IN SITU DETECTION OF CRTC-MAML2 TRANSLOCATION EXPRESSION IN MUCOEPIDERMOID CARCINOMA

Abstract

<h3>Background</h3> The heterogeneity of salivary gland neoplasms, within and between histologic types, presents a major diagnostic challenge. Mucoepidermoid carcinoma (MEC), the most common salivary gland cancer in adults, children, and adolescents, is associated with the presence of a novel CRTC1-MAML2 fusion gene. The translocation can be detected by fluorescence in situ hybridization or reverse transcription polymerase chain reaction but without information regarding transcript level, identification of the cell type(s) harboring the translocation and histologic architecture is not preserved. This study describes, for the first time, a novel in situ chromogenic assay, BaseScope, to detect CRTC1-MAML2 translocation expression. <h3>Objective</h3> Design a novel BaseScope probe targeting the novel exon-exon junction in the CRTC1-MAML2 fusion transcript, determine expression levels of the transcript, and identify specific cell types harboring the translocation. <h3>Methods</h3> Formalin-fixed paraffin-embedded tissues and known fusion-positive and -negative human MEC cells were subjected to the assay. <h3>Results</h3> The CRTC1-MAML2 RNA transcript was detected in known fusion-positive cells but not in fusion-negative cells. In MEC tissues distinct fusion events, in the form of punctate red dots, were detected in all tumor grades and all cell types. Interestingly, the translocation was specifically identified in tumor cells that had direct contact with tumor stroma or nerve invasion. No positive staining was seen in normal tissue or surrounding stroma and no unique morphological features were noted in negative cases. <h3>Conclusions</h3> The BaseScope assay accurately detects the CRTC1-MAML2 fusion translocation and thus provides an alternative chromogenic technique, for easy use in routine clinical labs, to aid accurate diagnosis of MEC.