Amyloidosis is a group of rare diseases caused by extracellular deposition of insoluble misfolded fibrillary protein aggregates termed amyloid. It occurs in both localized and systemic forms (Visual Abstract 14.1). The kidney is the organ most frequently affected by systemic amyloidosis (Visual Abstract 14.2). To date, 36 proteins have been implicated as the main constituents in the formation of amyloid deposits, 14 of which are known to cause clinically significant kidney disease (Table 14.1).¹,²,³

All amyloid deposits are primarily composed of a single protein precursor along with several common minor nonfibrillary elements such as glycosaminoglycans and serum amyloid P component.⁴ Regardless of the precursor protein, all amyloid fibrils share a similar diameter (7-13 nm) and core structure (β-sheets) that
confer the property to bind Congo red dye in an ordered intercalation pattern leading to pathognomonic birefringence when viewed under polarized light, typically described as apple green in color but also yellow to orange at times.²,⁵

Several different mechanisms underlie the formation of an amyloid. These include mutations (both sporadic and inherited) within the precursor proteins, leading to improper folding and/or resistance to degradation, and overabundance of precursor proteins due to either increased production or reduced clearance. Overabundance can be due to malignant conditions such as in the immunoglobulin-related forms of amyloidosis, which includes AL (light chains only), AH (heavy chains only), and AHL (heavy- and light-chain overproductions) or the nonmalignant prolonged production of proteins such as serum amyloid protein A, an acute-phase reactant, in response to prolonged inflammation/infection in AA amyloid. Reduced protein clearance is exemplified by the accumulation of β2-microglobulin in the setting of long-term dialysis. Regardless of the underlying mechanism, the common result is the accumulation of precursor proteins leading to the formation of insoluble amyloid plaques resulting in tissue damage and organ dysfunction. The most common forms of amyloidosis affecting the kidneys include AL, AA, and ALECT2, associated with the deposition of leukocyte chemotactic factor 2 (LECT2), and some hereditary forms of amyloidosis. Usually, β2-microglobulin and transthyretin (TTR) amyloidoses do not affect the kidneys.

The histologic diagnosis of amyloidosis is a two-step process. The initial step consists of the identification of the amyloid deposits in the kidney tissue, followed by the determination of the specific amyloid protein involved—amyloid protein typing. Under light microscopy, amyloid deposits demonstrate a characteristic appearance, regardless of their protein composition. Amyloid deposits typically appear as acellular, pale eosinophilic material when stained with hematoxylin-eosin, whereas they are weakly positive for periodic acid-Schiff, and commonly, non-argyrophilic on the silver stain (Figure 14.1).² A positive Congo red stain is the gold standard in the identification of amyloidosis; the deposits show a red or salmon color by light microscopy, with red, green, or yellow birefringence when viewed under polarized light. Electron microscopy demonstrates extracellular fibrils that are randomly arranged and are 7 to 13 nm in diameter (Figure 14.2).

Within the kidney, amyloid predominantly involves the glomeruli (97% of cases) but also commonly involves the vasculature (85%) and interstitium (58%).⁶ Glomerular involvement is characterized by mesangial deposits in nearly all cases and frequent deposition along the capillary walls as well. Early glomerular involvement may be extremely subtle and may only be detected by Congo red staining. With the progression of the disease, replacement of the mesangial matrix by amyloid deposits is seen. Along the capillary walls, amyloid deposits may be found in a subendothelial location, sometimes associated with the duplication of the glomerular basement membrane, and in a subepithelial location, often forming amyloid spicules. Those are characterized by long, feathery spikes on the outer aspect of the glomerular basement membrane on the silver stain, mainly found in AL and AA amyloidoses (Figure 14.3). When the glomerular involvement is extensive, global obliteration of the tuft by amyloid deposits may be seen.

Interstitial involvement, with the accumulation of a similar pale material, is a predominant finding in ALECT2 amyloidosis and is also frequent in AL and AA
amyloidoses. Tubular basement membrane involvement may also be seen. Arteriolar and arterial deposits are often present. Of note, rare cases of amyloidosis may show vascular involvement only.

Identification of the biochemical nature of the amyloid protein involved is critical in the subsequent management of amyloidosis. This can be accomplished through several methods based on immune stains and proteomic analyses. Immune stains—or antibody-based methods, including immunohistochemistry and immunofluorescence—are the most commonly available techniques and are often the first-line approach in clinical practice. They have relatively good diagnostic yield for AL and AA amyloidoses when rigorous technical conditions are observed, and interpretation is made by an experienced pathologist. Frozen section immunofluorescence can identify the causative immunoglobulin chain in immunoglobulin-related forms of amyloid by demonstrating restriction of staining for one immunoglobulin chain, most commonly the λ light chain (Figure 14.4). Immunofluorescence for fibrinogen shows bright glomerular staining in fibrinogen α-1-associated amyloidosis (AFib). Immunofluorescence on paraffin sections may be performed when frozen tissue is not available. Although immunohistochemistry on paraffin sections using antibodies to specific amyloid protein performs poorer than does immunofluorescence, it can be helpful when adequately handled by demonstrating AA and amyloid transthyretin (ATTR) amyloidoses. Immunoelectron microscopy is another highly sensitive but not commonly available immune method of amyloid protein identification.

The current method of choice for amyloid type determination is mass spectrometry performed on formalin-fixed, paraffin-embedded tissue after laser microdissection.⁷,⁸ Mass spectrometry identifies the amyloid precursor protein using computational determination of a large number of peptides from the amyloidogenic protein, by matching identified peptide masses with a database of proteins. In addition, mass spectrometry detects amyloid signature proteins such as apolipoprotein E and serum amyloid P component. One important advantage of this technique is that it allows for the identification of all amyloid proteins. It also provides superior sensitivity and specificity for the identification of immunoglobulin-derived amyloid.⁹ However, the technical complexity of this assay, including the requisite validation and expertise, limits the number of centers that can perform this procedure and the accessibility to this method.⁴ At this time, mass
spectrometry should be performed for amyloid typing at least in cases where routine immunofluorescence/immunohistochemistry finding is negative/equivocal, when immune typing of amyloid deposits does not match clinical findings, and for the detection of less common amyloid types.

The clinical presentation of amyloidosis is vague and variable, reflecting its insidious nature, the diversity of its underlying pathogenesis and variability of organ involvement. Symptoms such as fatigue, weight loss, and loss of exercise capacity can be initial signs of disease, with subsequent development of additional symptoms such as dyspnea, edema, syncope, and ascites thereafter. More organ-centric manifestations can develop with disease progression such as heart failure with cardiac involvement, nephrotic syndrome with kidney involvement, and neuropathy and/or autonomic dysfunction with nervous system involvement.

Clinical evidence of kidney amyloidosis most commonly manifests as proteinuria with or without concomitant kidney dysfunction. The full spectrum of proteinuria can be encountered, and around a third of the patients demonstrate nephrotic syndrome.² Albumin constitutes the majority of proteinuria. Kidney dysfunction alone tends to be predominant in forms of amyloid that typically spare the glomeruli such as vascular-limited AL and ALECT2.² Rarely, involvement of the peri-collecting duct tissue can lead to nephrogenic diabetes insipidus.¹⁰

Neurologic symptoms are common (especially in AL amyloid) and can manifest as both peripheral sensory and autonomic dysfunction, a combination of symptoms otherwise rarely seen outside of severe diabetes. Autonomic symptoms can initially present as impotence in men with progression to postural hypotension, early satiety, and gastrointestinal (GI) transit issues (diarrhea and/or constipation). Peripheral symptoms present as symmetric, ascending neuropathy.

Certain constellations of symptoms that should raise a high index of clinical suspicion for amyloidosis include heart failure with either concomitant nephrotic syndrome or carpal tunnel syndrome (especially in older adults), the combination of peripheral and autonomic neuropathy, thick-walled heart failure with low- or normal-voltage electrocardiogram (ECG), and recurrent carpal tunnel syndrome.

On physical examination, evidence of amyloidosis can manifest as edema, ascites, anasarca, periorbital purpura, and nail dystrophy. Carpal tunnel syndrome (particularly with bilateral involvement) can be due to many forms of amyloidosis, whereas the involvement of other soft tissues (macroglossia, submandibular soft tissue infiltration, muscular pseudohypertrophy, and/or enlargement of salivary glands) is typically caused by AL amyloidosis.⁴ Localized AL can produce mass effects from clonal B-cell populations in immunoglobulin-related amyloid. It can occur in sites such as the respiratory tract, bladder, eyelids, and skin and typically represents an indolent process that is amenable to local surgical measures and seldom progresses to systemic disease. β2-microglobulin amyloid has a predilection for joint involvement (commonly scapulohumeral, carpal, and cervical spine) and carpal tunnel syndrome, but can also present with pathologic fractures due to bone cysts, nonspecific GI symptoms due to visceral organ involvement, and, rarely, heart failure or arrhythmia due to cardiac involvement.