研究人類端粒四股結構與BMVC衍生物的交互作用

Investigation of the interactions between Human Telomeric G-Quadruplex and BMVC derivatives

陳政維(Cheng-Wei Chen)

指導教授 ： 張大釗

國立臺灣大學/理學院/化學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.02064

摘要

中文摘要
人類端粒 (telomere)位於細胞核內染色體 (chromosome) 的末端。其序列為六個核苷酸5' - d(TTAGGG) - 3'形成的重複串聯 (tandem repeat)，實驗已證明會形成G-四股結構 (G-Quadruplex , G4)。癌症細胞的端粒酶 (telomerase)在維持端粒長度時，需將DNA的3'端打開成為單股 (single strand)。若G4的結構能被穩定，那麼便可以抑制端粒酶的功能。因此，科學家們認為：若能有效設計G-四股結構穩定劑 (G - Quadruplex Stabilizer) 來穩定住G4結構，將有機會成為治療癌症的方法。
評估G-四股結構穩定劑的方式是非常重要的，傳統上使用CD來量測G4 DNA的解旋溫度，實驗過程中會大幅改變溫度 (如5 ~ 100˚C)，所以篩選出的G4穩定劑未必適用於人體生理實際溫度。因此我們藉由恆溫滴定測焓儀 (ITC) 能夠在恆溫37˚C下進行實驗這項特點，設計互補競爭法ITC Assay，評估了小分子穩定G4結構的能力。
接下來我們想瞭解不同分子結構的G-四股結構穩定劑，是否會影響與G4結合的模式。本論文從實驗室設計的BMVC衍生物中挑選4種類似分子結構的化合物。結構主要可分為Core及Side Chain兩部分。Core為BMVC與o-BMVC Core，兩者為同分異構物 (isomer)。Side Chain為含有4個碳原子的支鍊 4C以及含有ethylene glycol 的支鏈 6C2O。4C較疏水，6C2O較親水。我們使用ITC在定溫下測量小分子的熱力學結合參數。實驗結果顯示小分子的結合模式與Core的關聯性較強，而Side Chain並沒有顯著的影響。
我們進而探討功能性Side Chain。實驗室曾報導含ethylene glycol支鏈的BMVC-6C2O小分子具有將非平行結構G4轉換成平行結構的能力，經過轉換的G4結構穩定度能大幅提升。從上一部分已知不同Core的小分子，結合模式較不相同，那麼o-BMVC-6C2O也有轉換結構的能力嗎 ? 若能夠轉換，那麼BMVC-6C2O與o-BMVC-6C2O兩者在轉換G4結構的速率上，是否會有所差異 ? 以及轉換成平行結構的G4，穩定度的比較又是何種情形 ? 實驗結果顯示，2種小分子都會對G4結構進行轉換，將非平行G4結構轉換為平行G4結構。轉換速率及轉換完後的平行G4結構穩定度，都會因為Core的不同而產生差異性。
本論文使用互補競爭法ITC Assay在生理溫度37˚C恆溫的條件下，對G-四股結構穩定劑的效力進行評估。並以G4 ligand分子結構與熱力學參數之間的關聯性為基礎，進一步探討與動力學結構轉換現象的相關性。我們提供給未來研究者一個設計G-四股結構穩定劑的研究方法，也期望未來研究者能藉此篩選出穩定效力更佳的小分子。

關鍵字

人類端粒G-四股結構 ； G-四股結構穩定劑 ； 恆溫滴定測焓儀 ； 解旋溫度 ； 熱力學結合參數

並列摘要

Abstract
Human telomeric DNA consist of tandem repeats of the hexanucleotide d(TTAGGG)n which could form G-quadruplex structure under physiological conditions. Importantly, the formation of G-quadruplexes has been shown to inhibit the activity of the telomerase which is aberrant in cancer cell. Thus, design a G-quadruplex ligand which can target and further stabilize G-quadruplex structures has becoming a potential anticancer strategy.
A novel method is necessary to examine the G-quadruplex ligands with emerging number of small molecules reported in recent years. In general, thermal melting analysis is a typical method used, in which the unfolding of G-quadruplex is monitored by changes in spectroscopic signal. By addition of G-quadruplex ligand, one can obtain the increment in melting temperature (Tm) of G-quadruplexes. However, the major issue of thermal melting analysis method is the dependence on large temperature change. At high temperature, some compounds would induce high-order structure of G-quadruplexes and melting analysis result may not be correspondent to the ability of G-quadruplex ligand at the physiological temperature. Moreover, thermal melting analysis cannot measure additional reliable thermodynamic data, e.g. binding enthalpy, to further design or modify present G-quadruplex ligand. Therefore, we here introduce isothermal titration calorimetry (ITC) to investigate the thermodynamic binding feature of our G-quadruplex ligand, BMVC derivatives bind to human telomeric G-quadruplex, Tel23 dTAG3(T2AG3)3 under isothermal condition. In addition, we used hybridization competition ITC Assay to further evaluate ligand’s ability to stabilize G4 structures.
In order to study and compare G4 ligands, we have selected four G-quadruplex ligands with similar structure, which can be compared by two aspects: Core and Side Chain. The Cores of two isomers are different, one is para-pyridium structure of BMVC Core and the other is ortho-pyridium structure o-BMVC Core. And, two different Side Chain are used, one is hydrophobic four carbon chain, 4C, and the another is hydrophilic triethylene glycol chain, 6C2O. We use ITC to estimate four ligand’s thermodynamic binding parameter. From ITC result, we found out the relationship between G-quadruplex ligand structure and its binding mode. The binding thermodynamic feature of G-quadruplex ligand, for ortho-pyridium structure o-BMVC Core is more enthalpy driven and for para-pyridium structure of BMVC Core is more entropy driven. The difference of side chain has minor effect on thermodynamic feature. It shows core structure of G-quadruplex ligand is the major drive for their binding stability efficiency. Moreover, we further obtain the similar binding heat capacity value for similar core structure that agrees this result.
However, what is the minor effect on the different side chain of ligand ? We further discuss functional side chain’s effect. We have previously reported that BMVC - 6C2O, which constitutes para-pyridium structure of BMVC, can induce structural conversion from nonparallel to parallel. From the previous result, we have known that BMVC-derivatives with different Core have different thermodynamic binding driving force. Thus, we question if the core of ligand governs the binding force, does o-BMVC- 6C2O, which contains different core structure, induce such a structure change from nonparallel G4 to parallel G4 ? If so, do they have different conversion rate and parallel G4 stability? Our data shows that both ligand with 6C2O unit can make structure conversion from nonparallel to parallel. However, conversion rate and stability have major difference which presumably results from the core structure differences.
In conclusion, base on the isothermal titration calorimetry (ITC) assay which provides thermodynamic parameter for ligand-G-quadruplex binding and the results offer thermodynamic information of the G-quadruplex ligand structure with the additional kinetic study of structural conversion, these information on ligand-G4 stability can give more reliable way to design better and potential G-quadruplex ligand for anticancer drug design.

並列關鍵字

telomere ； G-quadruplex ； G-quadruplex stabilizer ； anti-cancer drugs ； isothermal titration calorimetry ； melting temperature

參考文獻

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