DNA contents and chromosomes of clonal lines of transformed rat liver epithelial cells and of cells from their derived tumors

Identification of candidate liver tumor suppressor genes from human 11p11.2-p12

We have previously described a functional model for identification of human liver tumor suppressor genes in which human chromosome 11 was introduced into rat liver epithelial tumor cell lines via microcell-mediated chromosome transfer, producing microcell hybrid (MCH) cell lines that exhibit suppression of tumorigenicity in vivo. Chromosome deletion mapping studies identified a 950-kb region of 11p11.2-p12 that was retained in all suppressed MCH cell lines, suggesting that this region may harbor one or more genes with liver tumor suppressor function. In this study, we generated a comprehensive transcription map of the 11p11.2-p12 liver tumor suppressor region through examination of 142 expressed sequence tag (EST) markers among a group of suppressed MCH cell lines. Of 142 ESTs examined, 19 were localized within the 11p11.2-p12 liver tumor suppressor region. RT-PCR analysis of gene expression for these 19 ESTs among an index panel of suppressed MCH cell lines (n = 3) identified 11 potential candidate liver tumor suppressor genes. Examination of candidate gene expression among six additional suppressed MCH cell lines reduced the number of potential candidate genes to three (stSG30184, stSG10014, and stSG29748). Northern blot analysis of suppressed MCH cell lines and derived tumor cell lines suggested stSG30184 as the best candidate liver tumor suppressor gene. The 3.7 kb stSG30184 transcript was expressed by all suppressed MCH cell lines, but expression was extinguished coordinately with reexpression of tumorigenicity by these cells, consistent with a tumor suppressor gene. Subsequent characterization of this EST indicates that it is a novel transcript with expression in a broad range of tissue types. Further characterization of the genes identified in this study will provide a greater understanding of their role in the molecular pathogenesis of neoplastic liver disease.

Genomic fluidity is a necessary event preceding the acquisition of tumorigenicity during spontaneous neoplastic transformation of WB-F344 rat liver epithelial cells

The genomic evolution of a cohort of WB-F344 rat liver epithelial cell lineages undergoing spontaneous neoplastic transformation was followed to define the mechanistic relationship between genomic instability and progression to the neoplastic phenotype. Eighteen independent populations of WB-F344 cells (initiated from a single diploid-founding population) were subjected to 12 cycles of selective growth at confluent cell density, and cellular DNA contents were measured after each selection cycle. Flow cytometry demonstrated significant gains in the amount of G1 DNA after selection cycles 3, 6, and 7 in 44% (8 of 18), 89% (16 of 18), and 39% (7 of 18) of the cell populations, respectively. All populations subsequently lost DNA and returned to a diploid or pseudo-diploid DNA content within 1 to 2 selection cycles after the appearance of an increased DNA content. Additionally, appearance and subsequent disappearance of aneuploid or tetraploid subpopulations was observed in 11% (2 of 18) and 83% (15 of 18) of the experimental lineages, respectively. Although perturbations of G1 DNA content were apparent as early as selection cycle 3, at least 8 cycles of selective growth were required for the acquisition of tumorigenicity. While the independent lineages demonstrated significant fluctuations in G1 DNA content between selection cycles 3 and 8, the majority (11 of 13) of the populations contained a diploid or pseudo-diploid DNA content at the time tumorigenicity was expressed. Genomic instability preceded the acquisition of tumorigenic potential in rat liver epithelial cells subjected to selective growth conditions of maintenance at confluence, and may be required for its expression.

Localization of a putative liver tumor suppressor locus to a 950-kb region of human 11p11.2-p12 using rat liver tumor microcell hybrid cell lines

We previously demonstrated that a locus (or loci) linked to the D11S436 marker, which is within the approximately 6-Mb cen-p12 region of human chromosome 11, suppresses the tumorigenic potential of some rat liver epithelial tumor microcell hybrid (MCH) cell lines. To more precisely map this putative liver tumor suppressor locus, we examined 25 loci from human chromosome 11 in suppressed MCH cell lines. Detailed analysis of these markers revealed a minimal area of overlap among the suppressed MCH cell lines corresponding to the chromosomal region bounded by (but not including) microsatellite markers D11S1319 and D11S1958E and containing microsatellite markers D11S436, D11S554, and D11S1344. Direct examination of the kang ai 1 (KA/1) prostatic adenocarcinoma metastasis suppressor gene (which is closely linked to D11S1344) produced evidence suggesting that this locus was not responsible for tumor suppression in this model system. In addition, our data strongly suggested that the putative liver tumor suppressor locus was distinct from other known 11p tumor suppressor loci, including the multiple exotoses 2 locus (at 11p11.2-p12), Wilms' tumor 1 locus (at 11p13), and Wilms' tumor 2 locus (at 11p15.5). The results of this study significantly narrowed the chromosomal location of the putative liver tumor suppressor locus to a region of human 11p11.2-p12 that is approximately 950 kb. This advance forms the basis for positional cloning of candidate genes from this region and, in addition, identified a number of chromosomal markers that will be useful for determining the involvement of this locus in the pathogenesis of human liver cancer.

Suppression of the tumorigenic phenotype of a rat liver epithelial tumor cell line by the p11.2-p12 region of human chromosome 11

Comparative chromosomal mapping studies and investigations of tumor-associated chromosomal abnormalities suggest that the development of hepatic tumors in humans and rats may share a common molecular mechanism that involves inactivation of the same tumor suppressor genes or common genetic loci. We investigated the potential of human chromosomes 2 and 11 to suppress the tumorigenic phenotype of rat liver epithelial tumor cell lines. These tumor cell lines (GN6TF and GP7TB) display elevated saturation densities in culture, efficiently form colonies in soft agar, and produce subcutaneous tumors in 100% of syngeneic rat hosts with short latency periods. Introduction of human chromosome 11 by microcell fusion markedly altered the tumorigenicity and the transformed phenotype of GN6TF cells. In contrast, the tumorigenic potential and phenotype of GP7TB cells was unaffected by the introduction of human chromosome 11, indicating that not all rat liver tumor cell lines can be suppressed by loci carried on this chromosome. Introduction of human chromosome 2 had little or no effect on the tumorigenicity or cellular phenotype of either tumor cell line, suggesting the involvement of chromosome 11-specific loci in the suppression of the GN6TF tumor cell line. The GN6TF-11neo microcell hybrid cell lines displayed significantly reduced saturation densities in monolayer cultures, and their ability to grow in soft agar was completely inhibited. Although GN6TF-11neo cells ultimately formed tumors in 80-100% of syngeneic rat hosts, the latency period for tumor formation was much longer. Molecular characterization of GN6TF-11neo microcell hybrid cell lines indicated that some of the clonal lines had spontaneously lost significant portions of the introduced human chromosome, partially delineating the chromosomal location of the putative tumor suppressor locus to the region between the centromere and 11p12. Molecular examination of microcell hybrid-derived tumor cell lines further defined the minimal portion of human chromosome 11 capable of tumor suppression in this model system to the region 11p11.2-p12.