Identification and quantitative analysis of human transthyretin variants in human serum by Fourier transform ion-cyclotron resonance mass spectrometry

Transthyretin (TTR) is a homotetrameric protein involved in thyroid hormone transport in blood and in retinol binding in the central nervous system. More than 80 point mutations in this protein are known to be associated with the formation of amyloid deposits and systemic amyloidotic pathologies. Age at onset varies according to the mutation but considerable variations also occur for subjects carrying the same mutation. Moreover, wild-type TTR forms amyloid deposits in systemic senile amyloidosis, a geriatric disorder. An accurate diagnostic and the choice of therapeutic options depend on the identification of the specific mutation. Previous characterization of TTR variants by mass spectrometry required the use of antibodies for sample enrichment. We developed a novel assay based on ultra high-resolution mass spectrometry to identify human TTR variants. The method, requiring a very low sample amount, is based on SDS-PAGE fractionation of human serum, followed by peptide mass fingerprinting by MALDI-FTICR-MS (matrix assisted laser desorption ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry). Moreover, it is possible to perform a relative quantification of wild type and mutant TTR forms by mass spectrometry. The method was tested and validated with the V30M mutant, involved in familial amyloidotic neuropathy of Portuguese type.