Cellular processing of the amyloidogenic cystatin C variant of hereditary cerebral hemorrhage with amyloidosis, Icelandic type

AN important gap in our understanding of the pathogenesis of the amyloidoses is the identification of the cellular events that lead from synthesis of an amyloid precursor protein to its conversion to the amyloid fiber subunit. We address this question by characterizing the effects of an amyloidogenic mutation on the intracellular processing of its protein product, The protein, a mutant of the cysteine protease inhibitor cystatin C, is the amyloid precursor protein in Hereditary Cerebral Hemorrhage with Amyloidosis-Icelandic type (HCHWA-I). The amyloid fibers are composed of mutant cystatin C (L68Q) that lades the first 10 amino acids. We have previously shown that processing of wild-type cystatin C entails formation of a transient intracellular dimer that dissociates prior to secretion, such that extracellular cystatin C is monomeric. We report here that the cystatin C mutation engenders several alterations in its intracellular trafficking. It forms a stable intracellular dime that is partially retained in the endoplasmic reticulum and degraded. The bulk of mutant cystatin C that is secreted does not dissociate and is secreted as an inactive dimer. Thus, formation of the stable mutant cystatin C dimer is an early event in the pathogenesis of this disease.