Glycosylation of immunoglobulin light chains associated with amyloidosis

Current status and prospect of anti-amyloid fibril therapy in AL amyloidosis

Amyloid light-chain (AL) amyloidosis is a rare hematological disease that produces abnormal monoclonal immunoglobulin light chains to form amyloid fibrils that are deposited in tissues, resulting in organ damage and dysfunction. Advanced AL amyloidosis has a very poor prognosis with a high risk of early mortality. The combination of anti-plasma cell therapy and amyloid fibrils clearance is the optimal treatment strategy, which takes into account both symptoms and root causes. However, research on anti-amyloid fibrils lags far behind research on anti-plasma cells, and there is currently no approved treatment that could clear amyloid fibrils. Nevertheless, anti-amyloid fibril therapies are being actively investigated recently and have shown potential in clinical trials. In this review, we aim to outline the preclinical work and clinical efficacy of fibril-directed therapies for AL amyloidosis.

Performance Characteristics and Limitations of the Available Assays for the Detection and Quantitation of Monoclonal Free Light Chains and New Emerging Methodologies

Light chain measurements form an essential component of the testing strategy for the detection and monitoring of patients with suspected and/or proven plasma cell disorders. Urine-based electrophoretic assays remain at the centre of the international guidelines for response assessment but the supplementary role of serum-free light chain (FLC) assays in response assessment and the detection of disease progression due to their increased sensitivity has been increasingly recognised since their introduction in 2001. Serum FLC assays have also been shown to be prognostic across the spectrum of plasma cell disorders and are now incorporated into risk stratification scores for patients with monoclonal gammopathy of undetermined significance (MGUS), smouldering multiple myeloma, and light chain amyloidosis (AL amyloidosis), as well as being incorporated into the criteria for defining symptomatic multiple myeloma. There are now multiple different commercially available serum FLC assays available with differing performance characteristics, which are discussed in this review, along with the implications of these for patient monitoring. Finally, newer methodologies for the identification and characterisation of monoclonal FLC, including modifications to electrophoretic techniques, mass spectrometry-based assays and Amylite, are also described along with the relevant published data available regarding the performance of each assay.

Role of the mechanisms for antibody repertoire diversification in monoclonal light chain deposition disorders: when a friend becomes foe

The adaptive immune system of jawed vertebrates generates a highly diverse repertoire of antibodies to meet the antigenic challenges of a constantly evolving biological ecosystem. Most of the diversity is generated by two mechanisms: V(D)J gene recombination and somatic hypermutation (SHM). SHM introduces changes in the variable domain of antibodies, mostly in the regions that form the paratope, yielding antibodies with higher antigen binding affinity. However, antigen recognition is only possible if the antibody folds into a stable functional conformation. Therefore, a key force determining the survival of B cell clones undergoing somatic hypermutation is the ability of the mutated heavy and light chains to efficiently fold and assemble into a functional antibody. The antibody is the structural context where the selection of the somatic mutations occurs, and where both the heavy and light chains benefit from protective mechanisms that counteract the potentially deleterious impact of the changes. However, in patients with monoclonal gammopathies, the proliferating plasma cell clone may overproduce the light chain, which is then secreted into the bloodstream. This places the light chain out of the protective context provided by the quaternary structure of the antibody, increasing the risk of misfolding and aggregation due to destabilizing somatic mutations. Light chain-derived (AL) amyloidosis, light chain deposition disease (LCDD), Fanconi syndrome, and myeloma (cast) nephropathy are a diverse group of diseases derived from the pathologic aggregation of light chains, in which somatic mutations are recognized to play a role. In this review, we address the mechanisms by which somatic mutations promote the misfolding and pathological aggregation of the light chains, with an emphasis on AL amyloidosis. We also analyze the contribution of the variable domain (V_L) gene segments and somatic mutations on light chain cytotoxicity, organ tropism, and structure of the AL fibrils. Finally, we analyze the most recent advances in the development of computational algorithms to predict the role of somatic mutations in the cardiotoxicity of amyloidogenic light chains and discuss the challenges and perspectives that this approach faces.

Analysis of monoclonal immunoglobulins from bone marrow plasma cells using immunopurification and LC-MS

Introduction

Clonal plasma cells secrete immunoglobulins, each with the exact same amino acid sequence, that are referred to as monoclonal immunoglobulins. The monoclonal heavy chain and light chain secreted from clonal plasma cells have the same molecular mass prior to the addition of post-translational modifications (PTMs) since their amino acid sequences are the same.

Objective

To examine the molecular masses of monoclonal light chains and heavy chains isolated directly from the cytoplasm of bone marrow (BM) plasma cells and compare them to the serum derived monoclonal heavy and light chains.

Methods

Using immunopurification and LC-MS we compared the molecular masses of immunoglobulins immunopurified from a patient's serum to those immunopurified from the cytoplasm of their BM plasma cells.

Results

Our findings demonstrate that the light chain molecular masses were identical whether they were obtained from serum or plasma cell cytoplasm. However, the heavy chain molecular masses did not match in bone marrow and serum due to differences in glycosylation, a common post-translational modification (PTM) found on the heavy chain.

Conclusion

The data presented here show that by using LC-MS to analyze monoclonal immunoglobulins (also referred to as miRAMM) additional phenotype information is obtained at the cellular level which is complementary to other more common techniques such as flow cytometry and histopathology.

Antibodies gone bad - the molecular mechanism of light chain amyloidosis

Light chain amyloidosis (AL) is a systemic disease in which abnormally proliferating plasma cells secrete large amounts of mutated antibody light chains (LCs) that eventually form fibrils. The fibrils are deposited in various organs, most often in the heart and kidney, and impair their function. The prognosis for patients diagnosed with AL is generally poor. The disease is set apart from other amyloidoses by the huge number of patient-specific mutations in the disease-causing and fibril-forming protein. The molecular mechanisms that drive the aggregation of mutated LCs into fibrils have been enigmatic, which hindered the development of efficient diagnostics and therapies. In this review, we summarize our current knowledge on AL amyloidosis and discuss open issues.

[Nephropathy associated with monoclonal immunoglobulins: From clonal expansion B to renal toxicity of pathological immunoglobulins]

Germinal center regulation pathways are often involved in lymphomagenesis and myelomagenesis. Most of the lymphomas (and multiple myeloma) derive from post-germinal center B-cells that have undergone somatic hypermutation and class switch recombination. Hence, B-cell clonal expansion can be responsible for the presence of a monoclonal component (immunoglobulin) of variable titer which, owing to physicochemical properties, can provoke pathologically defined entities of diseases. These diseases can affect any functional part of the kidney, by multiple mechanisms, either well known or not. The presence of renal deposition is influenced by germinal gene involved, immunoglobulin primary structure, post-translational modifications and microenvironmental interactions. The two ways immunoglobulin can cause kidney toxicity are (i) an excess of production (overcoming catabolism power by proximal tubule epithelial cells) with an excess of free light chains within the distal tubules and a subsequent risk of precipitation due to local physicochemical properties; (ii) by structural characteristics that predispose immunoglobulin to a renal disease (whatever their titer). The purpose of this manuscript is to review literature concerning the pathophysiology of renal toxicities of clonal immunoglobulin, from molecular B-cell expansion mechanisms to immunoglobulin renal toxicity.

An N-glycosylation hotspot in immunoglobulin κ light chains is associated with AL amyloidosis

Immunoglobulin light chain (AL) amyloidosis is caused by a small, minimally proliferating B-cell/plasma-cell clone secreting a patient-unique, aggregation-prone, toxic light chain (LC). The pathogenicity of LCs is encrypted in their sequence, yet molecular determinants of amyloidogenesis are poorly understood. Higher rates of N-glycosylation among clonal κ LCs from patients with AL amyloidosis compared to other monoclonal gammopathies indicate that this post-translational modification is associated with a higher risk of developing AL amyloidosis. Here, we exploited LC sequence information from previously published amyloidogenic and control clonal LCs and from a series of 220 patients with AL amyloidosis or multiple myeloma followed at our Institutions to define sequence and spatial features of N-glycosylation, combining bioinformatics, biochemical, proteomics, structural and genetic analyses. We found peculiar sequence and spatial pattern of N-glycosylation in amyloidogenic κ LCs, with most of the N-glycosylation sites laying in the framework region 3, particularly within the E strand, and consisting mainly of the NFT sequon, setting them apart with respect to non-amyloidogenic clonal LCs. Our data further support a potential role of N-glycosylation in determining the pathogenic behavior of a subset of amyloidogenic LCs and may help refine current N-glycosylation-based prognostic assessments for patients with monoclonal gammopathies.

The potential role of mass spectrometry for the identification and monitoring of patients with plasma cell disorders: Where are we now and which questions remain unanswered?

Mass spectrometry (MS) techniques provide a highly sensitive methodology for the assessment and monitoring of paraproteins compared to standard electrophoretic techniques. The International Myeloma Working Group (IMWG) recently approved the use of intact light chain matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) in lieu of immunofixation in the clinical assessment of patients and the assessment of patients enrolled on clinical trials. The increased sensitivity of these assays may help to detect and monitor monoclonal proteins (MP) in many patients with previously non-measurable disease, will reduce complete response (CR) rates and increase detection of low-level MP. The ability to track the unique mass or amino acid sequence of the MP also eliminates interference from therapeutic monoclonal antibodies (tmAbs) in most patients with IgG kappa myeloma. The intact light chain assays also provide structural information about the monoclonal light chain, including the presence of N-linked glycosylation, which has been shown to be commoner on amyloidogenic light chains and may have prognostic significance in monoclonal gammopathy of undetermined significance (MGUS). In this review, we discuss these issues alongside differences in the analytical and practical aspects related to the different MS assays under development and the challenges for MS.

The characteristics of patients with kidney light chain deposition disease concurrent with light chain amyloidosis

The type of monoclonal light chain nephropathy is thought to be largely a function of the structural and physiochemical properties of light chains; hence most affected patients have only one light chain kidney disease type. Here, we report the first series of kidney light chain deposition disease (LCDD) concomitant with light chain amyloidosis (LCDD+AL), with or without light chain cast nephropathy (LCCN). Our LCDD+AL cohort consisted of 37 patients (54% females, median age 70 years (range 40-86)). All cases showed Congo red-positive amyloid deposits staining for one light chain isotype on immunofluorescence (62% lambda), and LCDD with diffuse linear staining of glomerular and tubular basement membranes for one light chain isotype (97% same isotype as the amyloidogenic light chain) and ultrastructural non-fibrillar punctate deposits. Twelve of 37 cases (about 1/3 of patients) had concomitant LCCN of same light chain isotype. Proteomic analysis of amyloid and/or LCDD deposits in eight revealed a single light chain variable domain mutable subgroup in all cases (including three with separate microdissections of LCDD and amyloid light chain deposits). Clinical data on 21 patients showed proteinuria (100%), hematuria (75%), kidney insufficiency and nephrotic syndrome (55%). Extra-kidney involvement was present in 43% of the patients. Multiple myeloma occurred in 68% (about 2/3) of these patients; none had lymphoma. On follow up (median 16 months), 63% developed kidney failure and 56% died. The median kidney and patient survivals were 12 and 32 months, respectively. LCDD+AL mainly affected patients 60 years of age or older. Thus, LCDD+AL could be caused by two pathological light chains produced by subclones stemming from one immunoglobulin light chain lambda or kappa rearrangement, with a distinct mutated complementary determining region.

Role of mutations and post-translational modifications in systemic AL amyloidosis studied by cryo-EM

Systemic AL amyloidosis is a rare disease that is caused by the misfolding of immunoglobulin light chains (LCs). Potential drivers of amyloid formation in this disease are post-translational modifications (PTMs) and the mutational changes that are inserted into the LCs by somatic hypermutation. Here we present the cryo electron microscopy (cryo-EM) structure of an ex vivo λ1-AL amyloid fibril whose deposits disrupt the ordered cardiomyocyte structure in the heart. The fibril protein contains six mutational changes compared to the germ line and three PTMs (disulfide bond, N-glycosylation and pyroglutamylation). Our data imply that the disulfide bond, glycosylation and mutational changes contribute to determining the fibril protein fold and help to generate a fibril morphology that is able to withstand proteolytic degradation inside the body.

Inflammatory conditions promote a switch of oligosaccharyltransferase (OST) catalytic subunit isoform expression

Oligosaccharyltransferase (OST) complex catalyzes the N-glycosylation of nascent polypeptides in the endoplasmic reticulum. Glycoproteins are critical for normal cell-cell interactions, especially during an immune response. Abnormal glycosylation is an insignia of several inflammatory diseases. However, the mechanisms that regulate the differential N-glycosylation are not fully understood. The OST complex can be assembled with one out of two catalytic subunits, STT3A or STT3B, which have different enzymatic properties. In this work, we investigated the expression of STT3A and STT3B in several mouse models such as a crossbreeding of normal and abortion-prone mice and an intestinal inflammation model. These animals were either exposed or not to acoustic stress (acute or chronic). The expression of the isoforms was analysed by immunohistochemistry and protein immunoblot. Finally, we investigated the gene regulatory elements employing public databases. Results demonstrated that inflammation alters the balance between the expression of both isoforms in the affected tissues. In homoeostatic conditions, STT3A expression predominates over STT3B, especially in epithelial cells. This relation is reversed as a consequence of inflammation. An increase in STT3B activity was associated to the generation of mannose-rich N-glycans. Accordingly, this type of N-glycans were found to decorate diverse inflamed tissues. The STT3A and STT3B genes are differentially regulated, which could account for the differences in the expression levels observed here. Our results support the idea that these isoforms could play different roles in cellular physiology. This study opens the possibility of studying the STT3A/STT3B expression ratio as a biomarker in acute inflammation or chronic diseases.

The Process of Amyloid Formation due to Monoclonal Immunoglobulins

Monoclonal antibodies secreted by clonally expanded plasma cells can form a range of pathologic aggregates including amyloid fibrils. The enormous diversity in the sequences of the involved light chains may be responsible for complexity of the disease. Nevertheless, important common features have been recognized. Two recent high-resolution structures of light chain fibrils show related but distinct conformations. The native structure of the light chains is lost when they are incorporated into the amyloid fibrils. The authors discuss the processes that lead to aggregation and describe how existing and emerging therapies aim to prevent aggregation or remove amyloid fibrils from tissues.

Progress in Pathogenesis of Immunoglobin A Nephropathy

The understanding of the pathogenesis of any disease is the key to effective and specific treatment of the disease. immunoglobulin A (IgA) nephropathy is an autoimmune disease of the kidney. Oxford MEST classification is commonly used to stratify patients according to the severity of the disease. Patients with IgA nephropathy seem to produce anti-GalNAc antibodies against a particularly defective IgA1. This immune complex deposits in the kidneys, leading to a type 3 hypersensitivity reaction which ultimately damages the kidneys. People of a certain genetic background and who experience upregulation of certain defective receptors seem to develop primary IgA nephropathy. Secondary IgA nephropathy could be due to dysbiosis of the microbiota in the gut, compromised gut immunity or other gut pathologies, pulmonary function abnormalities, or amyloidosis. Overproduction of IgA due to plasma cell dyscrasia or reduced clearance of IgA due to liver abnormalities could also be potential causes. Genes that predispose individuals to IgA nephropathy and intestinal abnormalities, such as Celiac disease, seem to overlap and these people tend to have a poorer prognosis and need to be placed on more intensive treatment regimens. IgA Vasculitis seems to be a systemic form of IgA nephropathy, whereby IgA deposits systemically and leads to multiple disease manifestations. Patients in high-risk groups could also be prophylactically screened for the disease and closely monitored by immunohistochemical methods such as an enzyme-linked immunosorbent assay (ELISA) or identified by genetic testing. Currently, the major treatment regimens involve supportive therapy or immunosuppressive therapy which has major side effects. More specific treatment methods such as monoclonal antibodies, immunoglobulin replacement therapy, or low-antigen-content diet could also be looked into as potential treatment options. Stem cell replacement, by way of bone marrow transplant and tonsillectomy, has been suggested as a treatment option in patients with indications.

Tracking of low disease burden in multiple myeloma: Using mass spectrometry assays in peripheral blood

Efforts over the last 5 years have demonstrated that it is technically feasible to detect low levels of monoclonal proteins in peripheral blood using mass spectrometry. These methods are based on the fact that an M-protein has a specific amino acid sequence, and therefore, a specific mass. This mass can be tracked over time and can serve as a surrogate marker of the presence of clonal plasma cells. This review describes the use of mass spectrometry to detect M-proteins in multiple myeloma to date, identifies the challenges of using this biomarker, and describes potential strategies to overcome these challenges. We discuss the work that must be done for these techniques to be incorporated into clinical practice for tracking of low disease burden in multiple myeloma.

Proteomics-inspired precision medicine for treating and understanding multiple myeloma

Introduction

Remarkable progress in molecular characterization methods has led to significant improvements in how we manage multiple myeloma (MM). The introduction of novel therapies has led to significant improvements in overall survival over the past 10 years. However, MM remains incurable and treatment choice is largely based on outdated risk-adaptive strategies that do not factor in improved treatment outcomes in the context of modern therapies.

Areas covered

This review discusses current risk-adaptive strategies in MM and the clinical application of proteomics in the monitoring of treatment response, disease progression, and minimal residual disease (MRD). We also discuss promising biomarkers of disease progression, treatment response, and chemoresistance. Finally, we will discuss an immunomics-based approach to monoclonal antibody (mAb), vaccine, and CAR-T cell development.

Expert opinion

It is an exciting era in oncology with basic scientific knowledge translating in novel therapeutic approaches to improve patient outcomes. With the advent of effective immunotherapies and targeted therapies, it has become crucial to identify biomarkers to aid in the stratification of patients based on anticipated sensitivity to chemotherapy. As a paradigm of diseases highly dependent on protein homeostasis, multiple myeloma provides the perfect opportunity to investigate the use of proteomics to aid in precision medicine.

Structural analysis of urinary light chains and proteomic analysis of hyaline tubular casts in light chain associated kidney disorders

Background

Monoclonal overproduction of kappa and/or lambda light chains might result in renal light chain deposition disease. Light chain associated cast nephropathy and renal AL-amyloidosis represent two further pathologies going along with monoclonal gammopathy of renal significance and multiple myeloma. While cast nephropathy often manifests with acute kidney injury, AL-amyloidosis is rather accompanied with chronic kidney disease.

Methods

Urine samples were collected from 17 patients with multiple myeloma or monoclonal gammopathy. The urine sediment was stained for cast morphology by H/E and light chain immunofluorescence. Following micro-selection of casts under microscope, proteomic analysis of casts was performed by mass spectrometry. Sucrose gradient sedimentation was employed and light chain architecture examined by immunoblotting. Uromodulin was measured by ELISA in sucrose gradient fractions.

Results

Urinary casts were observed of about 30 µm in diameter by H/E staining and under immunofluorescence microscopy. Casts with a diameter of 20 µm were observed as a novel variant. Proteome analysis showed that in addition to the expected light chain variants produced by the malignant clone of plasma cells, also histones such as H2B and cathepsin B were contained. Uromodulin was not detectable in urinary casts of all patients. All eleven patients with lambda light chains showed predominant dimerized light chains in the urine immunoblot. Six patients with kappa light chains presented with predominantly monomeric forms of light chains in the immunoblot. The densitometric evaluated ratio of lambda dimers vs. monomers was significantly higher (2.12 ± 0.75) when compared with the ratio of kappa dimers vs. monomers (0.64 ± 0.47), p = 0.00001. Aggregates of light chains separated in part into denser sucrose fractions.

Conclusion

This work on urinary casts and light chains demonstrates that hyaline tubular casts represent a complex formation of protein-protein aggregates with histones and cathepsin B identified as novel cast components. Apart from the proteomic composition of the casts, also the formation of the light chains and aggregates is of relevance. Dimerized light chains, which are typical for lambda paraproteins, might be less dialyzable than monomeric forms and may therefore identify patients less responsive to high cut-off dialysis.

The utility of MASS-FIX to detect and monitor monoclonal proteins in the clinic

The detection and quantification of monoclonal-proteins (M-proteins) are necessary for the diagnosis and evaluation of response in plasma cell dyscrasias. Immunoglobulin enrichment-coupled with matrix-assisted laser desorption ionization time-of-flight mass-spectrometry (MASS-FIX) is a simple and inexpensive method to identify M-proteins, but its clinical generalizability has not yet been elucidated. We compared MASS-FIX to protein electrophoresis (PEL), serum/urine immunofixation-electrophoresis (IFE), and quantitative serum free-light chain (FLC) for the identification of M-proteins in different clinical diagnoses. Paired serum and urine samples from 257 patients were tested. There were six patients for whom s-IFE and FLC ratio were positive and serum MASS-FIX was negative, but when serum and urine MASS-FIX results were combined, only one patient with light chain-MGUS was missed. Serum/urine-MASS-FIX detected M-proteins in 18 patients with negative serum/urine-PEL/IFE and serum-FLC, 10 of whom had multiple myeloma or AL amyloidosis, who were mistakenly thought to have complete hematologic response by serum/urine-PEL/IFE and serum-FLC. Nearly half of the AL amyloidosis patients had atypical spectra, which may prove to be a clue to the diagnosis and pathogenesis of the disease. In conclusion, MASS-FIX has a comparable sensitivity with PEL/IFE/FLC methods and can help inform the clinical diagnosis.

Crystalglobulinemia manifesting as chronic arthralgia and acute limb ischemia: A clinical case report

RATIONALE

Crystalglobulinemia is a rare disease caused by monoclonal immunoglobulins, characterized by irreversible crystallization on refrigeration. It causes systemic symptoms including purpura, arthralgia, and vessel occlusive conditions to be exacerbated by exposure to cold. We report a patient with crystalglobulinemia associated with monoclonal gammopathy of undetermined significance (MGUS) manifesting as chronic arthralgia and recurrent acute arterial occlusion.

PRESENTING CONCERNS

A 61-year-old man, who had been diagnosed with MGUS and who had arthralgia of unknown origin, presented with recurrent acute limb ischemia after surgical thromboembolectomy. Refrigeration of his serum formed precipitates that looked like needle-shaped crystals. These crystals did not dissolve with warming, which is not a characteristic of cryoglobulins. Skin biopsy results showed crystal-liked eosinophilic bodies in small vessels and we diagnosed crystalglobulinemia.

INTERVENTION AND OUTCOMES

Although he underwent above-knee amputation, he was treated with a bortezomib and dexamethasone-based chemotherapeutic regimen, following lenalidomide maintenance therapy. Finally, he achieved complete remission and serum crystalglobulins diminished.

LESSONS

Monoclonal gammopathy, previously diagnosed as MGUS, can cause systemic symptoms and thrombotic conditions by producing pathologic immunoglobulins, such as crystalglobulins. In such situations, MGUS, even when it has not progressed to multiple myeloma, can be a target of aggressive chemotherapy. Crystalglobulinemia should be considered for patients with monoclonal gammopathy manifesting as systemic and thrombotic symptoms exacerbated by cooling.

The Kinetic Stability of a Full-Length Antibody Light Chain Dimer Determines whether Endoproteolysis Can Release Amyloidogenic Variable Domains

Light chain (LC) amyloidosis (AL amyloidosis) appears to be caused by the misfolding, or misfolding and aggregation of an antibody LC or fragment thereof and is fatal if untreated. LCs are secreted from clonally expanded plasma cells, generally as disulfide-linked dimers, with each monomer comprising one constant and one variable domain. The energetic contribution of each domain and the role of endoproteolysis in AL amyloidosis remain unclear. To investigate why only some LCs form amyloid and cause organ toxicity, we measured the aggregation propensity and kinetic stability of LC dimers and their associated variable domains from AL amyloidosis patients and non-patients. All the variable domains studied readily form amyloid fibrils, whereas none of the full-length LC dimers, even those from AL amyloidosis patients, are amyloidogenic. Kinetic stability-that is, the free energy difference between the native state and the unfolding transition state-dictates the LC's unfolding rate. Full-length LC dimers derived from AL amyloidosis patients unfold more rapidly than other full-length LC dimers and can be readily cleaved into their component domains by proteases, whereas non-amyloidogenic LC dimers are more kinetically stable and resistant to endoproteolysis. Our data suggest that amyloidogenic LC dimers are kinetically unstable (unfold faster) and are thus susceptible to endoproteolysis that results in the release amyloidogenic LC fragments, whereas other LCs are not as amenable to unfolding and endoproteolysis and are therefore aggregation resistant. Pharmacologic kinetic stabilization of the full-length LC dimer could be a useful strategy to treat AL amyloidosis.

IgA nephropathy caused by unusual polymerization of IgA1 with aberrant N-glycosylation in a patient with monoclonal immunoglobulin deposition disease

Immunoglobulin A nephropathy (IgAN) is a form of chronic glomerulonephritis characterized by the deposition of IgA immune complexes in the glomerular region. The cause of IgAN is unknown, but multiple mechanisms have been suggested. We previously reported a rare case of mesangioproliferative glomerulonephritis in a patient with monoclonal immunoglobulin deposition disease associated with monoclonal IgA1. In this study, we performed the detailed analyses of serum IgA1 from this patient in comparison with those from patients with mIgA plasma cell disorder without renal involvement and healthy volunteers. We found unusual polymerization of IgA1 with additional N-glycosylation distinctive in this patient, which was different from known etiologies. Glycan profiling of IgA1 by the lectin microarray revealed an intense signal for Wisteria floribunda agglutinin (WFA). This signal was reduced by disrupting the native conformation of IgA1, suggesting that the distinct glycan profile was reflecting the conformational alteration of IgA1, including the glycan conformation detected as additional N-glycans on both the heavy and light chains. This unusually polymerized state of IgA1 would cause an increase of the binding avidity for lectins. WFA specifically recognized highly polymerized and glycosylated IgA1. Our results of analysis in the rare case of a patient with monoclonal immunoglobulin deposition disease suggest that the formation of unusually polymerized IgA1 is caused by divergent mechanisms including multiple structural alterations of glycans, which contributes to IgA1 deposition and mesangium proliferation.

The peptidic middle molecules: is molecular weight doing the trick?

Chronic kidney disease (CKD) is characterized by a gradual endogenous intoxication caused by the progressive accumulation of bioactive compounds that in normal conditions would be excreted and/or metabolized by the kidney. Uremic toxicity now is understood as one of the potential causes for the excess of cardiovascular disease and mortality observed in CKD. An important family of uremic toxins is that of the peptidic middle molecules, with a molecular weight ranging between 500 and 60,000 Da, which makes them, as a consequence, difficult to remove in the process of dialysis unless the dialyzer pore size is large enough. This review provides an overview of the main and best-characterized peptidic middle molecules and their role as potential culprits of the cardiometabolic complications inherent to CKD patients.

A strategy for synthesis of pathogenic human immunoglobulin free light chains in E. coli

Monoclonal immunoglobulin light chains are normally synthesized in excess compared to the heavy chain partners and can be detected in serum and urine ("free" LC). Occasionally free LC are per se cause of organ toxicity, as in free LC-related disorders. In AL amyloidosis, the most common of these conditions, free LC with peculiar biophysical properties related to their primary structure damage target organs and organize in amyloid fibrils. Unlimited availability of well-characterized free LC is instrumental to investigate the toxic effect of these proteins and to study their interactions with targets. We present a straightforward strategy to obtain recombinant monoclonal free LC by using a bacterial system. These proteins, expressed as inclusion bodies, were subjected to solubilization and refolding procedures to recover them in native form. To minimize differences from the circulating natural LC, full-length recombinant LC were expressed, i.e. complete of variable and constant regions, with the original amino acid sequence along the entire protein, and with no purification tags. The strategy was exploited to generate free LC from three AL amyloidosis patients. After purification, recombinant proteins were biochemically characterized and compared to the natural Bence Jones protein isolated from one of the patients. Results showed that the recombinant free LC were properly folded and formed homodimers in solution, similar to the natural Bence Jones protein used for comparison. Furthermore, as proof of pathogenicity, recombinant proteins formed amyloid fibrils in vitro. We believe that the present strategy represents a valuable tool to speed research in free LC-related disorders.

Localized AL amyloidosis: a suicidal neoplasm?

Although AL amyloidosis usually is a systemic disease, strictly localized AL deposits are not exceptionally rare. Such case reports form a considerable body of published articles. Although both AL amyloidosis types are formed from an N-terminal segment of a monoclonal immunoglobulin light chain, a typical localized AL amyloid differs from the systemic counterpart by the morphological appearance of the amyloid, and presence of clonal plasma cells and of giant cells. In this article it is pointed out that localized AL amyloidosis ('amyloidoma') represents a true plasma cell neoplasm and not a pseudotumor. The pathogenesis of localized AL amyloidosis may differ from that of the systemic type, a suggestion underlined by the fact that localized AL amyloidosis of kappa type is as common as that of lambda origin, in contrast to the systemic form where lambda chains constitute the overwhelming majority of cases. It is suggested that oligomeric assemblies of the produced immunoglobulin light chain are toxic to plasma cells, which in this way commit suicide.

Phosphorylation as a tool to modulate aggregation propensity and to predict fibril architecture

Despite the importance of post-translational modifications in controlling the solubility and conformational properties of proteins and peptides, precisely how the aggregation propensity of different peptide sequences is modulated by chemical modification remains unclear. Here we have investigated the effect of phosphorylation on the aggregation propensity of a 13-residue synthetic peptide incorporating one or more phosphate groups at seven different sites at various pH values. Fibril formation was shown to be inhibited when a single phosphate group was introduced at all seven locations in the peptide sequence at pH 7.5, when the phosphate group is fully charged. By contrast, when the same peptides were analysed at pH 1.1, when the phosphate is fully protonated, fibrils from all seven peptide sequences form rapidly. At intermediate pH values (pH 3.6) when the phosphate group is mono-anionic, the aggregation propensity of the peptides was found to be highly dependent on the position of the phosphate group in the peptide sequence. Using this information, combined with molecular dynamics (MD) simulations of the peptide sequence, we provide evidence consistent with the peptide forming amyloid fibrils with a class 7 architecture. The results highlight the potential utility of phosphorylation as a method of reversibly controlling the aggregation kinetics of peptide sequences both during and after synthesis. Moreover, by exploiting the ability of the phosphate group to adopt different charge states as a function of pH, and combining experimental insights with atomistic information calculated from MD simulations as pH is varied, we show how the resulting information can be used to predict fibril structures consistent with both datasets, and use these to rationalise their sensitivity of fibrillation kinetics both to the location of the phosphate group and its charge state.

Progesterone induces a switch in oligosaccharyltransferase isoform expression: consequences on IgG N-glycosylation

The presence of additional N-glycans in the Fab region of IgG has shown to dramatically modify the properties and functionality of these molecules including changes in antibody affinity and stability. However, the underlying molecular mechanism responsible for the presence or absence of these glycans remains unknown. Polypeptide N-linked glycosylation is catalyzed in the lumen of the endoplasmic reticulum by the oligosaccharyltransferase complex. Mammalian cells can express two isoforms of the oligosaccharyltransferase catalytic subunit (STT3-A and STT3-B), which are endowed with distinct enzymatic properties. In this work we employed a murine hybridoma cell culture to study whether the expression of STT3 isoforms could be modulated by progesterone, thus altering the pattern of IgG N-glycosylation. We found that progesterone induces a switch of STT3 isoform expression, increasing IgG N-glycosylation. These effects were dependent on the progesterone-induced blocking factor (PIBF), whose concentration was modulated by progesterone. PIBF was previously found to be an immunomodulatory molecule relevant for the maintenance of pregnancy. We concluded that the STT3-B/STT3-A ratio modulates the N-glycosylation level of IgG, in agreement with previous data showing that full N-glycosylation of polypeptides requires cooperation between both catalytic isoforms. This work provides the first evidence that STT3 isoforms can be hormonally modulated, with marked consequences on IgG N-glycosylation.

Renal lesions associated with plasma cell dyscrasias: practical approach to diagnosis, new concepts, and challenges

CONTEXT

Patients with plasma cell dyscrasias (myeloma) may exhibit a variety of renal manifestations as a result of damage from circulating light- and heavy-chain immunoglobulin components produced by the neoplastic plasma cells. The renal alterations can occur in any of the renal compartments, and in a significant number of the cases more than one compartment is affected. Research in the laboratory has helped considerably in providing a solid conceptual understanding of how renal damage occurs.

OBJECTIVES

To detail advances that have been made in the diagnosis of these conditions and to provide an account of research accomplishments that have solidified diagnostic criteria. The new knowledge that has been acquired serves to provide a solid platform for the future design of new therapeutic interventions aimed at ameliorating or abolishing the progressive renal damage that typically takes place.

DATA SOURCES

Translational efforts have substantially contributed to elucidate mechanistically the molecular events responsible for the renal damage. The spectrum of renal manifestations associated with plasma cell dyscrasias has expanded significantly in the last 10 years. Diagnostic criteria have also been refined. This information has been summarized from work done at several institutions.

CONCLUSIONS

A number of significant challenges remain in the diagnosis of these conditions, some of which will be discussed in this article. Dealing with these challenges will require additional translational efforts and close cooperation between basic researchers, clinicians, and pathologists in order to improve the diagnostic tools available to renal pathologists and to acquire a more complete understanding of clinical and pathologic manifestations associated with these conditions.

Heterogeneity in primary structure, post-translational modifications, and germline gene usage of nine full-length amyloidogenic kappa1 immunoglobulin light chains

Immunoglobulin light chain amyloidosis is a protein misfolding disease in which a monoclonal immunoglobulin (Ig) light chain (LC) with a critically folded beta-conformation self-aggregates to form highly ordered, nonbranching amyloid fibrils. The insoluble nature of amyloid fibrils ultimately results in the extracellular deposition of the LC in tissues and organs throughout the body. Structural features that confer amyloidogenic properties on an Ig LC likely include amino acid sequence variations and post-translational modifications, but the specific natures of these changes remain to be defined. As part of an exploration of the effective range of amyloidogenic modifications, this study details the structural and genetic analyses of nine kappa1 LC proteins. Urinary LCs were purified by size exclusion chromatography using FPLC, and structural analyses were performed by electrospray ionization, matrix-assisted laser desorption/ionization, and tandem mass spectrometry. RT-PCR amplification, cloning, and sequencing of the monoclonal LC genes were accomplished using bone marrow-derived mRNA. Clinical data were reviewed retrospectively. Characterization of the urinary kappa1 LCs revealed extensive post-translational modification in all proteins, in addition to somatic mutations expected on the basis of results from genetic analyses. Post-translational modifications included disulfide-linked dimerization, S-cysteinylation, glycosylation, fragmentation, S-sulfonation, and 3-chlorotyrosine formation. Genetic analyses showed that several LC variable region germline gene donors were represented including O18/O8, O12/O2, L15, and L5. Clinical features included soft tissue, cardiac, renal, and hepatic involvement. This study demonstrated the extensive heterogeneity in primary structure, post-translational modifications, and germline gene usage that occurred in nine amyloidogenic kappa1 LC proteins.

Nonsecretory multiple myeloma with amyloidosis. A case report and review of the literature

We report a case concerning a 49-year-old female patient with thoracic pain. X-rays showed a single osteolytic lesion on the right seventh rib. The excision of the rib disclosed a plasmocytic plasmocytoma with extensive amyloidosis. Serum and urine protein electrophoresis were both negative for monoclonal gammopathy. Bone marrow biopsy showed that 80% of the marrow had been replaced by plasma cells. A diagnosis of nonsecretory multiple myeloma was made. Immunohistochemistry revealed amyloid light (AL) amyloidosis of kappa-light chain origin. The relationship between nonsecretory multiple myeloma and amyloidosis is discussed.

Biclonal systemic AL-amyloidosis with one glycosylated and one nonglycosylated AL-protein

The amyloid fibril protein AL was isolated from the spleen of a patient with systemic amyloidosis. Size-exclusion chromatography of the solubilized amyloid fibrils revealed a distinct, retarded asymmetric peak. The symmetrical part of the peak showed on SDS-PAGE two positive periodic acid Schiff-staining bands at 14 and 16 kDa. Staining with Coomassie Brilliant Blue revealed in addition two proteins with masses of 13 and 20 kDa. The 14 and 16 kDa bands were the strongest ones. N-Terminal analyses of the four blotted bands showed that the N-termini were the same in all cases. Elucidation of the amino acid sequence established an AL-chain of 157 residues as well as a fragment covering positions 188-207 of the constant region. Two tryptic peptides derived from the same region, positions 25-46, showed an identical sequence, except for position 34 where both alanine and threonine residues occurred. Monosaccharide compositional analysis of the threonine-containing peptide revealed an oligosaccharide in the N-glycosylation site, position 32-34. Mass analysis of the glycopeptide verified the oligosaccharide. The AL-chains belong to the kappa 3a germline gene and verifies that the glycosylated chain is a mutated form. The AL-chains differ from that of the germline in 14 positions. The J-segment is of JkappaIII and is mutated in position 106.

Protein aggregation

Protein aggregation occurs in vivo as a result of improper folding or misfolding. Diverse diseases arise from protein misfolding and are now grouped under the term "protein conformational diseases", including most of the neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, the prion encephalopathies and Huntington's disease, as well as cystic fibrosis, sickle cell anemia and other less common conditions. The hallmark event in these diseases is a change in the secondary and/or tertiary structure of a normal, functional protein, leading to the formation of protein aggregates with various supramolecular organizations. In most cases the aggregates are organized in structurally well-defined fibrils forming amyloid deposits. The crucial feature of the amyloidogenic proteins is their structural instability induced either by mutations, post-translational modifications, or local conditions, such as pH, temperature, and co-solutes. The conformational change may promote the disease either by gain of a toxic activity or by the lack of biological function of the natively folded protein. As different molecular mechanisms are involved in the formation of the various forms of protein aggregates, the laboratory diagnostic approach remains frequently elusive.

Identification and location of a cysteinyl posttranslational modification in an amyloidogenic kappa1 light chain protein by electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry

Amyloid-deposited light chain (AL) amyloidosis is correlated with the overproduction of a monoclonal immunoglobulin light chain protein by a B-lymphocyte clone. Since the amyloid fibril deposits in AL amyloidosis most often consist of the N-terminal fragments of the light chain, the majority of studies have focused on the determination of the primary structure of the protein, and reducing agents have been used routinely in the initial purification process. In this study, two light chain proteins were isolated and purified, without reduction, from the urine of a patient diagnosed with kappa 1 (kappa1) AL amyloidosis. One protein had a relative molecular mass of 12,000 and the other 24,000. Electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry, in combination with enzymatic digestions, were used to verify the amino acid sequences and identify and locate posttranslational modifications in these proteins. The 12-kDa protein was confirmed to be the N-terminal kappa1 light chain fragment (variable region) consisting of residues 1-108 or 1-109 and having one disulfide bond. The 24-kDa protein was determined to be the intact kappa1 light chain containing a cysteinyl posttranslational modification at Cys214 and disulfide bonds located at Cys23-Cys88, Cys134-Cys194, and Cys214-Cys. The methods used in this report enable high-sensitivity determination of amino acid sequence and variation in intact and truncated light chains as well as posttranslational modifications. This approach facilitates consideration of the effect of cysteinylation on the native protein structure and the potential involvement of this modification in AL amyloidosis.