Lysozyme as an amyloid fibril protein together with transthyretin in the ligamentum flavum in association with lumbar spinal stenosis.

BACKGROUND Transthyretin amyloid (ATTR) is frequently found in aging ligaments and may represent an early sign of systemic ATTR amyloidosis, particularly of wild-type origin. Amyloid deposits of other compositions are commonly observed alongside ATTR deposits.

METHODS Ligamentum flavum samples obtained during surgery for lumbar spinal stenosis, containing distinct ATTR amyloid deposits as well as amyloid of unknown composition, were analyzed using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), electron microscopy, immunohistochemistry, and immunofluorescence confocal microscopy. In vitro fibril formation and potential cross-seeding by the two major proteins, transthyretin and lysozyme, were assessed with the thioflavin-T assay.

RESULTS A close topographic association between ATTR deposits and elastic material was evident. Distinct ATTR amyloid was intermingled with non-ATTR amyloid. HPLC-MS/MS identified three major amyloid fibril proteins with high score: transthyretin (TTR), apolipoprotein AI, and lysozyme. Congo red positivity combined with immunoreactivity for TTR and lysozyme demonstrated that ATTR predominated, though distinct lysozyme amyloid deposits were also present. Only minimal apolipoprotein AI-positive amyloid was detected. In vitro, cross-seeding between TTR and lysozyme was not observed; however, lysozyme inhibited TTR fibrillation.

CONCLUSION In lumbar spinal stenosis, ATTR amyloid deposits are predominant but are often intermixed with other amyloid deposits, notably those of lysozyme origin.