Abstract P3-08-03: Dissecting the heterogeneity of metaplastic breast cancer: A morphological, immunohistochemical and genomic analysis of a large cohort

Although rare, Metaplastic Breast Carcinomas (MBC) account for significant global breast cancer mortality. This subgroup is extremely heterogeneous and by definition exhibits metaplastic change to squamous and/or mesenchymal elements, including but not limited to spindle, squamous, chondroid, osseous and rhabdomyoid elements. The WHO working group recognizes that the current classification is inadequate and in the interim, has suggested a purely descriptive classification. The mixed epithelial-mesenchymal morphology has led to speculation that MBC represent 'stem cell tumours'; in support of this, MBC have been shown to have a CD44+/CD24-/low phenotype. Clinically, patients present with tumours that are larger (higher stage), have increased likelihood of distant metastases at presentation and overall, have a reduced 5-year survival rate compared to Invasive Carcinoma-NST. Hence, this is a unique subtype with poor outcome but without a robust classification or understanding of the biology to aid clinical management. We present a detailed morphological, immunohistochemical and genomic analysis of a large series of MBC (n=347), as amassed through the Asia-Pacific MBC consortium. We consider our morphological dissection using the WHO subtyping guidelines and show that an increasing number of phenotypes in a mixed MBC (classified as WHO_1) significantly associates with a poor prognosis. Immunohistochemical analysis showed that a pure spindle (WHO_5) is significantly less likely to express vimentin, CK5/6, CK14, and CK19 than a mixed WHO_1 with spindle features. Similarly, a WHO_1 with chondroid features is less likely to express EGFR than WHO_1 with chondroid features and rhabdoid or osseous differentiation. Across the cohort, positivity for the AE1/3 antibody and a lack of EGFR expression both significantly associate with a better outcome. We report no significant association between patient age at diagnosis and breast cancer specific survival, nor between age and specific WHO MBC subtypes. We report a significant association between WHO_1 types and increasing tumour grade, and also between tumour size and grade, with tumour size being a highly significant prognostic indicator in this cohort. Our exome sequencing confirms a significant enrichment for TP53 and PTEN mutations in MBC, and intriguingly for concurrent mutations of TP53, PTEN and PIK3CA. A novel enrichment for NF1 mutations is also presented. In summary, we provide a thorough assessment of a large cohort of MBC, including morphology, survival, IHC and exome sequencing, and present our analysis contextualized by the WHO guidelines, extending the existing knowledge base of this rare tumour type. Citation Format: McCart Reed AE, Kalaw E, Nones K, Bettington M, Lim M, Bennett J, Johnstone K, Kutasovic JR, Kazakoff S, Xu QC, Saunus JM, Reid LE, Black D, Niland C, Ferguson K, Gresshoff I, Raghavendra A, Liu JC, Kalinowski L, Reid AS, Davidson M, Pearson JV, Yamaguchi R, Harris G, Tse G, Papadimos D, Pathmanathan R, Pathmanathan N, Tan PH, Fox S, O'Toole S, Waddell N, Simpson PT, Lakhani SR. Dissecting the heterogeneity of metaplastic breast cancer: A morphological, immunohistochemical and genomic analysis of a large cohort [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-08-03.