SWI/SNF complex loss facilitates gene silencing during NSCLC development

基因表达的表观遗传改变在人非小细胞肺癌（NSCLC）的发生发展中起着重要作用。最近的研究表明，具有一个或多个表观遗传沉默基因的肿瘤患者通常表现出较差的总体生存率。大多数报告都集中在两个主要的机制来解释表观遗传修饰，DNA甲基化和组蛋白修饰的改变。然而，核小体在基因启动子位置的改变代表了基因可以被表观遗传调控的另一个重要机制。事实上，一些研究现在已经暗示染色质重塑复合体在人类肿瘤发展的表观遗传沉默的发生。特别是，SWI/SNF（交配型开关/蔗糖非发酵）染色质重塑复合体似乎是促成非小细胞肺癌表观遗传改变的强有力候选。SWI/SNF复合体最初是在酵母中发现的，它通过atp依赖的过程重塑核小体，从而改变染色质结构。

Epigenetic changes in gene expression play an important role in the development and progression of human non-small cell lung carcinoma (NSCLC). Recent studies have shown that patients with tumors possessing one or more epigenetically silenced genes often show a poorer overall survival. Most reports have focused upon two major mechanisms to account for epigenetic modifications, DNA methylation and alterations in histone modifications. However, altered nucleosome positioning at gene promoters represent another important mechanism by which genes can be epigenetically regulated. Indeed, several studies have now implicated chromatin-remodeling complexes in the genesis of epigenetic silencing in human tumor development. In particular, the SWI/SNF (mating type switch/sucrose nonfermenting) chromatin remodeling complex appears like a strong candidate for contributing to epigenetic alterations in NSCLC. Originally identified in yeast, the SWI/SNF complex alters chromatin structure by remodeling nucleosomes through an ATP-dependent process. Loss or expression of the SWI2 ATPase homologs, BRG1 and BRM, by promoter methylation and/or mutations of either or both genes occurs in ~25% of human NSCLC cell lines and ~10% of primary human NSCLCs. Importantly, several groups including our own have shown a correlation between loss of expression of BRG1/BRM and poor prognosis in NSCLC patients. Our published reports and preliminary results demonstrate that reexpression of BRG1 or BRM in deficient NSCLC cells induces expression of many epigenetically silenced genes. Therefore, we hypothesize that loss of SWI/SNF complex activity represents a novel mechanism for gene silencing during NSCLC development. To test this hypothesis, we propose a synergistic research plan using cell culture and genetically engineered mouse models. Specifically, we will determine which gene promoters are activated by BRG1 and/or BRM after reexpression in deficient NSCLC cell lines, discover gene promoters that undergo silencing after loss of BRG1 expression in NSCLC cell lines and assess the effects of BRG1 and/or BRM loss on tumor development in a genetically engineered mouse model for NSCLC. The successful completion of the proposed studies will provide valuable insights into the mechanisms of epigenetic silencing during NSCLC development and of SWI/SNF chromatin remodeling as well as generate a novel genetically engineered animal model for further basic and translational studies. Furthermore, if DNMT or HDAC inhibitors are not effective in reversing gene silencing in the subset of NSCLCs that lack BRG1 and BRM expression, they may require a novel approach for treatment and prevention of progression. By identifying the unique chromatin changes that occur in these tumors and how they differ from BRG1 and/or BRM-positive tumors, we can initiate rational drug design studies to find the reagents to treat this deadly disease.