Focal Light Chain Proximal Tubulopathy Complicated by Monoclonal Gammopathy of Undetermined Significance/Smoldering Multiple Myeloma Successfully Diagnosed by Immunofluorescence on Pronase-Digested Paraffin Section: Reports of Two Cases and Review of the Literature.

Light chain proximal tubulopathy (LCPT) is a rare type of paraprotein-related disease (PRDs) characterized by monoclonal free light chain (FLC) deposition in proximal tubular epithelial cells (PTECs). A diagnosis of LCPT requires identification of FLC deposition in PTECs; however, FLC luminescence defects in immunofluorescence staining using frozen tissue (IF-F), regarded as "masked LCPT", are occasionally encountered. We describe two cases of focal masked LCPT in monoclonal gammopathy of undetermined significance (MGUS) or smoldering multiple myeloma (SMM) diagnosed by IF in formalin-fixed, paraffin-embedded tissue sections following pronase digestion (IF-P) rather than by IF-F. Case 1 was a 66-year-old woman who exhibited renal dysfunction with IgG-λ monoclonal proteinemia, and Case 2 was a 69-year-old man who exhibited renal dysfunction with IgG-κ type monoclonal proteinemia. In both cases, renal pathology showed focal tubular damage consisted of swelling and desquamation of PTECs. FLC deposition in PTECs was detectable by IF-P but not by IF-F. Consequently, an appropriate diagnosis by IF-P led the patients to receive chemotherapy immediately. These two cases indicate that LCPT can be present even if tubular injury is focal and PRD is not severe. According to a literature review of 33 cases, including our 2 cases, focal LCPT complicated by MGUS/SMM is relative rare. In PRD, evaluation with IF-P is desirable for assessing LCPT when FLC deposition is undetected by IF-F despite characteristic degenerative PTECs. We consider that early and definitive diagnosis of LCPT by IF-P rather than IF-F might result in favorite outcome since physicians could smoothly decide treatment strategy.