Amyloidosis

General Considerations

(Note: Amyloid can be studied in the context of cell injury or immunological diseases. For this reason, we have included the same material in both Cell Injury and Immunopathology.)

Amyloidosis is not a single disease entity but rather a diverse group of disease processes characterized by extracellular tissue deposits, in one or many organs, of protein materials which are generically termed amyloid. Amyloid is distinguished grossly by a starch-like staining reaction with iodine (thus the term amyloid), microscopically by its extracellular distribution and tinctorial and optical properties when stained with Congo red, and by its protein fibril structure as shown by electron microscopy and x-ray crystallography (see Table-1).

Table-1

Amyloid deposition may be either a primary (idiopathic) process without known antecedent or secondary to some other condition and may be localized to one specific site or generalized throughout the body (systemic), usually with fatal consequences. Although considerable overlap is seen in the organ distribution of various forms of amyloid, primary amyloidosis tends to involve mesodermal tissues, most frequently affecting peripheral nerves, skin, tongue, joints, heart, and liver while secondary amyloidosis mainly affects parenchymatous organs, such as spleen, kidneys, liver, and adrenals.

Amyloid deposits typically contain three components. Amyloid protein fibrils account for about 90% of the amyloid material and comprise one of several different types of proteins with the capacity to fold into what are called "beta-pleated" sheet fibrils, a unique protein configuration with binding sites for Congo red. In addition, amyloid deposits are intimately associated with the amyloid P (pentagonal) component (AP), a glycoprotein related to normal serum amyloid P (SAP), and are closely associated with sulfated glycosaminoglycans (GAG), complex carbohydrates of connective tissue. 

Systemic forms of amyloid are derived from circulating protein precursors by conversion from soluble into insoluble (fibrillar) form. By convention, amyloid fibril types are designated by two letters: A for amyloid followed by a letter for the chemical type. There are two, chemically distinct, major types of amyloid protein fibrils designated AL and AA, respectively, and several minor types unrelated to AL or AA. AL (amyloid light chain) fibrils associated mainly with multiple myeloma are related to monoclonal immunoglobulin light chains synthesized by abnormal plasma cells. AA fibrils associated mainly with chronic inflammatory diseases are related to the nonimmunoglobulin amyloid associated (AA) protein and its serum precursor (SAA), an acute phase reactant synthesized by liver cells.

Classification

The classification of amyloidosis is based upon the tissue distribution of amyloid deposits (local or systemic amyloidosis), the absence or presence of preexisting disease (primary or secondary amyloidosis), and the chemical type of amyloid protein fibril (see Table-2). 

Table-2 

Classifications of Amyloidosis
 
 

Amyloidosis Related to Monoclonal Ig Light Chains. AL amyloid is derived from monoclonal Ig light chains, usually of lambda type, produced by abnormal clones of Ig-secreting plasma cells (B cells) AL type of amyloidosis may be primary or may occur secondary to multiple myeloma or some other monoclonal gammopathy (immunocyte dyscrasia). It is the most common type of amyloidosis seen in the U.S. today.

AL type of amyloidosis occurs in about 5-10% of patients who have preexisting or coexisting multiple myeloma. Multiple myeloma is seen mainly in patients over 40 years of age (median age of 60 years) and, next to metastatic carcinoma, is the most common malignant tumor of bone. It is a malignant tumor of plasma cells which arises in the bone marrow, permeates the medullary cavity, erodes the bone cortex, and is characterized by multiple osteolytic ("punched out") lesions of vertebrae, skull, ribs, pelvis, and other bones and by narrow-banded electrophoretic peaks of monoclonal IgG (less commonly IgA , rarely IgD or IgE) in the serum and free light chains of the same kappa or lambda type in the urine (Bence-Jones proteinuria). An identical, patient-specific, free monoclonal light chain protein is also usually present in myeloma serum but, being smaller than albumin molecules, readily passes into the urine. Overall, about 70% of myeloma patients have both serum monoclonal Ig and urinary light chains, and the remaining patients have urinary light chains alone without serum monoclonal Ig. 

The AL fibrils are derived from circulating light chains by proteolytic cleavage and conversion to an insoluble form. The organ distribution of AL deposits is usually generalized (systemic) and conforms to either the primary or secondary patterns previously noted. 

AL type of amyloidosis is also associated with some other rare monoclonal gammopathies (plasma cell/B immunocyte dyscrasias), such as solitary myeloma (of bone or soft tissue), Waldenstrom's macroglobulinemia, or heavy chain disease in which there are also sometimes an increased production of free light chains that become deposited as amyloid.

Noteworthy, the majority of patients who develop AL type of amyloidosis apparently do so in the absence of clinically overt myeloma or other predisposing disease, and such cases are commonly referred to as primary or idiopathic amyloidosis. Nevertheless, in long term follow up, a substantial proportion of these patients do manifest overt, monoclonal Ig-producing plasma cell or lymphoid cell dyscrasias, such as myeloma, macroglobulinemia, or lymphoma.

Amyloidosis Associated with Inflammatory or Infectious Diseases. The amyloid deposits in this form of amyloidosis have a systemic distribution and contain AA (amyloid-associated) fibrils which are related to the nonimmunoglobulin AA protein and its serum protein precursor (SAA), an acute phase reactant synthesized by hepatic cells. Also called reactive or secondary amyloidosis, this form of amyloidosis occurs mainly as a complication of long standing inflammatory diseases, most frequently rheumatoid arthritis (Å 5-10% of rheumatoid patients) and also dermatomyositis, scleroderma, regional enteritis, and ulcerative colitis.

Prior to the antibiotic era, chronic tissue-destructive infectious diseases, such as tuberculosis, chronic osteomyelitis, and bronchiectasis, were the most common antecedants of secondary amyloidosis. Now, amyloidosis often develops as a complication of skin and lung abscesses occurring in subcutaneous heroin abusers.

Reactive-type amyloidosis may also occur in association with cancer, such as Hodgkin's disease and renal cell carcinoma.

Other Amyloids and Disease Associations. 

1. Amyloid associated with hemodialysis (AH). The systemic amyloid deposition of beta2-microglobulin (beta2-m), a normal serum protein, occurs as a complication of long-term dialysis in patients with chronic renal failure because this protein does not pass through conventional dialysis membranes. 
2. Amyloid associated with familial Mediterranean fever. The systemic deposition of AA fibrils occurs in familial Mediterranean fever, an autosomal recessive disorder seen in individuals of Sephardic Jewish, Armenian, and Arabic descent. 
3. Amyloid associated with familial amyloid neuropathies (AF). Amyloid deposition of a mutant form of transthyretin, a normal serum protein that transports thyroxin and retinol (vitamin A), occurs in peripheral nerves in familial amyloid polyneuropathy, an autosomal dominant disorder occurring in different parts of the world (Sweden, Portugal, Japan, and the U.S.). 
4. Localized deposits of amyloid. 


Endocrine-related. Localized amyloid deposits are associated with hormones produced by certain endocrine tumors and endocrine glands, such as medullary carcinoma of the thyroid gland (procalcitonin), islet cell tumors of the pancreas, and the islets of Langerhans (islet associated polypeptide, IAPP) in patients with type II diabetes mellitus.

Age-related. Amyloid deposits of transthyretin occur in the heart of elderly patients with senile cardiac amyloidosis. Beta amyloid protein is deposited in the cerebral blood vessels and plaques of patients with senile cerebral amyloidosis and Alzheimer's disease.

Pathogenesis of Amyloidosis

Amyloidosis is not one disease but a diverse group of diseases of acquired or hereditary origin and characterized by the extracellular deposition of one of several different types of protein fibrils with similar properties and called amyloid. Despite their biochemical diversity, amyloid proteins share unique features: fibril ultrastructure, cross beta x-ray diffraction pattern, and staining characteristics (with Congo red).

The mechanisms of amyloid formation, although not well understood, can be summarized as follows:

Figure 3 - Formation and deposition of AL and AA amyloid.

Pathology

Systemic amyloidosis (AA) related to chronic inflammation tends to involve parenchymatous organs, such as kidneys, spleen, liver, and adrenals, while amyloidosis (AL) related to myeloma tends to affect mesodermal or other tissues, such as heart, gastrointestinal tract, peripheral nerves, skin, and tongue. Nevertheless, the overlap in organ involvement is such that the different forms of systemic amyloidosis are not distinguishable on that basis alone.

Grossly, organs extensively infiltrated by amyloid are usually enlarged and have a pale, waxy ("lardaceous") or varnished appearance and tough consistency. The iodine test for amyloid is done by applying iodine solution to the washed cut surface of the organ: amyloid typically stains mahogany-brown, and this color reaction changes to blue ( a "starch-like" reaction) after the application of dilute sulfuric acid.

Amyloidosis of the left atrial endocardium. 

Cross section of amyloid myocardium stained with Lugol's iodine solution.

The staining reactions of amyloid reflect its complex composition. Amyloid deposits in tissues typically stain as follows: homogeneously pink (as do other eosinophilic hyaline materials) with H&E; metachromatically (as do sulfated glycosaminoglycans) with crystal violet and similar dyes; and positively (as do glycoproteins) with periodic acid-Schiff. A diagnostic criterion, amyloid stains pink or orange with Congo red and, further, with Congo red staining shows green birefringence by polarizing microscopy, specific and unique properties shared by all amyloids due to their beta-pleated fibrillar structure.

Amyloid deposition (green birefringence) in tongue. Congo red. Polarizing microscopy.

Legend Structure of an amyloid fibril, depicting the beta-pleated sheet structure and binding sites for the Congo red dye, which is used for diagnosis of amyloidosis. 

The distribution of amyloid deposits is extracellular, closely related to the connective tissue framework of involved organs, and often interposed between parenchymal cells and their blood supply.

Kidneys. Grossly, amyloid kidneys are usually enlarged, pale, and smooth surfaced and have a tough consistency. On cortical transection, the glomeruli (barely visible as pink dots in the normal kidney) may be seen as enlarged, waxy, gray dots.

Primary amyloidosis of kidneys. 

Histologically, the mesangium and capillary basement membrane of glomeruli are the most frequent renal sites of amyloid deposition, followed by involvement of arteriolar and arterial walls and peritubular interstitial tissues. As the extent of the amyloid deposition increases, the glomerular capillary tufts become obliterated and replaced by functionless spherical masses of amyloid material

Amyloidosis of kidney. Most of this glomerulus is the site of homogeneous pink "soft fluffy" deposits of amyloid located focally in the widened mesangial regions and in the thickened capillary basement membranes. H&E.

Amyloidosis of kidney. This photomicrograph taken with plane polarized light shows diffuse amyloid deposition ( green birefringence) in the glomerular tufts throughout the mesangial regions. Congo red. Polarizing microscopy.

Amyloidosis of kidney. Congo red - polarizing microscopy.

Amyloidosis of kidney. In this photomicrograph, the glomerulus shows diffuse amyloid deposition in the widened mesangial, or intercapillary, regions and in the thickened capillary basement membranes. Glomerular capillary lumens are narrowed by concentric and eccentric deposits of amyloid material. PASH

Amyloidosis of kidney. As the amyloid deposition continues, the glomerular tufts become obliterated and replaced by spherical masses of amyloid material. H&E.

By electron microscopy, the first deposition of amyloid fibrils in the kidney is seen on the endothelial side of the glomerular capillary basement membrane (subendothelial deposits).

Liver. The amyloid liver is usually grossly enlarged, pale, smooth surfaced, and firm and, when sectioned, has sharp rigid edges. 

Microscopically, amyloid is initially deposited in the spaces (of Disse) between the hepatocytes and vascular sinusoids. As more amyloid accumulates, it compresses the hepatic cords and sinusoids. The hepatic cords undergo nutritional and pressure atrophy and become displaced or replaced by bands and nodules of amyloid.

Amyloidosis of liver. The hepatic cords are diminished in size and are compressed, fragmented, and replaced by amyloid deposition (pink). H&E.

Amyloidosis of liver. The hepatic parenchyma is infiltrated and replaced by nodular accumulations of amyloid (pink) in which remnants of hepatic cords are seen. H&E.

Spleen. Amyloidosis of the spleen has two different anatomical patterns. Most commonly, the amyloid deposition is limited to the splenic follicles, resulting in the gross appearance of a moderately enlarged spleen dotted with gray nodules (so called "sago" spleen). Alternatively, the amyloid deposits may spare the follicles and mainly infiltrate the red pulp sinuses, producing a large, firm spleen mottled with waxy discolorations ("lardaceous" spleen). 

Amyloidosis of the spleen

Heart. Amyloidosis of the heart may accompany systemic amyloid deposition or localized organ involvement (amyloidosis of aging). Histologically, the deposits are located between the myocardial fibers or in the walls of the coronary arteries

Amyloidosis of heart. Amyloid infiltrates the muscular wall of a coronary artery. Congo red. 

Other Organs. Amyloid deposition may involve the adrenals, thyroid, skin, tongue, and other portions of the alimentary tract. In the adrenals, extracellular amyloid deposits encompass, compress, and replace the cortical cells. A comparable pattern of amyloid deposition is seen in the thyroid. The alimentary tract may be involved at any level, from the tongue to the rectum (submucosa).

Amyloidosis of adrenal gland. Amyloid deposits surround, compress, and replace some cortical cells and infiltrate the wall of a small blood vessel (at one corner of the photomicrograph). Congo red.

Amyloidosis of tongue (interstitial tissue). Amyloid infiltrates the capillary walls and narrows the lumens of some of them. H&E.

Clinical Aspects

The clinical manifestations of amyloidosis vary from minimal to life threatening, depending upon the organs involved (see Table-3).

Table-3