Detection of genomic alterations in carcinogen-induced mouse liver tumors by DNA fingerprint analysis

Genotype-phenotype relationships in hepatocellular tumors from mice and man

Experimentally induced liver tumors in mice harbor activating mutations in either Catnb (beta-catenin) or Ha-ras, according to the carcinogenic treatment. We have now investigated by microarray analysis the gene expression profiles in tumors of the two genotypes. In total, 364 genes or expressed sequences with aberrant expression relative to normal liver were identified, but only 30 of these demonstrated unidirectional changes in both tumor types. Several functional clusters were identified that involve changes in amino acid utilization and ammonia disposition in Catnb-mutated tumors as opposed to alterations in lipid and cholesterol metabolism in Ha-ras-mutated tumors. Moreover, several genes coding for inhibitory molecules within the Wnt-signaling pathway were upregulated in Catnb-mutated tumors, suggesting induction of a negative feedback loop, whereas Ha-ras-mutated tumors showed alterations in the expression of several genes functional in monomeric G-protein signaling. We conclude that mouse hepatoma cells adopt different evolutionary strategies that allow for their selective outgrowth under variable environmental conditions. Human hepatocellular cancers (HCC) lack RAS mutations but are frequently mutated in CTNNB1, the human Catnb ortholog. The set of genes aberrantly expressed in Catnb-mutated mouse tumors was used to screen, by expression profiling, for dysregulation of orthologous genes within a panel of 25 HCCs, of which 10 were CTNNB1-mutated. HCCs with activated beta-catenin displayed a gene expression profile that was similar to Catnb-mutated mouse tumors but distinct from the other human HCCs. In conclusion, expression fingerprints may be used for diagnostic purposes and potential new therapeutic intervention strategies. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index/html).

DNA fingerprint analysis reveals differences in mutational patterns in experimentally induced rat peritoneal tumors, depending on the type of environmental mutagen

We performed tumor DNA fingerprint analysis using the synthetic minisatellite probe S3315x2 based on the 33.15-repeat unit. The aim of the study was to investigate fingerprinting patterns of peritoneal tumors induced experimentally in Wistar rats by two carcinogens with unknown mechanism of action (crocidolite asbestos and nickel powder) and, as a positive control, benzo[a]pyrene. The carcinogens were administered intraperitoneally into rats. The banding patterns obtained with DNA from 71 peritoneal tumors were compared to the corresponding normal tissues. DNA derived from peritoneal tumors induced by the three carcinogens differed with respect to mutation frequencies and mutation patterns. The mutation frequencies in these tumors, revealed by DNA fingerprinting, were 18.2% for benzo[a]pyrene, 14.8% for crocidolite asbestos, and 40.9% for nickel powder. The alterations detected in the banding pattern of benzo[a]pyrene-induced peritoneal tumors were exclusively additional bands. On the contrary, in the DNA from asbestos-induced peritoneal tumors, only deletions of bands were observed on the autoradiographs. In the DNA from nickel-induced peritoneal tumors, both types of mutations occurred. The different mutation frequencies and mutation patterns appear to discriminate between benzo[a]pyrene, crocidolite asbestos, and nickel powder, and may be related to the mechanisms of action of these compounds.

Analysis of loss of heterozygosity in murine hepatocellular tumors

Because allelotype analysis of tumors has been important in the identification of new tumor suppressor genes, here we studied loss of heterozygosity (LOH) in a well-defined animal experimental system. We analyzed spontaneous liver tumors from C3HHc x C57BL/6J (B6C3F1) mice and urethane-induced hepatocellular tumors from (C3H/He x Mus spretus) x C57BL/6JBy (HSB) interspecific mice. A total of 95 different genetic markers were tested: 13 in 24 B6C3F1 tumors, 76 in 58 HSB tumors, and six in both groups. Minisatellite finger-printing analysis detected one case of LOH and less than 1% genomic rearrangements in polymorphic and nonpolymorphic bands, respectively. There were no changes at hepatocellular susceptibility loci or at markers homologous to loci frequently lost in human hepatocellular carcinomas. Therefore, our results suggest that LOH and genomic rearrangements are uncommon in mouse hepatocellular tumors.

Demonstration by DNA fingerprint analysis of genomic instability in mouse BALB 3T3 cells during cell transformation

We employed DNA fingerprint analysis to monitor DNA rearrangements in BALB 3T3 cells transformed spontaneously or by treatment with 3-methylcholanthrene (MCA) and UV-C. The effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in combination with MCA was also examined. Twenty-three spontaneously transformed cells, 28 induced transformed cells (18 by 1 microgram/ml MCA, six by 5 micrograms/ml MCA, and four by UV-C), and 31 non-transformed subclones were isolated from parental BALB 3T3 A31-1-1 cells. The DNAs were digested with HinfI and subjected to DNA fingerprint analysis with three multi-locus minisatellite probes, Per-6, Core, and Ins. Per-6 was the most effective probe for detecting DNA rearrangements. Rearranged bands detected by the Per-6 probe were observed in 9/31 (29%) of non-transformed subclones, 14/23 (61%) of spontaneously transformed cells, 16/18 (89%) of cells transformed by 1 microgram/ml of MCA, 6/6 (100%) of cells transformed by 5 micrograms/ml MCA, and 4/4 (100%) of UV-C-transformed cells. Higher numbers of DNA rearrangements (> or = 3) occurred most frequently in the induced transformed cells. TPA enhanced the frequency of DNA rearrangements in cells transformed by MCA. These data indicate that (1) genomic DNA in BALB 3T3 cells is unstable and susceptible to rearrangement, (2) its instability is elevated during cell transformation, and (3) MCA and UV-C induce DNA rearrangements, and TPA enhances the effect of the former, probably via the recombination process. DNA fingerprint analysis is valuable for monitoring genomic instability during cell transformation.

p53 mutations are absent from carcinogen-induced mouse liver tumors but occur in cell lines established from these tumors

Mutations in the p53 gene are frequent genetic alterations in human hepatocellular carcinomas. We have examined, by single-strand conformation polymorphism analysis of polymerase chain reaction products, a total of 93 carcinogen-induced liver tumors from mice of three different strains (C3H/He, C57BL/6J, and B6C3F1) for the presence of p53 aberrations. Hepatoma lines, established from 12 liver tumors, were also included in the analysis. While structural aberrations of the p53 gene were not detected in any of the primary mouse liver tumors analyzed, single-base substitutions occurred at different locations within the p53 gene in three of the cell lines during in vitro propagation. One hepatoma line carried two point mutations on separate alleles. All four mutations were either G:C----C:G or C:G----G:C transversions. Mutations at codon 61 of the c-Ha-ras gene, which were frequent in primary liver tumors from C3H/He and B6C3F1 mice, were not detected in the hepatoma lines. Our data indicate (i) that c-Ha-ras but not p53 mutations play an important role during the early stages of mouse hepatocarcinogenesis and (ii) that p53 mutations confer a selective growth advantage to the mutated hepatoma cells in vitro.