Abstract P4-04-08: Genomic and transcriptomic characterization of papillary carcinomas of the breast

Abstract

Abstract Introduction: Papillary carcinomas (PCs) are a rare (<1%) special histological type of breast cancer that often affects postmenopausal women, and has an overall favorable outcome. Based on their histological characteristics, these tumors are classified into three different subtypes, namely encapsulated papillary carcinomas (EPC), solid papillary carcinomas (SPC) and invasive papillary carcinomas (IPC). In this study, we sought i) to investigate whether PCs constitute a molecular entity distinct from grade- and ER-matched invasive ductal carcinomas of no special type (IDC-NST) at the transcriptomic level, ii) to investigate whether EPC, SPC and IPC display distinct transcriptomic profiles, iii) to characterize the repertoire of copy number alterations in the different subtypes of PC, and iv) to identify recurrent fusion genes that may be potential drivers of this disease. Material and methods: DNA and RNA were extracted from microdissected PCs (4 SPCs, 5 IPCs and 7 EPCs) and grade- and ER-matched IDC-NSTs (RNA only). 16 PCs and 16 grade- and ER-matched IDC-NSTs were subjected to gene expression profiling using the Illumina Human HT-12 v4 platform. Genes differentially expressed between the PC subtypes were identified using SAM, and functional annotation of these genes was performed using DAVID. Intrinsic molecular subtypes were determined using the PAM50 single sample predictor. Copy number profiling was performed using Affymetrix Human SNP 6.0 arrays with DNA extracted from 16 PCs. In addition, 8 PCs (3 IPCs, 3 EPCs, 2 SPCs) were subjected to paired-end massively parallel RNA sequencing (Illumina GAIIx). Putative expressed fusion transcripts were identified using validated algorithms (i.e. deFuse and Chimerascan), and confirmed by reverse transcription PCR. Results: PCs were preferentially of histological grade I/II (82%) and ER-positive (100%). Unsupervised analysis revealed that PC subtypes show a high degree of similarity at the transcriptomic level, and form clusters distinct from grade- and ER-matched IDC-NSTs. Compared with IDC-NSTs, PCs displayed reduced expression of genes related to cell motility, adhesion and extracellular matrix. PAM50 subtyping classified 87.5%, 50% and 100% of EPCs, SPCs and IPCs as of luminal subtypes, respectively. 12.5% of EPCs were classified as of basal-like subtype, and 50% of SPCs as of HER2-enriched subtype. At the genomic level, PC subtypes displayed similar patterns of gene copy number aberrations. Five in-frame fusion genes, USF1-CCDC38, MDC1-SFTA2, DLD-LMBR1, PDCL-DENND1A and SUGT1-NOL6, were identified and validated in PCs, however none of these were recurrent in the cases included in this study. Conclusion: Our results demonstrate that the majority of PCs are of luminal subtype, and support the contention that at the transcriptomic level, PCs are distinct from grade- and ER-matched IDC-NSTs. Our findings also demonstrate that unlike some other histological special types of breast cancer, PCs are not underpinned by a highly recurrent expressed fusion gene. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-04-08.