Hybrid Tetramers of Wild-Type and Amyloidogenic H88R Transthyretin Challenge the Monomer Paradigm
Hereditary transthyretin amyloidosis (ATTRv) is driven by mutations that destabilize the native tetrameric transthyretin (TTR), promoting monomer formation and amyloid aggregation. The H88R variant has been considered fully monomeric, yet its behavior in heterozygous patients has remained unclear. Here, we demonstrate that H88R TTR can form hybrid tetramers with wild-type chains, both in vitro and in patient sera. We considered all possible tetramers, along with various trimer and dimer constructions, and found that the stability of hybrid tetramers containing one or two mutant chains is not significantly reduced in comparison with of the wild-type tetramer, suggesting a practically unhindered entry for H88R TTR monomers into such hybrid tetrameric assemblies, facilitating their secretion. Mass spectrometry confirms incorporation of H88R chains into tetramers in vitro, and show that the H88R TTR variant is present in the serum of carriers albeit in low concentration. We propose that the scarcity of this variant is the result of its retention in the endoplasmic reticulum and present a model for the association of H88R TTR with the endoplasmic reticule chaperone Binding immunoglobulin Protein (BiP). These findings revise the current monomeric view of H88R TTR, with direct implications for the efficacy of tetramer-stabilizing therapeutics. Our results highlight a delicate balance between cellular retention and hybridization, informing mechanistic understanding and treatment strategies in heterozygous ATTRv patients.