Pathogenesis of AA amyloidosis

INTRODUCTION — AA amyloid results from the deposition in tissue of serum amyloid A (SAA) protein, which is a major acute phase reactant. Amyloidosis encompasses a group of diseases caused by misfolding and extracellular accumulation of proteins as fibrillar deposits [1,2]. These fibrils stain with Congo red and produce pathognomonic green birefringence when viewed by microscopy under crossed polarized light. The process of amyloid formation and deposition causes tissue toxicity and progressive organ dysfunction.

The pathogenesis of AA amyloidosis is presented here. The clinical manifestations, diagnosis, and treatment of this disorder, an overview of amyloidosis, and the pathogenesis of other forms of amyloidosis are discussed separately. (See "Causes and diagnosis of AA amyloidosis and relation to rheumatic diseases" and "Treatment of AA (secondary) amyloidosis" and "Overview of amyloidosis".)

AMYLOID PRECURSORS AND AMYLOIDOGENESIS — All forms of amyloidosis, including AA amyloidosis, are characterized by codeposition of other molecules, including:

●Serum amyloid P component (SAP), a member of the pentraxin family that includes C-reactive protein [2]

●Glycosaminoglycans, notably heparan sulfate [3]

●Apolipoproteins (E and A4) [4]

There are 36 different human precursor proteins known to be deposited in a fibrillar configuration as amyloid, 14 associated with systemic disease [5]. Common examples include:

●Serum amyloid A (SAA) protein in AA amyloidosis

●Monoclonal immunoglobulin light chains in primary or immunoglobulin light chain (AL) amyloidosis (see "Monoclonal immunoglobulin deposition disease")

●Beta-2 microglobulin in dialysis-related amyloidosis (see "Dialysis-related amyloidosis")

●Transthyretin in wild-type transthyretin (TTR) amyloidosis (previously known as senile cardiac amyloidosis)

●Several different apolipoproteins among the hereditary amyloidoses (see "Genetic factors in the amyloid diseases")

A number of factors have been identified that contribute to the pathogenesis of AA amyloid:

●Sustained increase in the serum concentration of SAA in chronic inflammatory states [6]

●Post-translational modifications that may influence synthesis or destabilize SAA and favor its misfolding and aggregation [7]

●Interruption of ligand binding to various receptors as a result of proteolysis at position 76 of SAA [8]

●The intrinsic propensity of SAA subsequences to misfolding [9]

A combination of these factors may determine the amyloidogenicity. Nonetheless, undetermined environmental and genetic factors must also be involved as only a minority of patients with sustained inflammation and persistent elevation of SAA levels develop AA amyloidosis [2].

Cofactors, such as glycosaminoglycans and SAP, may play a role in predisposition to amyloid formation via one or more of the following effects [10-14]:

●Modulation of fibrillogenesis by direct binding to specific domains of subunit proteins or their precursors

●Stabilization of the fibril and protection from degradation

●Localization to specific organs or tissue sites by binding to matrix components or receptor molecules

●Secondary effects on the metabolism of precursor protein, leading to the accumulation of degradation products with amyloidogenic potential

●Modulation of proteolytic events that may facilitate fibril formation

SERUM AMYLOID A PROTEIN — AA amyloid results from the deposition in tissue of serum amyloid A (SAA) protein, which is a major acute phase reactant [7,15]. (See "Acute phase reactants".)

Types — Several forms of SAA have been identified in serum. These include acute phase (SAA1 and SAA2) and constitutive (SAA4) isoforms, allelic variants, and post-translational modifications of these gene products [1,7,15]. Acute phase SAA proteins (SAA1 and SAA2) are apolipoproteins, primarily associated with high-density lipoprotein (HDL), and are also expressed extrahepatically (eg, synovial membrane) in the absence of HDL [16]. However, during the inflammatory response, SAA may exchange with low-density lipoproteins [17], and a small fraction binds to retinol-binding protein and indirectly to transthyretin [18].

Expression of SAA1 and SAA2 is induced by a number of factors, particularly interleukin (IL) 6, but also IL-1, tumor necrosis factor (TNF), lipopolysaccharide (LPS), and several transcription factors (pERK1/2, pjNK, p38), notably including SAA-activating factor (SAF)-1 [19-21].

●SAA1 is the precursor protein in most individuals with AA amyloid [22].

●Binding of heparin to SAA1 displaces it from HDL, perhaps contributing to the mechanism by which heparin and heparan sulfate facilitate formation of AA amyloid [3].

●SAA1 has three alleles, designated SAA1.1, SAA1.3, and SAA1.5, defined by amino acid substitutions at positions 52 and 57 of the molecule [23]. SAA2 has two alleles (alpha and beta) that differ from each other by a single base pair substitution at codon 71 resulting in an adenine substitution of guanine [24].

●The frequency of these alleles varies between populations and homozygosity for SAA1. SAA1 has been associated with an increased risk of AA amyloidosis in diseases such as rheumatoid arthritis and familial Mediterranean fever in White individuals, perhaps because of increased susceptibility to proteolytic cleavage by specific metalloproteinases [25,26].

●In the Japanese population, specific single nucleotide polymorphisms (SNPs) of the gene for SAA1.3 have been associated with increased gene transcription, AA amyloidosis complicating rheumatoid arthritis, adult-onset Still's disease, and susceptibility to familial Mediterranean fever [27,28].

●The carrier frequency for MEFV variants and familial Mediterranean fever in Armenia ranges up to one in three persons; Armenian patients homozygous for SAA1-alpha have a sevenfold higher risk of developing renal AA amyloidosis [29].

Possible function — The acute phase response, as reflected in the SAA gene, has been conserved for half a billion years throughout vertebrate evolution. Whereas SAA1 and SAA2 genes are found in most mammalian species due to conversion/duplication of an ancestral gene, fowl and fish have only a single acute phase response SAA, yet some species (eg, Pekin duck, some dolphins) are still susceptible to developing AA amyloid [21]. In humans, SAA is one of the most dynamic components with a potential increase in circulating concentration of more than 1000-fold, implying that it has important functions in the inflammatory response. Nonetheless, the exact contributions SAA proteins play in host defense remain somewhat opaque [7].

SAA proteins may increase the affinity of HDLs for macrophages and adipocytes during the acute phase response, a property termed "reverse cholesterol metabolism" [15,30,31]. Other properties include extracellular matrix binding [32]; opsonization of Gram-negative bacteria [33]; chemoattractant activity for monocytes, neutrophils, and lymphocytes [34]; induction of the release of proinflammatory cytokines from neutrophils [35,36]; and platelet effects [37,38]. Pro- and antiinflammatory effects of SAA vary with regard to the blood pool of hepatically derived acute phase SAA; local effects of tissue- and macrophage-derived SAA; the use of recombinant SAA for many in vitro studies; and whether SAA is bound to HDL or delipidated [39].

Ligands — SAA interacts specifically with a wide range of ligands, either bound to the surface of HDLs or in a lipid-free form [40]. The former include cholesterol (residues 1 to 18 and 40 to 63 of SAA), HDL (residues 1 to 18), calcium (residues 48 to 51), laminin (residues 29 to 33), fibronectin (residues 39 to 41), cystatin c (residues 86 to 104), and heparin/heparan sulfate (two binding sites within residues 17 to 49 and 77 to 103) [21]; the last ligand, in particular, has been targeted for the development of therapeutic agents for the treatment of AA amyloid [41]. (See "Treatment of AA (secondary) amyloidosis".)

Some SAA binding may occur selectively within the joint space of patients with rheumatoid arthritis, as both acute phase and constitutive isoforms of SAA appear to be synthesized by synovial tissue [16,42]. High levels of both SAA and its receptor, formyl peptide receptor-like 1 (FPRL1), have been demonstrated in the rheumatoid synovium, and a link to leukocyte infiltration, including the influx of CCR6-expressing Th17 cells, was confirmed by the ability of SAA to upregulate CCR20 expression of rheumatoid arthritis synoviocytes [43]. Acute phase SAA may directly stimulate the production of TNF-alpha and acute phase pentraxin production by synoviocytes [44]; SAA can mediate inflammatory and angiogenic effects by interaction with toll-like receptors (TLR2 and TLR4) [39,45]. Inhibition of IL-6-mediated induction of acute phase SAA in rheumatoid synovium by JAK/STAT (Janus kinase/signal transducer and activator of transcription) inhibitors being developed for clinical use provides another mechanism for abrogating SAA that may have potential for use in inflammatory arthritis, as well as in AA amyloidosis [46].

PATHOGENESIS OF AA AMYLOIDOSIS — Common causes of AA amyloidosis are rheumatoid arthritis, chronic infections (particularly in resource-limited countries), and autoinflammatory disorders [47,48]. (See "Causes and diagnosis of AA amyloidosis and relation to rheumatic diseases".)

AA amyloid occurs in various mammalian and avian species and can be induced by chronic inflammatory stimuli. The best-studied model for this disease is amyloid induction by casein/azocasein injections in certain genetically susceptible strains of mice [49]. Although most animal models for AA amyloid parallel human disease with predominant fibril deposition in spleen, liver, and kidney, chickens develop a unique articular amyloidosis when injected with Enterococcus faecalis [50,51]. Transmissibility of systemic AA amyloidosis by a seeding-nucleation process (so-called amyloid-enhancing factor [AEF]) has been identified in the pathogenesis of bovine, avian, mouse, and cheetah AA amyloid [52].

Principal pathogenic factors — Based upon findings in these animal models and results in patients, the principal pathogenic factors in AA amyloidosis (figure 1) appear to be the following [1,53,54]:

●Overproduction of both high-density lipoprotein (HDL)-associated and lipid-free serum amyloid A (SAA) as a consequence of acute and chronic inflammation.

●Proteolytic processing of SAA to AA, with a major cleavage occurring at position 76, thereby releasing the carboxyterminal third of the molecule [55]. The in vitro demonstration of internalization of SAA by macrophages, followed by intracellular proteolysis, and later by release of amyloidogenic peptides into the extracellular space [56], suggests that these events precede fibril formation. However, there is still uncertainty about the order of events in vivo [12,13].

●Focus on the intralysosomal pathway and acidification as promoting SAA aggregation has provided evidence for oligomers and other prefibrillar soluble aggregates of SAA as contributing to disease pathogenesis, toxicity, and the potential value of therapies that can inhibit aggregation and clear oligomers [57,58]

●Intrinsic fibrillogenic properties of SAA subsequences, particularly involving the amino-terminal end of the molecule [59,60].

Increased synthesis — There is increased synthesis of SAA with inflammation due to elevated levels of proinflammatory cytokines. With rheumatoid arthritis, for example, this increase in cytokine levels correlates with synovitis, which, in turn, may stimulate synoviocytes to produce SAA [16,34,42,61]. These events lead to elevated levels in joint fluid relative to serum [42]. In addition, SAA may directly potentiate rheumatoid inflammation via the induction of matrix metalloproteinases (MMP1 and MMP3) by synovial cells and chondrocytes [42,44,62].

Although most acute phase response SAA originates in the liver, SAA1/2 may also be produced by macrophages, kidney, lung, adipocytes, mammary glands, synovial cells, and intestinal epithelial cells (IECs) (figure 1). Early studies demonstrated SAA to be a major adipokine [63], relevant to the lipoprotein redistribution in obesity [64], inflammation [24], and in some instances, the development of AA amyloidosis [65]. Studies of SAA upregulation in mice and zebrafish, and the development of double and triple SAA knockout mice, have focused attention on possible roles in regulating inflammation and facilitating autoimmunity in inflammatory bowel disease (IBD). In mice, acute phase response SAA production by IECs may be induced by intra-ileal colonization by specific segmented filamentous microbes, triggered by interleukin (IL) 22-producing type 3 innate lymphoid cells, with SAA in turn promoting T helper (Th) 17 differentiation and increased IL-17 produced by these Th cells. This scenario provides a mechanism for SAA modulation of the inflammasome response in IBD, as well as autoimmune disease systemically [66-70].

The importance of increased SAA synthesis has been demonstrated by the correlation between the serum SAA concentration and disease course in patients with established AA amyloidosis. This was illustrated in a report of 80 patients with AA amyloid (mostly due to juvenile idiopathic arthritis or to rheumatoid arthritis) who were prospectively followed for a median of four years; the systemic amyloid load was assessed by yearly serum amyloid P component (SAP) scintigraphy [71]. The following findings were noted (see "Treatment of AA (secondary) amyloidosis"):

●Forty-two patients had median serum SAA concentration within the reference range (<10 mg/L); amyloid deposits regressed in 25 and stabilized in 14. Among patients with renal disease at baseline, proteinuria typically fell while the serum creatinine concentration was either stable or improved. Two patients who required dialysis remained on dialysis.

●The outcomes were variable in the other patients. However, among those in whom the serum SAA was persistently above 50 mg/L, the amyloid load usually increased and organ function deteriorated. In one patient who underwent renal transplantation, proteinuria and renal amyloid deposits recurred within 36 months.

●Amyloid deposition increased rapidly in patients with relapse of the underlying inflammatory disease. This observation is consistent with an underlying susceptibility to amyloidosis.

Processing — All forms of AA amyloidosis are associated with increased levels of SAA in blood [72]. In rheumatoid arthritis, however, SAA levels are increased in patients both with and without amyloidosis [73]. This indicates that additional factors (perhaps genetic and environmental) must be involved in pathogenesis [74,75], a feature of pathogenesis underscored by the variable incidence of AA amyloidosis complicating hereditary autoinflammatory disorders [76]. Other factors potentially include aberrant degradation of SAA to AA protein [77] and the accumulation of proteolytic cleavage products attributable to specific enzymes (eg, MMP1) in blood and/or tissue [78].

Intrinsic properties — The type and size of the fragments that are formed may also determine both the amyloidogenic potential and the site of tissue deposition [79,80]. Biochemical analysis has shown that smaller fragments are more likely to deposit in the glomeruli, while larger fragments may preferentially deposit in the blood vessels [80]. Cryogenic electron microscopy (CryoEM) studies of AA amyloid fibrils purified from tissue compared with those formed from recombinant peptides suggest that proteolytic selection may be responsible for the dominance of morphologies more likely to damage tissue [81].

Accelerated deposition — In murine models of amyloidosis, the deposition phase of disease can be greatly accelerated by injection of AEF, an activity that is probably due to the AA fibrils, peptides, and oligomers that provide a template for amyloid deposition and that is transmissible between animals [1,52,82,83]. The relevance of these observations to the role of SAA in inflammation and the induction of AA amyloid in susceptible individuals with inflammatory diseases remains to be determined [52,84].

SUMMARY

●Amyloidosis is a general term that refers to the predominantly extracellular tissue deposition of fibrils composed of low molecular weight subunits (5 to 25 kD) derived from any of more than 30 diverse serum proteins; these proteins have little in common with regard to their primary structure or metabolism. The fibrils have a predominantly antiparallel beta-pleated sheet configuration; they can be identified on biopsy specimens by tinctorial properties that result from the binding of specific dyes, such as Congo red. All forms of amyloidosis are characterized by codeposition of other substances, and cofactors may play a role in amyloid formation. (See 'Introduction' above.)

●Systemic AA amyloid results from the deposition in tissue of serum amyloid A (SAA) protein, which is a major acute phase reactant. SAA proteins may function to increase the affinity of high-density lipoproteins (HDLs) for macrophages and adipocytes during the acute phase response. SAA binds specifically to several ligands, including cholesterol, HDL, calcium, laminin, and heparin/heparan sulfate. (See 'Serum amyloid A protein' above.)

●Common causes of AA amyloidosis are rheumatoid arthritis, chronic infections (particularly in underdeveloped countries), and autoinflammatory disorders. (See 'Pathogenesis of AA amyloidosis' above.)

●The principal pathogenic factors in AA amyloidosis include overproduction of both HDL-associated and lipid-free SAA in both blood and tissue as a consequence of acute and chronic inflammation; proteolytic processing of SAA to AA, which releases the carboxyterminal third of the molecule and which may involve internalization of SAA by macrophages, intracellular proteolysis, and release of amyloidogenic peptides into the extracellular space; and intrinsic fibrillogenic properties of the molecule (figure 1). (See 'Principal pathogenic factors' above.)