Instability of the amyloidogenic cystatin C variant of hereditary cerebral hemorrhage with amyloidosis, Icelandic type.

[hereditary cerebral hemorrhage with amyloidosis]

A cystatin C variant with L 68 Q substitution and a truncation of 10 NH 2 -terminal residues is the major constituent of the amyloid deposited in the cerebral vasculature of patients with the Icelandic form of hereditary cerebral hemorrhage with amyloidosis (HCHWA -I ). Variant and wild type cystatin C production , processing , secretion , and clearance were studied in human cell lines stably overexpressing the cystatin C genes . Immunoblot and mass spectrometry analyses demonstrated monomeric cystatin C in cell homogenates and culture media . While cystatin C formed concentration-dependent dimers , the HCHWA -I variant dimerized at lower concentrations than the wild type protein . Amino-terminal sequence analysis revealed that the variant and normal proteins produced and secreted are the full-length cystatin C . Pulse-chase experiments demonstrated similar levels of normal and variant cystatin C production and secretion . However , the secreted variant cystatin C exhibited an increased susceptibility to a serine protease in conditioned media and in human cerebrospinal fluid , explaining its depletion from the cerebrospinal fluid of HCHWA -I patients . Thus , the amino acid substitution may induce unstable cystatin C with intact inhibitory activity and predisposition to self-aggregation and amyloid fibril formation .