%0 Generic
%A Beumers, Lukas
%C Heidelberg
%D 2023
%F heidok:33065
%K Breast cancer, Therapy resistance, Tumor heterogeneity
%R 10.11588/heidok.00033065
%T Clonal heterogeneity of endocrine therapy resistance in breast cancer
%U https://archiv.ub.uni-heidelberg.de/volltextserver/33065/
%X Breast cancer is the most common malignancy in women worldwide and roughly two-thirds of breast tumors are characterized by estrogen receptor (ER) α expression allowing for a targeted treatment approach by suppressing estrogen signaling. Clinically, suppression of estrogen signaling is achieved by Tamoxifen or Fulvestrant which prevent the receptor’s activation or aromatase inhibitors which prevent the formation of (peripheral) estrogen. Unfortunately, relapses are observed in up to 41% of cases and tumor heterogeneity has recently been implicated in therapeutic resistance. To analyze endocrine therapy resistance on a clonal level, I used a previously developed barcoded in vitro model. There, two endocrine therapy sensitive ER+ breast cancer cell lines had been transduced with a barcode library. These cells had then been rendered resistant to Tamoxifen treatment (TAMR) or long-term estrogen deprivation (LTED), the latter to mimic clinically used aromatase inhibitors. Barcode analysis of complex cell pools suggested that endocrine therapy resistance arose either due to the selection of pre-existing clones or the rewiring of initially treatment-persisting cells. In the next step, I isolated and characterized endocrine therapy sensitive and resistant clones from biological replicates using phenotypic assays, RNA-Sequencing and Mass spectrometry-based (phospho-)proteomics. Phenotypically, endocrine therapy resistant clones showed either weak or strong proliferative capacity. On pathway, transcription factor and kinase activity level, I observed heterogeneity between the cell lines utilized resembling inter-tumor heterogeneity and between endocrine therapy resistant clones isolated from each cell line resembling intra-tumor heterogeneity. Activation of the unfolded protein response (UPR) was a private event, which was only observed in a single TAMR population, and correlated with sensitivity to the proteasome inhibitor Bortezomib. Conversely, TAMR and LTED populations shared the activation of multiple protein kinase C (PKC) isoforms, however to different degrees. Treatment with the pan-PKC inhibitor Sotrastaurin preferentially reduced cellular viability of endocrine therapy resistant populations showing stronger PKC activation. Finally, my in vitro findings were supported by clinical findings from the CPTAC-BRCA cohort. Generally, strong heterogeneity between individual patients was evident on pathway, transcription factor and kinase activity levels. After I had deconvoluted the cohort on a per patient basis, I identified patients with estrogen independent tumors showing UPR and PKC activation, closely resembling my in vitro models. Taken together, in the presented PhD thesis I could identify private and shared clonal endocrine therapy resistance drivers with clinical importance.