Key Information
Amyloid light-chain (AL) amyloidosis is the most prevalent type of diagnosed systemic amyloidosis in Western countries, characterized by the deposition of misfolded immunoglobulin light chains (LCs) in various organs, most commonly the heart and kidneys. Circulating free LC (FLC) measurement, which can be performed by mass spectrometry or antibody-based techniques, is a crucial tool for AL amyloidosis diagnosis, risk assessment, and management. Additionally, diagnosing AL amyloidosis requires accurate detection of LC deposits in tissues. In addition to immunohistochemical techniques, mass spectrometry-based methods are now available.
Keywords: free light-chains, AL amyloidosis, plasma cell disorder, heart failure, amyloid fibrils, immunoglobulin
1. Introduction
Amyloidosis is a disease caused by tissue deposition of insoluble protein fibrils composed of misfolded proteins. The term ‘amyloid’ was first introduced into human pathophysiology by Rudolf Virchow in 1854 to describe a pathologic substance initially thought to be related to starch or cellulose, but was subsequently identified as being composed of proteins [1external link, opens in a new tab]. Amyloid fibrils are composed of precursor proteins that undergo self-assembly into a β-sheet-rich conformation.
Proteins normally assume a well-defined three-dimensional conformation essential for their biological function and stability. Following polypeptide synthesis, proteins are vulnerable to adopting unstable, unfolded states and thus require molecular chaperones to ensure correct folding [2external link, opens in a new tab]. Various extracellular stressors can disrupt protein conformation, promoting the unfolding of polypeptide chains [3external link, opens in a new tab]. The precursor protein needs to be in an intermediate partially unfolded form in order to start amyloidogenesis [4external link, opens in a new tab]. While such intermediates typically refold spontaneously into their native conformation, they may occasionally misfold into aberrant, non-native structures. These misfolded proteins may be degraded via the proteasomal system or alternatively may adopt a β-sheet-rich structure, capable of self-association and subsequent polymerization into amyloid fibrils. In AL amyloidosis, this process is primarily driven by the thermodynamic instability of immunoglobulin light chains [5external link, opens in a new tab].
Depending on the specific amyloidogenic precursor involved, amyloid deposits in tissues give rise to distinct forms of amyloidosis. The most prevalent type, especially in Western countries [6external link, opens in a new tab,7external link, opens in a new tab] is amyloid light-chain (AL) amyloidosis, which is defined by the accumulation of free immunoglobulin light chains (FLCs) of antibodies in the tissue as a result of a plasma cell dysfunction. There are two types of clonal light-chains (LCs): lambda (found in 70–80% of individuals) and kappa (20–30%).