Hereditary cystatin C amyloid angiopathy

Hereditary cystatin C amyloid angiopathy is an autosomal dominant, highly penetrant disease of young adults in which a mutant cystatin C protein deposits as amyloid in the small arteries of the brain, causing recurrent lobar haemorrhages and early death.

It is essentially confined to families of Icelandic origin, presents in normotensive people in their twenties or thirties, and has no approved disease-modifying therapy, although high-dose N-acetylcysteine has shown encouraging biomarker effects.

Terminology and classification

• The standard name is hereditary cystatin C amyloid angiopathy, abbreviated HCCAA, and the historical name is hereditary cerebral haemorrhage with amyloidosis, Icelandic type, abbreviated HCHWA-I.
• ACys amyloidosis and cystatin C amyloid angiopathy are synonyms, and the disease is classified among the hereditary cerebral amyloid angiopathies together with the Dutch, Flemish, Italian and Arctic types.
• ICD-10 uses the dagger and asterisk combination E85.4 with I68.0, and ICD-11 assigns the specific code 8B22.3 for cerebral amyloid angiopathy.
• Other identifiers are OMIM 105150, Orphanet ORPHA:85458, MONDO:0007098, GARD 10266, UMLS C1510489 and MedGen C1527338.
• Unlike sporadic cerebral amyloid angiopathy of the elderly, HCCAA is a monogenic disease with complete or near-complete penetrance and an early, fatal course.
• The disorder is also a systemic amyloidosis in a mild sense, because mutant cystatin C deposits can be found in skin, salivary glands and other tissues, which provides a biopsy route for diagnosis and monitoring.

History

• The Icelandic physician Arni Arnason described the disease in the 1930s after noticing that 79 young people in western Iceland were dying of brain haemorrhage before the age of 40, and he documented the familial pattern in detail.^[1]
• For decades the condition was known as hereditary cerebral haemorrhage with amyloidosis, Icelandic type, and the amyloid protein remained unidentified until biochemical studies of Icelandic brain vessels.
• In 1989 Levy and colleagues demonstrated that stroke in Icelandic patients was linked to a mutation in the cystatin C gene, identifying the first hereditary amyloid angiopathy with a known gene product.^[2]
• Cell biology and animal model work in 2006 established that mutant cystatin C is secreted, misfolds and forms amyloid fibrils in the vascular wall, with accompanying smooth muscle loss.^[3]
• A 2008 genealogical and linkage disequilibrium study showed that all known copies of the mutation descend from a single founder and that carrier lifespan fell sharply over two centuries as living conditions changed.^[4]
• In 2025 a nonrandomized phase 2a trial of high-dose N-acetylcysteine reported safety and reduced skin biomarkers, and the European Medicines Agency granted orphan designation to an acetylcysteine amide product in August 2025.^[5]

Epidemiology

• HCCAA is found almost exclusively in people of Icelandic descent, with most families traced to a small region in the west of the country, and only a few cases reported abroad in descendants of emigrants.
• Genetic genealogy has identified 15 families, of which 9 have been confirmed by DNA analysis, and it is estimated that more than 300 individuals inherited the mutation over the past centuries.
• The number of living carriers is small and has been estimated at a few dozen, so the disease meets the European definition of an ultra-rare disorder.
• Among carriers born after 1900, the age at first symptom ranges from 16 to 79 years, most first haemorrhages occur between 20 and 40 years and only 2 to 4 percent survive beyond 50.^[1]
• Contemporary carriers who reach adulthood have an average age at death of about 30 years, a dramatic reduction compared with earlier generations that has been attributed to lifestyle change.^[4]
• Because the mutation is dominant and penetrance is high, nearly every carrier develops haemorrhagic disease if they live long enough, and there is no sex predilection.
• A pathological survey identified 127 affected people in eight Icelandic families, all originating from the same western region, and this founder population remains the basis of every published prevalence estimate.
• The disease has been reported in Icelandic emigrants to North America and Europe, but no clusters have been described outside the Icelandic diaspora, so incidence and mortality rates remain unpublished.

Pathophysiology

• The causative variant is L68Q in CST3 on chromosome 20p11.21, which substitutes glutamine for leucine at position 68 of the mature cystatin C protein.
• Cystatin C is a cysteine protease inhibitor produced by most nucleated cells, and the mutant protein is less stable, more prone to dimerisation and able to form amyloid fibrils under physiological conditions.
• Amyloid deposits in leptomeningeal and cortical arterioles, and the vessel wall loses smooth muscle cells and becomes fragile, predisposing to lobar haemorrhage and to impaired perivascular drainage.
• A 2026 neuropathology study showed basement membrane remodelling with fibroblast activation and cystatin C aggregation in cerebral vessels, suggesting that matrix changes accompany and possibly amplify the amyloid deposition.^[6]
• Deposits are also found in skin, salivary glands and other systemic vessels, and skin biopsy staining for mutant cystatin C correlates with cerebral disease and has been used as a pharmacodynamic marker.
• Histology shows amyloid angiopathy with thickening of arteriolar walls, microaneurysms, fibrinoid necrosis and old and recent microhaemorrhages, often with white matter rarefaction.

Clinical features

• The typical presentation is a lobar intracerebral haemorrhage in a normotensive adult in the third or fourth decade, often with headache, vomiting, focal deficit or impaired consciousness.
• Haemorrhages recur, may be multiple or simultaneous, and can affect any lobe, and asymptomatic microbleeds and cortical superficial siderosis accumulate between clinical events.
• Seizures, transient focal episodes, progressive cognitive decline and stepwise dementia develop as the disease advances, and some patients present with cognitive change before any overt bleed.
• Because carriers are young and previously healthy, the first event is often initially attributed to trauma, vascular malformation or illicit drug use before the family history is elicited.
• Neurological deficits depend on the site and size of the bleed, and survivors may have hemiparesis, aphasia, visual field loss, ataxia or epilepsy requiring rehabilitation.
• Systemic symptoms are usually absent, although skin and salivary gland deposits can be demonstrated pathologically, and routine blood tests including inflammatory markers are normal.

Diagnosis

• The diagnosis should be suspected in any young normotensive patient with lobar haemorrhage, multiple microbleeds, leukoencephalopathy or a family history of early stroke.
• Brain magnetic resonance imaging with susceptibility-weighted sequences shows lobar microbleeds, cortical superficial siderosis and white matter hyperintensities, and angiography excludes arteriovenous malformations and aneurysms.
• Genetic testing for the CST3 L68Q variant confirms the diagnosis, and targeted testing is preferred over panel testing because the mutation is specific and the population is small.
• Skin biopsy with immunohistochemistry for mutant cystatin C supports the diagnosis when genetic testing is delayed and is also used to monitor treatment response.
• The differential diagnosis includes the Dutch type of hereditary cerebral amyloid angiopathy, sporadic cerebral amyloid angiopathy, hypertensive small vessel disease, cerebral vasculitis, cavernous malformation and Reversible cerebral vasoconstriction syndrome.
• Familial screening, predictive testing and genetic counselling are offered to at-risk relatives, and testing of minors is generally deferred until the individual can consent.

Management

• There is no approved disease-modifying therapy, and acute haemorrhage is managed with standard neurocritical care, blood pressure control and reversal of any anticoagulation.
• Antiplatelet drugs and anticoagulants should generally be avoided because of the high risk of recurrent haemorrhage, and decisions about antithrombotic therapy require specialist assessment.
• High-dose N-acetylcysteine, 2400 mg daily for nine months, was safe and well tolerated in 17 carriers, with five cerebral bleeds during treatment but no permanent sequelae and no deaths.^[5]
• Treatment significantly reduced disease-specific biomarkers in skin, including collagen IV, supporting a biological effect that now requires confirmation in larger and longer trials.^[5]
• Rehabilitation, epilepsy management, cognitive support and advance care planning are important because survivors often have accumulating deficits and a shortened life expectancy.
• Carriers should be followed in a specialised centre with serial imaging, neurological assessment, skin biomarker sampling and counselling about pregnancy, employment and driving.
• Blood pressure should be maintained below 130/80 mmHg and any acute neurological event requires urgent non-contrast computed tomography and specialist stroke care, because recurrent lobar haemorrhage is the main cause of death.
• N-acetylcysteine should not be used outside clinical trials until efficacy is confirmed, and patients who choose it should be enrolled in a registry with serial skin biomarker measurements.

Research and controversies

• The mechanism by which a single amino acid substitution produces vascular amyloid remains incompletely understood, and whether clearance of deposits can reverse vessel damage is unknown.
• Why carrier lifespan fell after the nineteenth century is debated, and the leading explanation involves changes in diet, physical activity and cardiovascular risk factors rather than genetic drift.^[4]
• A 2022 Icelandic hypothesis proposed that a ketogenic diet in past centuries protected carriers and that modern carbohydrate-rich diets accelerated disease, but this remains speculative.^[7]
• Biomarker validation, optimal N-acetylcysteine dose, combination with other reducing agents and trial design in an ultra-rare population are the main methodological challenges.
• International collaboration and patient registries are essential because no single centre can recruit enough carriers for an adequately powered randomized trial.
• Long-term outcomes such as haemorrhage rate, cognition, quality of life and survival must be measured, and natural history data are still being assembled from Icelandic genealogies and clinical records.