Pleomorphic adenomas with unbalanced chromosomal abnormalities have an increased in vitro lifetime

WIF1, an inhibitor of the Wnt pathway, is rearranged in salivary gland tumors

Chromosome rearrangements involving 12q13-15 are frequent among several tumors, including pleomorphic adenomas. The common molecular target for these aberrations is the HMGA2 gene, but various fusion partners of HMGA2 have been reported in tumors. Here we report the identification of the WNT inhibitory factor 1 (WIF1) gene as a novel HMGA2 fusion partner in a salivary gland pleomorphic adenoma. In normal salivary gland tissue WIF1 is expressed at a high level and HMGA2 is not expressed. However, in the pleomorphic adenoma expressing the HMGA2/WIF1 fusion transcript, we observed re-expression of HMGA2 wild-type transcripts and very low levels of WIF1 expression. These data suggest a possible synergistic effect between upregulation of HMGA2 and downregulation of WIF1. We screened 13 additional benign and malignant salivary gland tumors and detected WIF1 rearrangement in one out of two carcinomas ex-pleomorphic adenoma analyzed. In this malignant tumor, the rearrangement of one WIF1 allele coexists with loss of the other allele, a classic signature of a tumor suppressor gene. WIF1 is an antagonist of the Wnt signaling pathway, which plays a critical role in human cancer. In transgenic mouse models, Wnt activation leads to a high frequency of benign and malignant salivary gland tumors. To our knowledge, this is the first report suggesting that WIF1 is a recurrent target in human salivary gland oncogenesis and that downregulation of WIF1 plays a role in the development and/or progression of pleomorphic adenomas.

In vitro transformation of cell lines from human salivary gland tumors

Explanted cells from salivary gland tumors are particularly difficult to propagate in vitro and not efficiently immortalized by agents such as simian virus 40. Human papillomavirus 16 (HPV16) has been widely used to transform cells of epithelial origin, but its use for salivary gland cell transformation has not been described. In this study, we employed viral constructs containing the E6/E7 genes of HPV16 to infect and stably transform 9 salivary gland tumor cell cultures. Four of the tumor cell cultures were derived from benign tumors and 5 from malignant tumors. All of the original cell cultures were diploid; however, 6 contained subpopulations of cells with structural abnormalities. All 9 cell cultures were successfully transformed, and 8 were immortalized. The resulting cell lines have decreased serum requirements, exhibit a high proliferation rate, are E6/E7-positive and form colonies in soft agar. Immuno-histochemical and molecular studies confirmed that the transformed cells were indeed epithelial/myoepithelial in origin. All of the transformed cell lines had a diploid or near-diploid karyotype, and 2 contained the original translocated chromosomes in all cells. Our report represents a new application of the E6/E7 system in immortalizing salivary gland cell cultures, resulting in retention of the cellular features found in the native tissue without a general destabilization of the karyotype. These types of tissue culture resources should prove useful for positional cloning and functional studies of genes involved in salivary gland oncogenesis.

Deletions of the short arm of chromosome 3 in solid tumors and the search for suppressor genes

The concept that cells can become malignant upon the elimination of parts of chromosomes inhibiting cell division dates back to Boveri in 1914. Deletions occurring in tumor cells are therefore considered a first indication of possible locations of tumor suppressor gene. Approaches used to localize and identify the paradigm of tumor suppressors, RB1, have also been applied to localize tumor suppressor genes on 3p, the short arm of chromosome 3. This review discusses the methodological advantages and limitations of the various approaches. From a review of the literature on losses of 3p in different types of solid tumors it appears that some tumor types show involvement of the same region, while between others the regions involved clearly differ. Also discussed are results of functional assays of tumor suppression by transfer of part of chromosome 3 into tumor cell lines. The likelihood that a common region of deletions would contain a tumor suppressor is strongly enhanced by coincidence of that region with a chromosome fragment suppressing tumorigenicity upon introduction in tumor cells. Such a situation exists for a region in 3p21.3 as well as for one or more in 3p12-p14. The former region is considered the location of a lung cancer suppressor. The same gene or a different one in the same region may also play a role in the development of other cancers including renal cell cancer. In the latter cancer, there may be additional roles of the VHL region and/or a 3p12-p14 region. The breakpoint region of a t(3;8) originally found to be constitutively present in a family with hereditary renal cell cancer now seems to be excluded from such a role. Specific genes on 3p have been suggested to act as suppressor genes based on either their location in a common deletion region, a markedly reduced expression or presence of aberrant transcripts, their capacity to suppress tumorigenicity upon transfection in to tumor cells, the presumed function of the gene product, or a combination of several of these criteria. A number of genes are evaluated for their possible role as a tumor suppressor according to these criteria. General agreement on such a role seems to exist only for VHL. Though hMLH1 plays an obvious role in the development of specific mismatch repair-deficient cancers, it cannot revert the tumor phenotype and therefore cannot be considered a proper tumor suppressor. The involvement of VHL and MLH1 also in some specific hereditary cancers allowed to successfully apply linkage analysis for their localization. TGFBR2 might well have a tumor suppressor function. It does reduce tumorigenicity upon transfection. Other 3p genes coding for receptor proteins THRB and RARB, are unlikely candidates for tumor suppression. Present observations on a possible association of FHIT with tumor development leave a number of questions unanswered, so that provisionally it cannot be considered a tumor suppressor. Regions that have been identified as crucial in solid tumor development appear to be at the edge of synteny blocks that have been rearranged through the chromosome evolution which led to the formation of human chromosome 3. Although this may merely represent a chance occurrence, it might also reflect areas of genomic instability.

Detection of hidden structural rearrangements by FISH in pleomorphic adenomas

Previous cytogenetic analysis of pleomorphic adenomas of the salivary glands has revealed a subgroup of tumors with interstitial deletions of 8q with breakpoints at q12 and q21-23. In this paper we have re-examined four adenomas with confirmed or suspected deletions of 8q by FISH. Painting of interphase nuclei and metaphase chromosomes with whole chromosome libraries revealed that in each of the four cases the deleted chromosome 8 material was found inserted or translocated onto other chromosomes. In two of the cases we were also able to resolve several other complex rearrangements. Our FISH data thus suggest that gross deletions of 8q do not occur in pleomorphic adenomas. These observations are of crucial importance for the molecular interpretations of the 8q12 rearrangements. The fact that all rearrangements with breakpoints at 8q12 seem to be translocations or insertions strongly indicates that they involve a similar molecular mechanism. Our findings illustrate that chromosome painting is a valuable and useful adjunct to conventional cytogenetic analysis of solid tumors.

Submicroscopic deletions of 3p sequences in pleomorphic adenomas with t(3;8)(p21;q12)

A subgroup of benign pleomorphic adenomas of the salivary glands is characterized by translocations, or on rare occasions deletions, with breakpoints at 3p21. We have applied restriction fragment length polymorphism (RFLP) analysis to assess the frequency of allelic losses at four different loci located within 3p21-->p25 in 35 pleomorphic adenomas, 18 of which were also karyotyped. Parallel analysis of constitutional and tumor DNAs in informative tumors revealed that all patients retained heterozygosity in their tumor DNA at the D3S2 and RAF1 loci. Among the 29 tumors informative for THRB three showed loss of heterozygosity (LOH). All three tumors had a t(3;8)(p21;q12). Of the 23 tumors informative for D3F15S2, one showed LOH. This tumor also had a t(3;8)(p21;q12). To further map the deletions in relation to the 3p21 translocation breakpoint, we also sublocalized the THRB locus. Using in situ hybridization we assigned the gene to 3p24.1-3. The fact that none of the tumors with loss of 3p alleles showed cytogenetic evidence of deletions indicates that the losses are submicroscopic, probably interstitial, and in most cases distal to the 3p21 breakpoint. This was confirmed in one case with loss of a THRB allele where both proximal (D3F15S2) and distal (RAF1) markers retained heterozygosity. Our results suggest that deletion of 3p sequences might be of progressional importance in a subset of pleomorphic adenomas with t(3;8)(p21;q12).

Leiomyoma cells with 12q15 aberrations can be transformed in vitro and show a relatively stable karyotype during precrisis period

Tumor cells from three uterine leiomyomas showing translocations involving 12q14-15 were transformed by transfection using the "early regions" of the SV40 genome. The cells had a higher proliferative capacity, were able to form colonies in soft agar, and showed an increased growth potential. Karyotype analyses of these transformed leiomyoma cells showed that the cells had retained the initial t(12;14) and t(12;15).