Abstract P1-09-07: Breast cancer initiating cells express functional ROR1, which can be targeted by cirmtuzumab to potentially mitigate the risk of relapse after therapy
Authors: Zhang S , Zhang H , Ghia EM , Liu G , Tripple V , Xu S , Cui B , Widhopf G , Yu J , Schwab R , Messer K , Parker BA , Kipps TJ 
Published: February 2018

Abstract
Although initially responsive to chemotherapy, patients with advanced breast cancer often relapse, generally with incurable metastatic disease. This may be due to a subpopulation of tumor cells, called cancer-initiating cells, or cancer stem cells (CSCs), which are relatively resistant to chemotherapy and have self-renewing and tumor-initiating capacities. Prior studies in our laboratory found that CSCs may express ROR1, an onco-embryonic, tyrosine-kinase-like orphan receptor, which we found could bind Wnt5a to activate non-canonical Wnt-signaling (Proc Nat Acad Sci USA 111:17266, 2014). Interrogation of the transcriptomes of breast-cancer cells obtained from patients before and after paclitaxel therapy revealed that chemotherapy treatment enhanced cancer-cell expression of ROR1, along with genes induced by activation of Rho-GTPases (e.g. RhoA, cdc42, and Rac1). We found that primary breast-cancer patient-derived xenografts with high-level expression of ROR1 were enriched for cells that had activated Rho-GTPases and stem-cell-like gene-expression signatures. Furthermore, we found that treatment of breast cancer cell lines with Wnt5a induced ROR1-dependent activation of Rho-GTPases andAKT and induced high-level protein expression of BMI1, also known as polycomb group RING finger protein 4 (PCGF4) or RING finger protein 51 (RNF51); Wnt5a also enhanced the capacity of breast cancer cell lines to form spheroids. All these effects could be inhibited by cirmtuzumab, a humanized high-affinity anti-ROR1 mAb, which can block Wnt5a signaling. We find that ROR1-positive breast cancer cells have a greater capacity to form spheroids or engraft immune-deficient mice than did ROR1-negative cancer cells isolated from the same PDX tumor. Treatment of immune-deficient mice bearing breast-cancer PDX with paclitaxel reduced tumor volumes but enhanced expression of ROR1 and other CSC markers, such as aldehyde dehydrogenase 1 (ALDH1). Moreover, the breast cancer cells surviving such paclitaxel treatment had increased activation of Rho-GTPases and AKT, and increased expression of BMI1, relative to that of breast cancer cells obtained from the same primary tumor prior to therapy. On the other hand, treatment of such mice with cirmtuzumab also reduced breast cancer PDX tumor volumes, but the remaining cells had reduced expression of ROR1 and CSC markers and had impaired capacity to re-engraft immune-deficient mice. Finally, therapy with cirmtuzumab and paclitaxel was more effective in eradicating breast-cancer PDX than treatment with either agent alone. Collectively, these findings support use of cirmtuzumab in combination with conventional anti-cancer drugs to improve the outcome of patients with advanced breast cancer.
Citation Format: Zhang S, Zhang H, Ghia EM, Liu G, Tripple V, Xu S, Cui B, Widhopf G, Yu J, Schwab R, Messer K, Parker BA, Kipps TJ. Breast cancer initiating cells express functional ROR1, which can be targeted by cirmtuzumab to potentially mitigate the risk of relapse after therapy [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-09-07.