Abstract #2194: OTX2 in medulloblastoma: its function and targets

Medulloblastoma is the most common malignant brain tumor in children. The homeobox gene OTX2 has been implicated in medulloblastoma pathogenesis, as OTX2 is amplified in some medulloblastomas and highly overexpressed in the majority of tumors. However, little is known about the precise function of OTX2 in medulloblastoma or its downstream targets. Therefore, we generated doxycycline-inducible cell line models in which we can either overexpress ectopic OTX2 or silence endogenous OTX2. We used these cell lines to investigate the function of OTX2 and to identify its downstream targets. When OTX2 was overexpressed in cell lines without endogenous OTX2 expression (MED8A and DAOY), these cell lines displayed a senescence-like phenotype: the proliferation decreased, the morphology changed and senescence-associated ß-galactosidase activity was detected. Expression profiling at different time points after induction of OTX2 in the MED8A cell line revealed an initial upregulation of G2/M related genes, whereas G1/S related genes were downregulated. We hypothesized that this imbalance in cell cycle regulation contributes to oncogenic stress, which will activate the P53 pathway with senescence as a result. Indeed, expression profiling and reporter assays showed activation of the P53 pathway after induction of OTX2 in these MED8A cells. Silencing endogenous OTX2 in the D425 cell line, which has an amplification of OTX2, resulted in growth arrest and differentiation. Expression profiling of these cells following OTX2 silencing in time showed downregulation of cell cycle genes as well as upregulation of differentiation genes. To distinguish between direct and indirect targets, we also performed chromatin immunoprecipitation with microarray analyses. Our results give a first glance at the OTX2 transcriptional network and its distinct function in medulloblastoma.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2194.