Spontaneous tumorigenesis in mice defective in the MTH1 gene encoding 8-oxo-dGTPase

Oxygen radicals, which can be produced through normal cellular metabolism, are thought to play an important role in mutagenesis and tumorigenesis. Among various classes of oxidative DNA damage, 8-oxo-7,8-dihydroguanine (8-oxoG) is most important because of its abundance and mutagenicity. The MTH1 gene encodes an enzyme that hydrolyzes 8-oxo-dGTP to monophosphate in the nucleotide pool, thereby preventing occurrence of transversion mutations. By means of gene targeting, we have established MTH1 gene-knockout cell lines and mice. When examined 18 months after birth, a greater number of tumors were formed in the lungs, livers, and stomachs of MTH1-deficient mice, as compared with wild-type mice. The MTH1-deficient mouse will provide a useful model for investigating the role of the MTH1 protein in normal conditions and under oxidative stress.

Importance of DNA repair in carcinogenesis: evidence from transgenic and gene targeting studies

We have generated transgenic mice by introducing copies of the E. coli O6-methylguanine-DNA methyltransferase gene, ada. Liver extracts from homozygotes demonstrate about three times the control enzyme activity and increase up to about eight-fold can be induced by treatment with zinc, since the metal-responsive metallothionein promoter is attached to the ada gene. Furthermore, studies of liver carcinogenesis in our transgenic mice demonstrated significantly reduced rates of development of hepatocellular tumors after treatment with dimethylnitrosamine or diethylnitrosamine. It is well known that xeroderma pigmentosum (XP) patients are deficient in DNA repair. The availability of XPA (XP group A complementing) knockout mice has enabled us to investigate the functional role of the XPA nucleotide excision repair gene in carcinogenesis in vivo, first using the mouse skin as a model system. XPA-/- mice demonstrated skin ulcers 5-7 days after 7,12-dimethylbenz[a]anthracene (DMBA) treatment and papilloma development within 4 weeks prior to promotion, skin tumor incidence being also much higher than in heterozygous and wild-type mice. Experiments targeting the lung, liver and tongue have also been conducted to answer the question of whether the internal organs of these mice are also susceptible to chemical carcinogens. For lung carcinogenesis, mice were instilled intratracheally with a small dose of benzo[a]pyrene. The pulmonary tumor incidence in XPA-/- mice was significantly higher than in XPA+/- and XPA+/+ mice. XPA-/- mice were also found to be have enhanced sensitivity to aflatoxin B1 regarding liver tumor induction. In addition, administration of 4-nitroquinoline-1-oxide in drinking water for 50 weeks resulted in tongue tumors only in XPA-/- mice. These studies, thus, provided convincing evidence that XPA mice are also sensitive to carcinogenesis in organs other than the skin.

Morphological and physiological restorations of hereditary form of dilated cardiomyopathy by somatic gene therapy

TO-2 strain hamsters with dilated cardiomyopathy, gene deletion of delta-sarcoglycan (SG) and no expression of alpha-, beta-, gamma-, and delta-SG proteins are useful for developing the potential gene therapy of intractable heart failure. We prepared recombinant adeno-associated virus vector including normal delta-SG gene driven by CMV promoter and intramurally administered in vivo. The transfected myocardium induced robust expression of both transcript and transgene for 2/3 period of the animal's life expectancy. Immunostaining demonstrated reexpression of not only delta-SG but also other three SGs in 40% cells in the transfected region and normalization of the diameter of transduced cardiomyocytes. Hemodynamic study revealed preferential amelioration of the diastolic indices (LVEDP, the dP/dt(min) and CVP). These results provide the first evidence that supplementation of a specific gene with efficient and sustained transfection capability restores the genetic, morphological, and functional deteriorations.

Xeroderma pigmentosum group A gene action as a protection factor against 4-nitroquinoline 1-oxide-induced tongue carcinogenesis

To test the hypothesis that nucleotide excision repair (NER) plays a protective role in chemical carcinogenesis in internal organs, xeroderma pigmentosum group A gene-deficient (XPA(-/-)) mice, heterozygous (XPA(+/-)) and wild-type (XPA(+/+)) mice were orally administered 0.001% 4-nitroquinoline 1-oxide (4NQO) in their drinking water and compared. After 50 weeks of 4NQO exposure, tongue squamous cell carcinomas (SCCs) occurred in XPA(-/-) mice only, no tumors being observed in XPA(+/-) and XPA(+/+) animals. Of the XPA(-/-) mice 86% had tumors and 100% demonstrated multiple foci of dysplastic epithelium in the tongue. Accumulation of p53 protein was immunohistochemically detected in 56% of the SCCs. Mutational analysis of the p53 gene (exons 4-10) in carcinoma DNA revealed missense mutations in exons 5 and 9 in four of 20 samples. Our results clearly demonstrate that the NER gene XPA acts as a defensive factor against 4NQO-induced tongue carcinogenesis in vivo.

Protection against malignant progression of spontaneously developing liver tumors in transgenic mice expressing O(6)-methylguanine-DNA methyltransferase

To study the effect of O(6)-methylguanine-DNA methyltransferase (MGMT) on carcinogenesis, we have previously generated MGMT transgenic mice overexpressing the bacterial MGMT gene, ada, and demonstrated that high MGMT levels in the liver suppress induction of liver tumors after treatment with an alkylating hepatocarcinogen. To examine the effects of life-long elevation of MGMT activity on mouse spontaneous liver tumor development, ada-transgenic and control non-transgenic mice were compared. We also examined mutations at codon 61 of the H-ras oncogene, reported as a hot spot in mouse liver tumors, using a direct DNA sequencing method. The results revealed no significant difference in tumor incidence or mutation spectrum, but interestingly, ada-transgenic mice were found to have fewer malignant tumors and survived longer, indicating a possible protective role of MGMT against malignant conversion.

Genetic identification of bilateral primary or metastatic nonpapillary renal cell carcinoma

OBJECTIVE

To clarify the clonality of bilateral tumours by genetic analysis of bilateral renal cell carcinomas (RCCs) using the VHL gene, which is inactivated in approximately 60% of RCCs and which plays a causal role in the development of most cases of nonpapillary RCC.

PATIENTS AND METHODS

The study included 20 patients; seven had von Hippel-Lindau disease, three had papillary RCC and 10 had nonpapillary RCC. Paraffin-embedded blocks of tumour tissue were obtained from two of the three patients with papillary RCC and from nine of 10 with nonpapillary disease; all three exons of VHL were examined by direct sequencing.

RESULTS

As reported previously, no VHL mutations were found in papillary tumours. However, in five of the nine nonpapillary cases, VHL mutations were identified in tumours on one or both sides. Three of the tumours had the same mutation on both sides, confirming a common origin. In the remaining two patients, the mutation status differed between the sides, confirming a bilateral primary origin. The former cases were characterized by a relatively large tumour on one side and multiple tumours on the other.

CONCLUSIONS

In nonpapillary RCC multiplicity may suggest a metastatic origin. Such genetic information will be useful in treating and following patients with bilateral renal tumours.

Possible rare involvement of O6-methylguanine formation as a significant mutational factor in mouse urinary bladder carcinogenesis models

O6-methylguanine is known as one of the major premutagenic lesions in the human and rodent carcinogenesis process. O6-methylguanine-DNA methyltransferase (MGMT), which repairs methylated guanine bases, might prevent the G:C to A:T transition, and transgenic mice carrying this MGMT gene have been reported to be less sensitive to the carcinogenicity of certain alkylating agents. Here we utilized MGMT transgenic mice to assess the significance of O6-methylguanine formation during urinary bladder carcinogenesis. In experiment 1, 100 and 60 ppm N-butyl-N(4-hydroxybutyl)nitrosamine was given for 20 weeks to transgenic and non-transgenic mice in their drinking water. The incidences of urinary bladder carcinomas were not different between transgenic mice and non-transgenic mice. The mutational spectrum of the p53 gene was evaluated by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and direct sequencing. The pattern of p53 mutations of transgenic and non-transgenic mice did not differ, and the frequencies of mutations were 40% and 42%, respectively. G:C to A:T transition mutations were particularly infrequent (1 of 14 mutations, 7%). In experiment 2, N-methyl-N-nitrosourea, which might induce O6-methylguanine in affected alleles, was given once a week, 3 times (total 5 mg) by direct instillation into the urinary bladder through an abdominal incision. No significant neoplastic lesions were detected, although the experiment was limited by severe toxicity of the treatment. p53 immunostaining was done and there was no difference in transgenic and non-transgenic mice. These results suggest that O6-methylguanine formation might not be a significant mutational factor in these mouse urinary bladder carcinogenesis models.

Mice deficient in the nucleotide excision repair gene XPA have elevated sensitivity to benzo[a]pyrene induction of lung tumors

This study is focused on chemical induction of lung tumors in xeroderma pigmentosum group A gene (XPA)-deficient mice to clarify the role of nucleotide excision repair (NER) in internal organs. Six-week-old female XPA-/-, XPA(+/-) and XPA(+/+) mice were instilled intratracheally with benzo[a] pyrene (B[a]P). A total of 68 surviving XPA mice treated with B[a]P were examined at month 16. The pulmonary adenoma incidence in XPA(-/-) mice was significantly higher than that in XPA(+/+) mice (71 versus 35%). Similarly, tumor multiplicity was elevated and, in addition, only XPA(-/-) mice had lung carcinomas. These results provide the first evidence that a deficiency in the NER gene XPA leads to enhanced tumorigenesis in the lung after exposure to B[a]P.

In vivo detection of ultraviolet photoproducts and their repair in purkinje cells

We previously developed a highly sensitive method to assess in situ repair kinetics of ultraviolet (UV)-induced DNA photoproducts in epidermal cells using monoclonal antibodies specific for cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4) photoproducts (64PPs) by immunohistochemistry. In order to determine whether nucleotide excision repair capacity is operative in postmitotic mature neurons, brain surfaces of adult mice were exposed to UVB, and induction and removal of CPDs and 64PPs in Purkinje cell DNA were assessed immunohistochemically. UVB penetrated brain tissue to a depth sufficient to allow quantitative study. CPDs but not 64PPs were clearly detectable in the nuclei of Purkinje cells at doses >500 J/m2, in a dose-dependent manner. A time course experiment showed a statistically significant decrease of CPDs with time after irradiation. Although there was no apparent removal on Day 1, about half of CPDs were removed within 5 days, and the repair was essentially completed by Day 10. We conclude that non-dividing cerebellar neuronal cells can indeed repair UV-induced DNA damage, but with relatively low efficiency as compared with dividing epidermal cells.

Eel WT1 sequence and expression in spontaneous nephroblastomas in Japanese eel

Nephroblastomas spontaneously developing in Japanese eel reared at farms for 5 to 9months after collection from the wild [Masahito et al., Cancer Res., 52 (1992) 2575-2579] were investigated to cast light on the role of Wilms' tumor 1 gene (WT1) in eel kidney tumorigenesis. Cloning of the WT1 counterpart, EWT1, revealed that conservation of an alternative splice II site, located between the third and fourth zinc fingers, was conserved. The zinc finger domain was highly conserved. The transregulator region, sequences corresponding to exons 4 and 5 in WT1, were lacking in EWT1 cDNA. EWT1 was found to be expressed in kidney, testis and spleen and in situ hybridization revealed dark-stained immature cells in elver kidney to be positive. Although no EWT1 gene mutations were found in 38 eel nephroblastomas, 26 polymorphic nucleic acid changes were observed. Aberrant WT1 expression was noted in epithelial (12 out of 27; 44%) and nephroblastic cell histological types (three out of five; 60%) of eel nephroblastomas. On in situ hybridization the EWT1 expressive cells resembled human blastema cells, similar to those in human Wilms' tumor. These data demonstrated strong signals that the EWT1 protein may function in the development of eel kidney and play a role in genesis of nephroblastomas as in mammals.

Benzo[a]pyrene carcinogenicity is lost in mice lacking the aryl hydrocarbon receptor

The contribution of the aryl hydrocarbon receptor (AhR) in induction of a battery of xenobiotic-metabolizing enzymes has been studied extensively. However, no direct proof has been obtained that it plays a role in modulating carcinogenesis. To address the question of whether AhR is required for tumor induction, we have investigated the response of AhR-deficient mice to benzo[a]pyrene (B[a]P), a widely distributed environmental carcinogen. B[a]P treatment induced expression of the cytochrome P450 gene Cyp1a1 in the skin and liver of AhR-positive mice bearing +/+ and +/- genotypes and did not induce expression of the cytochrome P450 gene Cyp1a1 in AhR-null mice in either skin or liver. In contrast, Cyp1a2 gene expression was positive in liver irrespective of the presence or absence of the AhR gene, or B[a]P treatment, although its inducibility was lost in the AhR(-/-) mouse. All AhR-positive male mice of both +/+ and +/- genotypes that received subcutaneous injection of B[a]P (2 mg) on the first and the eighth days had developed subcutaneous tumors at the site of injection at the end of the 18-week experiment. In contrast, no tumors were apparent in any of the AhR-deficient mice. Likewise, topical application of B[a]P (200 microg) at weekly intervals to the skin of female mice for 25 weeks produced skin tumors only in the AhR-positive mice. Thus the carcinogenic action of B[a]P may be determined primarily by AhR, a transcriptional regulator of the gene for CYP1A1. The results of the present study provide direct evidence that AhR is involved in carcinogenesis.

Strain- and age-dependent loss of sarcoglycan complex in cardiomyopathic hamster hearts and its re-expression by delta-sarcoglycan gene transfer in vivo

The delta-sarcoglycan (SG) gene is deleted in hamsters with hereditary cardiomyopathies. Immunological analyses of heart before, but not after, the progression of cardiomyopathy (CM) revealed that the BIO 14.6 strain, a model of hypertrophic CM, heterogeneously preserved alpha- and gamma-SG with loss of beta- and delta-SG. In contrast, the TO-2 strain, a model of dilated CM, did not show either SG. Furthermore, in vivo transfer of the full length delta-SG gene to TO-2 hearts expressed all four SGs. Thus, this age- and strain-dependent features suggest a more feasible setting for TO-2 than BIO 14.6 to verify both CM progression and the efficacy of gene therapy.

Inhibition of skin cancer by IP6 in vivo: initiation-promotion model

A two-stage mouse skin carcinogenesis model was used to examine the effects of IP6 on initiation and promotion phases of tumorigenesis. Seven week old ICR female mice were divided into 6 groups, each consisting of 20 animals. Initiation was performed by a single application of the carcinogen 7,12-dimethyl benz(a)anthracene (DMBA) (50 micrograms) to the back skin. Three weeks later, local application of the promoter TPA was started (2.5 micrograms, 2 x/week) and continued up to the end of the experiment (22 weeks). Mice were also administered 2% IP6 in drinking water over the entire duration of the experiment, or during the initiation (initial 3 weeks) or promotion (final 19 weeks) periods only. The animals consuming IP6 during the initiation stage showed an approximately 50% reduction in the mean number of papillomas per animal, as well as in the number of tumor bearing mice. However, no such inhibition was observed when IP6 was given during the tumor promotion stage. In a separate experiment the effects of IP6 on epithelial cell growth were assessed by BrdU labeling at several time points. Statistically significant inhibition of cell proliferation was observed during the initiation stage (one week after DMBA treatment) in the group given IP6. No inhibition was evident during the promotion stage.

Precise identification of gene products in hearts after in vivo gene transfection, using Sendai virus-coated proteoliposomes

Both efficient gene transfer and the exact identification of gene product are required for gene therapy. Gene transfection of green fluorescence protein (GFP) might be useful for the reporter. After in vivo cotransfection of GFP and beta-galactosidase (beta-Gal) genes in Sendai virus-coated proteoliposomes to rat hearts, we compared the sensitivity and specificity of three methods: GFP detection, histochemical staining (HC) of beta-Gal activity, and immunostaining (IS) of the beta-Gal protein. Fluorescence microscopy and double staining of HC and IS revealed that both GFP and IS were equally sensitive and fourfold superior to HC at the peak of gene expression. However, different from skeletal muscle, the GFP of transfected cardiomyocytes showed two demerits: the fluorescence quenching due to the intense staining of beta-Gal activity, and nonspecific autofluorescence from myocardium. Thus, specific IS would be so far the most reliable to identify the gene product in heart.

Loss of transcription factor IRF-1 affects tumor susceptibility in mice carrying the Ha-ras transgene or nullizygosity for p53

The transcription factor IRF-1 has been implicated in tumor suppression: IRF-1 suppresses cell transformation and mediates apoptosis in vitro. Here we show that the loss of IRF-1 alleles per se has no effect on spontaneous tumor development in the mouse but dramatically exacerbates previous tumor predispositions caused by the c-Ha-ras transgene or by nullizygosity for p53. Grossly altered tumor spectrum, as compared to p53-null mice, was also observed in mice lacking both IRF-1 and p53, and cells from these mice show significantly higher mutation rate. Our results suggest that IRF-1 is a new member of the tumor susceptibility genes.

cDNA cloning and developmental expression of the porcine homologue of WT1

Wilms' tumors occur most frequently in swines as sporadic tumors. To clarify the role of WT1 gene in the genesis of Wilms' tumors and genitourinary development, we have isolated the porcine homologue of the human WT1 gene (pWT1) and analyzed its expression in various organs including the kidney. The open reading frame of pWT1 cDNA was extremely homologous to the human counterpart: 94% identical at the nucleotide level and 98% at the polypeptide level. In particular, the zinc finger region was more than 97% similar to human WT1 gene at the nucleotide level and 100% at the polypeptide level. pWT1 mRNA was found to be expressed in new-born kidney, spleen, testis, and embryonic kidneys, suggesting a possible association of pWT1 with the development of the genitourinary system. In conclusion, the nucleotide sequence and expression patterns in organs of pWT1 were similar to those of human WT1. Therefore, swines could provide good models for analyzing the contributions of WT1 gene to genitourinary development and genesis of Wilms' tumors.

Separation of killing and tumorigenic effects of an alkylating agent in mice defective in two of the DNA repair genes

Alkylation of DNA at the O6-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. The enzyme O6-methylguanine-DNA methyltransferase repairs O6-methylguanine as well as a minor methylated base, O4-methylthymine, in DNA. Mouse lines deficient in the methyltransferase (MGMT) gene are hypersensitive to both the killing and to the tumorigenic effects of alkylating agents. We now show that these dual effects of an alkylating agent can be dissociated by introduction of an additional defect in mismatch repair. Mice with mutations in both alleles of the MGMT gene and one of the mismatch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) as are wild-type mice, in terms of survival, but do have numerous tumors after receiving MNU. In contrast to MGMT-/- MLH1(+/+) mice with decrease in size of the thymus and hypocellular bone marrow after MNU administration, no conspicuous change was found in MGMT-/- MLH1(-/-) mice treated in the same manner. Thus, killing and tumorigenic effects of an alkylating agent can be dissociated by preventing mismatch repair pathways.

p16INK4 gene mutations are relatively frequent in ampullary carcinomas

A high incidence of gene mutations or deletions of p16INK4, a cell cycle regulator which inhibits the activity of cyclin-dependent kinase 4/cyclin D complex and blocks the G1-to-S transition, has been reported in pancreato-biliary tract cancers. In order to investigate p16INK4 gene alterations in sporadic ampullary carcinomas, 17 sporadic ampullary carcinomas were examined. After histological diagnosis, DNA samples extracted separately from both cancerous and normal paraffin-embedded tissues were investigated. Loss of heterozygosity (LOH) was investigated utilizing 3 microsatellite markers on 9p21-22, and a mutational analysis was performed by cloning and sequencing. LOH was observed in 3 cases (17.6%) and somatic mutations with retention of heterozygosity were found in 7 cases (41.2%). Of note was that two mutations resulted in truncated incomplete proteins and one was a point mutation at the consensus site in the conserved ankyrin repeats, which would be crucial for function. Although two-hit inactivation was not evident in any of the mutation cases and further investigation would be needed to elucidate the role of altered p16INK4, these results suggest that the p16INK4 gene mutations are relatively frequent and its inactivation might be important in ampullary carcinogenesis.

Frequent somatic mutations of the APC and p53 genes in sporadic ampullary carcinomas

Although a close relation of somatic mutations of the adenomatous polyposis coli gene with ampullary carcinomas in familial adenomatous polyposis patients has been reported, the possible association with sporadic ampullary neoplasms has not been fully examined. We have therefore investigated loss of heterozygosity at the adenomatous polyposis coli locus and the mutational status of a portion of the adenomatous polyposis coli gene, including the mutation cluster region, in 17 ampullary carcinomas of non-familial adenomatous polyposis patients. Alteration of the adenomatous polyposis coli gene was found in 8 of 17 (47.1%) cases, as missense or insertion mutations, with or without loss of heterozygosity. Additional investigation of p53 (exons 5-8) and K-ras (codons 12 and 13) gene mutations revealed a striking mutational pattern of the p53 gene. Nine of the 17 cases demonstrated a total of 12 mutations, 6 clustered at codon 189 and 3 at codon 166. Furthermore, 5 of the 12 mutations were nonsense mutations. Regarding the K-ras gene, 4 of the 17 (23.5%) cases had mutations in codon 12, 3 of the 4 cases being derived from the intraduodenal bile duct. The findings indicate that alterations of the adenomatous polyposis coli and the p53 genes are relatively frequent in sporadic ampullary carcinomas. In particular, the clustering at specific p53 codons might offer an etiological clue to clarify ampullary carcinogenesis. Mutations of the K-ras gene, on the other hand, might be characteristic of intraduodenal bile duct origin.

High incidence of nitrosamine-induced tumorigenesis in mice lacking DNA repair methyltransferase

The enzyme O6-methylguanine-DNA methyltransferase repairs alkylation-induced DNA damage, O6-methylguanine and O4-methylthymine, the former being formed more frequently. Previously, by means of gene targeting, we generated mice in which alleles for methyltransferase were disrupted. We now use these mouse lines, which are totally deficient in methyltransferase activity, to examine protective effects of the enzyme against tumor formation. In gene-targeted female mice given an i.p. injection of 5 mg/kg of dimethylnitrosamine, a larger number of liver and lung tumors occurred, as compared with normal female mice treated in the same manner. In male mice given a lower dose of carcinogen, the difference between normal and gene-targeted mice was statistically insignificant although more tumors did form in the gene-targeted mice. Methyltransferase apparently afforded protection from nitrosamine-induced tumorigenesis.

Loss of p53 is an early event in induction of brain tumors in mice by transplacental carcinogen exposure

Experimental carcinogenesis studies using p53-deficient mice have suggested that loss of function of this tumor suppressor gene is generally not an early event but is rather related to tumor progression. However, the biological functions of p53 and the accumulating evidence of alteration in human tumors imply a possible role for loss of p53 in the initial stages of tumorigenesis. Ethylnitrosourea administration to p53-heterozygous pregnant mice resulted in rapid development of primary brain tumors, which are extremely rare in mice, in 70% of the p53-null offspring. Brain tumors also developed later in 4% of heterozygous mice, but they had lost the wild-type allele. Thus, loss of normal p53 gene expression is of direct significance to early events in brain tumorigenesis, and this tumor suppressor gene may protect embryos from DNA damage in the brain induced by transplacental carcinogen exposure.

Mutations of the Ki-ras, p53 and APC genes in adenocarcinomas of the human small intestine

In contrast to the origins of colorectal carcinomas, the mechanisms of carcinogenesis in the small intestine remain unclear. We therefore analyzed the mutational status of the Ki-ras, p53, and adenomatous polyposis coli (APC) genes in primary carcinomas of the small intestine and compared the mutation patterns with those established for colorectal cancers. DNA was extracted from 15 formalin-fixed, paraffin-embedded lesions. Codons 12, 13 and 61 of the Ki-ras gene, exons 5-8 of the p53 gene, and codons 1268-1569, which contain the mutation cluster region (MCR) of the APC gene, were amplified by means of PCR, subcloned and sequenced. Mutations of the Ki-ras and p53 genes were observed in 8 (53.3%) and 4 lesions (26.7%), respectively. The mutational frequency of the Ki-ras gene in the present series of small intestinal carcinomas was similar, while that of the p53 gene was slightly lower than the reported frequencies for colorectal carcinomas. Only one case showed a mutation of the APC gene, involving an insertional mutation of an adenine at codons 1554-1556 with formation of a stop codon immediately downstream. Since the occurrence of an APC mutation is considered an early event in colorectal carcinogenesis, our findings indicating an extremely low frequency of such changes in and around the MCR suggest that carcinomas of the small intestine arise via a genetic pathway distinct from that involved in the development of carcinomas of the colorectum.

Mutational analysis of the p53 and K-ras genes and allelotype study of the Rb-1 gene for investigating the pathogenesis of combined hapatocellular-cholangiocellular carcinomas

Because combined hepatocellular-cholangiocellular carcinoma is rare and its biological features and pathogenesis have not been well established, we investigated alterations of the p53, K-ras and Rb-1 genes, as well as expression patterns of carcinoembryonic antigen and keratin, in seven combined hepatocellular-cholangiocarcinomas out of 557 hepatocellular carcinomas autopsied at Tokyo University during 30 years. Mutations of the p53 gene were found in two cases, at codon 244 (GGC to TGC) in the cholangiocellular carcinoma component of case 1 (mixed type, showing an intimate intermingling of both elements) and at codon 234 (TAC to AAC) in both components of case 5 (combined type, consisting of contiguous but independent masses of both elements). Mutation of the K-ras gene (codon 12, GGT to GAT) was seen only in the cholangiocellular carcinoma component of clinically apparent double cancer, case 6. Allelic alteration of the Rb-1 gene was observed in two cases, deletion of both alleles in the hepatocellular carcinoma component of case 3 (combined type) and replication error of the same pattern in both components of case 4 (mixed type). Immunohistochemical analysis showed that the hepatocellular carcinoma components of five cases (cases 2, 3, 5, 6, 7) were immunoreactive for keratin, suggesting biliary epithelial transformation. In four of the five cases (cases 3 and 5 combined, case 7 mixed and case 6 double cancer), cholangiocellular carcinoma components were also positive for keratin. These results suggest that both components of combined hepatocellular-cholangiocarcinoma have the same genetic and phenotypic character and might have arisen from the same origin in some cases.

Somatic mutations of the APC gene in sporadic hepatoblastomas

Hepatoblastoma is a rare hepatic malignancy that occurs in children with an average age of 2 or 3 years and is known to be one of the extracolonic manifestations of familial adenomatous polyposis. Only a single hepatoblastoma with a germ-line mutation of the adenomatous polyposis coli (APC) gene has been reported thus far. To elucidate the possible roles of APC gene alterations in sporadic hepatoblastomas, we examined loss of heterozygosity (LOH) at the APC and MCC loci and performed a sequencing analysis of a part of the APC gene, including the mutation cluster region, in 13 hepatoblastomas of non-familial adenomatous polyposis patients. LOH at the APC and/or MCC loci was observed in four of seven (57%) informative cases. Of the 13 cases, somatic mutations were detected in 8 (61.5%), with 9 (69%) cases showing genetic alterations in the APC gene as LOH or somatic mutations. Two cases demonstrated double mutations. Furthermore, the nature of the somatic mutations observed in the present study was unusual because 9 of the 10 mutations were missense, with only 1 case featuring a frame-shift mutation due to an insertion. Previous reports have described almost all (>90%) mutations of the APC gene in colorectal tumors to result in a truncated APC protein due to either frame-shift or nonsense mutations. These findings suggest that a mutation of the APC gene may play an important role in the genesis of sporadic hepatoblastomas, and the mechanisms of APC gene alteration may be different from those reported previously for colorectal tumors.

Detection of ultraviolet photoproducts in mouse skin exposed to natural sunlight

In the present study, we for the first time investigated the formation of ultraviolet (UV) photoproducts, cyclobutane pyrimidine dimers (CPDs), pyrimidine-pyrimidone (6-4) photoproducts (64PPs) and Dewar isomers, in vivo in shaved and depilated C3H/HeN mouse skin exposed to natural sunlight (NSL) at noon for 5 min to 1 h in mid-summer, using a highly sensitive immunohistochemical method. This method permits the quantitative analysis of UV-photoproducts in formalin-fixed, paraffinembedded sections with specific antibodies against CPDs, 64PPs and Dewar isomers. We demonstrated that the induction of CPDs in vivo in mouse skin by NSL was exposure time-dependent, but the accumulation of 64PPs or Dewar isomers was comparatively low in the skin sections from mice exposed to NSL in vivo. The results indicate that CPDs are the main photoproducts in vivo induced by sunlight and that their formation and repair may be important in connection with carcinogenesis in sun-exposed areas of human skin.

Quantitative detection of ultraviolet light-induced photoproducts in mouse skin by immunohistochemistry

UVB-induced cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4)photoproducts [(6-4)photoproducts] in mouse skin DNA were quantitatively measured using an immunohistochemical approach with a computer-aided color image analyzer. The skins of the C3H/HeN mice were irradiated with ultraviolet B (UV-B, 280-320 nm), and processed to give conventional formalin-fixed, paraffin-embedded histologic sections. Routine immunohistochemistry clearly demonstrated a dose-dependent induction of both photoproducts. CPDs were detectable at doses > or = 125 J/m2, while for (6-4)photoproducts, the minimal dose at which they were detectable was 250 J/m2 in the present study. A time course study showed that the repair of (6-4)photoproducts was more rapid than that of CPDs, and that epidermal cells had a higher capacity for their removal than dermal cells. About half of the (6-4)photoproducts were excised within the first 24 h after the irradiation, and the process was essentially complete by 72 h. In contrast, there was no apparent removal (less than 10%) of CPDs in the first 24 h and they only completely disappeared from the epidermal cells at 120 h after irradiation. The effect of DNA dilution due to increased turnover of epidermal cells after UV-B irradiation was evaluated by quantitative immunohistochemical measurement of the time course of bromodeoxy-uridine (BrdUrd) incorporated into nuclei at 2 days post irradiation when the proliferation reaches a peak. The removal of photoproducts was more marked than the decrease in BrdUrd staining. Our results suggest that mouse skin cells can repair both (6-4)photoproducts and CPDs, but with considerably lower efficiency, especially in the latter case, then human or monkey skin cells.

Inhibitory effect of probucol on nephrotoxicity induced by ferric nitrilotriacetate (Fe-NTA) in rats

Effects of dietary probucol on renal damage induced by a renal carcinogen, ferric nitrilotriacetate (Fe-NTA), in male Wistar rats were quantitatively investigated with a computer-mediated image analyzer. The kidneys of rats fed a 1% probucol diet were protected from necrosis and lipid peroxidation induced by a single i.p. treatment with Fe-NTA solution at 5 mg Fe/kg body wt and were significantly resistant to a higher dose. For the parameter of lipid peroxidation, Schiff's staining method was utilized to demonstrate the extent of formation of aldehydes, products of lipid peroxidation. Thus following injection of Fe-NTA solution at 10 mg Fe/kg body wt the average areas of tubular necrosis were 85.8% versus 56.9% and the positive areas for Schiff's staining were 15.3% versus 5.6% in the renal cortices of rats fed control of 1% probucol diets respectively. These results indicate that probucol provides protection against Fe-NTA-induced nephrotoxicity through its antioxidant properties. In addition to being a cholesterol-lowering drug, useful for the control of hypercholesterolemia, probucol may therefore be beneficial for prevention and treatment of various pathogenic processes including those leading to carcinogenesis.

A polymorphism at codon 160 of human O6-methylguanine-DNA methyltransferase gene in young patients with adult type cancers and functional assay

O6-methylguanine-DNA methyltransferase (MGMT) plays an important role in repair of alkylating agent-induced DNA damage. Among the alkylation products of DNA, O6-methylguanine is one of the most critical lesions leading to the induction of mutations. The enzyme MGMT transfers the methyl group from O6-methylguanine of DNA to its own cysteine residue. Although mutations of other DNA repair genes involved in nucleotide excision repair and mismatch repair have been proven to be related to human tumorigenesis, the question of whether MGMT gene mutation might play a role in human carcinogenesis has hitherto not been elucidated. If there is a population with decreased enzyme activity due to defective MGMT gene, the affected individuals should be at risk of developing cancer early in life because of an increased susceptibility to alkylating agents. To test this hypothesis, germ line mutations of the MGMT gene were investigated in 12 young patients with adult type cancers (mean, 16.7 years old, 8 hepatocellular carcinomas, 3 gastric cancers, 1 cholangiocellular carcinoma) and 28 elderly patients who died of non-cancer diseases as controls (mean, 66 years old). A point mutation at codon 160 in exon 5, GGA to AGA, converting glycine to arginine was found in three of the young patients (25%), while the same mutation was found in three out of 28 (10.7%) in the control group. The mutated codon was located 15 codons from a functional cysteine residue toward the carboxyl terminal. Investigation of enzyme function, even in cases of bi-allelic mutation, revealed comparable activities for both mutated and wild type MGMT. Thus, we conclude the mutation is a normal polymorphism of the MGMT gene, present in approximately 15% of the population, although this does not rule out a possible influence in other tissues.

Inhibitory effect of probucol on benzo[a]pyrene induced lung tumorigenesis

The effects of probucol, a clinically used cholesterol lowering and antioxidant drug, on benzo[a]pyrene (B[a]P) induced pulmonary and forestomach tumorigenesis as well as induction of colonic aberrant crypt foci (ACF) in female A/J mice was investigated. Diet containing 1% probucol fed prior to, during and after 8 bi-weekly 1 mg/mouse oral intubations of B[a]P, reduced the number of pulmonary adenomas by 52% (P < 0.001) and the number of forestomach tumors by 31%. The 0.06% probucol diet also resulted in decreased tumor formation but the differences did not reach statistical significance. Both probucol diets significantly reduced the numbers of large ACF, putative preneoplastic lesions of the colon mucosa, but showed no effects on the total numbers of ACF. The results of this study suggest that probucol may also be useful as a chemopreventive agent, in addition to being a cholesterol lowering and anti-atherogenic drug with low toxicity.

High incidence of ultraviolet-B-or chemical-carcinogen-induced skin tumours in mice lacking the xeroderma pigmentosum group A gene

Xeroderma pigmentosum (XP) is an autosomal recessive disorder characterized by a high frequency of skin cancer on sun-exposed areas, and neurological complications. XP has a defect in the early step(s) of nucleotide-excision repair (NER) and consists of eight different genetic complementation groups (groups A-G and a variant). We established XPA (group-A XP) gene-deficient mice by gene targeting of mouse embryonic stem (ES) cells. The XPA-deficient mice showed neither obvious physical abnormalities nor pathological alterations, but were defective in NER and highly susceptible to ultraviolet-B- or 9,10-dimethyl-1,2-benz[a]anthracene-induced skin carcinogenesis. These findings provide in vivo evidence that the XPA protein protects mice from carcinogenesis initiated by ultraviolet or chemical carcinogen. The XPA-deficient mice may provide a good in vivo model to study the high incidence of skin carcinogenesis in group A XP patients.

A mutational hot spot in the p53 gene is associated with hepatoblastomas

Hepatoblastomas generally appear in children aged 2 or 3 years old and arise from apparently normal, non-cirrhotic liver. To elucidate any possible role of p53 mutations in their genesis, we amplified and sequenced exons 5 to 8 of the p53 gene in 10 cases of hepatoblastoma. Somatic mutations were detected in 9 cases, in eight of which a common point mutation at the first-base position of codon 157 was found, resulting in an amino-acid substitution of phenylalanine for valine. Two missense mutations in codon 244, and one each in codons 273 and 279, were also found, with 3 hepatoblastomas having double mis-sense mutations. Out of the total of 12 mutations, 11 were G-to-T transversions. One was a G-to-A transition and guanines were always present on the transcribed strand. Furthermore, p53 over-expression was immunohistochemically observed in 7 out of 9 cases with p53 gene mutations, although the staining pattern was focal and heterogeneous. The findings suggest that particular environmental mutagens may be involved in mutagenesis of the p53 gene in some cases of hepatoblastomas and that p53 mutations at a specific site may play an important role in the genesis of this disease.

Mutations of the p53 gene and p53 protein overexpression are associated with sarcomatoid transformation in renal cell carcinomas

Renal cell carcinomas sometimes show sarcomatoid transformation, thus comprising both sarcomatous and carcinomatous components. Such sarcomatoid renal cell carcinomas are highly malignant with pronounced proliferative activity. The present investigation was conducted to assess the mutational status of the p53 and H-ras genes independently in carcinomatous and sarcomatous portions of individual tumors, applying PCR, subcloning, and sequencing to 14 cases. Sarcomatoid portions showed an extremely high mutation rate for the 53 gene (11 of 14, 78.6%) with two mutational hot spots at codons 278 (8 of 14, 57.1%) and 244 (6 of 14, 42.9%). Five cases showed double mutations, four cases had mutations at codons 278 and 244, and one case had mutations at codons 278 and 248. In contrast, the carcinomatous portions demonstrated a low mutation rate for the p53 gene (2 of 14, 14.3%) and no double mutations were detected. Ten cases showed genetic heterogeneity in the p53 gene between the two tumor components. Furthermore, p53 overexpression was immunohistochemically observed only in those components with p53 mutations, mainly in the sarcomatoid portions. No H-ras mutations were observed. The findings strongly suggest that p53 mutations leading to overexpression of p53 protein are closely associated with sarcomatoid transformation in renal cell carcinomas.

Detection of active UV-photoproduct repair in monkey skin in vivo by quantitative immunohistochemistry

Ultraviolet-induced cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4)photoproducts in DNA were quantitatively measured in monkey skin using an immunohistochemical method with two specific monoclonal antibodies. The skins of Cynomolgus monkeys (Macaca fascicularis) were irradiated with UV light and processed for preparation of conventional formalin-fixed, paraffin-embedded histological sections. Both of the photoproducts were detectable in the nuclei of epidermal cells at doses of 500 J/m2 for UVB and 50 J/m2 for UVC, respectively, nuclear staining being clearly dose-dependent. Time course studies also showed a statistically significant decrease in nuclear staining with time after exposure to either UVB or UVC irradiation. Although only 30% of CPDs were removed from DNA in the first 24 h, about half of the (6-4) photoproducts were repaired within 3 h post-UV irradiation. Staining completely disappeared by 48 h in the (6-4) photoproduct case and by 72 h in the case of CPDs. The results suggest that epidermal cells of monkey skin can efficiently repair UV-photoproducts in DNA, but that the capacity is slightly less than in man.

DNA adduct formation and assessment of aberrant crypt foci in vivo in the rat colon mucosa after treatment with N-methyl-N-nitrosourea

N-Nitroso-compound DNA adduct formation in vivo and occurrence of aberrant crypt foci (ACF) were studied in the rat colon mucosa after a single, local treatment with a carcinogen, N-methyl-N-nitrosourea (MNU), using a simple surgical approach. A segment of F344 rat colon was ligated to make a pouch and injected with MNU solution. For the study of DNA adduct formation, the solution contained 50 microCi of [3H]MNU. The results demonstrated that similar ranges of carcinogen dose, i.e. 0.15 x 10(-2) - 1.5 x 10(-2) M MNU, could induce both DNA adduct formation and appearance of ACF in the rat colon with both parameters showing a nearly linear dose dependence. HPLC analysis revealed the DNA adducts to include both 7-methylguanine (7-mGua) and O6-methylguanine (O6-mGua) with the 7-mGua/O6-mGua ratio being 8.2-11.3:1 in the system used. Assessment of ACF development from 4 to 16 weeks after MNU treatment at a dose of 7.5 x 10(-2) M showed the numbers to increase up to the 8th week, followed by a decrease at weeks 12 and 16, when 40% of the ACF counted at the peak time point were still present. The percentage of large ACF (> or = 4 crypts/ACF) significantly increased with time. These results indicate a clear relation between DNA adducts and preneoplastic lesions, i.e. ACF. In conclusion, DNA adduct formation and ACF can be efficiently and simply detected in vivo by using the method described in the present paper.

O6-methylguanine-DNA methyltransferase activity in the human lung persists with advancing age

O6-methylguanine-DNA methyltransferase (MGMT) plays an important role in the repair of DNA lesions induced by alkylating carcinogens in a wide range of animals. To determine the relationship between DNA repair activity and tumor susceptibility with advancing age the activity of MGMT was measured in normal lung tissue extracts from 66 patients with primary or metastatic lung cancer obtained at surgery. The age of the patients whose lung MGMT activity was measured ranged from 40 to 79 years (males) and from 40 to 80 years (females), and the values varied over 10-fold in both sexes (33 males and 33 females). Our results indicate that the MGMT enzyme protein is expressed at appreciable levels throughout life, and on age-associated rapid decline is not a feature in the human lung. Thus, any age-associated increase in lung cancer incidence cannot be ascribed to any change in MGMT activity. In order to assess other possible factors which might exert a modulating influence we examined the MGMT activity in relation to sex, tumor type and smoking. However, in none of these cases was any significant correlation evident.

O6-methylguanine-DNA methyltransferase protects against nitrosamine-induced hepatocarcinogenesis

We previously generated transgenic C3H/HeN mice by introducing the Escherichia coli O6-methylguanine-DNA methyltransferase (MGMT, DNA-O6-methylguanine:protein-L-cysteine S-methyltransferase, EC2.1.1.63) gene, ada, attached to the Chinese hamster metallothionein I gene promoter. One transgenic mouse line expressing both ada-specific mRNA and Ada protein could be propagated over many generations in a homozygous state with respect to the integrated DNA. Liver extracts from transgenic homozygous mice have consistently demonstrated about 3 times the control activity of normal mice. Furthermore, in the transgenic homozygotes treated with ZnSO4, activity is increased to 6-8 times the normal level in mice and is equivalent to that for man. To examine whether these increased levels of MGMT activity can actually decrease the susceptibility of animals to N-nitroso compounds, we studied liver carcinogenesis in our transgenic mice expressing high amounts of MGMT. Groups of transgenic and nontransgenic mice, each comprising about 200 suckling animals (14 +/- 1 days old), were divided each into eight subgroups, providing paired groups of transgenic and nontransgenic mice. They received an i.p. injection of ZnSO4 to induce MGMT, and 10 hr thereafter were given an i.p. injection of either dimethylnitrosamine or diethylnitrosamine. Liver tumor development was quantitatively assessed at 7-11 months. Here, we report statistically significant reduction of tumor formation in transgenic mice of four of the six paired groups that received treatment. The remaining two demonstrated results in line with dose dependence. Therefore, our data indicate that MGMT can indeed protect animals from low-dose exposure to environmental alkylating carcinogens.

Tumor promotion by fecapentaene-12 in a rat colon carcinogenesis model

Fecapentaenes are a group of fecal mutagens produced by anaerobic microflora of the colon. The potential of fecapentaene-12 (FP-12) to promote tumor development was tested in a rat colon carcinogenesis model using N-methyl-N-nitrosourea (MNU) as the initiating agent. Two groups of female F-344 rats were initiated by intrarectal instillations of MNU (2 mg in 0.5 ml H2O, 3 times a week, for 3 weeks; MNU and MNU + FP-12 groups). Two additional groups (FP-12 and Control) were given H2O without carcinogen. In the post-initiation phase, rats of the MNU + FP-12 and FP-12 groups were intrarectally administered 400 ng of FP-12 in 0.5 ml T-E buffer, twice a week, for 24 weeks, whereas the MNU and Control groups received the vehicle only. Tumors were found only in the MNU and MNU + FP-12 groups, their number being higher in the latter. The number of carcinoma bearing rats as well as the average number of carcinomas per rat were significantly higher (P < 0.05) in the MNU + FP-12 group as compared to the MNU-alone values. Aberrant crypt foci (ACF) were found in all carcinogen-treated rats, including those that did not contain tumors, whereas none were observed in the FP-12 and Control groups. The average number of ACF/cm2 was also significantly higher in the MNU+FP-12 group, as was the case for the average number of ACF containing > 10 aberrant crypts per focus. These findings suggest that FP-12 can express promoting activity in chemical induced colon carcinogenesis.

Age dependence of O6-methylguanine-DNA methyltransferase activity and its depletion after carcinogen treatment in the teleost medaka (Oryzias latipes)

O6-Methylguanine-DNA methyltransferase (O6-MT) is considered to play an important role in the repair of DNA lesions induced by alkylating carcinogens in a wide range of animals. The activity of O6-MT was compared in liver extracts from the teleost medaka (Oryzias latipes) at various ages (3-5 years old) reared under natural conditions. O6-MT activity decreased significantly with advancing age. When medaka were exposed continuously to the alkylating agent methylazoxymethanol (MAM) acetate at levels of 0.1, 0.15 and 0.3 ppm in water, O6-MT activity was markedly reduced from days 1 to 7, with a slight increase thereafter. Furthermore, when fish were exposed to MAM acetate at levels of 1-2 ppm for 1 h and then maintained in normal tap water, O6-MT activity remained suppressed for 2 weeks, followed by a partial recovery.

Characterization of O6-methylguanine-DNA methyltransferase in transgenic mice introduced with the E. coli ada gene

The characteristics of O6-methylguanine-DNA methyltransferase (O6-MTase) produced in transgenic mice, in which the introduced E. coli ada gene was expressed under the control of the metallothionein promoter, were investigated. Liver extracts from transgenic homozygotes showed approximately 3 times the control activity, a marked increase of up to about 8 times the non-transgenic control levels being observed 10 h after zinc treatment. Examination of the substrate specificity of the enzyme revealed that the activity in the transgenic mice is due to the introduced foreign gene. The enzyme possessed methylphosphotriester-DNA methyltransferase as well as O6-MTase, characteristic of the E. coli Ada protein. Comparison of differences in biological response between transgenic and non-transgenic mice after treatment with the alkylating carcinogen methylnitrosourea (MNU) at various doses revealed transgenic mice to be more capable of repairing O6-MTase activity, only showing signs of exhaustion at very high levels of exposure. In non-transgenic mice, on the other hand, the basal level of O6-MTase was low, and the activity was hardly detectable when the animals were treated with MNU.

Immunological detection of in vivo aflatoxin B1-DNA adduct formation in rats, rainbow trout and coho salmon

Immunological detection of carcinogen-DNA adducts in organs or tissues should prove a particularly useful approach for monitoring carcinogen exposure, for characterization of carcinogen binding to DNA and for investigating DNA repair processes in vivo. In one of a series of experiments aimed at raising antibodies against several carcinogen-modified DNAs, rabbits were immunized in our laboratory with aflatoxin B1 (AFB1)-modified DNA. After the titer and specificity of the antibodies produced were checked against standards by the enzyme-linked immunosorbent assay (ELISA) method, they were used to investigate DNA extracted from livers of rats and rainbow trout (Salmo mykiss) injected with tritium-labeled AFB1 at doses of 1-2 mg/kg (rats) or 0.1-0.5 mg/kg (rainbow trout). Adduct levels were compared using both radioactivity and ELISA methods. Positive DNA binding could be detected in both rats and rainbow trout by the immunological method at similar levels to those estimated with the radioactive analysis. To throw light on possible mechanisms underlying the wide variation in response to aflatoxins among salmonid fish, DNA extracted from the livers of rainbow trout (susceptible species) and coho salmon (Oncorhynchus kisutch) (less susceptible species) were compared following AFB1 treatment. A high rate of DNA binding was observed in rainbow trout, whereas significantly lower values were evident in coho salmon, suggesting a direct relationship between binding levels and susceptibility to mycotoxin carcinogenicity.

Establishment of highly metastatic variants of murine colon adenocarcinoma 26 under serum-free conditions

Tumor cell variants were established in vitro by treating a metastatic variant of murine colon adenocarcinoma 26 (P-26-select) in serum-free RPMI 1640 medium for 6 to 10 weeks. Five tumor cell lines were established from independent cultures and designated PS-1 to PS-5. The PS cells showed higher growth potential in vitro under serum-free or low-serum conditions than the parental P-26-select. In comparison to P-26-select, the PS cell lines possessed enhanced lung-colonizing ability after i.v. inoculation and metastasized spontaneously to the lung following s.c. inoculation into the flank or the right fore-footpads of BALB/c mice. Metastatic nodules were also observed in the liver. Spleens and livers of the tumor-bearing mice were enlarged mainly as a result of hematopoiesis, suggesting the production of CSF-like substance(s) by the tumor cells. The PS cells secreted increased amounts of putative autocrine growth factor(s) and CSF-like substance(s) in vitro. These characteristics might be related to the in vivo metastatic ability of the PS cells.

Species and organ differences in DNA adduct formation and repair after treatment with 4-hydroxyaminoquinoline 1-oxide

4-Hydroxyaminoquinoline 1-oxide (4HAQO) demonstrates obvious organotropic and species specificity in its carcinogenesis and the present investigation concerns 4HAQO DNA adduct formation and repair as studied in various organs of four animal species (rats, mice, guinea pigs and hamsters). Three hours after an iv injection of 10 mg per kg body weight of tritium-labeled 4HAQO, the major organs were removed and used for assessment of label incorporation in the DNA. The results showed that the DNA binding levels generally correlated well with the reported species and organ specificity of 4HAQO tumorigenesis. For example, rats showed highest DNA binding in the pancreas and kidney, major target organs. The levels of DNA binding in the liver were invariably low in all 4 animal species, in agreement with the lack of hepatocarcinogenicity associated with 4HAQO exposure. A clear relationship between DNA adduct formation and carcinogen dose was also found after treatment of mice with 4HAQO at doses of 1, 5, 10 and 20 mg per kg body weight in all tissues (pancreas, kidney and lung) except for the liver. Comparison of DNA repair processes in rats, a highly susceptible species, and hamsters, a resistant species in terms of 4HAQO carcinogenicity, revealed highest formation and slowest removal of adducts in the target organs of the rat. In the hamster organs and the rat lung and liver, DNA adduct formation was generally low and in the case of elevation in the initial phase, quickly removed.

DNA adduct formation and unscheduled DNA synthesis in rat esophagus in vivo after treatment with N-methyl-N-nitrosourea

An in vivo method for assessment of DNA adduct formation and unscheduled DNA synthesis (UDS) in the esophagus of rats was devised. Small ventral incisions were made in the neck and upper abdomen regions of 6 week old F344 rats and ligation of the esophagus with thread at the two extreme ends performed to make an esophageal pouch. For the DNA adduct formation study, a solution (0.5 ml) containing various concentrations of N-[3H]methyl-N-nitrosourea ([3H]MNU) was injected into the pouch. DNA binding levels were calculated from radioactivity of the isolated DNA and dose-dependent DNA adduct formation could be detected 2 h after the treatment with MNU. By HPLC analysis, both 7-methylguanine (7-mGua) and O6-methylguanine (O6-mGua) adducts were identified in the esophageal DNA, the ratio of 7-mGua/O6-mGua being 5.7-12:1. For UDS measurement, a solution containing MNU plus [3H]thymidine (200 microCi/ml) was similarly injected into the pouch. UDS was dose-dependently demonstrated as silver grains over the nuclei of the epithelial cells by autoradiography. The results thus showed that MNU, when injected into the esophageal lumen, can penetrate the surface mucosa, react with the epithelial cell DNA and induce DNA adduct formation and DNA repair synthesis dose-dependently.

Enhanced O6-methylguanine-DNA methyltransferase activity in transgenic mice containing an integrated E. coli ada repair gene

The E. coli ada gene encodes O6-methylguanine DNA methyltransferase (O6MTase) which repairs the methylation of guanine at the O6 position in DNA. After recombination with a Chinese hamster metallothionein I gene promoter, the ada gene was microinjected into C3H/HeN mouse zygotes. Eventually, transgenic mice containing the ada fusion DNA were generated. The integrated ada DNA complex was transmitted to the progeny in a mode conforming to tandem integration at a single chromosome site, and homozygotes were also obtained from an inter-transgenic mouse cross. RNA transcripts of the chimeric ada gene were identified in the livers of these transgenic mice using dot and Northern blot analyses. O6MTase activity was increased in the liver of transgenic mice of line No. 708, and was more than 3 times the activity found in non-transgenic mice, especially in the transgenic homozygotes. The ada gene product was detected in the liver of a transgenic homozygote by immunoblot analysis. These transgenic mice have great potential for analysis of the role played by O6MTase in chemical carcinogenesis.

Immunohistochemical detection of carcinogen-DNA adducts and DNA repair in mouse skin

4-Hydroxyaminoquinoline 1-oxide (4HAQO) and (+/-)-trans-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BP-DE)-DNA adducts were immunohistochemically demonstrated in the nuclei of mouse skin using antibodies directed against carcinogen (4HAQO or BP) modified DNA. The specificity of the immunostaining was confirmed by several tests, including preincubation of the antibody with carcinogen modified DNA or related molecules, and digestion of the sections with DNase. Subcutaneous injection of 4HAQO dissolved in isotonic solution into an isolated portion of the mouse skin clamped off with ring-shaped forceps resulted in dose-dependent generation of DNA adducts in the nuclei of epithelial cells, fibroblasts, and panniculus carnosus cells. BP-DNA adducts could also be similarly detected dose-dependently in the nuclei of skin cells after local application of BP-DE. Nuclear staining was absent in animals injected with isotonic solution alone, and the intensity of staining correlated well with the level of unscheduled DNA synthesis (UDS) demonstrated autoradiographically after treatment with 4HAQO. Killing of mice at different time points after a single injection of 4HAQO revealed a gradual decrease in the intensity of the staining. Thus the postulated generation and repair of DNA adducts can be followed at the cellular level using the presently described method.