DIPG-21. INDUCTION OF MITOTIC ABNORMALITIES AND BMI-1 MODULATION TO TREAT DIFFUSE INTRINSIC PONTINE GLIOMA

Diffuse intrinsic pontine glioma (DIPG) is a poor-prognosis pediatric brain tumor with a median survival of less than one year. No effective therapy is currently available, and no therapeutic advances have been made in several decades. BMI-1 is a member of the multimeric protein complex Polycomb repressor complex 1 (PRC1). It has been implicated in self-renewal of normal and cancer cells, and in DNA damage signaling. We have previously identified BMI-1 as a potential therapeutic target in DIPG and have shown that BMI-1 is highly expressed in DIPG tumors regardless of histone 3 subtype. In the present study, we show that the modulation of BMI-1 leads to DNA damage, M phase cell cycle arrest, chromosome abnormalities and cell death. Furthermore, modulation of BMI-1 sensitizes DIPG patient-derived stem-like cells to ionizing radiation (IR). Treatment of DIPG stem-like cells with PTC596, a BMI-1 modulator, and IR, impairs the kinetics of DNA damage response (DDR). Both DDR foci formation and resolution were delayed, resulting in further reduction in cell viability compared with either treatment alone. In vivo, treatment of mice bearing DIPG xenografts with PTC596 leads to decreased tumor volume and growth kinetics, increased in-tumor apoptosis and sustained animal survival benefit. Gene expression analysis indicates that BMI-1 expression correlates positively with DIPG stemness and BMI-1 signature. Together our findings indicate that BMI-1 modulation is associated with mitotic abnormalities, impaired DDR and cell death, supporting the combination of BMI-1 modulation and radiation as a promising novel therapy to treat children with DIPG.