Acute lymphoblastic leukemia (ALL) is a malignancy that most often represents the maturation arrest and clonal expansion of a Iymphoid precursor at a specific stage in early human B cell development. Despite impressive progress in delineating genetic abnormalities in ALL, the contribution of the abnormal proteins (e.g., fusion oncoproteins derived from chromosomal translocations) to the dysregulated growth of neoplastic B cell precursors is largely unknown. The last several years have witnessed an explosion of new knowledge regarding regulation of the cell cycle in mammalian cells. Major classes of molecules that play a fundamental role in positive and negative regulation of cell cycle progression have been discovered and characterized. These include the cyclin-dependent kinases (CDK) and their regulatory cyclin subunit partners, and two major classes of CDK inhibitors. Interestingly, one of the CDK inhibitor family members designated p16INK4 is deleted in 20-40% of cases of newly diagnosed ALL. Therefore, since p16INK4 has been shown to be a tumor suppressor gene in the mouse, a similar role for this protein may exist in ALL. Many human cancers, including ALL, respond to external growth stimuli in the context of the microenvironment wherein expansion of the neoplastic clone initially occurs. In the case of ALL, bone marrow stomal cells are probably a source of membrane-associated or extracellular matrix-associated survival/growth signals that are essential for expansion of the leukemic cells in vivo. The major goal of this application is to link bone marrow stromal cell-derived growth stimuli with dysregulated growth of ALL cells mediated by the cyclin DCDK/retinoblastoma (Rb) pathway. To our knowledge there are no reported models demonstrating that activation of the cyclin D/CDK/Rb pathway occurs in a tumor cell, following signaling mediated via direct cell-cell contact in a model that potentially recapitulates the microenvironment of the tumor. The applicants have established a pre-B ALL cell line, designated BLIN-2, that maintains a strict requirement on bone marrow stromal cells for survival and growth. This cell line will be used to: 1)characterize the role of the cyclin D/CDK/Rb pathway in bone marrow stromal cell-dependent growth, and 2) to identify and functionally characterize the ligand/receptor interactions that support the survival and growth of BLIN-2. Additional studies will determine the relevance of the BLIN-2 model to additional cases of ALL. The long-term goal of the project is to identify and characterize the bone marrow stromal cell ligands and their counter receptors on ALL cells that regulate the growth of this type of human cancer.
