Sheng Yushou Center of Cell Biology and Immunology, School of Life Sciences and Biotechnology, Shanghai Jiao Tong University, Shanghai, China.
CAS Key Laboratory of Computational Biology, Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Shanghai, China.
J Biol Chem. 2024 Aug;300(8):107566. doi: 10.1016/j.jbc.2024.107566. Epub 2024 Jul 11.
Mixed lineage leukemia-fusion proteins (MLL-FPs) are believed to maintain gene activation and induce MLL through aberrantly stimulating transcriptional elongation, but the underlying mechanisms are incompletely understood. Here, we show that both MLL1 and AF9, one of the major fusion partners of MLL1, mainly occupy promoters and distal intergenic regions, exhibiting chromatin occupancy patterns resembling that of RNA polymerase II in HEL, a human erythroleukemia cell line without MLL1 rearrangement. MLL1 and AF9 only coregulate over a dozen genes despite of their co-occupancy on thousands of genes. They do not interact with each other, and their chromatin occupancy is also independent of each other. Moreover, AF9 deficiency in HEL cells decreases global TBP occupancy while decreases CDK9 occupancy on a small number of genes, suggesting an accessory role of AF9 in CDK9 recruitment and a possible major role in transcriptional initiation via initiation factor recruitment. Importantly, MLL1 and MLL-AF9 occupy promoters and distal intergenic regions, exhibiting identical chromatin occupancy patterns in MLL cells, and MLL-AF9 deficiency decreased occupancy of TBP and TFIIE on major target genes of MLL-AF9 in iMA9, a murine acute myeloid leukemia cell line inducibly expressing MLL-AF9, suggesting that it can also regulate initiation. These results suggest that there is no difference between MLL1 and MLL-AF9 with respect to location and size of occupancy sites, contrary to what people have believed, and that MLL-AF9 may also regulate transcriptional initiation in addition to widely believed elongation.
混合谱系白血病融合蛋白（MLL-FPs）被认为通过异常刺激转录延伸来维持基因激活并诱导 MLL，但其中的机制尚未完全了解。在这里，我们表明，MLL1 和 AF9（MLL1 的主要融合伙伴之一）主要占据启动子和远端基因间区域，表现出类似于 RNA 聚合酶 II 在 HEL 中的染色质占据模式，HEL 是人红白血病细胞系，没有 MLL1 重排。尽管 MLL1 和 AF9 共同占据了数千个基因，但它们仅共同调节十几个基因。它们彼此不相互作用，其染色质占据也彼此独立。此外，在 HEL 细胞中 AF9 的缺乏会降低全局 TBP 占据，而在少数基因上降低 CDK9 占据，这表明 AF9 在 CDK9 募集中起辅助作用，并且可能通过起始因子募集在转录起始中起主要作用。重要的是，MLL1 和 MLL-AF9 占据启动子和远端基因间区域，在 MLL 细胞中表现出相同的染色质占据模式，并且在 iMA9 中，MLL-AF9 的缺乏降低了 TBP 和 TFIIE 在 MLL-AF9 的主要靶基因上的占据，iMA9 是一种可诱导表达 MLL-AF9 的小鼠急性髓系白血病细胞系，这表明它也可以调节起始。这些结果表明，与人们的看法相反，MLL1 和 MLL-AF9 在占据位点的位置和大小方面没有区别，并且 MLL-AF9 除了广泛认为的延伸作用外，还可能调节转录起始。
