探討TERRA相關蛋白與端粒重組之關係

Abstract

端粒是位於染色體末端重複性序列，其功能為防止染色體末端之間產生融合 (fusion) 現象，進而維持染色體的穩定性。端粒DNA是由subtelomeric序列以及末端G-rich重複性序列組成。大部分真核細胞會利用端粒酶來維持端粒長度，其中端粒是由RNA及蛋白質組成的核蛋白，其會利用RNA為模板進行端粒的延長。除此之外，端粒長度也可以透過同源重組 (homologous recombination)方式維持，稱為alternative lengthening of telomeres (ALT)。在酵母菌中，先前研究發現，ALT機制會受到端粒的轉錄產物調控，此轉錄產物稱為telomeric repeat-containing RNA (TERRA)。TERRA會取代原本端粒DNA的雙股結構，與DNA形成RNA:DNA hybrid，稱為R-loop結構，並藉此結構促進端粒重組的發生。然而，具體R-loop如何參與在端粒重組過程中尚不清楚。基於R-loop中具有RNA的組成，因此RNase有可能透過其酵素活性處理R-loop結構，進一步調控端粒的重組。因此在我的研究中，我以找到參與在端粒重組中的RNase為目標。利用基因剔除的方式在端粒酶缺失的酵母菌中觀察目標RNase的剔除是否影響端粒重組的發生。在篩選的19個RNase中，我發現CCR4、XRN1以及RAI1三個RNase在基因剔除後會影響端粒重組。另外，由於RNA-binding proteins也是調控RNA穩定性及分布位置的重要因素，因此我也測試TERRA-binding proteins是否影響端粒重組。利用TERRA pull down assay - iDRiP所得數據，我一共針對18個TERRA-binding proteins做測試。然而，我並未在此部分發現調控端粒重組的蛋白質。總結以上，我找到三個基因，CCR4、XRN1以及RAI1會影響端粒重組，而此結果可以幫助我們日後更深入瞭解TERRA及R-loop如何影響端粒重組的過程。

Telomeres locate at the ends of eukaryotic chromosomes. They are essential to prevent chromosomal end-to-end fusion and maintain chromosome integrity. Telomeric DNA is composed of subtelomeric elements and short tandem repeated G-rich sequences. Most eukaryotes utilize telomerase to elongate telomeric DNA. Telomerase is a ribonucleoprotein that uses its RNA component as a template to replicate telomeric DNA. Telomere lengths can also be maintained through a recombination-based mechanism, called the alternative lengthening of telomere (ALT). Using Saccharomyces cerevisiae as a model system, previously we and others have found that the ALT pathway is regulated by telomere transcripts, termed telomeric repeat-containing RNA (TERRA). TERRA in the form of R-loop structure facilitates the homologous recombination process in the ALT pathway. However, the detail mechanism of how R-loop structure is involved in telomere recombination is unclear. Since the R-loop structure contains an RNA component, it is likely that specific ribonucleases involved in processing R-loop might impact telomere recombination. Here I applied genetic approach to screen and identify RNases that regulate telomere recombination. By deleting RNase genes into yeast cells lacking telomerase, I tested whether these deletions affect telomere recombination. Among a total of 19 RNases analyzed, I found XRN1, CCR4, and RAI1 affected telomere recombination. I also selected 18 genes encoding TERRA-associated proteins, that was identified from iDRiP analysis, to test their effects on telomere recombination. I found none of these 18 genes affected telomere recombination. Together I have identified three genes XRN1, CCR4, and RAI1 that affected telomere recombination. The results could help us understand the mechanism of how TERRA regulates telomere recombination in the future.