Benign salivary gland tumors: a cytogenetic study of 21 cases

Chromosome Translocations, Gene Fusions, and Their Molecular Consequences in Pleomorphic Salivary Gland Adenomas

Salivary gland tumors are a heterogeneous group of tumors originating from the major and minor salivary glands. The pleomorphic adenoma (PA), which is the most common subtype, is a benign lesion showing a remarkable morphologic diversity and that, upon recurrence or malignant transformation, can cause significant clinical problems. Cytogenetic studies of >500 PAs have revealed a complex and recurrent pattern of chromosome rearrangements. In this review, we discuss the specificity and frequency of these rearrangements and their molecular/clinical consequences. The genomic hallmark of PA is translocations with breakpoints in 8q12 and 12q13-15 resulting in gene fusions involving the transcription factor genes PLAG1 and HMGA2. Until recently, the association between these two oncogenic drivers was obscure. Studies of the Silver−Russel syndrome, a growth retardation condition infrequently caused by mutations in IGF2/HMGA2/PLAG1, have provided new clues to the understanding of the molecular pathogenesis of PA. These studies have demonstrated that HMGA2 is an upstream regulator of PLAG1 and that HMGA2 regulates the expression of IGF2 via PLAG1. This provides a novel explanation for the 8q12/12q13-15 aberrations in PA and identifies IGF2 as a major oncogenic driver and therapeutic target in PA. These studies have important diagnostic and therapeutic implications for patients with PA.

Particular aspects in the cytogenetics and molecular biology of salivary gland tumours - current review of reports

Salivary gland tumours are a group of lesions whose heterogeneity of biological and pathological features is widely reflected in the molecular aspect. This is demonstrated by an increasing number of studies in the field of genetics of these tumours.

The aim of this study was to collect the most significant scientific reports on the cytogenetic and molecular data concerning these tumours, which might facilitate the identification of potential biomarkers and therapeutic targets. The analysis covered 71 papers included in the PubMed database. We focused on the most common tumours, such as pleomorphic adenoma, Warthin tumour, mucoepidermoid carcinoma, and others. The aim of this study is to present current knowledge about widely explored genotypic alterations (such as PLAG1 gene in pleomorphic adenoma or MECT1 gene in mucoepidermoid carcinoma), and also about rare markers, like Mena or SOX10 protein, which might also be associated with tumourigenesis and carcinogenesis of these tumours.

Copy number gain at 8q12.1-q22.1 is associated with a malignant tumor phenotype in salivary gland myoepitheliomas

Salivary gland myoepithelial tumors are relatively uncommon tumors with an unpredictable clinical course. More knowledge about their genetic profiles is necessary to identify novel predictors of disease. In this study, we subjected 27 primary tumors (15 myoepitheliomas and 12 myoepithelial carcinomas) to genome-wide microarray-based comparative genomic hybridization (array CGH). We set out to delineate known chromosomal aberrations in more detail and to unravel chromosomal differences between benign myoepitheliomas and myoepithelial carcinomas. Patterns of DNA copy number aberrations were analyzed by unsupervised hierarchical cluster analysis. Both benign and malignant tumors revealed a limited amount of chromosomal alterations (median of 5 and 7.5, respectively). In both tumor groups, high frequency gains (> or =20%) were found mainly at loci of growth factors and growth factor receptors (e.g., PDGF, FGF(R)s, and EGFR). In myoepitheliomas, high frequency losses (> or =20%) were detected at regions of proto-cadherins. Cluster analysis of the array CGH data identified three clusters. Differential copy numbers on chromosome arm 8q and chromosome 17 set the clusters apart. Cluster 1 contained a mixture of the two phenotypes (n = 10), cluster 2 included mostly benign tumors (n = 10), and cluster 3 only contained carcinomas (n = 7). Supervised analysis between malignant and benign tumors revealed a 36 Mbp-region at 8q being more frequently gained in malignant tumors (P = 0.007, FDR = 0.05). This is the first study investigating genomic differences between benign and malignant myoepithelial tumors of the salivary glands at a genomic level. Both unsupervised and supervised analysis of the genomic profiles revealed chromosome arm 8q to be involved in the malignant phenotype of salivary gland myoepitheliomas.

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.

PLAG1 gene alterations in salivary gland pleomorphic adenoma and carcinoma ex-pleomorphic adenoma: a combined study using chromosome banding, in situ hybridization and immunocytochemistry

Pleomorphic adenoma is the most common benign tumor of the salivary glands. It has marked histological diversity with epithelial, myoepithelial and mesenchymal-type cells arranged in a variety of architectural and differentiation patterns. Pleomorphic adenoma gene 1 (PLAG1), shown to be consistently rearranged in pleomorphic adenomas, is activated by chromosomal translocations involving 8q12, the chromosome region that is most frequently affected in these tumors. In this study, we evaluated PLAG1 involvement in salivary gland tumorigenesis by determining the frequency of its alterations in a selected group of 20 salivary gland tumors: 16 pleomorphic adenomas and four carcinomas ex-pleomorphic adenoma, having in common the presence of karyotypic chromosome 8 deviations, either structural, with 8q12 rearrangements, or numerical, with gain of chromosome 8. PLAG1 status was analyzed using in situ hybridization techniques, on metaphase cells, by fluorescence detection and/or interphase cells in paraffin sections, by chromogenic detection. Except for one pleomorphic adenoma case (5%) that lacked PLAG1 involvement, 17 tumors (85%), (14 pleomorphic adenomas and three carcinomas ex-pleomorphic adenoma) showed intragenic rearrangements of PLAG1 and the remaining two cases (10%), (one pleomorphic adenoma and one carcinoma ex-pleomorphic adenoma), had chromosome trisomy 8 only. To further investigate the role of PLAG1 on pleomorphic adenomas tumorigenesis, as well as the putative morphogenesis mechanism, we attempted to identify the cell types (epithelial vs myoepithelial) carrying 8q12/PLAG1 abnormalities by a combined phenotypic/genotypic analysis in four cases (three pleomorphic adenoma and one carcinoma ex-pleomorphic adenoma) characterized by 8q12 translocations and PLAG1 rearrangement. In these cases, both cells populations carried PLAG1 rearrangements. This finding further supports the pluripotent single-cell theory, which postulates that the tumor-initiated, modified myoepithelial cell, evolves into the varied somatic cell phenotypes present in pleomorphic adenoma, and reinforces the role of PLAG1 on the tumorigenesis of benign and malignant pleomorphic adenoma.

Pleomorphic adenoma of the parotid in childhood

INTRODUCTION

Pleomorphic adenoma is a benign growth that most often appears in the parotid gland where it is the most frequent type of tumour. Its appearance in childhood is rare and there are no large series available in this age group for comparison with its presentation in adults. Surgical treatment is similar in children and in adults; whilst the prognosis is good, they may become malignant and metastasize.

MATERIAL AND METHOD

In this study, we present seven parotid pleomorphic adenomas in childhood treated in our Department and a review of the literature. The following data were evaluated: sex, age, affected side, clinical symptoms and period of evolution of the symptoms, surgical technique applied and complications observed.

RESULTS

The mean age at presentation was 12.4 years. All patients were males with the right side affected in five patients. The treatment was conservative superficial parotidectomy in five patients, conservative total parotidectomy in one, and an enucleation including a wide safety margin in the last case. None of the patients exhibited any permanent postoperative complication. The mean follow-up was 41.3 months (26-58 months).

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.

High-resolution DNA flow cytometry in papillary cystadenoma lymphomatosum (Warthin's tumour)

Twenty-eight examples of papillary cystadenoma lymphomatosum (Warthin's tumour) of the parotid gland were analysed by high-resolution DNA flow cytometry. The mean coefficient of variation was found to be 1.19% (SD: 0.41). All tumours were DNA diploid. These results did not correspond with expected deviations based on published chromosomal studies. Also, the homogeneously low S-phase fractions (mean: 4.8%; SD: 2.7) found did not support the hypothesis of etiologically distinctive subgroups in these tumours.

Cytogenetic characterisation of Warthin's tumour

Warthin's tumour is a peculiar subtype of monomorphic adenomas of the salivary glands, frequently cystic, and that characteristically associates an epithelial glandular cell component to a dense lymphoid infiltrate. Short-term cultures from 12 Warthin's tumours of salivary glands, including 5 previously reported cases were successfully karyotyped and clonal numerical and/or structural changes were detected in 7 of them (58%). 3 cases showed numerical abnormalities with loss of chromosomes Y (2 cases) and X (1 case). The remaining 4 abnormal cases presented the following structural changes: complex translocation t(11;19;16)(q21;p12;p13.3); reciprocal translocations t(6;8)(p23;q22) and t(6;15)(p21;q15) (2 cases); and 1p22, 3p26, 11p13 changes. In 1 case, clonal numerical deviations (+ 7 and -Y) were concurrent with the structural rearrangement t(6;8). Two of these aberrations are suggested to be Warthin's tumour-associated: 11q;19p translocation has already been described in 3 cases, and structural rearrangements of 6p23 have also been reported in another case. Our study extends the cytogenetic information about Warthin's tumour and identifies two recurrent abnormalities --6p rearrangements and t(11;19)--specific for this salivary neoplasm.

Malignant salivary gland neoplasms: a cytogenetic study of 19 cases

A group of 19 malignant salivary gland neoplasms of various histological types (mucoepidermoid carcinoma, acinic cell carcinoma, adenoid cystic carcinoma, epithelial-myoepithelial carcinoma, myoepithelial carcinoma, basal cell adenocarcinoma, carcinoma ex-pleomorphic adenoma, ductal carcinoma, adenocarcinoma not otherwise specified and undifferentiated carcinoma) were cytogenetically investigated. Previous karyotypic information revealed deletion of the long arm of chromosome 6, loss of chromosome Y and the gain of chromosome 8 as the most recurrent deviations found in these neoplasms. Clonal chromosome aberrations were detected in 11 cases of this series. In 7 of them there were only numerical deviations (gain of chromosomes 2, 7, 8, 10 and X and loss of chromosomes 18, 21 and Y) without concomitant structural anomalies. Structural rearrangements such as t(2;7), t(6;16), t(6;9) and t(1;1) translocations were found in two mucoepidermoid carcinomas, one adenoid cystic carcinoma and one ductal carcinoma, respectively. The wide spectrum of changes found in this group of neoplasms may reflect the diversity in their histogenesis and differentiation phenotypes.