Fluorescence in situ hybridization mapping of breakpoints in pleomorphic adenomas with 8q12-13 abnormalities identifies a subgroup of tumors without PLAG1 involvement

Insights into the molecular alterations of PLAG1 and HMGA2 associated with malignant phenotype acquisition in pleomorphic adenoma

Pleomorphic adenoma (PA) is the most common neoplasm of the salivary gland, presenting with a variety of histological features. In some cases, PA can undergo malignant transformation to carcinoma ex pleomorphic adenoma (CXPA). The transition from PA to CXPA is associated with complex molecular alterations, particularly involving the pleomorphic adenoma gene 1 (PLAG1) and high mobility group protein gene (HMGA2). This review investigates the molecular alterations of PLAG1 and HMGA2 in all domains in the malignant transformation of PA. Our analysis highlights that these markers are key alterations in the etiopathogenesis of PA and CXPA, with gene fusion and amplification being frequently reported mechanisms. Although the exact role of PLAG1 and HMGA2 in the oncogenic process remains unclear, further studies on the HMGA2 and PLAG1, are needed particularly in HMGA2-PLAG1-IGF2 which is proving to be a potential pathway for the development of clinically applicable therapies, especially for CXPA management.

Pediatric epithelial salivary gland tumors: spectrum of histologies and cytogenetics at a children's hospital

There are conflicting reports regarding the relative frequency of benign and malignant epithelial salivary gland tumors in children. There are only a few reports of the cytogenetic abnormalities in the pleomorphic adenomas (PA) that arise in children, and even less information regarding the pleomorphic adenoma gene 1 (PLAG1) and high motility group A2 (HMGA2 ) histochemical staining in PAs, or their correlation with histologic types (stromal vs epithelial predominance). A retrospective 14 year review of epithelial salivary gland tumors encountered at a children's hospital identified 13 tumors: 12 PAs and 1 acinic cell carcinoma (ACC). No mucoepidermoid carcinomas were identified. Tumors arose in the parotid (7) and other sites (2 submandibular, 4 minor). Ten PAs in our cohort had cytogenetic studies. Four were normal, 5 involved 8q12, and 1 involved 12q13. Immunohistochemistry identified an additional 2 PAs with PLAG1 staining, and 5 additional PAs with HMGA2 staining. One tumor with ins(18;8)(q21.1;q12q22.2) had no PLAG1 staining, but stained with HMGA2. This ins(18;8) may not have involved the PLAG1 gene. There was no demonstrable correlation of 8q12/PLAG1 staining or 12q13/HMGA2 staining with histologic type. Thus we found abnormalities in either 8q12/PLAG1 staining or 12q13/HMGA2 staining in all PAs. The HMGA2 staining in 50% of PAs suggests that it may be more frequently involved in PAs than previously thought based on cytogenetic studies, at least in children.

A link between the expression of the stem cell marker HMGA2, grading, and the fusion CRTC1-MAML2 in mucoepidermoid carcinoma

Recently, the concept of cancer stem cells and their expression of embryonic stem cell markers has gained considerable experimental support. In this study, we examined the expression of one such marker, the high-mobility group AT-hook 2 gene (HMGA2) mRNA, in 53 formalin-fixed, paraffin-embedded mucoepidermoid carcinomas (MEC) and four normal parotid tissues using quantitative real-time RT-PCR (qPCR). MECs are often characterized by the fusion gene CRTC1-MAML2, the detection of which is an important tool for the diagnosis and prognosis of MEC. For detection of the CRTC1-MAML2 fusion transcript, we performed RT-PCR. The mean expression level of HMGA2 was higher in fusion negative (302.8 +/- 124.4; n = 14) than in positive tumors (67.3 +/- 13.1; n = 39). Furthermore, the fusion-negative tumors were often high-grade tumors and the HMGA2 expression level rose with the tumor grade (low: 43.7 +/- 11.0, intermediate: 126.2 +/- 28.3, and high: 271.2 +/- 126.5). A significant difference was found in the HMGA2 expression levels between the different grading groups (one-way ANOVA, P = 0.04) and among the fusion-negative and -positive tumors (t-test, P = 0.05), indicating that the expression level of HMGA2 was closely linked to grading, the presence/absence of the CRTC1-MAML2 fusion, and the tumor behavior of MECs. These findings offer further evidence for the theory that the MEC group comprises two subgroups: one group with the CRTC1-MAML2 fusion, which is a group with a moderate aggressiveness and prognosis, and the other group lacking that fusion corresponding to an increased stemness, and thus, higher aggressiveness and worse prognosis.

Heterogeneity of PLAG1 gene rearrangements in pleomorphic adenoma

Pleomorphic adenoma (PA), a benign mixed salivary gland tumor, has been associated with abnormal karyotypes in up to 70% of cases, with nonrandom involvement of 8q12, the locus of the pleomorphic adenoma (PLAG1) gene. In this study, cytogenetics and fluorescence in situ hybridization (FISH) were used to investigate PLAG1 involvement in PA from seven patients. There were two males and five females ranging in age from 25 to 65 years. Samples of parotid gland tissue from the tumor sites, set up as solid tumor cultures, showed a normal karyotype in two cases [46,XY;46,XX] and cytogenetic abnormalities in five cases (71%). The abnormalities comprised one variant translocation [t(1;4;8)(p32;q35;q12)], two classic translocations [t(5;8)(p13;q12)], one novel deletion [del(12)(p11.2p12.1)], and a novel insertion [ins(9;8)(p22;q12q21.1)]. FISH was performed in all cases by using two probes from the RP11 library, flanking PLAG1; a sequence 1.48 megabases (Mb) upstream and another 2.27 Mb downstream, covering a total area of 3.8 Mb. The PLAG1 gene was intact and normally situated in four cases - the 46,XY, 46,XX, del(12p), and one t(5;8). PLAG1 was disrupted in three cases - one t(5;8), ins(9;8), and t(1;4;8). In addition, genomic instability was seen in two cases, one with PLAG1 amplification in the form of a homogeneously staining region, and the other in der(8) ring formation. The data provide further unique cases showing the complexity of PLAG1 gene rearrangements in PA.

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.

CHCHD7-PLAG1 and TCEA1-PLAG1 gene fusions resulting from cryptic, intrachromosomal 8q rearrangements in pleomorphic salivary gland adenomas

Pleomorphic salivary gland adenomas are characterized by recurrent chromosome rearrangements of 8q12, leading to activation of the PLAG1 oncogene. Here we demonstrate that CHCHD7-PLAG1 is a novel and recurrent gene fusion generated by a cytogenetically cryptic rearrangement in pleomorphic adenomas. CHCHD7 is a newly identified member of a multifamily of proteins containing a conserved (coiled coil 1)-(helix 1)-(coiled coil 2)-(helix 2) domain. Northern blot analysis revealed that the gene is ubiquitously expressed. Its biological function is unknown and the gene has hitherto not been associated with neoplasia. CHCHD7 and PLAG1 are located head-to-head about 500 bp apart in 8q12. Molecular analyses of 27 tumors revealed CHCHD7-PLAG1 fusions in three tumors, two of which had t(6;8) and t(8;15) translocations as the sole anomalies and one a normal karyotype. FISH analyses of interphase nuclei and nuclear chromatin fibers of a fourth adenoma with a normal karyotype revealed that a second fusion partner gene, TCEA1, located about 2 Mb centromeric to PLAG1, also is fused to PLAG1 as a result of a cryptic 8q rearrangement. The breakpoints in both fusions occur in the 5'-noncoding regions of the genes, leading to activation of PLAG1 by promoter swapping/substitution. Western blot and immunohistochemical analyses demonstrated that the PLAG1 protein was overexpressed in epithelial, myoepithelial, and mesenchymal-like tumor cells in tumors with both fusions. Our findings further emphasize the significance of PLAG1 activation in pleomorphic adenomas and demonstrate that the gene is more frequently activated than previously anticipated.

Fusion oncogenes and tumor type specificity--insights from salivary gland tumors

Salivary gland tumors are frequently characterized by recurrent chromosome translocations, which have recently been shown to result in pathogenetically relevant fusion oncogenes. These genes encode novel fusion proteins as well as ectopically expressed normal or truncated proteins, and are found in both benign and malignant salivary gland tumors. The major targets of the translocations are DNA-binding transcription factors (PLAG1 and HMGA2) involved in growth factor signaling and cell cycle regulation, and coactivators of the Notch (MAML2) and cAMP (TORC1) signaling pathways. Identification of these fusion oncogenes has contributed to our knowledge of molecular pathways leading to epithelial tumors in general, and to salivary gland tumors in particular. Interestingly, the fusions in salivary gland tumors do not seem to be as tumor type specific as those in leukemias and sarcomas. Instead, they may function by activating basic transformation pathways that can function in multiple cell types. The downstream gene products of these fusions will be important targets for development of new intracellular therapeutic strategies.

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.

Multiple reciprocal translocations in salivary gland mucoepidermoid carcinomas

Mucoepidermoid carcinoma, the most common human malignant salivary gland tumor, can arise from both major and minor salivary glands, including sites within the pulmonary tracheobronchial tree. We performed comparative genomic hybridization (CGH) and spectral karyotyping (SKY) on two tumor cell lines: H3118, derived from tumor originating in the parotid gland, and H292, from tumor in the lung. In both cell lines, CGH showed a partial gain within the short arm of chromosome 7 and SKY revealed the presence of the previously reported reciprocal translocation t(11;19)(q21;p12). Additional chromosomal rearrangements were found in both cell lines, including three more reciprocal translocations in cell line H292 [t(1;16), t(6;8)x2] and three other reciprocal translocations in cell line H3118 [t(1;7), t(3;15), and t(7;15)]. A review of the literature of other reported cases of mucoepidermoid carcinomas analyzed with standard G-banding techniques, as well as distinct benign salivary gland tumors, such as pleomorphic adenomas and Warthin tumor, confirmed the presence of a karyotype dominated by reciprocal translocations. Four chromosomal bands were involved in chromosomal translocations in both cell lines: 1q32, 5p15, 7q22, and 15q22. Fluorescence in situ hybridization studies showed that the breakpoints in these four bands were often within a few megabases of each other. The involvement of similar chromosomal bands in breakpoints in these two cell lines suggests that these regions may be predisposed or selected for chromosomal rearrangements in this tumor type. The presence of multiple reciprocal translocations in both benign and malignant salivary gland tumors may also suggest a particular mechanism within mucous or serous glands mediating chromosomal rearrangements.

Molecular analysis of nonrandom 8q12 deletions in acute lymphoblastic leukemia: identification of two candidate genes

Acute lymphoblastic leukemia is the most common malignancy in childhood. High-resolution allelotyping performed in our laboratory showed new chromosomal sites of nonrandom deletions. We have focused our work on 8q12 deletions, which we have found in about 4% of patients (eight of 205 informative cases). These deletions were of small size (less than 1 Mb) in all but one patient, and the deleted region common to all patients was delineated between two microsatellite markers (D8S1113 and D8S1763). This region was sequenced entirely from two overlapping bacterial artificial chromosomes. The common deleted region (120 kb) had a low GC content (37%), was composed more than 50% of LINE sequences, and contained only two candidate genes. The centromeric deletion borders were clustered within an interval of 33 kb between two microsatellite markers. This interval contains the first exon of an HMG-1-related gene (KIAA0808) and a putative gene, DL8q12, predicted to encode a protein with 231 amino acid residues with no homolog in protein databases. Analysis of the available mRNA from lymphoblastic cells of two patients with 8q12 deletions using common polymorphisms in the 3' UTR of KIAA0808 showed monoallelic expression of this gene. Identification of a biallelic polymorphism in the first exon of DL8q12 showed that this gene was deleted in two of four informative cases. Sequencing of the exons of both genes from all patients with 8q12 deletions did not show any mutation, which suggests that neither of these genes behaves as a classic tumor suppressor gene.

Translocation, deletion/amplification, and expression of HMGIC and MDM2 in a carcinoma ex pleomorphic adenoma

Carcinoma ex pleomorphic adenoma (CexPA) is a carcinoma developing within a pre-existing benign pleomorphic adenoma (PA). Here we describe the identification and characterization of a series of genetic events leading to translocation, deletion/amplification, and overexpression of the HMGIC and MDM2 genes in a CexPA at an early stage of development. The tumor had a pseudodiploid stemline karyotype with a del(5)(q22-23q32-33) and a t(10;12)(p15;q14-15). In addition, there were several sidelines with double minute chromosomes (dmin) or homogeneously staining regions (hsr). Fluorescence in situ hybridization (FISH) mapping revealed that the 12q14-15 breakpoint was located centromeric to HMGIC and that the entire gene was juxtaposed to the der(10) chromosome. Detailed analysis of cells with dmin and hsr revealed that HMGIC and MDM2 were deleted from the der(10) and that the dmin and hsr were strongly positive for both genes. Southern blot analysis confirmed that both HMGIC and MDM2 were amplified and that no gross rearrangements of the genes had occurred. Immunostaining revealed that the HMGIC protein was highly overexpressed particularly in the large polymorphic cells within the carcinomatous part of the tumor. These findings suggest that amplification and overexpression of HMGIC and possibly MDM2 might be important genetic events that may contribute to malignant transformation of benign PA.

Fusion genes in solid tumors

Tumor development in different cell types and tissue locations involves many pathways, distinct genes and exogenous factors. Tumor type-specific chromosome rearrangements resulting in fusion genes or promoter swapping are believed to be involved in the early development of many tumor types. They are present in almost all cases of a particular tumor type and cases have been described that carry only tumor type-specific translocations without any signs of other cytogenetic changes. The mechanisms behind chromosome rearrangements in solid tumors are largely unknown. Radiation is an important factor in thyroid carcinomas but no com-$bmon sequence motifs are made out in the break points of solid tumors. The fusion genes found in sarcomas are dominated by the transcription factor type of genes with the TLS/FUS and EWS series of fusion genes as the largest group. More than 50% of papillary thyroid carcinomas carry fusion proteins with tyrosine kinase activity. Rearrangements involving HMGIC, HMGIY, and PLAG1 are common in benign mesenchymal tumors and salivary gland adenomas. Many recurrent tumor translocations show a strict specificity for tumor type. This specificity can most likely be explained by the specific sets of target genes that are deregulated by the fusion gene products. Identification of the downstream target genes is currently the object of intense research and may provide us with information that will help design better diagnostic tools and eventually find a cure for these diseases.