What is AA (inflammatory) amyloidosis?

Updated: Jan 08, 2021
  • Author: Jefferson R Roberts, MD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

Amyloidosis comprises of a heterogeneous group of diseases in which normally soluble plasma proteins are deposited in the extracellular space in an abnormal, insoluble, fibrillar form.

Amyloid A (AA) amyloidosis is the most common form of systemic amyloidosis worldwide. [1]  It is characterized by extracellular tissue deposition of fibrils that are composed of fragments of serum amyloid A (SAA) protein, a major acute-phase reactant protein, produced predominantly by hepatocytes. AA amyloidosis occurs in the course of a chronic inflammatory disease of either infectious or noninfectious etiology, hereditary periodic fevers, and with certain neoplasms such as Hodgkin lymphoma and renal cell carcinoma. [3]

In developing countries, the most common instigator of AA amyloidosis is chronic infection; in industrialized societies, rheumatic diseases, such as rheumatoid arthritis (RA), are the usual stimuli. The United States is a major exception to this in that immunoglobulin-related amyloid light chain type (AL) of amyloidosis is more frequent than AA as the cause of systemic amyloid deposition.

The major sites of involvement in AA amyloidosis are the kidney, liver, and spleen. Clinically overt disease typically develops when renal damage occurs, manifesting as proteinuria, nephrotic syndrome, or derangement in renal function.

The tissue fibril consists of a 7500-dalton cleavage product of the SAA protein, which is an acute phase reactant, and like C-reactive protein, is synthesized by hepatocytes under the transcriptional regulation of cytokines including interleukin (IL)-1, IL-6 and tumor necrosis factor (TNF). [5, 6]  Under the influence of the inflammatory cytokine IL-6, hepatic transcription of the messenger ribonucleic acid (mRNA) for SAA may increase 1000-fold when exposed to an inflammatory stimulus.

Intact circulating SAA (molecular weight 12,500 dalton) is complexed with high-density lipoproteins (HDL). During the course of inflammation, the apolipoprotein SAA (apoSAA) apparently displaces apolipoprotein A1 (apoA1) from the HDL particles and facilitates HDL-cholesterol uptake by macrophages.

Several lines of evidence have indicated that the conversion of SAA into amyloid fibrils occurs through its specific interaction with heparan sulphate, a ubiquitously expressed glycosaminoglycan component of the extracellular matrix. SAA specifically binds to heparan sulfate (HS) glycosaminoglycan, a common constituent of all types of amyloid deposits that has been shown to facilitate conformational transition of a precursor to beta-pleated sheet structure. [8]

The protein has also been shown to be chemotactic for neutrophils, and it stimulates degranulation, phagocytosis, and cytokine release in these cells.

Until relatively recently, the erythrocyte sedimentation rate (ESR) and the serum C-reactive protein (CRP) level were used to monitor inflammation clinically. Current data suggest that, under some circumstances, changes in SAA may be a better measure. Increases in both CRP and SAA have been associated with active atherosclerotic coronary artery disease and cited as evidence for the inflammatory nature of that disease process. SAA also has been used to monitor the dissemination of malignancy.

For information on other types of amyloidosis, see Amyloidosis.


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