Unique Cell Surface Expression of Receptor Tyrosine Kinase ROR1 in Human B-Cell Chronic Lymphocytic Leukemia

• Published in: Clinical cancer research Vol. 14; no. 2; pp. 396 - 404
• Main Authors: BASKAR, Sivasubramanian, KWONG, Kayin, HOFER, Thomas, LEVY, Jessica M, KENNEDY, Michael G, LEE, Elinor, STAUDT, Louis M, WILSON, Wyndham H, WIESTNER, Adrian, RADER, Christoph
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA American Association for Cancer Research 15.01.2008
• Subjects: Antineoplastic agents; B-Lymphocytes - enzymology; Biological and medical sciences; Biomarkers, Tumor; Blood Cells - enzymology; Cell Line, Tumor; Cell Membrane - enzymology; cell surface antigen; Hematologic and hematopoietic diseases; Humans; leukemia; Leukemia, Lymphocytic, Chronic, B-Cell - enzymology; Leukemias. Malignant lymphomas. Malignant reticulosis. Myelofibrosis; Leukocytes, Mononuclear - enzymology; Medical sciences; new target; Pharmacology. Drug treatments; Receptor Protein-Tyrosine Kinases - blood; Receptor Protein-Tyrosine Kinases - metabolism; receptor tyrosine kinase; Receptor Tyrosine Kinase-like Orphan Receptors; tumor antigen; Human; Chronic lymphocytic leukemia; Lymphoproliferative syndrome; Receptor tyrosine kinase; Malignant hemopathy; B-Lymphocyte; Gene expression; Cell surface; Cancer
• ISSN: 1078-0432; 1557-3265
• DOI: 10.1158/1078-0432.CCR-07-1823

Purpose: Gene expression profiling identified receptor tyrosine kinase ROR1, an embryonic protein involved in organogenesis, as a signature gene in B-cell chronic lymphocytic leukemia (B-CLL). To assess the suitability of ROR1 as a cell surface antigen for targeted therapy of B-CLL, we carried out a comprehensive analysis of ROR1 protein expression. Experimental Design: Peripheral blood mononuclear cells, sera, and other adult tissues from B-CLL patients and healthy donors were analyzed qualitatively and quantitatively for ROR1 protein expression by flow cytometry, cell surface biotinylation, Western blotting, and ELISA. Results: ROR1 protein is selectively expressed on the surface of B-CLL cells, whereas normal B cells, other normal blood cells, and normal adult tissues do not express cell surface ROR1. Moreover, cell surface expression of ROR1 is uniform and constitutive, i.e., independent of anatomic niches, independent of biological and clinical heterogeneity of B-CLL, independent of B-cell activation, and found at similar levels in all B-CLL samples tested. The antibody binding capacity of B-CLL cell surface ROR1 was determined to be in the range of 10 3 to 10 4 molecules per cell. A portion of B-CLL cell surface ROR1 was actively internalized upon antibody binding. Soluble ROR1 protein was detectable in sera of <25% of B-CLL patients and a similar fraction of healthy donors at concentrations below 200 ng/mL. Conclusions: The restricted, uniform, and constitutive cell surface expression of ROR1 protein in B-CLL provides a strong incentive for the development of targeted therapeutics such as monoclonal antibodies.