Pediatric Hematology and Oncology, Hannover Medical School, Hannover 30625, Germany;
Genes Dev. 2014 Apr 15;28(8):858-74. doi: 10.1101/gad.233791.113.
Although regulation of stem cell homeostasis by microRNAs (miRNAs) is well studied, it is unclear how individual miRNAs genomically encoded within an organized polycistron can interact to induce an integrated phenotype. miR-99a/100, let-7, and miR-125b paralogs are encoded in two tricistrons on human chromosomes 11 and 21. They are highly expressed in hematopoietic stem cells (HSCs) and acute megakaryoblastic leukemia (AMKL), an aggressive form of leukemia with poor prognosis. Here, we show that miR-99a/100∼125b tricistrons are transcribed as a polycistronic message transactivated by the homeobox transcription factor HOXA10. Integrative analysis of global gene expression profiling, miRNA target prediction, and pathway architecture revealed that miR-99a/100, let-7, and miR-125b functionally converge at the combinatorial block of the transforming growth factor β (TGFβ) pathway by targeting four receptor subunits and two SMAD signaling transducers. In addition, down-regulation of tumor suppressor genes adenomatous polyposis coli (APC)/APC2 stabilizes active β-catenin and enhances Wnt signaling. By switching the balance between Wnt and TGFβ signaling, the concerted action of these tricistronic miRNAs promoted sustained expansion of murine and human HSCs in vitro or in vivo while favoring megakaryocytic differentiation. Hence, our study explains the high phylogenetic conservation of the miR-99a/100∼125b tricistrons controlling stem cell homeostasis, the deregulation of which contributes to the development of AMKL.
尽管 microRNA（miRNA）对干细胞稳态的调控作用已经得到了深入研究，但人们对于单个 miRNA 如何在基因组内通过相互作用来诱导整合表型仍知之甚少。miR-99a/100、let-7 和 miR-125b 是人类 11 号和 21 号染色体上两个 tricistron 中的编码基因。它们在造血干细胞（HSCs）和急性巨核细胞白血病（AMKL）中高度表达，后者是一种预后不良的侵袭性白血病。在这里，我们发现 miR-99a/100∼125b tricistron 作为一个多顺反子转录物被同源盒转录因子 HOXA10 反式激活。对全基因组表达谱分析、miRNA 靶标预测和通路结构的综合分析表明，miR-99a/100、let-7 和 miR-125b 通过靶向四个受体亚基和两个 SMAD 信号转导物，在转化生长因子 β（TGFβ）通路的组合阻断处功能上趋同。此外，肿瘤抑制基因腺瘤性息肉病基因（APC）/APC2 的下调稳定了活性 β-连环蛋白并增强了 Wnt 信号。通过调节 Wnt 和 TGFβ 信号之间的平衡，这些 tricistronic miRNA 的协同作用促进了体外或体内小鼠和人类 HSCs 的持续扩增，同时有利于巨核细胞分化。因此，我们的研究解释了 miR-99a/100∼125b tricistron 控制干细胞稳态的高度系统发育保守性，其失调导致 AMKL 的发生。
