Fibrillary Light Chain Proximal Tubulopathy: A Distinct Subtype Within the Spectrum of Light Chain Proximal...

Area of Science:

• Nephrology
• Pathology
• Immunology

Background:

• Light chain proximal tubulopathy (LCPT) is a rare kidney disorder linked to monoclonal gammopathy, traditionally identified by crystalline inclusions.
• A poorly understood variant, fibrillary LCPT, features non-amyloid fibrillar aggregates, causing diagnostic challenges.
• Existing classification systems lack precision for fibrillary LCPT, necessitating a refined approach.

Purpose of the Study:

• To elucidate the clinicopathologic characteristics of fibrillary LCPT.
• To refine the classification of LCPT subtypes based on ultrastructural findings.
• To improve diagnostic accuracy and guide therapeutic strategies for LCPT.

Main Methods:

• Retrospective analysis of 41 biopsy-proven LCPT cases.
• Reclassification based on ultrastructural features into crystalline, fibrillary, amyloid, and lysosomal indigestion LCPT subtypes.
• Utilized light microscopy, immunofluorescence (conventional and pronase-digested paraffin), and immunoelectron microscopy for diagnosis and κ light chain restriction detection.

Main Results:

• Fibrillary LCPT is characterized by Congo red-negative, 6-18 nm cytoplasmic fibrils within proximal tubular cells, not visible by light microscopy.
• Immunoelectron microscopy achieved 100% sensitivity for detecting κ light chain restriction in fibrillary LCPT.
• Fibrillary LCPT frequently associates with monoclonal gammopathy of renal significance (MGRS) and Fanconi syndrome (FS), often presenting with reduced eGFR and sometimes acute kidney injury.

Conclusions:

• Fibrillary LCPT is a distinct, non-amyloid entity within the LCPT spectrum, defined by specific ultrastructural fibril characteristics and κ light chain restriction.
• A proposed four-tier ultrastructural classification (crystalline, fibrillary, amyloid, lysosomal indigestion) enhances diagnostic precision for LCPT.
• Clone-directed therapy can stabilize renal function and improve tubular dysfunction in fibrillary LCPT, though proteinuria reduction may be less pronounced than in crystalline LCPT.