Characterizing triple negative breast cancer subpopulations for developing novel targeted therapies

Breast cancer is a multifaceted disease that affects 1 in every 8 women. Triple negative breast cancer (TNBC) accounts for ~15-20% of all diagnosed breast cancers and is characterized by the absence of ER, PR, and HER2 on the tumor cell surface. As most cancer therapies to date target these cell surface receptors, TNBC is the only subtype of breast cancer without a targeted therapy and thus prognosis for it remains poor. The heterogeneity of TNBC also makes finding a targeted therapy particularly difficult. This work focuses on different methods of targeting distinct subpopulations of TNBC in order to identify potential novel therapeutic nodes to exploit as targeted therapies. The first chapter describes the use of a directed siRNA synthetic lethality screen to target vulnerabilities associated with basal TNBC, the most common TNBC subtype. The screen identified multiple dependency genes associated with RNA splicing, particularly those in the U4/U6.U5 tri-snRNP complex (PRPF8, PRPF38A). Depletion of these genes or the upstream splicing inhibitor E7107 in basal TNBC cell lines resulted in intronic retention and altered splicing of transcripts in pathways necessary for TNBC survival, including mitosis and apoptosis. In vivo, E7107 hindered the growth of both basal cell line and patient derived xenographs, a phenotype that was enhanced with the addition of the proteasome inhibitor bortezomib. This suggests that splicing and proteasome inhibition could be an effective basal TNBC treatment. The second chapter investigates the role of G-Protein Pathway Suppressor 2 (GPS2) as a tumor suppressor in the PI3K/AKT pathway in TNBC. Previous work has shown that GPS2 acts as a negative regulator of this pathway through inhibition of Ubc13-mediated activation of AKT in the insulin signaling pathway. In this study, MDA-MB231-GPS2KO cells were found to have increased proliferative, migratory, and invasive properties, which were rescued upon treatment with the allosteric AKT inhibitor MK2206. In vivo, GPS2 depleted cells conferred greater tumor burden in an orthotopic mouse model that was also responsive to AKT inhibition. Transcriptomic analysis showed significant overlap between MB231-GPS2KO and MB231 cells modified to have constitutively active AKT, indicating that the phenotypes observed in MB231-GPS2KO were at least in part due to loss of GPS2-mediated regulation of AKT activation. These studies point to GPS2 as a potential biomarker for a subclass of breast cancers that would be responsive to PI3K-class inhibitor drugs. In sum, these studies elucidate interactions and processes that seem to specifically adversely affect TNBC cells, which broaden our knowledge of TNBC biology and its potential weaknesses.2022-03-03T00:00:00