The genome-wide maps of H3K27me3 in MLL-AF10 (Tip60 flox/flox, Cre-ERT2) cells (MATIP cells) with or without the treatment of 4-OHT. The genome-wide maps of H3K27me3 in MLL-AF10 (Tip60 flox/flox, Cre-ERT2) cells (MATIP cells) with or without the treatment of 4-OHT

Chromosome translocations involving the MLL gene are common rearrangements in leukemia. Such translocations fuse the MLL 5’-region in frame to partner genes , and the resultant fusion proteins can cause MLL-related leukemia. MLL-fusions activate transcription of target genes such as the HoxA cluster and Meis1, but the underlying mechanisms remain elusive. We found that the MLL-AF10 fusion recruits Tip60 to the Hoxa9 locus, where it acetylates H2A.Z, thereby promoting Hoxa9 expression. Following conditional deletion of Tip60, hypoacetylation of H2A.Z was accompanied by recruitment of Ezh2, the catalytic subunit of PRC2, suggesting that nucleosomes with hypoacetylated H2A.Z are the preferential targets of Ezh2. Our findings suggest that MLL-AF10 achieves active chromatin states by recruiting Tip60, which acetylates H2A.Z to prevent gene silencing by Ezh2. Overall design: The comprisons of H3K27me3 genome-wide distribution between 4-OHT-treated and non-treated (control) MLL-AF10 (Tip60 flox/flox, Cre-ERT2) cells.

涉及MLL基因的染色体易位是白血病中常见的染色体重排事件。此类易位会将MLL基因的5'端区域以可读框方式与伙伴基因融合，所生成的融合蛋白可引发MLL相关白血病。MLL融合蛋白可激活靶基因的转录，例如HoxA基因簇（HoxA cluster）与Meis1基因（Meis1），但其背后的分子机制仍有待阐明。本研究发现，MLL-AF10融合蛋白可将Tip60蛋白（Tip60）招募至Hoxa9基因座（Hoxa9 locus），使其对组蛋白H2A.Z（H2A.Z）进行乙酰化修饰，进而促进Hoxa9基因的表达。在对Tip60进行条件性敲除后，组蛋白H2A.Z的低乙酰化状态伴随多梳抑制复合体2（PRC2）的催化亚基Ezh2蛋白（Ezh2）的招募，这表明携带低乙酰化H2A.Z的核小体是Ezh2的优先作用靶点。本研究结果表明，MLL-AF10通过招募Tip60来构建活跃的染色质状态：Tip60对H2A.Z进行乙酰化修饰，从而阻止Ezh2介导的基因沉默。实验整体设计：比较经4-羟基他莫昔芬（4-OHT）处理与未处理（对照组）的MLL-AF10（Tip60 flox/flox，Cre-ERT2）细胞中全基因组范围内H3K27me3的分布情况。