Targeting MYC and exploring the role of mitochondrial metabolism in childhood neuroblastoma

Abstract: MYCN is a member of the MYC family of proto-oncogenes, encoding transcription factors (c-MYC, MYCN and L-MYC) that play crucial roles for normal cellular functions and during development. However, the expression of MYC (here referring to c-MYC and MYCN) is found elevated in a large number of human cancers where it is implicated in most aspects of tumorigenesis and correlates to poor clinical outcome. Neuroblastoma is a heterogenous childhood cancer of the sympathetic nervous system. Tumors harboring amplification of the MYCN gene are highly aggressive and these patients have a poor prognosis. Consequently, new treatments directed against high MYC expressing tumors could help to improve the survival rates of these children. In Paper I, we screened 80 chemotherapeutic drugs and small chemical compounds to assess their selectivity against MYC-overexpression, using cancer cells with conditional c-MYC or MYCN expression. Positive hits belonged to distinct classes of chemical agents acting on selective cellular processes, including RNA, DNA and protein synthesis and turnover, and those inhibiting microtubules and topoisomerases. These results may provide indications for future drug development and treatment optimization towards MYC. One important goal in cancer research is to identify small molecules, which can interfere with MYC’s function, since today, no therapeutically relevant therapy acting directly against MYC exists. In Paper II we demonstrated that a previously identified c-MYC binding molecule, 10058-F4, showed selectivity towards high MYCN expressing neuroblastoma cells and resulted in prolonged survival in a MYCN-driven transgenic mouse model of neuroblastoma. In Paper IV, we further demonstrated that 10058-F4 and a few additional c-MYC-binding small molecules bind directly to the corresponding region of MYCN, and that their binding affinities correlated with the level of growth suppression in cells. Metabolic rewiring is an important feature in aggressive tumors. In Paper II we showed that downregulation of MYCN in neuroblastoma cells leads to accumulation of cytoplasmic lipid droplets caused by mitochondrial dysfunction. In this regard, MYCN was found to be linked with an overall elevated mitochondrial metabolism important for mediating tumor aggressiveness in neuroblastoma. In Paper III, we carried out a systematic investigation of metabolic alterations associated with MYCN in neuroblastoma, using patient gene expression data, quantitative proteomics and functional studies of metabolic pathway fluxes. MYCN was found to positively regulate glycolysis, respiration as well as oxidation of exogenous fatty acids in neuroblastoma cells, suggesting that MYCN mediates metabolic plasticity, which could account for an important survival mechanism during neuroblastoma tumor progression. Together the work comprised in this thesis support the development of targeted therapy against MYCN and identified MYCN-induced metabolic signals as a potential approach to target high risk neuroblastoma.