Role of MYCN in retinoblastoma : From carcinogenesis to tumor progression

Abstract: Retinoblastoma, a pediatric malignancy of the retina, is primarily driven by the bi-allelic inactivation of the RB1gene. However, a subset of cases are characterized by proficient RB1 functions but with MYCN copy number mutations, suggesting an alternative oncogenic mechanism in the absence of RB1 mutations. The aim of this thesis is to investigate the intricate molecular and cellular pathways implicated in retinoblastoma, with a particular focus on the role of MYCN expression and its interplay with the cell cycle and apoptotic pathways.In Paper I, we explored the regulatory mechanisms underpinning MYCN-induced retinoblastoma using aRB1-proficient MYCN-overexpressing in vivo model in embryonic chicken retina and MYCN-transformed cells in culture. Our findings revealed that MYCN overexpression led to a significant upregulation of E2F levels, thereby dysregulating the cell cycle and mimicking the mechanistic phenotype of RB1-deficient tumors. Inhibition on E2f DNA-binding activity efficiently normalized growth and apoptosis in MYCN-transformed cells in culture. Despite RB1 proficiency, the elevated E2F levels induced a neoplastic behavior in retinal cells, indicating a novel mechanism of retinoblastoma carcinogenesis independent of RB1 inactivation.Paper II employed single-cell RNA sequencing to dissect the cellular composition of MYCN-driven retinoblastoma in chicken in vivo model, revealing a predominant origin in cone photoreceptor progenitors. This finding suggested a cell-type-specific vulnerability to MYCN-induced transformation. The research further identifies a notable heterogeneity within the MYCN-transformed cells, with a subset of cells exhibiting non-cone photoreceptor features but features of other neurons like ganglion cells. A cluster was also identified withelevated expression of genes related to malignancy and tumor progression, including UBE2C and TOP2A. This suggested a link between MYCN overexpression and tumor development, potentially mediated through the E2F pathway.In Paper III, the focus shifted to the interplay between MYCN expression, E2f activity, and the p53 pathway in human retinoblastoma cell lines exhibiting both RB1 deficiency and MYCN amplification. By modulating E2f and p53 pathway activities using chemical inhibitors, we demonstrated the essential role of MYCN expression level in mediating p53-driven growth inhibition and highlighted the independent effects of E2f inhibition and p53 activation by a Mdm2 inhibitor.Together, these studies illuminate the intricate molecular pathways involved in MYCN-amplified retinoblastoma, emphasizing the pivotal role of MYCN in disrupting cell cycle regulation and promoting tumorigenesis. These insights not only advance our understanding of retinoblastoma pathogenesis but also provide potential therapeutic targets within the MYCN-E2F axis, offering novel treatment strategies in MYCN-amplified retinoblastoma.