Neuroendocrine Unit, Massachusetts General Hospital and Harvard Medical School, 55 Fruit Street, Boston, Massachusetts 02114, USA.
Endocrinology. 2010 Mar;151(3):939-47. doi: 10.1210/en.2009-0657. Epub 2009 Dec 23.
Maternally expressed gene 3 (MEG3) is an imprinted gene highly expressed in the human pituitary. However, MEG3 expression is lost in human gonadotroph-derived pituitary adenomas and most human tumor cell lines. Expression of MEG3 in tumor cells results in growth suppression, p53 protein increase, and activation of p53 downstream targets. The MEG3 gene encodes a noncoding RNA of approximately 1700 nucleotides. There are 12 different MEG3 gene transcripts, generated by alternative splicing. They contain the common exons 1-3 and exons 8-10, but each uses one or more exons 4-7 in a different combination in the middle. MEG3 isoform expression patterns are tissue and cell type specific. Functionally, each isoform stimulates p53-mediated transactivation and suppresses tumor cell growth. We analyzed the secondary RNA folding structure of each MEG3 isoform, using the computer program mfold. All MEG3 RNA isoforms contain three distinct secondary folding motifs M1, M2, and M3. Deletion analysis showed that motifs M2 and M3 are important for p53 activation. Furthermore, a hybrid MEG3 RNA, containing a piece of artificially synthesized sequence different from the wild type but folding into a similar secondary structure, retained the functions of both p53 activation and growth suppression. These results support the hypothesis that a proper folding structure of the MEG3 RNA molecule is critical for its biological functions. This study establishes for the first time the structure-function relationship of a large noncoding RNA and provides a first look into the molecular mechanisms of the biological functions of a large noncoding RNA.
母系表达基因 3（MEG3）是一种在人类垂体中高度表达的印记基因。然而，MEG3 的表达在人类促性腺激素来源的垂体腺瘤和大多数人类肿瘤细胞系中丢失。在肿瘤细胞中表达 MEG3 会导致生长抑制、p53 蛋白增加和 p53 下游靶标的激活。MEG3 基因编码约 1700 个核苷酸的非编码 RNA。有 12 种不同的 MEG3 基因转录本，由选择性剪接产生。它们包含共同的外显子 1-3 和外显子 8-10，但每个外显子 4-7 在中间以不同的组合使用一个或多个。MEG3 亚型的表达模式具有组织和细胞类型特异性。在功能上，每个亚型都刺激 p53 介导的反式激活并抑制肿瘤细胞生长。我们使用计算机程序 mfold 分析了每个 MEG3 亚型的二级 RNA 折叠结构。所有 MEG3 RNA 亚型都包含三个不同的二级折叠基序 M1、M2 和 M3。缺失分析表明，基序 M2 和 M3 对 p53 激活很重要。此外，一种含有与野生型不同但折叠成类似二级结构的人工合成序列片段的杂交 MEG3 RNA 保留了 p53 激活和生长抑制的功能。这些结果支持这样一种假设，即 MEG3 RNA 分子的适当折叠结构对于其生物学功能至关重要。本研究首次建立了一个大的非编码 RNA 的结构-功能关系，并首次探讨了一个大的非编码 RNA 的生物学功能的分子机制。
