Thermodynamic Instability of Human λ6 Light Chains: Correlation with Fibrillogenicity

Abstract

Certain types of human light chains have the propensity to deposit pathologically as amyloid fibrils as evidenced by the preferential association of monoclonal λ6 proteins with AL amyloidosis. However, the molecular features that render such proteins amyloidogenic have not been elucidated. Based upon the demonstrated relationship between the thermodynamic stability of light chains and their propensity to aggregate in vitro, we have initiated studies where the thermodynamic properties and fibrillogenic potential of two recombinant (r) V_λ6 molecules were compared. The first protein was generated from cDNA cloned from marrow-derived plasma cells from a patient (Wil) who had AL amyloidosis and renal amyloid deposits; the second was from a patient (Jto) with multiple myeloma in whom the λ6 protein was deposited not as amyloid but in the form of renal tubular casts. The thermodynamic stabilities of rV_λ6Wil and -Jto were determined from chaotropic and thermal denaturation studies. Based upon the ΔG_H2O, ΔH, ΔG_25°C, T_m, and C_m values, the rV_λ6Wil was less stable than its nonamyloidogenic counterpart, rV_λ6Jto. Measurement of fibril formation using a novel in vitro fibril forming assay demonstrated that although both rV_λ6 proteins formed fibrils in vitro, Wil had a shorter lag time and exhibited faster kinetics under physiologic conditions. Comparative amino acid sequence analyses of these two components and other λ6 amyloid-associated light chains revealed that the Jto protein had certain primary structural features that we posit contributed to its increased stability and thus rendered this protein nonamyloidogenic. Our studies provide the first evidence that stabilizing interactions within the V_L domain can influence the kinetics of light chain fibrillogenicity.