Heterogeneous structural and pathogenic properties of transthyretin dimeric variants.

Transthyretin amyloidosis (ATTR) is caused by the deposition of transthyretin (TTR) aggregates. While unstable TTR monomers, which are dissociated from the native tetramer, are known to be the primary amyloidogenic precursors of ATTR, the pathological role of other intermediates, such as dimers and oligomers, remains unclear. This study aimed to characterize two disease-associated TTR variants, A19D and S112I, known to exist predominantly as dimers. Biochemical and biophysical analyses were employed to demonstrate that A19D and S112I form stable, β-sheet-rich dimeric structures, yet exhibit distinct structural stabilities and aggregation propensities. Using nuclear magnetic resonance (NMR) spectroscopy and small-angle X-ray scattering (SAXS), we revealed that these dimers adopt heterogeneous structural conformations rather than a uniform dimeric state. Notably, A19D displays characteristics of a highly unstable and amyloidogenic dimeric intermediate, whereas S112I forms a more stable dimer with locally disordered regions and shows an unfolding behavior similar to that of monomeric TTR. Together, our findings highlight the structural heterogeneity of non-native dimers in TTR aggregation and provide insights into their potential roles in ATTR.