Polycomb group protein BMI1 prevents neuroblastoma cells from DNA-damage-induced apoptotic cell death

BMI1 is a polycomb protein and its overexpression has been correlated with cancer development and aggressiveness. We previously reported that v-myc avian myelocytomatosis viral oncogene neuroblastoma-derived homolog (MYCN)-induced BMI1 positively regulated neuroblastoma (NB) cell proliferation via the transcriptional suppression of tumor suppressors in NB cells. In order to evaluate the potential of BMI1 as a new target for NB therapy, we herein examined the effects of BMI1 reductions on NB cell differentiation and apoptotic NB cell death. BMI1 knockdown (KD) up to 7 days in NB cells significantly induced their differentiation. BMI1 depletion up to 14 days significantly induced apoptotic NB cell death along with the activation of p53, increases in p73, and induction of p53 family downstream molecules. BMI1 depletion in vivo markedly suppressed NB xenograft tumor growth. A pathological analysis using the TUNEL assay indicated the induction of apoptotic cell death. BMI1 reductions activated ATM and increased γ-H2AX in NB cells. Importantly, these DNA damage signals and apoptotic cell death were not canceled by transduction of polycomb group molecules EZH2 and RING1B. Further, EZH2 or RING1B knockdown could not induce apoptotic NB cell death at the same level of BMI1 KD. Together, BMI1 appears to be a promising target of molecular therapy for NB tumors and our report clarified, for the first time, the shared role of PcG proteins in DNA damage response of NB cells. RNA was extracted at 8-14 days after shBMI1-expressing lentiviral infection, and a microarray analysis was performed using the HG-U133 plus 2.0 arrays were used to deteremine the genes perturbed by knockdown of BMI1.