转录后激活c-Myc调控CML干细胞的非Bcr/Abl依赖新机制研究

Chronic myeloid leukemia(CML) is caused by leukemia stem cells (LSCs) that harbor the BCR-ABL,a constitutively active fusion tyrosine kinase.It progresses from a chronic phase (CP) to blast crisis (BC), but the molecular alterations underlying the progression of CML are poorly characterized. Although the inhibition of BCR-ABL kinase activity by tyrosine kinase inhibitor(TKI)is highly effective in prolonging the CP, it is difficult to completely cure the disease and has poor effect on BC patients. The persistence remaining of TKI-insentive LSCs that are not fully addicted to BCR-ABL may account for this phenomenon. Recent important advances in CML LSCs suggest that abnomal activation of oncogene c-Myc play an crucial role, but the underlying mechanism needs to be clarified. It has been proved that oncogene MDM2 is up-regulated in the patients of BC. Our previous studies show MDM2 can bind mRNA and post-trancriptionally regulate expression of some oncogenes.There are reports that coordinated silencing of c-Myc mediated miR-29 transcription by HDAC in lymphoma and acute myeloid leukemia and decreased miR-29 expression in CML cell line. Our preliminary data in Ph(+)ALL cells and CML primary cells showed that the expression of MDM2 is related with c-Myc and sensitivity to nilotinib. So,we try to demonstrate the hypothesis by using CML cell lines ,primary cells and mouse models that MDM2 could bind c-Myc mRNA and activate its protein translation at a post transcription level, then c-Myc could recruit HDAC to surpress the transcription of miR-29. This mechanism of abnormal activation of c-Myc may represent a unique BCR-ABL independent mechanisms that regulate the characteristics of CML LSCs. It might provide novel targets for non-TKI therapeutic strategy to elimination of CML LSCs.

最新进展提示c-Myc激活在调控CML 白血病干细胞（LSCs）中至关重要，但其机制不清。调控CML LSCs 非Bcr/Abl依赖机制的存在使当前CML酪氨酸激酶抑制剂（ TKI）治疗效果受到限制。本课题拟在CML急变细胞系中证实c-Myc激活受癌基因MDM2转录后调控，并与下游肿瘤抑制microRNA miR-29的启动子结合，招募组蛋白乙酰化酶（HDAC）抑制miR-29转录。并在CML原代细胞及LSCs，Bcr/Abl转基因小鼠CML模型中的白血病细胞及LSCs中检测TKI治疗压力下MDM2、c-Myc及miR-29的表达，利用NSG免疫缺陷小鼠研究不同表达模式细胞的白血病植入能力，初步证实c-Myc异常激活机制是调控CML干细胞的非Bcr/Abl依赖新机制之一。本课题首次研究MDM2对c-Myc的转录后调控作用及CML中miR-29的调控机制，能为清除CML干细胞提供新治疗靶点。

虽然酪氨酸激酶抑制剂（ TKI）治疗CML取得很大成功，仍有部分患者最终进展至急变期，治疗效果差。文献报道CML急变期中MDM2、c-Myc高表达，而非Bcr/Abl依赖性转录后调节是CML中c-Myc异常激活的重要途径，但机制不清。本课题在CML急变细胞系和患者原代细胞中证明急变期MDM2、c-Myc高表达，miR-29b低表达，且MDM2与c-Myc表达呈正相关，与miR-29b表达负相关。并在CML细胞系中首次证明MDM2可与c-Myc mRNA结合，可在转录后水平调控c-Myc，使其蛋白表达增高。而且MDM2、c-Myc可负调控miR-29b的表达。另外文献报道和本课题也发现c-Myc可增加MDM2转录，上调MDM2表达，故两者形成一正反馈，抑制miR-29b表达，参与疾病的进展。在CML急变细胞系K562（p53缺失）中转染MDM2 SiRNA质粒降低MDM2表达，可抑制CML细胞增殖，诱导凋亡。总之，本课题初步阐明MDM2可转录后激活c-Myc调控miR-29b，参与CML疾病进展，为治疗提供新靶点。