ova基因在转座子沉默和生殖干细胞分化中的作用

Transposable elements or transposons are DNA pieces that can move around within the genome and are therefore potential threat to genome stability and faithful transmission of the genetic information in the germline. The rampant retrotransposons spread will cause insertion mutagenesis, DNA double strand breaks and genomic instability. The increasing copy number of retrotransposons is highly risky to individuals, as it is prone to ectopic recombination, invertion and deletion. These are hallmarks of cancer. Accordingly, self-defense mechanisms have been evolved in the metazoan germline to silence transposons, and the primary mechanism requires the germline-specific noncoding small RNAs named Piwi-interacting RNA (piRNAs), which are in complex with Argonaute family of PIWI proteins (the piRNA-RISC complexes) to silencing transposons. Piwi represses TEs mainly though the transcriptional level. Piwi-piRISC recognizes the nascent transposon transcripts by base pair complementarity, thereby recruiting Panx and general silencing machinery to induce the heterochromatin formation in the transposon source regions. But it is still unclear how general silencing machinery is recruited. From a female sterile screen in Drosophila, we identified a novel gene called ova, whose mutation caused female to have rudimentary ovaries. Ova functions in Escort cells to restrict BMP signaling, thereby facilitates GSCs differentiation. In escort cells, loss of Ova leads to upregulation of BMP ligand dpp, which in turn blocks GSCs further differentiation. In the germline, loss of Ova leads to a dramatic increase of some transposons. Ova is also a suppressor of position-effect variation (PEV), suggesting that it is required for heterochromatin formation. Studies on ova molecular mechanism will provide us a opportunity to understand how piRNAs recruit general silencing machinery to transposon regions and the roles of stem cell niche in regulating stem cell differentiation.

转座子是基因组中可以移动的DNA片段。活跃的转座现象会造成基因插入突变，DNA双链断裂，诱发基因组不稳定性，增加基因组重排的风险。这些是癌细胞的一种显著特征。相应的，生物体进化出自我防御转座子的机制——通过piRNA和Piwi家族蛋白来实现对转座子的沉默。Piwi-piRNA识别转座子转录产物后，招募Panx蛋白和基因沉默因子对转座子区域进行沉默，但是现在并不清楚招募基因沉默因子的具体机制。我们发现ova基因缺失后，在正常护卫细胞中被抑制的BMP信号配体dpp显著上调，增强的BMP信号抑制了生殖干细胞的分化。在生殖细胞中，ova基因的缺失将导致部分转座子mRNA出现明显的上调。进一步研究表明ova是位置效应的抑制者，说明ova参与了异染色质的形成。研究ova的分子功能给我们提供了一个揭示piRNA诱导异染色质形成的分子机制的机会，也能使我们更加理解干细胞微环境对于干细胞分化调控的功能。

转座子是基因组中可移动的DNA片段。在人类基因组中，转座子和重复序列占据了接近50%，而可编码基因仅仅占据约1%。转座子的激活会造成DNA插入突变，基因组不稳定，并且与肿瘤，衰老和神经退行性疾病的发生相关。因此生物体进化出了一种以小RNA为中心的防御机制来沉默转座子，叫piRNA通路。 piRNA通过序列互补配对识别转座子mRNA并且招募下游组蛋白修饰因子来抑制转座子表达。但是我们对这些组蛋白修饰因子的相互配合并不清楚。我们通过果蝇卵巢肿瘤模型和生殖力筛选在发现一个新的基因，进一步遗传分析确定是CG5694，命名为ovaries absent (ova)。通过遗传上位分析，ova在piRNA通路核心因子piwi基因下游。Ova识别结合在转座子区域H3K9三甲基化修饰上的异染色质蛋白HP1a，然后作为一个桥梁分子招募H3K4去甲基化酶dLSD1，对转座子启动子区域去甲基化，实现对转座子的抑制。此外，Ova还可以通过类似的机制抑制基因的表达。如果果蝇卵巢的护卫细胞缺失ova基因，会导致生殖干细胞自我更新信号的去抑制，形成生殖干细胞样肿瘤。我们的研究揭示了Ova作为桥梁分子促进组蛋白修饰因子HP1a和dLSD1配合实现对基因和转座子的抑制，揭示了抑制性组蛋白修饰H3K9me3和激活性组蛋白修饰H3K4me2之间的相互联系。