Abstract P3-06-07: Recurrent but not pathognomonic fusion genes in mucinous carcinomas of the breast

Introduction: Mucinous carcinoma of the breast (MCB) is a rare histologic subtype of estrogen receptor (ER)-positive invasive carcinoma, and is characterized by tumor cells floating in pools of mucin. Despite their characteristic histology, MCBs are heterogeneous at the genetic level and no driver genetic alterations have been identified. Notably, MCBs lack the common genetic alterations found in ER-positive invasive ductal carcinomas (i.e. 16q losses, 1q gains and PIK3CA mutations). Fusion genes have been reported in breast cancer, including highly recurrent or pathognomonic fusions genes, such as the ETV6-NTRK3 and MYB/MYBL1 rearrangements in secretory carcinoma and adenoid cystic carcinoma, respectively. In this study we sought to define whether MCBs would be underpinned by a pathognomonic fusion gene. Materials and methods: Seven pure mucinous A (hypocellular), seven pure mucinous B (hypercellular), and the mucinous component of a mixed MCB were microdissected, and subjected to RNA extraction followed by RNA-sequencing for fusion gene discovery. Read pairs supporting chimeric transcripts were identified using INTEGRATE, FusionCatcher and STARfusion. The Bayesian driver probability of the candidate fusion genes was annotated using OncoFuse. In-frame fusion gene candidates with a high driver probability were validated using orthogonal methods (RT-PCR and Sanger sequencing). Results: Our analysis identified fusion genes in 47% (7/15) of the MCBs analyzed (29% (2/7) of type A MCBs, 57% (4/7) type B MCBs and in the mucinous component of one mixed MCB). The OAZ1-CSNK1G2 and RFC4-LPP fusion genes were identified in 20% (3/15) and 13% (2/15) of the cases, respectively. The OAZ1-CSNK1G2 chimeric transcript results in the truncation of the kinase domain of CSNK1G2, which represses ER transactivation. The RFC4-LPP chimeric transcript leads to the fusion of exons 1-3 of RFC4 and exons 5-11 of LPP, where the LIM domains of LPP are conserved. LPP is a known partner of fusion genes identified in mesenchymal tumors and reported to mediate TGF-β induced breast oncogenesis via its LIM domain. Additional validated, potentially pathogenic fusion genes identified in MCBs involved kinases, phosphatases or regulators of tyrosine kinase receptor signaling, such as IRAK3-PPM1H (n=1), GIGYF2–GFRA3 (n=1) and PHF20-FAM217B (n=1). Conclusions: MCBs harbor fusion genes in almost half of the cases with two fusions being recurrent, involving primarily genes encoding kinases, phosphatases or receptor tyrosine kinase signaling regulators. Nevertheless, due to the lack of recurrence of a specific fusion gene, this special histologic type of breast cancer is unlikely to be underpinned by a highly recurrent/ pathognomonic pathogenic fusion gene. Citation Format: Pareja F, Gularte-Merida R, Da Cruz Paula A, Brown D, Geyer FC, Piscuoglio S, Marchio C, Vincent-Salomon A, Brogi E, Weigelt B, Reis-Filho JS. Recurrent but not pathognomonic fusion genes in mucinous carcinomas of the breast [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 P3-06-07.