Chromosomal patterns in a benign human neoplasm, the mixed salivary gland tumour

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.

Roles of fusion genes in digestive system cancers: dawn for cancer precision therapy

For advanced and advanced tumors of the digestive system, personalized, precise treatment could be a lifesaving medicine. With the development of next-generation sequencing technology, detection of fusion genes in solid tumors has become more extensive. Some fusion gene targeting therapies have been written into the guidelines for digestive tract tumors, such as for neurotrophic receptor tyrosine kinase, fibroblast growth factor receptor 2. There are also many fusion genes being investigated as potential future therapeutic targets. This review focuses on the current detection methods for fusion genes, fusion genes written into the digestive system tumor guidelines, and potential fusion gene therapy targets in different organs to discuss the possibility of clinical treatments for these targets in digestive system tumors.

Activation of PLAG1 and HMGA2 by gene fusions involving the transcriptional regulator gene NFIB

The pleomorphic adenoma (PA), which is the most common salivary gland neoplasm, is a benign tumor characterized by recurrent chromosome rearrangements involving 8q12 and 12q14-15. We have previously shown that the PLAG1 and HMGA2 oncogenes are the targets of these rearrangements. Here, we have identified previously unrecognized subsets of PAs with ins(9;8)/t(8;9) (n = 5) and ins(9;12)/t(9;12) (n = 8) and breakpoints located in the vicinity of the PLAG1 and HMGA2 loci. RNA-sequencing and reverse transcriptase (RT)-PCR analyses of a case with an ins(9;8) revealed a novel NFIB-PLAG1 fusion in which NFIB exon 4 is linked to PLAG1 exon 3. In contrast to the developmentally regulated PLAG1 gene, NFIB was highly expressed in normal salivary gland, indicating that PLAG1 in this case, as in other variant fusions, is activated by promoter swapping. RT-PCR analysis of three PAs with t(9;12) revealed two tumors with chimeric transcripts consisting of HMGA2 exon 4 linked to NFIB exons 9 or 3 and one case with a fusion linking HMGA2 exon 3 to NFIB exon 9. The NFIB fusion events resulted in potent activation of PLAG1 and HMGA2. Analysis of the chromatin landscape surrounding NFIB revealed several super-enhancers in the 5'- and 3'-parts of the NFIB locus and its flanking sequences. These findings indicate that PLAG1 and HMGA2, similar to MYB in adenoid cystic carcinoma, may be activated by enhancer-hijacking events, in which super-enhancers in NFIB are translocated upstream of PLAG1 or downstream of HMGA2. Our results further emphasize the role of NFIB as a fusion partner to multiple oncogenes in histopathologically different types of salivary gland tumors.

IGF2/IGF1R Signaling as a Therapeutic Target in MYB-Positive Adenoid Cystic Carcinomas and Other Fusion Gene-Driven Tumors

Chromosome rearrangements resulting in pathogenetically important gene fusions are a common feature of many cancers. They are often potent oncogenic drivers and have key functions in central cellular processes and pathways and encode transcription factors, transcriptional co-regulators, growth factor receptors, tyrosine kinases, and chromatin modifiers. In addition to being useful diagnostic biomarkers, they are also targets for development of new molecularly targeted therapies. Studies in recent decades have shown that several oncogenic gene fusions interact with the insulin-like growth factor (IGF) signaling pathway. For example, the MYB-NFIB fusion in adenoid cystic carcinoma is regulated by IGF1R through an autocrine loop, and IGF1R is a downstream target of the EWSR1-WT1 and PAX3-FKHR fusions in desmoplastic small round cell tumors and alveolar rhabdomyosarcoma, respectively. Here, we will discuss the mechanisms behind the interactions between oncogenic gene fusions and the IGF signaling pathway. We will also discuss the role of therapeutic inhibition of IGF1R in fusion gene driven malignancies.

Unusual Interstitial Deletion of the 8q 12 Band in a Case of Acute Monocytic Leukemia

The authors describe a case of acute monocytic leukemia with a clonal deletion of the 8q12 band as a single chromosomal aberration. On the basis of this and other reports from the literature, they suggest that the 8q1 region, hitherto considered significantly involved in solid tumors, may be important also for hematologic malignancies.

Molecular Pathology: Predictive, Prognostic, and Diagnostic Markers in Salivary Gland Tumors

Although initial attempts at using ancillary studies in salivary gland tumor classification were viewed with skepticism, numerous advances over the past decade have established a role for assessment of molecular alterations in the diagnosis and potential prognosis and treatment of salivary gland tumors. Many monomorphic salivary tumors are now known to harbor defining molecular alterations, usually translocations. Pleomorphic, high-grade carcinomas tend to have complex alterations that are often further limited by inaccuracy of initial classification by morphologic and immunophenotypic features. Next-generation sequencing techniques have great potential in many aspects of salivary gland tumor classification and biomarker discovery.

Pleomorphic salivary adenoma in an adolescent

This case study illustrates an unusual condition in childhood, pleomorphic salivary adenoma in a 12-year-old. The management, pathology and treatment are discussed with emphasis on the practitioner to be vigilant for rare conditions. A review of the literature on childhood salivary tumours illustrates the rare nature of this pathology.

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.

Oncogene activation: c-raf-1 gene mutations in experimental and naturally occurring tumors

We demonstrate here consistent point mutations of the c-raf-1 proto-oncogene, within a small region of the kinase domain, in a mouse model for chemical tumor induction. This is the first demonstration of point mutated raf genes in vivo, and the first isolation of activating in vivo point mutations in the kinase domain of a proto-oncogene. The specific region where these mutations are clustered also has biological significance. This is precisely the region where 5/5 independently generated monoclonal antibodies raised against Raf-1 map to [29], and predictions based upon the crystal structure of A kinase identify this as the substrate pocket. The tumors examined show a selective specificity for Raf-1 mutations in that another family of genes, the ras proto-oncogenes which are frequently activated by point mutation in both animal and human tumors [15-21,26], is not involved. Our consistent finding of Raf-1 mutations in a mouse tumor model also has consequences for further evaluation of the role of Raf-1 in human tumor development, as it emphasizes the need to examine c-raf-1 at the sequence level. In fact preliminary screening of human lung tumors indicates point mutations at amino acid 533 (John Lyons, personal communication). Finally, the cumulative data on the critical role of Raf-1 in signal transduction and the occurrence of oncogenic Raf-1 in tumors [32-41] highlight this enzyme as an attractive target for development of novel anticancer regimens.

Translocation (X;12)(q27;q14) in a lipoma

Cytogenetic analysis by short-term culture of a lipoma from the posterior neck region of a 63-year-old man showed a t(X;12)(q27;q14) as the sole chromosomal abnormality. Rearrangement of band 12q14 is nonrandom in lipomas, although its involvement with a sex chromosome has not been reported earlier.

Involvement of 9p in metastatic ovarian adenocarcinomas

By a direct method and after in vitro culture, we cytogenetically analyzed five ovarian adenocarcinomas, using six samples of ascitic fluid. The structural aberrations included rearrangements of chromosomes 1 and 3, in agreement with the results of other researchers. The long arm of chromosome 6 was rearranged in three samples, but no translocation t(6;14) was found. Rearrangements involving 9p were present in all cases, with breakpoints at p13 or p22-23.

Trisomy of chromosome 12 in a case of thecoma of the ovary

Cytogenetic analysis was performed after short-term tissue culture of a thecoma of the ovary. Trisomy of chromosome 12 was revealed as the sole chromosome abnormality. This is the first report of a chromosomal aberration in thecoma of the ovary.

Cytogenetic investigations on a cell line derived from a carcinoma arising in a salivary gland pleomorphic adenoma

This article discusses the results of cytogenetic investigations of a cell line derived from a malignant tumor arising in a benign salivary gland pleomorphic adenoma. Initial karyotypic studies became possible with cells of the second in vitro passage and revealed the existence of hypertetraploidy. Furthermore, a marked polysomy 7 and a translocation involving 12q13-15, a breakpoint also frequently seen in benign pleomorphic adenomas, were noteworthy. During long term culture, the number of chromosomes per cell decreased as well as the number of copies of chromosome 7. Cell tumorigenicity was proved by heterotransplantation to nude mice. The resulting tumors were karyotyped again. No significant changes of the chromosome number or of the degree of polysomy 7 were found compared to the cells before heterotransplantation. In contrast, the cells from the nude mice tumors showed a remarkable number of different isochromosomes probably indicating an unknown factor supporting the generation of isochromosomes. The consistent presence of the t(12;?)(q13-15;?) makes the cell line well suited for molecular studies of this breakpoint region.

Autoantibodies bind solubilized calcium channel-omega-conotoxin complexes from small cell lung carcinoma: a diagnostic aid for Lambert-Eaton myasthenic syndrome

Serum autoantibodies found by radioimmunoassay in 27 of 52 patients with the Lambert-Eaton myasthenic syndrome (LES) bound specifically to a soluble omega-conotoxin binding component of a voltage-gated Ca2+ channel (VGCC) complex extracted from small cell lung carcinoma (SCC). These antibodies were not found in 43 control patients with other neurologic diseases, including myasthenia gravis, peripheral neuropathies, and amyotrophic lateral sclerosis, or in 9 patients with endocrine autoimmunity, but they were found in 2 of 21 control patients with SCC without a history of LES, 1 of whom had severe autonomic neuropathy. Seropositivity was more frequent in patients with LES who had evidence of a primary lung cancer (76%) than in those with other neoplasms or without evidence of cancer (30%). Antigens extracted from SCC tumor lines derived from patients with and without LES and from a human neuroblastoma line yielded results that were highly correlated. A control extract of colonic carcinoma (derived from a patient with LES) yielded negative results. The data implicate a tumor-associated VGCC as the autoimmunizing stimulus in a subset of patients with LES and provide the first direct evidence that the VGCC complex in SCC is a target for some LES antibodies. The serologic test described should be a useful aid in diagnosing LES.

Relationship of cytogenetic subtypes of salivary gland pleomorphic adenomas with patient age and histologic type

Sixty-nine salivary gland pleomorphic adenomas were fully karyotyped by G-banding including 40 tumors previously reported. The cytogenetic results allowed to distinguish between three major chromosomal subgroups characterized by either an apparently normal karyotype or by rearrangements of chromosome 8q12 or by rearrangements of chromosome 12q14-15. Significant correlations of these karyotypic groups with the age of the patients and histologic subgroups of the pleomorphic adenomas were found. Furthermore, our analysis revealed that the three main cytogenetic subgroups showed an uneven geographical distribution. Based on these results we have concluded that the cytogenetic groups represent different etiologic entities.

Giant-cell tumor of bone, stage II, displaying translocation t(12;19)(q13;q13)

A new case of giant-cell tumour (GCT) of bone with benign histological features, clinical stage II, has been reviewed with immunohistochemistry and electron microscopy. After short-term tissue culture the karyotype, using G-banding techniques, presented a consistent translocation t(12;19)(q13;q13). Nude mice xenografts of the tumour were unsuccessful after 6 months of follow-up. Presence of such chromosomal rearrangement may be related to locally aggressive, histologically benign giant-cell tumors of bone.

A specific translocation, t(12;14)(q14-15;q23-24), characterizes a subgroup of uterine leiomyomas

We have cytogenetically investigated short-term cultures initiated from 34 uterine leiomyomas, all of which were histologically completely benign. Clonal chromosome abnormalities were detected in five cases, a normal female complement in 22, whereas, in the remaining seven tumors no karyotype could be established. Apparently identical reciprocal translocations, t(12;14)(q14-15;q23-24), were found as the sole abnormality in four tumors. The fifth abnormal case contained a t(2;14)(p11;p11). We conclude that chromosome aberrations may be found in myomas of the uterus, and that t(12;14)(q14-15;q23-24) characterizes a subset of these tumors.

Chromosomal composition of a series of 22 human low-grade gliomas

G-banded chromosomal analysis was performed on direct and/or in vitro cultures of 22 low-grade gliomas, including nine grade I-II astrocytomas, nine oligodendrogliomas, one mixed tumor oligodendroglioma-astrocytoma, and three ependymomas. Normal diploid stem lines were present in most astrocytomas and oligodendrogliomas, whereas, all three ependymomas displayed polyploid modal numbers. However, secondary cell lines showed the presence of clonal recurrent numerical abnormalities, mainly polysomy 7, monosomy 10 and 22, and loss of the Y chromosome. Clonal structural rearrangements were present with a low incidence; they mainly involved chromosomes #1 and #7. These patterns of chromosome involvement seem to correlate with the scarce previous cytogenetic banding data available from low-grade gliomas. They are also similar to the chromosome alterations found in high-grade gliomas.

Heterogeneity and balance of chromosomes in human cell line M-HeLa-76: analysis of 100 karyotypes

The cell subline M-HeLa-76 with the modal chromosome number 49 was used to define the level of karyotypic variability. By means of a G-banding technique, 100 metaphase plates were analyzed. Among them, 11 different karyotypes were found to range from 2% to 34%. Most probable evolutionary pathways of cell karyotypes within the cell subline were demonstrated. Determination of the origin of all marker chromosomes (32 in all) and unique structural rearrangements (16) revealed a nonrandom distribution of chromosome breaks. These are primarily centromeric regions of chromosomes 3 and 5. Identification of the overall chromosome material in the cells of each karyotype via karyotype reconstruction showed their close similarity in spite of differences in the sets of normal and marker chromosomes. On the basis of reconstructed karyotypes (RK) of each cell group, a generalized reconstructed karyotype (GRK) of the cell line was obtained. This permitted the conclusion of a balanced chromosome set in a cell population as a whole and obligatory retention of diploidy in all chromosomes of a normal set. The role of variability and selection in the evolution of cell populations in vitro is discussed.

Structural rearrangements of chromosome Nr 8 involving 8q12--a primary event in pleomorphic ademona of the parotid gland

Nine pleomorphic adenomas of the human parotid gland were investigated. Within this series the group of cases having clonal aberrations of chromosome Nr 8 predominates. The occurrence of cases with trisomy-8 and cases with structural rearrangements involving a breakpoint in 8q11-8q13 allows a further subdivision of this group of tumors. Our special interest in this paper is devoted to the latter group. The hypothesis is proposed that in these cases the chromosomal rearrangement is the primary event in tumorigenesis, leading to activation of a so far unknown oncogene located most likely at 8q12. The translocations to different recipient chromosomes may serve as sign posts to transcriptionally active chromosomal domains in the salivary gland.

Cytogenetic clones in a recurrent neurofibroma

Chromosome studies were performed on a plexiform neurofibroma arising in a probable von Recklinghausen's disease patient, who also showed a de novo constitutional reciprocal translocation, t(1;22)(p32;q11). Banding analysis of the metaphases obtained from two primary cultures in vitro showed the presence of five cytogenetic clones, characterized by different chromosomal rearrangements. In addition to t(1;22), marker chromosomes involved pairs 1, 2, 3, 5, 8, 9, 10, 12, 16, and X. These findings suggest a possible polyclonal evolution in this neurofibroma.

Aberrations of chromosome 8 in mixed salivary gland tumors--cytogenetic findings on seven cases

Cytogenetic findings on seven mixed salivary gland tumors are reported herein. The involvement of chromosome #8 in clonal chromosome aberrations in five of the seven tumors was particularly noteworthy. Four tumors had translocations involving chromosome #8 and one or two other chromosomes (#3, #7, #9, #13). The fifth showed a deletion of parts of the long arm of chromosome #8. In an attempt to define the critical segment on chromosome #8, we have identified the part between 8q11 and 8q13 as the critical region involved in all rearrangements. Thus far, our results confirm the results of the Swedish group, though the percentage of cases having #8 abnormalities is somewhat higher in our small series. The relationship between the two groups of cases, those with and those without chromosome abnormalities, will be discussed.

The complete coding sequence of the human A-raf-1 oncogene and transforming activity of a human A-raf carrying retrovirus

The complete 606 amino acid sequence of the human A-raf oncogene has been deduced from the 2453 nucleotide sequence of a human T cell cDNA. A cysteine-rich region located near the amino terminus, which is highly conserved in A-raf and c-raf, shows significant homology with protein kinase C. A 5' deleted fragment of the cDNA has been incorporated into a murine retrovirus which endows the virus with the ability to transform cells in vivo and in vitro. Functionally, human A-raf is similar to v-raf and v-mos in that transformation is independent of ras gene function.

Translocations involving the X chromosome in solid tumors: presentation of two sarcomas with t(X;18)(q13;p11)

Cytogenetic analysis of two poorly differentiated sarcomas revealed clonal chromosomal rearrangements involving several autosomes and the X chromosome. Both tumors displayed a common abnormality, t(X;18)(q13;p11). This finding is discussed in relation to published X/autosomes translocations in solid tumors.

A neuroendocrine tumor of the small intestine with a karyotype of 46,XY,t(11;22)

Direct preparations of cells from a neuroendocrine carcinoma of the small intestine reveal a karyotype of 46,XY,t(11;22)(q25;q12). Though only 13 cells could be analyzed from both direct and 24 hour preparations, all revealed the translocation; no normal cell line was seen. The patient had not been treated at the time of biopsy. A similar translocation recently has been reported in a number of cases of Ewing's sarcoma and in peripheral neuroepithelioma. This common occurrence, along with the observation that a rearranged chromosome #22 with breakpoints at q11 or q12 are observed in other neoplasias including leukemia, suggests a possible causative role for these phenomena.

Intermediate filaments in cultured human pleomorphic adenomas. An immunohistochemical study

Primary cultures of 9 benign human pleomorphic adenomas were analyzed by immunofluorescence and antibodies against the different intermediate filaments and microfilaments. The cultured cells were also cytogenetically characterized by G- and C-banding techniques. The adenoma cells expressed at least three different classes of intermediate filaments, viz. prekeratin, vimentin and glial fibrillary acidic protein (GFA). No correlation between chromosomal pattern and intermediate filament expression was found. The frequency of cells stained for each of the different antibodies varied considerably among the tumours. Prolonged culturing appeared to induce a reduction of cells positive for prekeratin. This was probably a true loss of antigens and not an effect of overgrowth of stromal cells. It is concluded that the cells studied, on the basis of several criteria, are of neoplastic origin. This in vitro system appears to be well suited for further histogenetic studies on pleomorphic adenomas.

Ultrastructural similarities of adenoid cystic carcinoma and pleomorphic adenoma

Ultrastructural examination of five adenoid cystic carcinomas, three breast and two salivary gland, reveals identical patterns of tumour cell differentiation, organization and distribution of cellular products (Zaloudek, Oertel & Orenstein 1984). In both sites, there is proliferation of two populations of cells, one with characteristics and organization of duct-type luminal epithelial cells and a second that forms the principal proliferating component and has the overall organization and appearance that would suggest that they represent modified myoepithelial cells. Recent ultrastructural studies also indicate that tumour cell types and histological organization similar to those described for adenoid cystic carcinoma occur during histodifferentiation of salivary gland pleomorphic adenoma (Dardick et al. 1983a, b). The characteristic histological pattern of adenoid cystic carcinoma is dependent on the formation of pseudolumina containing proteoglycans and reduplicated basal lamina. Similar, but smaller, lumina of like organization and contents are evident in some cases of pleomorphic adenoma. Both the ultrastructural similarities of the tumour cell types and their organization, in adenoid cystic carcinoma and pleomorphic adenoma, suggest that these tumours have a similar histogenetic basis. The fact that one lesion is malignant and the other benign does not preclude common types of tumour cells and developmental processes.

Fragile sites and structural rearrangements in cancer

We retracted information from a computerized databank which contains the cytogenetic findings of 17,000 patients with leukemia and lymphoma. Cytogenetic data from patients with solid tumors were compiled from Dr. Mitelman's catalogue on "Chromosome aberrations in cancer". We compared the observed distribution of breaks in chromosome bands involved in structural rearrangements with the random distribution of breaks generated by Monte Carlo simulation and showed that a majority but not all of the bands known to contain a fragile site are involved in structural aberrations in cancer and that some of them are associated with specific chromosome structural changes in specific types of cancer.

Inversion of chromosome 14 marks human T-cell chronic lymphocytic leukaemia

Rare cases of chronic lymphocytic leukaemia (CLL) in man stem from the malignant proliferation of T cells. The disease is usually more aggressive clinically than B-cell-derived CLL. Various haematological tumours are associated with specific chromosome aberrations (for example, refs 1, 2). Only limited numbers of T-cell CLL patients have so far been studied cytogenetically and, whereas chromosome 12 seems particularly to be involved in B-cell CLL, several markers have been found in T-cell tumours. Recently, by stimulating malignant clones with different mitogens, novel chromosome abnormalities have been detected in T-cell CLL. Using the same approach for additional cases of T-cell CLL, we now report that the most consistent chromosome change is an inversion of the long arm of chromosome 14, inv(14)(q11 q32), in four of five patients. Another remarkable chromosome aberration is trisomy for the long arm of chromosome 8, found in three of five patients.

Specificity of the involvement of chromosomes 8 and 12 in human mixed salivary-gland tumours

Using the G-banding technique the chromosomal patterns in 10 in vitro-cultured human mixed salivary gland tumours were studied. When surveying the results from the present 10 adenomas together with previous results from 31 adenomas, the following characteristic features were found. Mixed tumours probably always originate with a normal diploid stemline. This stemline type was also found to predominate during progression. However, about 41% of the tumours showed abnormal stemlines characterized by highly specific, usually structural, clonal aberrations. The deviations in these abnormal stemlines seemed to preferentially affect certain chromosome types, viz. Nos. 8 and 12. This non-random pattern of deviations - not found earlier in either benign or malignant tumour types in man - indicates that certain regions of, especially, chromosomes 8 and 12 are selectively affected in mixed tumours. Among other things, changes of gene dosage and/or oncogene activation are mentioned as factors possibly influencing the characteristics of this non-random pattern.

i(12p): specific chromosomal marker in seminoma and malignant teratoma of the testis?

A similar small marker chromosome, frequently present in duplicate, was seen in direct preparations and short-term cultures of each of ten seminomas, one combined seminoma and teratoma, and one malignant teratoma of the testis. In the four most favorable tumors (seminomas) this chromosome was identified as an i(12p). The findings may point to a chromosomal change that is specific for malignant testicular tumors.

Specificity and implications of ring chromosomes and dicentrics in benign mixed salivary gland tumours

Among 61 human pleomorphic adenomas subjected to chromosomal study by banding methods, two rare cases were detected, one having a dicentric marker chromosome and the other a ring chromosome included in the stemline karyotype. Excepting meningiomas, such marker types have previously only been observed in different types of malignant neoplasms. However, as in meningiomas, the occurrence of dicentrics or rings has no implications as regards the question of malignancy. Both our mixed tumours showed structural changes affecting one chromosome No. 8. These findings, along with similar observations in other pleomorphic adenomas, are discussed in relation to recently advanced hypotheses about the role of oncogenes and changes of gene dosage in the genesis and/or development of mixed tumours.

An unusual parotid tumor with histogenetic implications for salivary gland neoplasms

This case report describes an unusual parotid salivary gland tumor in which major proliferating elements arise in relation to well-differentiated, branching ducts. Both light and electron microscopic examination indicates that these ducts are formed by two distinct cell layers, an inner row of luminal epithelial cells and an outer layer of modified myoepithelial cells. It is from this outer layer of cells that discrete clusters of uniform basaloid cells develop. Although the majority of basaloid nests were relatively compact, a few showed a small number of clear, slightly widened, intercellular spaces which, ultrastructurally, were lined with basal lamina. Particularly in one area of the lesion, clusters of basaloid cells showed increasing numbers and sizes of these intercellular spaces with the gradual formation of an adenoid cystic pattern of differentiation. Even in this situation, however, luminal epithelial cells forming ducts could still be identified. The bidirectional cellular differentiation, the origin of basaloid clusters in relation to modified myoepithelial cells, and the mode of development of intercellular spaces resulting in an adenoid cystic pattern all have important implications for the histogenesis of salivary gland tumors.