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

Abstract

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 (VL) 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 (CL). The mechanistic impact of CL mutations is not yet understood in the context of AL amyloidosis. Our analysis reveals that the FOR005 CL mutation influences the amyloid pathway in specific ways: (1) folding and stability of the patient CL domain are strongly impaired; (2) the mutation disrupts the LC dimer interface and weakens dimerization; (3) the CL mutation promotes proteolytic cleavage of the LC monomers resulting in an isolated, amyloidogenic VL 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 CL domain support a model for AL amyloidosis in which the CL domain plays a protective role and in which proteolytic cleavage precedes amyloid formation.

Truncation of the constant domain drives amyloid formation by immunoglobulin light chains

“…This observation strongly suggests that the amyloidogenic potential of specific V L s can be disclosed after removal of C L . In vitro evidence, indeed, supports the critical role of the C L domain in modulating stability, dimerization, and aggregation of AL LCs ( 24 , 25 , 26 ). Coherently with the hypothesis that proteolysis of the constant domain may have a pathogenic role, AL LCs have been shown to be more susceptible to proteolysis compared to nonamyloidogenic ones ( 10 , 12 , 13 , 27 ).…”

“…It is otherwise well known ( 48 ), and confirmed in this study, that isolated V L domains of amyloidogenic LCs easily form amyloid fibrils in vitro ; however, V L -AL55–derived fibrils lack the parallel β-strand configuration of their ex vivo counterpart. On the contrary, full-length LCs are extremely resistant to in vitro fibrillar conversion ( 12 , 24 ).…”

“…Structural evidence based on cryo-EM shows that the core of purified AL fibrils substantially consists of the V L ( 16 , 17 , 18 ). In addition, whereas the full-length LCs from AL patients’ are resistant to in vitro fibril formation, the corresponding isolated V L rapidly converts into amyloid aggregates under physiological experimental conditions ( 12 , 15 , 19 , 20 , 21 , 22 , 23 , 24 ). This observation strongly suggests that the amyloidogenic potential of specific V L s can be disclosed after removal of C L .…”

Complete variable domain sequences of monoclonal antibody light chains identified from untargeted RNA sequencing data

“…Identification of the complete IGC L region may also be possible using sequencing platforms that acquire longer reads. Identifying full-length IGC L sequences would address the roles of the LC constant domain in amyloidosis and other disorders ( 68 – 72 ).…”

Role of the mechanisms for antibody repertoire diversification in monoclonal light chain deposition disorders: when a friend becomes foe

“…Other mutations have been shown to promote aggregation by altering the homodimer interface that LCs tend to form in solution ( Figure 1E ), shifting the equilibrium towards free monomers, which are typically thermodynamically less stable ( 219 – 221 ). This finding is in line with studies showing that the association of the LC in a stable dimer protects it from misfolding and aggregation ( 222 , 223 ).…”