Research ReportParenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy

- This is a post-mortem study of brain pathology in HCCAA patients.
- The study addressed the topographical distribution and severity of brain cystatin C deposition.
- This study revealed focal parenchymal cystatin C deposits for the first time.
- The study addressed the neuroinflammatory response to cystatin C deposition.
- The neuroinflammatory response was limited to the vicinity of arteries and focal deposits.

Hereditary Cystatin C Amyloid Angiopathy (HCCAA) is an amyloid disorder in Icelandic families caused by an autosomal dominant mutation in the cystatin C gene. Mutant cystatin C forms amyloid deposits in brain arteries and arterioles which are associated with changes in the arterial wall structure, notably deposition of extracellular matrix proteins. In this post-mortem study we examined the neuroinflammatory response relative to the topographical distribution of cystatin C deposition, and associated haemorrhages, in the leptomeninges, cerebrum, cerebellum, thalamus, and midbrain of HCCAA patients. Cystatin C was deposited in all brain areas, grey and white matter alike, most prominently in arteries and arterioles; capillaries and veins were not, or minimally, affected. We also observed perivascular deposits and parenchymal focal deposits proximal to affected arteries. This study shows for the first time, that cystatin C does not exclusively form CAA and perivascular amyloid but also focal deposits in the brain parenchyma. Haemorrhages were observed in all patients and occurred in all brain areas, variable between patients. Microinfarcts were observed in 34.6% of patients. The neuroinflammatory response was limited to the close vicinity of affected arteries and perivascular as well as parenchymal focal deposits. Taken together with previously reported arterial accumulation of extracellular matrix proteins in HCCAA, our results indicate that the central nervous system pathology of HCCAA is characterised by the formation of a glial scar within and around affected arteries.