Date Published: March 28, 2017
Publisher: Public Library of Science
Author(s): Emily B. Martin, Angela Williams, Craig Wooliver, R. Eric Heidel, Sarah Adams, John Dunlap, Marina Ramirez-Alvarado, Luis M. Blancas-Mejia, Ronald H. Lands, Stephen J. Kennel, Jonathan S. Wall, Patrick van der Wel.
Monoclonal free light chain (LC) proteins are present in the circulation of patients with immunoproliferative disorders such as light chain (AL) amyloidosis and multiple myeloma (MM). Light chain-associated amyloid is a complex pathology composed of proteinaceous fibrils and extracellular matrix proteins found in all patients with AL and in ~10–30% of patients who presented with MM. Amyloid deposits systemically in multiple organs and tissues leading to dysfunction and ultimately death. The overall survival of patients with amyloidosis is worse than for those with early stage MM.
We have developed a sensitive binding assay quantifying the recruitment of full length, patient-derived LC proteins by synthetic amyloid fibrils, as a method for studying their amyloidogenic potential. In a survey of eight urinary LC, both AL and MM-associated proteins were recruited by synthetic amyloid fibrils; however, AL-associated LC bound significantly more efficiently (p < 0.05) than did MM LCs. The LC proteins used in this study were isolated from urine and presumed to represent a surrogate of serum free light chains. The binding of LC to synthetic fibrils in this assay accurately differentiated LC with amyloidogenic propensity from MM LC that were not associated with clinical amyloid disease. Notably, the LC from a MM patient who subsequently developed amyloid behaved as an AL-associated protein in the assay, indicating the possibility for identifying MM patients at risk for developing amyloidosis based on the light chain recruitment efficacy. With this information, at risk patients can be monitored more closely for the development of amyloidosis, allowing timely administration of novel, amyloid-directed immunotherapies—this approach may improve the prognosis for these patients.
Monoclonal plasma cell proliferation is associated with a continuum of gammopathies characterized by the presence of a clonal plasma cell population in the bone marrow and the presence of intact monoclonal immunoglobulin and/or free light chain (LC) proteins in the serum [1–5]. In the US, the prevalence of monoclonal gammopathy of undetermined significance (MGUS), a pre-malignant state, is 4.2% in Caucasians over the age of 50, with 20% of those secreting only monoclonal light chain (LCMGUS) . Longitudinal studies have demonstrated that LCMGUS precedes LC-associated multiple myeloma (MM) and that both conditions may lead to light chain-associated (AL) amyloidosis, a devastating protein misfolding disorder characterized by the systemic deposition of extracellular amyloid fibrils composed of LC proteins . The genetic, biochemical and physiological factors that dictate which MGUS and MM patients will develop clinical LC amyloidosis are presently unknown. However, in addition to enigmatic host factors, the propensity of the monoclonal serum free light chain to aggregate into amyloid fibrils is a critically important factor [8, 9].
Our theoretical understanding of LC amyloidogenesis comes almost exclusively from in vitro studies of fibrillogenesis using recombinant VL fragments of κ4 [20, 36–39], λ6 [21, 40, 41] and κ1[18, 19, 42–44] LC subgroups produced in E. coli. Fibril formation from these components often requires harsh conditions such as low pH, agitation, or the presence of chaotropes, indicating the need for a denaturing milieu to initiate protein misfolding and, consequently, fibril formation. Although there are rare exceptions [19, 45], these studies demonstrated a general inverse correlation between VL folding free energy and the propensity for in vitro fibrillogenesis, such that less stably folded VL domains more readily form fibrils. Thus, de novo fibrillogenesis from soluble VL domains can be rapid and requires misfolding of the VL, allowing structured self-association into thermodynamically stable multimers that act as templates, or seeds, for the recruitment of additional VL domains. Template-mediated seeding is a universal paradigm in amyloid-related pathologies that underlies the proposed transmissibility of amyloid diseases [46–50].