Concurrent κ light chain proximal tubulopathy and crystal-storing histiocytosis in monoclonal gammopathy of renal significance: a rare case report with 4-year follow-up and literature review.

Abstract

Background: Light chain proximal tubulopathy (LCPT) and crystal-storing histiocytosis (CSH) are rare forms of monoclonal immunoglobulin-associated kidney disease, representing either variants of monoclonal gammopathy of renal significance (MGRS) or renal complications of multiple myeloma (MM). Concurrent LCPT and suspected CSH associated with κ light chain MGRS is exceptionally rare, with only three comparable cases reported worldwide. This CARE guideline-compliant report describes the clinical, laboratory and histopathological features of this rare dual renal lesion.

Case presentation: A 61-year-old Chinese woman presented with Fanconi syndrome (FS), chronic kidney disease (CKD) and isolated tubular proteinuria. Serum and urine immunofixation electrophoresis showed elevated monoclonal IgGκ. Bone marrow examination revealed mild plasmacytosis (< 10%), with no hypercalcemia, myeloma bone lesions, cast nephropathy or SLiM criteria-defined myeloma events, confirming MGRS and excluding MM and smoldering multiple myeloma (SMM).

Diagnostic assessment: Renal biopsy electron microscopy demonstrated dense crystalline inclusions in proximal tubular epithelial cells and interstitial histiocytes. Immunohistochemistry and immunoelectron microscopy confirmed strong κ light chain restriction. These findings are highly suggestive of concurrent LCPT and crystal-storing histiocytosis (CSH). However, definitive confirmation of CSH by CD68 immunohistochemistry-the gold standard for histiocytic lineage identification-could not be performed due to exhaustion of the limited archival biopsy tissue. This case is therefore reported as "CSH suspected based on ultrastructural and immunoelectron microscopy criteria.

Intervention and outcome: The patient received first-line CyBorD (cyclophosphamide, bortezomib, and dexamethasone) chemotherapy, achieving partial hematological remission (PR) and marked improvement of Fanconi syndrome. Renal function remained normal and stable throughout the 48-month follow-up period.

Conclusions: We report this rare κ light chain MGRS case with concurrent LCPT and suspected CSH, which showed favorable responses to CyBorD with durable renal stability. This case highlights integrated diagnostic strategies and boosts clinical awareness of this rare monoclonal gammopathy-related renal injury.