Diagnosis of renal AL amyloidosis using DAPI

Amyloidosis is often overlooked because its clinical manifestations can mimic those of more-common diseases. It is important to get a precise diagnosis as early as possible for the prevention of further organ damages. Amyloidosis is a disorder caused by deposition of insoluble abnormal amyloid. The kidney is a frequent site of amyloid deposition. The amyloid fibrils have a characteristic appearance and generate birefringence under polarized light when stained with the Congo red dye. Classification of amyloidosis is based on the precursor protein that forms the amyloid fibrils and the distribution of amyloid deposits as either systemic or localized. Involvement of amyloid fibrils in kidneys mainly occurs as amyloid light-chain (AL) or amyloid A (AA) amyloidosis. The potassium permanganate method with Congo red staining was once used widely to discriminate AL and AA amyloidoses, but thismethod has a problem of false positive results. We found that extracellular and cytoplasmic glomerular 4’,6-diamidino-2-phenylindole (DAPI)-positive areas were clearly consistent with amyloid deposition in AL amyloidosis. In contrast, the overlapping staining was not seen in AA amyloidosis. Therefore, we propose that DAPI staining readily distinguishes AL renal amyloidosis from AA renal amyloidosis as a simple and reproducible histochemical method.