Heterogeneous epigenetic regulation of HACE1 in Burkitt- Lymphoma-derived cells

We examined the consequences of 3-deazaneplanocin A (DZNep) on HACE1 expression in human Burkitt- Lymphoma-derived cells to investigate fundamental molecular mechanisms that control its expression. We treated the human Burkitt- Lymphoma-derived cells lines Ramos and Raji with DZNep and examined HACE1 mRNA expression by RT-PCR. We also studied the effect of DZNep on the methylation of lysine 9 and 27 of histone 3 (H3K27me3 and H3K9me2) associated with the CpG88 and CpG177 islands of the HACE1 promoters by chromatin immunoprecipitation and quantitative PCR. CpG88 (hypomethylated) of the HACE1 promoter was enriched for histone marks H3K27me3 and H3K9me2 whereas CpG177 (hypermethylated) was only enriched for H3K9me2. DZNep treatment increased HACE1 gene expression which was further increased by the addition of trichostatine A (TSA), a promising therapeutic compound for the treatment of human B-Lymphoma. Histone methylation (both H3K9me2 and H3K27me3) of the HACE1 promoter concomitantly decreased. Our experiments suggest that HACE1 can be downregulated by methylation of its promoter region chromatin (H3K27me3 and H3K9me2), making HACE1 a potential target for DZNep combined with TSA. These results highlight the heterogeneity of HACE1 regulation in B-lymphoma and suggest that successful drug-induced restoration of epigenetically silenced tumor suppressor genes will require accurate characterization of cell type- and locus-specific gene silencing mechanisms.