Quantification of Cleaved β2-Microglobulin in Serum from Patients Undergoing Chronic Hemodialysis

Background: Patients on chronic hemodialysis are prone to develop amyloid deposits of misfolded β2-microglobulin (β2M) in osteoarticular tissues. β2M with various deletions/truncations and chemical modifications has been found together with structurally intact β2M in extracts of β2M amyloid fibrils. The state of the circulating population of β2M molecules has not been characterized previously with high-resolution methods. Methods: We used immunoaffinity–liquid chromatography–mass spectrometry analysis of serum samples to examine whether structurally modified β2M is generated in the circulation. In addition, we developed an immunoassay for the quantification of a cleaved β2M variant in biological fluids based on novel monoclonal antibodies and applied this assay to patient and control sera. Results: A specific alteration compatible with the generation of lysine-58–cleaved and truncated β2M (ΔK58-β2M) was found in the sera of many (20%–40%) dialysis patients but not in control sera or sera from patients with cerebral amyloidosis (Alzheimer disease). Applied to patient sera, specific immunoassays revealed that dialysis, as expected, significantly lowered the total β2M concentration, but the concentrations of ΔK58-β2M remained unchanged after dialysis. The results also show that patients dialyzed with less biocompatible membranes have higher serum concentrations of cleaved β2M (mean, 8.5, 1.8, and 0.7 mg/L in cuprophane membrane-dialyzed, polysulfone membrane-dialyzed, and control sera, respectively). Conclusions: This study for the first time demonstrates and assigns the structure of a specific β2M variant in sera from dialysis patients. Because this variant is conformationally unstable in vitro, it may be involved in in vivo amyloidogenesis.