Mechanism of BRG1 silencing in primary cancers.
Mechanism of BRG1 silencing in primary cancers.
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DOI：
10.18632/oncotarget.10593
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发表时间：
2016-08-30
期刊：
影响因子：
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通讯作者：
Reisman D
中科院分区：
文献类型：
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作者：
Marquez-Vilendrer SB;Thompson K;Lu L;Reisman D
BRG1 （SMARCA4）是一种已知的肿瘤抑制因子，是SWI/SNF染色质重塑复合体的关键亚基，在许多癌症类型中沉默。研究表明，BRG1在癌症来源的细胞系中发生突变，这导致了BRG1在原发性人类肿瘤中也发生突变的断言。然而，brg1缺陷肿瘤的测序显示缺乏突变；因此，肿瘤中BRG1沉默的原因仍然是一个谜。我们对许多肿瘤微阵列进行了免疫组织化学（IHC），以表征不同肿瘤类型中BRG1丢失的频率。我们还通过测序基因组DNA和mRNA来分析brg1缺陷肿瘤。然后，我们测试了几种不同的表观遗传药物，包括抑制AKT通路的药物，是否可以在BRG1阴性细胞系（即BRG1缺乏突变）中诱导BRG1表达。我们发现BRG1阴性细胞系的一个子集也表现出BRG1的异常剪接，并且在至少30%的BRG1缺陷肿瘤中，BRG1的表达似乎由于BRG1的异常剪接而受到抑制。由于大多数BRG1缺陷肿瘤缺乏突变或剪接缺陷，可能导致BRG1表达缺失，这表明BRG1沉默的其他机制。为此，我们分析了3个BRG1缺失的非突变癌细胞系，发现BRG1在这些细胞系中通过抑制AKT通路而被诱导。我们发现BRG1的缺失与原发肿瘤中E-cadherin的缺失和Vimentin的上调有关，这就解释了为什么BRG1的缺失与多种肿瘤类型的不良预后相关。
BRG1 (SMARCA4) is a documented tumor suppressor and a key subunit of the SWI/SNF chromatin remodeling complex that is silenced in many cancer types. Studies have shown that BRG1 is mutated in cancer-derived cell lines, which led to the assertion that BRG1 is also mutated in primary human tumors. However, the sequencing of BRG1-deficient tumors has revealed a paucity of mutations; hence, the cause of BRG1 silencing in tumors remains an enigma. We conducted immunohistochemistry (IHC) on a number of tumor microarrays to characterize the frequency of BRG1 loss in different tumor types. We also analyzed BRG1-deficient tumors by sequencing the genomic DNA and the mRNA. We then tested if BRG1 expression could be induced in BRG1-negative cell lines (i.e., that lack mutations in BRG1) after the application of several different epigenetic agents, including drugs that inhibit the AKT pathway. We found that a subset of BRG1-negative cell lines also demonstrated aberrant splicing of BRG1, and in at least 30% of BRG1-deficient tumors, BRG1 expression appeared to be suppressed due to aberrant BRG1 splicing. As the majority of BRG1-deficient tumors lack mutations or splicing defects that could drive BRG1 loss of expression, this suggests that other mechanisms underlie BRG1 silencing. To this end, we analyzed 3 BRG1-deficient nonmutated cancer cell lines and found that BRG1 was inducible in these cell lines upon inhibition of the AKT pathway. We show that the loss of BRG1 is associated with the loss of E-cadherin and up-regulation of Vimentin in primary tumors, which explains why BRG1 loss is associated with a poor prognosis in multiple tumor types.