Mutations in Specific Structural Regions of Immunoglobulin Light Chains Are Associated with Free Light Chain Levels in Patients with AL Amyloidosis

Abstract

BackgroundThe amyloidoses are protein misfolding diseases characterized by the deposition of amyloid that leads to cell death and tissue degeneration. In immunoglobulin light chain amyloidosis (AL), each patient has a unique monoclonal immunoglobulin light chain (LC) that forms amyloid deposits. Somatic mutations in AL LCs make these proteins less thermodynamically stable than their non-amyloidogenic counterparts, leading to misfolding and ultimately the formation of amyloid fibrils. We hypothesize that location rather than number of non-conservative mutations determines the amyloidogenicity of light chains.Methodology/Principal FindingsWe performed sequence alignments on the variable domain of 50 κ and 91 λ AL light chains and calculated the number of non-conservative mutations over total number of patients for each secondary structure element in order to identify regions that accumulate non-conservative mutations. Among patients with AL, the levels of circulating immunoglobulin free light chain varies greatly, but even patients with very low levels can have very advanced amyloid deposition.ConclusionsOur results show that in specific secondary structure elements, there are significant differences in the number of non-conservative mutations between normal and AL sequences. AL sequences from patients with different levels of secreted light chain have distinct differences in the location of non-conservative mutations, suggesting that for patients with very low levels of light chains and advanced amyloid deposition, the location of non-conservative mutations rather than the amount of free light chain in circulation may determine the amyloidogenic propensity of light chains.

Laboratory testing in monoclonal gammopathy of renal significance (MGRS)

“…Some patient develops cast nephropathy with FLC levels of 157 mg/dL (1570 mg/L) while others at 6960 mg/dL (69,600 mg/L) [21]. Similar differences have also been noted in AL and MIDD [22,23]. The reduction of the M-protein is primarily achieved by cytotoxic therapy such as chemotherapy (low and high dose) and radiation in localized disease.…”

Site-directed Mutagenesis Reveals Regions Implicated in the Stability and Fiber Formation of Human λ3r Light Chains

“…The comparison of amyloidogenic and nonamyloidogenic V L sequences has allowed other researchers to identify certain amino acid changes that destabilize the V L domain (12)(13)(14)(15)(16). The introduction of some of these mutations into nonamyloidogenic V L domains decreases their stability and increases their tendency to aggregate and form amyloid fibers in vivo (17).…”

Effect of single point mutations in a form of systemic amyloidosis

“…19 Each unit of the folded REI light-chain protein is build out of nine bstrands (A-I, following the notation in Nowak et al), wherein the b-strands E, F, G, H, and I belongs to b-sheet 1 and A, B, C, and D to b-sheet 2. 2 The bsheets are arranged in a sandwich model as shown in Figure 5. There is a disulfide bond between Cys23 and Cys88, and a salt bridge between Arg61 and Asp82.…”

“…1,2 The mechanism of fibril formation and the cause of toxicity of these fibrils, or their oligomeric intermediates, is not yet understood. 3,4 Most of the research over the last decades has focused on neurodegenerative diseases such as Alzheimer's, Huntington's, or Parkinson's disease.…”

“…14 Light-chain proteins tend to form dimers where each chain consists of a constant C-terminal domain (C L ) and a variable N-terminal domain (V L ). 2 In the majority of patients, the amyloid deposits are made of the V L domain. 15 Unfolding and aggregation of this domain can be caused by change in pH, temperature, protein concentration, or certain mutations.…”