Abstract：
Abnormal expansion of trinucleotide CGG repeats is responsible for Fragile X syndrome. AGG interruptions in CGG repeat tracts were found in most healthy individuals, suggesting a crucial role in preventing disease-prone repeat expansion. Previous biophysics studies emphasize a difference in the secondary structure affected by AGG interruptions. However, the mechanism of how AGG interruptions impede repeat expansion remains elusive. We utilized single-molecule fluorescence resonance energy transfer spectroscopy to investigate the structural dynamics of CGG repeats and their AGG-interrupted variants. Tandem CGG repeats fold into a stem-loop hairpin structure with the capability to undergo a conformational rearrangement to modulate the length of the overhang. However, this conformational rearrangement is much more retarded when two AGG interruptions are present. Considering the significance of hairpin slippage in repeat expansion, we present a molecular basis suggesting that the internal loop created by two AGG interruptions acts as a barrier, obstructing the hairpin slippage reconfiguration. This impediment potentially plays a crucial role in curbing abnormal expansion, thereby contributing to the genomic stability.
摘要：
三核苷酸CGG重复的异常扩增是造成脆性X综合征的原因。在大多数健康个体中发现CGG重复序列中的AGG中断，提示在预防疾病倾向的重复扩张中起关键作用。先前的生物物理学研究强调了受AGG中断影响的二级结构的差异。然而,AGG中断如何阻碍重复扩张的机制仍然难以捉摸。我们利用单分子荧光共振能量转移光谱来研究CGG重复及其AGG中断变体的结构动力学。串联CGG重复序列折叠成茎-环发夹结构，能够进行构象重排以调节突出端的长度。然而,当存在两个AGG中断时，这种构象重排更加延迟。考虑到重复扩张中发夹滑动的重要性，我们提出了一个分子基础，表明由两个AGG中断产生的内部回路充当屏障，阻碍发夹滑移重新配置。这种障碍可能在遏制异常扩张中起着至关重要的作用，从而有助于基因组的稳定性。
