Resumen de Unraveling luminal breast cancer heterogeneity
Metastasis is the major cause of death from Breast Cancer (BCa). During its progression, divergent opinions have arisen regarding the hormonal status of the BCa subtypes. The mammary gland is composed of different cell linages, including the basal and the luminal cells (LCs). The last ones can be divided in estrogen receptor positive (ER+) and negative (ER-). Luminal tumors, which usually express ER+, may reappear after a long period of time on a process called latency or dormancy. Of importance, in Luminal BCa it is unknown which population, and which is the driver of tumor initiation. It has been shown that Luminal A tumors can suffer molecular changes and switch to the Luminal B type at the metastatic site. Whether these changes are passenger or have consequences on latency and metastasis it is still unclear.
We hypothesize that tumor heterogeneity found in ER+ luminal BCa is the consequence of either i) pre-existing ER+ and ER– luminal cells heterogeneity or ii) a high degree of plasticity in luminal cells acquired upon treatment resistance or metastasis. Critically, distinguishing between these two options will help address how to target the founding cellular populations for therapy options for this otherwise fatal disease.
The main objectives of this thesis were to establish ER+ BCa organoids to model luminal heterogeneity, to decipher the different ER+ Luminal BCa tumor initiating populations and finally, to study the different ER+ Luminal BCa cellular hierarchies.
We conclude that we establish ER+ luminal BCa cell lines and Patient Derived Organoids (PDOs) and, that these PDOs must be treated with estradiol, forskolin and hydrocortisone to maintain resemblance to original tumors and capture tumor heterogeneity. We also have shown that Organoid technology shows to be a powerful tool to recapitulate cellular heterogeneity in luminal BCa as represented by sc-RNA-seq. By means of CRISPR/Cas9 gene editing, we report endogenous ER expression, allowing us to purify these ER+ cells without permeabilization. Moreover, we identify one cluster enriched in CD44high, CD24low, SOX9low and KRT18low expression by sc-RNA-seq. In addition, we report a specific gene signature that defines the tumor initiation cells in ER+ luminal BCa. ERlow organoids show faster differentiation capacity and more faithfully recapitulate original ER expression heterogeneity and, ERlow Sore6 reporter positive population is enriched in TIC markers. On the other hand, ERhigh subpopulation correlates with higher proliferation index and associates with higher risk of bone metastasis in ER+ tumors.
