ACys Amyloidosis

Mendelian MONDO:0007098 Pathograph 14 Show in embeddings browser hereditary disease amyloidosis cerebral amyloid angiopathy

ACys amyloidosis (hereditary cystatin C amyloid angiopathy, HCCAA) is an ultra-rare autosomal dominant cerebral amyloidosis caused by a single Icelandic founder variant, CST3 L68Q. The substitution destabilizes cystatin C and drives three-dimensional domain-swapped dimerization and oligomerization, producing amyloid that deposits in the walls of brain arteries and arterioles together with excess basement-membrane extracellular matrix. Progressive arterial wall thickening and smooth muscle cell loss culminate in recurrent lobar intracerebral hemorrhage in young normotensive adults, with dementia and paralysis, and death at a mean age of about 30 years. Amyloid is deposited systemically - notably in skin basement membrane - but clinical manifestations are essentially restricted to the brain, and the systemic deposits are exploited diagnostically. Distinguish carefully from the Dutch type of hereditary cerebral hemorrhage with amyloidosis (HCHWA-D), which is an APP-related amyloid-beta disease and not part of this entry.

Ask OpenScientist

Ask a research question about ACys Amyloidosis. OpenScientist will conduct autonomous deep research using the Disorder Mechanisms Knowledge Base and PubMed literature (typically 10-30 minutes).

Submitting...

Do not include personal health information in your question. Questions and results are cached in your browser's local storage.

1
Inheritance
10
Pathophys.
3
Histopath.
6
Phenotypes
3
Gaps
14
Pathograph
1
Genes
1
Variants
3
Medical Actions
3
Differentials
👪

Inheritance

1
Autosomal dominant inheritance HP:0000006
HCCAA segregates as a highly penetrant autosomal dominant trait; all known carriers are heterozygous for the same CST3 L68Q founder variant.
Autosomal dominant inheritance
Show evidence (2 references)
PMID:18566660 SUPPORT Human Clinical
"Hereditary cystatin C amyloid angiopathy (HCCAA) is an autosomal dominant disease with high penetrance, manifest by brain hemorrhages in young normotensive adults."
Directly establishes autosomal dominant inheritance with high penetrance for HCCAA.
PMID:2602420 SUPPORT Human Clinical
"The Alu I marker has been used to show that this mutation is transmitted only in the affected members in all eight families investigated, proving that the mutated cystatin C gene causes HCCAA."
Co-segregation of the CST3 variant with disease across eight families establishes the dominant Mendelian relationship.
?

Discussions and Knowledge Gaps

3
Why does HCCAA still lack an animal model, and what would a faithful model have to reproduce?
KNOWLEDGE GAP OPEN hccaa_no_animal_model
Attached to
pathophysiology#Cystatin C Amyloid Deposition in Cerebral Arterial Walls pathophysiology#Arterial Wall Matrix Accumulation and Thickening
Essentially all mechanistic knowledge of HCCAA comes from human post-mortem tissue, patient-derived cells, and in vitro structural biology. The absence of an animal model is stated explicitly in the vascular pathology literature, and it is why the temporal ordering of amyloid deposition, matrix accumulation, and smooth muscle cell loss remains inferred from cross-sectional autopsy material rather than demonstrated longitudinally.
Show evidence (1 reference)
PMID:23973860 SUPPORT Human Clinical
"The aetiology of HCCAA pathology is not clear and there is, at present, no animal model of the disease."
States both the unresolved aetiology and the absence of an animal model.
Are basement membrane changes an early driver that facilitates cystatin C deposition, or a consequence of deposition already underway?
INTERPRETATION OPEN hccaa_basement_membrane_primacy
Attached to
pathophysiology#Cystatin C Amyloid Deposition in Cerebral Arterial Walls pathophysiology#Arterial Wall Matrix Accumulation and Thickening pathophysiology#Systemic Extracerebral Cystatin C Deposition
This entry currently draws the edge from amyloid deposition to matrix accumulation, following the arterial post-mortem description. The skin biopsy study argues the opposite direction - that basement membrane change is early and facilitates deposition - and reports collagen IV increased to a similar extent in all carrier biopsies regardless of deposit stage. Both directions are supported by cross-sectional human material only, and resolving the order would require longitudinal sampling or a model system that does not yet exist.
Show evidence (1 reference)
PMID:28067897 SUPPORT Human Clinical
"suggest that BM changes are early and important events in HCCAA pathogenesis that could facilitate cystatin C deposition and aggregation"
States the alternative causal ordering that this discussion records as unresolved.
Which specific environmental or life-style exposure accounts for the halving of L68Q carrier life span between 1825 and 1900?
KNOWLEDGE GAP OPEN hccaa_environmental_modifier
Attached to
pathophysiology#Recurrent Lobar Intracerebral Hemorrhage
The genealogical evidence for an environmental modifier is strong and replicated across independent extended families, and it implies that a preventive strategy may exist. But the responsible exposure has never been identified, so no ECTO term can be bound and the effect cannot be modeled mechanistically. A parent-of-origin effect emerging over the same period is an additional unexplained observation.
Show evidence (1 reference)
PMID:18566660 SUPPORT Human Clinical
"At the same time, a parent-of-origin effect emerged, whereby maternal inheritance of the mutation was associated with a 9 year reduction in life span relative to paternal inheritance."
Records the unexplained parent-of-origin effect that accompanies the life-span shift.
⚙

Pathophysiology

10
CST3 L68Q Founder Variant
A single T-to-A substitution in codon 68 of CST3 replaces leucine with glutamine in cystatin C. Every known HCCAA carrier is heterozygous for this one Icelandic founder allele, so the disorder has a uniform initiating genetic lesion.
CST3 hgnc:2475 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves CST3 (hgnc:2475). hgnc:2475 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Destabilized L68Q Cystatin C Precursor The L68Q substitution sits near the inhibitory region of cystatin C and yields a conformationally destabilized variant protein.
Show evidence (2 references)
PMID:2602420 SUPPORT Human Clinical
"We have used the full length cystatin C cDNA probe (Abrahamson et al., 1987) to demonstrate a mutation in the codon for leucine at position 68, which abolishes an Alu I restriction site in cystatin C gene of the HCCAA patients."
Identifies the codon-68 CST3 mutation as the initiating lesion in HCCAA patients.
PMID:16612982 SUPPORT Human Clinical
"The same mutation in cystatin C, L68Q, has been found in all patients examined so far pointing to a common founder."
Establishes L68Q as the single founder allele shared by all examined patients.
Destabilized L68Q Cystatin C Precursor
Variant cystatin C is the amyloidogenic precursor of this disease, the disorder-specific substitution for the module's generic destabilized precursor. The variant protein purified from patient amyloid is cystatin C (gamma-trace) carrying the position-68 substitution, and structural work shows the mutation destabilizes the folded monomer.
CST3 hgnc:2475 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves CST3 (hgnc:2475). hgnc:2475 is a gene from the HUGO Gene Nomenclature Committee.
protein folding GO:0006457 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal protein folding (GO:0006457). GO:0006457 is a biological process from the Gene Ontology. ⚠ ABNORMAL
cysteine-type endopeptidase inhibitor activity GO:0004869 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves cysteine-type endopeptidase inhibitor activity (GO:0004869). GO:0004869 is a molecular function from the Gene Ontology.
Downstream
Domain-Swapped Dimerization and Oligomerization Destabilization of the monomer favours the partially unfolded intermediate that permits three-dimensional domain swapping.
Intracellular Retention of Variant Cystatin C The variant protein is poorly secreted and is retained in association with cells, which lowers extracellular cystatin C and raises the intracellular pool.
Show evidence (2 references)
PMID:3517880 SUPPORT Human Clinical
"A gamma-trace variant protein is the major constituent of the amyloid fibrils in patients from Iceland with hereditary cerebral hemorrhage with amyloidosis."
Identifies variant cystatin C (gamma-trace) as the amyloid subunit in patient tissue, establishing it as the amyloidogenic precursor.
PMID:11276250 SUPPORT In Vitro
"The structure of the three-dimensional domain-swapped dimers shows how the L68Q mutation destabilizes the monomers and makes the partially unfolded intermediate less unstable."
Crystallographic work provides the destabilization mechanism that makes the L68Q precursor misfolding-prone.
Intracellular Retention of Variant Cystatin C
Variant cystatin C accumulates within cells rather than being secreted, which depletes cerebrospinal fluid cystatin C and is proposed to raise the local concentration that drives aggregation. This is a secretion/trafficking arm that runs alongside the intrinsic destabilization arm.
protein folding GO:0006457 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal protein folding (GO:0006457). GO:0006457 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Domain-Swapped Dimerization and Oligomerization The authors propose that the raised intracellular concentration promotes aggregation and fibril formation; the intermediate steps in patient brain are not established.
Show evidence (1 reference)
PMID:9445375 SUPPORT Human Clinical
"The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished and cultivated monocytes from affected individuals accumulate cystatin C."
Documents both the reduced extracellular (CSF) pool and the cellular accumulation that define this node.
Domain-Swapped Dimerization and Oligomerization
Cystatin C dimerizes by three-dimensional domain swapping, in which two partially unfolded monomers exchange structural elements to reconstitute monomer-like domains. Open-ended propagation of the same mechanism links molecules into higher aggregates. Oligomer formation - not the monomer - is the species that accumulates in the cerebral vasculature.
protein folding GO:0006457 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal protein folding (GO:0006457). GO:0006457 is a biological process from the Gene Ontology. ⚠ ABNORMAL
protein homodimerization activity GO:0042803 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves increased protein homodimerization activity (GO:0042803). GO:0042803 is a molecular function from the Gene Ontology. ↑ INCREASED
Downstream
Cystatin C Amyloid Deposition in Cerebral Arterial Walls Aggregation-prone variant protein forms heavy amyloid deposits in the walls of small cerebral arteries and arterioles.
Show evidence (3 references)
PMID:11276250 SUPPORT In Vitro
"The dimerization occurs through three-dimensional domain swapping, a mechanism for forming oligomeric proteins."
Establishes the specific misfolding/oligomerization mechanism substituted for the module's generic beta-sheet oligomerization step.
PMID:11276250 SUPPORT In Vitro
"Higher aggregates may arise through the three-dimensional domain-swapping mechanism occurring in an open-ended fashion in which partially unfolded molecules are linked into infinite chains."
Extends the dimer mechanism to higher-order aggregates, the amplifying step of the chain.
PMID:40163249 SUPPORT Human Clinical
"One of the most important drivers of the disease is the aggregation of L68Q-hCC into amyloid oligomers, as the monomer does not accumulate in the cerebral vasculature."
Identifies the oligomer, rather than the monomer, as the disease-driving species, which is what makes this node the therapeutic target.
Cystatin C Amyloid Deposition in Cerebral Arterial Walls
The rate-limiting lesion of HCCAA. Variant cystatin C amyloid deposits selectively in the walls of brain arteries and arterioles - grey and white matter alike, and in leptomeningeal as well as parenchymal vessels - while capillaries and veins are spared. Perivascular and parenchymal focal deposits also occur.
vascular associated smooth muscle cell CL:0000359 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves vascular associated smooth muscle cell (CL:0000359). CL:0000359 is a cell type from the Cell Ontology.
amyloid fibril formation GO:1990000 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased amyloid fibril formation (GO:1990000). GO:1990000 is a biological process from the Gene Ontology. ↑ INCREASED
Downstream
Arterial Wall Matrix Accumulation and Thickening Amyloid deposits are accompanied by excess basement-membrane extracellular matrix within the arterial wall.
Vascular Smooth Muscle Cell Loss and Microvascular Degeneration Smooth muscle cells progressively disappear from vessel walls as cystatin C accumulates.
Perivascular Glial Scar Formation The neuroinflammatory response is confined to the immediate vicinity of affected arteries and their focal deposits.
Systemic Extracerebral Cystatin C Deposition The same aggregation process deposits amyloid in extracerebral tissues, most usefully in skin.
Show evidence (3 references)
PMID:26115583 SUPPORT Human Clinical
"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."
Post-mortem topographical mapping establishes the arterial selectivity of deposition in patient brain.
PMID:9063500 SUPPORT Human Clinical
"We found that cystatin C amyloid immunoreactivity was present not only in cerebral cortical and leptomeningeal vessels, but also in white matter parenchymal vessels."
Immunohistochemistry documents the anatomical distribution of cystatin C amyloid across cortical, leptomeningeal, and parenchymal vessels.
PMID:26115583 SUPPORT Human Clinical
"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."
Extends deposition beyond the vessel wall to parenchymal focal deposits.
Arterial Wall Matrix Accumulation and Thickening
Continued amyloid and extracellular matrix deposition - collagen IV, aggrecan, thickened laminin, intimal thickening over a frayed elastic layer - progressively expands and stiffens the arterial wall. This is the disorder-specific form of the module's progressive tissue amyloid accumulation, and it is what converts molecular aggregation into a structural vascular lesion.
vascular associated smooth muscle cell CL:0000359 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves vascular associated smooth muscle cell (CL:0000359). CL:0000359 is a cell type from the Cell Ontology.
extracellular matrix organization GO:0030198 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal extracellular matrix organization (GO:0030198). GO:0030198 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Recurrent Lobar Intracerebral Hemorrhage Thickened, amyloid-laden arterial walls occlude or rupture, producing brain hemorrhage.
Show evidence (2 references)
PMID:23973860 SUPPORT Human Clinical
"Our results show that excess deposition of extracellular matrix proteins in cerebral arteries of HCCAA is a prominent feature of the disease and may play an important role in its pathogenesis."
Establishes excess matrix deposition as a prominent structural feature of the affected arteries.
PMID:16612982 SUPPORT Human Clinical
"The amyloid deposition in the vessel walls causes thickening of the walls leading to occlusion or rupture and resulting in brain hemorrhage."
Connects wall thickening to the two vascular outcomes, occlusion and rupture.
Vascular Smooth Muscle Cell Loss and Microvascular Degeneration
Smooth muscle cells are progressively lost from the media and adventitia of affected vessels. Solubilized cystatin C amyloid extracted from patient leptomeninges kills cultured cerebral smooth muscle cells, supplying a direct cytotoxic mechanism for the withdrawal of these cells observed in patient tissue. The denuded, structurally incompetent wall is what fails under pressure.
vascular associated smooth muscle cell CL:0000359 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves vascular associated smooth muscle cell (CL:0000359). CL:0000359 is a cell type from the Cell Ontology.
Downstream
Recurrent Lobar Intracerebral Hemorrhage Loss of the muscular wall component leaves the vessel unable to withstand luminal pressure.
Show evidence (2 references)
PMID:9063500 SUPPORT Human Clinical
"Cystatin C deposition within the vascular media and adventitia, with associated vessel wall injury as manifested by sm cell loss, represents microvascular degeneration that leads to cerebral hemorrhage."
Names smooth muscle cell loss as the vessel wall injury that leads to hemorrhage.
PMID:17963746 SUPPORT In Vitro
"To evaluate possible cytotoxicity in this condition solubilized cystatin C amyloid extracted from HCHWA-I leptomeninges was applied to cerebral smooth muscle cells in culture and was found to kill the cells."
Supplies the direct cytotoxicity mechanism; note this is a culture experiment, so the in vivo magnitude of this contribution is not established.
Perivascular Glial Scar Formation
A spatially restricted neuroinflammatory reaction forms a glial scar within and around affected arteries and their perivascular and parenchymal focal deposits. The response is local rather than diffuse, tracking the deposits themselves.
astrocyte CL:0000127 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves astrocyte (CL:0000127). CL:0000127 is a cell type from the Cell Ontology.
Show evidence (1 reference)
PMID:26115583 SUPPORT Human Clinical
"The neuroinflammatory response was limited to the close vicinity of affected arteries and perivascular as well as parenchymal focal deposits."
Establishes the restricted spatial distribution of the neuroinflammatory response.
Systemic Extracerebral Cystatin C Deposition
Variant cystatin C also deposits outside the central nervous system - skin, lymph nodes, testis, spleen, salivary glands, adrenal cortex. In skin it begins in the dermal-epidermal basement membrane, colocalizes with collagen IV, and is associated with increased upper-dermal fibroblast density. Clinically this arm is essentially silent, but it is the basis for skin biopsy as a staging and pharmacodynamic assay.
fibroblast CL:0000057 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves fibroblast (CL:0000057). CL:0000057 is a cell type from the Cell Ontology.
Show evidence (3 references)
PMID:28067897 SUPPORT Human Clinical
"The density of fibroblasts in the upper dermis of carrier skin was increased, whereas the distribution of other cell types examined did not differ compared with control biopsies."
Identifies the selective fibroblast change in carrier skin.
PMID:39054254 SUPPORT Human Clinical
"The pathogenesis of the disease in carriers is very similar in the CNS and in the skin based on autopsy studies, thus skin biopsies can be used to monitor the progression of the disease by quantifying the cystatin C immunoreactivity."
Establishes the CNS-skin correspondence that licenses skin biopsy as a disease monitor.
PMID:16612982 SUPPORT Human Clinical
"Although the amyloid can be detected outside the brain, the clinical manifestation is restricted to the brain, and usually consists of repeated hemorrhages leading to paralysis."
Confirms that despite systemic deposition, clinical disease is restricted to the brain.
Recurrent Lobar Intracerebral Hemorrhage
The clinical end state. Amyloid-laden arteries rupture, producing intracerebral hemorrhage in normotensive young adults. Hemorrhages recur with increasing frequency and severity, accompanied by dementia and paralysis, and are fatal at a mean age of about 30 years. Microinfarcts are also found in a substantial minority at autopsy.
Show evidence (3 references)
PMID:8737928 SUPPORT Human Clinical
"The disorder has an autosomal dominant mode of inheritance and causes fatal brain hemorrhage in normotensive young adults."
States the defining clinical consequence, including its occurrence in normotensive individuals.
PMID:26115583 SUPPORT Human Clinical
"Haemorrhages were observed in all patients and occurred in all brain areas, variable between patients. Microinfarcts were observed in 34.6% of patients."
Post-mortem series quantifying hemorrhage occurrence and the microinfarct fraction.
PMID:39054254 SUPPORT Human Clinical
"Most L68Q carriers have clinical symptoms characterized by recurrent hemorrhages and dementia, between the age of 20-30 years."
Establishes recurrence and the accompanying dementia in the typical age window.
✶

Histopathology

3
Amyloid deposition in cerebral arteries and arterioles
Heavy amyloid deposits in the walls of small cerebral arteries and arterioles, most prominent in the media of parenchymal vessels and the adventitia of leptomeningeal vessels, with sparing of capillaries and veins.
Show evidence (1 reference)
PMID:9063500 SUPPORT Human Clinical
"Cystatin C deposition was more prominent in the media of parenchymal vessels and in the adventitia of leptomeningeal vessels."
Defines the layer-specific distribution of deposits seen on immunohistochemistry.
Extracellular matrix accumulation in the arterial wall
Affected leptomeningeal arteries show collagen IV and aggrecan deposition, thickened laminin distribution, intimal thickening with a frayed elastic layer, and variable loss of endothelial integrity.
Show evidence (1 reference)
PMID:23973860 SUPPORT Human Clinical
"Other structural abnormalities were thickening of the laminin distribution, intimal thickening concomitant with a frayed elastic layer, and variable reduction in the integrity of endothelia."
Enumerates the additional structural wall abnormalities found at post-mortem.
Cystatin C deposition in skin basement membrane
In carriers, cystatin C deposits first in the dermal-epidermal basement membrane and extend to other basement membrane regions as disease advances, always in close association with collagen IV.
Show evidence (1 reference)
PMID:28067897 SUPPORT Human Clinical
"When the deposits were more advanced, they extended to other BM regions in the skin."
Documents the staged extension of skin basement membrane deposition with disease progression.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for ACys Amyloidosis Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

6
Cardiovascular 1
Stroke HP:0001297 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Stroke (HP:0001297), qualified as temporality recurrent. HP:0001297 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:8737928 SUPPORT Human Clinical
"The amyloid deposition leads to arterial damage with single or multiple strokes."
Directly attributes single or multiple strokes to the amyloid arterial damage.
Nervous System 3
Dementia FREQUENT HP:0000726 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dementia (HP:0000726), qualified as course progressive. HP:0000726 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:39054254 SUPPORT Human Clinical
"Most L68Q carriers have clinical symptoms characterized by recurrent hemorrhages and dementia, between the age of 20-30 years."
Dementia is reported in most carriers, supporting a frequent band.
Hemiparesis FREQUENT HP:0001269 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hemiparesis (HP:0001269). HP:0001269 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:16612982 SUPPORT Human Clinical
"the clinical manifestation is restricted to the brain, and usually consists of repeated hemorrhages leading to paralysis"
Paralysis is described as the usual outcome of the repeated hemorrhages, supporting a frequent band. Note the source says paralysis generically rather than specifying hemiparesis.
Personality changes OCCASIONAL HP:0000751 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Personality changes (HP:0000751). HP:0000751 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:16612982 SUPPORT Human Clinical
"Sometimes the initial signs of hemorrhage are dementia and personality changes."
The qualifier sometimes maps to an occasional frequency band.
Other 2
Cerebral hemorrhage VERY_FREQUENT HP:0001342 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral hemorrhage (HP:0001342), qualified as temporality recurrent. HP:0001342 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:26115583 SUPPORT Human Clinical
"Haemorrhages were observed in all patients and occurred in all brain areas, variable between patients."
Hemorrhage was present in every patient of this post-mortem series, which supports a very frequent band.
Cerebral amyloid angiopathy OBLIGATE HP:0011970 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral amyloid angiopathy (HP:0011970). HP:0011970 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:8737928 SUPPORT Human Clinical
"This variant protein has an increased tendency to aggregate and forms heavy depositions of amyloid in the walls of the small arteries and arterioles of the brain."
Cerebral amyloid angiopathy is the constitutive lesion of the disease rather than a variable feature.
🧬

Genetic Associations

1
CST3
Gene: CST3 hgnc:2475 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is CST3 (hgnc:2475). hgnc:2475 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
PMID:2602420 SUPPORT Human Clinical
"HCCAA is the first human disorder known to be caused by an abnormal gene for a cysteine proteinase inhibitor."
Establishes CST3, a cysteine proteinase inhibitor gene, as the causative gene.
PMID:39054254 SUPPORT Human Clinical
"HCCAA is dominantly inherited, caused by L68Q mutation in the cystatin C gene, leading to aggregation of the cystatin C protein."
Confirms the dominant CST3 L68Q aetiology and links it to protein aggregation.
Variants (1)
CST3 L68Q
Founder missense variant in exon 2 of CST3 (T-to-A at the codon for leucine 68), replacing leucine with glutamine. Carried heterozygously by every known HCCAA patient and traced to a single common ancestor.
Show evidence (1 reference)
PMID:8097919 SUPPORT Human Clinical
"HCCAA has been shown to be caused by a T-->A point mutation in the codon for leucine at position 68 in exon 2 of the cystatin C gene, which results in a leucine-->glutamine amino acid substitution in the cystatin C molecule."
Specifies the exact nucleotide and amino acid change and its exon location.
💊

Medical Actions

3
N-Acetylcysteine
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: N-acetyl-L-cysteine CHEBI:28939 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses N-acetyl-L-cysteine (CHEBI:28939). CHEBI:28939 is a therapeutic agent from Chemical Entities of Biological Interest.
High-dose N-acetylcysteine (2400 mg daily) was assessed in a phase 2a open-label trial in 17 L68Q-CST3 carriers. It was safe and well tolerated, raised reduced glutathione, lowered plasma high-molecular-weight cystatin C aggregates, and reduced disease-associated biomarkers in skin. It is an investigational, biomarker-supported candidate preventive strategy, not an established therapy - the trial was nonrandomized, uncontrolled for clinical outcome, and cerebral bleeds still occurred during treatment.
Mechanism Target:
INHIBITS Domain-Swapped Dimerization and Oligomerization — NAC treatment decreased high-molecular-weight cystatin C aggregate levels in plasma, consistent with action on the aggregation/oligomerization node rather than on established deposits.
Show evidence (1 reference)
PMID:40163249 SUPPORT Human Clinical
"Secondary outcomes included a significant increase in reduced glutathione levels and decreased high-molecular-weight cystatin C aggregate levels in plasma after NAC treatment."
Provides the pharmacodynamic evidence that NAC acts on the cystatin C aggregate pool.
Show evidence (2 references)
PMID:40163249 SUPPORT Human Clinical
"In this single-center nonrandomized clinical trial, NAC was safe and well tolerated and decreased disease-associated biomarker and amyloid deposition, suggesting NAC may offer a preventive strategy against HCCAA."
The trial's own conclusion, which is framed as suggestive of a preventive strategy rather than as established clinical benefit.
PMID:40163249 SUPPORT Human Clinical
"Five cerebral bleeds occurred during the treatment period without permanent neurological sequela; no death occurred."
Records that hemorrhages still occurred on treatment, which qualifies any claim of clinical efficacy.
Genetic counseling
Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Autosomal dominant transmission of a single founder allele in a small number of known Icelandic families makes counseling and predictive testing central to management of at-risk relatives.
Show evidence (1 reference)
PMID:8097919 SUPPORT Human Clinical
"One hundred ninety-one individuals have now been screened for the HCCAA causing mutation, including a fetus for prenatal diagnosis."
Documents established family screening and prenatal diagnosis practice, which is delivered through genetic counseling.
Supportive care
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
No disease-modifying therapy is established. Management is supportive, aimed at the consequences of recurrent hemorrhage.
Show evidence (1 reference)
PMID:39054254 SUPPORT Human Clinical
"No therapy is available for this devastating disease."
Establishes the absence of disease-modifying therapy, which is why care remains supportive.
🌍

Environmental Factors

1
Twentieth-century Icelandic lifestyle change
Obligate L68Q carriers born between 1825 and 1900 experienced a fall in life span from about 65 years to the modern average of about 30, and a parent-of-origin effect emerged over the same period. Because the shift appears in several independent extended families many generations after the founding mutation, the authors infer an environmental cause tied to changes in Icelandic life-style. The specific exposure has not been identified, so no ECTO term is bound here.
Show evidence (1 reference)
PMID:18566660 SUPPORT Human Clinical
"Intriguingly, the genealogies reveal that obligate L68Q carriers born 1825 to 1900 experienced a drastic reduction in life span, from 65 years to the present-day average."
Quantifies the historical shift in carrier life span that defines this environmental observation.
Mechanism Target:
MODULATES Recurrent Lobar Intracerebral Hemorrhage — An unidentified environmental factor associated with modern Icelandic life-style is inferred to accelerate the clinical course, but the mediating biology is unknown.
Show evidence (1 reference)
PMID:18566660 SUPPORT Human Clinical
"As these trends can be observed in several different extended families, many generations after the mutational event, it seems likely that some environmental factor is responsible, perhaps linked to radical changes in the life-style of Icelanders during this period."
The environmental attribution is an inference from genealogical data and is stated by the authors as likely rather than demonstrated, so this is recorded as partial support.
🔬

Biochemical Markers

2
Cerebrospinal fluid cystatin C
Show evidence (1 reference)
PMID:9445375 SUPPORT Human Clinical
"The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished"
Directly reports the reduced CSF concentration in patients.
Plasma high-molecular-weight cystatin C aggregates
Show evidence (1 reference)
PMID:40163249 SUPPORT Human Clinical
"Secondary outcomes included a significant increase in reduced glutathione levels and decreased high-molecular-weight cystatin C aggregate levels in plasma after NAC treatment."
Reports the plasma aggregate measurement and its treatment-associated decrease.
🔬

Diagnosis

4
CST3 molecular genetic testing
Detection of the CST3 L68Q founder variant is definitive. Because the mutation abolishes an AluI site, RFLP analysis historically provided a simple and accurate assay, and it supports presymptomatic and prenatal diagnosis.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: Presence of the CST3 L68Q allele establishes the diagnosis of HCCAA.
Show evidence (1 reference)
PMID:8097919 SUPPORT Human Clinical
"Since the HCCAA-causing mutation abolishes an AluI restriction site in the cystatin C gene, analysis of this AluI restriction fragment-length polymorphism (RFLP) enables simple and accurate molecular diagnosis of HCCAA."
Establishes the molecular assay and its diagnostic accuracy claim.
Skin biopsy for cystatin C immunoreactivity
Because skin and CNS pathogenesis correspond closely, quantifying cystatin C immunoreactivity in skin biopsies provides an accessible measure of disease progression, and it was used as the primary biomarker readout in the NAC trial.
biopsy procedure NCIT:C15189 NCI Thesaurus (NCIT)
Results: Cystatin C immunoreactivity in skin, initially confined to the dermal-epidermal basement membrane, tracks disease stage.
Show evidence (1 reference)
PMID:39054254 SUPPORT Human Clinical
"thus skin biopsies can be used to monitor the progression of the disease by quantifying the cystatin C immunoreactivity"
States the validated use of skin biopsy for disease monitoring.
Cerebrospinal fluid cystatin C measurement
Markedly reduced CSF cystatin C was one of the earliest available measures in living carriers. It has been superseded by molecular testing and is not specific on its own.
Lumbar Puncture NCIT:C15327 NCI Thesaurus (NCIT)
Results: CSF cystatin C is markedly diminished in HCCAA patients.
Show evidence (1 reference)
PMID:9445375 SUPPORT Human Clinical
"The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished"
Supports the biochemical observation, but the study reports no diagnostic performance characteristics.
Brain computed tomography
CT imaging is used to confirm intracranial hemorrhage when new neurological symptoms develop in a known carrier.
computed tomography NCIT:C17204 NCI Thesaurus (NCIT)
Results: Demonstrates intracranial hemorrhage underlying new neurological symptoms.
Show evidence (1 reference)
PMID:40163249 SUPPORT Human Clinical
"A computed tomography (CT) scan of the brain was acquired if new onset of neurological symptoms occurred."
Documents the trial protocol's use of CT to detect hemorrhage on symptom onset.
📈

Progression

3
Presymptomatic carrier state
Age: childhood to early adulthood
Carriers are clinically well before the first hemorrhage, but cystatin C already deposits in skin basement membrane, which is why skin biopsy can stage preclinical disease.
Show evidence (1 reference)
PMID:28067897 SUPPORT Human Clinical
"We found that cystatin C deposition in minimally affected samples was limited to the basement membrane (BM) between the dermis and epidermis."
Establishes that early, minimally affected carriers already show restricted basement-membrane deposition before advanced disease.
First symptomatic hemorrhage
Age: third decade
Most carriers present with their first intracerebral hemorrhage in their twenties.
Show evidence (1 reference)
PMID:40163249 SUPPORT Human Clinical
"Most carriers of this sequence variation experience their first intracerebral hemorrhage in their 20s."
Directly dates the typical first hemorrhage to the third decade.
Recurrent hemorrhage and neurological decline
Age: third to fourth decade Duration: about 5 years after the first bleed
Hemorrhages recur with increasing frequency and severity, accompanied by dementia and paralysis, and death occurs at a mean age of about 30 years.
Show evidence (1 reference)
PMID:39054254 SUPPORT Human Clinical
"If the carriers survive the first hemorrhage, the frequency and severity of the hemorrhages tend to increase, resulting in death at average of 30 years with mean number of major hemorrhages ranging from 3.2 to 3.9 over a 5-year average life span."
Quantifies recurrence, mean number of major hemorrhages, and mean age at death.
📊

Prevalence

1
Iceland
Cases In Literature Ultra Rare
HCCAA is confined to a small number of Icelandic families descending from a common founder; case counts rather than population rates are what the literature reports.
Show evidence (2 references)
PMID:8097919 SUPPORT Human Clinical
"Thirty-six individuals belonging to nine Icelandic families have been found to have the mutation and it is highly probable that these families descend from a common ancestor."
Reports the identified carrier count and the restriction to a small set of related Icelandic families.
PMID:16612982 SUPPORT Human Clinical
"Hereditary cystatin C amyloid angiopathy (HCCAA) is a rare, fatal amyloid disease in young people in Iceland caused by a mutation in cystatin C, which is an inhibitor of several cysteine proteinases, such as cathepsins S, B, and K."
Characterizes HCCAA as a rare disease geographically restricted to Iceland.
🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from ACys Amyloidosis:

Sporadic cerebral amyloid angiopathy
Overlapping Features Age-related sporadic CAA is amyloid-beta based and affects the elderly, whereas HCCAA presents with hemorrhage in the third decade and is cystatin C based. Cystatin C does colocalize with amyloid-beta in sporadic CAA, so immunostaining for cystatin C alone does not distinguish the two.
Show evidence (1 reference)
PMID:9063500 SUPPORT Human Clinical
"Cystatin C is also found to colocalize with amyloid beta/A4 protein in cerebral vessel walls of patients with Alzheimer's disease (AD), sporadic CAA, and hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D)."
Establishes the colocalization that makes cystatin C staining alone non-discriminating between these entities.
Hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D)
Overlapping Features HCHWA-D is caused by an APP variant and deposits amyloid-beta, not cystatin C. The shared HCHWA acronym is a recognized source of confusion between two genetically and biochemically distinct diseases; only the Icelandic type is modeled in this entry.
Show evidence (1 reference)
PMID:9063500 SUPPORT Human Clinical
"Cystatin C is also found to colocalize with amyloid beta/A4 protein in cerebral vessel walls of patients with Alzheimer's disease (AD), sporadic CAA, and hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D)."
Names HCHWA-D as a separate entity from HCCAA within the same paper, supporting the distinction.
Hypertensive intracerebral hemorrhage
Overlapping Features HCCAA hemorrhage occurs in normotensive young adults, which separates it from hypertensive deep intracerebral hemorrhage.
Show evidence (1 reference)
PMID:8737928 SUPPORT Human Clinical
"The disorder has an autosomal dominant mode of inheritance and causes fatal brain hemorrhage in normotensive young adults."
The explicit normotensive qualifier is what distinguishes HCCAA from hypertensive hemorrhage.
{ }

Source YAML

click to show 
name: ACys Amyloidosis
creation_date: '2026-08-18T10:00:00Z'
category: Mendelian
synonyms:
- hereditary cystatin C amyloid angiopathy
- HCCAA
- hereditary cerebral hemorrhage with amyloidosis, Icelandic type
- HCHWA-Icelandic type
- cystatin amyloidosis
- CST3-related amyloidosis
description: >-
  ACys amyloidosis (hereditary cystatin C amyloid angiopathy, HCCAA) is an
  ultra-rare autosomal dominant cerebral amyloidosis caused by a single Icelandic
  founder variant, CST3 L68Q. The substitution destabilizes cystatin C and drives
  three-dimensional domain-swapped dimerization and oligomerization, producing
  amyloid that deposits in the walls of brain arteries and arterioles together
  with excess basement-membrane extracellular matrix. Progressive arterial wall
  thickening and smooth muscle cell loss culminate in recurrent lobar
  intracerebral hemorrhage in young normotensive adults, with dementia and
  paralysis, and death at a mean age of about 30 years. Amyloid is deposited
  systemically - notably in skin basement membrane - but clinical manifestations
  are essentially restricted to the brain, and the systemic deposits are
  exploited diagnostically. Distinguish carefully from the Dutch type of
  hereditary cerebral hemorrhage with amyloidosis (HCHWA-D), which is an
  APP-related amyloid-beta disease and not part of this entry.
disease_term:
  preferred_term: ACys amyloidosis
  term:
    id: MONDO:0007098
    label: ACys amyloidosis
parents:
- hereditary disease
- amyloidosis
- cerebral amyloid angiopathy
inheritance:
- name: Autosomal dominant inheritance
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    HCCAA segregates as a highly penetrant autosomal dominant trait; all known
    carriers are heterozygous for the same CST3 L68Q founder variant.
  evidence:
  - reference: PMID:18566660
    reference_title: A drastic reduction in the life span of cystatin C L68Q carriers due to life-style changes during the last two centuries.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hereditary cystatin C amyloid angiopathy (HCCAA) is an autosomal dominant disease with high penetrance, manifest by brain hemorrhages in young normotensive adults."
    explanation: >-
      Directly establishes autosomal dominant inheritance with high penetrance
      for HCCAA.
  - reference: PMID:2602420
    reference_title: Mutation in the cystatin C gene causes hereditary brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The Alu I marker has been used to show that this mutation is transmitted only in the affected members in all eight families investigated, proving that the mutated cystatin C gene causes HCCAA."
    explanation: >-
      Co-segregation of the CST3 variant with disease across eight families
      establishes the dominant Mendelian relationship.
prevalence:
- population: Iceland
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    HCCAA is confined to a small number of Icelandic families descending from a
    common founder; case counts rather than population rates are what the
    literature reports.
  evidence:
  - reference: PMID:8097919
    reference_title: Molecular diagnosis of hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thirty-six individuals belonging to nine Icelandic families have been found to have the mutation and it is highly probable that these families descend from a common ancestor."
    explanation: >-
      Reports the identified carrier count and the restriction to a small set of
      related Icelandic families.
  - reference: PMID:16612982
    reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hereditary cystatin C amyloid angiopathy (HCCAA) is a rare, fatal amyloid disease in young people in Iceland caused by a mutation in cystatin C, which is an inhibitor of several cysteine proteinases, such as cathepsins S, B, and K."
    explanation: >-
      Characterizes HCCAA as a rare disease geographically restricted to Iceland.
progression:
- phase: Presymptomatic carrier state
  age_range: childhood to early adulthood
  notes: >-
    Carriers are clinically well before the first hemorrhage, but cystatin C
    already deposits in skin basement membrane, which is why skin biopsy can
    stage preclinical disease.
  evidence:
  - reference: PMID:28067897
    reference_title: Pathological changes in basement membranes and dermal connective tissue of skin from patients with hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found that cystatin C deposition in minimally affected samples was limited to the basement membrane (BM) between the dermis and epidermis."
    explanation: >-
      Establishes that early, minimally affected carriers already show restricted
      basement-membrane deposition before advanced disease.
- phase: First symptomatic hemorrhage
  age_range: third decade
  notes: >-
    Most carriers present with their first intracerebral hemorrhage in their
    twenties.
  evidence:
  - reference: PMID:40163249
    reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Most carriers of this sequence variation experience their first intracerebral hemorrhage in their 20s."
    explanation: >-
      Directly dates the typical first hemorrhage to the third decade.
- phase: Recurrent hemorrhage and neurological decline
  age_range: third to fourth decade
  duration: about 5 years after the first bleed
  notes: >-
    Hemorrhages recur with increasing frequency and severity, accompanied by
    dementia and paralysis, and death occurs at a mean age of about 30 years.
  evidence:
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "If the carriers survive the first hemorrhage, the frequency and severity of the hemorrhages tend to increase, resulting in death at average of 30 years with mean number of major hemorrhages ranging from 3.2 to 3.9 over a 5-year average life span."
    explanation: >-
      Quantifies recurrence, mean number of major hemorrhages, and mean age at
      death.
pathophysiology:
- name: CST3 L68Q Founder Variant
  biological_scale: MOLECULAR
  role: trigger
  description: >-
    A single T-to-A substitution in codon 68 of CST3 replaces leucine with
    glutamine in cystatin C. Every known HCCAA carrier is heterozygous for this
    one Icelandic founder allele, so the disorder has a uniform initiating
    genetic lesion.
  genes:
  - preferred_term: CST3
    term:
      id: hgnc:2475
      label: CST3
  evidence:
  - reference: PMID:2602420
    reference_title: Mutation in the cystatin C gene causes hereditary brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have used the full length cystatin C cDNA probe (Abrahamson et al., 1987) to demonstrate a mutation in the codon for leucine at position 68, which abolishes an Alu I restriction site in cystatin C gene of the HCCAA patients."
    explanation: >-
      Identifies the codon-68 CST3 mutation as the initiating lesion in HCCAA
      patients.
  - reference: PMID:16612982
    reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The same mutation in cystatin C, L68Q, has been found in all patients examined so far pointing to a common founder."
    explanation: >-
      Establishes L68Q as the single founder allele shared by all examined
      patients.
  downstream:
  - target: Destabilized L68Q Cystatin C Precursor
    causal_link_type: DIRECT
    description: >-
      The L68Q substitution sits near the inhibitory region of cystatin C and
      yields a conformationally destabilized variant protein.
    evidence:
    - reference: PMID:3517880
      reference_title: Amyloid fibrils in hereditary cerebral hemorrhage with amyloidosis of Icelandic type is a variant of gamma-trace basic protein (cystatin C).
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "It has an amino acid substitution (glutamine for leucine) at position 58 (position 68 in gamma-trace numbering), which is near the proposed active site of related proteins"
      explanation: >-
        Locates the amino acid substitution in the amyloid-forming variant
        protein purified from patient tissue.
- name: Destabilized L68Q Cystatin C Precursor
  conforms_to: amyloidogenesis#Amyloidogenic Precursor Protein
  biological_scale: MOLECULAR
  role: trigger
  description: >-
    Variant cystatin C is the amyloidogenic precursor of this disease, the
    disorder-specific substitution for the module's generic destabilized
    precursor. The variant protein purified from patient amyloid is cystatin C
    (gamma-trace) carrying the position-68 substitution, and structural work
    shows the mutation destabilizes the folded monomer.
  genes:
  - preferred_term: CST3
    term:
      id: hgnc:2475
      label: CST3
  biological_processes:
  - preferred_term: protein folding
    term:
      id: GO:0006457
      label: protein folding
    modifier: ABNORMAL
  molecular_functions:
  - preferred_term: cysteine-type endopeptidase inhibitor activity
    term:
      id: GO:0004869
      label: cysteine-type endopeptidase inhibitor activity
  evidence:
  - reference: PMID:3517880
    reference_title: Amyloid fibrils in hereditary cerebral hemorrhage with amyloidosis of Icelandic type is a variant of gamma-trace basic protein (cystatin C).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A gamma-trace variant protein is the major constituent of the amyloid fibrils in patients from Iceland with hereditary cerebral hemorrhage with amyloidosis."
    explanation: >-
      Identifies variant cystatin C (gamma-trace) as the amyloid subunit in
      patient tissue, establishing it as the amyloidogenic precursor.
  - reference: PMID:11276250
    reference_title: Human cystatin C, an amyloidogenic protein, dimerizes through three-dimensional domain swapping.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The structure of the three-dimensional domain-swapped dimers shows how the L68Q mutation destabilizes the monomers and makes the partially unfolded intermediate less unstable."
    explanation: >-
      Crystallographic work provides the destabilization mechanism that makes the
      L68Q precursor misfolding-prone.
  downstream:
  - target: Domain-Swapped Dimerization and Oligomerization
    causal_link_type: DIRECT
    description: >-
      Destabilization of the monomer favours the partially unfolded intermediate
      that permits three-dimensional domain swapping.
    evidence:
    - reference: PMID:11276250
      reference_title: Human cystatin C, an amyloidogenic protein, dimerizes through three-dimensional domain swapping.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "The structure of the three-dimensional domain-swapped dimers shows how the L68Q mutation destabilizes the monomers and makes the partially unfolded intermediate less unstable."
      explanation: >-
        Links monomer destabilization directly to the domain-swapped dimer that
        initiates aggregation.
  - target: Intracellular Retention of Variant Cystatin C
    causal_link_type: DIRECT
    description: >-
      The variant protein is poorly secreted and is retained in association with
      cells, which lowers extracellular cystatin C and raises the intracellular
      pool.
    evidence:
    - reference: PMID:9445375
      reference_title: 'Hereditary cystatin C amyloid angiopathy: monitoring the presence of the Leu-68-->Gln cystatin C variant in cerebrospinal fluids and monocyte cultures by MS.'
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "These results support the conclusion that the mutated cystatin C is retained in association with the monocytes and not secreted."
      explanation: >-
        Mass-spectrometric analysis of patient-derived monocyte cultures shows
        retention rather than secretion of the variant protein.
- name: Intracellular Retention of Variant Cystatin C
  biological_scale: CELLULAR
  role: amplifier
  description: >-
    Variant cystatin C accumulates within cells rather than being secreted,
    which depletes cerebrospinal fluid cystatin C and is proposed to raise the
    local concentration that drives aggregation. This is a secretion/trafficking
    arm that runs alongside the intrinsic destabilization arm.
  biological_processes:
  - preferred_term: protein folding
    term:
      id: GO:0006457
      label: protein folding
    modifier: ABNORMAL
  evidence:
  - reference: PMID:9445375
    reference_title: 'Hereditary cystatin C amyloid angiopathy: monitoring the presence of the Leu-68-->Gln cystatin C variant in cerebrospinal fluids and monocyte cultures by MS.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished and cultivated monocytes from affected individuals accumulate cystatin C."
    explanation: >-
      Documents both the reduced extracellular (CSF) pool and the cellular
      accumulation that define this node.
  downstream:
  - target: Domain-Swapped Dimerization and Oligomerization
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The authors propose that the raised intracellular concentration promotes
      aggregation and fibril formation; the intermediate steps in patient brain
      are not established.
    evidence:
    - reference: PMID:9445375
      reference_title: 'Hereditary cystatin C amyloid angiopathy: monitoring the presence of the Leu-68-->Gln cystatin C variant in cerebrospinal fluids and monocyte cultures by MS.'
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "An increased intracellular concentration would presumably promote the aggregation and denaturation of the mutated cystatin C, leading to the formation of amyloid fibrils and cell death."
      explanation: >-
        The link is explicitly framed by the authors as a presumption, so this
        edge is recorded as partial support with unknown intermediates.
- name: Domain-Swapped Dimerization and Oligomerization
  conforms_to: amyloidogenesis#Protein Misfolding and Beta-Sheet Oligomerization
  biological_scale: MOLECULAR
  role: amplifier
  description: >-
    Cystatin C dimerizes by three-dimensional domain swapping, in which two
    partially unfolded monomers exchange structural elements to reconstitute
    monomer-like domains. Open-ended propagation of the same mechanism links
    molecules into higher aggregates. Oligomer formation - not the monomer - is
    the species that accumulates in the cerebral vasculature.
  biological_processes:
  - preferred_term: protein folding
    term:
      id: GO:0006457
      label: protein folding
    modifier: ABNORMAL
  molecular_functions:
  - preferred_term: protein homodimerization activity
    term:
      id: GO:0042803
      label: protein homodimerization activity
    modifier: INCREASED
  evidence:
  - reference: PMID:11276250
    reference_title: Human cystatin C, an amyloidogenic protein, dimerizes through three-dimensional domain swapping.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The dimerization occurs through three-dimensional domain swapping, a mechanism for forming oligomeric proteins."
    explanation: >-
      Establishes the specific misfolding/oligomerization mechanism substituted
      for the module's generic beta-sheet oligomerization step.
  - reference: PMID:11276250
    reference_title: Human cystatin C, an amyloidogenic protein, dimerizes through three-dimensional domain swapping.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Higher aggregates may arise through the three-dimensional domain-swapping mechanism occurring in an open-ended fashion in which partially unfolded molecules are linked into infinite chains."
    explanation: >-
      Extends the dimer mechanism to higher-order aggregates, the amplifying step
      of the chain.
  - reference: PMID:40163249
    reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "One of the most important drivers of the disease is the aggregation of L68Q-hCC into amyloid oligomers, as the monomer does not accumulate in the cerebral vasculature."
    explanation: >-
      Identifies the oligomer, rather than the monomer, as the disease-driving
      species, which is what makes this node the therapeutic target.
  downstream:
  - target: Cystatin C Amyloid Deposition in Cerebral Arterial Walls
    causal_link_type: DIRECT
    description: >-
      Aggregation-prone variant protein forms heavy amyloid deposits in the walls
      of small cerebral arteries and arterioles.
    evidence:
    - reference: PMID:8737928
      reference_title: The molecular pathology of hereditary cystatin C amyloid angiopathy causing brain hemorrhage.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This variant protein has an increased tendency to aggregate and forms heavy depositions of amyloid in the walls of the small arteries and arterioles of the brain."
      explanation: >-
        Directly connects the aggregation tendency of the variant to arterial
        wall amyloid deposition.
- name: Cystatin C Amyloid Deposition in Cerebral Arterial Walls
  conforms_to: amyloidogenesis#Amyloid Fibril Formation and Extracellular Deposition
  biological_scale: TISSUE
  role: central_effector
  description: >-
    The rate-limiting lesion of HCCAA. Variant cystatin C amyloid deposits
    selectively in the walls of brain arteries and arterioles - grey and white
    matter alike, and in leptomeningeal as well as parenchymal vessels - while
    capillaries and veins are spared. Perivascular and parenchymal focal deposits
    also occur.
  cell_types:
  - preferred_term: vascular associated smooth muscle cell
    term:
      id: CL:0000359
      label: vascular associated smooth muscle cell
  biological_processes:
  - preferred_term: amyloid fibril formation
    term:
      id: GO:1990000
      label: amyloid fibril formation
    modifier: INCREASED
  evidence:
  - reference: PMID:26115583
    reference_title: Parenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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."
    explanation: >-
      Post-mortem topographical mapping establishes the arterial selectivity of
      deposition in patient brain.
  - reference: PMID:9063500
    reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found that cystatin C amyloid immunoreactivity was present not only in cerebral cortical and leptomeningeal vessels, but also in white matter parenchymal vessels."
    explanation: >-
      Immunohistochemistry documents the anatomical distribution of cystatin C
      amyloid across cortical, leptomeningeal, and parenchymal vessels.
  - reference: PMID:26115583
    reference_title: Parenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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."
    explanation: >-
      Extends deposition beyond the vessel wall to parenchymal focal deposits.
  downstream:
  - target: Arterial Wall Matrix Accumulation and Thickening
    causal_link_type: DIRECT
    description: >-
      Amyloid deposits are accompanied by excess basement-membrane extracellular
      matrix within the arterial wall.
    evidence:
    - reference: PMID:23973860
      reference_title: Deposition of collagen IV and aggrecan in leptomeningeal arteries of hereditary brain haemorrhage with amyloidosis.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Examination of post-mortem samples revealed extensive changes in the walls of affected arteries characterised by deposition of extracellular matrix constituents, notably collagen IV and the proteoglycan aggrecan."
      explanation: >-
        Post-mortem arterial wall analysis identifies the specific matrix
        constituents accumulating alongside amyloid.
  - target: Vascular Smooth Muscle Cell Loss and Microvascular Degeneration
    causal_link_type: DIRECT
    description: >-
      Smooth muscle cells progressively disappear from vessel walls as cystatin C
      accumulates.
    evidence:
    - reference: PMID:9063500
      reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Smooth muscle (sm) cells were few or could not be identified within vessel walls showing extensive cystatin C deposition, suggesting progressive loss of these cells as cystatin C accumulates."
      explanation: >-
        Directly correlates extent of cystatin C deposition with smooth muscle
        cell loss in patient vessels.
  - target: Perivascular Glial Scar Formation
    causal_link_type: DIRECT
    description: >-
      The neuroinflammatory response is confined to the immediate vicinity of
      affected arteries and their focal deposits.
    evidence:
    - reference: PMID:26115583
      reference_title: Parenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "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."
      explanation: >-
        Establishes glial scar formation as a consequence localized to affected
        arteries.
  - target: Systemic Extracerebral Cystatin C Deposition
    causal_link_type: DIRECT
    description: >-
      The same aggregation process deposits amyloid in extracerebral tissues,
      most usefully in skin.
    evidence:
    - reference: PMID:16612982
      reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Mutated cystatin C forms amyloid, predominantly in brain arteries and arterioles, but also to a lesser degree in tissues outside the central nervous system such as skin, lymph nodes, testis, spleen, submandibular salivary glands, and adrenal cortex."
      explanation: >-
        Enumerates the extracerebral tissues in which the same amyloid is
        deposited.
- name: Arterial Wall Matrix Accumulation and Thickening
  conforms_to: amyloidogenesis#Progressive Tissue Amyloid Accumulation
  biological_scale: TISSUE
  role: effector
  description: >-
    Continued amyloid and extracellular matrix deposition - collagen IV,
    aggrecan, thickened laminin, intimal thickening over a frayed elastic layer -
    progressively expands and stiffens the arterial wall. This is the
    disorder-specific form of the module's progressive tissue amyloid
    accumulation, and it is what converts molecular aggregation into a
    structural vascular lesion.
  cell_types:
  - preferred_term: vascular associated smooth muscle cell
    term:
      id: CL:0000359
      label: vascular associated smooth muscle cell
  biological_processes:
  - preferred_term: extracellular matrix organization
    term:
      id: GO:0030198
      label: extracellular matrix organization
    modifier: ABNORMAL
  evidence:
  - reference: PMID:23973860
    reference_title: Deposition of collagen IV and aggrecan in leptomeningeal arteries of hereditary brain haemorrhage with amyloidosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our results show that excess deposition of extracellular matrix proteins in cerebral arteries of HCCAA is a prominent feature of the disease and may play an important role in its pathogenesis."
    explanation: >-
      Establishes excess matrix deposition as a prominent structural feature of
      the affected arteries.
  - reference: PMID:16612982
    reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The amyloid deposition in the vessel walls causes thickening of the walls leading to occlusion or rupture and resulting in brain hemorrhage."
    explanation: >-
      Connects wall thickening to the two vascular outcomes, occlusion and
      rupture.
  downstream:
  - target: Recurrent Lobar Intracerebral Hemorrhage
    causal_link_type: DIRECT
    description: >-
      Thickened, amyloid-laden arterial walls occlude or rupture, producing brain
      hemorrhage.
    evidence:
    - reference: PMID:16612982
      reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The amyloid deposition in the vessel walls causes thickening of the walls leading to occlusion or rupture and resulting in brain hemorrhage."
      explanation: >-
        States the causal path from wall thickening to rupture and hemorrhage.
- name: Vascular Smooth Muscle Cell Loss and Microvascular Degeneration
  biological_scale: CELLULAR
  role: effector
  description: >-
    Smooth muscle cells are progressively lost from the media and adventitia of
    affected vessels. Solubilized cystatin C amyloid extracted from patient
    leptomeninges kills cultured cerebral smooth muscle cells, supplying a direct
    cytotoxic mechanism for the withdrawal of these cells observed in patient
    tissue. The denuded, structurally incompetent wall is what fails under
    pressure.
  cell_types:
  - preferred_term: vascular associated smooth muscle cell
    term:
      id: CL:0000359
      label: vascular associated smooth muscle cell
  evidence:
  - reference: PMID:9063500
    reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cystatin C deposition within the vascular media and adventitia, with associated vessel wall injury as manifested by sm cell loss, represents microvascular degeneration that leads to cerebral hemorrhage."
    explanation: >-
      Names smooth muscle cell loss as the vessel wall injury that leads to
      hemorrhage.
  - reference: PMID:17963746
    reference_title: Solubilized cystatin C amyloid is cytotoxic to cultured human cerebrovascular smooth muscle cells.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "To evaluate possible cytotoxicity in this condition solubilized cystatin C amyloid extracted from HCHWA-I leptomeninges was applied to cerebral smooth muscle cells in culture and was found to kill the cells."
    explanation: >-
      Supplies the direct cytotoxicity mechanism; note this is a culture
      experiment, so the in vivo magnitude of this contribution is not
      established.
  downstream:
  - target: Recurrent Lobar Intracerebral Hemorrhage
    causal_link_type: DIRECT
    description: >-
      Loss of the muscular wall component leaves the vessel unable to withstand
      luminal pressure.
    evidence:
    - reference: PMID:9063500
      reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cystatin C deposition within the vascular media and adventitia, with associated vessel wall injury as manifested by sm cell loss, represents microvascular degeneration that leads to cerebral hemorrhage."
      explanation: >-
        Explicitly identifies this microvascular degeneration as leading to
        cerebral hemorrhage.
- name: Perivascular Glial Scar Formation
  biological_scale: TISSUE
  role: effector
  description: >-
    A spatially restricted neuroinflammatory reaction forms a glial scar within
    and around affected arteries and their perivascular and parenchymal focal
    deposits. The response is local rather than diffuse, tracking the deposits
    themselves.
  cell_types:
  - preferred_term: astrocyte
    term:
      id: CL:0000127
      label: astrocyte
  evidence:
  - reference: PMID:26115583
    reference_title: Parenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The neuroinflammatory response was limited to the close vicinity of affected arteries and perivascular as well as parenchymal focal deposits."
    explanation: >-
      Establishes the restricted spatial distribution of the neuroinflammatory
      response.
- name: Systemic Extracerebral Cystatin C Deposition
  biological_scale: TISSUE
  role: effector
  description: >-
    Variant cystatin C also deposits outside the central nervous system - skin,
    lymph nodes, testis, spleen, salivary glands, adrenal cortex. In skin it
    begins in the dermal-epidermal basement membrane, colocalizes with collagen
    IV, and is associated with increased upper-dermal fibroblast density.
    Clinically this arm is essentially silent, but it is the basis for skin
    biopsy as a staging and pharmacodynamic assay.
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:28067897
    reference_title: Pathological changes in basement membranes and dermal connective tissue of skin from patients with hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The density of fibroblasts in the upper dermis of carrier skin was increased, whereas the distribution of other cell types examined did not differ compared with control biopsies."
    explanation: >-
      Identifies the selective fibroblast change in carrier skin.
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The pathogenesis of the disease in carriers is very similar in the CNS and in the skin based on autopsy studies, thus skin biopsies can be used to monitor the progression of the disease by quantifying the cystatin C immunoreactivity."
    explanation: >-
      Establishes the CNS-skin correspondence that licenses skin biopsy as a
      disease monitor.
  - reference: PMID:16612982
    reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Although the amyloid can be detected outside the brain, the clinical manifestation is restricted to the brain, and usually consists of repeated hemorrhages leading to paralysis."
    explanation: >-
      Confirms that despite systemic deposition, clinical disease is restricted
      to the brain.
- name: Recurrent Lobar Intracerebral Hemorrhage
  conforms_to: amyloidogenesis#Organ Dysfunction
  biological_scale: ORGANISM
  role: consequence
  description: >-
    The clinical end state. Amyloid-laden arteries rupture, producing
    intracerebral hemorrhage in normotensive young adults. Hemorrhages recur with
    increasing frequency and severity, accompanied by dementia and paralysis, and
    are fatal at a mean age of about 30 years. Microinfarcts are also found in a
    substantial minority at autopsy.
  evidence:
  - reference: PMID:8737928
    reference_title: The molecular pathology of hereditary cystatin C amyloid angiopathy causing brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder has an autosomal dominant mode of inheritance and causes fatal brain hemorrhage in normotensive young adults."
    explanation: >-
      States the defining clinical consequence, including its occurrence in
      normotensive individuals.
  - reference: PMID:26115583
    reference_title: Parenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Haemorrhages were observed in all patients and occurred in all brain areas, variable between patients. Microinfarcts were observed in 34.6% of patients."
    explanation: >-
      Post-mortem series quantifying hemorrhage occurrence and the microinfarct
      fraction.
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Most L68Q carriers have clinical symptoms characterized by recurrent hemorrhages and dementia, between the age of 20-30 years."
    explanation: >-
      Establishes recurrence and the accompanying dementia in the typical age
      window.
phenotypes:
- category: Neurological
  name: Cerebral hemorrhage
  description: >-
    Recurrent lobar intracerebral hemorrhage in normotensive young adults is the
    defining manifestation.
  phenotype_term:
    preferred_term: Cerebral hemorrhage
    term:
      id: HP:0001342
      label: Cerebral hemorrhage
    temporality: RECURRENT
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:26115583
    reference_title: Parenchymal cystatin C focal deposits and glial scar formation around brain arteries in Hereditary Cystatin C Amyloid Angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Haemorrhages were observed in all patients and occurred in all brain areas, variable between patients."
    explanation: >-
      Hemorrhage was present in every patient of this post-mortem series, which
      supports a very frequent band.
- category: Neurological
  name: Cerebral amyloid angiopathy
  description: >-
    Variant cystatin C amyloid deposits in the walls of cerebral arteries and
    arterioles, the defining vascular pathology.
  phenotype_term:
    preferred_term: Cerebral amyloid angiopathy
    term:
      id: HP:0011970
      label: Cerebral amyloid angiopathy
  frequency: OBLIGATE
  evidence:
  - reference: PMID:8737928
    reference_title: The molecular pathology of hereditary cystatin C amyloid angiopathy causing brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This variant protein has an increased tendency to aggregate and forms heavy depositions of amyloid in the walls of the small arteries and arterioles of the brain."
    explanation: >-
      Cerebral amyloid angiopathy is the constitutive lesion of the disease
      rather than a variable feature.
- category: Neurological
  name: Stroke
  description: >-
    Amyloid-mediated arterial damage produces single or multiple strokes.
  phenotype_term:
    preferred_term: Stroke
    term:
      id: HP:0001297
      label: Stroke
    temporality: RECURRENT
  evidence:
  - reference: PMID:8737928
    reference_title: The molecular pathology of hereditary cystatin C amyloid angiopathy causing brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The amyloid deposition leads to arterial damage with single or multiple strokes."
    explanation: >-
      Directly attributes single or multiple strokes to the amyloid arterial
      damage.
- category: Neurological
  name: Dementia
  description: >-
    Progressive dementia accompanies recurrent hemorrhage and may be an initial
    presenting sign.
  phenotype_term:
    preferred_term: Dementia
    term:
      id: HP:0000726
      label: Dementia
    clinical_course: PROGRESSIVE
  frequency: FREQUENT
  evidence:
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Most L68Q carriers have clinical symptoms characterized by recurrent hemorrhages and dementia, between the age of 20-30 years."
    explanation: >-
      Dementia is reported in most carriers, supporting a frequent band.
- category: Neurological
  name: Hemiparesis
  description: >-
    Repeated hemorrhages leave motor deficits, described in the clinical
    literature as paralysis.
  phenotype_term:
    preferred_term: Hemiparesis
    term:
      id: HP:0001269
      label: Hemiparesis
  frequency: FREQUENT
  evidence:
  - reference: PMID:16612982
    reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the clinical manifestation is restricted to the brain, and usually consists of repeated hemorrhages leading to paralysis"
    explanation: >-
      Paralysis is described as the usual outcome of the repeated hemorrhages,
      supporting a frequent band. Note the source says paralysis generically
      rather than specifying hemiparesis.
- category: Neuropsychiatric
  name: Personality changes
  description: >-
    Personality change, alongside dementia, can be the initial sign of
    hemorrhage.
  phenotype_term:
    preferred_term: Personality changes
    term:
      id: HP:0000751
      label: Personality changes
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:16612982
    reference_title: 'Hereditary cystatin C amyloid angiopathy: genetic, clinical, and pathological aspects.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sometimes the initial signs of hemorrhage are dementia and personality changes."
    explanation: >-
      The qualifier sometimes maps to an occasional frequency band.
histopathology:
- name: Amyloid deposition in cerebral arteries and arterioles
  description: >-
    Heavy amyloid deposits in the walls of small cerebral arteries and
    arterioles, most prominent in the media of parenchymal vessels and the
    adventitia of leptomeningeal vessels, with sparing of capillaries and veins.
  evidence:
  - reference: PMID:9063500
    reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cystatin C deposition was more prominent in the media of parenchymal vessels and in the adventitia of leptomeningeal vessels."
    explanation: >-
      Defines the layer-specific distribution of deposits seen on
      immunohistochemistry.
- name: Extracellular matrix accumulation in the arterial wall
  description: >-
    Affected leptomeningeal arteries show collagen IV and aggrecan deposition,
    thickened laminin distribution, intimal thickening with a frayed elastic
    layer, and variable loss of endothelial integrity.
  evidence:
  - reference: PMID:23973860
    reference_title: Deposition of collagen IV and aggrecan in leptomeningeal arteries of hereditary brain haemorrhage with amyloidosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other structural abnormalities were thickening of the laminin distribution, intimal thickening concomitant with a frayed elastic layer, and variable reduction in the integrity of endothelia."
    explanation: >-
      Enumerates the additional structural wall abnormalities found at
      post-mortem.
- name: Cystatin C deposition in skin basement membrane
  description: >-
    In carriers, cystatin C deposits first in the dermal-epidermal basement
    membrane and extend to other basement membrane regions as disease advances,
    always in close association with collagen IV.
  evidence:
  - reference: PMID:28067897
    reference_title: Pathological changes in basement membranes and dermal connective tissue of skin from patients with hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "When the deposits were more advanced, they extended to other BM regions in the skin."
    explanation: >-
      Documents the staged extension of skin basement membrane deposition with
      disease progression.
biochemical:
- name: Cerebrospinal fluid cystatin C
  notes: >-
    CSF cystatin C is markedly reduced in HCCAA patients, consistent with
    retention of the variant protein rather than secretion. Reduced CSF cystatin
    C was one of the earliest diagnostic measures available for living carriers.
  evidence:
  - reference: PMID:9445375
    reference_title: 'Hereditary cystatin C amyloid angiopathy: monitoring the presence of the Leu-68-->Gln cystatin C variant in cerebrospinal fluids and monocyte cultures by MS.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished"
    explanation: >-
      Directly reports the reduced CSF concentration in patients.
- name: Plasma high-molecular-weight cystatin C aggregates
  notes: >-
    High-molecular-weight cystatin C aggregates are measurable in plasma and
    decreased after high-dose N-acetylcysteine, making them a candidate
    pharmacodynamic marker of the aggregation node.
  evidence:
  - reference: PMID:40163249
    reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Secondary outcomes included a significant increase in reduced glutathione levels and decreased high-molecular-weight cystatin C aggregate levels in plasma after NAC treatment."
    explanation: >-
      Reports the plasma aggregate measurement and its treatment-associated
      decrease.
genetic:
- name: CST3
  gene_term:
    preferred_term: CST3
    term:
      id: hgnc:2475
      label: CST3
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    A single T-to-A substitution in codon 68 of CST3 replaces leucine with
    glutamine (L68Q) in cystatin C. This is the sole known cause of HCCAA; every
    examined patient carries the same founder allele, heterozygously, and the
    mutation abolishes an AluI restriction site, which historically enabled
    simple RFLP-based diagnosis.
  variants:
  - name: CST3 L68Q
    description: >-
      Founder missense variant in exon 2 of CST3 (T-to-A at the codon for leucine
      68), replacing leucine with glutamine. Carried heterozygously by every
      known HCCAA patient and traced to a single common ancestor.
    evidence:
    - reference: PMID:8097919
      reference_title: Molecular diagnosis of hereditary cystatin C amyloid angiopathy.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "HCCAA has been shown to be caused by a T-->A point mutation in the codon for leucine at position 68 in exon 2 of the cystatin C gene, which results in a leucine-->glutamine amino acid substitution in the cystatin C molecule."
      explanation: >-
        Specifies the exact nucleotide and amino acid change and its exon
        location.
  evidence:
  - reference: PMID:2602420
    reference_title: Mutation in the cystatin C gene causes hereditary brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HCCAA is the first human disorder known to be caused by an abnormal gene for a cysteine proteinase inhibitor."
    explanation: >-
      Establishes CST3, a cysteine proteinase inhibitor gene, as the causative
      gene.
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HCCAA is dominantly inherited, caused by L68Q mutation in the cystatin C gene, leading to aggregation of the cystatin C protein."
    explanation: >-
      Confirms the dominant CST3 L68Q aetiology and links it to protein
      aggregation.
environmental:
- name: Twentieth-century Icelandic lifestyle change
  description: >-
    Obligate L68Q carriers born between 1825 and 1900 experienced a fall in life
    span from about 65 years to the modern average of about 30, and a
    parent-of-origin effect emerged over the same period. Because the shift
    appears in several independent extended families many generations after the
    founding mutation, the authors infer an environmental cause tied to changes
    in Icelandic life-style. The specific exposure has not been identified, so no
    ECTO term is bound here.
  influences_mechanisms:
  - target: Recurrent Lobar Intracerebral Hemorrhage
    environmental_effect: MODULATES
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      An unidentified environmental factor associated with modern Icelandic
      life-style is inferred to accelerate the clinical course, but the mediating
      biology is unknown.
    evidence:
    - reference: PMID:18566660
      reference_title: A drastic reduction in the life span of cystatin C L68Q carriers due to life-style changes during the last two centuries.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "As these trends can be observed in several different extended families, many generations after the mutational event, it seems likely that some environmental factor is responsible, perhaps linked to radical changes in the life-style of Icelanders during this period."
      explanation: >-
        The environmental attribution is an inference from genealogical data and
        is stated by the authors as likely rather than demonstrated, so this is
        recorded as partial support.
  evidence:
  - reference: PMID:18566660
    reference_title: A drastic reduction in the life span of cystatin C L68Q carriers due to life-style changes during the last two centuries.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Intriguingly, the genealogies reveal that obligate L68Q carriers born 1825 to 1900 experienced a drastic reduction in life span, from 65 years to the present-day average."
    explanation: >-
      Quantifies the historical shift in carrier life span that defines this
      environmental observation.
treatments:
- name: N-Acetylcysteine
  description: >-
    High-dose N-acetylcysteine (2400 mg daily) was assessed in a phase 2a
    open-label trial in 17 L68Q-CST3 carriers. It was safe and well tolerated,
    raised reduced glutathione, lowered plasma high-molecular-weight cystatin C
    aggregates, and reduced disease-associated biomarkers in skin. It is an
    investigational, biomarker-supported candidate preventive strategy, not an
    established therapy - the trial was nonrandomized, uncontrolled for clinical
    outcome, and cerebral bleeds still occurred during treatment.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: N-acetyl-L-cysteine
      term:
        id: CHEBI:28939
        label: N-acetyl-L-cysteine
  target_mechanisms:
  - target: Domain-Swapped Dimerization and Oligomerization
    treatment_effect: INHIBITS
    description: >-
      NAC treatment decreased high-molecular-weight cystatin C aggregate levels
      in plasma, consistent with action on the aggregation/oligomerization node
      rather than on established deposits.
    evidence:
    - reference: PMID:40163249
      reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Secondary outcomes included a significant increase in reduced glutathione levels and decreased high-molecular-weight cystatin C aggregate levels in plasma after NAC treatment."
      explanation: >-
        Provides the pharmacodynamic evidence that NAC acts on the cystatin C
        aggregate pool.
  evidence:
  - reference: PMID:40163249
    reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this single-center nonrandomized clinical trial, NAC was safe and well tolerated and decreased disease-associated biomarker and amyloid deposition, suggesting NAC may offer a preventive strategy against HCCAA."
    explanation: >-
      The trial's own conclusion, which is framed as suggestive of a preventive
      strategy rather than as established clinical benefit.
  - reference: PMID:40163249
    reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Five cerebral bleeds occurred during the treatment period without permanent neurological sequela; no death occurred."
    explanation: >-
      Records that hemorrhages still occurred on treatment, which qualifies any
      claim of clinical efficacy.
- name: Genetic counseling
  description: >-
    Autosomal dominant transmission of a single founder allele in a small number
    of known Icelandic families makes counseling and predictive testing central
    to management of at-risk relatives.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:8097919
    reference_title: Molecular diagnosis of hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "One hundred ninety-one individuals have now been screened for the HCCAA causing mutation, including a fetus for prenatal diagnosis."
    explanation: >-
      Documents established family screening and prenatal diagnosis practice,
      which is delivered through genetic counseling.
- name: Supportive care
  description: >-
    No disease-modifying therapy is established. Management is supportive, aimed
    at the consequences of recurrent hemorrhage.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No therapy is available for this devastating disease."
    explanation: >-
      Establishes the absence of disease-modifying therapy, which is why care
      remains supportive.
diagnosis:
- name: CST3 molecular genetic testing
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Detection of the CST3 L68Q founder variant is definitive. Because the
    mutation abolishes an AluI site, RFLP analysis historically provided a simple
    and accurate assay, and it supports presymptomatic and prenatal diagnosis.
  results: Presence of the CST3 L68Q allele establishes the diagnosis of HCCAA.
  evidence:
  - reference: PMID:8097919
    reference_title: Molecular diagnosis of hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since the HCCAA-causing mutation abolishes an AluI restriction site in the cystatin C gene, analysis of this AluI restriction fragment-length polymorphism (RFLP) enables simple and accurate molecular diagnosis of HCCAA."
    explanation: >-
      Establishes the molecular assay and its diagnostic accuracy claim.
- name: Skin biopsy for cystatin C immunoreactivity
  diagnosis_term:
    preferred_term: biopsy procedure
    term:
      id: NCIT:C15189
      label: Biopsy Procedure
  description: >-
    Because skin and CNS pathogenesis correspond closely, quantifying cystatin C
    immunoreactivity in skin biopsies provides an accessible measure of disease
    progression, and it was used as the primary biomarker readout in the NAC
    trial.
  results: >-
    Cystatin C immunoreactivity in skin, initially confined to the
    dermal-epidermal basement membrane, tracks disease stage.
  evidence:
  - reference: PMID:39054254
    reference_title: 'The historical background of hereditary cystatin C amyloid angiopathy: Genealogical, pathological, and clinical manifestations.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "thus skin biopsies can be used to monitor the progression of the disease by quantifying the cystatin C immunoreactivity"
    explanation: >-
      States the validated use of skin biopsy for disease monitoring.
- name: Cerebrospinal fluid cystatin C measurement
  diagnosis_term:
    preferred_term: Lumbar Puncture
    term:
      id: NCIT:C15327
      label: Lumbar Puncture
  description: >-
    Markedly reduced CSF cystatin C was one of the earliest available measures in
    living carriers. It has been superseded by molecular testing and is not
    specific on its own.
  results: CSF cystatin C is markedly diminished in HCCAA patients.
  evidence:
  - reference: PMID:9445375
    reference_title: 'Hereditary cystatin C amyloid angiopathy: monitoring the presence of the Leu-68-->Gln cystatin C variant in cerebrospinal fluids and monocyte cultures by MS.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished"
    explanation: >-
      Supports the biochemical observation, but the study reports no diagnostic
      performance characteristics.
- name: Brain computed tomography
  diagnosis_term:
    preferred_term: computed tomography
    term:
      id: NCIT:C17204
      label: Computed Tomography
  description: >-
    CT imaging is used to confirm intracranial hemorrhage when new neurological
    symptoms develop in a known carrier.
  results: Demonstrates intracranial hemorrhage underlying new neurological symptoms.
  evidence:
  - reference: PMID:40163249
    reference_title: 'N-Acetylcysteine for Hereditary Cystatin C Amyloid Angiopathy: A Nonrandomized Clinical Trial.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A computed tomography (CT) scan of the brain was acquired if new onset of neurological symptoms occurred."
    explanation: >-
      Documents the trial protocol's use of CT to detect hemorrhage on symptom
      onset.
differential_diagnoses:
- name: Sporadic cerebral amyloid angiopathy
  description: >-
    Age-related sporadic CAA is amyloid-beta based and affects the elderly,
    whereas HCCAA presents with hemorrhage in the third decade and is cystatin C
    based. Cystatin C does colocalize with amyloid-beta in sporadic CAA, so
    immunostaining for cystatin C alone does not distinguish the two.
  evidence:
  - reference: PMID:9063500
    reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cystatin C is also found to colocalize with amyloid beta/A4 protein in cerebral vessel walls of patients with Alzheimer's disease (AD), sporadic CAA, and hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D)."
    explanation: >-
      Establishes the colocalization that makes cystatin C staining alone
      non-discriminating between these entities.
- name: Hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D)
  description: >-
    HCHWA-D is caused by an APP variant and deposits amyloid-beta, not cystatin
    C. The shared HCHWA acronym is a recognized source of confusion between two
    genetically and biochemically distinct diseases; only the Icelandic type is
    modeled in this entry.
  evidence:
  - reference: PMID:9063500
    reference_title: Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cystatin C is also found to colocalize with amyloid beta/A4 protein in cerebral vessel walls of patients with Alzheimer's disease (AD), sporadic CAA, and hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D)."
    explanation: >-
      Names HCHWA-D as a separate entity from HCCAA within the same paper,
      supporting the distinction.
- name: Hypertensive intracerebral hemorrhage
  description: >-
    HCCAA hemorrhage occurs in normotensive young adults, which separates it from
    hypertensive deep intracerebral hemorrhage.
  evidence:
  - reference: PMID:8737928
    reference_title: The molecular pathology of hereditary cystatin C amyloid angiopathy causing brain hemorrhage.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder has an autosomal dominant mode of inheritance and causes fatal brain hemorrhage in normotensive young adults."
    explanation: >-
      The explicit normotensive qualifier is what distinguishes HCCAA from
      hypertensive hemorrhage.
discussions:
- discussion_id: hccaa_no_animal_model
  prompt: >-
    Why does HCCAA still lack an animal model, and what would a faithful model
    have to reproduce?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Cystatin C Amyloid Deposition in Cerebral Arterial Walls
  - pathophysiology#Arterial Wall Matrix Accumulation and Thickening
  rationale: >-
    Essentially all mechanistic knowledge of HCCAA comes from human post-mortem
    tissue, patient-derived cells, and in vitro structural biology. The absence
    of an animal model is stated explicitly in the vascular pathology
    literature, and it is why the temporal ordering of amyloid deposition,
    matrix accumulation, and smooth muscle cell loss remains inferred from
    cross-sectional autopsy material rather than demonstrated longitudinally.
  evidence:
  - reference: PMID:23973860
    reference_title: Deposition of collagen IV and aggrecan in leptomeningeal arteries of hereditary brain haemorrhage with amyloidosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The aetiology of HCCAA pathology is not clear and there is, at present, no animal model of the disease."
    explanation: >-
      States both the unresolved aetiology and the absence of an animal model.
- discussion_id: hccaa_basement_membrane_primacy
  prompt: >-
    Are basement membrane changes an early driver that facilitates cystatin C
    deposition, or a consequence of deposition already underway?
  kind: INTERPRETATION
  status: OPEN
  attaches_to:
  - pathophysiology#Cystatin C Amyloid Deposition in Cerebral Arterial Walls
  - pathophysiology#Arterial Wall Matrix Accumulation and Thickening
  - pathophysiology#Systemic Extracerebral Cystatin C Deposition
  rationale: >-
    This entry currently draws the edge from amyloid deposition to matrix
    accumulation, following the arterial post-mortem description. The skin biopsy
    study argues the opposite direction - that basement membrane change is early
    and facilitates deposition - and reports collagen IV increased to a similar
    extent in all carrier biopsies regardless of deposit stage. Both directions
    are supported by cross-sectional human material only, and resolving the order
    would require longitudinal sampling or a model system that does not yet
    exist.
  evidence:
  - reference: PMID:28067897
    reference_title: Pathological changes in basement membranes and dermal connective tissue of skin from patients with hereditary cystatin C amyloid angiopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "suggest that BM changes are early and important events in HCCAA pathogenesis that could facilitate cystatin C deposition and aggregation"
    explanation: >-
      States the alternative causal ordering that this discussion records as
      unresolved.
- discussion_id: hccaa_environmental_modifier
  prompt: >-
    Which specific environmental or life-style exposure accounts for the
    halving of L68Q carrier life span between 1825 and 1900?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Recurrent Lobar Intracerebral Hemorrhage
  rationale: >-
    The genealogical evidence for an environmental modifier is strong and
    replicated across independent extended families, and it implies that a
    preventive strategy may exist. But the responsible exposure has never been
    identified, so no ECTO term can be bound and the effect cannot be modeled
    mechanistically. A parent-of-origin effect emerging over the same period is
    an additional unexplained observation.
  evidence:
  - reference: PMID:18566660
    reference_title: A drastic reduction in the life span of cystatin C L68Q carriers due to life-style changes during the last two centuries.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At the same time, a parent-of-origin effect emerged, whereby maternal inheritance of the mutation was associated with a 9 year reduction in life span relative to paternal inheritance."
    explanation: >-
      Records the unexplained parent-of-origin effect that accompanies the
      life-span shift.
notes: >-
  Naming caution: HCHWA without a type qualifier is ambiguous. HCHWA-Icelandic
  type (this entry, ACys, CST3 L68Q, cystatin C amyloid) and HCHWA-Dutch type
  (APP, amyloid-beta) are distinct diseases. Literature searches on the
  unqualified acronym return both. ACys is also represented as the ACys subtype
  of the Cerebral_Amyloid_Angiopathy entry, which models the CAA umbrella; this
  entry is the standalone disease, paralleling the existing ADan_amyloidosis
  entry.