Xpa knockout mice

High levels of oxidatively generated DNA damage 8,5'-cyclo-2'-deoxyadenosine accumulate in the brain tissues of xeroderma pigmentosum group A gene-knockout mice

Xeroderma pigmentosum (XP) is a genetic disorder associated with defects in nucleotide excision repair, a pathway that eliminates a wide variety of helix-distorting DNA lesions, including ultraviolet-induced pyrimidine dimers. In addition to skin diseases in sun-exposed areas, approximately 25% of XP patients develop progressive neurological disease, which has been hypothesized to be associated with the accumulation of an oxidatively generated type of DNA damage called purine 8,5'-cyclo-2'-deoxynucleoside (cyclopurine). However, that hypothesis has not been verified. In this study, we tested that hypothesis by using the XP group A gene-knockout (Xpa^-/-) mouse model. To quantify cyclopurine lesions in this model, we previously established an enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (CdA-1) that specifically recognizes 8,5'-cyclo-2'-deoxyadenosine (cyclo-dA). By optimizing conditions, we increased the ELISA sensitivity to a detection limit of ˜one cyclo-dA lesion/10⁶ nucleosides. The improved ELISA revealed that cyclo-dA lesions accumulate with age in the brain tissues of Xpa^-/- and of wild-type (wt) mice, but there were significantly more cyclo-dA lesions in Xpa^-/- mice than in wt mice at 6, 24 and 29 months of age. These findings are consistent with the long-standing hypothesis that the age-dependent accumulation of endogenous cyclopurine lesions in the brain may be critical for XP neurological abnormalities.

DICER- and MMSET-catalyzed H4K20me2 recruits the nucleotide excision repair factor XPA to DNA damage sites

The endoribonuclease DICER facilitates chromatin decondensation during lesion recognition following UV exposure. Chitale and Richly show that DICER mediates the recruitment of the methyltransferase MMSET, which catalyzes the dimethylation of histone H4 at lysine 20 and facilitates the recruitment of the nucleotide excision repair factor XPA.

Ultraviolet (UV) irradiation triggers the recruitment of DNA repair factors to the lesion sites and the deposition of histone marks as part of the DNA damage response. The major DNA repair pathway removing DNA lesions caused by exposure to UV light is nucleotide excision repair (NER). We have previously demonstrated that the endoribonuclease DICER facilitates chromatin decondensation during lesion recognition in the global-genomic branch of NER. Here, we report that DICER mediates the recruitment of the methyltransferase MMSET to the DNA damage site. We show that MMSET is required for efficient NER and that it catalyzes the dimethylation of histone H4 at lysine 20 (H4K20me2). H4K20me2 at DNA damage sites facilitates the recruitment of the NER factor XPA. Our work thus provides evidence for an H4K20me2-dependent mechanism of XPA recruitment during lesion recognition in the global-genomic branch of NER.

Mouse Models of Human Familial Cancer Syndromes

As many as 5% of human cancers appear to be of hereditable etiology. Of the more than 50 characterized familial cancer syndromes, most involve disease affecting multiple organs and many can be traced to one or more abnormalities in specific genes. Studying these syndromes in humans is a difficult task, especially when it comes to genes that may manifest themselves early in gestation. It has been made somewhat easier with the development of genetically engineered mice (GEM) that phenotypically mimic many of these inheritable human cancers. The past 15 years has seen the establishment of mouse lines heterozygous or homozygous null for genes known or suspected of being involved in human cancer syndromes, including APC, ATM, BLM, BRCA1, BRCA2, LKB1, MEN1, MLH, MSH, NF1, TP53, PTEN, RB1, TSC1, TSC2, VHL, and XPA. These lines not only provide models for clinical disease and pathology, but also provide avenues to investigate molecular pathology, gene-gene and protein-tissue interaction, and, ultimately, therapeutic intervention. Possibly of even greater importance, they provide a means of looking at placental and fetal tissues, where genetic abnormalities are often first detected and where they may be most easily corrected. We will review these mouse models, examine their usefulness in medical research, and furnish sources of animals and references.

The RASSF1A tumor suppressor regulates XPA-mediated DNA repair

RASSF1A may be the most frequently inactivated tumor suppressor identified in human cancer so far. It is a proapoptotic Ras effector and plays an important role in the apoptotic DNA damage response (DDR). We now show that in addition to DDR regulation, RASSF1A also plays a key role in the DNA repair process itself. We show that RASSF1A forms a DNA damage-regulated complex with the key DNA repair protein xeroderma pigmentosum A (XPA). XPA requires RASSF1A to exert full repair activity, and RASSF1A-deficient cells exhibit an impaired ability to repair DNA. Moreover, a cancer-associated RASSF1A single-nucleotide polymorphism (SNP) variant exhibits differential XPA binding and inhibits DNA repair. The interaction of XPA with other components of the repair complex, such as replication protein A (RPA), is controlled in part by a dynamic acetylation/deacetylation cycle. We found that RASSF1A and its SNP variant differentially regulate XPA protein acetylation, and the SNP variant hyperstabilizes the XPA-RPA70 complex. Thus, we identify two novel functions for RASSF1A in the control of DNA repair and protein acetylation. As RASSF1A modulates both apoptotic DDR and DNA repair, it may play an important and unanticipated role in coordinating the balance between repair and death after DNA damage.

Slow accumulation of mutations in Xpc-/- mice upon induction of oxidative stress

XPC is one of the key DNA damage recognition proteins in the global genome repair route of the nucleotide excision repair (NER) pathway. Previously, we demonstrated that NER-deficient mouse models Xpa(-/-) and Xpc(-/-) exhibit a divergent spontaneous tumor spectrum and proposed that XPC might be functionally involved in the defense against oxidative DNA damage. Others have mechanistically dissected several functionalities of XPC to oxidative DNA damage sensitivity using in vitro studies. XPC has been linked to regulation of base excision repair (BER) activity, redox homeostasis and recruitment of ATM and ATR to damage sites, thereby possibly regulating cell cycle checkpoints and apoptosis. XPC has additionally been implicated in recognition of bulky (e.g. cyclopurines) and non-bulky DNA damage (8-oxodG). However, the ultimate contribution of the XPC functionality in vivo in the oxidative DNA damage response and subsequent mutagenesis process remains unclear. Our study indicates that Xpc(-/-) mice, in contrary to Xpa(-/-) and wild type mice, have an increased mutational load upon induction of oxidative stress and that mutations arise in a slowly accumulative fashion. The effect of non-functional XPC in vivo upon oxidative stress exposure appears to have implications in mutagenesis, which can contribute to the carcinogenesis process. The levels and rate of mutagenesis upon oxidative stress correlate with previous findings that lung tumors in Xpc(-/-) mice overall arise late in the lifespan and that the incidence of internal tumors in XP-C patients is relatively low in comparison to skin cancer incidence.

Delayed neuromotor recovery and increased memory acquisition dysfunction following experimental brain trauma in mice lacking the DNA repair gene XPA

OBJECT

This study investigates the outcome after traumatic brain injury (TBI) in mice lacking the essential DNA repair gene xeroderma pigmentosum group A (XPA). As damage to DNA has been implicated in neuronal cell death in various models, the authors sought to elucidate whether the absence of an essential DNA repair factor would affect the outcome of TBI in an experimental setting.

METHODS

Thirty-seven adult mice of either wild-type (n = 18) or XPA-deficient ("knock-out" [n = 19]) genotype were subjected to controlled cortical impact experimental brain trauma, which produced a focal brain injury. Sham-injured mice of both genotypes were used as controls (9 in each group). The mice were subjected to neurobehavoral tests evaluating learning/acquisition (Morris water maze) and motor dysfunction (Rotarod and composite neuroscore test), pre- and postinjury up to 4 weeks. The mice were killed after 1 or 4 weeks, and cortical lesion volume, as well as hippocampal and thalamic cell loss, was evaluated. Hippocampal staining with doublecortin antibody was used to evaluate neurogenesis after the insult.

RESULTS

Brain-injured XPA(-/-) mice exhibited delayed recovery from impairment in neurological motor function, as well as pronounced cognitive dysfunction in a spatial learning task (Morris water maze), compared with injured XPA(+/+) mice (p < 0.05). No differences in cortical lesion volume, hippocampal damage, or thalamic cell loss were detected between XPA(+/+) and XPA(-/-) mice after brain injury. Also, no difference in the number of cells stained with doublecortin in the hippocampus was detected.

CONCLUSIONS

The authors' results suggest that lack of the DNA repair factor XPA may delay neurobehavioral recovery after TBI, although they do not support the notion that this DNA repair deficiency results in increased cell or tissue death in the posttraumatic brain.

Mouse models of inherited cancer syndromes

Animal models of cancer have been instrumental in understanding the progression and therapy of hereditary cancer syndromes. The ability to alter the genome of an individual mouse cell in both constitutive and inducible approaches has led to many novel insights into their human counterparts. In this review, knockout mouse models of inherited human cancer syndromes are presented and insights from the study of these models are highlighted.

DNA polymerase beta-dependent long patch base excision repair in living cells

We examined a role for DNA polymerase beta (Pol beta) in mammalian long patch base excision repair (LP BER). Although a role for Pol beta is well known in single-nucleotide BER, information on this enzyme in the context of LP BER has been limited. To examine the question of Pol beta involvement in LP BER, we made use of nucleotide excision repair-deficient human XPA cells expressing UVDE (XPA-UVDE), which introduces a nick directly 5' to the cyclobutane pyrimidine dimer or 6-4 photoproduct, leaving ends with 3'-OH and 5'-phosphorylated UV lesion. We observed recruitment of GFP-fused Pol beta to focal sites of nuclear UV irradiation, consistent with a role of Pol beta in repair of UV-induced photoproducts adjacent to a strand break. This was the first evidence of Pol beta recruitment in LP BER in vivo. In cell extract, a 5'-blocked oligodeoxynucleotide substrate containing a nicked 5'-cyclobutane pyrimidine dimer was repaired by Pol beta-dependent LP BER. We also demonstrated Pol beta involvement in LP BER by making use of mouse cells that are double null for XPA and Pol beta. These results were extended by experiments with oligodeoxynucleotide substrates and purified human Pol beta.

An Industry Perspective on the Utility of Short-Term Carcinogenicity Testing in Transgenic Mice in Pharmaceutical Development

International guidelines allow for use of a short-term cancer bioassay (twenty-six weeks) in transgenic mice as a substitute for one of the two required long-term rodent bioassays in the preclinical safety evaluation of pharmaceuticals. The two models that have gained the widest acceptance by sponsors and regulatory authorities are the CB6F1-RasH2 mouse hemizygous for a human H-ras transgene and the B6.129N5-Trp53 mouse heterozygous for a p53 null allele. The p53+/- model is of particular value for compounds with residual concern that genotoxic activity may contribute to tumorigenesis. The rasH2 model is an appropriate alternative without regard to evidence of genotoxic potential. Since results from a short-term bioassay can be obtained relatively early in drug development, there is the potential for more timely assessment of cancer risk for individuals in long-term clinical trials. Use of these models in preclinical safety evaluation also significantly reduces animal use, time, and manpower. Preliminary findings indicate that prediction of two-year rat bioassay outcomes based on data from chronic rat toxicity studies, together with early assessment of carcinogenic potential in short-term transgenic models, may have the potential to increase the timeliness and efficiency of strategies for the identification of human carcinogenic hazards.

Do heat stress and deficits in DNA repair pathways have a negative impact on male fertility?

In Europe up to one in four couples experience difficulty conceiving and in half of these cases the problem has been attributed to sub or infertility in the male partner. The development of assisted reproductive technologies (ART) such as in vitro fertilization and intra-cytoplasmic spermatozoa injection has allowed some such couples to achieve a pregnancy. Concerns have been raised over the increasing use of ART not least because of the discovery of elevated levels of DNA damage in sperm from subfertile men. The impact of damaged DNA originating in the male germ line is poorly understood, but is thought to contribute to early pregnancy loss (recurrent miscarriage), placental problems and have a long-term impact on the health of the offspring. DNA repair is essential for meiotic recombination and correction of DNA damage in germ cells and proteins involved in all the major repair pathways are expressed in the testis. In this review, we will consider evidence that the production of sperm containing damaged DNA can be the result of suboptimal DNA repair and/or a mild environmental insult, such as heat stress, and how studies in mice may give us insight into the origins and consequences of DNA damage in human sperm.

The case for 8,5'-cyclopurine-2'-deoxynucleosides as endogenous DNA lesions that cause neurodegeneration in xeroderma pigmentosum

Patients with the genetic disease xeroderma pigmentosum (XP) lack the capacity to carry out a specific type of DNA repair process called nucleotide excision repair (NER). The NER pathway plays a critical role in the repair of DNA damage resulting from ultraviolet (UV) radiation. A subset of XP patients develops a profound neurodegenerative condition known as XP neurological disease. Robbins and colleagues [Andrews A, Barrett S, Robbins J (1978) Xeroderma pigmentosum neurological abnormalities correlate with the colony forming ability after ultraviolet irradiation. Proc Natl Acad Sci U S A 75:1984-1988] hypothesized that since UV light cannot reach into the human brain, XP neurological disease results from some form of endogenous DNA damage that is normally repaired by the NER pathway. In the absence of NER, the damage accumulates, causing neuronal death by blocking transcription. In this manuscript, I consider the evidence that a particular class of oxidative DNA lesions, the 8,5'-cyclopurine-2'-deoxynucleosides, fulfills many of the criteria expected of neurodegenerative DNA lesions in XP. Specifically, these lesions are chemically stable, endogenous DNA lesions that are repaired by the NER pathway but not by any other known process, and strongly block transcription by RNA polymerase II in cells from XP patients. A similar set of criteria might be used to evaluate other candidate DNA lesions responsible for neurological diseases resulting from defects in other DNA repair mechanisms as well.

Transgenic and knockout mice for DNA repair functions in carcinogenesis and mutagenesis

Genetically modified mouse models with defects in DNA repair pathways, especially in nucleotide excision repair (NER) and mismatch repair (MMR), are powerful tools to study processes like carcinogenesis and mutagenesis. The use of mutant mice in these studies has many advantages over using normal wild type mice with respect to costs, number of animals, predictive value towards carcinogenic compounds and the duration of study. Short-term carcinogenicity assays still require considerable number of animals and extensive pathological analyses. Therefore, alternatives demanding less animals and shorter exposure times would be desirable. In this respect, one approach could be the use of transgenic mice harbouring marker genes, that can easily detect mutagenic features of carcinogenic compounds, especially when such models are in a DNA repair deficient background. Here, we review the progress made in the development and use of DNA repair deficient mouse models as replacements for long-term cancer assays and discuss the applicability of enhanced gene mutant frequencies as early indicators of tumourigenesis. Although promising models exist, there is still a need for more universally responding and highly sensitive mouse models, since it is likely that non-genotoxic carcinogens will go undetected in a DNA repair deficient mouse. One attractive candidate mouse model, having a presumptive broad detective range, is the Xpa/p53 mutant mouse model, which will be discussed in more detail.

Transgenic tumor models for carcinogen identification: the heterozygous Trp53-deficient and RasH2 mouse lines

Genetically altered mouse models (GAMM) for human cancers have been critical to the investigation and characterization of oncogene and tumor suppressor gene expression and function and the associated cancer phenotype. Similarly, several of the mouse models with defined genetic alterations have shown promise for identification of potential human carcinogens and investigation of mechanisms of carcinogen-gene interactions and tumorigenesis. In particular, both the B6.129N5-Trp53 mouse, heterozygous for a p53 null allele, and the CB6F1-RasH2 mouse, hemizygous for the human H-ras transgene, have been extensively investigated. Using 26-week exposure protocols at or approaching the maximum tolerated dose, the summary results to date indicate the potential for GAMM to identify and, possibly, classify chemicals of potential risk to humans using short-term carcinogenicity experiments. This IWGT session focused on: (1) the development of recommendations for genetic/molecular characterization required in animals, tissues, and tumors before and after treatment for identification of presumptive human carcinogens based on the current state of knowledge, (2) identification of data gaps in our current state of knowledge, and (3) development of recommendations for research strategies for further development of our knowledge base of these particular models. By optimization of protocols and identification of significant outcomes and responses to chemical exposure in appropriate short-term mechanism-based genetically altered rodent models, strategies for prevention and intervention may be developed and employed to the benefit of public health.

How nucleotide excision repair protects against cancer

Eukaryotic cells can repair many types of DNA damage. Among the known DNA repair processes in humans, one type--nucleotide excision repair (NER)--specifically protects against mutations caused indirectly by environmental carcinogens. Humans with a hereditary defect in NER suffer from xeroderma pigmentosum and have a marked predisposition to skin cancer caused by sunlight exposure. How does NER protect against skin cancer and possibly other types of environmentally induced cancer in humans?

A 32P-postlabeling assay for the oxidative DNA lesion 8,5'-cyclo-2'-deoxyadenosine in mammalian tissues: evidence that four type II I-compounds are dinucleotides containing the lesion in the 3' nucleotide

8,5'-Cyclopurine-2'-deoxynucleotides, which are strong blocks to mammalian DNA and RNA polymerases, represent a novel class of oxidative DNA lesion in that they are specifically repaired by nucleotide excision repair but not by base excision repair or direct enzymatic reversion. Previous studies using thin layer chromatography of (32)P-postlabeled DNA digests have detected several bulky oxidative lesions of unknown structure, called I-compounds, in DNA from normal mammalian organs. We investigated whether any of these type II I-compounds contained 8,5'-cyclo-2'-deoxyadenosine (cA). Two previously detected type II I-compounds were found to be dinucleotides of the sequence pAp-cAp and pCp-cAp. Furthermore, a modification of the technique resulted in detection of two additional I-compounds, pTp-cAp and pGp-cAp. Each I-compound isolated from neonatal rat liver DNA matched authentic (32)P-labeled cA-containing chromatographic standards under nine different chromatographic conditions. Their levels increased significantly after normal birth. The (32)P-postlabeling technique used here is capable of detecting 1-5 lesions/diploid mammalian cell. Thus, it should now be possible to detect changes of cA levels resulting from low level ionizing radiation and other conditions associated with oxidative stress, and to assess cA levels in tissues from patients with the genetic disease xeroderma pigmentosum who are unable to carry out nucleotide excision repair.

Reactivity of germ cell maturation stage-specific markers in spermatocytic seminoma: diagnostic and etiological implications

It is generally accepted that testicular seminomas and spermatocytic seminomas have separate pathogeneses, although the origin of these two types of germ cell tumors of the adult testis remains a matter of debate. Although an embryonic germ cell origin seems to be most likely for seminomas, a spermatogonia-spermatocyte origin has been suggested for spermatocytic seminoma. To shed more light on the etiology of spermatocytic seminomas, we undertook an immunohistochemical and molecular approach using SCP1 (synaptonemal complex protein 1), SSX (synovial sarcoma on X chromosome), and XPA (xeroderma pigmentosum type A) as targets. Although a stage-specific expression pattern has been reported for SCP1 and SSX in normal spermatogenesis, we demonstrate here that it also exists for XPA. In fact, immunohistochemistry shows that the proteins of SCP1 and XPA are specifically present in the stage of primary and pachytene spermatocytes. In contrast, SSX was found in spermatogonia and primary spermatocytes, as well as in germ cells, from at least the 17th week of intrauterine development onward. Although no protein encoded by any of these genes was detected in tumor cells of a series of testicular seminomas, all tested spermatocytic seminomas were positive, in agreement with expression analysis. These data support the model that seminomas originate from an embryonic germ cell, and they imply that the cell of origin of spermatocytic seminomas is at least capable of maturing to the stage of spermatogonia-pachytene spermatocyte.

Use of transgenic animals for carcinogenicity testing: considerations and implications for risk assessment

Advances in genetic engineering have created opportunities for improved understanding of the molecular basis of carcinogenesis. Through selective introduction, activation, and inactivation of specific genes, investigators can produce mice of unique genotypes and phenotypes that afford insights into the events and mechanisms responsible for tumor formation. It has been suggested that such animals might be used for routine testing of chemicals to determine their carcinogenic potential because the animals may be mechanistically relevant for understanding and predicting the human response to exposure to the chemical being tested. Before transgenic and knockout mice can be used as an adjunct or alternative to the conventional 2-year rodent bioassay, information related to the animal line to be used, study design, and data analysis and interpretation must be carefully considered. Here, we identify and review such information relative to Tg.AC and rasH2 transgenic mice and p53+/- and XPA-/- knockout mice, all of which have been proposed for use in chemical carcinogenicity testing. In addition, the implications of findings of tumors in transgenic and knockout animals when exposed to chemicals is discussed in the context of human health risk assessment.

DNA repair: counteragent in mutagenesis and carcinogenesis-- accomplice in cancer therapy resistance

DNA-reactive carcinogens and anticancer drugs induce many structurally distinct mutagenic and cytotoxic DNA lesions. The varying capability of normal and malignant cells to recognize and repair specific DNA lesions influences both cancer risk and the relative sensitivity or resistance of cancer cells towards cytotoxic agents. Using monoclonal antibody-based immunoanalytical assays, very low amounts of defined carcinogen-DNA adducts can be quantified in bulk genomic DNA, in individual genes, and in the nuclear DNA of single cells. DNA repair kinetics can, thus, be measured in a lesion-, gene-, and cell type-specific manner, and the DNA repair profiles of malignant cells can be monitored in individual patients. Even structurally very similar DNA lesions may be repaired with strongly differing efficiency. The miscoding DNA alkylation products O(6)-methylguanine and O(6)-ethylguanine, for example, differ only by one CH(2) group. These lesions are formed in DNA upon exposure to N-methyl-N-nitrosourea or N-ethyl-N-nitrosourea, both of which induce mammary adenocarcinomas in female rats at high yield. Unrepaired O(6)-alkylguanines in DNA cause G:C-->A:T transition mutations via mispairing during DNA replication. O(6)-methylguanines are repaired at a similar slow rate in both transcriptionally active (H-ras, beta-actin) and inactive genes (IgE heavy chain; bulk DNA) of the target mammary epithelia (which express the repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) at a very low level). In contrast, O(6)-ethylguanines are repaired approximately 20 times faster than O(6)-methylguanines in both DNA strands of the transcribed genes selectively via an AGT-independent, as yet unclarified excision mechanism. Accordingly, G:C-->A:T transitions resulting from the misreplication of an O(6)-methylated guanine at the second position of codon 12 (GGA) of H-ras represent a frequent "signature" mutation in rat mammary adenocarcinomas that develop after exposure to N-methyl-N-nitrosourea. However, this mutation is not observed when these tumors are induced by N-ethyl-N-nitrosourea, due to the fast repair of O(6)-ethylguanines in the H-ras gene. The key importance of "conventional" and "conditional" gene knockout technology for resolving the intricacies of the complex network of DNA repair pathways is briefly discussed.

Current status and use of short/medium term models for carcinogenicity testing of pharmaceuticals--scientific perspective

Short- and medium-term rodent bioassays have been proposed under ICH guidelines for use in testing for the carcinogenic potential of pharmaceuticals. Further evaluation of these models is needed urgently and coordinated efforts are in progress worldwide to expand the available database. Models currently being investigated include transgenic mice (Tg-rasH2, Tg.AC, p53(+/-), XPA(-/-)) and neonatal mice. As more data become available on the performance of these assays, regulatory and industry scientists will be faced with the difficult challenge of determining how the performance (accuracy) of each assay will be measured and deciding which assays have value in the risk assessment process.

Protection against methylation-induced cytotoxicity by DNA polymerase beta-dependent long patch base excision repair

Using a plasmid-based uracil-containing DNA substrate, we found that the long patch base excision repair (BER) activity of a wild-type mouse fibroblast extract was partially inhibited by an antibody to DNA polymerase beta (beta-pol). This suggests that beta-pol participates in long patch BER, in addition to single-nucleotide BER. In single-nucleotide BER, the deoxyribose phosphate (dRP) in the abasic site is removed by the lyase activity of beta-pol. Methoxyamine (MX) can react with the aldehyde of an abasic site, making it refractory to the beta-elimination step of the dRP lyase mechanism, thus blocking single-nucleotide BER. MX exposure sensitizes wild-type, but not beta-pol null mouse embryonic fibroblasts, to the cytotoxic effects of methyl methanesulfonate (MMS) and methylnitrosourea. Expression of beta-pol in the null cells restores the ability of MX to modulate sensitivity to MMS. The beta-pol null cells are known to be hypersensitive to MMS and methylnitrosourea, and in the presence of MX (i.e. under conditions where single-nucleotide BER is blocked) the null cells are still considerably more sensitive than wild-type. The data are consistent with a role of beta-pol in long patch BER, which helps protect cells against methylation damage-induced cytotoxicity.

Relevance of DNA repair to carcinogenesis and cancer therapy

DNA-reactive carcinogens and anticancer drugs induce many structurally distinct cytotoxic and potentially mutagenic DNA lesions. The capability of normal and malignant cells to recognize and repair different DNA lesions is an important variable influencing the risk of mutation and cancer as well as therapy resistance. Using monoclonal antibody-based immunoanalytical assays, very low amounts of defined carcinogen-DNA adducts can be quantified in bulk genomic DNA, individual genes, and in the nuclear DNA of single cells. The kinetics of DNA repair can thus be measured in a lesion-, gene-, and cell type-specific manner, and the DNA repair profiles of malignant cells can be monitored in individual patients. Even structurally very similar DNa lesions may be repaired with extremely different efficiency. The miscoding DNA alkylation products O6-methylguanine (O6-MeGua) and O6-ethylguanine (O6-EtGua), for example, differ only by one CH2 group. These lesions are formed in DNA upon exposure to N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), both of which induce mammary adenocarcinomas in female rats at high yield. Unrepaired O6-alkylguanines cause transition mutations via mispairing during DNA replication. O6-MeGua is repaired at a similar slow rate in transcribed (H-ras, beta-actin) and inactive genes (IgE heavy chain; bulk DNA) of the target mammary epithelia (which express the repair protein O6-alkylguanine-DNA alkyltransferase at a very low level). O6-EtGua, however, via an alkyltransferase-independent mechanism, is excised approximately 20 times faster than O6-MeGua from the transcribed genes selectively. Correspondingly, G:C-->A:T transitions arising from unrepaired O6-MeGua at the second nucleotide of codon 12 (GGA) of the H-ras gene are frequently found in MeNU-induced mammary tumors, but are absent in their EtNU-induced counterparts.