抑癌基因TWIST2负调控BCR-ABL恶性转化的研究

TWIST2 belongs to basic helix-loop-helix protein, which forms homodimer or heterodimer to act as an activator or repressor of transcription regulation. We have demonstrated that TWIST2 is a novel factor to regulate human normal hematopoiesis. DNA hypermethylation of TWIST2 in some acute myeloid leukemia (AML) patients leads to its repressed expression. TWIST2 inhibits the growth of AML cells mainly through the formation of homodimer. However the role of TWIST2 in other types of leukemia remains unclear. We found that the fusion protein BCR-ABL suppressed the expression of TWIST2, which drives human chronic myeloid leukemia (CML). In addition, TWIST2 expression was lower in CML patients than healthy donors, however no hypermethylation was detected in CML patients. TWIST2 was able to inhibit the growth of CML cells through the formation of both homodimer and heterodimer. In this study, we will delineate the role of TWIST2 in BCR-ABL transformation, identify the heterodimer containing TWIST2, and investigate the action of this heterodimer. These will uncover the role of TWIST2 as a tumor suppressor in BCR-ABL+ leukemic cells and deepen the understanding of CML molecular pathogenesis, which potentially provides new clues to improve the treatment of this disease.

TWIST2是bHLH（basic helix-loop-helix）蛋白，形成同源或异源二聚体发挥转录调控功能。我们首次证实TWIST2负调控人类正常造血发生，DNA甲基化导致其在部分急性髓细胞白血病（AML）患者中表达下降，TWIST2主要形成同源二聚体抑制AML细胞生长。然而，TWIST2在其他白血病中的功能与机制尚待阐明。预研显示诱发慢性髓细胞白血病（CML）的融合蛋白BCR-ABL抑制TWIST2表达，CML患者TWIST2表达较健康供体下降，但未发生DNA甲基化修饰；另外，TWIST2形成同源和异源二聚体抑制CML细胞生长。本项目将研究TWIST2在BCR-ABL恶性转化能力中的作用，研究BCR-ABL通过何种机制抑制TWIST2，免疫共沉淀-质谱鉴定TWIST2异源二聚体并研究其作用机制。将揭示CML中TWIST2的抑癌基因功能、增进对BCR-ABL诱发CML分子机制的理解。

以往研究中我们鉴定到TWIST2是人类造血发生的负向调控因子，然而它在慢性髓细胞白血病中的功能与作用机制尚不明确，因而有待继续深入研究。本研究中首先发现TWIST2可以在正常造血CD34+细胞中调控ID2的表达，且功能密切相关。TWIST2和ID2都在CML患者CD34+细胞中表达较正常对照细胞显著下降。TWIST2过表达抑制CML细胞的体内外生长且增强它们的伊马替尼（IM）反应性；正常CD34+细胞中TWIST2沉默与BCR-ABL协同增强这些细胞的集落生成能力（CFC）。TWIST2调控CML细胞生长与IM反应性需要激活ID2，这是通过TWIST2结合ID2启动子中的一个保守E-box序列实现的。BCR-ABL过表达能在BaF3细胞中减少TWIST2在细胞核的定位，构建核定位序列缺陷的TWIST2变体能减少TWIST2的核分布且失去抑制CML细胞体内外生长的能力。综上，我们的研究表明TWIST2/ID2轴在调控CML细胞生长和IM反应中发挥重要作用，也首次揭示BCR-ABL可能通过改变TWIST2的分布而增强其恶性转化能力。这些结果深化了对TWIST2抑癌基因功能和CML分子致病机制的认知。