Paternally Expressed Peg3 Controls Maternally Expressed Zim1 as a Trans Factor

The expression of two adjacent imprinted genes, Peg3 and Zim1, is inversely correlated: down-regulation of Peg3 coinciding with up-regulation of Zim1. The current study characterized this inverse correlation using a mutant allele targeting Peg3. According to the results, the mutation on the paternal allele of Peg3 resulted in a dramatic increase in the transcription levels of the maternal allele of Zim1, suggesting the involvement of unknown trans factors in this trans-allelic event. Subsequent ChIP experiments revealed that the protein encoded by Peg3 itself binds to the zinc finger exon of Zim1, which is modified with the repression mark H3K9me3. Interestingly, the levels of H3K9me3 on Zim1 are also reduced in the mutant cells lacking the protein PEG3, suggesting potential roles for PEG3 in establishing H3K9me3 on Zim1. Reintroducing PEG3 into the mutant cell restored down-regulation of Zim1, confirming the predicted repressor role for Peg3 on Zim1. Overall, these results demonstrated that paternally expressed Peg3 controls maternally expressed Zim1 as a trans factor. The current study also provides the first case for the trans-allelic interaction of two oppositely imprinted genes through their gene products.

两个相邻的印记基因（imprinted gene）Peg3与Zim1的表达呈负相关：Peg3的下调与Zim1的上调同时发生。本研究利用靶向Peg3的突变等位基因（allele）对这一负相关关联进行了系统表征。实验结果显示，Peg3父系等位基因的突变会导致Zim1母系等位基因的转录水平显著升高，提示该跨等位基因事件涉及未知的反式作用因子（trans factor）。后续的染色质免疫沉淀实验（ChIP, chromatin immunoprecipitation）发现，Peg3编码的蛋白质可直接结合Zim1的锌指外显子区域，该区域带有抑制性组蛋白修饰标记H3K9me3。值得注意的是，在缺失PEG3蛋白的突变细胞中，Zim1位点的H3K9me3修饰水平同样显著降低，这表明PEG3可能在Zim1位点的H3K9me3修饰建立过程中发挥作用。将PEG3重新导入该突变细胞后，可恢复Zim1的下调表达，进一步证实了Peg3对Zim1的转录抑制功能。综上，上述结果证明父系表达的Peg3可作为反式作用因子调控母系表达的Zim1。本研究同时首次报道了两个反向印记基因通过其基因产物发生跨等位基因相互作用的案例。