The Structural and Dynamic Insights into the Ala97Ser Amyloidogenic Mutation in Transthyretin

Transthyretin (TTR), a homo-tetrameric protein encoded by the TTR gene, can lead to amyloid diseases when destabilized by mutations. The TTR-Ala97Ser (A97S) mutation is the predominant pathogenic variant found in Han-Taiwanese patients and is associated with late-onset familial amyloid polyneuropathy (FAP), which presents a rapid progression of symptoms affecting peripheral nerves and the heart. In this study, we combined nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography to investigate how the A97S mutation impacts the structure and dynamics of TTR. Previous X-ray analyses indicated that the FG loop exhibits increased flexibility due to the mutation, evidenced by missing electron density and a reduced number of hydrogen bonds. Our NMR hydrogen-deuterium (H/D) exchange experiments provided additional insights, revealing that inter-residue hydrogen bonds among the FG loop residues are unstable in both wild-type (WT) and A97S TTR. Notably, the hydrogen bonds between G67 and S97 are unstable, influencing the stability of adjacent loops. This elongation of the FG loop is believed to contribute to increased flexibility and enhanced water-protein proton exchange, as observed in NMR relaxation and chemical exchange experiments. Our findings offer a comprehensive understanding of how the A97S mutation affects TTR structure and dynamics, providing new insights into its amyloidogenicity.