A unique case of neural amyloidoma diagnosed by mass spectrometry of formalin-fixed tissue using a novel preparative technique

We report here a unique amyloidoma of the radial nerve which could not be subtyped by available techniques, including immunohistochemistry and standard clinical and laboratory evaluation. In order to identify the amyloid monomer, we developed a novel preparative procedure designed to optimize conditions for liquid chromatography tandem mass spectrometry analysis of formalin-fixed/paraffin-embedded (FFPE) tissue. Subsequent mass spectrometric analysis clearly identified kappa light chain as the monomer, with no evidence of lambda light chain. Manual interpretation of the matched spectra revealed no evidence of polyclonality. This study also enabled detailed characterisation of twelve likely amyloid matrix components. Finally, our analysis revealed extensive hydroxylation of collagen type I but, unexpectedly, an almost complete lack of hydroxylated residues in the normally heavily-hydroxylated collagen type VI chains, pointing to structural/functional alterations of collagen VI in this matrix that could have contributed to the pathogenesis of this very unusual tumour. Given the high quality of the data here acquired using a standard quadrupole-time of flight tandem mass spectrometer of modest performance, the robust and straightforward preparative method described constitutes a competitive alternative to more involved approaches using state-of-the-art equipment.