Increased body temperature accelerates aggregation of the Leu-68-->Gln mutant cystatin C, the amyloid-forming protein in hereditary cystatin C amyloid angiopathy.

Hereditary cystatin C amyloid angiopathy is adominantly inherited disorder, characterized by dementia, paralysis, and deathfrom cerebral hemorrhage in early adult life. A variant of the cysteineproteinase inhibitor, cystatin C, is deposited as amyloid in the tissues of thepatients and their spinal-fluid level of cystatin C is abnormally low. Thedisease-associated Leu-68-->Gln mutant (L68Q) cystatin C has been produced inan Escherichia coli expression system and isolated by use of denaturing buffers,immunosorption, and gel filtration. Parallel physicochemical and functionalinvestigations of L68Q-cystatin C and wild-type cystatin C revealed that bothproteins effectively inhibit the cysteine proteinase cathepsin B (equilibriumconstants for dissociation, 0.4 and 0.5 nM, respectively) but differconsiderably in their tendency to dimerize and form aggregates. While wild-typecystatin C is monomeric and functionally active even after prolonged storage atelevated temperatures, L68Q-cystatin C starts to dimerize and lose biologicalactivity immediately after it is transferred to a nondenaturing buffer. Thedimerization of L68Q-cystatin C is highly temperature-dependent, with a rise inincubation temperature from 37 to 40 degrees C resulting in a 150% increase indimerization rate. The aggregation at physiological concentrations is likewiseincreased at 40 compared to 37 degrees C, by approximately 60%. These propertiesof L68Q-cystatin C have bearing upon our understanding of the pathophysiologicalprocess of hereditary cystatin C amyloid angiopathy. They might also be ofclinical relevance, since medical intervention to abort febrile periods ofcarriers of the disease trait may reduce the in vivo formation of L68Q-cystatinC aggregates.