Unusual manifestation of crystalline light chain tubulopathy in patient with multiple myeloma: case report and review of the literature

Abstract

Multiple myeloma (MM) is the second most common hematological malignancy, with an annual incidence in Europe and the USA of about 4–6 cases per 100,000. Several forms of renal disease are found in the course of MM, including: cast nephropathy, light chain (LC) deposition disease and primary amyloidosis. Less frequent forms include: acute and chronic tubulopathies, neoplastic plasma cell infiltration and interstitial nephritis. In this paper, we discuss a case of 53-year-old male patient with MM who presented with massive proteinuria (24 g/24 h), mild renal insufficiency (eGFR 43 mL/min), and Fanconi-like syndrome (as reflected by normoglycemic glycosuria). In kidney biopsy glomeruli were normal, whereas abundant AFOG-positive deposits were found in the cytoplasm of proximal tubular epithelial cells. These deposits were strongly positive for kappa light chains on immunofluorescence. Electron microscopy revealed electron-dense, intracytoplasmic crystalloid deposits of variable shape (needle-shaped, round and rectangular), and size in the proximal tubular cells. This unusual variant of microscopic renal lesions in the course of MM coupled with coincidence of Fanconi-like and nephrotic syndrome as a clinical manifestation has not been reported to date.

• Chronic kidney disease
• crystalline nephropathy
• fanconi syndrome
• multiple myeloma
• nephrotic syndrome

Introduction

Multiple myeloma (MM) is the second most common hematological malignancy with an annual incidence in Europe and the USA of about 4–6 cases per 100,000, commonly involving kidneys. It is characterized by a monoclonal proliferation of plasma cells and (in a vast majority of patients) the presence of a monoclonal (M) protein in serum and/or urine. Synthesis of monoclonal paraprotein constitutes the main pathological factor responsible for the evolution of renal disease in a course of MM.1,2 In a course of myeloma the synthesis rate of light chains (LCs) ranges between 3 and 85 g/day (as compared with approximately 0.9 g/day in physiology). It results in high concentration of LCs in glomerular filtrate that may exceed the maximal reabsorptive capacity of the proximal tubular cells. In addition, due to their altered physicochemical characteristics monoclonal LCs are nephrotoxic. A majority (approximately 70%) of pathogenic LCs is tubulopathic, and about one-third interacts with glomerular structures (glomerulopathic LCs). In some MM patients, more than one renal compartment is affected, and a mixture of different types of injury may be seen.

In a course of MM, the presence of tubulopathic monoclonal LCs is most commonly complicated by cast nephropathy (myeloma kidney), that is, typically manifested by the elevated serum creatinine and mild proteinuria. In a minority of cases, LCs may cause an acute tubular injury in a form of proximal tubulopathy or distal tubular dysfunction without cast formation nor significant tubulointerstitial inflammation, and with relatively well maintained glomerular filtration rate. In this form of renal injury, proximal tubular epithelial cells (PTEC) are more commonly affected. The reabsorption of pathogenic LCs may cause two types of injury. In most common one, the overload of the PTEC lysosomal system is followed by a release of lysosomal enzymes that causes an acute tubular necrosis. On rare occasions, the reabsorbed pathogenic LCs form intracellular crystals that impair tubular function, which apart from the rise in serum creatinine, is typically associated with the occurrence of glycosuria, aminoaciduria, hyperuricosuria, hyperphosphaturia, hypercalciuria, and type 2 renal tubular acidosis, typical constituents of an acquired Fanconi syndrome.3,4 This type of tubular damage may accompany other forms of myeloma-associated renal injury, and the clinical manifestations that predominate in those cases may be those related to the other conditions. Proteinuria, particularly a heavy one, is not regarded as typical manifestation of lesions localized predominantly in PTEC. The pure (isolated) proximal tubulopathy is considered very rare (∼1%), but due to its discrete symptomatology may be easily overlooked, since except for glucose most of mentioned substances are not routinely measured in urine.5

Herein, we report the case of MM patient presented with severe (nephrotic range) proteinuria, Fanconi syndrome and moderate renal insufficiency, whose renal biopsy revealed features of isolated crystalline tubulopathy with only minor and unspecific glomerular injury. We believe that such morphological appearance of MM-related nephropathy manifested in a form of heavy proteinuria has not been reported to date.

Case report

In November 2012, a 53-year-old man was admitted to the Department of Nephrology, Hypertension and Internal Medicine for the evaluation of proteinuria and renal function impairment diagnosed by general practitioner 4 weeks beforehand. He was in good general state, reporting recent shoulder and hip pain, right femoral bone pain, and 10 kg weight loss within a year. He presented with severe nephrotic syndrome (24 g/24 h). Interestingly, such a massive protein loss was not associated with hypoalbuminemia (his plasma albumin equaled 43.3 g/L) nor significant changes in lipid profile (suggesting rapid onset and short duration of proteinuria). Other clinical abnormalities were as follows: serum creatinine 1.8 mg/dL (to convert to μmol/L, multiply by 88.4), eGFR 43 mL/min. (using abbreviated MDRD formula), uric acid 1.6 mg/dL (normal value for local lab 3.4–7 mg/dL, to convert to μmol/L, multiply by 59.48). The patient presented also mild anemia (RBC 4.2 × 10⁶/μL, Hb 12.2 g/dL, Hct 36.6%) with marginally lowered platelet count (136 × 10³/μL). Spot urine analysis revealed proteinuria of 15.9 g/L and glycosuria (60 mg/dL, with normal blood glucose and no previous diagnosis of diabetes; to convert to mmol/L multiply by 0.055). We also measured plasma free LC, β2-microglobulin (β2M) and searched for urinary Bence–Jones protein obtaining the following results (in brackets—normal range of respective parameters in our lab): λLC 0.48 mg/L (5.71–26.3 mg/L), κLC 5990 mg/dL (3.3–19.4 mg/L), the free κ/λ ratio 12,479.17 (0.26–1.65) and β2M 7.64 mg/L (0.8–2.2 mg/L). Total protein was normal—74 g/L. Plasma protein electrophoresis showed hypogammaglobulinemia (5.1%) with a peak within the β2 globulin zone. Plasma IgM and IgG were unremarkable, but there was a significant increase in IgA concentration (1352 mg/dL; reference value: 70–400 mg/dL). X-ray revealed lytic lesions located in the mandible, right femur and hip bone; serum calcium and phosphate as well as alkaline phosphatase activity were within normal range. Abdominal ultrasound was described as normal (including size, shape and the morphology of both kidneys).

On the third day after admission, an ultrasound-guided kidney biopsy was performed. Tissue processing for light and electron microscopy was carried out in a routine manner. The immunomorphological studies for IgG, IgA, IgM, C3, C1q, fibrinogen and λLC, κLC were performed on the paraffin-embedded tissue. The light microscopy appearance was dominated by the presence of eosinophilic, orange in AFOG staining, weakly PAS—positive, and silver—negative deposits localized in the cytoplasm of PTEC (Figures 1 and 2). These deposits turned out to be strongly and solely positive for κLC on immunofluorescence (Figure 3). In the interstitium, there were few scattered small foci of mononuclear inflammation. There was neither interstitial fibrosis nor appreciable tubular atrophy. The morphology of all 17 glomeruli present in the sample was normal (Figure 4). Electron microscopy (EM) revealed electron-dense, intracytoplasmic crystalloid deposits of variable shape (needle-shaped, round and rectangular) and size ranging from 0.3 × 0.7–2 × 6 μm³ in the proximal tubular cells (Figure 5a and b). The ultrastructural examination revealed some mild, unspecific glomerular lesions in a form of segmental collapse, and GBM wrinkling, but no dense deposits were seen, and the podocyte foot processes were preserved. According to these findings, diagnosis of proximal crystalline tubulopathy with κLC restriction was made. That recognition was consistent with clinical signs of Fanconi-like syndrome, that is, normoglycemic glycosuria and low serum uric acid.

Figure 1. Eosinophilic, crystalloid deposits localized in the cytoplasm of PTEC (HE staining, original magnification 200×).

Figure 2. Orange, crystalloid deposits localized in the cytoplasm of PTEC (AFOG staining, original magnification 200×).

Figure 3. κLC deposits identified in PTEC (original magnification 200×).

Figure 4. Apparently normal glomerulus, with mild tuft hypertrophy (AFOG staining, original magnification 200×).

Figure 5. (a) and (b) Electron microscopy (EM) showing electron-dense, intracytoplasmic round and rectangular crystalloid deposits of variable size in the proximal tubular cells.

Bone marrow biopsy showed massive plasmocytosis (85%) consisted with a diagnosis of IgA kappa MM (grade IIIA according to International Staging System). At the time of present report, the patient is doing well, with serum creatinine of 1.4 mg/dL, uric acid 1.4 mg/dL, IgA 617 mg/dL, λLC 1.23 mg/L, κLC 3370 mg/dL, β2M 6.04 mg/L and spectacular reduction in proteinuria (to 0.8 g/L) after two cycles of CTD (cyclophosphamide, thalidomide, dexamethasone) chemotherapy with pamidronate.

Discussion

Renal involvement is a common and serious complication in the course of MM. In most cases, renal lesions are paraprotein-related.1,2 Type of renal pathology largely depends on the amount of LCs filtered through the filtration membrane (LC load), type of LC (λ vs. κ), their molecular size and electrical charge, interaction with other urine contents (i.e., calcium, Tamm–Horsfall protein) or certain receptors (hypothetic LC receptors, megalin, cubilin, etc.). Depending on these factors LCs may be deposited within glomerular structures, form amyloid fibers, interact with proximal tubules or lead to cast formation within distal nephron. Sometimes two or more renal abnormalities co-exist.6–8 LC accessing proximal tubule may be reabsorbed in excess by PTEC—their uptake is thought to be mediated by non-specific receptors localized on apical surfaces of PTEC, namely cubilin and megalin. Both λLC and κLC are found within PTEC deposits, although crystalline-like deposits almost exclusively are formed by κ chains (certain subtypes, such as subtype Vκ1 LC have been identified as having particular properties to form crystals).5,9 It has been postulated, that the mutations of the gene encoding CDR-L1 (complementarity-determining region L1 of LCs, contributing to formation of immunoglobulin antigen-binding site) may facilitate resistance to proteases, causing the accumulation of LC in PTEC lysosomes where they serve as a nidus for crystal formation. Such a resistance to proteolysis seems to be a peculiar characteristic of LC involved in the pathophysiology of Fanconi syndrome, since it was never documented in cast nephropathy patients or myeloma patients without renal involvement.10 In a case discussed morphological criteria of crystalline tubulopathy and certain clinical criteria of Fanconi-like syndrome (i.e., normoglycemic glycosuria, very low serum uric acid and low phosphate levels, which may indirectly suggest hyperuricosuria and hyperphosphaturia) were fulfilled.

Plasma cell dyscrasia-related crystalline tubulopathy is well-described in the medical literature, although almost in a form of single case reports or case series. Larsen et al. gathered own observations of three patients with LC proximal crystalline tubulopathy and analyzed more than 50 cases that were reported in literature before December 2010. Although additional cases were described and published after this date, crystalline tubulopathy is still considered rare.9 Larsen’s analysis provided more detailed insight into the epidemiology of this morphological entity from the perspective of a large nephropathology department: LC proximal tubulopathy was found in 13 of 322 biopsies taken from patients with LC deposition disorders (4%), but only in 3 cases (i.e., 0.93%) LC deposits in PTEC had crystalline appearance.9 It seems very probable that crystalline tubulopathy is being overlooked by pathologists since light microscopy findings are often very subtle and the visualization of crystallized LC by direct immunofluorescence is commonly obscured due to their relative inaccessibility to antibody binding. This later obstacle may be overcome with a immunofluorescence performed on formalin-fixed, paraffin-embedded sections preprocessed by enzymatic digestion.5,9 From the clinical perspective crystalline, tubulopathy may have various presentation forms, including Fanconi syndrome, incomplete Fanconi syndrome, and renal failure. Proteinuria, if present is most commonly mild, well below nephrotic range with the exception of cases in which crystalline tubulopathy is accompanied by other forms of kidney injury such as amyloidosis, or MIDD.5,9,11,12 In some crystalline tubulopathy cases, no appreciable disturbances in renal function were found.

In a majority of available reports, crystalline tubulopathy was associated with a distinct type of plasma cell disorder characterized by a slow progression of the tumor, usually monoclonal gammopathy of undetermined significance or low-mass (typically kappa LC-excreting) myeloma, although cases of high-mass myeloma, and even lymphocytic leukemia and B-cell lymphoma were also documented. In many of reported cases, the recognition of this morphological entity preceded the evolution of overt hematological disorder by several years,4,12 although a reversed sequence with bone marrow pathology being identified long before kidney biopsy performance has also been described.8

Apart from PTEC crystalline inclusions may also be found in other cell types, such as podocytes or renal histiocytes.5,11,13 Interestingly, in the course of plasma cell dyscrasias LC crystal depositions were also identified extrarenally, namely in marrow plasma cells, within eyelid and orbital masses, pleural and peritoneal surface cells, gallbladder surface columnar cells, thyroid follicular cells and neck mass.14,15 Crystal’s deposition in renal tissue is a phenomenon not specific to monoclonal gammopathies. It may accompany disorders associated with abnormal calcium and/or phosphate metabolism and enhanced excretion of calcium phosphate and calcium oxalate. It may also be seen in a course of several other metabolic disorders such as urate nephropathy, cystinosis, and 2,8-dihydroxyadenuria. The formation of intratubular crystals may also be drug-induced: it has been described as a side effect of the treatment with sulfadiazine, acyclovir, and indinavir.5,12

The main difference between our case and other reports on isolated crystalline tubulopathy is the presence of extremely high protein loss. Unfortunately, we are not able to discriminate the role of proximal tubules in our patient’s proteinuria. It is, however, tempting to speculate that PTEC damage contributed significantly to proteinuria, given the relatively well preserved structure of glomerulus on light microscopy and foot processes in EM. Saturation of cubilin- and megalin-mediated transport could be one of the mechanisms underlying proteinuria. Larsen et al. also reported LC proximal tubulopathy (with or without crystal formation) with unremarkable glomerular appearance (including lack of foot process effacement), except for one case in which coexistence with membranous nephropathy was found.9 In some available reports on MM-related proximal tubulopathy glomeruli were described as normal or were not mentioned, in others concomitant crystalloid deposits were also reported within podocytes.11

To the authors’ best knowledge, the unusual variant of microscopic renal lesions in the course of MM coupled clinically with coincidence of Fanconi-like and nephrotic syndrome has not been reported to date and deserves attention. Although there is a strict and undisputable relationship between tubular lesions and signs of Fanconi-like syndrome, we remain uncertain on the contribution of glomeruli and tubules to observed massive proteinuria.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.