MORC2蛋白M276I点突变促进三阴性乳腺癌侵袭转移的功能及机制研究

Triple-negative breast cancer (TNBC) is a specific subtype of breast cancer, which is defined by the absent expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). TNBC remains a major challenge for clinical management due to a high incidence of invasion and metastasis and no available therapeutic targets. Thus, elucidation of the underlying mechanisms and identification of potential therapeutic targets for TNBC invasion and metastasis are among the most important questions to be addressed in this field. Recently, we set out to investigate the gene mutation profiling of primary TNBC tumors using high-throughput RNA sequencing, and found that primary TNBC tumors from Chinese TNBC patients had a conserved mutation of G to C at position 828 of MORC family CW-type zinc finger 2 (MORC2) gene, resulting in its coding protein harboring a substitution of methionine to isoleucine at residue 276 (M276I). However, the biological function and related molecular mechanism for the patient-derived M276I mutation of chromatin remodeling protein MORC2 in TNBC development and progression remain unexplored. Preliminary functional data showed that the M276I mutation in MORC2 did not affect cell proliferation and cell-cycle progression of TNBC cells, but significantly enhanced the migratory, invasive and metastatic potential of TNBC cells in vitro and in vivo. However, the underlying molecular mechanism remains unknown. In the present study, we are aimed to address the functional role for the M276I mutation of MORC2 protein in the process of invasion and metastasis of TNBC, identify downstream target genes and interacting proteins of the mutant MORC2 (M276I), and examine the biological functions of these identified target genes and interacting proteins of MORC2 in TNBC invasion and metastasis mediated by the M276I mutation. The expected findings from this study will reveal the molecular mechanism for the M276I mutation of MORC2 protein in regulating invasion and metastasis of TNBC and provide new clue for development of personalized therapy of TNBC.

三阴性乳腺癌 (TNBC) 是一种具有高度侵袭转移潜能且缺乏靶向治疗的乳腺癌亚型，阐明其分子机制以及鉴定治疗新靶点是该领域亟需解决的关键科学问题之一。最近我们通过RNA测序分析发现，中国人群TNBC原发肿瘤组织中存在保守的染色质重塑蛋白MORC2 M276I点突变, 但该突变在TNBC进展中的功能及分子机制尚未见报道。前期预实验结果显示，M276I点突变在体内外显著增加TNBC细胞侵袭及转移潜能，但其分子机制尚不清楚。本研究拟采用多种实验模型和技术系统深入地研究M276I点突变在TNBC侵袭转移中的功能，阐明该突变对MORC2下游靶基因及相互作用蛋白的影响及其分子机制，并明确这些鉴定的靶基因及相互作用蛋白在M276I介导的TNBC侵袭转移中的作用。本研究的预期成果将为揭示MORC2蛋白M276I点突变促进TNBC侵袭转移的功能与分子机制以及研发TNBC个体化治疗策略提供新的思路。

MORC家族CW型锌指蛋白2（MORC Family CW-Type Zinc Finger 2，MORC2）属于核蛋白 MORC (Microrchidia) 家族，是一种染色质重塑相关蛋白。染色质重塑是指由染色质重塑复合物所介导，以染色质核小体改变为基本特征的表观遗传调控形式之一。染色质重塑可引起染色质开放性改变，进而影响DNA与转录因子、复制复合物及各种调节因子的可接近性改变，是调节细胞生命活动的重要机制之一。以往研究报道显示，MORC2的表达水平异常与多种肿瘤的发生发展相关，MORC2蛋白磷酸化修饰水平与胃癌预后相关。此外，MORC2多个点突变与神经系统疾病相关。以上研究结果提示MORC2在机体内发挥重要功能，亟需对其生物学功能作进一步研究并阐明其作用机制。本实验室前期研究发现，MORC2与乳腺癌发生发展及DNA损伤修复等密切相关，但是有关MORC2点突变在乳腺癌发生发展中的功能及机制方面尚未见研究报道。.乳腺癌是一种高度异质性恶性肿瘤，而三阴性乳腺癌（Triple-negative breast cancer, TNBC）是其中一种侵袭性强、复发和远处转移率高的亚型，具有无明确治疗靶点以及预后差等临床特点，是目前乳腺癌基础研究与临床实践中的难点。本课题研究发现，MORC2 蛋白的第 276位甲硫氨酸（Methionine，M）突变成异亮氨酸（Isoleucine, I）促进TNBC 细胞迁移、侵袭和肺转移潜能，但是不影响其增殖和肿瘤生长。机制探索发现M276I突变型MORC2与异质性核糖核蛋白M（heterogeneous nuclear ribonucleo-protein M, hnRNPM）结合增强，影响hnRNPM介导的CD44 分子mRNA剪接，导致CD44上皮型（CD44v）减少而CD44间充质型（CD44s）增加，最终驱动乳腺癌细胞发生上皮-间质化转变。在过表达M276I突变型MORC2的TNBC细胞中敲低hnRNPM，可减少MORC2招募至CD44的新生前体mRNA区域，降低剪切产物CD44s水平，进而降低突变型MORC2介导的TNBC细胞迁移、侵袭和肺转移潜能。综上所述，本研究发现MORC2中M276I突变可促进TNBC进展，提示生化实验研究对验证患者基因突变功能的重要性，为优化高度异质性TNBC患者的个性化治疗提供理论依据。