The Role of ROR1 Pseudokinase Signaling in Cancer

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is a Wnt signaling associated cell surface receptor that is classified as pseudokinase due to the alterations in kinase domain consensus motifs required for kinase activity. Although originally defined as an orphan receptor, Wnt5a is now established as a ligand for ROR1. Normally, ROR1 is mainly expressed during the development, and in the tissues of healthy adults its expression is minimal. However, ROR1 is expressed in many cancers, both hematological and solid malignancies.

ROR1 has been shown to promote cancer cell proliferation, migration and invasion. High ROR1 expression often correlates with a poor prognosis and ROR1 can even contribute to the development of drug resistance. In recent years, interest towards developing ROR1-targeted cancer therapies has been increasing and some successful approaches are already undergoing clinical trials. Still, relatively little is known about the actual signaling pathways activated downstream of ROR1.

For this thesis, ROR1 signaling was studied in mantle cell lymphoma (MCL), B- cell precursor acute lymphoblastic leukemia (BCP-ALL) and epithelial ovarian cancer (EOC) using cell line models and ex vivo primary cells. To study the effects of ROR1 downregulation, stable inducible ROR1 shRNA cell lines were generated from selected cell lines and used to analyze cell viability and the levels of putative downstream signaling proteins. For EOC models, the effects of glucocorticoid dexamethasone on ROR1 expression, signaling, and chemoresistance were also investigated. Finally, the drug sensitivity and resistance testing (DSRT) platform was used to explore how the ROR1 silencing affects the drug sensitivities of cancer cells. ROR1 expression was found to be heterogeneous in all studied cancer types. The binding of Wnt5a to ROR1 and its ability to induce ROR1-mediated downstream signaling was also further confirmed in MCL, BCP-ALL, and EOC. Small GTPases RhoA and Rac-1 were most consistently detected as possible ROR1 downstream effectors. ROR1 signaling was also conveyed through kinases AKT, MEK, and ERK. Additionally, ROR1 associates with NF-κB signaling in MCL and with transcriptional regulators YAP/TAZ and BMI-1 in EOC. YAP/TAZ and BMI-1 were also upregulated after a glucocorticoid treatment that increased ROR1 expression in EOC cells as well. The most notable findings from the DSRT indicate that combining ROR1-targeted therapies to Bcl-2 inhibitor venetoclax in B-cell malignancies or to SMAC-mimetics in EOC could have increased anti-cancer efficacy compared to single agent treatments.