Oncogenic Pathways in Lobular Breast Cancer

Breast cancer affects approximately 1 in 8 women in the Western world with more than one million new cases worldwide per year, of which 30% will eventually die. It is a heterogeneous disease with several histological and molecular characteristics within tumors and between patients. Invasive lobular breast cancer (ILC) is the second most common histologic subtype of breast cancer after invasive ductal breast cancer (IDC). A very distinguishing feature of ILC is the typical growth pattern of tumor cells in a loosely cohesive way which makes the diagnosis of this subtype very difficult. It has been reported several times that ILC has a worse prognosis than IDC and this is a clear sign of inadequate success rate of current treatment strategies against this malignancy. There is yet a lack of understanding in the molecular pathways associated with lobular cancer etiology. Several signaling pathways which play a key role in normal mammary gland development and homeostasis have been identified. The aim of this thesis was to investigate the involvement of some of these key pathways in ILC growth and maintenance to gain a better insight in this subtype. This is required to improve the existing strategies and to develop novel therapies. NOTCH signaling, a cell-cell communication pathway, has been shown to play an important role in breast cancer. We addressed the role of NOTCH signaling in ILC by performing in vitro and in vivo experiments. We observed a high frequency of NOTCH pathway activity in human ILC patient material. Importantly, in vitro inhibition of NOTCH signaling using γ-secretase inhibitors (GSI) led to proliferative arrest in both mouse and human ILC like cell lines. Furthermore, GSI treatment in a mouse allograft model for ILC inhibited NOTCH signaling and retarded tumor growth. Our data revealed NOTCH signaling as an important pathway for the growth and maintenance of ILC. Moreover, we investigated the associations between NOTCH and HIF-1α pathways in human breast cancer specimens. Co-expression of NOTCH and HIF-1α was associated with high grade/high proliferation. HIF-1α expression is low in classic but high in pleomorphic lobular cancers, which also frequently showed stromal HIF-1α expression. Although NOTCH and HIF signaling did not seem to be functionally associated, NOTCH pathway activation indicated worse prognosis. Furthermore, we performed an extensive analysis to determine p53 status, a well known tumor suppressor, in classic and more aggressive pleomorphic variants of ILC (PILC) and demonstrated that p53 mutations seem to occur more frequently in pleomorphic ILC than classic ILC. In order to gain more insight into the genetic profile of these two ILC subtypes, we performed multiplex ligation dependent probe amplification (MLPA) copy number analysis for a set of established breast cancer tumor suppressor and oncogenes in classic and pleomorphic ILC. Overall, there were fewer copy number changes in classic than in pleomorphic ILC. In conclusion, this research revealed attractive candidates for further research to improve the understanding on ILC development/maintenance and to investigate hypoxia and the NOTCH pathway as target for individualized treatment regimens in patients with ILC.