Loss of heterozygosity at chromosome 1q loci in rat mammary tumors

Absence of Ras mutations in rat DMBA-induced mammary tumors

Animal cancer models reduce genetic background heterogeneity and thus, may facilitate identification and analysis of specific genetic aberrations in tumor cells. Rat and human mammary glands have high similarity in physiology and show comparable hormone responsiveness. Thus, spontaneous and carcinogen (e.g., NMU and DMBA)-induced rat mammary models are valuable tools for genetic studies of breast cancer. In NMU-induced rat mammary tumors, activating mutations in Hras codon 12 have frequently been reported and are supposed to contribute to the mammary carcinogenic process. Involvement of Ras mutations in DMBA-induced tumors is less clear. In the present study we investigated the mutation status of the three Ras genes, Hras, Kras, and Nras, in DMBA-induced rat mammary tumors. We examined codons 12, 13, and 61 of all three genes for mutations in 71 tumors using direct sequencing method that in experimental conditions is sensitive enough to detect single nucleotide mutations even when present in only 25% of the test sample. No activating Ras gene mutation was found. Thus, in contrast to NMU-induced rat mammary tumor, tumorigenesis in DMBA-induced rat mammary tumors seems to be independent on activating mutations in the Ras genes. Our finding suggests that the genetic pathways selected in mammary tumor development are influenced by and perhaps dependent on the identity of the inducing agent, again emphasizing the importance of tumor etiology on the genetic changes in the tumor cells.

Genetic predisposition to 4NQO-induced tongue carcinogenesis in the rat

OBJECTIVE

This study aims to elucidate the genetic basis of predisposition to 4-nitroquinoline 1-oxide (4NQO)-induced tongue cancers (TCs).

MATERIALS AND METHODS

We have reported that inbred Dark-Agouti (DA) strain rats were highly susceptible to 4NQO-induced TCs, whereas Wistar/Furth (WF) rats were resistant to tongue squamous cell carcinomas induced by oral administration of 4NQO. Using size and number of the tumours as quantitative parameters, responsible host loci were analysed by an interval mapping of F2 intercross of DA and WF given carcinogenic regimen. Also, loss of heterozygosity (LOH) at these loci was analysed in tongue cancers in (DA x WF) F1.

RESULTS

We identified and mapped 5 significant quantitative trait loci (QTL), the Tongue squamous cell carcinoma 1-5 (Tscc1-5), and several other suggestive QTL that determine susceptibility to 4NQO-induced TC. Study of TCs induced in (DA x WF)F1 rats revealed a high frequency of LOH in the chromosomal regions of Tscc2, 3, and 4 and also of suggestive QTL on chromosomes 5 and 6. The fact that LOH was found only in larger TCs indicates that LOH occurred in the process of tumour progression. In most LOH, the allele of the resistant WF strain was lost, suggesting that these loci may encode tumour suppressor genes. In larger TCs, in addition to LOH, point mutations and the methylation of possible candidate genes were accumulated.

CONCLUSION

These observations indicate that the 4NQO-induced TC in the rat is a multifactorial disease of a polygenic trait. This model will be useful to understand the complicated genetic basis of predisposition to oral cancers.

Recurrent allelic imbalance at the rat Pten locus in DMBA-induced fibrosarcomas

The tumor-suppressor gene PTEN (phosphatase and tensin homolog) is frequently inactivated in different types of human tumors. Less is known about the involvement of the homologous gene Pten in animal model systems of cancer. By sequencing one of the introns of rat Pten, we found an informative intragenic PCR marker suitable for genetic studies. Through use of this marker, the position of Pten in the genetic linkage map was localized to the distal part of rat chromosome 1 (RNO1) by analysis of F2 progeny from an intercross between inbred strains BN and LE. Subsequently, 22 markers from this region (including the intragenic Pten marker) were used to study the occurrence of allelic imbalance in distal RNO1 in fibrosarcomas that had been induced by DMBA in F1(BNxLE) rats. The analysis revealed that allelic imbalance was common in the vicinity of Pten, and there was loss or reduction of one of the Pten alleles in more than 60% of the fibrosarcomas. DNA sequencing was preformed to investigate whether the Pten allele remaining in the tumors was inactivated by mutation. However, no mutations were detected in the genomic sequence of Pten exons 5 to 9 in any of the fibrosarcomas, and normal mRNA transcripts were expressed in all tumors. Thus, based on the targeted selection for loss of Pten observed in some of these tumors and the absence of inactivation of the remaining allele, we suggest that haploinsufficiency of Pten may be an important factor in rat DMBA-induced fibrosarcomas.

Carcinogenesis modifier loci in rat tongue are subject to frequent loss of heterozygosity

Rats of the DA strain are highly susceptible to 4NQO-induced TCs, whereas WF rats are barely susceptible. In (DA x WF)F2 rats, 5 QTL, Tscc1-5, are responsible for most of the phenotypic variations, though they do not account for all of the phenotypic differences between WF and DA rats. Analysis of 40 tongue tumors >5 mm in diameter from (DA x WF)F1 rats for LOH at the Tscc loci revealed a high frequency of LOH in chromosomal regions where the Tscc2, -3 and -4 loci map. In most cases of LOH, the allele of the barely susceptible WF strain was lost, suggesting that these loci in the WF strain encode tumor-suppressor genes. Analysis of the same tumors for somatic mutations in oncogenes indicated frequent alteration of Ha-ras, which maps in the Tscc3 region, but rare mutation of the p15(INK4B) and p16(INK4A) genes or the p53 and Msh2 genes. Frequent LOH was also found on rat chromosomes 5 (RNO5) and 6 (RNO6). Tumors of large size accumulated LOH at multiple loci, suggesting the involvement of Tscc loci in tumor progression.

Five quantitative trait loci affecting 4-nitroquinoline 1-oxide-induced tongue cancer in the rat

In our previous study, Dark-Agouti (DA) rats were found to be highly susceptible to 4-nitroquinoline 1-oxide (4NQO)-induced tongue carcinoma (TC), whereas Wistar / Furth (WF) rats were barely susceptible. Interval mapping analysis of reciprocal backcross rats showed two quantitative trait loci (QTL) on rat chromosomes (RNO) 1 and 19. In the present study, a composite interval mapping analysis was applied to 4NQO-induced TC in 130 (DA x WF) F2 rats, demonstrating five independent QTL, Tongue squamous cell carcinoma 1 - 5 (Tscc1 - 5), responsible for phenotypic differences in the size and number of TCs in the two strains. Two of these QTL were mapped on RNO1, and the others were mapped on RNO4, 14, and 19. The DA allele at these loci consistently yielded semidominant susceptibility to TC. Out of the five loci detected in this F2 generation, Tscc1 and 2 were identical to Stc1 and Rtc1 described in our previous study, but the other three were novel. We propose a new nomenclature consistent with their function. Genome-wide screening of the F2 progeny also suggested the presence of three additional QTL on RNO5, 6, and 10. The possible roles of these loci in tongue carcinogenesis are discussed.

Genomewide assessment of genetic alterations in DMBA-induced rat sarcomas: cytogenetic, CGH, and allelotype analyses reveal recurrent DNA copy number changes in rat chromosomes 1, 2, 4, and 7

Rat sarcomas, induced by subcutaneous injections of 7,12-dimethylbenz[a]anthracene (DMBA), were studied with the objective of identifying critical chromosome regions associated with tumorigenesis. We employed three genomewide screening techniques-cytogenetics, CGH, and allelotyping-in 19 DMBA-induced sarcomas in F1 (BN/Han x LE/Mol) rats. The most conspicuous finding in the cytogenetic analysis was a high incidence of trisomy for rat chromosome 2 (RNO2). Signs of gene amplification (hsr) were also seen in several tumors. The CGH analysis revealed that gains in copy number were much more common than losses. The gains mostly affected RNO2 (10/19), RNO12q (7/19), and RNO19q (5/19), as well as the proximal part of RNO4 (8/19) and the distal part of RNO7 (7/19). Reduction in copy number was seen in RNO17 (2/19). For the allelotyping, we used 318 polymorphic microsatellite marker loci covering the entire genome. We identified regions of allelic imbalance affecting most of the rat chromosomes. The highest incidences of recurrent allelic imbalance were observed at loci in certain regions in RNO1, 2, 4, and 7 and at lower incidences in parts of RNO12, 16, 18, and 19. The combined results suggested that genetic alterations detected in RNO2 and RNO12 usually corresponded to complete or partial trisomy, whereas those in RNO1 and RNO7 seemed to involve regional deletions and/or gains. Furthermore, we could confirm that copy number gains occur proximally in RNO4, where a previous study showed amplification of the Met oncogene in a subset of these tumors.

Host genes controlling the susceptibility and resistance to squamous cell carcinoma of the tongue in a rat model

Development of tongue carcinoma (TC) in rats by 4-nitroquinoline 1-oxide (4NQO), a potent carcinogen, is under host genetic control. The inbred Dark-Agouti (DA) strain rats showed a much higher susceptibility to TC than the Wistar-Furth (WF) strain. The author's previous study on crosses between two strains postulated a susceptibility gene in DA and a resistance gene in WF rats. This hypothesis was confirmed by the genetic analysis of the backcrosses to either parent and F2 with a simple sequence repeat polymorphism analysis. In the crosses between the DA and WF strains of rats, two major independently segregating host loci that influenced the cancer development by application of 4NQO positively or negatively were identified and mapped. DA rats had a semidominant susceptibility gene, Stc1, closely linked with D19Mit9 on chromosome 19, which was on the segment syntenic to human chromosome 16. In contrast, WF rats had a semidominant resistance gene, Rtc1, closely linked with D1Rat320 on chromosome 1, which is syntenic to human chromosome 11. The presence of other susceptibility and resistance genes on some chromosomes of both DA and WF rats was suspected, and they will be clarified in the near future. These findings provide powerful evidence that chemically induced tongue carcinogenesis is a multigenetic event.

Genomic imbalance in rat mammary gland carcinomas induced by 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine

2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), a compound found in cooked meat, is a mammary gland carcinogen in female Sprague-Dawley rats. PhIP-induced rat mammary gland carcinomas were examined for mutations in several genes (exons) known to regulate cell growth and apoptosis, including p53 (4-8), p21(Waf1) (coding region), Apc (14, 15), B-catenin (3), E-cadherin (9,13,15), Bcl-x (coding region), Bax (3), IGFIIR (28), and TGFBIIR (3). DNA from 30 carcinomas was examined by single-strand conformation polymorphism analysis, but no mutations were detected in these genes or gene regions. DNA from carcinomas and matching normal tissue were further screened for allelic imbalance by using a polymerase chain reaction-based approach with primers to known microsatellite regions located throughout the rat genome. Of 53 markers examined, 12 revealed allelic imbalance. Microsatellite instability (MSI) was detected at two markers, one on chromosome 4 and one on chromosome 6. Sixty-five percent and 96% of all carcinomas examined (N=23) showed MSI at these loci on chromosomes 4 and 6, respectively, supporting the notion that MSI plays a role in PhIP-induced mammary carcinogenesis. Loss of heterozygosity (LOH), an indication of a possible tumor suppressor gene, was observed at 10 markers distributed on chromosomes 3, 10, 11, 14, and X. The frequency of LOH at these markers was 75-94%, supporting that the regions of allelic imbalance were largely similar for the PhIP-induced carcinomas examined in this study. When PhIP-induced carcinomas from rats placed on high-fat and low-fat diet were compared, no unique regions of allelic imbalance or statistical differences in the frequency of allelic imbalance were observed. Therefore, the high-fat diet, known to be a promoter of PhIP-induced rat mammary carcinogenesis, did not appear to influence allelic imbalance in the carcinomas. Interestingly, 7,12-dimethylbenz[a]-anthracene-induced mammary carcinomas did not show allelic imbalance at 11 of the 12 loci that showed allelic imbalance in PhIP-induced carcinomas. These findings suggest that distinct chemical carcinogens induce different patterns of allelic imbalance during rat mammary carcinogenesis. Since several of the known genes involved in carcinogenesis did not harbor mutations in PhIP-induced carcinomas, further studies are needed to clarify the critical genes involved in PhIP-induced mammary carcinogenesis and to determine whether regions of LOH harbor potentially novel tumor suppressor genes involved in this disease.

Allele-specific losses of heterozygosity on chromosomes 1 and 17 revealed by whole genome scan of ethylnitrosourea-induced BDIX x BDIV hybrid rat gliomas

The induction of neural tumors by N-ethyl-N-nitrosourea (EtNU) in inbred strains of rats has evolved as a valuable model system of developmental stage- and cell type-dependent oncogenesis. Tumor yield and latency times are strongly influenced by genetic background. Compared with BDIX rats, BDIV rats are relatively resistant to the induction of brain tumors by EtNU, with a lower tumor incidence and latency periods prolonged by a factor of 3. To characterize genetic abnormalities associated with impaired tumor suppressor gene function in neuro-oncogenesis, losses of heterozygosity (LOHs) and microsatellite instability (MI) were investigated in brain tumors induced by EtNU in (BDIV x BDIX) F(1) and F(2) rats. The polymerase chain reaction was used to amplify 55 polymorphic microsatellite markers spanning the entire rat genome. The tumors displayed different histologies and grades of malignancy, corresponding to part of the spectrum of human gliomas. MI was not observed in any of the tumors. LOH of rat chromosome 1q was predominantly detected in oligodendrogliomas and mixed gliomas, with a 30% incidence in informative cases. 11p15.5, the human genome region syntenic to the consensus region of LOHs observed on rat chromosome 1, has been shown to be involved in the formation of gliomas in humans. Furthermore, rat brain tumors of different histologies often showed allelic imbalances on chromosome 17p. In both cases of LOH, there was a clear bias in favor of the parental BDIV allele, suggesting the involvement of tumor suppressor genes functionally polymorphic between the two rat strains.

A comparative analysis of allelic imbalance events in chemically induced rat mammary, colon, and bladder tumors

In this paper, patterns of allelic imbalances (Als) in chemically induced rat mammary, colon, and bladder tumors from (Wistar Furth x Fischer 344)F1 rats are described and compared. Male F1 rats were administered azoxymethane (AOM), and colon tumors were collected at 58 wk after treatment. Female F1 rats were given either N-nitroso-N-methylurea (NMU) or N-butyl-(hydroxybutyl)-nitrosoamine (BBN), and mammary and bladder tumors were collected at 15 and 52 wk after treatment, respectively. DNA was extracted from a subset of 18 of the largest tumors from each group, and a genome scan was performed by using polymerase chain reaction and 90 polymorphic microsatellite markers. Als, such as loss of heterozygosity, gene duplication, and microsatellite instability, were observed at low frequencies in all of the tumor models. Thirty random Als were observed in the AOM-induced colon tumors but only four in the NMU-induced mammary tumors. In both these models, all the tumors were classified as adenocarcinomas, and most of the Als observed were confined to single tumors with atypical histopathology. In contrast, 27 random Als were identified in the BBN-induced bladder tumors. Als were observed in both transitional-cell carcinomas and papillomas, although most were in the carcinomas. Statistical analysis of the Al data revealed no significant nonrandom Als within or among the tumor models, although several of the infrequently observed Al events identified in the rat tumors may also be observed in the corresponding human tumor type.

Rat chromosome 1: regional localization of seven genes (Slc9a3, Srd5a1, Esr, Tcp1, Grik5, Tnnt3, Jak2) and anchoring of the genetic linkage map to the cytogenetic map

Seven genes were regionally localized on rat Chromosome (Chr) 1, from 1p11 to 1q42, and two of these genes were also included in a linkage map. This mapping work integrates the genetic linkage map and the cytogenetic map, and allows us to orient the linkage map with respect to the centromere, and to deduce the approximate position of the centromere in the linkage map. These mapping data also indicate that the Slc9a3 gene, encoding the Na+/H+ exchanger 3, is an unlikely candidate for the blood pressure loci assigned to rat Chr 1. These new localizations expand comparative mapping between rat Chr 1 and mouse or human chromosomes.

Genetic changes induced by heterocyclic amines

Clarification of the mutational fingerprints of HCAs offers a promising approach in the investigation of the role of heterocyclic amines (HCAs) in human carcinogenesis. We analyzed mutations in the tumor related genes of tumors induced by HCAs, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), which mainly yield DNA-adducts of C8-guanine. The G-->T transversion at codon 13-2nd position in Ha-ras was predominantly observed in mouse forestomach and rat Zymbal gland tumors induced by MeIQ. In contrast, various types of mutation were detected in the ras family genes of rat Zymbal gland tumors induced by IQ; the presence of a methyl group at position 4 of imidazo[4,5-f]quinoline gave rise to a remarkable difference in the mutational fingerprint. Apc mutations were detected in PhIP- and IQ-induced rat colon tumors, with incidences of 50% (4/8) and 15% (2/13), respectively. All five mutations detected in the four PhIP-induced tumors consisted of a guanine deletion from the 5'-GGGA-3' sequence, in contrast with T to C and C to T mutations in IQ-induced tumors. Four of these five mutations shared seven common nucleotides, -GTGGGAT- surrounding the guanine; indicating that PhIP leaves a characteristic mutational fingerprint in Apc. Colon tumors induced by PhIP were also found to have mutations in their microsatellite sequences, and similar results were detected in mammary gland tumors induced by PhIP, contrasting with no mutations in IQ-induced colon tumors and a very low frequency of mutations in 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumors. Although the mechanisms involved in the induction of microsatellite mutations are not known yet, microsatellite mutations which can also be detected in sporadic human tumors, including colon and breast tumors, were indicated to be a characteristic of PhIP. Mammary tumors induced by PhIP showed loss of heterozygocity (LOH) at the distal part of chromosome 10, which shows synteny with the distal part of human chromosome 17, where LOH frequently occurs in human breast cancer. In conclusion, each heterocyclic amine leave a mutational fingerprint which is specific to each compound. Since the tumor-related genes involved in PhIP-induced tumors have characteristics in common with those in human cancers, further detailed analysis will provide us with useful information on mutational fingerprints, and on the possible contribution of PhIP to human colon cancer.

Consistent allelic loss on mouse chromosome 7 distal to tyrosinase in 4-nitroquinoline-1-oxide-induced oral cavity tumors with loss of heterozygosity at Ha-ras-1

We have previously shown that all CBA/J mice exposed to 4-nitroquinoline-1-oxide (4NQO) eventually develop oral cavity squamous cell carcinomas, and two-thirds of these tumors have Ha-ras-1 (Hras1) point mutations at codon 12. Half of the tumors with Hras1 mutations have loss of heterozygosity (LOH) at Hras1. In the study reported here, seven tumors with LOH at Hras1, six heterozygous for Hras1, and six without Hras1 mutations were analyzed to define the extent of LOH on chromosome (Chr) 7. Microsatellite polymorphisms present in CBA/J mice were used as informative allelic markers. Tumors with LOH at Hras1 showed consistent allelic loss at the distal portion of Chr 7. The boundary of allelic loss lay between the tyrosinase and hemoglobin beta chain loci, which are 6 cM apart. None of the tumors that remained heterozygous for Hras1 or had no Hras1 mutations had evidence of chromosomal loss involving Chr 7. Because LOH was only detected in advanced lesions long after exposure to 4NQO had ceased, we presume that the chromosomal alterations by which LOH occurred were independent of the carcinogen exposure. The development of LOH in only half of the tumors with Hras1 point mutations suggests that LOH was not caused by the initial Hras1 point mutation but was a highly selected event during tumorigenesis.

Allelic imbalance in mammary carcinomas induced by either 7,12-dimethylbenz[a]anthracene or ionizing radiation in rats carrying genes conferring differential susceptibilities to mammary carcinogenesis

To identify and compare the genetic lesions associated with tumorigenesis in rats carrying the mammary carcinoma suppressor (MCS) 1 gene, we induced mammary carcinomas in (Wistar Furth (WF) x Copenhagen (Cop))F1 rats by using either 7,12-dimethylbenz[a]anthracene (DMBA) or radiation. The tumors were screened for allelic imbalances by using polymerase chain reaction and 65 polymorphic microsatellite markers spanning the genome. No allelic imbalance was detected at the mapped location of MCS-1 on chromosome 2; however, a scan of the genome revealed random allelic imbalances in the radiation-induced tumors. In addition, non-random loss of heterozygosity (LOH) on chromosome 1 in the DMBA-induced tumors was documented. We then screened three other subsets of DMBA- and radiation-induced mammary carcinomas from (WF x Fischer (F344))F1, (Wistar Kyoto x F344)F1, and (F344 x Cop)F1 rats for imbalance on chromosomes 1 and 2. No allelic imbalance was detected in the MCS-1 region of chromosome 2 in any of the tumors screened. Nonrandom imbalance on chromosome 1 was detected but only in the DMBA-induced tumors from the (F344 x Cop)F1 rats. Thus, only Cop-derived F1 rats have mammary tumors with the chromosome 1 imbalance; however, the imbalance does not favor the Cop parental allele. We also analyzed the DMBA-induced tumors with LOH at chromosome 1 for Ha-ras codon 61 mutation and found no association. These results suggest that loss of the MCS-1 Cop allele is not required for tumor formation, that the genetic background of the F1 rat appears to influence the type of genetic lesion identified in the mammary tumors, and that there is no association between Ha-ras codon 61 mutation and chromosome 1 imbalance in our model system.

Allelotypic and cytogenetic characterization of chemically induced mouse mammary tumors: high frequency of chromosome 4 loss of heterozygosity at advanced stages of progression

Loss of heterozygosity (LOH) is one of the most common genetic abnormalities in cancer. To define the role of LOH and chromosomal abnormalities at various stages of mouse mammary cancer progression, we analyzed the allelotypes and karyotypes of primary mammary tumors induced in CD2F, mice by two basic protocols, the classical multiple-dose 7,12-dimethylbenz[a]anthracene (DMBA) protocol and a novel protocol of combined medroxyprogesterone acetate (MPA) and DMBA. The advantage of the latter protocol is that its latency for tumor development is much shorter and its tumor incidence is higher than those of DMBA alone. To study more advanced stages of mammary tumor progression, we also analyzed mouse mammary tumors that had acquired autonomous growth and were transplantable into syngeneic hosts. The allelotypic studies were performed by means of microsatellite length polymorphism analysis with a minimum of two simple-sequence repeat markers per chromosome. We observed that MPA-DMBA-induced mammary adenocarcinomas, which in general arose earlier because of the growth promotion exerted by MPA, did not show any significant LOH and were essentially diploid. Tumors induced by DMBA alone, which on average took longer to develop, showed a higher frequency of allelic losses. LOH on chromosome 11 was observed in 30% of the cases. Chromosomes 4 and 8 were affected in 25% and 20% of the tumors, respectively. Interestingly, advanced stages of mammary tumor progression, represented by transplantable mammary tumors, showed a much higher level of genomic instability, specifically a very high frequency (66%) of LOH on chromosome 4. These findings indicate that chromosome 4 harbors a gene whose inactivation may play a role in the acquisition of more aggressive characteristics such as autonomous growth and transplantation ability.

Karyotype evolution of the clonal rat liver cell line CL 52 during progression in vitro and in vivo

Liver cell line CL 52, derived from a diethylnitrosamine-treated rat at the stage of preneoplasia/early neoplasia, had an inconspicuous 2n karyotype when analyzed 6 months after in vitro propagation. Malignant progression was accompanied by cytogenetic alterations of chromosomes 1, 3, and 11. In addition, trisomy of chromosomes 4, 6, and 7 led to a significant reduction of the tumor latency period after retransplantation. During 4 years of cytogenetic observation, the once clonal 2n population showed a characteristic karyotype evolution: loss of diploidy, occurrence of polyploid sidelines, deletions followed by unbalanced rearrangements, clonal diversification, and selection of the in vitro most rapidly growing or in vivo most malignant cell type. The karyotype alterations in the four sublines of CL 52 are discussed with special reference to oncogenesis-related genes assigned to the involved rat chromosomes 1, 3, 11, 12, 4, 6, 7, 10. The observed karyotype evolution of this cell line exemplifies genetic/ chromosome instability of carcinogen-induced preneoplastic/early neoplastic liver cells and provides a tool for analyzing, under controlled conditions, stage-dependent sequences of molecular genetic alterations in liver carcinogenesis.

Microsatellite instability and loss of heterozygosity on chromosome 10 in rat mammary tumors induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

Microsatellite instability (MI) and loss of heterozygosity (LOH) were examined in mammary tumors induced in Sprague-Dawley x F344 F1 female rats by 2-amino-l-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Examination of 62 microsatellite loci revealed MI in nine of 15 (60%) PhIP-induced mammary tumors, and five of these MI-positive tumors had mutations in more than one microsatellite locus. In contrast, two of 12 (17%) 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumors were MI positive but had mutations at only one locus each. Further, by using 37 polymorphic markers specific LOH was observed in four of 15 PhIP induced mammary tumors on distal parts of rat chromosome 10, which is homologous to human chromosome 17q with no background level of LOH. Similarly, DMBA-induced mammary tumors showed specific LOH on the same region of chromosome 10. These data suggest that mismatch-repair deficiency and loss of chromosome 10 are involved in carcinogenesis of PhIP-induced rat mammary tumors.