Abstract 4880: CASZ1, a neuroblastoma tumor suppressor gene, is epigenetically silenced by EZH2

Abstract

Abstract Casz1 is a C2H2 Zn-finger transcriptional factor that localizes to a region of LOH on Chr1p36.22 in tumors of Neuroblastoma patients who have a poor prognosis. Moreover, high CASZ1 expression is found in tumors of patients with a good prognosis while tumors from patients with a poor prognosis express low levels of CASZ1 (58 patients, p= 2.07×10-6). Studies indicate that CASZ1 functions as a tumor suppressor gene. Our finding that treatment of NB cells with HDAC inhibitors results in a 10-fold increase in CASZ1 expression suggests that epigenetic silencing contributes to the functional loss of CASZ1 expression from the remaining Chr1p36.22 allele. In studying the mechanism of CASZ1 epigenetic silencing, we found a bivalent chromatin modification region around the CASZ1 Transcription Start Site (TSS) that contains both a H3K27 methylation mark as well as an H3K4 methylation mark. The H3K27 methylation is thought to be a gene repressive marker, which is catalyzed by Polycomb repressor complex 2 (PRC2) proteins. PRC2 contains three core components, EZH2, EED and SUZ12, in which EZH2 (enhancer of zeste 2) is a methyltransferase catalyzing H3K27 methylation. Using Chromatin Immunoprecipitation (ChIP) we identified a 5-fold decrease in binding of EZH2, EED and SUZ12 to the bivalent region after HDAC inhibitor treatment. Consistent with decreased PRC2 binding is the finding of decreased tri-methylation of H3K27 and an increase in H3K4me3, a histone mark associated with transcriptionally active genes. The changes in histone modifications are associated with the activation of the CASZ1 gene. RNA silencing of EZH2 also leads to increases in CASZ1 expression. Consistent with this, CASZ1 expression is also up-regulated (3- fold) in EZH2 knockout MEFs compared to EZH2 wild type MEFs. This indicates that CASZ1 expression on the remaining allele is silenced by epigenetic modifications mediated by EZH2. High expression of EZH2 and EED are also found in tumor samples of neuroblastoma patients with a poor prognosis (58 patients, p=2.6×10-3 for EED and p=5.87×10-4 for EZH2). This is consistent with a model in which aberrant epigenetic gene silencing of the tumor suppressor gene CASZ1 contributes to neuroblastoma tumorigenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4880.