Age-dependent modulation of tissue-specific repair activity for 3-methyladenine and O6-methylguanine in DNA in inbred mice

Novel mechanism whereby nuclear factor kappaB mediates DNA damage repair through regulation of O(6)-methylguanine-DNA-methyltransferase

O(6)-Methylguanine-DNA-methyltransferase (MGMT) and nuclear factor kappaB (NF-kappaB) are two key effectors associated with the development of resistance to alkylating agent-based chemotherapy. This prompted us to hypothesize that NF-kappaB might be involved in MGMT regulation. Consistent with this hypothesis, we have discovered two putative NF-kappaB binding sites within the MGMT promoter region and showed a specific and direct interaction of NF-kappaB at each of these sites. Forced expression of the NF-kappaB subunit p65 in HEK293 cells induced an increase in MGMT expression whereas addition of the NF-kappaB super repressor DeltaNIkappaB completely abrogated the induction. We also found a significant correlation between the extent of NF-kappaB activation and MGMT expression in the glioma cell lines and the human glial tumors tested and showed that it was independent of MGMT promoter methylation. Our results are of potential clinical significance because we show that cell lines with ectopic p65 or high constitutive NF-kappaB activity are less sensitive to nitrosourea treatment and that suppression of MGMT activity with O(6)-benzylguanine completely abolishes the chemoresistance acquired by NF-kappaB. The findings of our study strongly suggest that NF-kappaB plays a major role in MGMT regulation and that MGMT is most probably the major player in NF-kappaB-mediated chemoresistance to alkylating agents.

O6-methylguanine-DNA methyltransferase deficiency in developing brain: implications for brain tumorigenesis

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is a cardinal defense against the mutagenic and carcinogenic effects of alkylating agents. We have reported evidence that absence of detectable MGMT activity (MGMT(-) phenotype) in human brain is a predisposing factor for primary brain tumors that affects ca. 12% of individuals [J.R. Silber, A. Blank, M.S. Bobola, B.A. Mueller, D.D. Kolstoe, G.A. Ojemann, M.S. Berger, Lack of the DNA repair protein O(6)-methylguanine-DNA methyltransferase in histologically normal brain adjacent to primary brain tumors, Proc. Natl. Acad. Sci. U.S.A. 93 (1996) 6941-6946]. We report here that MGMT(-) phenotype in the brain of children and adults, and the apparent increase in risk of neurocarcinogenesis, may arise during gestation. We found that MGMT activity in 71 brain specimens at 6-19 weeks post-conception was positively correlated with gestational age (P<or=0.0015). Moreover, the proportion of specimens exhibiting MGMT(-) phenotype (MGMT content<0.42 fmol/10(6)cells or 255 molecules/cell) declined progressively from 76% (16/21) at 6-8 weeks to 13% (1/8) at 15-19 weeks. All liver specimens that accompanied MGMT(-) brain (15/15) had measurable MGMT activity, demonstrating that the phenotype was not systemic in these cases. In contrast to MGMT, apurinic endonuclease, DNA polymerase beta and lactate dehydrogenase activities were found in every brain extract assayed, and showed no significant relationship with gestational age. The observed gestational pattern has at least two implications for neurocarcinogenesis. (1) Early in development, brain tissue that has MGMT(-) phenotype and is rapidly proliferating may be especially vulnerable to alkylation-induced mutations, including mutations that lead to brain tumors. (2) Persistence of prenatal MGMT deficiency into postnatal life in a sub-population of individuals may increase brain tumor risk. Our findings provide possible mechanistic insight into epidemiologic data associating maternal alkylating agent exposure with brain tumor incidence.

Base excision repair and the central nervous system

Reactive oxygen species generated during normal cellular metabolism react with lipids, proteins, and nucleic acid. Evidence indicates that the accumulation of oxidative damage results in cellular dysfunction or deterioration. In particular, oxidative DNA damage can induce mutagenic replicative outcomes, leading to altered cellular function and/or cellular transformation. Additionally, oxidative DNA modifications can block essential biological processes, namely replication and transcription, triggering cell death responses. The major pathway responsible for removing oxidative DNA damage and restoring the integrity of the genome is base excision repair (BER). We highlight herein what is known about BER protein function(s) in the CNS, which in cooperation with the peripheral nervous system operates to control physical responses, motor coordination, and brain operation. Moreover, we describe evidence indicating that defective BER processing can promote post-mitotic (i.e. non-dividing) neuronal cell death and neurodegenerative disease. The focus of the review is on the core mammalian BER participants, i.e. the DNA glycosylases, AP endonuclease 1, DNA polymerase beta, X-ray cross-complementing 1, and the DNA ligases.

Differential gene expression of early and late passage retinal pigment epithelial cells

We examined the gene expression profiles of retinal pigment epithelial (RPE) cells which were aged in vitro by repeated passage. RPE cells from human eyes were cultured to passage 3-5 (early passage) or 19-21 (late passage) and used to study gene expression profiles by cDNA microarray. Results from microarray analysis were further confirmed by real-time PCR. Microarray analysis showed gene expression changes among 588 known genes. The expression levels of 15 genes (2.6%) increased in late passage RPE cells, while 43 genes (7.3%) decreased using a two-fold criterion. These differentially expressed genes encompassed many functional classes. A small number of stress genes, such as clusterin, replication protein A and Ku80, were up-regulated. The down-regulated genes included many enzymes of energy and biomolecule metabolism as well as cell cycle proteins and cell adhesion proteins. Results from real-time PCR were generally consistent with microarray findings. The expression levels of the examined angiogenic factors were either unchanged or down-regulated. Comparing early (p=3-5) and late (p=9-12) passage RPE cells, several categories of differentially expressed genes were identified. However, there was no enhanced expression of known angiogenic factors.

Role of nucleotide- and base-excision repair in genotoxin-induced neuronal cell death

Base-excision (BER) and nucleotide-excision (NER) repair play pivotal roles in protecting the genomes of dividing cells from damage by endogenous and exogenous agents (i.e. environmental genotoxins). However, their role in protecting the genome of post-mitotic neuronal cells from genotoxin-induced damage is less clear. The present study examines the role of the BER enzyme 3-alkyladenine DNA glycosylase (AAG) and the NER protein xeroderma pigmentosum group A (XPA) in protecting cerebellar neurons and astrocytes from chloroacetaldehyde (CAA) or the alkylating agent 3-methyllexitropsin (Me-Lex), which produce ethenobases or 3-methyladenine (3-MeA), respectively. Neuronal and astrocyte cell cultures prepared from the cerebellum of wild type (C57BL/6) mice or Aag(-/-) or Xpa(-/-) mice were treated with 0.1-50 microM CAA for 24h to 7 days and examined for cell viability, DNA fragmentation (TUNEL labeling), nuclear changes, and glutathione levels. Aag(-/-) neurons were more sensitive to the acute (>20 microM) and long-term (>5 microM) effects of CAA than comparably treated wild type neurons and this sensitivity correlated with the extent of DNA fragmentation and nuclear changes. Aag(-/-) neurons were also sensitive to Me-Lex at comparable concentrations of CAA. In contrast, Xpa(-/-) neurons were more sensitive than either wild type or Aag(-/-) neurons to CAA (>10 microM), but less sensitive than Aag(-/-) neurons to Me-Lex. Astrocytes from the cerebellum of wild type, Aag(-/-) or Xpa(-/-) mice were essentially insensitive to CAA at the concentrations tested. These studies demonstrate that BER and NER are required to protect neurons from genotoxin-induced cell death.

CpG methylation-dependent repression of the human O6-methylguanine-DNA methyltransferase gene linked to chromatin structure alteration

The mechanism of inactivation of the O6-methylguanine-DNA methyltransferase (MGMT), responsible for repair of mutagenic and cytotoxic O6-alkylguanine, in Mex- tumor cells, is not completely understood. We have examined the role of CpG methylation in the human MGMT promoter in a luciferase (luc) reporter plasmid and associated alteration in chromatin structure. Methylation of 16% CpG sequences in promoter and flanking sequences in the plasmid with HpaII methylase reduced luciferase activity by 10-12-fold, while methylation of all CpG sites, including those in the luc coding sequence, as well as the promoter sequence blocked expression completely. Repression of luc expression due to partial but not complete CpG methylation could be reversed by histone deacetylase inhibitor trichostatin A (TSA). However, 5-azacytidine, which reverses CpG methylation, but not TSA, could reactivate silent MGMT gene in Mex- HeLa MR cells. Furthermore, chromatin immunoprecipitation (ChIP) assay showed reduced level of acetylation of H4 histone bound to the methylated promoter compared with the non-methylated promoter. These results suggest that complete repression of the MGMT gene in Mex- cells requires methylation of CpG sequences in both promoter and neighboring regions of the gene, resulting in inactive, condensed chromatin state of the gene.

Attenuation of DNA polymerase beta-dependent base excision repair and increased DMS-induced mutagenicity in aged mice

The biological mechanisms responsible for aging remain poorly understood. We propose that increases in DNA damage and mutations that occur with age result from a reduced ability to repair DNA damage. To test this hypothesis, we have measured the ability to repair DNA damage in vitro by the base excision repair (BER) pathway in tissues of young (4-month-old) and old (24-month-old) C57BL/6 mice. We find in all tissues tested (brain, liver, spleen and testes), the ability to repair damage is significantly reduced (50-75%; P<0.01) with age, and that the reduction in repair capacity seen with age correlates with decreased levels of DNA polymerase beta (beta-pol) enzymatic activity, protein and mRNA. To determine the biological relevance of this age-related decline in BER, we measured spontaneous and chemically induced lacI mutation frequency in young and old animals. In line with previous findings, we observed a three-fold increase in spontaneous mutation frequency in aged animals. Interestingly, lacI mutation frequency in response to dimethyl sulfate (DMS) does not significantly increase in young animals whereas identical exposure in aged animals results in a five-fold increase in mutation frequency. Because DMS induces DNA damage processed by the BER pathway, it is suggested that the increased mutagenicity of DMS with age is related to the decline in BER capacity that occurs with age. The inability of the BER pathway to repair damages that accumulate with age may provide a mechanistic explanation for the well-established phenotype of DNA damage accumulation with age.

Spontaneous hepatocellular carcinoma is reduced in transgenic mice overexpressing human O6- methylguanine-DNA methyltransferase

O(6)-methylguanine (O(6)mG) is a potent mutagenic and procarcinogenic DNA lesion. Organisms have evolved with a DNA repair mechanism that largely ameliorates the deleterious effects of O(6)mG through a direct reversal mechanism by a protein termed O(6)-methylguanine-DNA methyltransferase (MGMT). However, the contribution of O(6)mG to carcinogenesis, in the absence of known exposure to agents that produce it, has not been defined. Nontransgenic C3HeB male mice have a high frequency of spontaneous liver tumors. Transgenic CeHeB/FeJ mice expressing human MGMT (hMGMT) were generated that had elevated hepatic MGMT activity. The spontaneous development of hepatocellular carcinoma was significantly reduced in those mice expressing hMGMT compared with nontransgenic C3HeB/FeJ male mice. No differences were detected in spontaneous mutant frequencies in lacI transgenes in mice carrying hMGMT compared with that without hMGMT but the proportion of GC to AT transition mutations was lower in the transgenic mice carrying hMGMT as well as lacI. Tumors that arose in C3HeB/FeJ transgenic mice were largely deficient in hMGMT protein as determined by immunohistochemistry with a monoclonal antibody directed against hMGMT. Together these data indicate that spontaneous O(6)mG lesions induced hepatocellular carcinogenesis in C3HeB/FeJ male mice. These transgenic mice represent a rare example of reduced spontaneous carcinogenesis.

Health span and life span in transgenic mice with modulated DNA repair

One way to better understand the contribution of DNA repair, DNA damage, and mutagenesis in aging would be to enhance DNA repair activity, lower DNA damage, and lower mutagenesis. Because the repair protein O6-methylguanine-DNA methyltransferase (MGMT) acts alone and stoichiometrically, the human MGMT (hMGMT) cDNA was selected to test the feasibility of enhancing DNA repair activity in transgenic mice. MGMT activity is largely responsible for ameliorating the deleterious effects of O6-methylguanine (O6mG) lesions in DNA in a direct reversal mechanism. A transgene was constructed consisting of a portion of the human transferrin (TF) promoter and hMGMT cDNA such that hMGMT is expressed in transgenic mouse brain and liver. Expression of hMGMT was associated with a significant reduction in the occurrence of an age-related hepatocellular carcinoma in male mice at 15 months of age. Longitudinal and cross-sectional studies were initiated to determine whether the reduced incidence of hepatocellular carcinoma would impact median or maximum life span. The cross-sectional study performed on 15-month-old male animals confirmed the reduced occurrence of spontaneous hepatocellular carcinoma. At 30 months of age, however, the occurrence of hepatocellular carcinoma in at least one transgenic line was similar to that for nontransgenic animals. The longitudinal study is ongoing; however, at present no significant differences in life span have been detected. Tissues expressing the MGMT transgene also displayed greater resistance to alkylation-induced tumor formation. These results suggest that transgenes can be used to direct enhanced DNA repair gene expression and that enhanced expression can protect animals from certain spontaneous and induced tumors.

Transplacental mutagenicity of N-ethyl-N-nitrosourea at the hprt locus in T-lymphocytes of exposed B6C3F1 mice

Previous studies have compared age-related differences in total mutagenic burden in mice of differing age (preweanling, weanling, or young adult) after single intraperitoneal (i.p.) injections of ethylnitrosourea (ENU). The purpose of the present investigation was to determine the effects of time elapsed since treatment on the frequency of hprt mutant T-cells (Mf) from mice treated transplacentally with single acute vs. multiple split doses of ENU. To this end, pregnant C57BL/6 mice (n = 13-16/group), which had been bred to C3H males, were given i.p. injections of 40 mg ENU/kg bw in a single dose on day 18 of gestation, in a split dose of 6 mg ENU/kg bw on days 12 through 18 of gestation, or DMSO vehicle alone. Groups of pups were necropsied on days 10, 13, 15 (single dose only), 17, 20, 40, and 70 postpartum for T-cell isolations and hprt Mf measurements using the T-cell cloning assay. The time required to reach maximum Mfs in T-cells isolated from thymus of transplacentally treated animals was 2 weeks, the same time span as previously observed after ENU treatment of adult, weanling, and preweanling mice. Mfs in T-cells isolated from spleens of control animals averaged 2.1 +/- 0.3 (SE) x 10(-6). In spleens of mice treated transplacentally with ENU in a single dose, Mfs reached a maximum at 15 days postpartum [84.7 +/- 15.8 (SE) x 10(-6)] and decreased to lower but still elevated levels at 40 days postpartum. In spleens of mice treated transplacentally with ENU in a split dose, Mfs reached a maximum at 13 days postpartum [74.0 +/- 16.3 (SE) x 10(-6)] and decreased to background levels at 40 days postpartum. The areas under the curves describing the change in hprt Mfs over time for ENU-treated vs. control mice estimate the mutagenic potency for transplacental single- and split-dose exposures to be 1.9 and 0.8 x 10(3), respectively. Comparison of the mutagenic potency estimates for mice exposed to ENU in utero to 4-week-old mice given a similar dose of the same lot number of ENU indicates that the mouse is more susceptible to ENU-induced mutagenesis during fetal life.

Regulation of the human O(6)-methylguanine-DNA methyltransferase gene by transcriptional coactivators cAMP response element-binding protein-binding protein and p300

O(6)-Methylguanine-DNA methyltransferase (MGMT)(1), a ubiquitous DNA repair protein, removes O(6)-alkylguanine from DNA, including cytotoxic O(6)-chloroethylguanine induced by chemotherapeutic N-alkyl N-nitrosourea-type drugs, e.g. 1,3-bis(2-chloroethyl)-1-nitrosourea. Treating the pancreatic carcinoma cell line MIA PaCa-2 with trichostatin A (TSA), a specific inhibitor of histone deacetylase, increased MGMT mRNA and protein levels by 2-3-fold. Surprisingly, TSA treatment increased MGMT promoter-dependent luciferase activity by some 40-fold in a transient reporter expression assay. Deletion and point mutation analysis showed that two AP-1 binding sites in the MGMT promoter are involved in activation by TSA. Ectopic expression of the transcriptional coactivators cAMP response element-binding protein-binding protein (CBP) and p300, which have intrinsic histone acetyltransferase activity, enhanced luciferase expression. Overexpression of adenovirus E1A, which binds CBP/p300, strongly inhibited both basal and TSA-inducible MGMT promoter activity, while a mutant E1A, defective in binding CBP/p300, did not. Chromatin immunoprecipitation assays revealed that TSA treatment increased histone acetylation in the endogenous MGMT promoter region, which also showed association with CBP/p300. Taken together, our results indicate that targeted histone acetylation results in the remodeling of chromatin by recruitment of the coactivator CBP/p300, and constitutes an important step in regulating MGMT expression.

Deficiency of N-methylpurine-DNA-glycosylase expression in nonparenchymal cells, the target cell for vinyl chloride and vinyl fluoride

The ability to repair promutagenic damage resulting from exposure to carcinogens is a critical factor in determining quantitative relationships in carcinogenesis, including the target cell for neoplasia. One major pathway for the repair of alkylating agent-induced DNA damage involves removal of alkylated bases by N-methylpurine-DNA-glycosylase (MPG), the first enzyme in base excision repair. We have measured the expression level of MPG mRNA in liver, lung, and kidney of Sprague-Dawley rats as a function of age. A quantitative reverse transcriptase-polymerase chain reaction (QRT-PCR) method was used to measure cellular MPG mRNA. MPG mRNA was readily detectable in each tissue analyzed and the age-dependent and tissue specific expressions were not statistically different. The lowest amount of mRNA was measured in preweanling liver and the highest amounts were found in preweanling lung and kidney. Since MPG is reported to be responsible for excision of 1,N(6)-ethenoadenine and N(2),3-ethenoguanine, two promutagenic DNA adducts of vinyl chloride (VC) and vinyl fluoride (VF), we examined the regulation of this enzyme after carcinogen exposure. Expression of MPG was induced in rat liver by these carcinogens. In order to determine the repair capacity in different cell populations of liver, we measured MPG gene expression in isolated hepatocytes and nonparenchymal cells (NPC). The amount of MPG mRNA was 4.5-5 times higher in hepatocytes than in NPC of control rats. Induction of MPG expression was observed in hepatocytes of VF exposed-rats but not in NPC. The expression of MPG in NPC was only 15% of that of the hepatocytes from exposed rats. Western blots of MPG protein confirmed the cell type differences, but did not show increased protein in exposed vs. control liver and hepatocytes. Since metabolism of VC and VF requires CYP2E1, an enzyme exhibiting much greater activity in hepatocytes, formation of etheno adducts preferentially occurs in hepatocytes. These data suggest that cellular differences in the repair of N-alkylpurines may be a critical mechanism in the development of cell specificity in VC carcinogenesis.

3-methyladenine DNA glycosylases: structure, function, and biological importance

The genome continuously suffers damage due to its reactivity with chemical and physical agents. Finding such damage in genomes (that can be several million to several billion nucleotide base pairs in size) is a seemingly daunting task. 3-Methyladenine DNA glycosylases can initiate the base excision repair (BER) of an extraordinarily wide range of substrate bases. The advantage of such broad substrate recognition is that these enzymes provide resistance to a wide variety of DNA damaging agents; however, under certain circumstances, the eclectic nature of these enzymes can confer some biological disadvantages. Solving the X-ray crystal structures of two 3-methyladenine DNA glycosylases, and creating cells and animals altered for this activity, contributes to our understanding of their enzyme mechanism and how such enzymes influence the biological response of organisms to several different types of DNA damage.

Age-associated decreases in human DNA repair capacity: Implications for the skin

Multiple pathways are involved in accurate synthesis and distribution of DNA during replication, repair and maintenance of genomic integrity. An increased error rate, abovethe spontaneous mutation baseline, has been implicated in carcinogenesis and aging. Moreover, cytogenetic abnormalities are increased in Down's, Edwards', Patau's, and Klinefelter's syndromes with increasing maternal age, and in Marfan's and Apert's syndromes with paternal age. In response to DNA damage, multiple overlapping systems of DNA repair have evolved, preferentially repairing the transcribed strand within transcriptionally-active regions of the genome. These include direct reversal of dimers and specific adducts and pathways for base excision, nucleotide excision, and mismatch repair. A consensus has emerged that some DNA repair capacities decline with organism age, contradictory reports notwithstanding. As is the case for inborn defects in humans, knockout mice lacking components of nucleotide excision repair or DNA-damage checkpoint arrest have increased frequencies of skin and internal cancers, whereas mice overexpressing DNA repair genes have fewer spontaneous cancers. Oxidative stress and resultant free radicals can damage genomic and mitochondrial DNA; damage increases with age but decreases with caloric restriction. We review recent studies of long-lived C. elegans mutants which appear to involve metabolic attenuation, the role of telomere shortening and telomerase in cellular senescence, and the genetic bases of progeroid syndromes in humans. Finally, we discuss roles of extrinsic and intrinsic factors in skin aging, and their association with DNA damage, emphasizing preventive and protective measures and prospects for intervention by modulating DNA repair pathways in the skin.

Activation of human O6-methylguanine-DNA methyltransferase gene by glucocorticoid hormone

O6-methylguanine-DNA methyltransferase (MGMT), a ubiquitous DNA repair protein, removes the mutagenic DNA adduct O6-alkylguanine, which is synthesized both endogenously and after exposure to alkylnitrosamines and alkylating antitumor drugs such as 2-chloroethyl-N-nitrosourea (CNU). The MGMT gene is highly regulated in mammalian cells and its overexpression, observed in many types of tumor cells, is often associated with cellular resistance to CNU. Dexamethasone, a synthetic glucocorticoid hormone, was found to increase MGMT expression in HeLa S3 cells, concomitant with their increased resistance to CNU. Two putative glucocorticoid responsive elements (GREs) were identified in the human MGMT (hMGMT) promoter. Transient expression of the luciferase reporter gene driven by an hMGMT promoter fragment containing these GREs was activated by dexamethasone. DNase I footprinting assays demonstrated the binding of glucocorticoid receptor to these sequences. In vitro transcription experiment showed that these DNA sequences are functional in glucocorticoid receptor signal-mediated activation of transcription. These results suggest glucocorticoid-mediated induction of the MGMT gene contributes to high level expression of MGMT.

Quantitative reverse transcriptase polymerase chain reaction for measuring the N-methylpurine-DNA glycosylase mRNA level in rodent cells

A modified quantitative reverse transcriptase polymerase chain reaction (QRT-PCR) procedure was developed for measuring mRNA concentration, in rodent cells, of the N-methylpurine-DNA glycosylase (MPG), a ubiquitous DNA repair protein responsible for the removal of N-alkylpurines and ethenoadducts of adenine, guanine, and cytosine from DNA. The method, applicable for quantitation of any mRNA, is based on the standard approach of comparing the relative amounts of PCR products of the experimental mRNA and a known amount of an exogenous reference RNA which is nearly identical to the experimental RNA. However, unlike in the earlier procedures in which deletion or insertion sequences were added to the reference RNA template, which may affect the efficiency of PCR but are needed to generate different size PCR products, experimental and reference RNAs yield PCR products of the same size in the new method. However, prior digestion with EcoRI allows separation of the two products because a unique EcoRI site was created in the reference RNA vector by point mutations. The QRT-PCR procedure is particularly useful for studying expression of the MPG gene whose mRNA level is very low and difficult to quantitate by Northern blot analysis. The number of MPG mRNA molecules/cell in late log-phase cultures varied from about 6 to 30 in several rodent lines. The SSV-NRK rat cell line has 6 +/- 0. 2 molecules/cell, while mouse NIH3T3 cells have about 30 +/- 1 molecules/cell. If the mRNA level is indicative of the level of the active MPG enzyme, these results may imply a variation in the capacity of various lines to remove the cytotoxic and mutagenic adducts from DNA.

Lack of the DNA repair protein O6-methylguanine-DNA methyltransferase in histologically normal brain adjacent to primary human brain tumors

Exposure to exogenous alkylating agents, particularly N-nitroso compounds, has been associated with increased incidence of primary human brain tumors, while intrinsic risk factors are currently unknown. The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) is a major defense against the carcinogenicity of N-nitroso compounds and other alkylators. We report here that in 55% (64/117) of cases, histologically normal brain tissue adjacent to primary human brain tumors lacked detectable MGMT activity [methyl excision repair-defective (Mer-) status]. The incidence of Mer- status in normal brain tissue from brain tumor patients was age-dependent, increasing from 21% in children 0.25-19 years of age to 75% in adults over 50. In contrast, Mer- status was found in 12% (5/43) of normal brain specimens from patients operated for conditions other than primary brain tumors and was not age-dependent. The 4.6-fold elevation in incidence of Mer- status in brain tumor patients is highly significant (chi2 = 24; p < or = 0.001). MGMT activity was independent of age in the lymphocytes of brain tumor patients and was present in lymphocytes from six of nine tumor patients whose normal brain specimen was Mer-. DNA polymerase beta, apurinic/apyrimidinic endonuclease, and lactate dehydrogenase activities were present in all specimens tested, including Mer- specimens from brain tumor patients. Our data are consistent with a model of carcinogenesis in human brain in which epigenetically regulated lack of MGMT is a predisposing factor and alkylation-related mutagenesis is a driving force.

Purification and biochemical characterization of recombinant N-methylpurine-DNA glycosylase of the mouse

The mouse N-methylpurine-DNA glycosylase (MPG), responsible for the removal of most N-alkyladducts in DNA, was purified to homogeneity as a recombinant nonfusion protein from Escherichia coli. Only 10-15% of the protein was present in the soluble form in E. coli cells. The N-terminal amino acid sequence of the purified protein which lacks 48 residues from the amino terminus of the wild type protein was identical to that predicted from the nucleotide sequence. The glycosylase hydrolyzes 3-methyladenine (m3A), 7-methylguanine(m7G), and 3-methylguanine (m3G) from DNA, and the Km and kcat values were 130 nM and 0.8 min-1 for m3A, and 860 nM and 0.2 min-1 for m7G, respectively, when methylated calf thymus DNA was used as the substrate. A comparison of kcat/Km values for different bases indicates that the enzyme was more efficient in excising both m3A and m3G than m7G from methylated DNA. The enzyme showed moderate binding affinities (KA) for both methylated (5.8 x 10(7) M-1) and nonmethylated DNAs (4.2 x 10(7) M-1). The mouse protein has an extinction coefficient E280nm1% of 10.5 and a pI of 9.3. The enzyme activity was optimal in the presence of 100 mM NaCl, with a broad pH optimum of 8.5-9.5. The enzymatic release of both m3A and m7G was stimulated 50-75% by 0.5 mM MgCl2 and 0.02 mM spermine but inhibited by higher concentrations of these agents. Product inhibition by 40-50% of the reaction occurred in the presence of 10 mM m3A or m7G. However, 1.0 mM m3A stimulated release of m7G. The enzyme was inhibited by 60% in the presence of 0.9 mg/mL DNA which, at the same time, protected it from thermal inactivation.

Age and dose-dependent carcinogenic effects of N-nitrosomethylurea administered intraperitoneally in a single dose to young and adult female mice

Female Swiss-derived SHR mice aged 3 ("young") and 12 months ("adult") were exposed to a single i.p. administration of N-nitrosomethylurea (NMU) at one of four doses: 0, 10, 20, or 50 mg/kg. The mean survival time of the young mice so treated was 440, 325, 398 and 182 days, and of the adult mice 221, 249, 191, and 168 days respectively. The incidence of all kinds of tumours in young mice was 40%, 64%, 77%, and 40%, of malignant tumours 33%, 43%, 57% and 20% of lung adenomas 7%, 14%, 40%, and 20% and of papillomas of the forestomach 0%, 14%, 23%, and 3% respectively. In adult mice these figures were for all kinds of tumours 52%, 52%, 56%, and 50%, for malignant tumours 44%, 52%, 52%, and 40%, for lung adenomas 7%, 0%, 8%, and 20%, for papillomas of the forestomach 0%, 0%, 12%, and 0% respectively. The exposure of adult female mice to various doses of NMU did not significantly increase the incidence of tumours or of malignant tumour incidence in comparison to age-matched controls. At the same time the latency of fatal tumours was shorter in the adult groups than in the young groups. Histoautoradiography of tissues of intact young and adult mice showed that there are no statistically significant age-related differences in the labelling index of forestomach epithelium, endometrium and lung alveolar wall epithelium. Only the labelling index of hepatocytes was decreased in the liver of adult mice in comparison to young ones. Comparison of the present experimental results with the data available on DNA synthesis and on O6-methylguanine repair in target tissues suggests a requirement for individual monitoring of age-related changes of biomarkers in exposure to carcinogenic agents. The analysis of data on the dose dependence of the carcinogenic effect of NMU against the background of a multistage model suggests an age-related accumulation of stochastically damaged cells for some tissues.

Structure of the human 3-methyladenine DNA glycosylase gene and localization close to the 16p telomere

We recently reported the presence of four genes lying between the human alpha-globin gene cluster and the telomere of the short arm of chromosome 16 (16p). We now report that one of these genes encodes 3-methyladenine DNA glycosylase, an enzyme important in the repair of DNA after damage by alkylating agents. The gene comprises five exons, representation of which differs in independently isolated cDNA clones. Although the gene is widely expressed, the abundance of its mRNA is considerably higher in a colon adenocarcinoma cell line (HT29) than in other cell lines that were tested. The major positive erythroid-specific regulatory element controlling alpha-globin gene expression lies equidistant between the promoters of the alpha-globin genes and the 3-methyladenine DNA glycosylase gene. Interestingly, in contrast to the alpha-globin genes, expression of the 3-methyladenine DNA glycosylase gene is not influenced by the regulatory element in the human erythroleukemia cell line K562.

Age-related studies on the removal of 7-methylguanine from DNA of mouse kidney tissue following N-methyl-N-nitrosourea treatment

To investigate the effects of age on DNA repair of alkylation damage, C57BL/6NNia mice ranging from 9 months to 29 months of age were injected by the intraperitoneal route with single doses of N-methyl-N-nitrosourea (MNU). The rates of removal of 7-methylguanine (m7Gua) in nuclear DNA from kidney were determined at various intervals from 1 to 288 h after injection of either 25 mg or 50 mg MNU per kg body weight. Reversed phase HPLC with electrochemical detection was used to monitor adduct disappearance from DNA hydrolysates. The kinetics of m7Gua removal from DNA were at least biphasic. Evidence was obtained that there was a rapid removal of m7Gua occurring in the first 24 h after MNU administration, followed by a slow phase of removal with a t1/2 greater than 150 h. We assume that these two phases of m7Gua removal correspond to active repair of DNA by N-alkylglycosylases and to passive elimination via spontaneous hydrolysis, respectively. Young and old kidney tissues all exhibited significant repair of m7Gua (55-73% of the induced adducts were removed in the first 24 h), but a substantial fraction of m7Gua was removed slowly, indicating that there are methylated bases which were refractory to repair processes. At both doses of MNU studied, old tissues showed active repair of m7Gua that, within the limits of detection, had similar initial rates of removal as young tissues. However, old kidney did not remove this adduct with the same overall efficiency as young kidney. Therefore, the amount of m7Gua in the repair-resistant fraction was greater in the senescent tissues. The biochemical mechanisms responsible for the less efficient DNA repair in senescent kidney are not known, but we suggest that such differences are due in part to structural alterations in the chromatin.

Genomic damage and its repair in young and aging brain

A brief review of the available information concerning age-related genomic (DNA) damage and its repair, with special reference to brain tissue, is presented. The usefulness of examining the validity of DNA-damage and repair hypothesis of aging in a postmitotic cell like neuron is emphasized. The limited number of reports that exist on brain seem to overwhelmingly support the accumulation of DNA damage with age. However, results regarding the age-dependent decline in DNA-repair capacity are conflicting and divided. The possible reasons for these discrepancies are discussed in light of the gathering evidence, including some human genetic disorders, to indicate how complex is the DNA-repair system in higher animals. It is suggested that assessment of repair potential of neurons with respect to a specific damage in a specific gene might yield more definitive answers about the DNA-repair process and its role in aging.

Developmental regulation of the base excision repair enzyme uracil DNA glycosylase in the rat

The developmental regulation of the mammalian DNA-repair enzyme uracil DNA glycosylase was examined in the rat at specific intervals ranging from -4 days before to 106 days after birth. Enzyme activity was quantitated by in vitro biochemical assay. In the adult animal, as measured in crude cell extracts, three organs (liver, kidney and spleen) had significant levels of activity. In contrast, three organs (brain, heart and lung) had low activity. Partial purification of this enzyme identified one major species of molecular weight 32,700 Da, demonstrating the quantitation of the nuclear glycosylase. During development, with the exception of the liver, the specific activity of the glycosylase paralleled the regulation of DNA synthesis. In these organs the highest levels of the glycosylase and the rate of DNA replication were observed around the time of birth. In the liver, DNA replication was similarly regulated. However, glycosylase activity was minimal at early stages of life. Instead, maximal levels were observed at 14-21 days after birth. At that time DNA replication was severely reduced. These results demonstrate that individual organs express this DNA-repair enzyme in a distinct and specific pattern during development. Accordingly, the regulation of the uracil DNA glycosylase during development may provide a model system to examine the differential regulation of DNA-repair genes.

DNA damage and repair in brain: relationship to aging

The usefulness of conducting DNA damage and repair studies in a postmitotic tissue like brain is emphasized. We review studies that use brain as a tissue to test the validity of the DNA damage and repair hypothesis of aging. As far as the accumulation of age dependent DNA damage is concerned, the data appear to overwhelmingly support the hypothesis. However, attempts to demonstrate a decline in DNA repair capacity as a function of age are conflicting and equally divided. Possible reasons for this discrepancy are discussed. It is suggested that assessment of the repair capacity of neurons with respect to a specific type of damage in a specific gene might yield more definitive answers regarding the role of DNA repair potential in the aging process and as a longevity assurance system.

Characterization of cDNA encoding mouse DNA repair protein O6-methylguanine-DNA methyltransferase and high-level expression of the wild-type and mutant proteins in Escherichia coli

A mouse cDNA clone encoding O6-methylguanine-DNA methyltransferase (MGMT), responsible for repair of mutagenic O6-alkylguanine in DNA, was cloned from a lambda gt11 library. On the basis of an open reading frame in cDNA, the mouse protein contains 211 amino acids with a molecular mass of 22 kDa. The size and the predicted N-terminal sequence of the mouse protein were confirmed experimentally. The deduced amino acid sequence of the mouse MGMT is 70% homologous to that of the human MGMT. Cysteine-149 was shown to be the only alkyl acceptor residue in the mouse protein, in confirmation of the prediction based on conserved sequences of different MGMTs. Mouse MGMT protein is recognized by some monoclonal antibodies specific for human MGMT. Site-directed mutagenesis was utilized to reclone the mouse cDNA in a T7 promoter-based vector for overexpression of the native repair protein in Escherichia coli. The mouse protein has a tetrapeptide sequence, Pro-Glu-Gly-Val at positions 56-59, absent in the human protein. Neither deletion of this tetrapeptide nor substitution of valine-169 with alanine affected the activity of the mutant proteins.

Inducibility of the DNA repair gene encoding O6-methylguanine-DNA methyltransferase in mammalian cells by DNA-damaging treatments

The inducibility of the mammalian O6-methylguanine-DNA methyltransferase (MGMT) gene encoding the MGMT protein (EC 2.1.1.63) responsible for removal of the procarcinogenic and promutagenic lesion O6-alkylguanine from DNA was examined by an analysis of transcription of the MGMT gene following exposure of repair-competent (Mex+) and repair-deficient (Mex-) cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). While human and rodent Mex- cells (CHO-9, V79, HeLa MR) showed no detectable MGMT mRNA despite the presence of the gene in their genome, the amount of it in several Mex+ lines (NIH 3T3, HeLa S3, HepG2) paralleled their MGMT activity. However, none of these cell lines showed an increase in the MGMT mRNA level after treatment with various concentrations of MNNG. In contrast, MNNG-treated rat hepatoma cells, H4IIE and FTO-2B, both Mex+, had three- to fivefold more MGMT mRNA than the corresponding untreated controls as measured 12 to 72 h after alkylation. N-Methyl-N-nitrosourea, methyl methanesulfonate, N-hydroxyethyl-N-chloroethylnitrosourea, UV light, and X rays caused a similar accumulation of MGMT mRNA in rat hepatoma cells. Studies with inhibitors of RNA and protein synthesis indicate that the induced increase in the amount of MGMT mRNA was due to enhanced transcription of the gene. Furthermore, they revealed the turnover of the MGMT mRNA to be relatively low (half-life, greater than 7 h). Mutagen-induced increase of transcription of MGMT mRNA in H4IIE cells was accompanied by elevation of MGMT repair activity and resulted in reduction of mutation frequency after a challenge dose of MNNG. Although induction of MGMT mRNA transcription has been observed in two rodent hepatoma cell lines so far, this appears to be the first demonstration of inducibility of a mammalian gene encoding a clearly define DNA repair function. The transcription activation of the MGMT gene protects cells from the mutagenic effects of methylating agents.

Inducibility of the DNA repair gene encoding O6-methylguanine-DNA methyltransferase in mammalian cells by DNA-damaging treatments

The inducibility of the mammalian O6-methylguanine-DNA methyltransferase (MGMT) gene encoding the MGMT protein (EC 2.1.1.63) responsible for removal of the procarcinogenic and promutagenic lesion O6-alkylguanine from DNA was examined by an analysis of transcription of the MGMT gene following exposure of repair-competent (Mex+) and repair-deficient (Mex-) cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). While human and rodent Mex- cells (CHO-9, V79, HeLa MR) showed no detectable MGMT mRNA despite the presence of the gene in their genome, the amount of it in several Mex+ lines (NIH 3T3, HeLa S3, HepG2) paralleled their MGMT activity. However, none of these cell lines showed an increase in the MGMT mRNA level after treatment with various concentrations of MNNG. In contrast, MNNG-treated rat hepatoma cells, H4IIE and FTO-2B, both Mex+, had three- to fivefold more MGMT mRNA than the corresponding untreated controls as measured 12 to 72 h after alkylation. N-Methyl-N-nitrosourea, methyl methanesulfonate, N-hydroxyethyl-N-chloroethylnitrosourea, UV light, and X rays caused a similar accumulation of MGMT mRNA in rat hepatoma cells. Studies with inhibitors of RNA and protein synthesis indicate that the induced increase in the amount of MGMT mRNA was due to enhanced transcription of the gene. Furthermore, they revealed the turnover of the MGMT mRNA to be relatively low (half-life, greater than 7 h). Mutagen-induced increase of transcription of MGMT mRNA in H4IIE cells was accompanied by elevation of MGMT repair activity and resulted in reduction of mutation frequency after a challenge dose of MNNG. Although induction of MGMT mRNA transcription has been observed in two rodent hepatoma cell lines so far, this appears to be the first demonstration of inducibility of a mammalian gene encoding a clearly define DNA repair function. The transcription activation of the MGMT gene protects cells from the mutagenic effects of methylating agents.

Isolation and structural characterization of a cDNA clone encoding the human DNA repair protein for O6-alkylguanine

O6-Methylguanine-DNA methyltransferase (MGMT; DNA-O6-methylguanine:protein-L-cysteine S-methyltransferase, EC 2.1.1.63), a unique DNA repair protein present in most organisms, removes the carcinogenic and mutagenic adduct O6-alkylguanine from DNA by stoichiometrically accepting the alkyl group on a cysteine residue in a suicide reaction. The mammalian protein is highly regulated in both somatic and germ-line cells. In addition, the toxicity of certain alkylating drugs in tumor and normal cells is inversely related to the levels of this protein. The cDNA of the human gene, henceforth named MGMT, has been cloned in an expression vector on the basis of its rescue of a methyltransferase-deficient (ada-) Escherichia coli host. A 22-kDa active methyltransferase encoded entirely by the cDNA contains an amino acid sequence of 61 residues that bears 60-65% similarity with segments of E. coli methyltransferase (products of the ada and ogt genes), which encompass the alkyl-acceptor residues. The human cDNA has no sequence similarity with the ada and ogt genes, due in part to differences in codon usage, and shows no detectable homology with E. coli genomic DNA. However, it hybridizes with distinct restriction fragments of human, mouse, and rat DNAs. The lack of methyltransferase observed in many human cell lines is due to the absence of the MGMT gene or to lack of synthesis and/or stability of its 0.95-kilobase poly(A)+ RNA transcript.