Cys-10 mixed disulfide modifications exacerbate transthyretin familial variant amyloidogenicity: a likely explanation for variable clinical expression of amyloidosis and the lack of pathology in C10S/V30M transgenic mice?
The marked variation in clinical expression and age of familial amyloid disease onset is not well understood. One possibility is that metabolite modification(s) of a disease-associated mutant protein can change the energetics and propensity for misfolding, influencing the disease course. Each subunit of the transthyretin (TTR) tetramer has a single Cys residue that can exist in the SH form or as a mixed disulfide with the amino acid Cys or the peptide glutathione or fragments of the latter. The stability and amyloidogenicity of the clinically most important TTR variants (V30M and V122I) in their SH oxidation state were compared with those of their mixed disulfide adducts. All the Cys-10 mixed disulfide conjugates exhibited substantially decreased protein stability (urea, pH 7) and a higher rate and extent of amyloidogenesis (slightly acidic conditions). We also investigated the amyloidogenicity and stability of a C10S/V30M TTR double mutant which lacks the ability to make mixed disulfides, but retains the disease-associated V30M mutation. Unlike V30M TTR, this double mutant is nonamyloidogenic in transgenic mice. Our in vitro data reveal that the C10S/V30M and V30M TTR homotetramers have identical amyloidogenicity and stability, implying that Cys-10 mixed disulfide formation enhances amyloidogenesis in V30M transgenic mice. Given the high proportion of TTR subunits having mixed disulfide modifications in human plasma ( approximately 50%), and the data within demonstrating their increased amyloidogenicity, we submit that disulfide metabolite modifications have the potential to influence the course of amyloidoses, including TTR amyloidoses caused by mutations.