Notch signaling requiem : orchestral role of notch signaling in cancer and developmental disease

Abstract

Notch signaling is an evolutionary conserved contact-dependent cell-cell communication pathway. This “contact” spans from hydra to fruit flies to human; orchestrating development, homeostasis and cancer, thus the Requiem, a song of life and death. Upon the “contact” of Notch receptor and ligand, the intracellular domain NICD is released and translocates to the nucleus. NICD, together with the DNA binding protein CSL and other co-activators, activate downstream targets. In this thesis, I have investigated the role of Notch signaling in multiple contexts with a modular approach. This includes: the non-canonical role of CSL in breast cancer, crosstalk of Notch signaling with hypoxia signaling in cancer, canonical Notch signaling in blood development, a novel mouse model for Alagille syndrome, and the hyperactivated Notch during mammary development and tumourigenesis.

Here I phrase them in five sections of a requiem (Mozart’s Requiem, 1791): Introitus: In Paper I, we found that ablation of CSL unleashed a hypoxic response in normoxic conditions and enhanced tumour growth in breast cancer. A large part of the deregulated genes in the CSL null cell line is Notch independent. We demonstrated a non-canonical role of CSL and the possible implication of loss of CSL in breast cancer. Kyrie: In Paper II, we established that Notch signaling can modulate hypoxia signaling in multiple cancer cell types. By siRNA knocked down of HIF2α, we found that Notch signaling requires HIF2α for regulating a subset of Notch targets in medulloblastoma cells. Differences in the effect of N1ICD and N2ICD were also shown in the medulloblastoma cells. Lastly, we presented evidence of Notch signaling contributing to the HIF1α-to-HIF2α switch. Dies Irae: In Paper III, we revealed that canonical Notch signaling is dispensable in adult steady-state and stress myelo-erythropoiesis by ablating CSL in the myeloid lineage. Some of the Notch targets were derepressed in some of the progenitor stages, indicating CSL could act as a repressor in some contexts. Rex tremendae: In Paper IV, we established and characterized a mouse model for Alagille syndrome in human, recapitulating defects in multiple organ-systems. We showed a mutation in Jag1 caused delay differentiation and structural abnormalities in the bile ducts. From transcriptomics of mice and patients samples, we also found some commonly affected genes across species. Lastly, we discovered that the mutated Jag1 failed to bind to Notch1 and reduced the extent of Notch2 and Notch3 activation. Lacrymosa: In Paper V, we observed that hyperactive Notch in the luminal lineage during lactation cause defect in ductal development and led to mammary tumour development. Furthermore, we showed that this lineage can contribute to a large part of the mammary tumour.