Epigenetic Repression of the Adaptor Molecule LAT2 by the Leukemic Fusion Protein AML1/ETO.

Abstract

The chromosomal translocation (8;21) fuses the hematopoietic transcription factor AML1 (RUNX1) with ETO, resulting in the leukemia-specific chimeric protein AML1/ETO. This fusion protein represses transcription by recruiting a nuclear co-repressor complex containing HDACs and DNMT1 to its target promoters. Previously, we have identified a novel in vivo AML1/ETO target gene, LAT2 (NTAL/LAB/WBSCR5), which is involved in FcεR I, c-Kit, B cell- and T cell receptor signalling. Notably, LAT2 is strongly repressed in AML1/ETO positive cells including primary AML blasts, which was confirmed by others in several large AML cohorts. We have now addressed the molecular mechanisms of AML1/ETO-mediated LAT2 repression. AML1/ETO was induced by Ponasterone A in an ecdysone-inducible system in U937 cells (9/14/18 cell line). To deplete AML1/ETO in t(8;21)-positive cells, we electroporated Kasumi-1 cells with AML1/ETO siRNA. To interfere with epigenetic modifications more directly, cells were treated with the DNMT inhibitor decitabine (DAC) and 4 different HDAC inhibitors. LAT2 expression was determined by Northern Blot, qRT-PCR and Western Blot. HDAC occupation and the histone status of the LAT2 promoter was examined by chromatin immunoprecipitation (ChIP). LAT2 mRNA was downregulated already after 4 hours of conditional expression of AML1/ETO in 9/14/18 cells, and constitutively repressed in the AML1/ETO-positive Kasumi-1 and SKNO-1 cells. siRNA-mediated AML1/ETO depletion caused a 9-fold upregulation of LAT2 in Kasumi-1 cells, suggesting a possible direct mechanism of repression. To address this question, we performed ChIP assays for the LAT2 promoter after AML1/ETO induction in 9/14/18 cells. AML1/ETO inhibited acetylation of histone H3, H3K9 and H4, but did not affect trimethylation of H3K4. These changes were associated with the recruitment of HDAC2, but not HDAC1 and HDAC3, to the LAT2 promoter. The HDAC inhibitors MS-275, SAHA, TSA and valproic acid induced LAT2 mRNA in a dose-dependent manner in AML1/ETO-expressing Kasumi-1, with MS-275 being the most efficient inhibitor. MS-275 induced LAT2 expression also in t(8;21)-positive SKNO-1, but not in AML1/ETO-negative HL60 and U937 cells. LAT2 mRNA was also upregulated in a dose-dependent manner after DAC treatment in Kasumi-1 cells. The combination of DAC and MS-275 had a synergistic effect on inhibition of cell growth, acetylation of histones H3 and H4, and re-expression of LAT2 mRNA. MS-275-mediated re-expression of LAT2 was associated with an increase in acetylation of histone H3, H3K9, H4 and trimethylation of H3K4. The increase of activating histone modifications was associated with the release of HDAC1, HDAC2 and HDAC3 from the LAT2 promoter. In conclusion, the epigenetic changes of the LAT2 promoter caused by AML1/ETO could be pharmacologically reverted by inhibition of histone acetylation.