Fusion Transcripts that Characterize Malignancies of Salivary Gland Origin [Hot Topic]

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Joaquín García, M.D.

Salivary gland neoplasia is often regarded as one of the more challenging areas of diagnostic surgical pathology. Dramatic histomorphologic overlap between benign and malignant entities creates a risk for misclassification and inappropriate treatment. Four malignancies of salivary gland origin are characterized by recurrent fusion gene events: mucoepidermoid carcinoma, mammary analogue secretory carcinoma of salivary glands, adenoid cystic carcinoma, and hyalinizing clear cell carcinoma. Identification of fusion transcripts that encode novel or ectopically expressed oncoproteins in salivary gland tumors can improve diagnostic accuracy and selection of appropriate therapy.

Presenter and Credentials:
Joaquín García, M.D., Consultant in the Division of Anatomic Pathology, and Vice Chair of Laboratories and Medical Director of the Histology Laboratory at Mayo Clinic in Rochester, Minn.

Transcript

Welcome to Mayo Medical Laboratories Hot Topics. These presentations provide a short discussion of current topics and may be helpful to you in your practice. Today our topic is a discussion on the clinical utility of identifying fusion transcripts to accurately classify and treat salivary gland malignancies.

Presenter

Our speaker for this program is Dr. Joaquín García, a Consultant in the Division of Anatomic Pathology at Mayo Clinic in Rochester, Minnesota. Dr. García is also the Vice Chair of Laboratories and Medical Director of the Histology Laboratory.

Dr. Garcia, thank you for presenting today.

Thank you for the kind introduction.

Salivary gland neoplasia is often considered one of the more challenging areas of diagnostic surgical pathology. Given the overall rarity of salivary gland tumors, most surgical pathologists encounter them infrequently in practice. Naturally, this creates a risk for misclassification and inappropriate treatment. This Hot Topic discusses 4 malignancies of salivary gland origin characterized by recurrent fusion gene events: mucoepidermoid carcinoma, mammary analogue secretory carcinoma of salivary glands, adenoid cystic carcinoma, and hyalinizing clear cell carcinoma. We will emphasize the clinical utility of identifying fusion transcripts in salivary gland malignancies to aid in accurate classification.

Disclosure

I have no disclosures.

Utilization Message

As you view this presentation, consider the following important points regarding fusion transcripts that characterize malignancies of salivary gland origin:

  • How is the testing going to be used in your practice?
  • When should the tests be used?
  • How will results impact patient management?

Challenges in Contemporary Classification

Challenges in salivary gland pathology are rooted in the dramatic histomorphologic overlap within and between benign and malignant tumors. Although our understanding of the morphologic and immunophenotypic nuances of salivary gland neoplasia has improved over the years, even expert head and neck pathologists continue to struggle with a subset of cases using conventional surgical pathology techniques; that is, histomorphology, histochemistry, and immunohistochemistry.

This series of micrographs highlights the inherent difficulty in distinguishing high-grade glandular salivary malignancies from one another: adenosquamous carcinoma (on the left), salivary duct carcinoma (in the middle), and mucoepidermoid carcinoma (in the right). The overall rarity and histomorphologic overlap between salivary gland tumors makes classification difficult so cytogenetic and/or molecular testing are occasionally indicated.

Role of Fusion Transcripts in Oncology

The identification of unique molecular signatures in malignancy —such as fusion oncogenes—provides tremendous diagnostic, prognostic, and predictive value in the world of oncology. Historically, the menu of known fusion oncogenes was primarily comprised of sarcomas and hematologic tumors. For a significant period of time, it was thought that fusion transcripts were unusual in epithelial malignancies as they often exhibit complex karyotypes, and recurrent identifiable chromosomal rearrangements are not readily appreciated. Nevertheless, an increasing number of epithelial malignancies are associated with recurrent chromosomal rearrangements. This trend will likely continue as next-generation sequencing platforms are applied to explore the genetic profile of epithelial malignancies in both research and clinical domains.

Role of Fusion Transcripts in

Role of Fusion Transcripts in Salivary Gland Oncology

Our understanding of salivary gland pathogenesis was historically based on morphologic and immunophenotypic observations rather than the molecular events that precede them. In recent decades, we have seen the unveiling of disease-defining fusion transcripts that encode novel oncoproteins or ectopically expressed normal or truncated oncoproteins in several salivary gland tumors. These fusion oncogenes typically encode transcriptional coactivators, transcription factors, and tyrosine kinases.

Several epithelial malignancies of salivary gland origin frequently harbor fusion transcripts: mucoepidermoid carcinoma, mammary analogue secretory carcinoma of salivary glands, adenoid cystic carcinoma, and hyalinizing clear cell carcinoma. Although fusion genes have inspired targeted therapy in other oncologic settings, this is currently not the case in salivary gland oncology. At present, the clinical utility of identifying fusion transcripts in these tumors is restricted to diagnosis.

Mucoepidermoid Carcinoma

Mucoepidermoid carcinoma is characterized by the recurrent translocation t(11;19), which has not been observed in other neoplasms. The resulting fusion transcript, CRTC1-MAML2, is composed of exon 1 of CRTC1 and exons 2 to 5 of MAML2, both of which are transcriptional coactivators. CRTC1 belongs to a family of CREB coactivators. MAML2 belongs to the mastermind-like family of nuclear proteins and serves as a coactivator of Notch receptors. The resulting fusion transcript encodes a chimeric protein containing the CREB-binding coiled-coil domain of CRTC1 and the transactivation domain of MAML2.

Mucoepidermoid carcinoma is the most common malignant salivary gland tumor in both pediatric and adult populations. Although mucoepidermoid carcinoma is typically observed in the major salivary glands, it is occasionally seen in minor salivary gland sites as well.

The classic cellular constituents of mucoepidermoid carcinoma are mucous, epidermoid, and intermediate cells. The proportion of each cell type varies widely and several morphologic variants exist as well; such as oncocytic, sclerosing, cystic, and clear cell mucoepidermoid carcinoma. Differential diagnostic considerations for mucoepidermoid carcinoma range from benign to malignant lesions: necrotizing sialometaplasia, inverted ductal papilloma, cystadenoma, lymphoepithelial cyst, oncocytoma, adenosquamous carcinoma, and salivary duct carcinoma.

This image shows disruption of the MAML2 gene in a case of mucoepidermoid carcinoma. Intact MAML2 genes are represented by red and green signals that are immediately adjacent to one another—occasionally appearing as a yellow or fused signal. Disrupted MAML2 genes are represented by red and green signals that are significantly separated from one another.

Mammary Analogue Secretory Carcinoma of Salivary Glands

Mammary analogue secretory carcinoma of salivary glands is characterized by the same translocation, seen in secretory carcinoma of the breast, that is, t(12;15). The resulting fusion transcript, ETV6-NTRK3, encodes a chimeric tyrosine kinase that has the potential to activate 2 major cellular pathways: Ras-MAP kinase mitogenic pathway and phosphatidyl inositol-3-kinase AKT pathway. Interestingly, the fusion transcript observed in mammary analogue secretory carcinoma and secretory carcinoma of the breast is also seen in congenital or infantile fibrosarcoma, congenital mesoblastic nephroma, and in rare cases of acute myeloid leukemia.

Mammary analogue secretory carcinoma of salivary glands is a recently described entity that shows morphologic, immunophenotypic, and molecular overlap with secretory carcinoma of the breast. Prior to the characterization of mammary analogue secretory carcinoma by Skalova et al. in 2010, these cases were commonly misclassified as acinic cell carcinoma, cystadenocarcinoma, and adenocarcinoma not otherwise specified. In 2010, Skalova et al described mammary analogue secretory carcinoma as a relatively low-grade salivary gland malignancy that predominantly involves major salivary glands—primarily the parotid gland. Mammary analogue secretory carcinoma of salivary glands is typically a well-circumscribed and lobulated mass dissected by bands of fibrosis.

Tumor cells coalesce within an intricate network of cystic and microcystic spaces; cystic spaces characteristically contain homogenous or vacuolated pink material. Tumor cell nuclei are vesicular with finely granular chromatin, and occasional centrally located nucleoli. Mammary analogue secretory carcinoma exhibits a relatively consistent immunophenotype with strong and diffuse expression for S-100 and mammaglobin; however, these features are not specific to mammary analogue secretory carcinoma.

This image shows disruption of the ETV6 gene in a case of mammary analogue secretory carcinoma of salivary glands. Intact ETV6 genes are represented by red and green signals that are immediately adjacent to one another—occasionally appearing as  yellow or fused signal. Disrupted ETV6 genes are represented by red and green signals that are significantly separated from one another.

Adenoid Cystic Carcinoma

A significant percentage of adenoid cystic carcinoma cases harbor a recurrent translocation, t(6;9), that fuses the MYB oncogene with the NFIB transcription factor gene—most commonly MYB exon 14 fused to NFIB exons 8c or 9. MYB is a transcriptional regulator for cell proliferation, apoptosis, and differentiation. MYB activation has been observed in adenoid cystic carcinoma even in the absence of the MYB-NFIB fusion transcript, implying that MYB appears to be the key oncoprotein and other molecular pathways impacting its expression likely exist. The reported prevalence of MYB rearrangement in cases of adenoid cystic carcinoma has ranged from 28% to 86%. Importantly, the MYB-NFIB fusion transcript has not been reported in malignancies of the head and neck other than adenoid cystic carcinoma.

Adenoid cystic carcinoma is infrequently encountered in surgical pathology; however, it is often given diagnostic consideration because of its histomorphologic overlap with other salivary gland neoplasms and peculiar clinical course. Adenoid cystic carcinoma is notorious for its insidious growth—the exception being cases with high-grade transformation—yet almost invariably lethal clinical course. Adenoid cystic carcinoma is observed in both major and minor salivary glands of the head and neck, and has also been reported in virtually every anatomic subsite; skin, lung, breast, and vulva to name a few.

Adenoid cystic carcinoma is a biphasic salivary gland neoplasm composed of ductal and myoepithelial cell populations that classically shows tubular, cribriform, and/or solid architectural patterns.

Not uncommonly, the myoepithelial cell component secretes a basement membrane-like material that can be seen within glandular lumina, enveloping tumor cell nests, or embedded in stroma. Although this is occasionally a useful diagnostic clue, a similar extracellular matrix can be produced by other biphasic salivary gland neoplasms as well.

This image shows disruption of the MYB gene in a case of adenoid cystic carcinoma. Intact MYB genes are represented by red and green signals that are immediately adjacent to one another—occasionally appearing as yellow or fused signals. Disrupted MYB genes are represented by red and green signals that are significantly separated from one another.

Hyalinizing Clear Cell Carcinoma

Hyalinizing clear cell carcinoma is characterized by a translocation, t(12;22), that results in a fusion transcript, EWSR1-ATF1, in over 80% of cases. EWSR1 belongs to the TET family of transcription factors and ATF1 encodes a cyclic AMP-dependent transcription factor. Similar to the promiscuous fusion gene in mammary analogue secretory carcinoma, the fusion gene EWSR1-ATF1 has been noted in several other malignancies: clear cell sarcoma of tendons and aponeuroses, angiomatoid fibrous histiocytoma, and clear cell sarcoma of the gastrointestinal tract.

Hyalinizing clear cell carcinoma is one of the more diagnostically challenging entities encountered in head and neck pathology. The inconsistent classification of this problematic entity over recent decades has made it difficult to characterize its histomorphologic, immunophenotypic, and clinical profile. In retrospect, hyalinizing clear cell carcinoma was frequently misclassified as mucoepidermoid carcinoma, myoepithelial carcinoma, polymorphous low-grade adenocarcinoma, and adenocarcinoma not otherwise specified. The discovery of a disease-defining fusion transcript in hyalinizing clear cell carcinoma by Antonescu et al in 2011 brought clarity to the matter. Hyalinizing clear cell carcinoma is a rare, infiltrative, low-grade, monophasic salivary gland neoplasm. Tumor cells are frequently arranged in sheets, trabeculae, cords, and nests set within a markedly hyalinized background.

Tumor cells often demonstrate clear to eosinophilic cytoplasm. True glandular elements and chondromyxoid stroma are not features of hyalinizing clear cell carcinoma. A fundamental feature of hyalinizing clear cell carcinoma is its lack of immunoreactivity for myoepithelial markers such as S-100, SMA, desmin, and calponin, although focal staining has been reported infrequently.

This image shows disruption of the EWS gene in a case of hyalinizing clear cell carcinoma. Intact EWS genes are represented by red and green signals that are immediately adjacent to one another—occasionally appearing as a yellow or fused signal. Disrupted EWS genes are represented by red and green signals that are significantly separated from one another.

Conclusion

The discovery of molecular signatures in epithelial malignancies has reformatted the landscape of oncology. These discoveries have not uncommonly led to dramatic advances in diagnostic, prognostic, and predictive testing. Accordingly, surgical pathologists have become increasingly important to the practice of oncologic patient care. Salivary gland neoplasms are notoriously challenging to diagnose and treat, which makes the incorporation of cytogenetic and molecular classification paramount in a subset of cases. Although the clinical utility of fusion gene identification in salivary gland oncology is primarily restricted to diagnosis currently, prognostication and prediction of targeted therapy response will likely play a role in the future.

Thank you for your time.

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