fluorouracil and 2-chloroethyl-1-nitrosourea in mice

Previous studies have demonstrated that novel molecular combinations of 5-fluorouracil (5FU) and 2-chloroethyl-1-nitrosourea (CNU) have good preclinical activity and may exert less myelotoxicity than the clinically used nitrosoureas such as 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). This study examined the effect of O6-alkylguanine-DNA-alkyltransferase (ATase) depletion by the pseudosubstrate O6-benzylguanine (BG) on the anti-tumour activity and normal tissue toxicity in mice of three such molecular combinations, in comparison with BCNU. When used as single agents at their maximum tolerated dose, all three novel compounds produced a significant growth retardation of BCNU-resistant murine colon and human breast xenografts. This in vivo anti-tumour effect was potentiated by BG, but was accompanied by severe myelotoxicity as judged by spleen colony forming assays. However, while tumour resistance to BCNU was overcome using BG, this was at the expense of enhanced bone marrow, gut and liver toxicity. Therefore, although this ATase-depletion approach resulted in improved anti-tumour activity for all three 5-FU:CNU molecular combinations, the potentiated toxicities in already dose-limiting tissues indicate that these types of agents offer no therapeutic advantage over BCNU when they are used together with BG.

Recombinant adeno-associated virus-mediated expression of O6-alkylguanine-DNA-alkyltransferase protects human epithelial and hematopoietic cells against chloroethylating agent toxicity

Recombinant adeno-associated virus (rAAV) encoding the human O6-alkylguanine-DNA-alkyltransferase (hAT) protein and a selectable marker (Neo(r)) was used to transduce human cervical carcinoma (HeLa) cells and erythroleukemic (K562) cells and clones were selected using G418 (0.4 mg/ml). Thirteen HeLa clones were isolated, 9 of which survived for 2-3 months before cell death ensued, presumably owing to the loss of G418 resistance. Northern blot analysis of the remaining four clones, using a neo probe, showed high levels of RNA equivalent in size to the bicistronic RNA expected to be produced from this construct. Analysis of hAT activity showed that 2000-5000 fmol/mg protein was expressed relative to untransduced cells (800-900 fmol/mg protein). Cell survival analysis following exposure to the chloroethylating agent mitozolomide revealed that expression of hAT at levels two- to fourfold higher than background conferred significant resistance (p < 0.001) to the toxic effects of this drug. Two days following infection of K562 cells with the rAAV vector, immunoblot analysis showed that hAT protein was being produced. Three K562 clones, isolated using G418 selection, were studied in detail and were shown to express hAT activities of 1500, 1010, and 890 fmol/mg protein, respectively, at 40 days posttransduction (mock-transduced K562 cells contain <2 fmol of hAT/mg protein). As with HeLa cells, Northern blot analysis showed the production of an appropriately sized transcript and immunoblot analysis indicated that hAT protein was being produced. These clones were assayed for cell survival following exposure to mitozolomide. Expression of hAT at levels 800- to 1500-fold higher than background conferred significant resistance (p < 0.001) to the toxic effects of mitozolomide. We have therefore successfully conferred a protective advantage against mitozolomide toxicity to cells by rAAV-mediated hAT expression.

The antitumour activity of alkylating agents is not correlated with the levels of glutathione, glutathione transferase and O6-alkylguanine-DNA-alkyltransferase of human tumour xenografts. EORTC SPG and PAMM Groups

Twenty-three human xenografts, including five colon, five gastric, nine lung (three small cell lung cancer) and four breast carcinomas, were investigated for their sensitivity to nitrosoureas, dacarbazine (DTIC), cyclophosphamide (CTX) and cisplatin (DDP). In 12 cases, at least one of the drugs produced complete or partial remission, in 2, a minor regression was observed and in the other 9, treatment was ineffective. The level of sensitivity to each drug, using a score from 1 to 5, was correlated to three biochemical parameters reported to be involved in resistance to alkylating agents: glutathione (GSH), glutathione transferase (GST) and O6-alkylguanine-DNA-alkyltransferase (AGT). A wide variability was found in these parameters in the xenografts investigated. No correlation was found between any of the three parameters and sensitivity to the drugs used or between sensitivity to one drug and to any of the other drugs tested. These results illustrate the complexity of the question of resistance to alkylating agents and indicate that, at least in xenografts, the biochemical parameters examined are not predictive of response to alkylating agents.

Transgenic systems in studies on genotoxicity of alkylating agents: critical lesions, thresholds and defense mechanisms

Transgenic systems, both cell lines and mice with gain or loss of function, are being used in order to modulate the expression of DNA repair proteins, thus allowing to assess their contribution to the defense against genotoxic mutagens and carcinogens. In this review, questions have been addressed concerning the use of transgenic systems in elucidating critical primary DNA lesions, their conversion into genotoxic endpoints, low-dose effects, and the relative contribution of individual cellular functions in defense. It has been shown that the repair protein alkyltransferase (MGMT) is decisive for protection against methylating and chloroethylating compounds. Protection pertains also to tumor formation, as revealed by the response of MGMT transgenic and knockout mice. Overexpression of genes involved in base excision repair (N-methylpurine-DNA glycosylase, apurinic endonuclease, DNA polymerase beta) is in most cases not beneficial in increasing the protection level, whereas their down-modulation or inactivation increases cellular sensitivity. This indicates that non-repaired base N-alkylation lesions and/or repair intermediates possess genotoxic potential. Modulation of mismatch repair and poly(ADP)ribosyl transferase has also been shown to affect the cellular response to alkylating agents. Furthermore, the role of Fos, Jun and p53 in cellular defense against alkylating mutagens is discussed.

Modulation of O6-alkylating agent induced clastogenicity by enhanced DNA repair capacity of bone marrow cells

The murine bone marrow micronucleus assay has been used to examine (1) the potentiation of fotemustine and streptozotocin induced-clastogenicity by the O6-alkylguanine-DNA alkyltransferase (ATase) inactivator O6-benzylguanine (O6-beG) and (2) the level of protection afforded against this potentiation by retrovirus-mediated expression of an O6-beG-resistant mutant of human ATase (haTPA/GA) in mouse bone marrow. Both fotemustine and streptozotocin induced significantly higher levels of micronucleated polychromatic erythrocytes (p < 0.001 for the highest doses studied) compared to those seen in vehicle-treated animals. The number of micronuclei produced by either agent was dramatically elevated by pretreatment with O6-beG (p < 0.001). Furthermore, in myeloablated mice reconstituted with bone marrow expressing the O6-beG-resistant hATPA/GA as a result of retroviral gene transfer, the frequency of micronucleus formation following exposure of mice to otherwise clastogenic doses of fotemustine or streptozotocin, in the presence of O6-beG, wash highly significantly reduced (p < 0.001 for both agents) relative to that in mock transduced controls. These data clearly implicate O6-chloroethyl- and O6-methylguanine as clastogenic lesions in vivo and establish ATase as a major protective mechanism operating to reduce the frequency of such damage. The potentiation of drug induced clastogenicity by O6-beG suggests that the clinical use of this inactivator in combination with O6-alkylating agents, could substantially increase the risk of therapy related malignancy. Nevertheless the use of hATPA/GA as a protective mechanism via gene therapy may overcome this risk.

DNA repair functions in heterologous cells

Our genetic information is constantly challenged by exposure to endogenous and exogenous DNA-damaging agents, by DNA polymerase errors, and thereby inherent instability of the DNA molecule itself. The integrity of our genetic information is maintained by numerous DNA repair pathways, and the importance of these pathways is underscored by their remarkable structural and functional conservation across the evolutionary spectrum. Because of the highly conserved nature of DNA repair, the enzymes involved in this crucial function are often able to function in heterologous cells; as an example, the E. coli Ada DNA repair methyltransferase functions efficiently in yeast, in cultured rodent and human cells, in transgenic mice, and in ex vivo-modified mouse bone marrow cells. The heterologous expression of DNA repair functions has not only been used as a powerful cloning strategy, but also for the exploration of the biological and biochemical features of numerous enzymes involved in DNA repair pathways. In this review we highlight examples where the expression of DNA repair enzymes in heterologous cells was used to address fundamental questions about DNA repair processes in many different organisms.

Chemoprotection of normal tissues by transfer of drug resistance genes

The effectiveness of many types of antitumour agent is limited by (i) acute dose limiting cytotoxicity, principally myelosuppression but also lung, liver and gastrointestinal tract toxicity, (ii) the risk of therapy related secondary malignancy and (iii) the inherent or acquired drug-resistance of tumour cells. As the management of the acute toxic effects improve, the more insidious effects, and particularly haematological malignancies, are anticipated to increase. Furthermore, attempts to overcome tumour cell resistance to treatment can lead to increased collateral damage in normal tissues. One approach to circumventing both the acute toxic and chronic carcinogenic effects of chemotherapy would be to use gene therapy to achieve high levels of expression of drug resistance proteins in otherwise drug-sensitive tissues. To date the products of the multi-drug resistance (MDR-1) and the human O6-alkylguanine-DNA-alkyltransferase (ATase) gene have been used in preclinical experiments to demonstrate proof of principle, and the former of these is now being tested in a clinical situation. Here we discuss the potential of drug-resistance gene therapy to provide chemoprotection to normal tissues and examine the prospects for a dual approach which combines this with pharmacological sensitisation of tumours to chemotherapeutic agents.

The Escherichia coli AlkB protein protects human cells against alkylation-induced toxicity

Escherichia coli can ameliorate the toxic effects of alkylating agents either by preventing DNA alkylation or by repairing DNA alkylation damage. The alkylation-sensitive phenotype of E. coli alkB mutants marks the alkB pathway as an extremely effective defense mechanism against the cytotoxic effects of the SN2, but not the SN1, alkylating agents. Although it is clear that AlkB helps cells to better handle alkylated DNA, no DNA alkylation repair function could be assigned to the purified AlkB protein, suggesting that AlkB either acts as part of a complex or acts to regulate the expression of other genes whose products are directly responsible for alkylation resistance. However, here we present evidence that the provision of alkylation resistance is an intrinsic function of the AlkB protein per se. We expressed the E. coli AlkB protein in two human cell lines and found that it confers the same characteristic alkylation-resistant phenotype in this foreign environment as it does in E. coli. AlkB expression rendered human cells extremely resistant to cell killing by the SN2 but not the SN1 alkylating agents but did not affect the ability of dimethyl sulfate (an SN2 agent) to alkylate the genome. We infer that SN2 agents produce a class of DNA damage that is not efficiently produced by SN1 agents and that AlkB somehow prevents this damage from killing the cell.

bcl-2 delay of alkylating agent-induced apoptotic death in a murine hemopoietic stem cell line

Many cytotoxic agents kill cells by invoking a specific death pathway termed physiological cell death, or apoptosis. Treatment of a murine hemopoietic stem cell line, FDCP-mix, with methylmethanesulfonate (MMS) or N'-methyl-N'-nitrosourea (MNU) leads to death by apoptosis. Retroviral gene transfer was used to overexpress the bcl-2 oncogene in FDCP-mix cells, and this was associated with a delay in apoptosis in these cells after treatment with MNU and MMS and decreased sensitivity of colony formation to the cytotoxic effects of MMS. These data suggest an explanation for the refractory nature of bcl-2-expressing follicular lymphoma to cytotoxic chemotherapy and furthermore suggest that DNA-damaging antitumor therapy may contribute to the progression of disease.