LAUSR

Dear Editor, A 77-year-old man was referred to our department for a recent deterioration of renal function. Relevant medical history included hypertension, type 2 diabetes, and calreticulinmutated essential thrombocythemia with myelofibrosis. A monoclonal IgM spike, considered to correspond to a monoclonal gammopathy of undetermined significance (MGUS), had been diagnosed 25 years earlier. Physical examination revealed lower limb edema and isolated splenomegaly without enlarged palpable lymph nodes or peripheral neuropathy. Laboratory investigations revealed an impairment of renal function (creatinine = 170 μmol/L versus 115 μmol/L 2 years ago) associated with marked proteinuria (3.2 g/day) and normal albumin concentration. The patient also exhibited anemia (9.8 g/dl) and thrombocytosis (720,000/mm) but electrolyte concentrations were normal. Immunoelectrophoresis analysis demonstrated the presence of a circulating monoclonal IgMκ (4 g/ L). Immunonephelometric assays confirmed the presence of free light-chain kappa at a concentration of 41.7 mg/L with a κ/λ ratio of 1.45. Serum complement levels were within the normal range and no cryoglobulinemia was detected. Urine immunoelectrophoresis was positive for kappa light chain and albuminuria. A renal biopsy demonstrated diabetic glomerulopathy and focal and segmental glomerular sclerosis lesion (not shown), which could be responsible for the marked proteinuria. A striking interstitial infiltration of voluminous, round cells with inconspicuous eccentric nucleus and polychromatophilic cytoplasm on Masson’s trichrome staining was highly suggestive of the presence of Mott cells in the renal parenchyma (Fig. 1(a–b)). They were CD79a-positive, reflecting their lymphoplasmacytic lineage (Fig. 1(c)). Immunofluorescence assays revealed positive staining for the μ chain with κ light chain restriction (Fig.1(d)) in the Mott cells, demonstrating the neoplastic nature of these cells. Ultrastructural analysis showed Russel bodies, corresponding to cytoplasmic immunoglobulins inclusions without crystal formation (Fig.1(e)), and immunogold staining confirmed κ light chain restriction (Fig.1(f)). Computed tomography detected sub-centimeter lymphadenopathy and trephine biopsy of the bone marrow showed lymphoplasmacyt ic lymphoma. As Waldenst röm macroglobulinaemia (WM)-related nephropathy is an established indication for initiation of therapy [1], specific anti-Bruton tyrosine kinase treatment (Ibrutinib) was initiated. * Anissa Moktefi [email protected]