Protease resistance and binding of Ig light chains in myeloma-associated tubulopathies

Light chain proximal tubulopathy without crystals in a case of Burkitt lymphoma presenting with acute kidney injury

Acute kidney injury is a common manifestation of malignancies, either directly by the tumor or indirectly from anticancer therapy. The pathologic correlates of acute kidney injury in malignancies are many and can be diagnosed conclusively using kidney biopsy. We report a rare case of a patient with light chain proximal tubulopathy without crystals who presented with acute kidney injury. Kidney biopsy showed proximal tubulopathy without crystals with λ light chain restriction. Detailed investigations lead to a diagnosis of Burkitt lymphoma with surface λ light chain restriction.

Aggregates, crystals, gels, and amyloids: intracellular and extracellular phenotypes at the crossroads of immunoglobulin physicochemical property and cell physiology

Recombinant immunoglobulins comprise an important class of human therapeutics. Although specific immunoglobulins can be purposefully raised against desired antigen targets by various methods, identifying an immunoglobulin clone that simultaneously possesses potent therapeutic activities and desirable manufacturing-related attributes often turns out to be challenging. The variable domains of individual immunoglobulins primarily define the unique antigen specificities and binding affinities inherent to each clone. The primary sequence of the variable domains also specifies the unique physicochemical properties that modulate various aspects of individual immunoglobulin life cycle, starting from the biosynthetic steps in the endoplasmic reticulum, secretory pathway trafficking, secretion, and the fate in the extracellular space and in the endosome-lysosome system. Because of the diverse repertoire of immunoglobulin physicochemical properties, some immunoglobulin clones' intrinsic properties may manifest as intriguing cellular phenotypes, unusual solution behaviors, and serious pathologic outcomes that are of scientific and clinical importance. To gain renewed insights into identifying manufacturable therapeutic antibodies, this paper catalogs important intracellular and extracellular phenotypes induced by various subsets of immunoglobulin clones occupying different niches of diverse physicochemical repertoire space. Both intrinsic and extrinsic factors that make certain immunoglobulin clones desirable or undesirable for large-scale manufacturing and therapeutic use are summarized.

Specific impairment of proximal tubular cell proliferation by a monoclonal κ light chain responsible for Fanconi syndrome

BACKGROUND

Fanconi syndrome (FS) is a rare renal disorder featuring proximal tubule dysfunction that may occur following tubular reabsorption of a monoclonal light chain (LC), in patients with multiple myeloma. FS may precede the recognition of multiple myeloma by several years. In most cases, crystalline inclusions of monoclonal κ LCs are observed within the lysosomes of proximal tubular cells (PTCs) and probably participate in their functional alteration.

METHODS

To investigate the mechanism implicated in proximal tubule dysfunction, we compared the effects of κ LC-CHEB obtained from a patient with myeloma-associated FS to those of control κ LC-BON obtained from a patient without evidence of FS, on the viability and proliferation of two different PTC lines.

RESULTS

Our data suggest that the tubular atrophy in myeloma-associated FS does not result from increased apoptosis of PTCs, but from their impaired capacity to proliferate and renew. Indeed, in vitro incubation of cultured PTCs with physiological amounts of the nephrotoxic κ LC-CHEB was sufficient to cause a depression in DNA synthesis and in cell proliferation. This effect was observed neither with control κ LC-BON nor in the absence of κ LC.

CONCLUSIONS

The reduced turnover of PTCs may affect tubular repair and regeneration. In addition, the reduced proliferation of myeloma cells producing the same monoclonal κ LC might explain the frequent association of FS with smoldering multiple myeloma.

Crystalline nephropathies

CONTEXT

The kidney is a favored site for crystal deposition because of the high concentration of ions and molecules reached at the level of the renal tubules in the course of filtration. This review focuses on crystalline nephropathies in 4 broad categories: (1) dysproteinemia- associated, (2) drug-induced, (3) calcium-containing, and (4) metabolic or genetic.

OBJECTIVE

To provide a framework for accurate identification of the diverse types of crystals encountered in the kidney in order to formulate an appropriate differential diagnosis and guide additional testing and treatment.

DATA SOURCES

Review of pertinent published literature along with practical experience gained in a high-volume renal pathology laboratory.

CONCLUSIONS

Accurate identification of crystals encountered in the kidney is essential in detecting conditions ranging from hematologic malignancy to drug toxicity to metabolic disorders. Detailed clinical-pathologic correlation is needed to accurately diagnose the underlying cause of most crystalline nephropathies.

Light Chain Proximal Tubulopathy: Expanding the Pathologic Spectrum with and without Deposition of Crystalline Inclusions

Light chain proximal tubulopathy (LCPT) is an uncommon form of renal disease associated with dysproteinemias. It is characterized by intracytoplasmic deposition of crystallized mostly kappa monoclonal light chains in proximal tubules (PTs). Crystals are located within lysosomes by electron microscopy (EM). Rare lambda LCPT cases without crystals by EM were described. Retrospectively, we reviewed clinical, light microscopic (LM), immunofluorescence (IF), and EM findings in 9 cases) (8 males, 1 female; mean age 57 years (38–81)) with multiple myeloma. LM showed abundant cytoplasmic droplets in PT cells in all cases. Droplets were also present in the podocytes, endothelial and parietal cells in one case. IF revealed staining of crystals with kappa in 3 and lambda in 6. EM showed electron dense rectangular, rhomboid, or needle shaped crystals in PT cells in 3 cases (33%), one of which had crystals in podocytes and interstitial cells. Six lambda LCPT cases showed no crystals by EM (67%). This may reflect differences in the physicochemical properties of light chains. The mechanisms of crystal accumulation in these cells and the significance of this finding are unknown.

Renal impairment in multiple myeloma: presenting features in different departments

In order to analyze the cai]ses of delayed diagnosis and raise the level of early diagnosis of atypical multiple myeloma (MM), the differences of presenting features between the patients presented to nephrologists and those presented to hematologists were compared. MM patients in our hospital were studied retrospectively. Those who referred renal impairment were divided into two groups: group I presented to nephrologists prior to MM diagnosis (n=29) and group II presented to hematologists directly (n=62). The age, sex, initial symptoms, haematological and biochemical parameters, the phenotype of paraprotein, bone marrow biopsy and cytology were undertaken and analyzed. The results showed that the median time between the initial symptoms and diagnosis in the patients of group I was longer than that in group II (P<0.001); patients in group I had significantly lower incidence of bone pain (P<0.01) and worse renal function (P<0.05) on presentation. There were lower level of myeloma cells (P<0.05), lower incidence of hypergammaglobulinemia (P<0.05), lower positive rate of monoclonal immunoglobulin (M protein) (P<0.05) and M protein level (P<0.05) in the patients of group I than those in group II. The ratio of monoclonal to lambda monoclonal proteins in a population was 1:3.67 in patients of group I, whereas 1:0.90 in patients of group II (P<0.01). Moreover, patients with λ type had a higher degree of renal insufficiency than those with κ type (P<0.05). It was suggested that the median time between the initial symptoms and diagnosis in the patients presented to nephrologists was longer than that in those presented to hematologists; the patients presented to nephrologists had the lower incidence of bone pain, lower level of myeloma cells and M protein, which made early diagnosis more difficult; more patients presented to nephrologists had the majority of λ light chain type, moreover, patients with λ light chain type had a higher incidence of renal insufficiency.

The morphologic spectrum and clinical significance of light chain proximal tubulopathy with and without crystal formation

The renal diseases most frequently associated with myeloma include amyloidosis, monoclonal immunoglobulin deposition disease, and cast nephropathy. Less frequently reported is light chain proximal tubulopathy, a disease characterized by κ-restricted crystal deposits in the proximal tubule cytoplasm. Light chain proximal tubulopathy without crystal deposition is only loosely related to the typical light chain proximal tubulopathy, and little is known about this entity. A search was performed of the 10 081 native kidney biopsy samples processed by our laboratory over the past 2 years for cases that had light chain restriction limited to the proximal tubule cytoplasm. A total of 10 cases of light chain proximal tubulopathy without crystal deposition were found representing 3.1% of light chain-related diseases. Nine of these 10 showed λ-light chain restriction. Only three cases of light chain proximal tubulopathy with crystals were found accounting for 0.9% of light chain-related diseases. Two of these three were κ subtype. Plasma cell dyscrasia was unsuspected in seven of the 10 patients with light chain proximal tubulopathy without crystals at the time of renal biopsy. After the biopsy was reported, follow-up was available on 9/10 patients with 9/9 showing a plasma cell dyscrasia including 8/9 with multiple myeloma. We found that light chain proximal tubulopathy without crystal formation, despite being rarely described in the literature, is over three times more common than light chain proximal tubulopathy with crystal formation in our series. And given that it is often associated with previously unrecognized myeloma, it is a critically important diagnosis.

Ig-related renal disease in lymphoplasmacytic disorders: an update

Ig-related renal diseases occurring in lymphoplasmacytic disorders (LPD) cover a wide spectrum of renal lesions. Except for cast nephropathy, which is almost specific for multiple myeloma, similar renal lesions caused by deposition or precipitation of monoclonal Ig-related material may occur in the various types of LPD. Because the secreted Ig provides the link between the LPD and the kidney disease, the renal outcome is linked to efficacy of chemotherapy. In the past 10 years, considerable advances have occurred in chemotherapy regimens with the advent of new classes of drugs, which already result in markedly improved renal and vital survival.

Clinical approach to renal tubular acidosis in adult patients

Renal tubular acidosis (RTA) is a group of disorders observed in patients with normal anion gap metabolic acidosis. There are three major forms of RTA: A proximal (type II) RTA and two types of distal RTAs (type I and type IV). Proximal (type II) RTA originates from the inability to reabsorb bicarbonate normally in the proximal tubule. Type I RTA is associated with inability to excrete the daily acid load and may present with hyperkalaemia or hypokalaemia. The most prominent abnormality in type IV RTA is hyperkalaemia caused by hypoaldosteronism. This article extensively reviews the mechanism of hydrogen ion generation from metabolism of normal diet and various forms of RTA leading to disruptions of normal acid-base handling by the kidneys.

Plasma Cell Dyscrasia with Kappa Light-chain Crystals in Proximal Tubular Cells: A Histological, Immunofluorescent, and Ultrastructural Study

This is a case report of a 56-year-old man with plasma cell dyscrasia who presented with proximal tubulopathy manifested as kappa light-chain crystal deposition in the proximal convoluted tubular cells. This was associated with mild cellular damage. The crystals were seen as negative images with the hematoxylin-eosin and periodic acid-Schiff (PAS) stains. They were identified as kappa light-chains by immunofluorescent imaging and confirmed by immunoelectron microscopy. Ultrastructurally, the crystals appear to be located within lysosomes. No deposits of light-chains were seen elsewhere in the kidney biopsy.

HEMATOLOGY: ISSUES IN THE DIALYSIS PATIENT: Multiple Myeloma in End-Stage Renal Disease

Multiple myeloma is a common cause of chronic kidney disease (CKD). Patients with myeloma-related kidney disease but low levels of serum monoclonal proteins can be diagnosed with symptomatic myeloma in a simplified diagnostic classification. The presence and type of renal disease in myeloma is dependent on the light chain secreted. Treatment has recently changed and now includes the use of thalidomide and bisphosphonates. Thalidomide can cause hyperkalemia and bisphosphonates can cause renal failure in patients with CKD. Their use is not contraindicated, but they should be used with caution. High-dose melphalan with an autologous stem cell transplant is now the standard of care and should not be withheld from patients with CKD, even those on dialysis. This treatment can improve the renal disease, and this is more likely if treatment is started early. In patients with persistent dialysis dependence, renal transplantation can be performed if the patient has a complete remission.

Dangerous small B-cell clones

The detection of a monoclonal immunoglobulin in serum or urine usually raises concerns about the size of the underlying B-cell-derived clone and possible systemic effects caused by its expansion. However, a small clone can synthesize a very toxic protein, producing devastating systemic damage and protean clinical presentations. The resulting "monoclonal component-related diseases," although difficult to diagnose, may be progressive and even fatal. The monoclonal protein can aggregate and deposit systemically as occurs in light-chain amyloidosis, monoclonal immunoglobulin deposition disease, crystal-storing histiocytosis, and monoclonal cryoglobulinemia. Alternatively, some monoclonal proteins possess antibody activity toward autogenous antigens and cause chronic cold agglutinin disease, mixed cryoglobulinemia, and peripheral neuropathies. Other humoral mediators may contribute to neuropathy in variant disorders such as the POEMS (polyneuropathy, organomegaly, endocrinopathy, M protein, and skin changes) syndrome. The clone synthesizing the noxious monoclonal proteins is often small, and sensitive techniques may be required to detect these immunoglobulins. A delay in diagnosis can allow irreversible organ damage and dramatically shorten survival. Prompt recognition of suggestive signs and symptoms should trigger a thorough diagnostic approach to reach the correct diagnosis quickly, because this is the key to effective therapy. Although the treatment of these conditions is not optimal, significant advances have been made, improving the duration and quality of life.

Fanconi’s syndrome induced by a monoclonal Vκ3 light chain in Waldenström’s macroglobulinemia

Fanconi's syndrome (FS) is a disorder of sodium-dependent proximal tubule reabsorption, which may complicate plasma cell disorders producing a free monoclonal light chain (LC). FS often occurs in the setting of smoldering myeloma and features cytoplasmic crystalline inclusions of monoclonal kappa LC in proximal tubular cells and malignant plasma cells. Although the clinical and pathological presentation may vary, including lack of crystal formation, monoclonal kappa LCs that underlie FS show a striking genetic and biochemical homogeneity: they almost always belong to the Vkappa1 subgroup of variability and originate from 2 germline genes, O2/O12 or O8/O18. Their variable domain sequences present unusual hydrophobic residues, responsible for the resistance to proteolysis, which leads to LC accumulation in the endocytic compartment of proximal tubule cells. We report a patient with slowly progressive Waldenstrom's macroglobulinemia and full-blown FS with accumulation of a monoclonal kappa LC within proximal tubules, but no detectable crystalline organization. This LC, which belonged to the unusual Vkappa3 subgroup and derived from the L2/L16 germline gene, showed no common substitution with previously described FS kappaI LC and was sensitive to trypsin digestion. These data show that molecular and biochemical characteristics of kappa LCs in patients with FS are more heterogeneous than initially suspected. Mechanisms other than resistance of LCs to endosomal proteolysis probably are involved in the pathogenesis of FS-associated plasma cell dyscrasias.

Crystalloid deposits in the kidney. Case Report

Light chain-producing lymphoproliferative disorders such as multiple myeloma are frequently complicated by renal impairment. Typically, the renal biopsy of a patient with renal failure caused by multiple myeloma shows cast nephropathy, but occasionally crystals may be seen. We describe the case of a patient with acute renal failure caused by multiple myeloma in which, on renal and bone marrow biopsy, there were widespread crystalloid deposits. Crystalloid nephropathy is a very rare condition associated with multiple myeloma and other light chain-secreting disorders. An underlying lymphoproliferative disorder should be considered in the differential diagnosis if crystalloid deposits are seen on a renal or other tissue biopsy.

Acquired hypophosphatemic osteomalacia associated with multiple myeloma

Hypophosphatemic osteomalacia is a rare but important complication of multiple myeloma. In these cases, the pathophysiology of the phosphate renal wasting notably differs from oncogenic osteomalacia and is due to light-chain nephropathy, resulting in proximal tubular dysfunction which is not restricted to phosphate handling. These patients seems to have a distinct type of plasma cell disorder characterized by a slow progression of the tumor and by an early phase dominated by the metabolic complications of the renal proximal tubular dysfunction. For this reason hypophosphatemic osteomalacia is the presenting feature that leads to the diagnosis of multiple myeloma in most of these patients. Recognition of this complication is important, since supportive treatment with phosphate supplements and calcitriol may substantially alleviate pain and weakness associated with hypophosphatemia.

Dysproteinemia and the kidney

Dysproteinemia is a clinical state characterized by abnormal, often excessive, synthesis of immunoglobulin (Ig) molecules or subunits. Dysproteinemia results from clonal proliferation of plasma cells or B lymphocytes. The abnormal circulating Ig molecules or subunits (most commonly free light chains) reach the glomerulus via the systemic circulation and are associated with the development of a variety of pathologic lesions within the kidney. Free light chain molecules may pass through the glomerular basement membrane and form casts within distal tubular lumina (myeloma cast nephropathy) or form crystals within the cytoplasm of proximal tubules (light chain Fanconi syndrome). Alternatively, Ig molecules or subunits may form paraprotein tissue deposits and produce an array of pathologic lesions, most commonly amyloidosis and monoclonal Ig deposition disease. The pattern of renal parenchymal disease is determined by the unique properties of the Ig molecule or subunit. Each of the patterns of renal disease is in turn associated with unique, but frequently overlapping, clinical features and outcomes. This review emphasizes the pathologic, clinical, and prognostic differences among the patterns of renal parenchymal disease related to dysproteinemia.

Tamm-Horsfall glycoprotein: biology and clinical relevance

Tamm-Horsfall glycoprotein (THP) is the most abundant urinary protein in mammals. Urinary excretion occurs by proteolytic cleavage of the large ectodomain of the glycosyl phosphatidylinositol-anchored counterpart exposed at the luminal cell surface of the thick ascending limb of Henle's loop. We describe the physical-chemical structure of human THP and its biosynthesis and interaction with other proteins and leukocytes. The clinical relevance of THP reported here includes: (1) involvement in the pathogenesis of cast nephropathy, urolithiasis, and tubulointerstitial nephritis; (2) abnormalities in urinary excretion in renal diseases; and (3) the recent finding that familial juvenile hyperuricemic nephropathy and autosomal dominant medullary cystic kidney disease 2 arise from mutations of the THP gene. We critically examine the literature on the physiological role and mechanism(s) that promote urinary excretion of THP. Some lines of research deal with the in vitro immunoregulatory activity of THP, termed uromodulin when isolated from urine of pregnant women. However, an immunoregulatory function in vivo has not yet been established. In the most recent literature, there is renewed interest in the capacity of urinary THP to compete efficiently with urothelial cell receptors, such as uroplakins, in adhering to type 1 fimbriated Escherichia coli. This property supports the notion that abundant THP excretion in urine is promoted in the host by selective pressure to obtain an efficient defense against urinary tract infections caused by uropathogenic bacteria.

Light chain crystal deposition as a manifestation of plasma cell dyscrasias: the role of immunoelectron microscopy

Light chain crystal deposition disease is a rare and poorly characterized entity that can be confused with a number of different conditions, depending on where the disease process is manifested. The present study explored the role of ultrastructural immunogold labeling in the diagnosis of this condition. Seven cases of light chain crystal deposition (kappa light chain-related) are reported. Immunohistochemistry and immunofluorescence techniques play a rather limited role in the evaluation of these cases, as a result of the inability to detect monoclonal kappa light chains in association with the crystalline structures or high background staining. Ultrastructural labeling is the method of choice to fully characterize these cases. However, surgical pathologists must learn to recognize the findings associated with this condition to avoid misdiagnosis. If the diagnosis is at least suspected, then a complete hematologic workup may identify the underlying plasma cell dyscrasia. It must be emphasized that in some patients the plasma cell dyscrasia does not become clinically manifested until years after the diagnosis of light chain crystal deposition.

Generalized crystal-storing histiocytosis associated with monoclonal gammopathy: molecular analysis of a disorder with rapid clinical course and review of the literature

Crystal-storing histiocytosis (CSH) is a rare event in disorders associated with monoclonal gammopathy. The intracellular crystal formation is almost always accompanied by the expression of κ light chains. However, the exact mechanism for the storage has not been clarified until now. We report a case of generalized CSH in a 73-year-old man who presented with IgA κ paraproteinemia and paraproteinuria. The initially observed CSH in the bone marrow biopsy was associated with the clinical and pathomorphologic features of a monoclonal gammopathy of undetermined significance. The progression of disease could not be affected by steroid therapy and the patient died of septic shock 7 months after detection of CSH. At the time of autopsy there was evidence for multiple myeloma and generalized CSH. Two-dimensional gel electrophoresis of liver tissue combined with immunoblotting revealed the massive storage of heavy chains of α type and light chains of κ type, each in a monoclonal pattern. Analysis of the stored κ light chain by nanoelectrospray-ionization mass spectrometry indicated that it belongs to the variable κI variability subgroup. We identified some unusual amino acid substitutions including Leu59, usually important for hydrophobic interactions within a protein, at a position where it has never been previously described in plasma cell disorders. In conclusion, we present the first case of CSH with molecular identification of the stored κ subgroup and detection of unusual amino acid substitutions. Our results suggest that conformational alterations induced by amino acid exchanges represent a crucial pathogenic factor in CSH.

Crystal-storing histiocytosis involving the kidney in a low-grade B-cell lymphoproliferative disorder

Intracellular crystal formation in histiocytes in multiple myeloma and other lymphoproliferative disorders is an uncommon presentation. In all cases, crystal formation is associated with accumulation of histiocytes containing light chain or immunoglobulin inclusions, and the disorder has been termed crystal-storing histiocytosis. We report a case of crystal-storing histiocytosis affecting the kidney with prominent infiltration of the mesangium by large mononuclear and multinuclear cells that contained eosinophilic crystalline material in the setting of a low-grade B-cell lymphoproliferative disorder. The interstitium did not contain similar crystal-containing histiocytes. On electron microscopy, the mononuclear and multinuclear cells in the mesangium were filled with rhomboid and needle-shaped crystals. Immunofluorescence studies showed the material to be positive for lambda light chains but negative for kappa light chains. To our knowledge, this is the first report of crystal-storing histiocytosis involving the kidney in a low-grade B-cell lymphoproliferative disorder with an immunophenotype of a marginal zone lymphoma and of exclusive expansion of the mesangium by infiltrating histiocytes containing needle-shaped and rhomboid crystals that were positive for lambda light chains. We report this case to illustrate an unusual finding of mesangial infiltration by crystal-storing histiocytes, and we review the literature of renal involvement by crystal-storing histiocytosis and crystal deposition in lymphoproliferative disorders.

Malignancy and renal disease

A variety of renal diseases and electrolyte disorders may be associated with various malignancies or with treatment of malignancy with chemotherapeutic drugs or radiation. This article reviews renal disease in cancer patients, which constitutes a major source of morbidity and mortality.

Adult Fanconi syndrome secondary to light chain gammopathy. Clinicopathologic heterogeneity and unusual features in 11 patients

Fifty-seven cases of Ig light chain-associated Fanconi syndrome (FS) have been reported so far, mostly as isolated reports. The pioneering work by Maldonado and associates (35), who reviewed the first 17 cases in 1975, led to the unifying concept that patients with FS and Bence Jones proteinuria have a special form of plasma cell dyscrasia characterized by slow progression of the tumor and by prominent crystal formation in proximal tubule cells, in the absence of myeloma casts in the distal tubule. We carefully reappraised these characteristics in a series of 11 patients. Ten renal biopsy specimens were available for electron microscopy, adding to the 15 previously reported cases with ultrastructural studies. Moreover, 10 of the kappa light chains could be entirely or partially sequenced and tested for their resistance to cathepsin B, a lysosomal protease present in proximal tubule cells. Our series showed an unexpected clinicopathologic heterogeneity. Seven patients presented with the typical clinical and pathologic features of FS and low-mass myeloma or monoclonal gammopathy of undetermined significance (MGUS), in keeping with Maldonado et al's description. Crystals in bone marrow cells were detected in patients of this group, only. Three patients who presented with full-blown FS exhibited, however, the characteristic features of myeloma cast nephropathy in the setting of high-mass myeloma. One patient of this group also had numerous crystals in proximal tubule cells. The eleventh patient had complete FS with MGUS, but no crystals in proximal tubule cells even after electron microscopy. Contrasting with the clinicopathologic heterogeneity, genetic and biochemical analyses of the light chains showed a striking homogeneity. First, they all were of the kappa type. Second, 8 of 9 belonged to the V kappa I variability subgroup, which indicates that FS light chains are related by the sequence of their variable regions. Third, the 8 V kappa I light chain sequences most likely originated from only 2 germline genes, LCO2/012 and LCO8/018. Fourth, all 5 LCO2/012-derived sequences presented an unusual hydrophobic or nonpolar residue at position 30. These sequence peculiarities may account for unusual physicochemical properties of the light chains including the resistance of their variable domain V kappa to proteolysis by cathepsin B, observed in 7 of 9 patients in our series, while light chains isolated from patients with myeloma cast nephropathy are completely digested. Resistance of V kappa to proteolysis in FS patients can explain the accumulation of the light chain in the endocytotic compartment of the proximal tubule cells, leading to impairment of proximal tubule functions.

Acquired Fanconi's syndrome associated with monoclonal gammopathies

Franconi's syndrome is a rare complication of the monoclonal gammopathies characterized by diffuse failure in reabsorption at the level of the proximal renal tubule resulting in glycosuria, generalized aminoaciduria, and hypophosphatemia. Current evidence suggests monoclonal light chains that are incompletely digested in renal tubule lysosomes cause renal injury. The light-chain fragments serve as a nidus for crystal formation, and the crystals interfere with a broad range of apical membrane transporters. Franconi's syndrome is predominantly associated with monoclonal kappa in the urine, but rare instances of FS caused by lambda light chains have been described. Overt hematologic malignancies such as multiple myeloma, Waldenström's macroglobulinemia, or other lymphoproliferative disorders occur in one third of patients. Clinical manifestations include slowly progressive renal failure and bone pain caused by osteomalacia. The osteomalacia is caused by chronic hypophosphatemia and may be exacerbated by secondary hyperparathyroidism and renal tubular acidosis. Treatment consists of supplementation with phosphorus, calcium, and vitamin D. The osteomalacia is often completely reversible with mineral supplementation. Patients with an associated symptomatic malignancy or rapidly progressive renal failure may also benefit from chemotherapy. The prognosis is good in the absence of overt malignant disease.

Mutational Analysis in Murine Models for Myeloma-Associated Fanconi’s Syndrome or Cast Myeloma Nephropathy

We have designed an in vivo model in which murine hybridoma cell clones producing human Ig light chains (LC) are administred to mice. Depending on which monoclonal LC is expressed, this model mimicks either cast myeloma nephropathy or the pathological condition defined as myeloma-associated Fanconi’s syndrome (FS) with LC crystallization. Morphological alterations of the kidney cells are thus obtained in mice. All studied LC are closely related human monoclonal VκI proteins, which differ by a limited number of substitutions within the variable region. In the case of an FS monoclonal LC, we show that limited changes introduced through site-directed mutagenesis in the variable domain may suppress formation of intracellular crystals within tubular cells. We also show that multiple peculiarities of the variable region are simultaneously needed to allow LC crystallization; this property thus likely results from a unique LC tridimensional conformation imposed by concomitant somatic mutations of a specific germinally encoded framework.

Megalin knockout mice as an animal model of low molecular weight proteinuria

Megalin is an endocytic receptor expressed on the luminal surface of the renal proximal tubules. The receptor is believed to play an important role in the tubular uptake of macromolecules filtered through the glomerulus. To elucidate the role of megalin in vivo and to identify its endogenous ligands, we analyzed the proximal tubular function in mice genetically deficient for the receptor. We demonstrate that megalin-deficient mice exhibit a tubular resorption deficiency and excrete low molecular weight plasma proteins in the urine (low molecular weight proteinuria). Proteins excreted include small plasma proteins that carry lipophilic compounds including vitamin D-binding protein, retinol-binding protein, alpha(1)-microglobulin and odorant-binding protein. Megalin binds these proteins and mediates their cellular uptake. Urinary loss of carrier proteins in megalin-deficient mice results in concomitant loss of lipophilic vitamins bound to the carriers. Similar to megalin knockout mice, patients with low molecular weight proteinuria as in Fanconi syndrome are also shown to excrete vitamin/carrier complexes. Thus, these results identify a crucial role of the proximal tubule in retrieval of filtered vitamin/carrier complexes and the central role played by megalin in this process.

Kappa light chain-associated Fanconi's syndrome: molecular analysis of monoclonal immunoglobulin light chains from patients with and without intracellular crystals

Plasma cell dyscrasias may be responsible for Fanconi's syndrome, due to the toxicity of a free monoclonal kappa light chain toward kidney proximal tubules. Eight cases of Fanconi's syndrome were analyzed. We compared the structures of VkappaI variability subgroup V domains from five cases of Fanconi's syndrome and one myeloma without renal involvement. Among Fanconi cases, four putative structures were obtained after molecular modeling by homology, and the other had previously been refined by X-ray crystallography. The complete sequences of one VkappaI, one VkappaIII and N-terminal sequences of two VkappaI light chains, from patients with different forms of Fanconi's syndrome, were compared with four previously studied sequences. All three kappa chains responsible for a 'classical' form with intralysosomal crystals and a low mass myeloma, were encoded by the LCO2/O12 germline gene and had an unusual non-polar residue exposed to the solvent in the CDR-L1 loop. Of both VkappaI light chains from patients with Fanconi's syndrome without intracellular crystals, one derived from LCO2/O12 and the other from LCO8/O18 gene. Another feature that could be related to non-crystallization was the absence of accessible side chains in the CDR-L3 loop which is known to be implicated in dimer formation.

The structure of an entire noncovalent immunoglobulin kappa light-chain dimer (Bence-Jones protein) reveals a weak and unusual constant domains association

Monoclonal free light chains secreted in immunoproliferative disorders are frequently involved in renal complications, including a specific proximal tubule impairment, Fanconi's syndrome. The latter is characterized in most cases by intracellular crystallization including a light-chain variable-domain fragment which resists lysosomal proteases. Bence-Jones protein (BJP) DEL was isolated from a patient with myeloma-associated Fanconi's syndrome. The crystal structure of this human kappa immunoglobulin light-chain noncovalent dimer was determined using molecular replacement with the structure of molecule REI, as the variable domain, and that of BJP LOC as the constant domain. To our knowledge, DEL is the first complete kappa BJP structure described to date. The R-factor is 20.7% at 2.8 A resolution. The BJP DEL dimer was compared with other light-chain dimers and with Fab fragments with a kappa light chain. Although the domain-folding pattern was similar, the relative positions of the constant domains differed. BJP DEL showed a noncanonical quaternary structural arrangement which may be attributable to the poor CL-CL affinity and lack of an interchain disulfide bridge, combined with the conformational editing effect of the crystal-packing forces. Our results suggest that, in the absence of a disulfide bridge, most BJP CLs are probably mobile in solution. This may explain their high susceptibility to proteases and the absence of naturally occurring crystals for these dimers. Furthermore, these findings of an unusual quaternary structure of an immunoglobulin light-chain association extend our knowledge about the large and highly diverse structures of the immunoglobulin superfamily.