Amyloid fibrils derived from the apolipoprotein A1 Leu174Ser variant contain elements of ordered helical structure

Abstract

We recently described a new apolipoprotein A1 variant presenting a Leu174Ser replacement mutation that is associated with a familial form of systemic amyloidosis displaying predominant heart involvement. We have now identified a second unrelated patient with very similar clinical presentation and carrying the identical apolipoprotein A1 mutation. In this new patient the main protein constituent of the amyloid fibrils is the polypeptide derived from the first 93 residues of the protein, the identical fragment to that found in the patient previously described to carry this mutation. The X-ray fiber diffraction pattern obtained from preparations of partially aligned fibrils displays the cross-␤ reflections characteristic of all amyloid fibrils. In addition to these cross-␤ reflections, other reflections suggest the presence of well-defined coiled-coil helical structure arranged with a defined orientation within the fibrils. In both cases the fibrils contain a trace amount of full-length apolipoprotein A1 with an apparent prevalence of the wild-type species over the variant protein. We have found a ratio of full-length wild-type to mutant protein in plasma HDL of three to one. The polypeptide 1-93 purified from natural fibrils can be solubilized in aqueous solutions containing denaturants, and after removal of denaturants it acquires a monomeric state that, based on CD and NMR studies, has a predominantly random coil structure. The addition of phospholipids to the monomeric form induces the formation of some helical structure, thought most likely to occur at the C-terminal end of the polypeptide.

Liver biopsy discloses a new apolipoprotein A-I hereditary amyloidosis in several unrelated Italian families

“…Determination of wild-type and mutant species in total circulating apoA-I in 2 patients showed a 10:1 ratio or even less. This severe imbalance is in agreement with previously reported data for other amyloidogenic variants, 10 suggesting abnormal metabolism of the mutant compared with the wild-type protein. 10 -12 Although the role of liver biopsy in the evaluation of abnormal liver enzyme results in asymptomatic patients is still controversial, [13][14][15] in this case it was essential to establish a correct diagnosis, avoid further diagnostic procedures and unnecessary medications, and offer genetic counseling.…”

Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I

“…This indicates that the core structural elements within the two fibril types are similar. Diffraction patterns from ex vivo fibrils resulting from the apoA-I Leu174Ser mutation showed two closely spaced, major equatorial reflections in addition to evidence of coiled-coil structure (44); however, these features were not apparent in the diffraction patterns from MetO-apoA-I fibrils.…”

“…The ultrastructural morphology of amyloid fibrils formed from MetO-apoA-I is similar to those formed by apoC-II (28) but differs from the more classical fibril morphology observed for ex vivo apoA-I fibrils (44,47). Ultrastructural remodelling of fibrils may occur in vivo during proteolysis of the fibril subunits, or due to the presence of lipid or accessory proteins such as serum amyloid P that may explain the difference in gross appearance.…”

Probing the origins, diagnosis and treatment of amyloid diseases using antibodies

“…In some cases, it is likely that only certain segments of a polypeptide chain will be involved in the structural core of the fibrils with the remainder of the chain being organised in some other manner at the surface of the amyloid fibril [62]. In other cases, it is probable that most of the polypeptide chain is involved in the core of the fibril, as for example in the case of relatively short peptides [63].…”