Neuroblastoma Cell Differentiation: The Role of Neurotrophin Receptor Signaling and N-myc Expression

Sammanfattning: Neuroblastoma is a tumor of sympathetic nervous system derivation, mostly afflicting young children. This thesis is focused on a group of receptor proteins for neurotrophic factors, the Trk family, and on N-Myc, a transcription factor, all important in the formation of the sympathetic nervous system as well as in determining neuroblastoma patient outcome. Expression of trkA and trkC is linked to favorable prognosis, while amplification of the N-myc gene is strongly correlated to poor outcome. The role of trkA and trkC in neuroblastoma cell differentiation was studied using neuroblastoma cell lines constitutively expressing trkA or trkC. Stimulation of the trkA- and trkC-transfected cells with their cognate ligands resulted in differentiation of both cell clones, the differentiated trkC-transfected cells lacking important neuronal features present in the differentiated trkA-transfected cells. Signaling elicited by the two receptors was studied and differences described. The possible connection between expression of N-myc and trkB, another trk family member, was tested by examination of expression of these genes in a set of neuroblastomas and neuroblastoma cell lines. Results suggested high expression of N-myc per se to be insufficient to induce trkB expression. In other experiments, retinoic acid, an agent known to induce trkB expression was added to neuroblastoma cells in combination with the ligand of TrkB, brain-derived neurotrophic factor. Data from characterization of the resulting phenotype indicated that a sympathetic neuronal differentiation did not take place in these cells and alternative explanations were suggested. N-myc expression in the developing human sympathetic nervous system was studied using in situ hybridization and the role of N-myc in neuroblastoma cell was examined, utilizing non-N-myc-amplified neuroblastoma cells with constitutive N-myc overexpression as a model system. Overall, the capacity of these cells to differentitate morphologically in response to various differentiation protocols was retained, thus offering an explanation to the lack of correlation between N-myc expression and patient outcome in non-N-myc-amplified tumors.