Kidney Amyloidosis: Updates on Pathogenesis and Therapeutic Frontiers Free

Amyloidosis is a diverse group of uncommon diseases characterized by the misfolding of native or mutant proteins that accumulate in the extracellular space of different organs. To date, 42 proteins have been identified associated with amyloidosis, 14 linked to systemic disease, 24 to localized forms, and 4 capable of causing both [1, 2].

An analysis of 16,175 tissue samples using mass spectrometry revealed that immunoglobulin (Ig) light chain (AL) amyloidosis was the most prevalent (59%), followed by transthyretin (ATTR) (28.4%), leukocyte chemotactic factor 2 (ALECT2) (3.2%), serum amyloid A (SAA) (2.9%), and heavy chain (HC) amyloidosis (2.3%) [3]. Other types each represented 1% or less of cases. The most commonly affected organs, in descending order, were the heart, kidney, gastrointestinal (GI) tract, bone marrow, and fat aspirate/biopsy [3]. However, it is important to note that these frequencies can vary depending on the region. For instance, a French nationwide study on mass spectrometry-based proteomics examined 833 cases of amyloidosis, revealing the following prevalences: Ig (57.7%), ATTR (25.8%), amyloid A (AA) (6.1%), apolipoprotein AI (2.3%), semenogelin 1 (1.7%), with remaining types accounting for 1% or less [4].

While the International Society of Amyloidosis recognizes 14 types of kidney-related amyloidosis, clinicopathologic studies from the USA, using mass spectrometry, identified 11 amyloid types [1, 3]. AL amyloid accounted for 55% of kidney cases, followed by ALECT2 (19%), AA (15%), HC amyloidosis (5%), fibrinogen Aα-chain (AFib) (3.5%), and the remaining 6 types each representing approximately 1% or less [3]. Previous studies have documented diverse frequencies of kidney amyloidosis types, raising concerns about potential selection bias. This discrepancy may be attributed to the use of proteomics exclusively in cases unresolved by other methods for typing amyloid proteins. In a study by Said et al. [5], 474 cases of kidney amyloidosis were evaluated, revealing the following frequencies: Ig (85.9%), AA (7%), ALECT2 (2.7%), AFib (1.3%), and other types, each representing less than 1%. Notably, laser microdissection/mass spectrometry was necessary to determine the amyloid origin in 16% of cases [5]. Importantly, AL amyloidosis is recognized as one of the kidney lesions associated with monoclonal gammopathy of renal significance [6]. Table 1, adapted from Nader et al. [34], summarizes the different types of amyloidosis with kidney involvement. This review will cover three of the most common systemic types with a focus on AL, AA, and ATTR amyloidosis, alongside ALECT2 which is among the three most common types of kidney amyloidosis.

Amyloid proteins undergo a structural shift from their usual α helical form to an insoluble, proteolysis-resistant β-pleated sheet configuration. They accumulate in various tissues and cause local toxicity and progressive organ dysfunction [34, 35]. The specific amyloidogenic protein varies with the type of amyloidosis, and each type exhibits different organ involvement patterns (Table 1) [1].

Traditionally, it was believed that tissue toxicity was directly and primarily caused by amyloid fibrils. However, recent studies challenge this notion. Charalampous et al. [36] analyzed the kidney amyloid plaque proteome of 2,650 cases through laser microdissection and mass spectrometry. They found variations in total protein content across amyloidosis types, with non-AL types exhibiting higher proteomic content than AL types. The complement cascade was identified as a key pathway. Notably, AA, AFib, and apolipoprotein CII were identified as distinct proteomic entities, while AL and ALECT2 displayed significant heterogeneity [36]. Further investigation into the kidney AL plaque-specific proteome identified four distinct clusters. One of these clusters, linked to better kidney survival, had higher overall protein content but lower levels of LCs [36]. These findings underscore the complexity of kidney toxicity in amyloidosis and challenge conventional beliefs about kidney toxicity solely attributed to amyloid abundance.

In AL amyloidosis, unstable monoclonal free light chains (FLCs) are produced by small B-cell clones, mainly plasma cell clones found in the bone marrow [35]. Igs consist of two HCs and two LCs, with each LC containing a variable domain (VL) for antigen binding and a constant domain (Fig. 1). LCs can belong to either the lambda (λ) or kappa (κ) family, exhibiting diversity in amino acid sequence and structure that allows them to recognize various antigens. This also explains the diverse clinical presentations in AL amyloidosis [35].

The destabilization of the native Ig structure, driven by variations in the amino acid sequence of the VL, results in qualitative abnormalities in Ig. This, along with the quantitative abnormality of excessive LC production by abnormal plasma cells, contributes to AL amyloidosis development [35]. In AL patients, there is an overrepresentation of the λ family compared to the κ family, suggesting a close link between gene sequence and amyloid formation [35]. Certain germline genes, like IGVL6-57 (Vλ6a), IGVL3-01 (Vλ3r), and IGVL2-14 segments, are significantly overrepresented in AL [37]. Notably, λ-LCs with the VL region from the IGVL6-57 segment are prevalent in AL patients with kidney disease, while IGVL1-44 is more common in those with dominant heart involvement, indicating a potential association with organ tropism [38, 39].

In AA amyloidosis, the N-terminal segment of SAA protein acts as the precursor protein [8]. SAA is produced by the liver, macrophages, and endothelial cells as an acute-phase reactant. Therefore, AA amyloidosis typically affects individuals with chronic infectious or inflammatory conditions. In many studies, rheumatoid arthritis is the most common disease linked to AA amyloidosis [8]. Factors like SAA polymorphism, isoform variations, obesity, age, and genetic autoinflammatory diseases contribute to its development [8].

In ATTR amyloidosis, destabilized transthyretin (TTR) acts as the precursor protein. Under normal conditions, TTR is produced by the liver as a transporter of thyroid hormone and retinol-binding protein bound to vitamin A [35, 40, 41]. There are two types of ATTR amyloidosis: wild-type (ATTRwt) and hereditary or variant forms (ATTRv). In ATTRwt, aging is implicated as the trigger for misfolded TTR molecules, earning the label of “senile ATTR amyloidosis” [35]. ATTRv is an inherited autosomal dominant condition associated with over 120 different mutations in the TTR gene on chromosome 18 [32]. The most common mutation described in association with heart disease is the substitution of valine for isoleucine at position 142 (V142I) [31, 42]. The characterization of kidney involvement in ATTR remains incomplete, as reported prevalence rates range widely from 0% to 50% among symptomatic ATTRv cases across different cohorts [3, 31, 43‒45]. The V30M, V122I, and V142I mutations have been specifically linked to kidney disease in ATTRv, with the former one being the most frequently reported [32, 42].

Clinical symptoms associated with AL amyloidosis are often vague and nonspecific, including fatigue, peripheral edema, weight loss, exertional dyspnea, orthostatic hypotension, and abnormal bleeding [22]. Specific signs, such as macroglossia and periorbital purpura, occur in only 15% of patients and lack sensitivity [22]. The heart is involved in 75% of cases, kidneys in 57%, nerves in 22%, liver in 20%, and GI tract in 17% [21]. Within the first 6 months of diagnosis, 25% of patients died from end-stage organ failure linked to advanced amyloid deposition [22].

The most common kidney manifestation of AL amyloidosis is nephrotic syndrome [5, 21, 22]. Additionally, a reduction in estimated glomerular filtration rate (eGFR) with low-grade proteinuria, indicating limited vascular and/or interstitial involvement, has been described [23, 24]. Eirin et al. [23] reported 12 out of 234 cases of AL amyloidosis with limited vascular involvement. Therefore, it is essential to consider AL amyloidosis in cases of reduced eGFR with unexplained or low-grade proteinuria, especially when there is a concomitant monoclonal gammopathy of unknown significance [22]. The cardiac presentation includes diastolic dysfunction with echocardiography findings of concentric thickening of the left ventricle, often misdiagnosed as hypertrophy due to hypertension, and abnormal apical longitudinal strain [22].

AA amyloidosis affects the kidneys in 80–100% of the cases, primarily in the form of nephrotic syndrome [5, 7, 8]. Less common kidney manifestations, primarily documented through case reports, include isolated tubular damage leading to conditions such as nephrogenic diabetes insipidus, and nephrotic syndrome coexisting with Fanconi syndrome and renal tubular acidosis [9, 10]. If left untreated, patients often progress to end-stage kidney disease (ESKD), with over 10% reaching ESKD at the time of diagnosis [7]. Poor kidney survival has been associated with proteinuria >4 g/24 h and eGFR <35 mL/min [7]. Additionally, hepatic, splenic, and GI involvements are common, while cardiac and neurologic complications, although reported, are infrequent [8].

The heart and peripheral nerves are the organs most commonly affected in ATTR amyloidosis [22, 32, 46]. It typically presents with peripheral neuropathy with dysautonomia and subsequently progresses to cardiomyopathy [31]. Symptoms include palpitations, often associated with atrial fibrillation, exertional dyspnea, and edema. In ATTRwt amyloidosis, noncardiac symptoms include bilateral carpal tunnel syndrome, lumbar spinal stenosis, and biceps tendon rupture [22]. Notably, carpal tunnel syndrome can be present in both AL and ATTR amyloidosis; therefore, tissue typing is still required.

The impact of ATTR amyloidosis on the kidneys has not been well characterized but has been reported primarily in specific populations with hereditary or variant forms of ATTR (ATTRv) [3, 31, 42‒45]. Various mechanisms of kidney dysfunction have been identified, including indirect damage from cardiorenal syndrome, direct amyloid deposition, or a combination of both. Indirect damage linked to cardiorenal syndrome typically manifests as chronic kidney disease (CKD) with absent or mild proteinuria [42]. Conversely, the direct mechanism involves microalbuminuria, which progresses to overt proteinuria and eventual kidney failure over a span of approximately 10 years in one-third of cases [42]. In a small study, at least 15 nephropathic variants were described, with kidney involvement defined by CKD, proteinuria, or amyloid deposits [31]. The V30M, V122I, and V142I mutations have been specifically linked to kidney disease in ATTRv [32, 42].

ALECT2 kidney amyloidosis has a strong ethnic association, particularly with Hispanic populations [2, 29]. Other affected ethnic groups include Native Americans, First Nations of Canada, Egyptians, Indians, Chinese, and Sudanese [2, 30]. Almost all patients are homozygous for the G-allele (SNP rs31517) at nucleotide 172, resulting in a substitution of isoleucine with valine at position 40 [2]. However, a specific pathologic variant remains unidentified. ALECT2 is thought to be pathologically driven by hepatic over-production, and while this amyloidogenic protein is found in multiple organs, its primary clinical impact is kidney disease [2]. ALECT2 amyloidosis commonly presents with subnephrotic range proteinuria, often progressing to CKD and occasionally to ESKD [2, 29, 30]. Histologically, it is characterized by diffuse cortical interstitial lesions, with variable glomerular and vascular involvement (Table 1) [2, 29, 30].

Almost all patients with AL amyloidosis have a preexisting Ig abnormality, typically detected in the form of a monoclonal protein in serum or urine immunofixation or an abnormal FLC level [47]. Notably, elevated levels of amyloid FLCs have been documented in all patients at least 4 years prior to the manifestation of AL amyloidosis symptoms [48]. Histologic identification of amyloid fibrils and typing the amyloidogenic protein are required for diagnosis. Subcutaneous fat aspiration yields AL amyloid deposits in 80% to 100% of patients, lip biopsy in 60%, and bone marrow biopsy in 57% of patients [49, 50]. A bone marrow biopsy is essential to determine the plasma cell burden, which is usually low in AL amyloidosis unless associated with multiple myeloma or Waldenström macroglobulinemia [22].

Fat pad aspirate sensitivity varies in other types of amyloidosis (ATTRv: 67%, AA amyloidosis: 66%, ATTRwt: 14%) [50‒52]. Small excisional fat pad biopsy proves effective in AL amyloidosis but does not enhance diagnostic rates in other systemic amyloidosis types [53]. Therefore, direct organ biopsy may be necessary. Amyloid typing is performed through techniques like immunofluorescence (IF) or immunohistochemistry, or mass spectrometry, with the latter being the most sensitive and specific [5].

Amyloid deposits are typically found in the mesangium and capillary loops in glomerular amyloid deposition, with localization in different nephron areas depending on the type of amyloidosis (Table 1). These deposits are characterized by apple-green birefringence when stained with Congo red and viewed under polarized light [5]. They typically appear periodic acid-Schiff negative or weakly positive, trichrome blue or gray, and silver negative [5]. Electron microscopy shows randomly arrayed, nonbranching fibrils measuring 8–12 nm [5]. Given that therapy for systemic amyloidosis is type-specific, typing the amyloid protein is crucial.

Typing is done via IF or other immunohistochemistry methods, with additional techniques like mass spectrometry done in complex cases (Fig. 2). Direct IF on frozen tissue is commonly used, employing fluorescently labeled antibodies (Abs) against specific proteins, such as Igs (IgG, IgA, IgM), kappa (κ) and lambda (λ) LCs, and complement components (C3, C1q) [5]. Notably, Abs are directed against specific epitopes on the constant domains of LCs (Fig. 1), so if these epitopes are altered in amyloid deposits, IF staining will be negative [5]. Nonspecific staining can complicate accurate differentiation, with studies showing negative staining for κ and λ in 7–35% of kidney AL cases [5, 54]. Similarly, employing a commercially available Ab against SAA for diagnosis of AA amyloidosis may lead to false-positive results due to the Ab’s lack of specificity, and false-negative ones may occur due to epitope loss [8].

To address these challenges, pronase-treated tissue for IF is utilized to unmask hidden monoclonal Ig deposits and improve diagnostic accuracy. Studies suggest that pronase-treated tissue for IF can outperform traditional IF on frozen tissue in identifying amyloidosis and various renal conditions [55].

Laser microdissection and proteomic analysis by mass spectrometry stand as the gold standard, dissecting and sorting amyloid deposits by molecular weight [5]. Mass spectrometry exhibits the highest sensitivity and specificity among all methods for amyloid typing, identifying the origin of kidney amyloidosis in over 97% of cases [5]. However, not all centers have access to these advanced techniques. In a comparative study of 142 biopsy specimens, immunohistochemistry had a diagnostic accuracy of 76%, while mass spectroscopy was 94% [56].

Figure 2 summarizes the different kidney amyloid typing techniques and their accuracy. Figure 3 shows the kidneys’ histological involvement in ALECT2, AA and AL amyloidosis.

Amyloidosis primarily affects the heart and kidneys, with cardiac involvement playing a pivotal role in patient survival [22]. Cardiac evaluation includes an electrocardiogram, echocardiography, and contrast-enhanced cardiac magnetic resonance to quantify amyloid deposition [22, 57]. Cardiac biomarkers, including N-terminal pro-B-type natriuretic peptide and troponin-T (cTnT), are employed in staging systems [22, 57‒59].

Distinguishing between AL and ATTR amyloidosis involves a technetium pyrophosphate scan (Tc-PYP) or 3,3-diphosphono-1,2-propanodicarboxylic acid scan [22]. The Perugini staging system visually grades these scans, comparing cardiac and rib uptake on a scale from 0 to 3. Grade 0 indicates no uptake in the heart, grade 1 less uptake in the heart than in the rib, grade 2 equal uptake, and grade 3 greater uptake in the heart than the rib [60]. Higher Tc-PYP uptake characterizes cardiac ATTR amyloidosis. When ATTR amyloidosis is suspected based on radionuclide scans, genetic testing, typically via polymerase chain reaction sequencing of the TTR gene, is essential to exclude an inherited variant [22]. Notably, 23% of ATTR amyloidosis patients may have a monoclonal protein suggesting the possibility of AL amyloidosis [61]. Therefore, a Tc-PYP or 3,3-diphosphono-1,2-propanodicarboxylic acid scan is diagnostic for ATTR only when no monoclonal gammopathy is present. If both coexist, tissue diagnosis with mass spectroscopy is crucial for confirmation [22].

The current approach aims to limit amyloid protein production by targeting the plasma cell clone. Therapies are individualized based on organ involvement severity and clonal characteristics. Experts recommend assessing the eligibility for autologous stem cell transplant first, given studies demonstrating a survival of up to 20 years with high-dose melphalan and autologous stem cell transplant (HDM/SCT) [62, 63]. However, HDM/SCT has not been compared to newer and highly effective treatments like daratumumab, cyclophosphamide, bortezomib, and dexamethasone (Dara-CyBorD), which is now considered standard of care. In addition, eligibility criteria for HDM/SCT are center dependent and only 20% of newly diagnosed patients are eligible for this intensive treatment [63].

The ANDROMEDA trial showed high rates of hematologic response, with approximately 79% of patients having very good partial response to Dara-CyBorD [64]. Organ response rates were also promising, with an overall rate of 64% at a median follow-up of 17.6 months [64]. Responses were observed in 53% of cases with cardiac involvement and 83% with renal involvement, with respective median response times of 114 and 57 days [64]. Cytogenetic abnormalities, particularly t(11;14), are prevalent in AL amyloidosis and are responsive to Dara-CyBorD, whereas CyBorD alone has shown inferior outcomes [65, 66].

Plasma cells, the source of amyloidogenic proteins, are directly targeted by the proteasome inhibitor, bortezomib, and the anti-CD 38 monoclonal Ab, daratumumab. For patients with advanced disease or drug toxicity concerns, single agent daratumumab or immunomodulatory agents such as thalidomide, lenalidomide, or pomalidomide have been used [63].

The treatments for AL amyloidosis are challenged by the side effect profile of the drugs. Bortezomib can cause neurotoxicity, worsening peripheral and autonomic neuropathy, and pulmonary fibrosis [67]. While daratumumab is generally well tolerated, it may lead to side effects like lymphopenia, upper respiratory tract infections, and peripheral sensory neuropathy [65]. Adjustments in treatment are necessary for cases with advanced cardiac involvement or poor functional status [65].

New treatments aiming to remove amyloid deposits from affected organs using Abs and stabilizers of precursor amyloidogenic protein are undergoing clinical trials [63, 67]. A ≥25% eGFR decline at 6 months predicts poor kidney survival, while a ≥30% decrease in proteinuria or reaching <0.5 g/24 h represents kidney response. Hematologic remission at 6 months improves kidney outcomes [25].

In cases of AA amyloidosis, the treatment focuses on addressing the underlying inflammatory or infectious condition. IL-1 inhibitors approved for clinical use in the USA, including anakinra, canakinumab, and rilonacept, are not specifically approved for kidney AA amyloidosis [8]. However, reports indicate effectiveness, particularly in patients with colchicine-resistant familial Mediterranean fever (FMF)-associated AA amyloidosis [68]. Observational cohorts in FMF-associated AA amyloidosis have demonstrated benefits, especially among those with Cr levels <1.5 mg/dL at onset [69].

TNF-α inhibitors, such as etanercept, infliximab, certolizumab, and adalimumab, have been employed in AA amyloidosis treatment, with studies indicating improvement or stabilization of kidney function in the majority of patients [70, 71]. Tocilizumab, an IL-6 inhibitor, exhibits promise in treating patients with FMF, ankylosing spondylitis, rheumatoid arthritis, and other inflammatory conditions-associated AA amyloidosis that have shown resistance to prior therapeutic interventions, including colchicine, IL-1 inhibitors, and TNF-alpha inhibitors, among others. Encouragingly, successful outcomes have been consistently reported across various studies [72‒74].

The main treatments for ATTR amyloidosis, wild-type (ATTRwt), and hereditary or variant forms (ATTRv) involve ATTR stabilizers (like diflunisal, tafamidis, and acoramidis) and gene silencers (such as patisiran and inotersen) [75‒78]. Ongoing clinical trials are exploring ways to remove amyloid proteins from tissues in ATTR cases [79]. Because ATTR is mainly produced in the liver, patients with ESKD should be considered for simultaneous liver-kidney transplantation [31]. Notably, these treatments have been proven effective for polyneuropathy and cardiac involvement, making kidney transplant alone a viable option [78].

Treatment options for ALECT2 amyloidosis are limited. Using liver-directed RNA interference is an attractive pathway for treating this disease, analogous to the success achieved in treating conditions where the pathogenic protein originates in the liver, such as primary oxalosis and ATTR amyloidosis [76, 80]. Kidney transplantation holds promise despite occasional reports of donor-derived ALECT2 and rare disease recurrence; overall, there are no negative effects observed on allografts [2, 29].

Effective management of amyloidosis-associated edema involves restricting dietary sodium and fluids, while using high-dose loop diuretics. Prophylactic anticoagulation should be considered in cases of severe hypoalbuminemia (<2 g/dL), balancing bleeding risks associated with advanced disease [81]. Choosing appropriate antihypertensive medications poses challenges due to underlying low blood pressures and concurrent cardiomyopathy [81]. While angiotensin-converting enzyme inhibitors and angiotensin-receptor blockers are commonly employed to reduce proteinuria, their efficacy lacks evidence in renal amyloidosis [81]. Calcium channel blockers are contraindicated due to their negative inotropic effects, and in vitro studies show potential worsening of local tissue toxicity by the drug binding to amyloid fibrils in the heart [81, 82]. Caution is advised with beta-blockers (BBs), particularly in advanced stages where amyloid protein deposition in the myocardium and conduction system increases the risk of arrhythmias [78, 81]. Additionally, increasing heart rate compensates for reduced stroke volume in restrictive forms of cardiomyopathy, making BB poorly tolerated due to their inhibition of compensatory tachycardia and subsequent reduction in cardiac output. ACEi/angiotensin-receptor blocker and mineralocorticoid receptor antagonists have been used at low doses followed by careful up-titration and monitoring and are better tolerated than BBs [83].

Even though amyloidosis is described as a rare disease, understanding its complexities and clinical presentations is crucial due to the significant impact of delayed diagnosis on organ function and treatment outcomes. Despite the precision of advanced techniques like mass spectrometry, their availability remains limited. Recent studies using mass spectrometry have challenged traditional views, offering new insights into amyloidosis pathogenesis. While amyloid deposition toxicity remains central, there is growing recognition of the complement pathway’s role in tissue damage. This understanding paves the way for novel therapeutic strategies targeting key pathways implicated in disease progression. From plasma cell-directed therapies for AL amyloidosis to interventions addressing underlying inflammation in AA amyloidosis, and TTR stabilization approaches in ATTR amyloidosis, treatment paradigms are evolving. Moving forward, collaborative efforts among clinicians, pathologists, researchers, and industry stakeholders are paramount to further elucidate the complex pathophysiology of amyloidosis and provide timely targeted therapies that improve patient outcomes.

The authors have no conflicts of interest to declare.

There is no support/funding for this work.

C. Elena Cervantes wrote and reviewed the manuscript, and created the table and figures. Mohamed G. Atta reviewed and edited the manuscript.