Abstract P1-03-01: A novel model to study to metaplastic TNBC
Authors: Shah Vrutant , Stratton Sabrina , Jiang Shiming , Chang Jeffrey , Piwnica-Worms Helen , Moulder Stacy , Barton Michelle 
Published: March 2020

Background: Triple-Negative Breast Cancers (TNBC), a broad class of breast cancers lacking estrogen- and progesterone-receptor (ER and PR) response and HER2 amplification, remain refractory to hormone-based or molecularly targeted therapies. Within the TNBC subclass, carcinosarcomas or metaplastic breast cancers (MpBC) are a heterogeneous mixture of malignant epithelial and mesenchymal elements that develop highly aggressive, rare, poorly differentiated and chemorefractory tumors. There is an unmet need to develop innovative treatments for TNBC, in general, and MpBC, in particular. In human breast carcinomas, both protein and mRNA levels of tripartite motif containing protein 24 (TRIM24) are elevated, and aberrant expression of TRIM24 is linked to poor patient survival. Previous in vitro studies determined that TRIM24 is a multidomain protein with histone reader function via a PHD/bromodomain and E3 ligase function via a RING domain.

Method: To determine if TRIM24 is oncogenic in vivo, our lab created a mouse model that conditionally overexpresses (COE) TRIM24 in mammary epithelia (Trim24COE). Trim24COE mice develop tumors, which were subjected to pathology, immunohistochemistry, global transcriptional and translational profile. Additionally, we determined expression of TRIM24 in MpBC patient-derived tumor arrays and xenografts (PDX). Trim24COE tumor-derived primary cell lines were assessed for function and therapeutic response.

Results: Surprisingly, TRIM24 overexpression is sufficient to drive development of murine MpBC (70% of all tumors), which are highly penetrant and lack ER, PR and HER2 expression. Preliminary results also show highly specific expression of TRIM24 in MpBC PDXs, supporting TRIM24 as a relevant oncogene in human MpBC. Gene expression profiles of TRIM24-driven mice tumors, compared to MMTV-Cre controls, showed that EMT, glycolysis, angiogenesis and G2/M Checkpoint regulators are among the top up-regulated pathways. Selected proteomics profile (RPPA) also shows pathways related to glycolysis, G2/M checkpoint and E2F targets are top up-regulated pathways. We matched TRIM24 expression signatures from TRIM24-driven tumors to TNBC patients and found that TRIM24 signatures are positively correlated to MpBC subtypes. Using a PROTAC targeting TRIM24, we showed that degradation of TRIM24 is sufficient to reduce cell viability compared to control, both in TRIM24-driven primary cell lines and MpBC PDX cell suspension.

Conclusions: TRIM24 is a bromodomain histone reader and p53-targeting E3-ubiquitin ligase, which is sufficient to promote tumorigenesis in vivo. A majority of Trim24-driven tumors is classified as MpBC TNBC. Genomewide RNA expression and protein profiling showed TRIM24 overexpression activates EMT and G2/M regulators. Our preliminary results show the TRIM24-PROTAC is promising as a future therapeutic approach towards MpBC. Findings from our mouse model are relevant to tumors derived from MpBC patients, as PDXs show high expression of TRIM24 and respond to PROTAC treatment of TRIM24. These findings suggest exciting opportunities to exploit the Trim24COE mouse model as a means of understanding mechanisms of MpBC tumor development and for preclinical studies of new, epigenetic-based therapeutic approaches.

Citation Format: Vrutant Shah, Sabrina Stratton, Shiming Jiang, Jeffrey Chang, Helen Piwnica-Worms, Stacy Moulder, Michelle Barton. A novel model to study to metaplastic TNBC [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-03-01.