Chemical Biology Interface Program, Johns Hopkins University, Baltimore, MD, USA.
T. C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD, USA.
Nat Commun. 2024 Oct 9;15(1):8738. doi: 10.1038/s41467-024-52764-x.
In Huntington's Disease (HD) and related disorders, expansion of CAG trinucleotide repeats produces a toxic gain of function in affected neurons. Expanded huntingtin (expHTT) mRNA forms aggregates that sequester essential RNA binding proteins, dysregulating mRNA processing and translation. The physical basis of RNA aggregation has been difficult to disentangle owing to the heterogeneous structure of the CAG repeats. Here, we probe the folding and unfolding pathways of expHTT mRNA using single-molecule force spectroscopy. Whereas normal HTT mRNAs unfold reversibly and cooperatively, expHTT mRNAs with 20 or 40 CAG repeats slip and unravel non-cooperatively at low tension. Slippage of CAG base pairs is punctuated by concerted rearrangement of adjacent CCG trinucleotides, trapping partially folded structures that readily base pair with another RNA strand. We suggest that the conformational entropy of the CAG repeats, combined with stable CCG base pairs, creates a stick-slip behavior that explains the aggregation propensity of expHTT mRNA.
在亨廷顿病（HD）和相关疾病中，CAG 三核苷酸重复序列的扩展导致受影响神经元产生毒性功能获得。扩展的亨廷顿蛋白（expHTT）mRNA 形成聚集体，隔离必需的 RNA 结合蛋白，扰乱 mRNA 加工和翻译。由于 CAG 重复序列的异质结构，RNA 聚集的物理基础一直难以理清。在这里，我们使用单分子力谱技术探测 expHTT mRNA 的折叠和展开途径。正常的 HTT mRNAs 可以可逆且协同地展开，而具有 20 或 40 个 CAG 重复的 expHTT mRNAs 在张力较低时会滑动并非协同展开。CAG 碱基对的滑动由相邻 CCG 三核苷酸的协同重排打断，捕获易于与另一条 RNA 链碱基配对的部分折叠结构。我们认为，CAG 重复序列的构象熵与稳定的 CCG 碱基对相结合，产生了一种粘滑行为，解释了 expHTT mRNA 的聚集倾向。
