Raman microspectroscopy for label-free diagnosis of amyloid light-chain amyloidosis in various organs

Systemic amyloidosis is a group of diseases in which misfolded proteins aggregate as fibrous amyloid proteins with a β-sheet structure and deposit in organs, resulting in organ failure. Most types of amyloidosis have a poor prognosis, and prompt diagnosis is essential for treatment. Systemic immunoglobulin light-chain (AL) amyloidosis is a type of amyloidosis that occurs when abnormal immunoglobulin light-chain proteins are deposited in various organs and tissues. The deposition of amyloid proteins in tissues has traditionally been confirmed using Congo red staining and polarised light microscopy, which show apple-green birefringence. In this study, we aimed to verify whether amyloid deposition in the heart, kidney, rectum, duodenum and skin can be detected using Raman microspectroscopy. Serial sections were prepared from formalin-fixed paraffin-embedded tissue biopsy samples obtained from patients with systemic amyloidosis. One of the serial sections was stained with Congo red to confirm the deposition of amyloid proteins using polarised light microscopy, whereas the other was left unstained for Raman microspectroscopy. A characteristic peak at Raman shift of 1665–1680 cm⁻¹, which may represent a β-sheet structure of amyloid proteins, was recorded in the area where the amyloid deposition had been confirmed by Congo red staining. Based on the peak at 1640–1680 cm⁻¹, a colour map was obtained to detect amyloid protein-positive regions. Thus, amyloid protein detection using Raman microspectroscopy may be useful for rapid diagnosis of amyloidosis.