Effect of position-specific single-point mutations and biophysical characterization of amyloidogenic peptide fragments identified from lattice corneal dystrophy patients

Corneal stromal dystrophies are a group of genetic disorders that may be caused by mutations in the transforming growth factor β-induced (TGFBI) gene which results in the aggregation and deposition of mutant proteins in various layers of the cornea. The type of amino acid substitution dictates the age of onset, anatomical location of the deposits, morphological features of deposits (amyloid, amorphous powder or a mixture of both forms) and the severity of disease presentation. It has been suggested that abnormal turnover and aberrant proteolytic processing of the mutant proteins result in the accumulation of insoluble protein deposits. Using mass spectrometry, we identified increased abundance of a 32 amino acid-long peptide in the 4th fasciclin-like domain-1 (FAS-1) domain of transforming growth factor β-induced protein (amino acid 611-642) in the amyloid deposits of the patients with lattice corneal dystrophies (LCD). In vitro studies demonstrated that the peptide readily formed amyloid fibrils under physiological conditions. Clinically relevant substitution (M619K, N622K, N622H, G623R and H626R) of the truncated peptide resulted in profound changes in the kinetics of amyloid formation, thermal stability of the amyloid fibrils and cytotoxicity of fibrillar aggregates, depending on the position and the type of the amino acid substitution. The results suggest that reduction in the overall net charge, nature and position of cationic residue substitution determines the amyloid aggregation propensity and thermal stability of amyloid fibrils.