Proteomic profiling of amyloid deposits in advanced systemic light-chain (AL) amyloidosis: Insights into clonal light chains, chaperone proteins, and cardiac involvement

Abstract Background: Systemic light-chain (AL) amyloidosis with advanced cardiac involvement is associated with markedly reduced survival. The extent of organ deposition is highly correlated with the sequence specificity of the variable domain (VL) of the clonal immunoglobulin light chain. However, VL sequence alone does not fully account for the severity of organ involvement. To better elucidate the pathogenic mechanism underlying AL amyloid formation, we performed proteomic analysis of Congo red–positive amyloid deposits to characterize the composition of clonal light chains and associated chaperone proteins and to explore their relationship with cardiac involvement. Methods: Laser microdissection followed by LC-MS/MS was performed on Congo red–positive tissues from 76 newly diagnosed AL amyloidosis patients. A total of 425 proteins were quantified. Customized peptide libraries enabled precise typing of light chain VL domains. Unsupervised clustering was applied to proteomic profiles from patients with cardiac involvement. Results: Mass spectrometry revealed that successful VL subtyping relied on the presence of key chaperone proteins—Vitronectin (VTNC), Apolipoprotein A-IV (ApoA4), Apolipoprotein E (ApoE), Serum amyloid P-component (SAMP). When any of these proteins were absent, the subtyping failure rate increased from 4.4% to 62.5% (OR = 36.1, 95% CI 6.3–272; P 0.001), particularly among patients with lower NT-proBNP and earlier Mayo stage. The IGLV6-57 gene was the most commonly used VL among patients with advanced disease (19.7%). While univariate analysis suggested a 3.13-fold increased mortality risk associated with IGLV6-57 (HR = 3.13, 95% CI 1.11–8.82; P = 0.031), this association lost significance after adjusting for NT-proBNP and cTnT levels (HR = 1.97, 95% CI 0.69–5.65; P = 0.205). In cardiac-involved AL patients, apolipoproteins ApoE and ApoA4 were significantly upregulated (P 0.05), with progressive elevation from Mayo stage III to IV. A high-risk cluster (Cluster 1) was identified in these patients. Multivariate Cox regression confirmed Cluster 1 as an independent predictor of mortality (HR = 3.75, 95% CI 1.36–10.34; P = 0.011), enhancing the prognostic accuracy of Mayo staging. Differentially expressed proteins were enriched in the cholesterol metabolism pathway (P 0.05). Conclusion: In advanced AL amyloidosis with cardiac involvement, the deposited clonal light chains exhibit distinct sequence characteristics. Chaperone proteins—particularly apolipoproteins—appear essential for amyloid formation and are associated with the severity of organ involvement, highlighting their potential as biomarkers or therapeutic targets. Keywords:Systemic light-chain amyloidosis; Immunoglobulin light chain; Apolipoprotein; Proteomics; Cardiac involvement