Familial Renal Amyloidosis

Amyloidosis is a disorder of protein folding in which normally soluble proteins undergo a conformational change and are deposited in the extracellular space in an abnormal fibrillar form. ^[1] Accumulation of these fibrils causes progressive disruption of the structure and function of tissues and organs, and the systemic (generalized) forms of amyloidosis are frequently fatal. The conditions that underlie amyloid deposition may be either acquired or hereditary, and at least 20 different proteins can form amyloid fibrils in vivo. See the image below.

Familial renal amyloidosis. Proposed mechanism for amyloid fibril formation. The drawing depicts a generic amyloid fibril precursor protein (1) in equilibrium with a partially unfolded, molten, globulelike form of the protein (2) and its completely denatured state (3). Autoaggregation through the beta domains initiates fibril formation (4), providing a template for ongoing deposition of precursor proteins and for the development of the stable, mainly beta-sheet, core structure of the fibril (5). The amyloidogenic precursor proteins in patients with familial renal amyloidosis are thought to be less stable than their wild-type counterparts, causing them to populate intermediate, molten, globulelike states more readily.

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Kidney dysfunction is one of the most common presenting features of patients with systemic amyloidosis, and amyloid accumulation is the major pathological finding in approximately 2.5% of all native kidney biopsies. Most such patients have either reactive systemic (AA) amyloidosis or monoclonal immunoglobulin light-chain (AL) amyloidosis, but in the few remaining cases, the disease is hereditary.

The syndrome of familial systemic amyloidosis with predominant nephropathy is inherited in an autosomal dominant manner and was first described in a German family by Ostertag in 1932. ^[2] Research has shown that almost all patients with familial renal amyloidoses (FRA) are heterozygous for mutations in the genes for lysozyme, apolipoprotein AI, apolipoprotein AII, or fibrinogen A alpha-chain and that the amyloid fibrils in this condition are derived from the respective variant proteins. Both penetrance and the clinical phenotype can vary substantially among different families with the same mutation, and even within individual kindreds.

The pathogenesis of amyloid centers around off-pathway folding of the various amyloid fibril precursor proteins. These proteins can exist as two radically different stable structures: the normal soluble form and a highly abnormal fibrillar conformation.

All amyloid fibrils share a common core structure in which the subunit proteins are arranged in a stack of twisted, antiparallel, beta-pleated sheets lying with their long axes perpendicular to the fibril long axis. Proteins that can form amyloid transiently populate partly unfolded intermediate molecular states that expose the beta-sheet domain, enabling them to interact with similar molecules in a highly ordered fashion.

Propagation of the resulting low-molecular-weight aggregates into mature amyloid fibrils is probably a self-perpetuating process that depends only on a sustained supply of the fibril precursor protein. In some cases, the precursors undergo partial proteolytic cleavage; however, whether this occurs before, during, or after the formation of amyloid fibrils remains unknown.

Studies on hereditary amyloidosis have provided unique and valuable insights into the general pathogenesis of amyloid. Most of the variant proteins associated with hereditary amyloidosis differ from their wild-type counterparts by just a single amino acid substitution, although deletions and insertions also occur (see the Table below). ^{[3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]}

Table. Recognized Genotypes and Their Associated Phenotypes in Familial Renal Amyloidosis (Open Table in a new window)

Investigation of the variant amyloidogenic forms of lysozyme has been exceptionally informative because wild-type lysozyme is not associated with amyloidosis and has been thoroughly characterized. The amyloidogenic mutations give rise to amino acid substitutions that subtly destabilize the native fold so that, under physiological conditions, these variants readily visit partly unfolded states, promoting their spontaneous aggregation into amyloid fibrils.

The whole process of lysozyme amyloid fibril formation can be reversed. A soluble functional variant lysozyme has been recovered in vitro from preparations of isolated ex vivo amyloid fibrils that had been denatured and permitted to refold in the normal conformation. Wild-type apolipoprotein AI is inherently moderately amyloidogenic, and small amyloid deposits derived from it occur in aortic atherosclerotic plaques in 20-30% of middle-aged and elderly individuals.

Amyloid deposits in all different forms of the disease, both in humans and in nonhuman animals, contain the nonfibrillar glycoprotein amyloid P component (AP). AP is identical to and derived from the normal circulating plasma protein, serum amyloid P component (SAP), a member of the pentraxin protein family that includes C-reactive protein.

SAP consists of five identical subunits, each with a molecular mass of 25.462 d, which are noncovalently associated in a pentameric disklike ring. The SAP molecule is highly resistant to proteolysis, and, although not itself a proteinase inhibitor, its reversible binding to amyloid fibrils in vitro protects them against proteolysis. In contrast to its normal rapid clearance from the plasma, SAP persists for very prolonged periods within amyloid deposits. The possibility that SAP may contribute to the pathogenesis and/or persistence of amyloid deposits in vivo has been confirmed in studies on SAP knockout mice. ^[22]

Amyloid deposits accumulate in the extracellular space, progressively disrupting the normal tissue architecture and consequently impairing organ function. Amyloid deposits can also produce space-occupying effects at both microscopic and macroscopic levels. Although amyloid is inert in the sense that it does not stimulate either a local or systemic inflammatory response, some evidence suggests that the deposits exert cytotoxic effects and possibly promote apoptosis.

Strong clinical impressions exist that suggest the rate of accumulation of amyloid has a major bearing on organ function, which can be preserved for very long periods in the presence of an extensive but stable amyloid load. This may reflect adaptation to gradual amyloid accumulation or may relate to toxic properties of newly formed amyloid material.

Prospective studies with serial SAP scintigraphy, a specific and semiquantitative nuclear medicine technique for imaging amyloid deposits (see Workup/Imaging Studies), have confirmed that amyloid deposits are turned over constantly, albeit at a relatively low and variable rate. Therefore, the course of a particular patient's amyloid disease depends on the relative rates of amyloid deposition versus turnover. Amyloid deposits often regress when the supply of the respective fibril precursor protein is reduced, and, under favorable circumstances, this is accompanied by stabilization or recovery of organ function.

Many questions about amyloid deposition remain unanswered. Why only a small number of unrelated proteins form amyloid in vivo remains unclear, and, as yet, little is known about the genetic or environmental factors that determine individual susceptibility to amyloid or factors that govern its anatomical distribution and clinical effects. Hereditary amyloid deposition starts in the first or second decade in some patients, but possibly not until much later in life in other patients. In addition, the mechanism by which amyloid deposits are cleared and why the rate of this varies so substantially among patients are not understood.

Susceptibility to FRA is inherited in an autosomal dominant manner. In nearly all cases, the disease results from mutations in the genes encoding one of the following four plasma proteins:

• Lysozyme
• Apolipoprotein AI
• Apolipoprotein AII
• Fibrinogen A alpha-chain

In a small number of families, the cause has not yet been determined. Rare cases of renal amyloidosis associated with familial Mediterranean fever have been reported. ^{[23, 24]}

Lysozyme

Lysozyme is a ubiquitous bacteriolytic enzyme present in both external secretions and in leukocytes. Lysozyme mutations were identified as a cause of familial amyloidosis when, in 1993, amyloid fibrils in two British families were demonstrated to be derived from the lysozyme variants Asp67His and Ile56Thr, respectively. These represent the least common causes of FRA. The authors have identified a polymorphism encoding lysozyme Thr70Asn, which has an allele frequency of 5% in the British population and which has not been shown to be associated with amyloid deposition.

Apolipoprotein AI

Apolipoprotein AI is a major constituent of high-density lipoprotein (HDL) particles and participates in their central function of reverse cholesterol transport from the periphery to the liver. Approximately half of apolipoprotein AI is synthesized in the liver and half in the small intestine.

Variant forms of apolipoprotein AI are extremely rare in the general population and may be phenotypically silent or may affect lipid metabolism. In 1990, apolipoprotein AI Gly26Arg was identified as a cause of FRA. Since then, 12 other amyloidogenic apolipoprotein AI variants have been discovered. These are mostly other single amino acid substitutions but include deletions and deletion/insertions, not all of which are associated with clinical kidney disease (see Pathophysiology). The amyloid fibril subunit protein has comprised the first 90 or so N-terminal residues of apolipoprotein AI in all cases that have been studied, even when the variant residue(s) has been more distal.

In contrast to lysozyme and fibrinogen A alpha-chain types, wild-type apolipoprotein AI is itself weakly amyloidogenic, and the various amyloidogenic variants are likely to render apolipoprotein AI less stable and/or more susceptible to enzymatic cleavage, promoting an abundance of a fibrillogenic N-terminal fragments.

Another potential mechanism could be reduced lipid binding, thereby increasing the amount of free (and therefore relatively less stable) apolipoprotein AI in the plasma.

Apolipoprotein AII

Apolipoprotein AII is the second major constituent of human HDL particles, accounting for approximately 20% of HDL protein. Like apolipoprotein AI, apolipoprotein AII is synthesized predominantly by the liver and the intestines.

In 2001, apolipoprotein AII stop78Gly was isolated from the amyloid fibrils of a patient who died of kidney failure. ^[25]  Since then, an additional three mutations (encoding two protein variants) have been described in association with hereditary renal amyloidosis (see Pathophysiology).

Unlike serum amyloid A protein (another apolipoprotein and the amyloid precursor in AA amyloidosis) or apolipoprotein AI, in apolipoprotein AII amyloidosis, the protein fibrils are not derived from a cleavage fragment of the native precursor but instead consist of the whole protein plus a 21- amino-acid extension.

Fibrinogen

Fibrinogen is a multimeric 340-kd circulating glycoprotein composed of six peptide chains (two each of alpha, beta, and gamma types), all of which are synthesized in the liver. The alpha chains are the largest and are involved in cross-linking fibrin strands. Numerous alpha-chain variants have been recognized that are either silent or are associated with abnormal hemostasis.

Variant fibrinogen A alpha-chain Arg554Leu was first identified as an amyloid fibril protein in 1993. Since then, five other amyloidogenic mutations have been discovered (see the Table in Pathophysiology). All of these mutations are clustered within the carboxyl terminus of the gene in a relatively small portion of exon 5.

Two of these mutations result in frame shifts that terminate the protein prematurely at codon 548; one is a single nucleotide deletion in the third base of codon 524 and the other is a deletion at codon 522. A single-base transversion, resulting in the substitution of leucine for arginine at codon 554, has been reported in three families of Peruvian-Mexican, African American, and French Caucasian ethnic backgrounds. Residue 554 may be a mutational hot spot because other (nonamyloidogenic) mutations have also been identified at this position.

By far, the most common amyloidogenic variant is fibrinogen A alpha-chain Glu526Val, which has been found in numerous families of Irish, English, German, and Polish origin with FRA.

Genetic information is depicted in the images below.

Familial renal amyloidosis. An extended kindred with hereditary amyloidosis associated with fibrinogen A alpha-chain Glu526Val; disease penetrance is high in this particular family.

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Familial renal amyloidosis. Partial DNA sequence of the gene associated with fibrinogen A alpha-chain Glu526Val in a patient with familial renal amyloidosis, and a sequence from a healthy control. The mutation, which alters codon 526 from glutamic acid to valine, is marked with an arrow.

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Frequency

No systematic data address the frequency of FRA, but the condition is not as rare as previously thought. The lack of awareness of the condition and the frequent absence of a family history (owing to its variable penetrance) have contributed to substantial underdiagnosis. The incidence of amyloidosis has been estimated at 5 to 13 cases per million population per year; prevalence data are scarce, but one United Kingdom study suggested a rate of about 20 per million inhabitants. ^[26]

Since the authors introduced routine DNA screening into their investigations of patients with systemic amyloidosis at their facility in the United Kingdom, approximately 5% of patients with presumed AL primary amyloidosis have been diagnosed with hereditary lysozyme, apolipoprotein AI, or fibrinogen A alpha-chain amyloid. The amyloidosis is associated with the fibrinogen A alpha-chain variant Glu526Val in more than 80% of these patients. ^[27]

Race-, Sex-, and Age-related Demographics

Most patients are of northern European ancestry, but fibrinogen A alpha-chain amyloidosis has been reported in Peruvian-Mexican, Korean, and African-American families, and the authors are presently investigating a northern Indian family with uncharacterized FRA. Cases of fibrinogen A alpha-chain amyloidosis have been reported in two unrelated Chinese individuals. ^[28]

Gene carriage and the incidence of clinical disease are equal between men and women.

FRA may manifest any time from the first decade to old age but most typically does so in mid-adult life. The age at presentation, like other clinical features, varies among mutations and even within individual kindreds.

Many patients with FRA survive until the seventh decade or older, and most patients survive for at least 10 years after diagnosis. Life expectancy has increased substantially since kidney and liver transplantations have been introduced as treatments for these diseases. Liver transplantation is potentially curative in patients with fibrinogen A alpha-chain FRA and, possibly, in some patients with apolipoprotein AI amyloidosis.

The natural history of familial renal amyloidosis is a relentless gradual progression, leading to kidney and sometimes other organ failure and, eventually, death. Amyloid deposits can ultimately affect many organ systems, but they may be widespread and very extensive without causing symptoms.

The rate of progression and course of disease are extremely variable. Some patients with clinically overt involvement of multiple organs survive for many years or decades. Overall, the prognosis of patients with FRA is much better than that of those with acquired AA and AL amyloidosis.

Histological localization of amyloid deposits determines overall survival in patients with renal amyloidosis. In a study of 35 patients, the glomerulus was the most common and most severely affected renal compartment. Compared with patients without glomerular amyloid deposits, those with severe glomerular amyloidosis advanced more quickly towards end-stage kidney disease and premature death. ^[29]  

Acute kidney injury and chronic kidney disease can occur in the following forms of familial renal amyloidosis (FRA):

• FRA due to variant lysozyme
• FRA due to variant apolipoprotein AI
• FRA due to variant apolipoprotein AII
• FRA due to variant fibrinogen A alpha-chain

Acute and chronic liver failure can occur in the following forms of FRA:

• FRA due to variant apolipoprotein AI
• Potentially, FRA due to variant lysozyme and fibrinogen A alpha-chain (very rarely)

The following complications can occur in these forms of FRA:

• Restrictive cardiomyopathy - Some apolipoprotein AI and AII variants
• GI hemorrhage/perforation - Lysozyme FRA
• Progressive neuropathy - Some patients with apolipoprotein AI Gly26Arg and Leu178His

Patients should be advised to avoid any potential systemic insults such as dehydration, nephrotoxic drugs, and avoidable general anesthetics or surgery. Patients should not only be aware that first-degree relatives each have a 50% chance of carrying the gene but also that disease penetrance is highly variable.

For patient education information, see Amyloidosis.