Structural determinants in ApoA-I amyloidogenic variants explain improved cholesterol metabolism despite low HDL levels

Abstract

Twenty Apolipoprotein A-I (ApoA-I) variants are responsible for a systemic hereditary amyloidosis in which protein fibrils can accumulate in different organs, leading to their failure. Several ApoA-I amyloidogenic mutations are also associated with hypoalphalipoproteinemia, low ApoA-I and high-density lipoprotein (HDL)-cholesterol plasma levels; however, subjects affected by ApoA-I-related amyloidosis do not show a higher risk of cardiovascular diseases (CVD). The structural features, the lipid binding properties and the functionality of four ApoA-I amyloidogenic variants were therefore inspected in order to clarify the paradox observed in the clinical phenotype of the affected subjects. Our results show that ApoA-I amyloidogenic variants are characterized by a different oligomerization pattern and that the position of the mutation in the ApoA-I sequence affects the molecular structure of the formed HDL particles. Although lipidation increases ApoA-I proteins stability, all the amyloidogenic variants analyzed show a lower affinity for lipids, both in vitro and in ex vivo mouse serum. Interestingly, the lower efficiency at forming HDL particles is compensated by a higher efficiency at catalysing cholesterol efflux from macrophages. The decreased affinity of ApoA-I amyloidogenic variants for lipids, together with the increased efficiency in the cholesterol efflux process, could explain why, despite the unfavourable lipid profile, patients affected by ApoA-I related amyloidosis do not show a higher CVD risk.

Keywords: Amyloidosis; Apolipoprotein A-I; Cholesterol efflux; High-density lipoprotein; Hypoalphalipoproteinemia.