After 2 years, his proteinuria was attenuated from 3.52 to 3 g/24 h, serum creatinine increased from 0.90 to 3.5 mg/dL. Besides, serum alkaline phosphatase (ALP) and gamma-glutamyltransferase (γ-GT) increased from 94/40 to 289.9/215.5 U/L.
In our study, case 1 suffered from nephrotic syndrome, whereas case 2 presented with heavy proteinuria, progressive renal insufficiency, splenomegaly, and elevated levels of ALP and γ-GT. Immunostaining of renal biopsies in 2 hospitals revealed totally different results. This is because ApoA-1 is usually not tested in the panel of amyloid typing and immunostaining can be nonspecific or negative, as it may be confounded by background staining and loss of antigenic epitopes in the fibrillar conformation because of mutant protein or proteins with conformational changes. Furthermore, LMD and MS-based proteomic analysis, which is currently considered to be the criterion standard in identifying amyloid protein, demonstrated the ApoA-1 origin of the amyloid in both cases. Therefore, case 2 was diagnosed as having hereditary ApoA-1 amyloidosis with multiorgan injuries (kidney, spleen, and liver), a positive family history and a heterozygous mutation in APOA1 gene (Trp74Arg). As case 1 had a strong immunofluorescence staining of PLA2R, IgG, and C3, which were diffusely distributed along the glomerular capillary loop but had no underlying diseases found in the clinical screening, case 1 was then diagnosed as having nonhereditary ApoA-1-associated renal amyloidosis combined with idiopathic membranous nephropathy. Alternatively, a systemic amyloidosis may be present in case 1, but we cannot conclude it because of the unavailability of 123I-SAP scintigraphy in China, which is used to diagnose systemic amyloidosis. According to examinations already done, he was less likely to have amyloid infiltration in the organs other than kidney.
Both cases presented with nephrotic syndrome. Renal involvement is very common in hereditary ApoA-1 amyloidosis (Table 2), which is characterized by renal interstitial/medulla deposition of amyloid and slow progress, resulting in mild tubular proteinuria and a lowered urinary specific gravity.[2,7,11] Case 2 in this study had amyloid deposits in glomeruli and presented with nephrotic syndrome. He was exclusively diagnosed as having ApoA-1 amyloidosis, which resulted in nephrotic syndrome. In contrast, case 1 additionally suffered from idiopathic membranous nephropathy. Renal biopsy of case 1 revealed extensive glomerular, interstitial, and vascular amyloid deposits and subepithelial deposits, marked ApoA-1 amyloid deposits in interstitium, and PLA2R, IgG, and C3 deposits along the glomerular capillary loop. In addition, proteomic analysis demonstrated the ApoA-1 origin of the amyloid in glomeruli. Based on these findings, we consider that both ApoA-1-related renal amyloidosis and membranous nephropathy contributed to the renal disease and proteinuria of case 1. Besides, the proteinuria of case 1 in the follow-up decreased from 10.52 to 2.17 g/24 h under steroids/cyclosporin therapy, nephrotic syndrome in case 1 may be mainly attributed to membranous nephropathy and ApoA-1-related renal amyloidosis partially resulted in proteinuria. ApoA-1 amyloidosis complicated with certain membranous nephropathy or IgA nephropathy was previously found in patients with ApoA-1 Leu75Pro variant. It is important to identify other types of kidney diseases accompanied for appropriate treatment.
Diagnosis of ApoA-1 amyloidosis is challenging given that AopA-1 amyloidosis is rare and is usually not tested in amyloid typing. Clinical presentation varies widely depending on the organs involved. As ApoA-1 amyloidosis is diagnosed histologically, biopsies from target organs are required.[1,31] Congo red staining remains the criterion standard for defining amyloid. Conventional methods such as immunohistochemistry and immunofluorescence can identify the amyloid subtype but can be confounded by background staining caused by serum contamination and loss of antigenic determinants in the fibrillar conformation. Studies have shown that LMD and MS-based proteomic analysis can provide accuracy rates ranging from 98% to 100% in the diagnosis of the subtypes of amyloidosis, which are higher than those by immunohistochemistry staining (38%–87%) and immunofluorescence (65%–87%). In addition, genetic testing can differentiate between hereditary and nonhereditary amyloidosis, but the results must be interpreted combined with other findings (e.g., immunohistochemical or proteomic typing of the amyloid). Low plasma levels of ApoA-1 and HDL owing to the dysfunctional proteins can serve as an indicator of hereditary ApoA-1 amyloidosis. Besides, as serum amyloid P component (SAP) binds to all amyloid deposits, 123I-SAP scintigraphy can locate amyloid deposits in the body. Thus, combinations of the detailed clinical evaluation, histology, immunohistochemistry, proteomics, genetic analysis, and biochemical investigations are necessary for establishing the diagnosis of ApoA-1 amyloidosis.
ApoA-1 amyloidosis is a slowly progressive disease. The 2 patients in the present study were treated with glucocorticoid combined with cyclosporine and calcitriol combined with angiotensin-converting enzyme inhibitors, respectively. During follow-up, renal function of case 1 remained stable, whereas case 2 had an elevation of cholestasis indices and renal insufficiency. Presently, there is no effective drug for hereditary apoA-1 amyloidosis. Supportive treatment and organ transplantation are the major therapeutic approaches. New therapies targeting associated amyloid proteins show great potential in hereditary and localized amyloidosis. A phase 1 trial of CPHPC ((R)-1-[6-[(R)-2-carboxy-pyrrolidin-1-yl]-6-oxo-hexanoyl]pyrrolidine-2-carboxylic acid), which can delete SAP from the plasma followed by an anti-SAP antibody, revealed major reduction in liver amyloid with no serious adverse effect in a patient of apoA-1 amyloidosis, which provides a new treatment for apoA-1 amyloidosis.
In summary, we have described 2 cases of ApoA-1-related renal amyloidosis in Chinese Han population presenting with hereditary and nonhereditary forms. Besides, our study has demonstrated the usefulness of mass spectrometry in identifying the compositions of amyloid deposits, which will provide further insights into the ApoA-1-associated renal amyloidosis.
The authors thank Prof. Shaolin Shi for critical reading and editing of the manuscript and thank Translational Medicine Core facilities of Medical School of Nanjing University for performing LMD and MS-based proteomic analysis of renal biopsies.
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