The receptor-tyrosine-kinase-like orphan receptor 1 (ROR1) was identified by a polymerase chain reaction (PCR)-based search for tyrosine kinases similar to the tropomyocin receptor kinase (Trk) neurotropic receptors. ROR1 and a related protein, ROR2, were identified as orphan receptors with an extracellular Frizzled-like, cysteine-rich domain, an extracellular, membrane-proximal kringle domain, and an intracellular tyrosine-kinase-like domain. Both ROR proteins are primarily expressed during embryogenesis. In prior studies, we and others found that ROR1 was expressed by leukemia cells and some cancer cell lines, and was involved in cell survival. However, it was not known whether breast tumor cells expressed ROR1 or whether its expression had functional and clinical significance. In the present study we used a high-affinity mAb specific for ROR1 (named 4A5) to examine human breast cancers and investigate its functional role for tumor growth. We examined fresh-frozen tumor biopsy specimens or tissue microarrays of neoplastic and normal adult tissues for ROR1. The neoplastic cells of high proportions of human breast cancers (70%) expressed ROR1, which was not detected on non-neoplastic adult breast tissues. We interrogated available DNA microarray datasets on primary human breast cancers and cancer cell lines, and fount that breast cancer cell lines or primary breast cancers that expressed high-levels of ROR1 were more likely to lack expression of estrogen or progesterone receptors or HER2/Neu. Conceivably, tumors that are poorly differentiated are more likely to express ROR1. Patients who had primary breast cancers that expressed higher levels of ROR1 had a significantly shorter median survival than did patients with primary breast cancers that had low-to-negligible expression of ROR1. We silenced ROR1 expression in breast cancer cell lines to evaluate its function on tumors. When silenced for ROR1 for MDA-MB-231 cells that expressed ROR1, MDA-MB-231 cells were more sensitive to spontaneous apoptosis and had an impaired cell growth in vitro and in vivo. On the other hand, MCF-7 cells that were transduced to express ROR1 had more aggressive growth characteristics than did control MCF-7 cells. We investigated the mechanism by ROR1 induced cell survival and found that ROR1 could interact with casein kinase 1 epsilon (CK1α) to activate phosphoinositide 3-kinase-mediated AKT phosphorylation and cAMP-response-element-binding protein (CREB), which in turn induce expression of genes (CCNB1, BCL2, and CCND1) that can enhance resistance to apoptosis and/or promote tumor growth. Moreover, Wnt5a, a ligand of ROR1, could induce ROR1-dependent signaling and enhance cell growth. This study demonstrates that ROR1 is expressed in human breast cancers and has biological and clinical significance, indicating that it may be a potential target for breast cancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3868. doi:1538-7445.AM2012-3868