Pannell D, Osborne C S, Yao S, Sukonnik T, Pasceri P, Karaiskakis A, Okano M, Li E, Lipshitz H D, Ellis J
Programs in Developmental Biology, and Cancer and Blood Research, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8, Toronto, Canada.
EMBO J. 2000 Nov 1;19(21):5884-94. doi: 10.1093/emboj/19.21.5884.
Retrovirus vectors are de novo methylated and transcriptionally silent in mammalian stem cells. Here, we identify epigenetic modifications that mark retrovirus-silenced transgenes. We show that murine stem cell virus (MSCV) and human immunodeficiency virus type 1 (HIV-1) vectors dominantly silence a linked locus control region (LCR) beta-globin reporter gene in transgenic mice. MSCV silencing blocks LCR hypersensitive site formation, and silent transgene chromatin is marked differentially by a histone code composed of abundant linker histone H1, deacetylated H3 and acetylated H4. Retrovirus-transduced embryonic stem (ES) cells are silenced predominantly 3 days post-infection, with a small subset expressing enhanced green fluorescent protein to low levels, and silencing is not relieved in de novo methylase-null [dnmt3a-/-;dnmt3b-/-] ES cells. MSCV and HIV-1 sequences also repress reporter transgene expression in Drosophila, demonstrating establishment of silencing in the absence of de novo and maintenance methylases. These findings provide mechanistic insight into a conserved gene silencing mechanism that is de novo methylase independent and that epigenetically marks retrovirus chromatin with a repressive histone code.
逆转录病毒载体在哺乳动物干细胞中会发生从头甲基化并转录沉默。在此，我们鉴定出了标记逆转录病毒沉默转基因的表观遗传修饰。我们发现，小鼠干细胞病毒（MSCV）和1型人类免疫缺陷病毒（HIV-1）载体在转基因小鼠中会显著沉默一个相连的位点控制区（LCR）β-珠蛋白报告基因。MSCV沉默会阻断LCR超敏位点的形成，沉默的转基因染色质由丰富的连接组蛋白H1、去乙酰化的H3和乙酰化的H4组成的组蛋白密码进行差异标记。逆转录病毒转导的胚胎干细胞（ES细胞）在感染后3天主要发生沉默，一小部分细胞低水平表达增强型绿色荧光蛋白，并且在从头甲基化酶缺失的[dnmt3a-/-;dnmt3b-/-]ES细胞中沉默并未解除。MSCV和HIV-1序列在果蝇中也会抑制报告转基因的表达，表明在没有从头甲基化酶和维持甲基化酶的情况下沉默得以建立。这些发现为一种保守的基因沉默机制提供了机制上的见解，该机制不依赖于从头甲基化酶，并且通过抑制性组蛋白密码在表观遗传上标记逆转录病毒染色质。
