Inhibitors of DNA methylation and histone deacetylation independently relieve AML1/ETO-mediated lysozyme repression.

The human lysozyme (LZM) gene is highly methylated in LZM-nonexpressor immature myeloid and in nonmyeloid cells and unmethylated only in LZM-expressing cells. Extended methylation analyses of the CpG-poor 5' flanking region and of the exon 4 CpG island (both containing Alu elements) of the LZM gene were now performed. Marked demethylation was noted after treatment of AML1/ETO-positive Kasumi-1 cells with the DNA methyltransferase (DNMT) inhibitor 5-aza-2'-deoxycytidine (5-azaCdR), not associated with cellular differentiation. LZM mRNA in Kasumi-1, but not in several AML1/ETO-negative myeloid cell lines, was specifically and independently up-regulated upon treatment with 5-azaCdR and, to a lesser extent, with the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). Increased chromatin accessibility within the 5' LZM gene was observed concomitantly with 5-azaCdR-induced demethylation. In contrast, TSA treatment had no effect on chromatin accessibility, but, as shown by chromatin immunoprecipitation, resulted in increased acetylation of histones H3 and H4. Repression of LZM transcription is mediated by conditional AML1/ETO expression in an inducible cell line model (U-937), and is reversed by siRNA "knock-down" of AML1/ETO in Kasumi-1 cells (Dunne et al., Oncogene 25: 2006). Antagonization of LZM repression following conditional expression of AML1/ETO was achieved by TSA. In conclusion, we demonstrate complex interactions between DNA methylation and histone modifications in mediating LZM repression, which implicate AML1/ETO as one component involved in local chromatin remodeling. Interestingly, inhibitors of DNMTs and HDACs independently relieve repression of this CpG-poor gene in AML1/ETO-positive cells.