Roles of the Pas1 and Par2 genes in determination of the unique, intermediate susceptibility of BALB/cByJ mice to urethane-induction of lung carcinogenesis: differential effects on tumor multiplicity, size and Kras2 mutations

Mmp12 Is Upregulated by in utero Second-Hand Smoke Exposures and Is a Key Factor Contributing to Aggravated Lung Responses in Adult Emphysema, Asthma, and Lung Cancer Mouse Models

Matrix metalloproteinase-12 (Mmp12) is upregulated by cigarette smoke (CS) and plays a critical role in extracellular matrix remodeling, a key mechanism involved in physiological repair processes, and in the pathogenesis of emphysema, asthma, and lung cancer. While cigarette smoking is associated with the development of chronic obstructive pulmonary diseases (COPD) and lung cancer, in utero exposures to CS and second-hand smoke (SHS) are associated with asthma development in the offspring. SHS is an indoor air pollutant that causes known adverse health effects; however, the mechanisms by which in utero SHS exposures predispose to adult lung diseases, including COPD, asthma, and lung cancer, are poorly understood. In this study, we tested the hypothesis that in utero SHS exposure aggravates adult-induced emphysema, asthma, and lung cancer.

Methods: Pregnant BALB/c mice were exposed from gestational days 6–19 to either 3 or 10mg/m³ of SHS or filtered air. At 10, 11, 16, or 17weeks of age, female offspring were treated with either saline for controls, elastase to induce emphysema, house-dust mite (HDM) to initiate asthma, or urethane to promote lung cancer. At sacrifice, specific disease-related lung responses including lung function, inflammation, gene, and protein expression were assessed.

Results: In the elastase-induced emphysema model, in utero SHS-exposed mice had significantly enlarged airspaces and up-regulated expression of Mmp12 (10.3-fold compared to air-elastase controls). In the HDM-induced asthma model, in utero exposures to SHS produced eosinophilic lung inflammation and potentiated Mmp12 gene expression (5.7-fold compared to air-HDM controls). In the lung cancer model, in utero exposures to SHS significantly increased the number of intrapulmonary metastases at 58weeks of age and up-regulated Mmp12 (9.3-fold compared to air-urethane controls). In all lung disease models, Mmp12 upregulation was supported at the protein level.

Conclusion: Our findings revealed that in utero SHS exposures exacerbate lung responses to adult-induced emphysema, asthma, and lung cancer. Our data show that MMP12 is up-regulated at the gene and protein levels in three distinct adult lung disease models following in utero SHS exposures, suggesting that MMP12 is central to in utero SHS-aggravated lung responses.

Complex genetic control of lung tumorigenesis in resistant mice strains

The SM/J mouse strain is resistant to chemically‐induced lung tumorigenesis despite having a haplotype, in the pulmonary adenoma susceptibility locus (Pas1) locus, that confers tumor susceptibility in other strains. To clarify this inconsistent genotype‐phenotype correlation, we crossed SM/J mice with another resistant strain and conducted genome‐wide linkage analysis in the (C57BL/6J × SM/J)F2 progeny exposed to urethane to induce lung tumors. Overall, >80% of F2 mice of both sexes developed from 1 to 20 lung tumors. Genotyping of 372 F2 mice for 744 informative non‐redundant SNPs dispersed over all autosomal chromosomes revealed four quantitative trait loci (QTLs) affecting lung tumor multiplicity, on chromosomes 3 (near rs13477379), 15 (rs6285067), 17 (rs33373629) and 18 (rs3706601), all with logarithm of the odds (LOD) scores >5. Four QTLs modulated total lung tumor volume, on chromosome 3 (rs13477379), 10 (rs13480702), 15 (rs6285067) and 17 (rs3682923), all with LOD scores >4. No QTL modulating lung tumor multiplicity or total volume was detected in Pas1 on chromosome 6. The present study demonstrates that the SM/J strain carries, at the Pas1 locus, the resistance allele: a finding that will facilitate identification of the Pas1 causal element. More generally, it demonstrates that lung tumorigenesis is under complex polygenic control even in a pedigree with low susceptibility to this neoplasia, suggesting that the genetics of lung tumorigenesis is much more complex than evidenced by the pulmonary adenoma susceptibility and resistance loci that have, so far, been mapped in a small number of crosses between a few inbred strains.

Pulmonary tumors associated with the JC virus T-antigen in a transgenic mouse model

Many attempts to demonstrate the oncogenic role of the JC virus (JCV) have been partially successful in producing brain tumors, either by direct inoculation of JCV into the brain or in transgenic models in rodents. We previously reported the presence of JCV DNA with a relatively high incidence in pulmonary and digestive organs. However, we could not prove the oncogenic role of JCV. We prepared a transgene composed of the K19 promoter, specific to bronchial epithelium with the JCV T-antigen and established transgenic (TG) mice. Pulmonary tumors were detected without any metastasis in 2 out of 15 (13.3%) 16-month-old K19/JCV T-antigen TG mice. Using immunohistochemistry (IHC), these tumors showed JCV T-antigen, p53 and CK 19 expression, but not expression of nuclear and cytoplasmic β-catenin and insulin receptor substrate 1 (IRS1). IHC revealed the same expression pattern as in the bronchial epithelium of the TG mice. One tumor, which was examined with laser capture microdissection and molecular biological tools, demonstrated an EGFR mutation but not a K-ras mutation. We propose that the pulmonary tumors were derived from the JCV T-antigen in a TG mouse model. These findings shed light on pulmonary carcinogenesis.

Identification of myeloid-derived suppressor cells and T regulatory cells in lung microenvironment after Urethane-induced lung tumor

Tumor growth occurs by the imbalance between cells with effector function and cells with suppressor/regulatory functions. To investigate this scenario we administered the chemical carcinogen Urethane in BALB/c mice and followed these animals during 120 days to observe lung tumor development. In another set of experiments the same protocol was performed with the harvest of spleen, lung and blood at 20 and 30 days after Urethane injection. The lung was used for histology, spleen cells were evaluated for IFN-γ production, and serum nitrite was measured as an indirect form of nitric oxide (NO) evaluation. The spleen and lung-infiltrating cells were evaluated by flow cytometry for CD11b+Gr-1+ myeloid suppressor cells and CD4+FoxP3+ T regulatory cells. Urethane led to lung nodules development after 120 days and the time point evaluation showed that splenocytes stimulated ex vivo expressed higher levels of IFN-γ 20 days after the chemical injection. Also, the level of nitric oxide in serum was higher after 20 days of Urethane injection. There was no statistical difference in spleen cells percentages for CD11b+Gr-1+ and CD4+Foxp3+ in all groups. However, lung-infiltrating cells presented early (20 days) a higher expression of CD11b+Gr-1+ suggesting suppression at this site. In conclusion, it was possible to observe two distinct events at the very early time point after Urethane injection. In periphery there was an increase at the effector immune response (as depicted by IFN-γ-producing cells) and in tumor development site there was an increase at the suppressor cell (CD11b+Gr-1+) phenotype. Suppressor/regulatory cells are targets for cancer therapy.

ANALYSIS OF THEPar2MODIFIER OF PULMONARY ADENOMA FORMATION IN MICE

Inbred strains of mouse show various susceptibilities to spontaneous and chemical-induced lung tumorigenesis. Genetic analyses have revealed that lung tumor susceptibilities of inbred mouse strains are governed by quantitative trait loci (QLTs) located on multiple chromosomes. A major lung tumor resistance QLT, designated pulmonary adenoma resistance 2 (Par2), was mapped to the mouse chromosome 18 independently by several groups and accounted for up to 60% phenotype variance between susceptible A/J and more resistant BALB/c strains. The authors recently conducted studies to positionally clone the Par2 gene. This review summarizes the effort and progress towards the identification of Par2 candidates.

Lung tumor development in the presence of sphingosine 1-phosphate agonist FTY720

Urethane is a chemical carcinogen which causes lung tumorigenesis in mice with similarities to human adenocarcinoma (AC). The sphingosine 1-phosphate agonist FTY720 administered to mice in doses above 5 mg/kg/day has been able to prevent hepatocellular carcinoma and bladder cancer. We used BALB/c mice in urethane-induced lung cancer model to investigate the effects of a lower dose of FTY720 (1 mg/kg/day). The benefits of FTY720 were associated with the time point of the compound administration. FTY720 30 Group presented lower incidence and smaller area of lung nodules, decreased PCNA and increased Caspase-3 expressions. The findings in FTY720 0 Group (nodule multiplicity and area, PCNA expression) were similar to Urethane Group suggesting that the administration of the compound at early time point did not affect lung tumor development. FTY720 90 Group presented the biggest nodule area which was associated with increased PCNA and decreased Caspase-3 expressions. FTY720 (30 days and 90 days) administration decreased CD4 + splenocytes and blood lymphocytes which caused opposite effects in lung tumor development - impairment and improvement respectively.In conclusion, FTY720 in low dose did not provide lung tumor inhibition in mice but its administration 30 days after the chemical carcinogen (Urethane) injection was associated with impaired tumor development.

Association of KRAS polymorphisms with risk for lung adenocarcinoma accompanied by atypical adenomatous hyperplasias

The pulmonary adenoma susceptibility 1 (Pas1) gene affects susceptibility to the development of lung adenomas in mice with a subset of the adenomas progressing to adenocarcinoma (ADC). In this study, genotype distributions for 10 polymorphisms in the human counterparts for three mouse candidate Pas1 genes, KRAS, CASC1/LAS1 and LRMP, were examined in a hospital-based case-control study consisting of 364 lung ADC cases and 253 controls. All the ADC cases were subjected to lobectomy and subsequent pathological investigation of atypical adenomatous hyperplasia (AAH), a putative precursor for peripheral lung ADC, including bronchioloalveolar carcinoma, in the resected lobes. Eighty-one (22%) of the ADC cases carried at least one AAH lesion in addition to the primary ADC and 34 (9%) of them carried multiple AAH lesions. None of the 10 polymorphisms examined showed significant associations with overall lung ADC risk (P > 0.05). However, minor allele carriers for two polymorphisms in the KRAS gene, KRAS-1 and -6, showed significantly increased odds ratios (ORs) for ADC accompanied by multiple AAHs [OR = 3.0; 95% confidence interval (CI) = 1.4-6.2, P = 0.004 and OR = 2.4; 95% CI = 1.1-4.7, P = 0.02, respectively]. Minor haplotypes including the minor allele for the KRAS-6 polymorphism showed increased ORs for ADC accompanied by multiple AAHs, and KRAS transcripts from the minor allele for this polymorphism were more abundantly detected in lung tissues than those from the major allele. Thus, KRAS polymorphisms were indicated to be involved in risk for the development of AAHs that progress to ADC by causing differential KRAS oncogene expression in the lungs.

Genetic alterations in cancer knowledge system: analysis of gene mutations in mouse and human liver and lung tumors

Mutational incidence and spectra for genes examined in both human and mouse lung and liver tumors were analyzed using the National Institute of Environmental Health Sciences (NIEHS) Genetic Alterations in Cancer (GAC) knowledge system. GAC is a publicly available, web-based system for evaluating data obtained from peer-reviewed studies of genetic changes in tumors associated with exposure to chemical, physical, or biological agents, as well as spontaneous tumors. In mice, mutations in Kras2 and Hras-1 were the most common events reported for lung and liver tumors, respectively, whether chemically induced or spontaneous. There was a significant difference in Kras2 mutation incidence for spontaneous versus induced mouse lung tumors and in Hras-1 mutation incidence and spectrum for spontaneous versus induced mouse liver tumors. The major gene changes reported for human lung and liver tumors were in KRAS2 (lung only) and TP53. The KRAS2 mutation incidence was similar for spontaneous and asbestos-induced human lung tumors, while the TP53 mutation incidence differed significantly. Aflatoxin B1, hepatitis B virus, hepatitis C virus, and vinyl chloride all caused TP53 mutations in human liver tumors, but the mutation spectrum for each agent differed. The incidence of KRAS2 mutations in human compared to mouse lung tumors differed significantly, as did the incidence of Hras and p53 gene mutations in human compared to mouse liver tumors. Differences observed in the mutation spectra for agent-induced compared to spontaneous tumors and similarities in spectra for structurally similar agents support the concept that mutation spectra can serve as a "fingerprint" of exposure based on chemical structure.

Quantitative analysis of early chemically-induced pulmonary lesions in mice of varying susceptibilities to lung tumorigenesis

Inbred mice vary in their susceptibility to develop macroscopic, chemically-induced, pulmonary neoplasias. It is not known, however, whether microscopic lesions appear in resistant strains but do not grow or if no early lesions arise at all. We show herein that resistant C57BL/6J (B6) and intermediately resistant BALB/cByJ (BALB) mice form very few urethane-induced early microadenomas (i.e. adenomas larger than hyperplasic foci, but detectable only by light microscopy). Additionally, while all urethane-induced microadenomas in sensitive A/J mice gave rise to adenomas, most microscopic tumors induced in BALB mice by 2-stage, 3-methylcholanthrene/butylated hydroxytoluene carcinogenesis spontaneously regressed. The formation of microscopic lesions is thus genetically dependent, but whether they continue to grow or regress depends on how they were induced.

Genetic linkage between Pol iota deficiency and increased susceptibility to lung tumors in mice

Pol iota is a member of the Y-family DNA polymerases, characterized by their capacity for translesion DNA synthesis and low fidelity base incorporation, and has therefore been assumed to play important roles in mutagenesis and carcinogenesis. In fact, the mouse Pol iota gene is located within the Par2 (pulmonary adenoma resistance 2) locus on distal chromosome 18, which we have identified as a major susceptibility locus regarding urethane induction of pulmonary adenomas. Indeed, Pol iota has been suggested to be a candidate for Par2 from both the genetic and biological standpoints. Taking advantage of 129X1/SvJ mice naturally deficient in Pol iota due to a nonsense mutation within the coding region of the gene, we here analyzed urethane-treated (A/J x 129X1/SvJ)F(1) x A/J backcross and (A/J x 129X1/SvJ)F(2) intercross mice and observed the defective 129X1/SvJ Pol iota allele to be genetically linked with an increased susceptibility to lung tumors relative to the A/J allele. Thus, among the already known mouse Pol iota alleles, the defective 129X1/SvJ allele is associated exclusively with the highest susceptibility to lung tumors. The result indicates a possibility that the Pol iota gene may participate in error-free repair of damaged DNA and prevention of lung tumor development.

Pol ι Is a Candidate for the Mouse Pulmonary Adenoma Resistance 2 Locus, a Major Modifier of Chemically Induced Lung Neoplasia

In this study, we performed systematic candidate gene analyses of the Pulmonary adenoma resistance 2 locus. Differential gene expression in lung tissues and nucleotide polymorphisms in coding regions between A/J and BALB/cJ mice were examined using reverse transcription-PCR and direct sequencing. Although not all genes in the interval were analyzed at this moment due to the recent database updating, we have found that the Pol iota gene, encoding the DNA polymerase iota, contains 25 nucleotide polymorphisms in its coding region between A/J and BALB/cJ mice, resulting in a total of ten amino acid changes. Primer extension assays with purified BALB/cJ and A/J proteins in vitro demonstrate that both forms of Pol iota are active but that they may differ in substrate discrimination, which may affect the formation of Kras2 mutations in mouse lung tumors. Altered expression of POL iota protein and an amino acid-changing nucleotide polymorphism were observed in human lung cancer cells, suggesting a possible role in the development of lung cancer. Thus, our data support the Pol iota gene as a modifier of lung tumorigenesis by altering DNA polymerase activity.

Prevalence of liver tumours in HIV-1 tat-transgenic mice treated with urethane

The human immunodeficiency virus type 1 (HIV-1) Tat protein stimulates cell proliferation, inhibits apoptosis, displays angiogenic functions and is believed to be involved in the pathogenesis of Kaposi's sarcoma (KS) and other tumours arising in AIDS patients. Tat-transgenic (TT) mice, which constitutively express Tat in all tissues and organs, may therefore be predisposed to tumorigenesis. To test this hypothesis, we treated TT mice with urethane, a general carcinogen inducing tumours of various organs. The results indicate that, after injection of urethane, the incidence of lung tumours and lymphomas is not significantly different in the TT and control (CC) mice, whereas liver preneoplastic lesions and tumours show a significantly greater incidence in TT than in CC mice. This remarkable carcinogenic effect of urethane for the liver may be due to a tat-induced predisposition, manifested as a liver cell dysplasia (LCD), spontaneously affecting most of the TT mice. LCD may exert a promoting effect by stimulating proliferation of cell clones initiated by the mutagenic effect of urethane. In addition, LCD, which is associated with aneuploidy and chromosome instability, may enhance the progression to malignancy of the preneoplastic lesions induced by urethane. Interestingly, a significantly greater incidence of vascular ectasias and haemangiomas was detected in the liver of urethane-treated TT mice, most likely due to the marked angiogenic properties of Tat. This study suggests a role for Tat in the promotion and progression of tumours initiated by exogenous and endogenous carcinogens in HIV-1-infected patients, thereby contributing to the tumorigenesis in the course of AIDS.

Analysis of lung tumorigenesis in chimeric mice indicates the Pulmonary adenoma resistance 2 (Par2) locus to operate in the tumor-initiation stage in a cell-autonomous manner: detection of polymorphisms in the Poli gene as a candidate for Par2

The Pulmonary adenoma resistance 2 (Par2) locus of the BALB/cByJ mouse, located within 0.5 cM of chromosome 18, is responsible for reducing the mean multiplicity of urethane-induced lung tumors relative to those in C57BL/6J, A/J and C3H/HeJ mice. Thus, BALB/B6-Par2 congenic strain genetically identical to BALB/cByJ except carrying C57BL/6J Par2 alleles develops seven times more tumors than BALB/cByJ. To gain clues for identification of Par2 candidate genes, we analysed lung tumorigenesis in BALB/cByJ<-->BALB.B6-Par2 chimeric animals. Of 100 tumors induced by urethane in 16 chimeras, 82 originated from BALB.B6-Par2 cells, indicating the Par2 phenotype to be cell-autonomous. In addition, the BALB.B6-Par2- and BALB/cByJ-derived tumors were similar in mean size, implying that the phenotype is primarily expressed during initiation rather than in the promotion stage of carcinogenesis. Given these results, we surveyed a comprehensive mouse genome database and physically mapped Par2 within a 2.3 Mbp segment containing three known genes, Poli, Mbd2 and Dcc. Among those, the Poli seemed to be the most reasonable Par2 candidate, since it encodes an extremely error-prone DNA polymerase preferentially incorporating G or T opposite template T in vitro, reminiscent of the Kras2 activation because of an A to G or T point mutation within codon 61 with which most urethane-induced lung tumors are initiated. Indeed, our sequencing of Poli cDNAs from BALB/cByJ, C57BL/6J, A/J and C3H/HeJ lungs revealed 21 BALB/cByJ-specific single-nucleotide polymorphisms in the coding region accompanied by seven amino-acid substitutions and an elevated frequency of alternative splicing, while no polymorphisms associated with tumor susceptibility were found for either Mbd2 or Dcc. Notably, we obtained evidence that BALB/cByJ Par2 alleles may selectively decrease the frequency of Kras2-mutated tumors compared with C57BL/6J alleles. Consequently, the Poli is an intriguing Par2 candidate clearly deserving further evaluation.

Quantitative trait locus mapping of susceptibilities to butylated hydroxytoluene-induced lung tumor promotion and pulmonary inflammation in CXB mice

We have reported previously [Bauer,A.K. et al. (2001) Exp. Lung Res., 27, 197-216] that the 13 CXB recombinant inbred mouse strains derived from BALB/cByJ and C57BL/6J progenitors vary in their responsiveness to both lung tumor promotion and pulmonary inflammation induced by chronic administration of butylated hydroxytoluene (BHT). Herein we have applied these data, along with markers known to be polymorphic among these strains, to conduct linkage analysis of these susceptibilities. This enabled us to assign provisional quantitative trait loci (QTL) that govern these strain variations in susceptibility as a genetic approach to assessing the influence of inflammation on tumorigenesis. A Chr 15 (39.1-55.6 cM) QTL regulated susceptibility to two-stage carcinogenesis, a protocol in which chronic BHT exposure followed a single urethane injection; a similar QTL on Chr 15 (46.7-61.7 cM) influenced BHT induction of cyclooxygenase-2 (COX-2) expression. A Chr 18 (37-41 cM) QTL modulated both the number of lung tumors induced by 3-methylcholanthrene (MCA) injection with subsequent treatment with BHT as well as BHT-induced ingress of macrophages into airways. Other chromosomal sites that affected either the degree of BHT-elicited macrophage infiltration, Chr 9 (48-61 cM), or COX-2 induction, Chr 10 (59-65 cM), were reported to influence susceptibility to lung tumorigenesis in other strains. The fact that common chromosomal locations regulate both inflammation and carcinogenesis suggests a pathogenic role of inflammatory mediators in tumor development that may be exploited for chemoprevention of lung cancer.

Fine chromosomal localization of the mouse Par2 gene that confers resistance against urethane-induction of pulmonary adenomas

BALB/cByJ mice are 14 times more resistant to urethane-induction of pulmonary adenomas than the susceptible A/J strain. Our previous linkage analysis of (A/J x BALB/cByJ)F1 x A/J backcross mice provided statistical evidence that a major resistance locus of BALB/cByJ with a dominant effect, designated Par2 (Pulmonary adenoma resistance 2), exists within an approximately 25 cM section of distal chromosome 18. To facilitate molecular identification of the Par2 locus, the present study was conducted to finely localize its chromosomal position utilizing Par2-congenic mice. Male BALB/cByJ mice were mated with female C57BL/6J mice carrying recessive Par2 alleles and their male F1 progeny were backcrossed to female BALB/cByJ mice. A male backcross mouse heterozygous within the Par2 interval of 25 cM was randomly selected and again backcrossed to female BALB/cByJ mice. This backcross-selection cycle was simply repeated to produce semi-congenic mice with a general BALB/cByJ genetic background except for the Par2 interval, where the mice were heterozygous with paternal C57BL/6J alleles and maternal BALB/cByJ alleles. After the 6th or 7th backcross, nine male mice possessing a recombination within the paternal Par2 interval were retained and crossed to female A/J mice. Resultant progeny were treated with urethane and examined for lung tumor development in order to deduce the Par2 genotypes of the recombinants through linkage analysis. By comparing the deduced Par2 genotype of each recombinant with its recombinational breakpoint, the Par2 locus was confined to an approximately 0.5 cM region flanked by D18Mit103 and D18Mit188 loci. Our results indicate that fully congenic mice conventionally established by at least nine simple backcrosses or by the speed congenic method are not necessarily required for fine mapping of quantitative trait loci. In the case of the Par2 locus, we found that semi-congenic mice after as few as four simple backcrosses were useful for this purpose. The map information obtained in this study should enable subsequent positional cloning of the Par2 gene.

Genetic dissection of murine susceptibilities to liver and lung tumors based on the two-stage concept of carcinogenesis

Inbred mouse strains exhibit strain-specific susceptibilities to spontaneous and induced tumors, indicating that the individual risks for neoplastic development are largely under genetic control. Recent advances in linkage analysis have made it routine to chromosomally map the mouse genes responsible for the strain variations in tumor susceptibility using segregating crosses. It is also possible to characterize their biological functions using the positional information. These types of studies are still severely hampered for human cases due to the remarkable genetic heterogeneity and impossibility of experimental crosses. In this article, previous work on genetic susceptibility to mouse liver and lung tumors is reviewed in view of the classical two-stage concept of carcinogenesis. According to this central concept, the tumor susceptibility genes should affect either the first stage, 'initiation', or the second stage, 'promotion', or both. At least some genes seem to be specifically involved in initiation or promotion, in line with the fact that initiation and promotion are due, to a certain extent, to independent mechanisms. This notion should be also applicable to human carcinogenesis and may provide important clues for prevention of initiation and promotion in populations with a genetic predisposition for cancer development.

At least four loci and gender are associated with susceptibility to the chemical induction of lung adenomas in A/J x BALB/c mice

Four putative quantitative trait loci (QTLs) that influence susceptibility to the induction of lung adenomas by urethane in an F2 cross between A/J and BALB/cOlaHsd have been mapped. Following microsatellite typing of mice with resistant and susceptible phenotypes at 97 microsatellite marker loci, a major locus was identified on chromosome 18 with a lod score of 15. This was responsible for an 8- to 10-fold increase in tumor multiplicity in males and females, respectively, having the AA and CC genotypes at the D18Mit188 marker locus. It mapped close to Dcc (deleted in colorectal cancer). A locus on chromosome 4 (lod score 6.5) had the resistant allele in strain A/J and the susceptible allele in BALB/c, with a 14-fold difference in tumor multiplicity between mice of the AA and CC genotypes. This mapped close to the Cdkn2a (cyclin-dependent kinase inhibitor 2A) locus, which is commonly deleted in mouse lung tumors. Two loci with smaller effects (lod scores 3.03 and 3.25) were identified on chromosomes 1 and 11. There was also significant sexual dimorphism in tumor multiplicity both among 151 F2 hybrids and among 52 mice resulting from a backcross to strain A/J, with males having higher tumor counts than females.