The cerebral hemorrhage-producing cystatin C variant (L68Q) in extracellular fluids

A variant of the normal extracellular cysteine protease inhibitor cystatin C (L68Q-cystatin C), is the amyloid precursor in hereditary cystatin C amyl oid angiopathy (HCCAA). It has been suggested that the mutation causes cell ular entrapment of L68Q-cystatin C in vivo and that the variant protein is not secreted to extracellular fluids. In order to test this hypothesis, we used matrix-assisted laser desorption ionization time-of-flight mass spectr ometry in an effort to demonstrate the presence of L68Q- along with wildtyp e cystatin C in plasma and cerebrospinal fluid (CSF) of HCCAA-patients. Pla sma from all five investigated HCCAA-patients contained both L68Q- and wild type cystatin C. The presence of approximately equal amounts of cystatin C dimers and monomers was demonstrated in plasma from HCCAA-patients, whereas only monomers could be found in normal plasma. L68Q-wildtype-cystatin C he terodimers seem to be present in the dimeric cystatin C population. CSF fro m six HCCAA-patients also contained cystatin C-dimers and monomers, bur the dimeric fraction was minute. CSF from control patients did not contain dim eric cystatin C. These results suggest that the milieu of L68Q-cystatin C i s important for its stability and dimerization status and that certain mili eus might hinder its further development into oligomers, amyloid fibrils an d other precipiting aggregates.