Abstract P6-03-10: Genomic and transcriptomic heterogeneity in metaplastic breast carcinomas

Abstract

Abstract Introduction: Metaplastic breast carcinoma (MBC) is a rare form of triple-negative breast cancer (TNBC), accounting for approximately 0.2%-5% of all invasive breast cancers. These tumors are characterized by the presence of neoplastic cells displaying differentiation towards squamous epithelium or mesenchymal elements. MBCs are reported to have an aggressive clinical behavior, to exhibit a worse prognosis and to respond less frequently to conventional chemotherapy regimens than common forms of TNBCs. In this study, we sought to define whether morphologically distinct subgroups of MBCs would be underpinned by distinct gene expression or copy number profiles, and whether MBCs, akin to other special histologic types of TNBC (e.g. secretory carcinoma and adenoid cystic carcinoma), would be underpinned by a highly recurrent fusion gene. Methods: RNA and DNA samples were extracted from microdissected frozen MBCs (5 squamous, 5 spindle and 7 chondroid) and subjected to gene expression profiling using the Illumina Human HT-12 v4 platform and gene copy number profiling using the Affymetrix Human SNP 6.0 arrays, respectively. Genes differentially expressed between MBC subtypes were identified using SAM, and functional annotation of these genes was performed using Ingenuity Pathway Analysis. Intrinsic molecular subtypes were determined using the PAM50 and claudin-low intrinsic gene lists. In addition, all cases were subjected to paired-end massively parallel RNA-sequencing (Illumina GAIIx). Putative expressed fusion transcripts were identified using a validated algorithm (i.e. ChimeraScan), and confirmed by means of RT-PCR. Results: MBCs with spindle cell morphology were all classified as of claudin-low intrinsic subtype, whereas MBCs with chondroid or squamous cell metaplasia were classified as of normal breast-like, basal-like or claudin-low subtypes, suggesting that these morphologic subgroups are heterogeneous. Unsupervised analysis of microarray and RNA-sequencing gene expression data further demonstrated that MBCs with spindle cell differentiation displayed distinctive transcriptomic profiles, and formed clusters distinct from those enriched for MBCs with chondroid and squamous cell metaplasia. MBCs with spindle cell morphology preferentially expressed regulators of epithelial-to-mesenchymal transition including lower expression of E-cadherin and EpCAM. At the genomic level, MBC subtypes displayed patterns of gene copy number alterations similar to those of common forms of TNBCs from The Cancer Genome Atlas, and no significant differences were found among the distinct MBC subtypes. Nine in-frame fusion genes, TBL1XR1-PIK3CA, WAPL-CDHR1, MAP2K3-HMGCLL, PARG-BMS1, FN1-ICAM1, TNKS1BP1-SPARC, AAK1-ARNT2, MBTPS1-TCEANC2 and PSMA6-SHMT1 were identified and validated in the index cases, however none of these was found to be recurrent in the cases analyzed in this study. Conclusion: MBC subtypes, despite harboring similar patterns of gene copy number alterations, display significant transcriptomic differences, which may account for their distinct histologic features. Our findings also demonstrate that unlike other histologic special types of TNBC, MBCs are not underpinned by a highly recurrent expressed fusion gene. Citation Format: Piscuoglio S, Ng CKY, Cowell CF, Mariani O, Martelotto L, Natrajan R, Lim RS, Maher CA, Vincent-Salomon A, Weigelt B, Reis-Filho JS. Genomic and transcriptomic heterogeneity in metaplastic breast carcinomas. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P6-03-10.