Systemic light chain amyloidosis

Introduction

Systemic amyloidoses encompass a group of diseases characterized by deposition of abnormal, insoluble, misfolded, β-pleated protein fibrils in different organs. Amyloid protein positively stains with Congo red and demonstrates apple-green birefringence under polarized light. The deposition of the β-pleated amyloid fibrils causes disruption of tissue architecture and thereby results in organ dysfunction and eventually organ failure. The kidneys, heart, gastrointestinal tract, peripheral nerves, and liver are most commonly involved organs. Many different proteins can form amyloid fibrils and to date, 36 different proteins have been recognized to form amyloid fibrils in vivo. The different amyloidogenic proteins form the basis of classification of systemic amyloidosis. Systemic light chain (AL) amyloidosis (formerly known as primary amyloidosis ) is the most common type of systemic amyloidosis in the United States. AL amyloidosis may affect five to 12 people per million per year and the incidence may be higher based on autopsy results. Among 474 patients seen at the Mayo Clinic, 60% of patients were between 50 and 70 years of age and only 10% were under 50 years of age. Approximately 70% of patients with amyloidosis have kidney involvement at the time of presentation with 4% to 5% requiring dialysis. The precursor amyloidogenic protein in AL amyloidosis is a monoclonal immunoglobulin (Ig) light chain or its fragment produced by a clone of plasma cells in the bone marrow. The clonal plasma cell burden in the bone marrow is less than 10% in 50% of patients with AL amyloidosis and occurs in association with multiple myeloma in 10% to 15% of patients. The disease has an insidious onset with an extensive clinical heterogeneity. Therefore a high level of clinical suspicion is necessary to avoid a delay in diagnosis and initiation of therapy before irreversible organ damage develops.

Pathogenesis

Reduced folding stability is a unifying feature of amyloidogenic proteins. Polypeptide chains are synthesized in the endoplasmic reticulum, enter a funnel-like pathway, in which the conformational intermediates become progressively more organized as they merge, resulting in the most stable native state. Mutations result in destabilization of these polypeptides and in the extracellular environment, the mutant polypeptides change from a fully folded state to a partially folded state and then retrace the final part of the folding pathway, ultimately forming either a native or misfolded protein. Some of the extracellular influences that affect the fate of the mutated proteins and direct them toward the pathologic pathway include temperature, pH, metal ions, oxidation, and proteolysis. The partially folded proteins have high propensity to aggregate and form oligomers and eventually fibrils that are insoluble. The deposition of these insoluble fibrils in the extracellular space alters tissue architecture and causes amyloidosis.

The Ig light chain is the amyloidogenic precursor in AL amyloidosis. Solomon et al. used an in vivo animal model to investigate the nephrotoxic potential of Bence-Jones proteins from patients with AL amyloidosis or multiple myeloma. They could reproduce kidney lesions in mice similar to that observed in patients from whom the light chains were purified. The clonal plasma cells producing the amyloidogenic light chains undergo antigen driven selection resulting in a highly mutated Ig light chain variable (VL) domain. It is now believed that the alterations in the amino acid sequences in the Ig VL domain are responsible for the structure, stability, predisposition to aggregation, localization of deposits, and distinct ultrastructural organization of the fibrils. This explains the structural heterogeneity of the fibrils and the protean nature of the disease. Although definitive common structural motif has not yet been identified, both germline sequences of VL domain and acquired amino acid replacements during somatic mutations are thought to be involved in the reduction of the folding stability of the amyloidogenic light chains.

Intense research is currently being conducted on the physiochemical properties of amyloid fibrils to understand amyloidogenesis in the kidneys. Pathogenic light chains can affect the tubulointerstitium or the glomerulus. About 70% of the pathogenic light chains are deposited in the tubulointerstitial compartment and are called tubulopathic light chains, whereas the remainder of the pathogenic light chains are deposited in the mesangium and are called glomerulopathic light chains. The term AL amyloidosis was coined in 1970 by Glenner and colleagues, when he showed that light chains formed a type of amyloid. In 1981 Gise observed and documented glomerular amyloidosis begins in the mesangium. Mesangial cells of the kidneys play a key role in the pathogenesis of AL amyloidosis. They phenotypically transform as macrophages in the presence of pathogenic AL amyloidosis light chains and aid in the generation of more amyloid fibrils. In contrast, in light chain deposition disease, they have a phenotype similar to myofibroblasts. Ting et al. validated the pathogenesis of AL amyloidosis in vivo using an animal model, where mice were injected with amyloidogenic light chains purified from the urine of patients with biopsy-proven, light-chain associated glomerular amyloidosis. The sequential steps involved are internalization of the amyloidogenic light chains by mesangial cells using caveolae followed by trafficking to the mature lysosomal compartment where they aggregate because of their thermodynamic instability and form fibrils through proteolysis. The fibrils are then extruded into the extracellular space where they accumulate and disrupt normal mesangium. Increasing evidence is emerging that the precursor amyloidogenic proteins also have direct cytotoxicity and contribute to disease manifestations. Oligomers or protofibrils may mediate cellular toxicity through a mechanism that activates apoptosis in the cells of target tissues.

Although in the normal bone marrow there is a greater proportion of Kappa (κ) than Lambda (λ) expressing clonal plasma cells, λ isotype plasma cells are more commonly associated with amyloidosis (λ:κ is 3:1) and the λ VI light chains have shown to be those most frequently linked to amyloidosis in the kidneys. This association may be related to specific amino acid sequences in the Ig VL domain and hence λ VI light chains are associated with kidney involvement, κ with hepatic involvement, and λ VIII with soft tissue deposits.

Clinical presentation

AL amyloidosis initially presents with vague clinical features, such as generalized fatigue and weight loss. Suspicion arises only when it affects a specific organ system. The most frequently involved organ systems at the time of diagnosis include the kidneys and the heart presenting as nephrotic syndrome, with or without kidney dysfunction and congestive heart failure, respectively. However, AL amyloidosis can involve any organ system other than the central nervous system. The peripheral nervous system and gastrointestinal tract are also commonly involved.

Diagnosis

Because AL amyloidosis presents with a wide array of clinical features and has an insidious onset, physicians should have a high index of clinical suspicion for its timely diagnosis. Delay in diagnosis results in advanced organ dysfunction at the time of diagnosis, when the response to therapy and prognosis remain poor. AL amyloidosis should be suspected in any patient who presents with nondiabetic nephrotic syndrome, hepatomegaly with elevated alkaline phosphatase and normal liver on imaging, heart failure with preserved ejection fraction or nonischemic cardiomyopathy with normal cardiac silhouette on chest x-ray and hypertrophy on echocardiography, polyneuropathy with monoclonal protein, or monoclonal gammopathy of unknown significance (MGUS) with unexplained fatigue, weight loss. As a next step, electrophoresis and immunofixation of serum and urine, and serum-free light chain (FLC) assays should be performed. Amyloidosis is a histologic diagnosis. If a monoclonal protein is detected, then a bone marrow biopsy and abdominal fat pad aspiration are recommended next. Noninvasive fine needle aspiration of abdominal fat pad in combination with bone marrow biopsy may reveal amyloid deposits in more than 70% of patients. Other organs where noninvasive biopsies can be performed to aid diagnosis are salivary gland, rectum, gingiva, and skin. Bone marrow biopsy aids to determine the type and burden of plasma cells and rule in or out other plasma cell disorders, such as multiple myeloma and Waldenstrom macroglobulinemia. Negative bone marrow biopsy and fat pad aspiration do not rule out amyloidosis. Biopsy of the affected organ is often necessary to establish the diagnosis of amyloidosis in cases of high clinical suspicion. However, presence of amyloid deposits alone does not confirm the diagnosis of AL amyloidosis, because several other forms of amyloid deposition have been described that are caused by other proteins besides light chains. These non-AL amyloid proteins include transthyretin (senile amyloidosis) and serum amyloid A (AA amyloidosis). Therefore it is crucial for histologic diagnosis to be followed by identification of the type of amyloid fibril and the extent of organ involvement, because chemotherapy is contraindicated in non-AL amyloidosis. Identification of the amyloid protein may be accomplished by using immunohistochemistry, deoxyribonucleic acid (DNA) analysis, or protein sequencing and mass spectrometry. However, protein sequencing through mass spectrometry is considered gold standard. See Fig. 8.1 for the diagnostic algorithm of AL amyloidosis.