Abstract
Rhabdomyosarcoma comprises two major subtypes, fusion positive (PAX3-FOXO1 or PAX7-FOXO1) and fusion negative. To investigate the significance of DNA methylation in these subtypes, we analyzed methylation profiles of 37 rhabdomyosarcoma tumors and 10 rhabdomyosarcoma cell lines, as well as 8 normal tissues. Unsupervised clustering of DNA methylation clearly distinguished the fusion-positive and fusion-negative subsets. The fusion-positive tumors showed substantially lower overall levels of methylation compared with fusion-negative tumors. Comparison with the methylation pattern of normal skeletal muscle and bone marrow indicates that fusion-negative rhabdomyosarcoma is more similar to these normal tissues compared with fusion-positive rhabdomyosarcoma, and suggests that many of the methylation differences between these subtypes arise from 'aberrant' hyper- and hypomethylation events in fusion-positive rhabdomyosarcoma. Integrative methylation and gene expression analysis revealed that methylation differences between fusion-positive and fusion-negative tumors could either be positively or negatively associated with mRNA expression. There was no significant difference in the distribution of PAX3-FOXO1-binding sites between genes with and without differential methylation. However, the finding that PAX3-FOXO1-binding sites were enriched among genes that were both differentially methylated and differentially expressed suggests that the fusion protein interacts with DNA methylation to regulate target gene expression. An 11-gene DNA methylation signature, classifying the rhabdomyosarcoma tumors into fusion-positive and fusion-negative subsets, was established and validated by pyrosequencing assays. Notably, EMILIN1 (part of the 11-gene signature) showed higher methylation and lower mRNA expression in fusion-positive compared with fusion-negative tumors, and demonstrated demethylation and re-expression in multiple fusion-positive cell lines after treatment with 5-aza-2'-deoxycytidine. In conclusion, our study demonstrates that fusion-positive and fusion-negative rhabdomyosarcoma tumors possess characteristic methylation profiles that contribute to the expression differences between these fusion subtypes. These findings indicate an important relationship between fusion status and epigenetic changes in rhabdomyosarcoma, present a novel approach for ascertaining fusion status, and may identify new therapeutic targets in rhabdomyosarcoma.
摘要
横纹肌肉瘤主要由两种亚型组成,即基因融合阳性亚型(PAX3-FOXO1融合或 PAX7-FOXO1融合)和基因融合阴性亚型。为了研究这些亚型中DNA甲基化的意义,我们分析了37例横纹肌肉瘤、10个横纹肌肉瘤细胞系以及8例正常组织的甲基化谱系。在无监控的DNA甲基化聚类分析中清楚的将其分为基因融合阳性亚型和基因融合阴性亚型。与基因融合阴性肿瘤相比,基因融合阳性肿瘤显示显著较低的整体甲基化水平。与正常骨骼肌以及骨髓的甲基化类型相比,基因融合阴性横纹肌肉瘤相对于基因融合阳性横纹肌肉瘤来说与这些正常组织更相似,并且提示这两种亚型之间诸多的甲基化差异源于基因融合阳性横纹肌肉瘤中的异常高甲基化和低甲基化事件。整合甲基化和基因表达分析提示,基因融合阳性和基因融合阴性肿瘤之间甲基化状态的不同与mRNA表达阳性或阴性相关。分布有PAX3-FOXO1融合位点的基因中伴与不伴不同甲基化,两者之间无显著差异性。然而,发现PAX3-FOXO1融合位点在具有不同甲基化状态和不同表达的基因之间富集,提示融合蛋白干扰DNA甲基化,从而调节靶基因的表达。通过焦磷酸测序检测建立和确认了一种11-基因甲基化标记,其将横纹肌肉瘤分为基因融合阳性亚型和基因融合阴性亚型。值得注意的是,与基因融合阴性肿瘤相比,EMILIN1(11-基因标记的一部分)在基因融合阳性肿瘤中显示高甲基化状态以及低mRNA表达,同时发现复合性基因融合阳性细胞系在接受5-氮-2'-脱氧胞苷酸处理后发生去甲基化以及再表达。总而言之,我们的研究阐明了基因融合阳性和基因融合阴性横纹肌肉瘤具有特征性的甲基化谱系,甲基化使得这些基因融合亚型的表达出现差异。这些发现提示,在横纹肌肉瘤中基因融合状态与表观遗传学变化之间存在重要的关系,在确定基因融合状态方面存在一种新的方法,并且可能在横纹肌肉瘤中鉴定出新的治疗靶点。
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