释放启动子附近停滞的RNA聚合酶II是转录调控的关键一步。当前的观点认为，这一步依赖于P-TEFb的激酶活性。 MLL1是人类细胞中的一个组蛋白3赖氨酸4甲基转移酶。由该基因发生断裂并与一系列融合伙伴基因中的一个发生融合所形成的MLL融合基因能诱发混合细胞系白血病。主要的几个融合伙伴，包括AF4、AFF4、AF9、ENL和ELL，最近被发现与P-TEFb一起组成超级延伸复合体。然而，AF4、AFF4、AF9和ENL的功能以及相应的MLL融合蛋白维持靶基因的表达从而诱发白血病的分子机理到目前还不清楚。申请人前期的研究表明，AF4和AF9分别是释放启动子附近停滞的RNA聚合酶II和RNA聚合酶II的翻译后修饰的负调控因子；MLL-AF9可以将通常与增强子相结合的AF4直接招募到启动子。本项目将深入地阐明超级延伸复合体的功能以及主要的MLL融合蛋白维持基因表达的分子机理，因此兼具理论和实践意义。

Promoter-proximal pausing release of RNA polymerase II (Pol II) is a critical step of transcriptional regulation. It is currently believed that paused Pol II release is dependent on the kinase activity of P-TEFb. MLL1 is one of the histone 3 lysine 4 methyltransferases in human cells. MLL fusion genes, arising from the break of the MLL1 gene and the fusion of its 5’ fragment in frame to one of the fusion partner genes, are found in mixed lineage leukemias (MLLs). Major fusion partners of MLL1, including AF4, AFF4, AF9, ENL, and ELL, were recently found to be subunits of a multiprotein complex, which also contains P-TEFb. Given that ELL and P-TEFb are known elongation factors, the complex was named the super elongation complex. Yet, roles of AF4, AFF4, AF9, and ENL in transcriptional regulation and the molecular mechanisms underlying the corresponding MLL fusion proteins-mediated gene activation maintenance remain unclear. In preliminary studies, we found that AF4 is a negative regulator of promoter-proximal pausing release of Pol II, that AF9 mainly restrains gene expression, and that MLL-AF9 is capable of recruiting the normally promoter-associated AF4 to promoters. In this application, we propose to further elucidate roles of the super elongation complex in transcriptional regulation and the molecular mechanisms underlying those major MLL fusion proteins-mediated gene activation maintenance. Results obtained from our study will advance our current understanding of transcriptional regulation and facilitate the development of small molecule inhibitors for the treatment of MLLs.