Differential regulation of MCM7 and its intronic miRNA cluster miR-106b-25 during megakaryopoiesis induced polyploidy

Megakaryocytes exit from mitotic cell cycle and enter a phase of repeated DNA replication without undergoing cell division, in a process termed as endomitosis of which little is known. We studied the expression of a DNA replication licensing factor mini chromosome maintenance protein 7 (MCM7) and its intronic miR-106b-25 cluster during mitotic and endo-mitotic cycles in megakaryocytic cell lines and in vitro cultured megakaryocytes obtained from human cord blood derived CD34+ cells. Our results show that contrary to mitotic cell cycle, endomitosis proceeds with an un-coupling of the expression of MCM7 and miR-106b-25. This was attributed to the presence of a transcript variant of MCM7 which undergoes nonsense mediated decay (NMD). Additionally, miR-25 which was up regulated during endomitosis was found to promote megakaryopoiesis by inhibiting the expression of PTEN. Our study thus highlights the importance of a transcript variant of MCM7 destined for NMD in the modulation of megakaryopoiesis.巨核细胞（Megakaryocytes）退出有丝分裂细胞周期，进入反复进行DNA复制但不发生细胞分裂的阶段，这一过程被称为核内有丝分裂（endomitosis），目前对其机制的了解仍十分有限。我们研究了DNA复制许可因子微小染色体维持蛋白7（mini chromosome maintenance protein 7, MCM7）及其内含子源性miR-106b-25基因簇，在巨核细胞系以及从人脐带血来源CD34阳性（CD34+）细胞体外培养获得的巨核细胞的有丝分裂与核内有丝分裂周期中的表达情况。结果显示，与有丝分裂细胞周期不同，核内有丝分裂过程中MCM7与miR-106b-25的表达出现解偶联现象。该现象归因于一种可经无义介导的mRNA降解（nonsense mediated decay, NMD）途径降解的MCM7转录变体的存在。此外，在核内有丝分裂过程中上调的miR-25，可通过抑制磷酸酶与张力蛋白同源物（PTEN）的表达来促进巨核细胞生成。综上，本研究凸显了经无义介导降解的MCM7转录变体在调控巨核细胞生成过程中的重要作用。