染色體易碎症為一種常見的遺傳性疾病，其發病原因可以歸咎於人體中FMR1中CGG/CCG三核苷酸重複序列的擴張。在正常人體中，CGG/CCG的重複次數落在5至44之間；然而，在病人中，重複次數則會擴大至200以上。若CGG三核苷酸重複序列在FMR1基因中過度的擴張，將會觸發一種不正常的甲基化，稱為過甲基化；當基因過甲基化後，會造成組蛋白去乙醯化。經由這一系列不正常的生物修飾後，就會使FMR1基因失效。相較之下，在正常人體中的CGG/CCG序列之間會差入AGG/CCT序列。插入的頻率大約為9至10個CGG會出現一個AGG序列。
然而CGG三核苷酸重複序列的構型以及AGG序列會對一般的CGG序列所照成影響仍是未知的。因此我們利用單分子螢光共振能量轉移光譜研究插入AGG以及未插入AGG之CGG序列的構型。我們的研究結果顯示，CGG序列不能在生理條件下折疊成鳥嘌呤四具體聚體。僅有在重複次數少於12的CGG序列能夠在有鳥嘌呤四具體聚體誘導劑的環境下，才能夠形成鳥嘌呤四具體聚體結構。在一般情況下，CGG序列則是會穩定地摺疊成對齊型的髮夾型構型。插入AGG的CGG的序列則會使這種使這種穩定的髮夾型結構改變，變化成另一種髮夾型結構。以先前研究所提出的重複序列的擴張理論為基礎，我們提出一個AGG三重複序列可能在DNA擴張中所扮演的角色。在具有AGG插入之序列含有傾向於形成帶有露出核苷酸之髮夾型結構，此一結構較不利於序列擴張，且在插入AGG後並未觀察到此結構活動至較利於被擴張的對齊型髮夾型結構。我們認為此一結構與結構動態學上的指標性與AGG插入抑制DNA擴張的原因。

The expansion of CGG/CCG trinucleotides in the fragile X mental retardation (FMR1) gene leads to Fragile X syndrome (FXS), one of the most common genetic disorders. The number of CGG tract is in between 5 and 44 tandem repeat units in the healthy humans, while in the pathological samples, more than 200 repeat units were found in FMR1 gene. The overexpansion of CGG repeat would trigger hypermethylation, an abnormal DNA methylation and lead to inhibition of histone modification and epigenetic gene of FMR1 silence. In contrast, healthy individuals show that AGG/CCT interruptions exist in every 9-10 CGG/CCG trinucleotides. CGG repeat can fold into hairpin-like structures or G-quadruplexes, which are still under debating.
Moreover, how AGG interruption affect to d(CGG)n structure is remained elusive. We use single-molecule fluorescence resonance energy transfer (smFRET) spectroscopy to study the conformation of CGG repeat with and without the interruption by AGG insertions. Our results show that d(CGG) repeats do not form a G-quadruplex under the physiological condition until the G-quadruplex inducer (pyridostatin) was added to the short (<16 repeat units) CGG sequences. The stable d(CGG) hairpin structure is interfered by the AGG insertion and lead to become another hairpin structure. Based on the DNA expansion model from our previous study, we propose a structure and structural dynamics role of AGG insertion in DNA preventing expansion. The insertion of AGG makes the majority of the sequencing folded into a hairpin with overhang configuration, which has less tendency to be expanded. When the CGG triplet fold into the hairpin structure, the new generated CGG repeat would enter the hairpin region and the DNA polymerase won’t work. Once the DNA replication restart, it’ll cause the DNA expansion. According to our result, CGG repeat interrupted by AGG triplet tend to fold into the hairpin structure with overhang. Furthermore, this configuration failed to slip to a expansion-favored blunt-end hairpin configuration. The unique structure and structural dynamics caused by the AGG insertion to tandem (CGG) repeat prevents the disease caused by error-prone expansion.

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