Introduction

This section of the book focuses on paraprotein-related kidney disorders. In this chapter, we describe a general overview of paraproteins and their adverse effects on the kidneys with emphasis on proximal tubule disorders. In the following chapters, detailed discussions on other types of paraprotein-mediated kidney disorders, including cast nephropathy, monoclonal immunoglobulin light chain (AL) amyloidosis, light and heavy chain diseases, etc., are provided.

Paraprotein basics

Paraproteins are monoclonal light chains (LCs), heavy chains, or intact immunoglobulins detected in the serum or urine in abnormal quantities. The majority of the paraproteinemia-associated kidney disorders result from free monoclonal light chains (FLC), as these immunoglobulin fragments have relatively unrestricted access to kidneys and kidney tubules. There is an expanding spectrum of kidney disorders caused by either direct effects of FLCs on kidney cells or by deposition of intact immune globulins or their fragments in the glomeruli or along the kidney tubules ( Box 6.1 ).

Paraproteins are produced by plasma cells or B cells, and their presence in blood or urine indicates clonal proliferation of either of these cell lines, that is, multiple myeloma (plasma cells), or lymphomas (B cells). Thus paraproteinemias are typically associated with cancer of these cell lines. In some cases, however, the workup for their clonal origin may not meet the criteria for cancer and there may be no overt kidney disorder, a condition referred to as monoclonal gammopathy of unknown significance ( MGUS ). Upon closer scrutiny, many MGUS cases are found to have renal abnormalities, although without an obvious cancer identifiable as their source. Such conditions are termed monoclonal gammopathy of renal significance ( MGRS ). In most cases of MGRS, the initial diagnosis is suspected when a kidney disorder is diagnosed in the presence of monoclonal gammopathy, or when a kidney biopsy is reported to show monoclonal immunoglobulin deposition, and when clonal workup fails to meet the criteria for the diagnosis of overt multiple myeloma or a B cell tumor. ^{, ,} Paraproteins, whether produced by a cancer, such as myeloma, lymphoma, or leukemia, or by a small clone that does not meet the criteria for cancer, often affect the kidneys by various mechanisms that include disruption of transport systems, triggering inflammatory reactions in the kidney, ^{, , , –} cast formation, ^, or kidney deposition of organized or unorganized monoclonal proteins. ^{, , –} Renal involvement in paraproteinemia always implies a worse prognosis. ^,

Paraprotein basics

Paraproteins are monoclonal light chains (LCs), heavy chains, or intact immunoglobulins detected in the serum or urine in abnormal quantities. The majority of the paraproteinemia-associated kidney disorders result from free monoclonal light chains (FLC), as these immunoglobulin fragments have relatively unrestricted access to kidneys and kidney tubules. There is an expanding spectrum of kidney disorders caused by either direct effects of FLCs on kidney cells or by deposition of intact immune globulins or their fragments in the glomeruli or along the kidney tubules ( Box 6.1 ).

Paraproteins are produced by plasma cells or B cells, and their presence in blood or urine indicates clonal proliferation of either of these cell lines, that is, multiple myeloma (plasma cells), or lymphomas (B cells). Thus paraproteinemias are typically associated with cancer of these cell lines. In some cases, however, the workup for their clonal origin may not meet the criteria for cancer and there may be no overt kidney disorder, a condition referred to as monoclonal gammopathy of unknown significance ( MGUS ). Upon closer scrutiny, many MGUS cases are found to have renal abnormalities, although without an obvious cancer identifiable as their source. Such conditions are termed monoclonal gammopathy of renal significance ( MGRS ). In most cases of MGRS, the initial diagnosis is suspected when a kidney disorder is diagnosed in the presence of monoclonal gammopathy, or when a kidney biopsy is reported to show monoclonal immunoglobulin deposition, and when clonal workup fails to meet the criteria for the diagnosis of overt multiple myeloma or a B cell tumor. ^{, ,} Paraproteins, whether produced by a cancer, such as myeloma, lymphoma, or leukemia, or by a small clone that does not meet the criteria for cancer, often affect the kidneys by various mechanisms that include disruption of transport systems, triggering inflammatory reactions in the kidney, ^{, , , –} cast formation, ^, or kidney deposition of organized or unorganized monoclonal proteins. ^{, , –} Renal involvement in paraproteinemia always implies a worse prognosis. ^,

Historical background

The earliest association of a paraprotein with the kidney dates back to 1845 when Dr. William Macintyre found a unique nonalbumin protein in the urine of his patient Thomas Alexander McBean diagnosed with “mollities and fragilitas ossium,” now known as multiple myeloma . The significance of this finding with respect to kidney involvement and the source of this protein were not clearly understood. Both Dr. Macintyre and Dr. Thomas Watson, a consultant on the same case, sent a urine sample from the patient to Dr. Henry Bence Jones, a recognized clinical pathologist of his time. Dr. Watson in his letter to Bence Jones described the sample as containing a large amount of “animal matter” that precipitated when nitric acid was added, became clear when heated, and reappeared upon cooling. Bence Jones confirmed the physical properties of this “animal matter,” which he thought was “hydrated deutoxide of albumen,” and recommended that the presence of this protein to be looked for in the urine of patients with ‘mollities and fragilitas ossium.’ ^, This may possibly be the first instance of designating a urinary biomarker for a kidney disorder associated with a systemic disease, multiple myeloma. This protein later came to be known as “ Bence Jones protein ,” although Bence Jones himself was not aware of its source or its significance for kidney disease.

After the first characterization of plasma cells in 1895 by Marschalko, in 1900, Wright defined the “gelatiniform substance” found in the bones of patients with mollities as a tumor consisting of plasma cells. Korngold and Lipari, in 1956, 111 years after the first demonstration of Bence Jones protein, using Ouchterlony technique, identified two distinct antigenic types of Bence Jones proteins, and also demonstrated the presence of the same proteins in the sera of multiple myeloma patients. These two antigenic types were later named kappa (κ) and lambda (λ) LCs honoring Korngold and Lipari. ^, Edelman and Gally showed in 1962 that LCs derived from a patient with immunoglobulin G myeloma in the serum had the same amino acid sequence with the Bence Jones protein in the urine and share the same heat properties, finally solving the mystery of Bence Jones proteins. With precise characterization of the monoclonal proteins associated with multiple myeloma and other tumors, and the demonstration that myeloma proteins are toxic to kidneys, ^{, ,} it became possible to investigate kidney disorders associated with paraproteinemias and explore therapies for such disorders.

Proximal tubule disorders

Proximal tubule disorders are very common in myeloma and may be present with or without cast formation in the distal tubules. ^{, ,} The paraproteins responsible for proximal tubule disorders are almost exclusively immunoglobulin LCs. Although tubular involvement is sometimes present in AL amyloidosis or other paraprotein deposition disorders, they are rare, and do not usually feature as prominent components of the underlying disorders. These disorders are discussed in the following chapters; here we will focus on proximal tubule disorders associated with immunoglobulin LCs.

LCs are approximately 210 to 220 amino acid polypeptide subunits of immunoglobulins, smaller than albumin, and are relatively positively charged compared with albumin. Monomeric LCs are approximately 22 to 25 kDa and most κ-FLCs exist in monomeric state whereas λ-FLCs tend to form approximately 44 kDa dimers. Either way, FLCs are relatively unhindered in the glomerulus, and based on an estimated glomerular sieving coefficient of approximately .09, significant quantities are filtered and presented to the renal tubule. ^{, ,} In normal healthy individuals 100 to 600 mg of polyclonal FLCs may be filtered in the glomerulus, and only minute quantities, no more than 2 to 3 mg per day, is excreted in the urine, implying near complete reabsorption in the renal tubule. ^, It is now clear that FLCs reabsorption takes place in the proximal tubules mostly via receptor-mediated endocytosis, after binding to the tandem endocytic receptors megalin/cubilin; most of the internalized FLCs are catabolized into their amino acid constituents through the action of lysosomal enzymes in the lysosomes. ^{, , –}

In myeloma and in other clonal proliferative disorders, the overproduction of monoclonal LCs overwhelms the proximal tubules’ capacity to process all the filtered FLCs and overflow proteinuria ensues. In such situations, there is always evidence of stress on the endocytic apparatus of the kidney, often seen as droplets or cytoplasmic (vacuolar) inclusion of monoclonal FLCs demonstrated by immunofluorescence, or electron microscopy ( Fig. 6.1 ). Some myeloma patients have been observed to excrete up to 20 g per day of FLCs with minimal albuminuria with a dipstick test negative for proteinuria —a situation that often results in delayed diagnosis. Electron microscopically, the FLCs appear in the lysosomes and the clinical picture reflects lysosomal dysfunction, such as Fanconi syndrome (FS). Thus the kidney’s abnormalities seen in myeloma patients are related to the renal handling of FLCs and both proximal and distal tubule disorders are common. Although most of the distal tubule disorders are related to cast formation because of the interaction of FLCs with unique amino acid sequence, in their CDR3 domain with Tamm-Horsfall proteins (discussed separately), ^{, , , ,} proximal tubule disorders range from subtle tubule transport disorders to tubule cell death—apoptosis or necrosis, acute kidney injury, and tubulointerstitial nephritis ( Box 6.2 ). ^{, , , ,}

Kappa light chain proximal tubulopathy in a patient with monoclonal κ-FLC gammopathy diagnosed as monoclonal gammopathy of renal significance (patient did not meet criteria for multiple myeloma). A. Immunofluorescence demonstrates intracellular crystals staining for κ-light chain within the tubular epithelial cytoplasm. Staining for all other immune reactants was negative. B. Periodic acid-Schiff staining shows the proximal tubular epithelia are laden with intracellular crystalline inclusions. Magnification 600×. C. Electron microscopy demonstrating abundant electron-dense intracellular crystals within the cytoplasm of proximal tubular epithelium. D. Electron microscopy-higher magnification of the intracellular crystals, most of them larger than the adjacent mitochondria.

There is a broad range of renal disorders associated with FLC-proteinuria and significant variability exists in the structure, mostly in the variable region, V _L , of the involved FLCs. These disorders display marked heterogeneity ranging from subtle functional abnormalities to severe kidney failure. Some patients may have a modest degree of monoclonal gammopathy along with asymptomatic FLC proteinuria—often referred to as MGUS . Detailed investigations of such patients frequently reveal some renal abnormalities involving either the tubules or sometimes the glomeruli, a condition referred to as MGRS . Many of these patients on long-term follow-up develop overt myeloma. ^{, ,} The variability in both the type and the severity of the FLC-associated renal disorders can be linked to the specific sequences in the V _L of the involved FLCs; ^, however, these disorders also require overproduction of a clone of FLCs.

The most common proximal tubule disorder associated with monoclonal FLCs, whether in the setting of overt myeloma or MGRS, is proximal tubular acidosis (type 2) that may be accompanied by one or a combination of sodium-dependent transport abnormalities, such as bicarbonaturia (along with renal potassium wasting), glycosuria, aminoaciduria, phosphaturia, and hyperuricosuria; that is, partial or complete FS. ^{, , , , ,} Although the observations that FLCs inhibit sodium-dependent amino acid, glucose, and phosphate transports in renal brush border membrane vesicles and kidney proximal tubule cells in vitro imply a direct effect by possibly membrane-bound FLCs, ^{, ,} cytoplasmic deposition of crystalline or noncrystalline FLCs is often present in proximal tubule cells of patients with FS and FLC-proteinuria. FLC-associated FS is most frequently caused by κ-LCs restricted to the Vk I subgroup, often with extensive crystal formation in the proximal tubule cells (PTCs) (see Fig. 6.1 ), and is linked to the variable domain of the monoclonal LC, which interestingly, is also associated with resistance to proteolysis. There are also reports of FS associated with λ-FLCs, and again the expression of FS is linked to the V region of the involved FLCs. Sometimes FS can occur without any discernible FLC deposition, crystalline or noncrystalline, in the proximal tubule cells, and conversely, not all patients with crystalline deposition always have FS. The patients with FS generally display various degrees of acute tubule injury, a condition referred to as light chain proximal tubulopathy , which may be associated with overt or smoldering myeloma, occasionally with MGRS, and rarely with other neoplasms elaborating monoclonal FLCs. ^,

Animal experiments in vivo and studies in vitro have shown that some FLCs induce extensive apoptosis in proximal tubules, which may be a mechanism contributing to acute tubule injury. ^{, ,} Proximal tubule cells exposed to tubulopathic FLCs, isolated from patients with myeloma, elicit a range of cytotoxic and inflammatory responses, such as generation of reactive oxygen species, activation of the transcription factors, nuclear factor-κB, leading to transcription and release to medium of inflammatory cytokines (interleukins 6, 8, monocyte chemoattractant protein-1, tumor necrosis factor-α, transforming growth factor-β1, etc.), mediated through phosphorylation of mitogen-activated protein kinases (MAPK), especially p38 MAPK. ^{, , ,} FLC-exposed proximal tubule cells also undergo morphologic changes that include disruption of cytoskeletal organization, extensive vacuolization, and cell death (apoptosis and necrosis). Furthermore, the tubulopathic FLCs can induce phenotypical changes in proximal tubule epithelium, inducing loss of epithelial cell marker E-cadherin and acquisition of myofibroblast marker α-smooth muscle actin (i.e., epithelial-to-mesenchymal transformation (EMT)), suggesting that this phenomenon may contribute to the extensive tubulointerstitial fibrosis frequently seen in patients with myeloma, although EMT is difficult to identify in human kidney biopsies. Most of these cytotoxic or inflammatory responses associated with monoclonal FLCs appear to require their internalization in the cell, and could be prevented by maneuvers that inhibit FLC endocytosis, such as disrupting the clathrin-coated pathway, inhibiting vacuolar acidification, or knocking down megalin and cubilin expression in proximal tubule epithelia, ^{, , , ,} although the studies with brush-border membrane vesicles also suggest direct toxic effects by FLCs at the membrane l evel. ^,

Proximal tubular disorders seen in the setting of FLC paraproteinemia may exhibit many additional structural lesions other than crystalline and noncrystalline cytoplasmic deposition in the proximal tubule cells. Such lesions generally have not attracted much attention until recently. However, the recent biopsy studies suggest that proximal tubule findings are common. ^{, ,} For example, Ecotiere et al. reviewed kidney biopsies of 70 patients with myeloma, and observed varying degrees of interstitial fibrosis, tubule atrophy, and interstitial inflammation in more than 50% of biopsies. In a systematic review of 5410 kidney biopsies, Herrera reported that 2.5% had kidney lesions related to monoclonal gammopathies, and 46% of these demonstrated significant histopathologic changes in the proximal tubules. Herrera classified these lesions into four categories (see Box 6.1 ): (1) proximal tubulopathy without cytoplasmic inclusions (acute tubular necrosis, ATN variant); (2) tubulopathy associated with inflammatory reaction (acute tubular interstitial nephritis variant); (3) proximal tubulopathy associated with intracytoplasmic inclusions; and (4) proximal tubulopathy associated with “lysosomal indigestion/constipation.”

In summary, tubule abnormalities are very common in patients with FLC paraproteinemia and comprise functional and morphologic changes that range from subtle transport abnormalities, including FS, to acute kidney injury, as well as inflammatory responses that contribute to renal interstitial fibrosis and chronic kidney disease. These disorders can be seen in patients with overt myeloma, or sometimes in cases with monoclonal gammopathy that cannot be linked to a cancerous clone (MGRS). The proximal tubule changes are always associated with overproduction of FLCs, and the type and severity of the lesions are determined by the V region of the involved FLCs. The majority of these disorders also require FLC endocytosis by the proximal tubule cells, and appear potentially reversible if their endocytosis can be prevented. ^{, , , , ,}