Misguided transcriptional elongation causes mixed lineage leukemia.
Misguided transcriptional elongation causes mixed lineage leukemia.
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DOI：
10.1371/journal.pbio.1000249
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发表时间：
2009-11
期刊：
影响因子：
9.8
通讯作者：
Slany RK
中科院分区：
文献类型：
--
作者：
Mueller D;García-Cuéllar MP;Bach C;Buhl S;Maethner E;Slany RK
对引起侵袭性白血病的融合蛋白家族的活性的研究表明转录延伸是致癌转化的一种新机制。由组蛋白甲基转移酶混合谱系白血病（MLL）和各种不相关的融合伴侣组成的融合蛋白具有高度致白血病性。尽管它们普遍存在，特别是在儿童急性白血病中，但其转化机制的许多分子细节尚不清楚。在这里，我们提供了MLL融合的功能机制的见解，证明他们捕获的转录延伸复合物，已被发现与11 - 19白血病蛋白（ENL）。我们发现，这个复杂的由一个紧密的核心稳定递归蛋白质-蛋白质相互作用。该中心部分整合了组蛋白H3赖氨酸79甲基化、RNA聚合酶II（RNA Pol II）磷酸化和MLL融合伴侣以刺激转录延伸，如RNA拴系测定所证明的。免疫共沉淀表明，MLL融合被纳入到这个复杂的，导致组成性招聘的延伸活动MLL的目标位点。染色质免疫沉淀（ChIP）的同源异型盒基因A簇证实了MLL融合和转录水平的结合之间的密切关系。利用条件MLL融合的时间分辨ChIP挑出H3K79甲基化作为与靶表达相关的主要参数。MLL融合蛋白的存在还使RNA Pol II保持在活跃的延伸状态，并防止抑制性组蛋白甲基化在靶染色质上的积累。Hox基因座保持开放和生产的MLL融合活性的存在下，即使在强制分化的条件下。最后，MLL转化的细胞对RNA Pol II磷酸化的药理学抑制特别敏感，这表明MLL的潜在治疗。总之，我们发现异常转录延伸是致癌转化的一种新机制。基因的表达水平需要精确调节以确保正常功能。调节可以在基因表达的整个过程中的不同阶段施加，包括转录起始、转录物延伸和转录物加工。如果这些机制之一的控制失败，可能会发生异常基因表达，这可能会产生严重的后果，如细胞转化和癌症的发展。在这里，我们表明一类与混合谱系白血病（MLL）有关的异常融合蛋白劫持了转录延伸复合物。我们分析了这种转录延伸复合物的结构，并证明该复合物是由MLL融合蛋白靶向的基因，通常应该沉默，以允许造血细胞的成熟。我们表明，这种错误的启动导致相应基因的组成型表达，这可能导致在细胞高度增殖的前体细胞阶段抑制血细胞分化。这种异常的前体细胞先前已被证明对正常分化信号具有抗性，并形成白血病起始群体。我们进一步表明，携带MLL融合蛋白的细胞比不同病因的白血病细胞对转录延长的化学抑制更敏感。我们的研究结果提出转录延长作为一种新的致癌机制，并指出这种难以治愈的白血病的潜在特异性治疗。
Investigation of the activity of a family of fusion proteins that cause aggressive leukemia suggests transcriptional elongation as a new mechanism for oncogenic transformation. Fusion proteins composed of the histone methyltransferase mixed-lineage leukemia (MLL) and a variety of unrelated fusion partners are highly leukemogenic. Despite their prevalence, particularly in pediatric acute leukemia, many molecular details of their transforming mechanism are unknown. Here, we provide mechanistic insight into the function of MLL fusions, demonstrating that they capture a transcriptional elongation complex that has been previously found associated with the eleven-nineteen leukemia protein (ENL). We show that this complex consists of a tight core stabilized by recursive protein–protein interactions. This central part integrates histone H3 lysine 79 methylation, RNA Polymerase II (RNA Pol II) phosphorylation, and MLL fusion partners to stimulate transcriptional elongation as evidenced by RNA tethering assays. Coimmunoprecipitations indicated that MLL fusions are incorporated into this complex, causing a constitutive recruitment of elongation activity to MLL target loci. Chromatin immunoprecipitations (ChIP) of the homeobox gene A cluster confirmed a close relationship between binding of MLL fusions and transcript levels. A time-resolved ChIP utilizing a conditional MLL fusion singled out H3K79 methylation as the primary parameter correlated with target expression. The presence of MLL fusion proteins also kept RNA Pol II in an actively elongating state and prevented accumulation of inhibitory histone methylation on target chromatin. Hox loci remained open and productive in the presence of MLL fusion activity even under conditions of forced differentiation. Finally, MLL-transformed cells were particularly sensitive to pharmacological inhibition of RNA Pol II phosphorylation, pointing to a potential treatment for MLL. In summary, we show aberrant transcriptional elongation as a novel mechanism for oncogenic transformation. The expression level of a gene needs to be precisely adjusted to ensure proper function. Adjustments can be imposed at different stages during the overall process of gene expression, including transcription initiation, transcript elongation, and transcript processing. If control of one of these mechanisms fails, aberrant gene expression can occur, which may have severe consequences such as cellular transformation and the development of cancer. Here, we show that a class of aberrant fusion proteins that are causal in mixed-lineage leukemia (MLL) hijacks a transcriptional elongation complex. We analyze the architecture of this transcriptional elongation complex and demonstrate that the complex is targeted by MLL fusion proteins to genes that should normally be silenced to allow maturation of hematopoietic cells. We show that this mistargeting causes constitutive expression of the respective genes, which likely leads to inhibition of blood cell differentiation at a precursor cell stage in which the cells are highly proliferative. Such abnormal precursor cells have been shown previously to be resistant to normal differentiation signals and to form the leukemia-initiating population. We further show here that cells carrying MLL fusion proteins are more sensitive to chemical inhibition of transcriptional elongation than leukemic cells of different etiology. Our results propose transcriptional elongation as a new oncogenic mechanism and point to a potential specific therapy for this hard-to-cure leukemia.
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影响因子：
30.8
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