The transformation of
serum proteins into
Congo red-sensitive fibrillar material is requisite for the onset and progression of
amyloid disease. All the mechanisms which lead to the disease itself have not been elucidated, but our knowledge has increased significantly. It is apparent that in all types of
amyloid fibrils, three common features are displayed by the major
protein constituents. These are that the fibril
protein has a serum precursor, a high degree of anti-parallel beta-sheet conformation and a distinctive ultrastructure on electron microscopy. In the AL and AA forms of
amyloidosis, the putative precursors appear to undergo limited degradation to form the
protein component of
amyloid fibrils. It has been suggested that there may be certain primary structural characteristics inherent in precursor molecules which make them amyloidogenic, thus predisposing them to
amyloid fibril formation. This would include certain subtypes of
immunoglobulin light chains, possibly kappa I and lambda VI, in the AL type of
amyloidosis and one of the polymorphic SAA species, SAA2, which has been identified as the predominating isotype found in AA
amyloid fibrils. In AH
amyloidosis, the mechanism of
amyloid fibril formation appears to be simply a concentration phenomenon where elevated concentrations of B2-M are not handled normally and
amyloid deposition is the result. Amyloidogenesis in the hereditary form of systemic
amyloidosis may involve other factors in addition to the presence of a variant precursor
prealbumin as indicated by the delayed onset of the disease. It is evident that the elucidation of the mechanism(s) which governs the onset and progression of the
amyloidoses will allow future regulation and treatment of these all too often complex disorders.