A constant domain mutation in a patient-derived antibody light chain reveals principles of AL amyloidosis.

Light chain (AL) amyloidosis is a debilitating disease in which mutant antibody light chains (LC), secreted by aberrant plasma cell clones, misfold and form insoluble fibrils, which can be deposited in various organs. In the majority of cases, the fibrillar deposits consist of LC variable domains (V L ) containing destabilizing mutations compared to their germline counterparts. This is also true for the patient LC FOR005. However, this pathogenic LC sequence contains an additional mutation in the constant domain (C L ). The mechanistic impact of C L mutations is not yet understood in the context of AL amyloidosis. Our analysis reveals that the FOR005 C L mutation influences the amyloid pathway in specific ways: (1) folding and stability of the patient C L domain are strongly impaired; (2) the mutation disrupts the LC dimer interface and weakens dimerization; (3) the C L mutation promotes proteolytic cleavage of the LC monomers resulting in an isolated, amyloidogenic V L domain while dimeric LCs are not cleaved. The enhanced proteolysis rates and the inability of full-length LCs to form amyloid fibrils even in the presence of a destabilized C L domain support a model for AL amyloidosis in which the C L domain plays a protective role and in which proteolytic cleavage precedes amyloid formation.