Cleavage of the Anaplastic Lymphoma Kinase in Neuroblastoma

Abstract

Neuroblastoma, a tumor of the sympathetic nervous system, is responsible for 15% of all childhood cancer deaths. Genetic aberrations in the anaplastic lymphoma kinase (ALK) receptor tyrosine kinase have been implicated as a contributing factor in neuroblastoma pathogenesis. Of all neuroblastomas, however, only 10% of tumors involve activating mutations or amplification of ALK; by contrast, wild-type and non-amplified ALK is expressed in more than 90% of neuroblastomas, suggesting an alternative, mutation-independent mechanism of ALK activation. In Ba/F3 cells, proteolytic cleavage of the ectodomain of ALK promotes their proliferation. In this thesis, I demonstrate that proteolytic cleavage of ALK in neuroblastoma cells, leading to the shedding of the ALK ectodomain, results in the production of an activated, truncated, membrane-bound ALK fragment, leading to a functionally significant event in promoting the proliferation of neuroblastoma cells. I also conduct a preliminary screen of broad-spectrum matrix metalloproteinase inhibitors and demonstrate reduction of ALK cleavage in cells treated with these inhibitors. I then identify the ALK cleavage site using a novel monoclonal antibody (8G7) directed against the shed N-terminal ectodomain of ALK. In addition, I demonstrate the specificity and ability of this antibody to detect ALK cleavage in endogenous neuroblastoma cells, its ability to detect levels of cleaved ALK from patient sera, activate complement-mediated cytotoxicity, and in vivo ability to reduce tumor growth in a neuroblastoma mouse model. This thesis demonstrates the finding that cleavage of ALK augments the proliferative potential of neuroblastoma cells, while the specificity of 8G7 to the ALK ectodomain may reveal new diagnostic and therapeutic possibilities for treatment of neuroblastoma patients whose tumors express wild-type ALK.