eMedicine Specialties > Rheumatology > Miscellaneous Inflammatory Arthritis

Amyloidosis, Overview

Author: Robert O Holmes, Jr, DO, Fellow, Department of Rheumatology, Walter Reed Army Medical Center, Washington, DC
Coauthor(s): Bruce A Baethge, MD, Faculty, Texas A&M Medical School; Daniel R Jacobson, MD, Professor of Medicine, Boston University School of Medicine; Chief of Oncology, Veterans Affairs Boston Healthcare System; Jess D Edison, MD, Assistant Professor of Medicine, Uniformed Services University of Health Sciences; Staff Rheumatologist, Walter Reed Army Medical Center; Director, Inpatient Services, 115th Combat Support Hospital
Contributor Information and Disclosures

Updated: Jul 30, 2009

Definition of Amyloidosis

Definition of amyloid and amyloidosis

Amyloid is defined as in vivo deposited material distinguished by fibrillar electron micrographic appearance, amorphous eosinophilic appearance on hematoxylin and eosin staining (see Image 1), beta pleated sheet structure as observed by x-ray diffraction pattern, apple-green birefringence on Congo Red histological staining (see Image 2), and solubility in water and buffers of low ionic strength. All types of amyloid consist of a major fibrillar protein that defines the type of amyloid.

Amyloidosis is a clinical disorder caused by extracellular and or intracellular deposition of insoluble abnormal amyloid fibrils that alter the normal function of tissues. Approximately 10% of amyloidosis depots consist of components such as glycosaminoglycans (GAGs), apolipoprotein E (apoE), and serum amyloid P (SAP) component, while 90% of the depots consist of the amyloid fibrils that are formed by the aggregation of misfolded proteins. These proteins either arise from proteins expressed by cells at the deposition site (localized) or precipitate systemically after production at a local site (systemic).¹ In humans, about 23 different unrelated proteins are known to form amyloid fibrils in vivo.² Many mechanisms of protein function contribute to amyloidogenesis, including "nonphysiologic proteolysis, defective physiologic proteolysis, mutations involving changes in thermodynamic or kinetic properties, and pathways that are yet to be defined."²

Classification Systems: Historical (Clinical Based) and Modern (Biochemical Based)

Historical classification systems (clinical based)

Until the early 1970s, the idea of a single amyloid substance predominated. Various descriptive classification systems were proposed based on the organ distribution of amyloid deposits and clinical findings. Most classification systems included primary (ie, in the sense of idiopathic) amyloidosis, in which no associated clinical condition was identified, and secondary amyloidosis, ie, associated with chronic inflammatory conditions. Some classification systems included myeloma-associated, familial, and localized amyloidosis.

The modern era of amyloidosis classification began in the late 1960s with the development of methods to solubilize amyloid fibrils. These methods permitted chemical amyloid studies. Descriptive terms such as primary amyloidosis, secondary amyloidosis, and others (eg, senile amyloidosis), which are not based on etiology, provide little useful information and are no longer recommended.

Modern amyloidosis classification (biochemical based)

Amyloid is now classified chemically. The amyloidoses are referred to with a capital letter A (for amyloid) followed by an abbreviation for the fibril protein. For example, in most cases formerly called primary amyloidosis and in myeloma-associated amyloidosis, the fibril protein is an immunoglobulin light chain or light chain fragment (abbreviated L); thus, patients with these amyloidoses are now said to have light chain amyloidosis (AL). Names such as AL describe the protein (light chain), but not necessarily the clinical phenotype.¹

Similarly, in most cases previously termed senile cardiac amyloidosis and in many cases previously termed familial amyloid polyneuropathy (FAP), the fibrils consist of the transport protein transthyretin (TTR); these diseases are now collectively termed ATTR.

Proteins that form amyloid fibrils differ in size function, amino acid sequence, and native structure but become insoluble aggregates that are similar in structure and in properties. Protein misfolding results in formation of fibrils that show a common beta sheet pattern on x-ray diffraction. In theory, misfolded amyloid proteins can be attributed to infectious sources (prions), de novo gene mutations, errors in transcription, errors in translation, errors in posttranslational modification, or protein transport. For example, in ATTR, 100 different points of single or double mutations, or deletions in the TTR gene and several different phenotypes of FAP have been documented.³ Twenty-three different fibril proteins are described in human amyloidosis, with variable clinical features. The major types of human amyloid are outlined and discussed individually in the table below.

Human Amyloidoses

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Systemic Amyloidoses

A amyloidosis

The precursor protein in A amyloidosis (AA) is a normal-sequence apo-SAA (serum amyloid A protein) now called "A," which is an acute-phase reactant produced mainly in the liver in response to multiple cytokines.¹ "A" protein circulates in the serum bound to high-density lipoprotein.

AA occurs in various chronic inflammatory disorders, chronic local or systemic microbial infections, and occasionally with neoplasms. The frequency of amyloidosis has been shown to vary significantly in different ethnic groups.⁴ Some of the conditions associated with AA include the following:

• Rheumatoid arthritis (RA)
• Juvenile chronic arthritis
• Ankylosing spondylitis
• Psoriasis and psoriatic arthritis
• Still disease
• Behçet syndrome
• Familial Mediterranean fever
• Crohn disease
• Leprosy
• Osteomyelitis
• Tuberculosis
• Chronic bronchiectasis
• Castleman disease
• Hodgkin disease
• Renal cell carcinoma
• Carcinoma of the gastrointestinal, lung, or urogenital tract
• Cryopyrin-associated periodic syndromes (CAPS)

Organs that are typically involved include the kidney, liver, and spleen. Worldwide, AA is the most common systemic amyloidosis; it was formerly termed secondary amyloidosis. Therapy has traditionally been aimed at the underlying inflammatory condition to reduce the production of the precursor amyloid protein SAA. Disease modifying antirheumatic drugs (DMARDS) such as colchicine, a microtubule inhibitor and weak immunosuppressant, can prevent secondary renal failure due to amyloid deposition specifically in familial Mediterranean fever.

Newer therapies have become more targeted to avoid the cytotoxicity of older agents (chlorambucil, cyclophosphamide). Recently, the SAA amyloid seen in CAPS was reduced with a new biologic interleukin (IL)–1 beta trap called rilonacept. Tumor necrosis factor (TNF)–alpha is also thought to be involved in amyloid deposition.⁵ Aggressive use of newer biologic therapies for RA, such as etanercept (a TNF-alpha blocker), have been used to decrease the concentration of SAA, serum creatinine, creatinine clearance, and proteinuria in renal AA associated with RA.⁶

Additionally, SAA isoforms have been studied using high-resolution 2-dimensional gel electrophoresis and peptide mapping by reverse-phase chromatography, electrospray ionization tandem mass spectrometry, and genetic analysis down to the posttranslational modification level.⁷ SAA is coded by 4 genes— SAA1, SAA2, SAA3, and SAA4. The SAA1 gene contributes to most of the deposits and contains a single nucleotide polymorphism that defines at least 3 haplotypes. The saa1.3 allele was found to be a risk factor and a poor prognostic indicator in Japanese patients with RA. Genetic analysis has proven useful not only in selecting patients for biologic therapy but also in predicting outcome (see below).⁸

Light chain amyloidosis

The precursor protein is a clonal immunoglobulin light chain or light chain fragment. AL is a monoclonal plasma cell disorder closely related to multiple myeloma, as some patients fulfill diagnostic criteria for multiple myeloma. Typical organs involved include the heart, kidney, peripheral nerve, gastrointestinal tract, respiratory tract, and nearly any other organ. AL includes former designations of primary amyloidosis and myeloma-associated amyloidosis.

Treatment usually mirrors the management of multiple myeloma (ie, chemotherapy). Selected patients have received benefit from high-dose melphalan and autologous stem-cell transplantation, with reports of prolonged survival in recent studies. Alternative therapeutic approaches include thalidomide, lenalidomide, iododoxorubicin, etanercept, and rituximab.⁹ Iododoxorubicin, a molecule that binds to and solubilizes amyloid fibrils, is undergoing clinical study. For more information, see Amyloidosis, Immunoglobulin-Related.

Heavy chain amyloidosis

In a few cases, immunoglobulin chain amyloidosis fibrils contain only heavy chain sequences rather than light chain sequences, and the disease is termed heavy chain amyloidosis (AH) rather than AL. Electron microscopy may be helpful in the detection of small deposits and in the differentiation of amyloid from other types of renal fibrillar deposits.¹⁰ For more information, see Amyloidosis, Immunoglobulin-Related.

Transthyretin amyloidosis

The precursor protein is the normal- or mutant-sequence TTR, a transport protein synthesized in the liver and choroid plexus. TTR is a tetramer of 4 identical subunits of 127 amino acids each. Normal-sequence TTR forms amyloid deposits in the cardiac ventricles of elderly people (ie, >70 y); this disease was also termed senile cardiac amyloidosis. The prevalence of TTR cardiac amyloidosis increases progressively with age, affecting 25% or more of persons older than 90 years. Normal-sequence ATTR can be an incidental autopsy finding or can cause clinical symptoms (eg, heart failure, arrhythmias).

Point mutations in TTR increase the tendency of TTR to form amyloid. Amyloidogenic TTR mutations are inherited as an autosomal-dominant disease with variable penetrance. More than 100 amyloidogenic TTR mutations are known.¹¹ The most prevalent TTR mutations include TTR Val30Met (common in Portugal, Japan, and Sweden), and TTR Val122Ile (carried by 3.9% of African Americans). Amyloidogenic TTR mutations cause deposits primarily in the peripheral nerves, heart, gastrointestinal tract, and vitreous.

Mutant-sequence amyloidogenic TTR is treated with liver transplantation or supportive care. Liver transplantation should be performed in Val30Met-positive patients as early as possible.¹¹ For normal-sequence amyloidogenic TTR, the treatment is supportive care. For details, see Amyloidosis, Transthyretin-Related.

Beta₂ -microglobulin amyloidosis

The precursor protein is a normal beta₂ -microglobulin (β₂ M), which is the light chain component of the major histocompatibility complex. In the clinical setting, β₂ M is associated with dialysis and, rarely, renal failure in the absence of dialysis. β₂ M is normally catabolized in the kidney.

In patients with renal failure, the protein accumulates in the serum. Conventional dialysis membranes do not remove β₂ M; therefore, serum levels in patients on hemodialysis can vault to 30-60 times the reference range. Traut et al (2007) reported that patients using polyamide high-flux membranes had lower β₂ M concentrations than patients on low-flux dialyzers.¹² They postulated that the difference was mediated by an increase in β₂ M mRNA, lower concentrations of β₂ M released from the blood cells, and/or better β₂ M clearance in patients treated with high-flux dialyzers.¹²

Musculoskeletal involvement is common and is characterized by deposits in the carpal ligaments, synovium, and bone, resulting in carpal tunnel syndrome, destructive arthropathy, bone cysts, and fractures. Other organs involved include the heart, gastrointestinal tract, liver, lungs, prostate, adrenals, and tongue.

Treatment includes renal transplantation, which may arrest amyloid progression. For details, see Amyloidosis, Beta2M (Dialysis-Related).

Cryopyrin-associated periodic syndrome–associated amyloidosis

Familial cold autoinflammatory syndrome (FCAS), Muckle-Wells syndrome (MWS), and neonatal-onset multisystem inflammatory disease (NOMID) are all types of CAPS.

These disorders are typically associated with heterozygous mutations in the NLRP3 (CIAS1) gene, which encodes the cryopyrin (NALP3) protein, and are inherited in an autosomal-dominant manner.¹³ The inflammation in CAPS is driven by excessive release of IL-1.¹⁴ IL-1 release is normally regulated by an intracellular protein complex known as the inflammasome, which maps to a gene sequence called NLRP3. Mutations in NLRP3 may cause an aberrant cryopyrin protein inside the inflammasome, leading to the release of too much IL-1 and subsequent multisystem inflammation.

Patients with CAPS have chronically elevated levels of acute-phase reactants, especially serum amyloid A (SAA) and high-sensitivity C-reactive protein (hsCRP), due to increased IL-1 levels.^15,16,17 With elevated SAA combined with multisystem cytokine dysregulation, multisystemic amyloid deposition can be severe, with the most feared complication including renal failure. By blocking the action of IL-1 or down-regulating its production, inflammation and therefore amyloid deposition can be reduced.¹⁸

In a recent randomized double-blind CAPS therapy trial, a novel soluble decoy receptor called rilonacept was shown to provide rapid and profound symptom improvement in addition to measures of inflammation such as hsCRP and SAA levels.¹⁸ In the second part of the study, continued treatment with rilonacept maintained improvements, and discontinuation worsened disease activity.¹⁸

Muckle-Wells syndrome

MWS is another autoinflammatory syndrome secondary to a mutation in CIAS gene encoding cryopyrin, a component of the inflammasome that regulates the processing of IL-1. The IL-1 receptor antagonist anakinra has been shown to improve the signs and symptoms in MWS by decreasing serum CRP and SAA levels and cytokines such as IL-6, IL-8, IL-12, and IL-1. In some cases, it improved sensory deafness, as well as the laboratory values for markers of inflammation MWS.¹⁹

Hereditary Renal Amyloidoses

Hereditary amyloidoses encompass a group of conditions that each are related to mutations in a specific protein. The most common form is TTR amyloidosis (usually neuropathic), but nonneuropathic amyloidoses likely result from abnormalities in lysozyme, fibrinogen, alpha-chain, or apolipoprotein A-I and A-II.²⁰ Consider these diseases when a renal biopsy demonstrates amyloid deposition and when they are likely diagnoses (rather than AL or AA) because the family history suggests an autosomal-dominant disease. Again, the definitive diagnosis is made using immunohistologic staining of the biopsy material with antibodies specific for the candidate amyloid precursor proteins. A clinical correlation is required to diagnose amyloid types, even if a hereditary form is detected by amyloid protein typing.²¹ For details, see Amyloidosis, Familial Renal.

Apolipoprotein AI amyloidosis (apoAI) is an autosomal-dominant amyloidosis caused by point mutations in the apoAI gene. Usually, this amyloidosis is a prominent renal amyloid but can also form in many locations. ApoAI (likely of normal sequence) is the fibril precursor in localized amyloid plaques in the aortae of elderly people. ApoAI can manifest either as a nonhereditary form with wild-type protein deposits in atherosclerotic plaques or as a hereditary form due to germline mutations in the apoA1 gene.²² Currently, more than 50 apoAI variants are known, and 13 are associated with amyloidosis.²² As more gene locations are found, the clinical phenotypes are slowly being elucidated.

Fibrinogen amyloidosis (AFib) is an autosomal-dominant amyloidosis caused by point mutations in the fibrinogen alpha chain gene. If DNA sequences indicate a mutant amyloid precursor protein, protein analysis of the deposits must provide the definitive evidence in laboratories with sophisticated methods.²¹

Lysozyme amyloidosis (ALys) is an autosomal-dominant amyloidosis caused by point mutations in the lysozyme gene.

Apolipoprotein AII amyloidosis (AapoAII) is an autosomal-dominant amyloidosis caused by point mutations in the apoAII gene. The 2 kindreds described with this disorder have each carried a point mutation in the stop codon, leading to production of an abnormally long protein.

Central Nervous System Amyloidoses and Other Localized Amyloidoses

Central Nervous System Amyloidoses

Beta protein amyloid

The amyloid beta precursor protein (AbPP), which is a transmembrane glycoprotein, is the precursor protein in beta protein amyloid (Ab). Three distinct clinical settings are as follows:

1. Alzheimer disease has a normal-sequence protein, except in some cases of familial Alzheimer disease, in which mutant beta protein is inherited in an autosomal-dominant manner.
2. Down syndrome has a normal-sequence protein that forms amyloids in most patients by the fifth decade of life.
3. Hereditary cerebral hemorrhage with amyloidosis (HCHWA), Dutch type, is inherited in an autosomal-dominant manner. The beta protein contains a point mutation. These patients typically present with cerebral hemorrhage followed by dementia.

The accumulation of amyloid-β peptide (Aβ) in the brain, both in the form of plaques in the cerebral cortex and in blood vessels as cerebral amyloid angiopathy (CAA), causes progressive cognitive decline. Recently, experimental models and human clinical trials have shown that accumulation of Aβ plaques can be reversed by immunotherapy. Aβ immunization results in solubilization of plaque Aβ42, which, at least in part, exits the brain via the perivascular pathway, causing a transient increase in the severity of CAA. The extent to which these vascular alterations following Aβ immunization in Alzheimer disease are reflected in changes in cognitive function remains to be determined.²³

Prion protein amyloidosis

The precursor protein in prion protein amyloidosis (APrP) is a prion protein, which is a plasma membrane glycoprotein. The etiology is either infectious (ie, kuru, transmissible spongiform encephalitis [TSE]) or genetic (ie, Creutzfeldt-Jakob disease [CJD], Gerstmann-Strãussler-Scheinker [GSS] syndrome, fatal familial insomnia [FFI]). The infectious prion protein is a homologous protein encoded by a host chromosomal gene that induces a conformational change in a native protease-sensitive protein, increasing the content of beta-pleated sheets. The accumulation of these beta-pleated sheets renders the protein protease-resistant and therefore amyloidogenic.²⁴ Patients with TSE, CJD, GSS, and FFI carry autosomal-dominant amyloidogenic mutations in the prion protein gene; therefore, the amyloidosis forms even in the absence of an infectious trigger.

Similar infectious animal disorders include scrapie in sheep and goats and bovine spongiform encephalitis (ie, mad cow disease).

Cystatin C amyloidosis

The precursor protein in cystatin C amyloidosis (ACys) is cystatin C, which is a cysteine protease inhibitor that contains a point mutation. This condition is clinically termed HCHWA, Icelandic type.

ACys is autosomal dominant. The clinical presentation includes multiple strokes and mental status changes beginning in the second or third decade of life. Many patients die by age 40 years. This disease is documented in a 7-generation pedigree in northwest Iceland. The pathogenesis is one of mutant cystatin that is widely distributed in tissues, but fibrils form only in the cerebral vessels; therefore, local conditions must play a role in fibril formation.

Non–amyloid beta cerebral amyloidosis (chromosome 13 dementias)

Two syndromes (British and Danish familial dementia) that share many aspects of clinical Alzheimer disease have been identified. Findings include the presence of neurofibrillary tangles, parenchymal preamyloid and amyloid deposits, cerebral amyloid angiopathy, and amyloid-associated proteins. Both conditions have been linked to specific mutations on chromosome 13; they cause abnormally long protein products (ABri and ADan) that ultimately result in different amyloid fibrils.

Other Localized Amyloidoses

Gelsolin amyloidosis

The precursor protein in gelsolin amyloidosis (AGel) is the actin-modulating protein gelsolin. Amyloid fibrils include a gelsolin fragment that contains a point mutation. Two amyloidogenic gelsolin mutations are described. One example is Asp187Asn, which is endemic in southeast Finland.

Clinical characteristics include slowly progressive cranial neuropathies, distal peripheral neuropathy, and lattice corneal dystrophy.

Atrial natriuretic factor amyloidosis

The precursor protein is atrial natriuretic factor (ANF), a hormone that controls salt and water homeostasis; it is synthesized by the cardiac atria. Amyloid deposits are localized to the cardiac atria. This condition is highly prevalent in elderly people and is of generally little clinical significance. Atrial natriuretic factor amyloidosis (AANF) is most common in patients with long-standing congestive heart failure, presumably because of persistent ANF production. No relation exists to the amyloidoses that involve the cardiac ventricles (ie, AL, ATTR).

Keratoepithelin amyloidosis and lactoferrin amyloidosis

Point mutations occur in a gene termed BIGH3, which encodes keratoepithelin and leads to autosomal-dominant corneal dystrophies characterized by the accumulation of corneal amyloid. Some BIGH3 mutations cause amyloid deposits, and others cause nonfibrillar corneal deposits. Another protein, lactoferrin, is also reported as the major fibril protein in familial subepithelial corneal amyloidosis. The relationship between keratoepithelin and lactoferrin in familial corneal amyloidosis is not yet clear.

Calcitonin amyloid

In calcitonin amyloid (ACal), the precursor protein is calcitonin, a calcium regulatory hormone synthesized by the thyroid. Patients with medullary carcinoma of the thyroid may develop localized amyloid deposition in the tumors, consisting of normal-sequence procalcitonin (ACal). The presumed pathogenesis is increased local calcitonin production, leading to a sufficiently high local concentration of the peptide and causing polymerization and fibril formation.

Islet amyloid polypeptide amyloidosis

In islet amyloid polypeptide amyloidosis (AIAPP), the precursor protein is an islet amyloid polypeptide (IAPP), also known as amylin. IAPP is a protein secreted by the islet beta cells that are stored with insulin in the secretory granules and released in concert with insulin. Normally, IAPP modulates insulin activity in skeletal muscle. IAPP amyloid is found in insulinomas and in the pancreas of many patients with diabetes mellitus type 2.

Prolactin amyloid

In prolactin amyloid (Apro), prolactin or prolactin fragments are found in the pituitary amyloid. This condition is often observed in elderly people and has also been reported in an amyloidoma in a patient with a prolactin-producing pituitary tumor.

Keratin amyloid

Some forms of cutaneous amyloid react with antikeratin antibodies. The identity of the fibrils is not chemically confirmed in keratin amyloid (Aker).

Medin amyloid

Aortic medial amyloid occurs in most people older than 60 years. Medin amyloid (AMed) is derived from a proteolytic fragment of lactadherin, a glycoprotein expressed by mammary epithelium.

Nonfibrillar Components of Amyloid

All types of amyloid deposits contain not only the major fibrillar component (solubility in water, buffers of low ionic strength) but also nonfibrillar components that are soluble in conventional ionic strength buffers. The role of the minor components in amyloid deposition is not clear. These components do not appear to be absolutely required for fibril formation, but they may enhance fibril formation or stabilize formed fibrils.

The nonfibrillar components, contained in all types of amyloid, include the following:

• Pentagonal component
  • Pentagonal (P) component comprises approximately 5% of the total protein in amyloid deposits. This component is derived from the circulating SAP component, which behaves as an acute-phase reactant. The P component is one of the pentraxin group of proteins, with homology to C-reactive protein. In experimental animals, amyloid deposition is slowed without the P component.
  • Radiolabeled material homes to amyloid deposits; therefore, this component can be used in amyloid scans to localize and quantify amyloidosis and to monitor therapy response. Radiolabeled P component scanning has proven clinically useful in England, where the technology was developed, but it is available in only a few centers worldwide.
• Apolipoprotein E
  • ApoE is found in all types of amyloid deposits.
  • One allele, ApoE4, increases the risk for beta protein deposition, which is associated with Alzheimer disease. ApoE4 as a risk factor for other forms of amyloidosis is controversial.
  • The role of apoE in amyloid formation is not known.
• Glycosaminoglycans
  • GAGs are heteropolysaccharides composed of long unbranched polysaccharides that contain a repeating disaccharide unit. These proteoglycans are basement membrane components intimately associated with all types of tissue amyloid deposits. Amyloidotic organs contain increased amounts of GAGs, which may be tightly bound to amyloid fibrils. Heparan sulfate and dermatan sulfate are the GAGs most often associated with amyloidosis.
  • Heparan sulfate and dermatan sulfate have an unknown role in amyloidogenesis. Studies of AA and AL amyloid have shown marked restriction of the heterogeneity of the GAG chains, suggesting that particular subclasses of heparan and dermatan sulfates are involved.
  • Compounds that bind to heparan sulfate proteoglycans (eg, anionic sulfonates) decrease fibril deposition in murine models of AA and have been suggested as potential therapeutic agents.
• Other components found in some types of amyloid include complement components, proteases, and membrane constituents.

Mechanisms of Amyloid Formation

Amyloid protein structures

In all forms of amyloidosis, the cell secretes the precursor protein in a soluble form that becomes insoluble at some tissue site, compromising organ function. All the amyloid precursor proteins are relatively small (ie, molecular weights 4000-25,000) and do not share any amino acid sequence homology. The secondary protein structures of most soluble precursor proteins (except for SAA and chromosomal prion protein [Prpc]) have substantial beta pleated sheet structure, while extensive beta sheet structure occurs in all of the deposited fibrils.

In some cases, hereditary abnormalities (primarily point mutations) in the precursor proteins are always present (eg, lysozyme, fibrinogen, cystatin C, gelsolin). In other cases, fibrils form from normal-sequence molecules (eg, AL, β₂ M). In other cases, normal-sequence proteins can form amyloid, but mutations underlying inflammatory milieu accelerate the process (eg, TTR, beta protein precursor, CAPS).

Deposition location

In localized amyloidoses, the deposits form close to the precursor synthesis site; however, in systemic amyloidoses, the deposits may form either locally or at a distance from the precursor-producing cells. Amyloid deposits primarily are extracellular, but reports exist of fibrillar structures within macrophages and plasma cells.

Proteolysis and protein fragments

In some types of amyloidosis (eg, always in AA, often in AL, ATTR), the amyloid precursors undergo proteolysis, which may enhance folding into an amyloidogenic structural intermediate. Also, some of the amyloidoses may have a normal proteolytic process that is disturbed, yielding a high concentration of an amyloidogenic intermediate. For example, it was shown that the mast cells of allergic responses may also participate in the development of secondary or amyloid AA in chronic inflammatory conditions. Mast cells hasten the partial degradation of the SAA protein that can produce highly amyloidogenic N-terminal fragments of SAA.²⁵ However, factors that lead to different organ tropisms for the different amyloidoses are still largely unknown.

Whether the proteolysis occurs before or after tissue deposition is unclear in patients in whom protein fragments are observed in tissue deposits. In some types of amyloid (eg, AL, Aβ, ATTR), nonfibrillar forms of the same molecules can accumulate before fibril formation; thus, nonfibrillar deposits, in some cases, may represent intermediate deposition.

Approach To Diagnosing Amyloidosis

Pathologic diagnosis (Congo red staining and immunohistochemistry)

Immunocytochemical studies for amyloid should include stains for Congo Red apple green birefringence, hematoxylin and eosin staining (H&E) stains for amorphous material, kappa and lambda light chains, beta-amyloid A4 protein, TTR, beta 2-microglobulin, cystatin C, gelsolin, and immunoreactivity with antiamyloid AA antibody.²⁶

Amyloidosis is diagnosed when Congo red–binding material is demonstrated in a biopsy specimen. Because different types of amyloidosis require different approaches to treatment, determining only that a patient has a diagnosis of amyloidosis is no longer adequate. A clinical situation may suggest the type of amyloidosis, but the diagnosis generally must be confirmed by immunostaining a biopsy specimen. Antibodies against the major amyloid fibril precursors are commercially available. For example, AL, ATTR, and Aβ₂ M can present as carpal tunnel syndrome or gastrointestinal amyloidosis, but each has a different etiology and requires a different treatment approach.

Similarly, determining whether the amyloid is of the AL or ATTR type is often difficult in patients with cardiac amyloidosis because the clinical picture is usually similar. Without immunostaining to identify the type of deposited protein, an incorrect diagnosis can lead to ineffective and, perhaps, harmful treatment. Be wary of drawing diagnostic conclusions from indirect tests (eg, monoclonal serum proteins) because the results of these presumptive diagnostic tests can be misleading; for example, monoclonal serum immunoglobulins are common in patients older than 70 years, but the most common form of cardiac amyloidosis is derived from TTR.

Diagnosis by subcutaneous fat aspiration

For many years, rectal biopsy was the first procedure of choice. An important clinical advance was the recognition that the capillaries in the subcutaneous fat are often involved in patients with systemic amyloidosis and can often provide sufficient tissue for the diagnosis of amyloid, immunostaining, and, in some cases, amino acid sequence analysis; thus, biopsy of the organ with the most severe clinical involvement is often unnecessary.

For example, in cardiac amyloidosis, the definitive diagnosis of the type of amyloid can be made using an endomyocardial biopsy specimen, with Congo red and immunologic staining of the tissue sample. Alternatively, when noninvasive testing suggests cardiac amyloidosis, a specific diagnosis is often made by studying a subcutaneous fat aspiration instead of endomyocardial biopsy, thereby avoiding an invasive procedure.

Organ biopsies

When the subcutaneous fat aspiration biopsy does not provide information to reach a firm diagnosis, biopsy samples can be collected from other organs. In addition, an advantage to performing a biopsy of an involved organ (eg, kidney, heart) is that it definitively establishes a cause-and-effect relationship between the organ dysfunction and amyloid deposition.

It is important to recognize that not all biopsy sites offer the same sensitivity. The best sites to biopsy are the abdominal fat pad and rectal mucosa (approaching 90% sensitivity for fat pad and 73%-84% for rectal mucosa).²⁷ Other sites that are often sampled but have poor sensitivity for the diagnosis of amyloid include the salivary glands, skin, tongue, gingiva, stomach, and bone marrow.

Amyloid Arthropathy

RA and other autoimmune diseases (juvenile RA, spondyloarthropathies, autoinflammatory syndromes) can predispose to the deposition of amyloid fibrils.²⁸ Amyloidosis is actually a common cause of death in patients with ankylosing spondylitis. Amyloid deposition in inflammatory syndromes is amplified by the underlying inflammatory state, significantly increasing morbidity and mortality, especially in the case of renal amyloidosis.²⁹ However, autopsy study shows that amyloid arthropathy in RA is often undiagnosed in patients with long-standing severe RA.³⁰

Amyloid arthropathy is typically seen in the shoulders, knees, wrists, and elbows, especially because these joints are more easily aspirated to make tissue available for Congo Red and immunostaining. Amyloid arthropathy can actually mimic classic RA but usually lacks the intense distal synovitis and affects the hips and shoulder more than peripheral joints.²⁷ Synovial fluid found to contain amyloid fibrils, although not particularly inflammatory, with white blood cell counts on average less than 2000/µL, contains marked synovial villi hypertrophy. The classic "shoulder-pad" sign denotes end-stage amyloid deposits in the shoulder synovium and periarticular structures.

Another articular structure commonly affected by amyloid deposition is the carpal tunnel. Carpal tunnel syndrome can be the presenting of sign of primary or secondary forms of amyloid, as only minimal deposits are required to impair nerve conduction. The diagnosis can be made with biopsy at the time of carpal tunnel release surgery or other joint procedures. In a pathology review of 124 patients undergoing carpal tunnel release without the previous diagnosis or clinical signs of amyloidosis, 82% had amyloid deposition. At 10-year follow up, only two patients had systemic amyloidosis diagnosed after amyloid was discovered in their tenosynovium.³¹

Radiography can show irregularly shaped hyperlucencies, subchondral cysts, and erosions that correspond with low intensity signals on both T1 and T2 MRIs.³² Ultrasonographic investigations may also show lucencies, soft-tissue changes, increased thickness of tendons, and joint effusions with echogenic zones.³³

Up to 5% of patients with long-standing RA can develop systemic amyloidosis that usually presents as nephrotic syndrome.²⁷ Genetic studies can predict certain haplotypes that can increase the risk of developing amyloidosis 7-fold.²⁹ This increasing understanding of haplotypes and proteomics will hopefully lead to more specific therapies.

Interestingly, with the advent of newer, more effective therapies for inflammatory arthritis, the incidence of amyloidosis secondary to RA and other rheumatic conditions has recently decreased from 5% to less than 1%.²⁷

Multimedia

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Media file 1: Amorphous eosinophilic interstitial amyloid observed on a renal biopsy.

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Media file 2: Congo Red staining of a cardiac biopsy specimen containing amyloid, viewed under polarized light.

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Media file 3: Paucicellular birefringent material (amyloid) between skeletal muscle fibers of the tongue in light chain amyloidosis. (Courtesy Robert O. Holmes Jr, DO, and the Walter Reed Army Medical Center Pathology Department)

Keywords

amyloid diseases, primary amyloidosis, secondary amyloidosis, myeloma-associated amyloidosis, familial amyloidosis, localized amyloidosis, senile amyloidosis, senile cardiac amyloidosis, light chain amyloidosis, AL, familial amyloid polyneuropathy, transport protein transthyretin, TTR, ATTR, systemic amyloidosis, A amyloidosis, AA, heavy chain amyloidosis, AH, beta₂ -microglobulin amyloidosis, Aβ₂ M

familial renal amyloidosis, apolipoprotein AI amyloidosis, AapoAI, fibrinogen amyloidosis, AFib, lysozyme amyloidosis, ALys, apolipoprotein AII amyloidosis, AapoAII, beta protein amyloid, Ab, prion protein amyloidosis, APrP, cystatin C amyloidosis, ACys, gelsolin amyloidosis, AGel, atrial natriuretic factor amyloidosis, AANF, keratoepithelin amyloidosis, AKE, lactoferrin amyloidosis, ALac, calcitonin amyloidosis, ACal, islet amyloid polypeptide amyloidosis, AIAPP, prolactin amyloid, Apro, keratin amyloid, Aker, cryopyrin-associated periodic syndromes, CAPS, primary localized cutaneous nodular amyloidosis, PLCNA, amyloid arthropathy

The views expressed in this article are those of the authors and do not reflect the official policy of the Department of Army, Navy, Department of Defense, or U.S. Government.

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