Changing the Horizon of Endocrine Therapy Resistance

Over the past years cancer has become a major public health issue being the most common cause of death in the Netherlands. From all types of cancer, breast cancer is the most common cancer in females. The prognosis of breast cancer is dependent on stage at presentation, which encompasses tumor size, lymph node status and presence or absence of distant metastases. Besides staging and grading, the molecular classification of tumors is an important prognostic and predictive factor. The most known molecular factors are the hormonal receptor molecules estrogen receptor (ER) and progesterone receptor (PR). The majority of breast tumors is positive for one or both of these receptors. Over the past years, many therapies have been developed that target the ER (endocrine therapy) and have dramatically improved the prognosis of breast cancer. Approximately 5-10% of the patients present with distant metastases at the initial diagnosis. Additionally, approximately 30% of the patients initially diagnosed and treated for non-metastatic disease, depending on stage, develop distant metastases during follow-up despite prior intensive treatment. Metastatic breast cancer (MBC) is a highly heterogeneous disease for which many therapies have been developed over the years. For most patients with ER/PR-positive disease, endocrine therapy is the mainstay of treatment because of the favorable toxicity profile. Despite of all the developments and enormous progress in the understanding of the biological behavior, MBC is still an incurable disease due to occurrence of resistance to therapy. Although much is already known, there is still a high need to get more insight into the factors causing resistance to endocrine therapy in order to develop methods to overcome resistance and to identify biomarkers that can contribute to a more personalized treatment approach of individual patients.
One of the more recent discovered factors that might contribute to endocrine therapy resistance is Enhancer of Zeste Homolog 2 (EZH2). Increased expression of EZH2 has been associated with increased tumor cell proliferation and therefore worse survival. More knowledge is however needed to identify the exact role of EZH2 in endocrine therapy resistance in breast cancer and to what extent. Another reason underlying resistance in MBC is the constantly changing molecular make-up of tumor cells. Currently, the determination of predictive factors for treatment decision making is most commonly done in the primary tumor tissue while differences in clinically relevant molecular characteristics between the primary tumor and metastases are increasingly recognized. Characteristics of metastatic cells rather than of the primary tumor are therefore likely to be more important since the ultimate outcome of cancer patients is determined by the behavior of metastases. With respect to the expression of ER, tumor status differs in 20% when comparing the primary tumor with its metastases, which would lead to a relevant treatment change in a substantial number of patients. Therefore, repetitive analyses of metastatic tissue could probably lead to more appropriate information for response prediction. However, metastatic tissue is often hard to obtain and only possible through invasive procedures, which limits its use in clinical practice. Hence, the characterization of circulating tumor cells (CTCs) poses an attractive alternative. CTCs are tumor cells found in the peripheral blood and are thought to originate from either the primary tumor or metastases. They can be obtained through venipunctures and can serve as a ‘liquid biopsy’. Besides detection of CTCs a major clinical challenge encompasses characterizing CTCs in order to base clinical decision making more on the characteristics of metastatic cells. However, since the isolation of pure fractions of CTCs is a major technological challenge, molecular characterization of CTCs is difficult to achieve since its low frequency and presence amongst a substantial number of leukocytes. It is however worthwhile to strive to since it holds great promise in improving knowledge on mechanisms underlying resistance to endocrine therapy. Concerning the use of CTCs in better understanding endocrine therapy resistance not much is yet known.

This thesis is aimed at exploring putative factors that are involved in endocrine therapy resistance by using cell line models and both primary tumors and CTCs of patients. The contribution of EZH2 expression in the behavior and drug response of tumors has been investigated on multiple levels of expression and the importance of characterization of CTCs has been explored.