Stick-slip unfolding favors self-association of expanded HTT mRNA

• Published in: Nature communications Vol. 15; no. 1; pp. 8738 - 10
• Main Authors: O’Brien, Brett M., Moulick, Roumita, Jiménez-Avalos, Gabriel, Rajasekaran, Nandakumar, Kaiser, Christian M., Woodson, Sarah A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 09.10.2024; Nature Publishing Group; Nature Portfolio
• Subjects: 38/71; 38/90; 631/337/1645/501; 631/57/2265; Aggregation behavior; Base Pairing; Base pairs; Heterogeneous structure; Humanities and Social Sciences; Humans; Huntingtin; Huntingtin Protein - chemistry; Huntingtin Protein - genetics; Huntingtin Protein - metabolism; Huntington Disease - genetics; Huntington Disease - metabolism; Huntington's disease; Huntingtons disease; Mechanical properties; Molecular structure; mRNA; mRNA processing; multidisciplinary; Nucleic Acid Conformation; Polyglutamine; RNA Folding; RNA probes; RNA processing; RNA, Messenger - genetics; RNA, Messenger - metabolism; RNA-binding protein; Science; Science (multidisciplinary); Self-association; Single Molecule Imaging; Slip; Slippage; Spectroscopy; Toxic diseases; Trinucleotide repeat diseases; Trinucleotide Repeat Expansion - genetics; Trinucleotide repeats
• ISSN: 2041-1723; 2041-1723
• 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 (exp HTT ) 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 exp HTT mRNA using single-molecule force spectroscopy. Whereas normal HTT mRNAs unfold reversibly and cooperatively, exp HTT 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 exp HTT mRNA. Mechanical unfolding of huntingtin mRNA with optical tweezers reveals how CAG repeat expansion triggers base pair reshuffling and favors RNA aggregation that is a hallmark of Huntington’s Disease.