Amylothrombosis: a novel molecular mechanism correlating Systemic Amyloidosis with thromboembolic risk

Haemostasis is a finely balanced process, representing a physiological defense response to blood vessel wall injuries. It requires a precise regulation, to ensure clot formation only when necessary and clot dissolution once the vessel is healed. This delicate balance can become disrupted by different causes, leading to several pathological conditions, that can be either bleeding disorders or thrombotic disorders. A key component of haemostasis is blood coagulation, that involves a complex cascade of events where specific proteins, known as clotting factors, are activated in a cascade reaction to form a stable blood clot. At present, cardiovascular diseases (CVDs) represent one of the major challenges to public health due to their increasing global prevalence. These diseases, often linked to the loss of hemostatic balance and malfunctioning of the blood coagulation mechanisms, include a variety of conditions, where abnormal clot formation can obstruct the normal flow of blood to vital organs, such as heart and brain. Lifestyle factors, aging populations, and genetic predispositions contribute to the increasing incidence of CVDs, raising the need for further research on blood coagulation and haemostasis. Recent studies have increasingly focused the attention on the correlation between systemic amyloidosis and CVDs. Systemic amyloidosis, a group of diseases characterized by the abnormal deposition of amyloid fibrils in various organs and tissues, has increasingly been linked to cardiovascular complications such as cardiomyopathy, heart failure, arrhythmias, and disfunctions in the coagulation system, with a marked tendency to thrombosis. The present work aims to investigate, at a molecular level, the interactions between amyloid fibrils (or their precursor proteins) and the coagulation cascade, to explain how these interactions promote thrombosis. Understanding these pathways can pave the way for the development of targeted therapeutic approaches, that can reduce thrombotic risk associated with systemic amyloidosis, and improve the diagnosis and management of patients suffering from both systemic amyloidosis and cardiovascular diseases. In particular, in this work the attention is focused on the effect that the exposure of amyloid fibrils to the blood circulation has on the coagulation system, to define a new molecular mechanism underlying the activation of the coagulation cascade in ATTR and AL systemic amyloidosis. The interaction between fibrillar aggregates and the circulating coagulation factors represents a novel and yet unexplored area of research. Amyloid fibrils extracted from human cardiac tissues demonstrated the ability to promote clot formation both in whole blood and plasma, through the generation of α-thrombin, the key enzyme in the coagulation process that converts soluble fibrinogen into insoluble fibrin polymer. The effect observed with ex vivo amyloid fibrils was confirmed with in vitro-generated amyloid fibrils, which have a better characterized composition and chemical properties, and are useful for rationalizing the possible molecular mechanisms for the observed pro-thrombotic effect. From the data presented in this study, it is clear that FXII undergoes an autoactivation process in the presence of amyloid fibrils, representing the main triggering event for the activation of the coagulation cascade. Furthermore, the interaction between α-thrombin and hTTR(49-127), the highly amyloidogenic peptide that derives from proteolysis of hTTR, was found to significantly enhance the hydrolytic activity of the enzyme, thus revealing a novel mechanism responsible for increasing thrombotic risk in ATTR amyloidosis.