Abstract 2578: ROR1: A novel target for pre-B acute lymphoblastic leukemia and neuroblastoma

Current cytotoxic drugs may not improve the pediatric cancer mortality rates further without added toxicities. Newer targeted therapies are therefore needed to improve treatment without increased immediate and long-term toxicities. Several monoclonal antibodies (mAbs) against tumor cell surface antigens have been used in clinical trials, but none of the targeted cell surface antigens are restricted to tumor cells, accounting for some of their toxicities. Following the cloning of full length cDNA for human receptor tyrosine kinase-like orphan receptor 1(ROR1) from the neuroblastoma cell line, SH-SY5Y, gene expression profiling identified ROR1, an embryonic protein, as a signature gene in B-cell chronic lymphocytic leukemia. This was confirmed by comprehensive analysis of ROR1 protein expression. Neither ROR1 mRNA nor protein expression was observed in normal B cells or other B cell malignancies. In the present study, we evaluated whether ROR1 can serve as a novel cell surface antigen for mAb therapy in pediatric ALL and neuroblastoma. Lymphoblasts from peripheral blood or bone marrow of 38 patients with precursor B-cell (pre-B) ALL, 14 ALL cell lines, and 13 neuroblastoma cell lines were analyzed qualitatively for ROR1 protein expression by flow cytometry and Western blotting. Gene expression profiling data of pre-B ALL and neuroblastoma patients were analyzed for ROR1 mRNA expression. We found that ROR1 protein is highly expressed in 7 out of 14 ALL cell lines, and 6 out of 38 primary patient samples. Gene expression profiling suggests a correlation between ROR1 upregulation and t (1;19) translocation, which generates the E2A-PBX1 fusion protein. We confirmed that all pre-B ALL cell lines with this cytogenetic abnormality expressed cell surface ROR1. ROR1 expression was also found in one pre-B ALL cell line and one primary pre-B ALL sample with MLL rearrangement. Future experiments will explore this correlation in patient samples with known cytogenetic abnormalities. ROR1 expression was maintained in primary pre-B ALL cells after xenotransplantation in NOD/SCID mice, making this a suitable in-vivo model for investigating ROR1-targeted immunotherapy. All of the 13 neuroblastoma cell lines revealed cell surface expression of ROR1. Interestingly, gene expression profiling shows a lower overall survival for patients with high ROR1 expression despite not having N-myc amplified(p=0.015). Previous studies revealed that, with few exceptions, ROR1 mRNA and protein is not detectable in normal adult tissues. We are currently analyzing ROR1 mRNA and protein expression in normal pediatric tissues. Future in vitro and in-vivo studies will address whether mAbs that target ROR1 can efficiently kill pre-B ALL and neuroblastoma cells. Based on its restricted cell surface expression in neuroblastoma and in a subset of pre-B ALL, ROR1 merits further investigation as a potential target for naked and armed mAbs in childhood cancer therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2578. doi:10.1158/1538-7445.AM2011-2578