The phenylalanine-and-glycine repeats of NUP98 oncofusions form condensates that selectively partition transcriptional coactivators

Recurrent cancer-causing fusions of NUP98 produce higher-order assemblies known as condensates. How NUP98 oncofusion-driven condensates activate oncogenes remains poorly understood. Here, we investigate NUP98-PHF23, a leukemogenic chimera of the disordered phenylalanine-and glycine (FG)-repeats-rich region of NUP98 and the H3K4me3/2-binding PHD finger domain of PHF23. Our integrated analyses using mutagenesis, proteomics, genomics, and condensate reconstitution demonstrate that the PHD domain targets condensates to H3K4me3/2-demarcated developmental genes while FG repeats determine condensate composition and gene activation. FG repeats are necessary to form condensates that partition a specific set of transcriptional regulators, notably the KMT2/MLL H3K4 methyltransferases, histone acetyltransferases and BRD4. FG repeats are sufficient to partition transcriptional regulators and activate a reporter when tethered to a genomic locus. NUP98-PHF23 assembles the chromatin-bound condensates that partition multiple positive regulators, initiating a feed-forward loop of reading-and-writing active histone modifications. This network of interactions enforces an open chromatin landscape at proto-oncogenes, thereby driving cancerous transcriptional programs.NUP98的复发性致癌融合基因可形成被称为凝聚体（condensates）的高阶组装体。目前，NUP98致癌融合蛋白驱动的凝聚体如何激活癌基因的分子机制仍未被充分阐明。本研究针对NUP98-PHF23展开探究——该蛋白是一种致白血病嵌合蛋白，由NUP98中富含苯丙氨酸与甘氨酸（FG）重复序列的无序区域，以及PHF23的结合H3K4me3/2的PHD指结构域（PHD finger domain）拼接而成。本研究通过诱变分析、蛋白质组学、基因组学及凝聚体重构实验等多维度整合手段开展研究，结果证实：PHD指结构域可将凝聚体靶向至由H3K4me3/2标记的发育相关基因，而FG重复序列则决定凝聚体的组成与基因激活效率。FG重复序列是形成此类凝聚体的必要条件，该类凝聚体可特异性分隔一组转录调控因子，其中尤以KMT2/MLL家族的H3K4甲基转移酶、组蛋白乙酰转移酶及BRD4最为关键。当将FG重复序列锚定至基因组位点时，其即可独立完成转录调控因子的分隔并激活报告基因。NUP98-PHF23可组装结合染色质的凝聚体，该凝聚体可分隔多种正向调控因子，进而启动读取与写入活性组蛋白修饰的前馈环路。这一相互作用网络可在原癌基因（proto-oncogenes）位点维持开放染色质构象，最终驱动致癌性转录程序的启动与执行。