Sitosterolemia

Mendelian MONDO:0008863 Pathograph 15 Show in embeddings browser Inborn Error of Metabolism Lipid Metabolism Disorder

Sitosterolemia (phytosterolemia) is an autosomal recessive sterol-storage disorder caused by biallelic loss-of-function variants in ABCG5 or ABCG8, which encode the two halves of the G5G8 sterol-efflux transporter expressed on the apical membrane of hepatocytes and enterocytes. Loss of G5G8 removes the pump that normally excretes cholesterol and dietary plant/shellfish sterols (xenosterols) back into the intestinal lumen and into bile, so absorption of both cholesterol and plant sterols becomes unselective and unrestricted. The resulting accumulation of sitosterol and campesterol in plasma and tissues produces tendinous and cutaneous xanthomas, premature atherosclerosis, arthralgia, and a distinctive hematologic syndrome of stomatocytic hemolytic anemia and macrothrombocytopenia. It is frequently misdiagnosed as familial hypercholesterolemia or immune thrombocytopenia and, unlike familial hypercholesterolemia, responds poorly to statins but dramatically to dietary sterol restriction and the cholesterol-absorption inhibitor ezetimibe.

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1
Mappings
1
Inheritance
10
Pathophys.
12
Phenotypes
3
Gaps
15
Pathograph
2
Genes
6
Medical Actions
1
References
1
Deep Research
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Classifications

Harrison's Part
ENDOCRINOLOGY METABOLISM GENETICS ENVIRONMENT DISEASE
🔗

Mappings

MONDO
MONDO:0008863 sitosterolemia
skos:exactMatch MONDO
MONDO:0008863 (sitosterolemia) is the exact disease concept; its definition names biallelic ABCG5/ABCG8 variants as the cause.
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Inheritance

1
Autosomal recessive inheritance HP:0000007
Sitosterolemia is recessively inherited; affected individuals carry biallelic (homozygous or compound heterozygous) loss-of-function variants in either ABCG5 or ABCG8. The two genes lie head-to-head at 2p21 and their products form an obligate heterodimer, so a defect in either gene produces the same disease.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:14769702 SUPPORT Human Clinical
"Sitosterolemia is a recessively inherited disorder that results from mutations in either ABCG5 or G8 proteins, with hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
States the autosomal recessive inheritance and the ABCG5/ABCG8 causal basis.
PMID:23556150 SUPPORT Other
"Sitosterolemia is inherited in an autosomal recessive manner."
GeneReviews confirms autosomal recessive inheritance.
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Discussions and Knowledge Gaps

3
Why is sitosterolemia so phenotypically heterogeneous — ranging from almost asymptomatic individuals to children with severe hypercholesterolemia, accelerated atherosclerosis, and premature cardiac death — and what genotype/modifier factors (specific ABCG5 vs ABCG8 variants, residual transporter activity, diet, other lipid genes) determine severity?
KNOWLEDGE GAP OPEN gap_sitosterolemia_phenotypic_heterogeneity
Attached to
pathophysiology#Plasma and Tissue Plant Sterol Accumulation pathophysiology#Accelerated Atherosclerosis
Plasma plant-sterol levels do not fully predict clinical severity, and the functional impact of most ABCG5/ABCG8 variants on transporter structure-function is unknown, so the determinants of the extreme phenotypic range are unresolved. Clarifying them would improve risk stratification and the timing/intensity of sterol-lowering therapy.
Show evidence (2 references)
PMID:27104173 SUPPORT Other
"Patients with sitosterolemia show extreme phenotypic heterogeneity, ranging from almost asymptomatic individuals to those with severe hypercholesterolemia leading to accelerated atherosclerosis and premature cardiac death."
Documents the extreme phenotypic heterogeneity that defines the gap.
PMID:33807969 SUPPORT Other
"The impact of these variants on G5G8 structure and activity are largely unknown."
The unknown functional impact of most variants underlies the unexplained severity range.
Sitosterolemia is repeatedly misdiagnosed as familial hypercholesterolemia (especially in children) or as immune thrombocytopenia. What discriminating features should trigger a plant-sterol assay, and how does the statin non-response distinguish it?
INTERPRETATION OPEN interp_sitosterolemia_misdiagnosis_FH_ITP
Attached to
pathophysiology#Plasma and Tissue Plant Sterol Accumulation phenotypes#Thrombocytopenia
Because sitosterolemia shares hypercholesterolemia and xanthomas with familial hypercholesterolemia and shares low platelet counts with immune thrombocytopenia, it is frequently misclassified — and mistreated. A key discriminator is that, unlike familial hypercholesterolemia, sitosterolemia responds poorly to statins but dramatically to diet and ezetimibe; normolipemic xanthomas, giant platelets/stomatocytes on smear, and a treatment response mismatch should prompt a plant-sterol assay. Capturing this pitfall is important for a knowledge base intended to support diagnosis.
Show evidence (2 references)
PMID:29984642 SUPPORT Other
"making it possible for sitosterolemia to be misdiagnosed as homozygous FH, especially in pediatric patients."
Documents misdiagnosis as homozygous familial hypercholesterolemia, especially in children.
PMID:16029460 SUPPORT Human Clinical
"phytosterolaemia, which does not respond to standard statin treatment, can be diagnosed via the distinctive haematology described here, even when the cholesterol is normal"
The statin non-response and distinctive hematology are the discriminating features that should prompt testing.
Is the detailed platelet-passivation mechanism worked out in Abcg5/Abcg8-deficient mice (constitutive fibrinogen binding to activated integrin alphaIIbbeta3, integrin internalization, microparticle generation, filamin relocalization, and GPIba shedding) the operative mechanism of macrothrombocytopenia and bleeding in human sitosterolemia?
HUMAN MODEL MISMATCH OPEN model_mismatch_sitosterolemia_platelet_passivation
Attached to
pathophysiology#Sterol Intercalation into Blood Cell Membranes pathophysiology#Megakaryocyte Demarcation Membrane System Defect pathophysiology#Macrothrombocytopenia
The step-by-step molecular mechanism linking membrane sterol intercalation to platelet dysfunction is established in mouse models; direct human confirmation is limited. The strongest human bridge is a correlation between plasma soluble GPIba and plasma sitosterol in patient samples, which is suggestive but not a demonstration that the full murine mechanism operates in human platelets. Whether the murine platelet-passivation cascade is the human mechanism is the open question.
Proposed experiments
Human platelet-passivation mechanism verification
exp_sitosterolemia_human_platelet_passivation
Measure integrin alphaIIbbeta3 activation state, GPIba surface density/shedding, and platelet-derived microparticles in platelets from human sitosterolemia patients before and after ezetimibe-induced plant-sterol lowering, to test whether the murine platelet-passivation cascade operates in human platelets and reverses with treatment.
Platelet-membrane sterol versus phenotype correlation
exp_sitosterolemia_membrane_sterol_correlation
Correlate platelet-membrane free-sterol content with platelet size, hyperreactivity, and bleeding phenotype across a human patient cohort to establish the human dose-response between membrane sterol loading and platelet dysfunction.
Show evidence (1 reference)
PMID:23926302 SUPPORT Human Clinical
"Plasma levels of soluble GPIbα were strongly correlated with plasma sitosterol levels in samples from human sitosterolemic patients, implicating a similar mechanism of sterol-induced platelet passivation in the human disease."
The mechanism is defined in mice; the only direct human evidence is a correlation between soluble GPIba and sitosterol, so the translational validity of the full mechanism is the mismatch.
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Pathophysiology

10
G5G8 Sterol-Efflux Transporter Deficiency
Biallelic loss-of-function variants in ABCG5 or ABCG8 abolish the G5G8 heterodimer, a pair of ATP-binding-cassette half-transporters that assemble and traffic to the apical (canalicular/brush-border) membrane of hepatocytes and enterocytes, where they pump cholesterol and plant/shellfish sterols out of the cell into bile and into the intestinal lumen. Because either subunit is required for a functional pump, a defect in either gene inactivates sterol efflux.
hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology. enterocyte CL:0000584 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves enterocyte (CL:0000584). CL:0000584 is a cell type from the Cell Ontology.
ABCG5 hgnc:13886 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ABCG5 (hgnc:13886). hgnc:13886 is a gene from the HUGO Gene Nomenclature Committee. ABCG8 hgnc:13887 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ABCG8 (hgnc:13887). hgnc:13887 is a gene from the HUGO Gene Nomenclature Committee.
sterol transport GO:0015918 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased sterol transport (GO:0015918). GO:0015918 is a biological process from the Gene Ontology. ↓ DECREASED
ABC-type sterol transporter activity GO:0034041 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased ABC-type sterol transporter activity (GO:0034041). GO:0034041 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Intestinal Xenosterol Hyperabsorption Loss of enterocyte G5G8 efflux back into the gut lumen removes the brake on dietary sterol absorption.
Reduced Biliary Sterol Excretion Loss of hepatocyte G5G8 efflux into bile removes the route for hepatic disposal of xenosterols.
Show evidence (1 reference)
PMID:33807969 SUPPORT Other
"ABCG5 ABCG8 encodes a pair of ABC half transporters that form a heterodimer (G5G8), which then traffics to the surface of hepatocytes and enterocytes and promotes the secretion of cholesterol and xenosterols into the bile and the intestinal lumen."
Identifies the G5G8 heterodimer sterol-efflux transporter disrupted in sitosterolemia.
Intestinal Xenosterol Hyperabsorption
Without enterocyte G5G8 to pump absorbed sterols back into the lumen, the intestine absorbs both cholesterol and plant/shellfish sterols in an unselective, unrestricted manner. Normally the body excludes ~95% of dietary plant sterols; in sitosterolemia their fractional absorption rises markedly.
enterocyte CL:0000584 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves enterocyte (CL:0000584). CL:0000584 is a cell type from the Cell Ontology.
Downstream
Plasma and Tissue Plant Sterol Accumulation Hyperabsorbed xenosterols enter the circulation and accumulate.
Show evidence (1 reference)
PMID:16029460 SUPPORT Human Clinical
"the absorption of both cholesterol and plant-derived cholesterol-like molecules at the gut is unselective and unrestricted."
Describes the unselective, unrestricted intestinal sterol absorption central to the disease.
Reduced Biliary Sterol Excretion
Loss of hepatocyte canalicular G5G8 reduces secretion of cholesterol and plant sterols into bile, removing the principal route by which the liver would otherwise dispose of the excess absorbed xenosterols. Impaired biliary efflux compounds the intestinal hyperabsorption to drive net sterol retention.
hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
Downstream
Plasma and Tissue Plant Sterol Accumulation Reduced biliary disposal leaves absorbed sterols to accumulate in plasma and tissues.
Show evidence (1 reference)
PMID:14769702 SUPPORT Human Clinical
"hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
Documents decreased hepatic (biliary) excretion of plant sterols and cholesterol.
Plasma and Tissue Plant Sterol Accumulation
The combination of intestinal hyperabsorption and reduced biliary excretion produces markedly elevated plasma and tissue levels of plant sterols — especially sitosterol, campesterol, and stigmasterol (and their metabolites). This sustained xenosterol accumulation is the central biochemical state of the disease and the hub from which the vascular, cutaneous, and hematologic manifestations branch.
Downstream
Sterol Intercalation into Blood Cell Membranes Circulating free plant sterols partition into the plasma membranes of blood cells and their marrow precursors.
Xanthoma Formation Tissue sterol deposition in tendon and skin produces xanthomas.
Accelerated Atherosclerosis Deposition of plant sterols and cholesterol in the arterial wall drives premature atherosclerosis.
Show evidence (3 references)
PMID:33807969 SUPPORT Other
"Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
Establishes plasma and tissue xenosterol accumulation as the central disease state.
PMID:14769702 SUPPORT Human Clinical
"markedly elevated plasma and tissue sitosterol and campesterol levels"
Identifies sitosterol and campesterol as the accumulating sterols.
PMID:23556150 SUPPORT Other
"increased plasma concentrations of plant sterols (especially sitosterol, campesterol, and stigmasterol)"
GeneReviews names sitosterol, campesterol, and stigmasterol as the accumulating plant sterols.
Sterol Intercalation into Blood Cell Membranes
Free plant sterols intercalate into the plasma membranes of erythrocytes, platelets, and megakaryocytes, altering membrane sterol composition and biophysics. This membrane remodeling is the proximate cause of the two distinctive hematologic manifestations of sitosterolemia and is reversible when plant-sterol levels are lowered.
platelet CL:0000233 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves platelet (CL:0000233). CL:0000233 is a cell type from the Cell Ontology. erythrocyte CL:0000232 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves erythrocyte (CL:0000232). CL:0000232 is a cell type from the Cell Ontology.
Downstream
Megakaryocyte Demarcation Membrane System Defect Sterol-enriched membranes disrupt the megakaryocyte demarcation membrane system and platelet formation.
Stomatocytic Hemolysis Erythrocyte membrane sterol enrichment produces stomatocyte morphology and premature hemolysis.
Show evidence (1 reference)
PMID:23926302 SUPPORT Model Organism
"Intercalation of plant sterols into the plasma membrane therefore results in dysregulation of multiple platelet activation pathways, leading to macrothrombocytopenia and bleeding."
Establishes membrane intercalation of plant sterols as the mechanism disrupting blood-cell function.
Megakaryocyte Demarcation Membrane System Defect
Plant-sterol enrichment of the megakaryocyte membrane causes defective megakaryocyte development with deterioration of the demarcation membrane system (the internal membrane reservoir from which platelets are formed) — not a reduction in megakaryocyte numbers or progenitors. Critically, bone-marrow-transplant crossover experiments show the phenotype tracks the recipient: irradiated wild-type mice given Abcg5-null marrow have normal platelets, whereas Abcg5-null mice given wild-type marrow remain affected. The defect is therefore extrinsic — driven by the host plasma plant-sterol environment rather than being cell-intrinsic to the hematopoietic compartment — which is why it reverses when plant sterols are lowered.
megakaryocyte CL:0000556 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves megakaryocyte (CL:0000556). CL:0000556 is a cell type from the Cell Ontology.
Downstream
Macrothrombocytopenia The demarcation-membrane-system defect yields enlarged, reduced-count platelets.
Show evidence (2 references)
PMID:18156627 SUPPORT Model Organism
"defective megakaryocyte development with deterioration of the demarcation membrane system was evident."
Establishes the megakaryocyte demarcation-membrane-system defect (not reduced megakaryocyte numbers) as the lesion.
PMID:18156627 SUPPORT Model Organism
"Lethally irradiated wild-type mice transplanted with bone marrow from Abcg5(-/-) mice displayed normal platelets, whereas Abcg5(-/-) mice transplanted with wild-type bone marrow still showed macrothrombocytopenia."
The crossover result shows the phenotype tracks the recipient's plasma-sterol environment (extrinsic defect), not the donor marrow (not cell-intrinsic).
Macrothrombocytopenia
The megakaryocyte demarcation-membrane-system defect yields a reduced count of enlarged (giant) platelets — macrothrombocytopenia. In humans this is the platelet component of the "Mediterranean stomatocytosis/macrothrombocytopenia" presentation, and plasma soluble GPIba (a marker of the accompanying platelet passivation) correlates with plasma sitosterol. Because the driver is the plasma plant-sterol burden, the abnormality reverses when plant sterols are lowered. (The parallel sterol-induced platelet hyperreactivity/passivation that explains the bleeding tendency is elaborated in the associated model-mismatch discussion.)
platelet CL:0000233 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves platelet (CL:0000233). CL:0000233 is a cell type from the Cell Ontology.
Show evidence (3 references)
PMID:18156627 SUPPORT Model Organism
"accumulation of plant sterols is responsible for development of macrothrombocytopenia in sitosterolemia"
Establishes plant-sterol accumulation as the cause of macrothrombocytopenia.
PMID:16029460 SUPPORT Human Clinical
"Mediterranean stomatocytosis/macrothrombocytopenia is caused by an excess of phytosterols in the blood"
Confirms in humans that the macrothrombocytopenia is caused by excess phytosterols.
PMID:23926302 SUPPORT Human Clinical
"Plasma levels of soluble GPIbα were strongly correlated with plasma sitosterol levels in samples from human sitosterolemic patients"
Human correlation between plasma soluble GPIba and plasma sitosterol, bridging the mouse mechanism to human platelet passivation.
Stomatocytic Hemolysis
Enrichment of the erythrocyte membrane with plant sterols alters red-cell shape (stomatocytosis) and reduces cell survival, producing a hemolytic anemia frequently accompanied by splenomegaly. The stomatocytic hemolysis combined with macrothrombocytopenia constitutes the characteristic "Mediterranean stomatocytosis/macrothrombocytopenia" hematologic presentation of the disease.
erythrocyte CL:0000232 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves erythrocyte (CL:0000232). CL:0000232 is a cell type from the Cell Ontology.
Show evidence (1 reference)
PMID:27104173 SUPPORT Other
"Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
Documents hemolytic anemia with stomatocytosis as a hematologic manifestation.
Xanthoma Formation
Sterol deposition in tendons and skin produces tendinous and cutaneous xanthomas, the classic clinical sign that historically prompted sterol testing. In children xanthomas can appear even when cholesterol is normal (normolipemic xanthomas), a presentation that should trigger a plant-sterol assay.
macrophage CL:0000235 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves macrophage (CL:0000235). CL:0000235 is a cell type from the Cell Ontology.
Show evidence (1 reference)
PMID:29984642 SUPPORT Other
"The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
Identifies tendinous and cutaneous xanthomas as a main clinical feature.
Accelerated Atherosclerosis
Chronic accumulation of plant sterols and cholesterol drives accelerated atherosclerosis, which can present as premature coronary artery disease, aortic disease, and, in severe childhood-onset cases, premature cardiac death. The phenotype ranges from near-asymptomatic to severe, and the accelerated atherosclerosis is the principal cause of morbidity and mortality.
macrophage CL:0000235 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves macrophage (CL:0000235). CL:0000235 is a cell type from the Cell Ontology.
Show evidence (1 reference)
PMID:27104173 SUPPORT Other
"increased plant sterol levels, xanthomas, and accelerated atherosclerosis"
Names accelerated atherosclerosis as a defining consequence of plant-sterol accumulation.
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Pathograph

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

12
Blood 3
Hemolytic anemia OCCASIONAL HP:0001878 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hemolytic anemia (HP:0001878). HP:0001878 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:27104173 SUPPORT Other
"Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
Hemolytic anemia is a characteristic hematologic manifestation.
PMID:29984642 SUPPORT Other
"Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases."
Quantifies the hematologic abnormalities (hemolytic anemia and macrothrombocytopenia) at 25-35% of patients, supporting the frequency band.
Thrombocytopenia OCCASIONAL HP:0001873 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Thrombocytopenia (HP:0001873). HP:0001873 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29984642 SUPPORT Other
"Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases."
Supports thrombocytopenia as the platelet-count component of macrothrombocytopenia.
Abnormal bleeding HP:0001892 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal bleeding (HP:0001892). HP:0001892 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27104173 SUPPORT Other
"Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
Abnormal bleeding is listed among the hematologic manifestations.
Cardiovascular 2
Premature atherosclerosis and coronary artery disease Premature coronary artery atherosclerosis HP:0005181 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Premature coronary artery atherosclerosis (HP:0005181). HP:0005181 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:14769702 SUPPORT Human Clinical
"As a consequence of markedly elevated plasma and tissue sitosterol and campesterol levels, premature atherosclerosis develops."
Premature atherosclerosis is a direct consequence of the elevated sterols.
PMID:23556150 SUPPORT Other
"Premature atherosclerosis, which can lead to angina, aortic valve involvement, myocardial infarction, and sudden death"
GeneReviews details the severe cardiovascular sequelae of the premature atherosclerosis.
Splenomegaly OCCASIONAL HP:0001744 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Splenomegaly (HP:0001744). HP:0001744 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:27104173 SUPPORT Other
"Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
Splenomegaly accompanies the hemolytic component of the hematologic syndrome.
PMID:23556150 SUPPORT Other
"Arthritis, arthralgias, and splenomegaly may sometimes be seen"
GeneReviews notes splenomegaly is seen only sometimes, supporting the OCCASIONAL band.
Integument 1
Tendon and cutaneous xanthomas Tendon xanthomatosis HP:0010874 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tendinous xanthomatosis, annotated with Tendon xanthomatosis (HP:0010874), qualified as childhood onset. HP:0010874 is a phenotype from the Human Phenotype Ontology.
Onset: CHILDHOOD
Show evidence (2 references)
PMID:29984642 SUPPORT Other
"The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
Tendinous and cutaneous xanthomas are a main clinical feature.
PMID:23556150 SUPPORT Other
"Tendon xanthomas or tuberous (i.e., planar) xanthomas that can occur in childhood and in unusual locations (heels, knees, elbows, and buttocks)"
GeneReviews documents childhood-onset tendon xanthomas.
Metabolism 2
Hypercholesterolemia HP:0003124 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypercholesterolemia (HP:0003124). HP:0003124 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27104173 SUPPORT Other
"severe hypercholesterolemia have been reported in patients with sitosterolemia, especially in children."
Documents severe hypercholesterolemia, particularly in children.
Elevated hepatic transaminases Elevated circulating hepatic transaminase concentration HP:0002910 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated circulating hepatic transaminase concentration (HP:0002910). HP:0002910 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:23556150 SUPPORT Other
"one study has concluded that "idiopathic" liver disease could be undiagnosed sitosterolemia."
GeneReviews flags hepatic involvement, proposing sitosterolemia as a cause of "idiopathic" liver disease.
Constitutional 1
Arthralgia or arthritis OCCASIONAL HP:0002829 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Arthralgia (HP:0002829). HP:0002829 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:29984642 SUPPORT Other
"The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
Arthritis or arthralgia is listed among the clinical features.
PMID:23556150 SUPPORT Other
"Arthritis, arthralgias, and splenomegaly may sometimes be seen"
GeneReviews notes arthritis/arthralgias are seen only sometimes, supporting an OCCASIONAL rather than FREQUENT band.
Other 3
Tuberous (planar) xanthomas Tuberous xanthoma HP:0031290 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tuberous xanthoma (HP:0031290), qualified as childhood onset. HP:0031290 is a phenotype from the Human Phenotype Ontology.
Onset: CHILDHOOD
Show evidence (1 reference)
PMID:23556150 SUPPORT Other
"Tendon xanthomas or tuberous (i.e., planar) xanthomas that can occur in childhood and in unusual locations (heels, knees, elbows, and buttocks)"
GeneReviews documents tuberous (planar) xanthomas in childhood and in unusual locations.
Stomatocytosis HP:0004446 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Stomatocytosis (HP:0004446). HP:0004446 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:16029460 SUPPORT Human Clinical
"combines stomatocytic haemolysis with the presence of very large platelets"
Stomatocytes on smear are the erythrocyte-morphology component of the hematologic syndrome.
Giant platelets OCCASIONAL HP:0001902 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Giant platelets (HP:0001902). HP:0001902 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:29984642 SUPPORT Other
"the peripheral blood smear is essential and reveals giant platelets and stomatocytes."
Giant platelets are the platelet-morphology component of macrothrombocytopenia.
PMID:29984642 SUPPORT Other
"Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases."
The 25-35% figure covers macrothrombocytopenia (giant platelets + low count), supporting the frequency band.
🧬

Genetic Associations

2
ABCG5
Gene: ABCG5 hgnc:13886 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ABCG5 (hgnc:13886). hgnc:13886 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (1 reference)
PMID:33807969 SUPPORT Other
"Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
Supports ABCG5 as one of the two causal genes.
ABCG8
Gene: ABCG8 hgnc:13887 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ABCG8 (hgnc:13887). hgnc:13887 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (2 references)
PMID:33807969 SUPPORT Other
"Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
Supports ABCG8 as one of the two causal genes.
PMID:16029460 SUPPORT Human Clinical
"two pedigrees showed the common W361X mutation in ABCG8"
Documents a recurrent (common) ABCG8 nonsense allele, W361X, across unrelated pedigrees.
💊

Medical Actions

6
Ezetimibe
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: ezetimibe CHEBI:49040 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses ezetimibe (CHEBI:49040). CHEBI:49040 is a therapeutic agent from Chemical Entities of Biological Interest.
Ezetimibe, a cholesterol-absorption inhibitor (NPC1L1 antagonist), lowers intestinal absorption of both cholesterol and plant sterols, producing significant and progressive reductions in plasma plant sterols. In animal models and patients it also reverses the macrothrombocytopenia, tying the hematologic disease to the plant-sterol burden. It is a mainstay of therapy.
Mechanism Target:
INHIBITS Plasma and Tissue Plant Sterol Accumulation — By blocking intestinal sterol absorption, ezetimibe lowers the plasma and tissue plant-sterol accumulation that this node represents, which in turn relieves the downstream hematologic and vascular disease.
Show evidence (2 references)
PMID:14769702 SUPPORT Human Clinical
"CONCLUSIONS: Ezetimibe produced significant and progressive reductions in plasma plant sterol concentrations in patients with sitosterolemia, consistent with the hypothesis that ezetimibe inhibits the intestinal absorption of plant sterols as well as cholesterol, leading to reductions in plasma..."
Randomized trial establishing ezetimibe as a plant-sterol-lowering therapy.
PMID:18156627 SUPPORT Model Organism
"Treatment with the sterol absorption inhibitor ezetimibe rapidly reversed macrothrombocytopenia in Abcg5(-/-) mice concomitant with a strong decrease in plasma plant sterols."
Shows ezetimibe reverses the hematologic phenotype in a mouse model by lowering plant sterols.
Dietary Sterol Restriction
Action: dietary interventionNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is dietary intervention (NCIT:C15447). NCIT:C15447 is a clinical intervention from the NCI Thesaurus. Ontology label: Dietary Intervention NCIT:C15447
Dietary restriction of both cholesterol and plant sterols is a first-line intervention. Foods rich in plant sterols (vegetable oils, wheat germ, nuts, seeds, avocado, margarine, chocolate) and shellfish sterols are limited. Importantly, plant-stanol-fortified margarines and products marketed to lower cholesterol in the general population are contraindicated in sitosterolemia, because affected patients cannot excrete the stanols and their accumulation is exacerbated — an "agents to avoid" caveat unique to this disorder.
Mechanism Target:
INHIBITS Intestinal Xenosterol Hyperabsorption — Lowering dietary plant-sterol intake reduces the substrate available for the unrestricted intestinal absorption that drives the disease.
Show evidence (2 references)
PMID:27104173 SUPPORT Other
"The mainstay of therapy includes dietary restriction of both cholesterol and plant sterols and the sterol absorption inhibitor, ezetimibe."
Dietary restriction of cholesterol and plant sterols is a mainstay of therapy.
PMID:23556150 SUPPORT Other
"Margarines and other products containing stanols (e.g., campestanol and sitostanol) that are recommended for use by persons with hypercholesterolemia are contraindicated as they can exacerbate plant stanol accumulation."
GeneReviews flags plant-stanol products as contraindicated ("agents to avoid") because they exacerbate stanol accumulation.
Partial Ileal Bypass Surgery
Action: surgical procedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is surgical procedure (NCIT:C15329). NCIT:C15329 is a clinical intervention from the NCI Thesaurus. Ontology label: Surgical Procedure NCIT:C15329
Partial ileal bypass surgery, which interrupts enterohepatic sterol recycling, may be considered as a last resort in patients who respond poorly to maximal dietary and pharmacologic therapy.
Show evidence (1 reference)
PMID:23556150 SUPPORT Other
"Partial ileal bypass surgery may be considered as a last resort for those with poor response to maximal therapies."
GeneReviews lists partial ileal bypass surgery as a last-resort treatment.
Bile Acid Sequestrant
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: bile acid sequestrant NCIT:C98148 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses bile acid sequestrant (NCIT:C98148). NCIT:C98148 is a therapeutic agent from the NCI Thesaurus.
Bile acid sequestrants (e.g., cholestyramine, colestimide, colesevelam) interrupt enterohepatic sterol recycling and, together with a low-cholesterol diet, dramatically lower the hypercholesterolemia seen in sitosterolemia.
Show evidence (1 reference)
PMID:27104173 SUPPORT Other
"Hypercholesterolemia in patients with sitosterolemia is dramatically responsive to low cholesterol diet and bile acid sequestrants."
Bile acid sequestrants (with diet) dramatically lower the hypercholesterolemia.
Statin Therapy (Ineffective)
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
HMG-CoA reductase inhibitors (statins), the standard therapy for familial hypercholesterolemia, are largely ineffective in sitosterolemia because the disease is driven by sterol hyperabsorption rather than endogenous synthesis. The lack of statin response is a key discriminator from homozygous familial hypercholesterolemia and should prompt a plant-sterol assay.
Show evidence (1 reference)
PMID:16029460 REFUTE Human Clinical
"phytosterolaemia, which does not respond to standard statin treatment"
Statins do not work in sitosterolemia; this negative response is a diagnostic discriminator from familial hypercholesterolemia.
Disease Surveillance and Monitoring
Action: clinical chemistry assessmentNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is clinical chemistry assessment, annotated with Blood Chemistry Measurement (NCIT:C47868). NCIT:C47868 is a clinical intervention from the NCI Thesaurus. Ontology label: Blood Chemistry Measurement NCIT:C47868
Recommended surveillance begins at diagnosis on an annual basis: plasma plant sterols and cholesterol, xanthoma burden, CBC and platelet count, and liver transaminases; long-standing untreated patients also undergo noninvasive imaging to exclude coronary and carotid plaque.
Show evidence (2 references)
PMID:23556150 SUPPORT Other
"Begin monitoring at the time of diagnosis on an annual basis: plasma concentrations of plant sterols (primarily beta-sitosterol and campesterol) and cholesterol; the size, number, and distribution of xanthomas; and CBC and platelet count, and liver transaminases (for elevation)."
GeneReviews specifies the annual surveillance schedule (plant sterols, xanthomas, CBC/platelets, liver transaminases).
PMID:23556150 SUPPORT Other
"In persons with long-standing untreated sitosterolemia, noninvasive imaging is used to exclude coronary and carotid plaque as well as valvular atherosclerotic manifestations."
GeneReviews specifies noninvasive vascular imaging surveillance in long-standing untreated disease, matching the imaging clause in the description.
🔬

Biochemical Markers

1
Plasma sitosterol and campesterol (Elevated)
Context: Markedly elevated plasma plant sterols (sitosterol and campesterol) are the defining biochemical abnormality of the disease and the pharmacodynamic readout used to monitor response to sterol-lowering therapy. Plant sterols are measured by gas chromatography (with mass spectrometry), which distinguishes them from cholesterol.
Pathograph Readouts
Pharmacodynamic Marker Of Plasma and Tissue Plant Sterol Accumulation Positive Pharmacodynamic
Higher plasma sitosterol and campesterol indicate greater plant-sterol accumulation; decreases after cholesterol-absorption inhibition indicate a reduced plant-sterol burden. The FDA lists plasma sitosterol and campesterol as a validated surrogate endpoint for homozygous sitosterolemia.
Plasma sitosterol and campesterol
Traditional Validated Surrogate Endpoint
Patients with homozygous sitosterolemia (phytosterolemia)
Plasma sitosterol and campesterol
Traditional Validated Surrogate Endpoint
Patients with homozygous sitosterolemia (phytosterolemia)
Show evidence (1 reference)
PMID:14769702 SUPPORT Human Clinical
"Ezetimibe produced significant and progressive reductions in plasma plant sterol concentrations in patients with sitosterolemia, consistent with the hypothesis that ezetimibe inhibits the intestinal absorption of plant sterols as well as cholesterol, leading to reductions in plasma concentrations."
Supports plasma plant sterols as a treatment-responsive pharmacodynamic readout.
Show evidence (1 reference)
PMID:14769702 SUPPORT Human Clinical
"Sitosterol concentrations decreased by 21% (P<0.001) in patients treated with ezetimibe compared with a nonsignificant 4% rise in those on placebo (between-group P<0.001)."
Measures plasma sitosterol (and campesterol) as treatment-responsive markers.
🔬

Diagnosis

2
Plasma Phytosterol (Plant Sterol) Assay
Diagnosis rests on demonstrating grossly elevated plasma plant sterols (sitosterol, campesterol) by gas chromatography/mass spectrometry, which separates plant sterols from cholesterol. A plant-sterol assay should be performed in patients with normolipemic xanthomas, hypercholesterolemia responding unexpectedly well to diet or cholesterol-absorption inhibitors or poorly to statins, or unexplained hemolytic anemia with macrothrombocytopenia.
clinical chemistry assessment NCIT:C47868 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:27104173 SUPPORT Other
"Plant sterol assay should be performed in patients with normocholesterolemic xanthomas, hypercholesterolemia with unexpectedly good response to dietary modifications or to cholesterol absorption inhibitors, or hypercholesterolemia with poor response to statins, or those with unexplained..."
States the clinical indications for the diagnostic plant-sterol assay.
PMID:23556150 SUPPORT Other
"The diagnosis of sitosterolemia is established in a proband with greatly increased plant sterol concentrations in plasma and/or by identification of biallelic pathogenic (or likely pathogenic) variants in ABCG5 and/or ABCG8."
GeneReviews states the diagnostic criteria (elevated plasma plant sterols and/or biallelic ABCG5/ABCG8 variants).
ABCG5/ABCG8 Molecular Genetic Testing
Biallelic pathogenic variants in ABCG5 or ABCG8 confirm the diagnosis. Targeted or high-throughput sequencing of the two genes is increasingly used as a first-line diagnostic approach.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Show evidence (1 reference)
PMID:29984642 SUPPORT Other
"recent advances in high-throughput sequencing, especially in the use of targeted sequencing of pre-specified genes, have begun to be incorporated in the first-line approach in the field of genetic disorders."
Supports targeted/high-throughput sequencing of the causal genes as a first-line diagnostic route.
📊

Prevalence

1
Worldwide
Unknown Unknown
Sitosterolemia is of unknown, likely markedly under-estimated prevalence: it is widely under-recognized because patients are misclassified as familial hypercholesterolemia or immune thrombocytopenia. Population allele-frequency analyses suggest true prevalence is much higher than the ~100 molecularly confirmed cases historically reported, but published point estimates are inconsistent across sources and are therefore not asserted here.
Show evidence (1 reference)
PMID:29984642 SUPPORT Other
"Sitosterolemia is a recessive inherited metabolic disorder of unknown prevalence, characterized by increased levels of plasma plant sterols."
States that the prevalence of sitosterolemia is unknown.
{ }

Source YAML

click to show 
name: Sitosterolemia
category: Mendelian
creation_date: "2026-07-29T19:20:00Z"
synonyms:
- Phytosterolemia
- Beta-sitosterolemia
- Shellfish sterolemia
description: >-
  Sitosterolemia (phytosterolemia) is an autosomal recessive sterol-storage
  disorder caused by biallelic loss-of-function variants in ABCG5 or ABCG8,
  which encode the two halves of the G5G8 sterol-efflux transporter expressed
  on the apical membrane of hepatocytes and enterocytes. Loss of G5G8 removes
  the pump that normally excretes cholesterol and dietary plant/shellfish
  sterols (xenosterols) back into the intestinal lumen and into bile, so
  absorption of both cholesterol and plant sterols becomes unselective and
  unrestricted. The resulting accumulation of sitosterol and campesterol in
  plasma and tissues produces tendinous and cutaneous xanthomas, premature
  atherosclerosis, arthralgia, and a distinctive hematologic syndrome of
  stomatocytic hemolytic anemia and macrothrombocytopenia. It is frequently
  misdiagnosed as familial hypercholesterolemia or immune thrombocytopenia and,
  unlike familial hypercholesterolemia, responds poorly to statins but
  dramatically to dietary sterol restriction and the cholesterol-absorption
  inhibitor ezetimibe.
disease_term:
  preferred_term: sitosterolemia
  term:
    id: MONDO:0008863
    label: sitosterolemia
parents:
- Inborn Error of Metabolism
- Lipid Metabolism Disorder
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0008863
      label: sitosterolemia
    mapping_predicate: skos:exactMatch
    mapping_source: MONDO
    mapping_justification: >-
      MONDO:0008863 (sitosterolemia) is the exact disease concept; its
      definition names biallelic ABCG5/ABCG8 variants as the cause.
references:
- reference: PMID:23556150
  title: "Sitosterolemia."
  tags:
  - GeneReviews
inheritance:
- name: Autosomal recessive inheritance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    Sitosterolemia is recessively inherited; affected individuals carry biallelic
    (homozygous or compound heterozygous) loss-of-function variants in either
    ABCG5 or ABCG8. The two genes lie head-to-head at 2p21 and their products
    form an obligate heterodimer, so a defect in either gene produces the same
    disease.
  evidence:
  - reference: PMID:14769702
    reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sitosterolemia is a recessively inherited disorder that results from mutations in either ABCG5 or G8 proteins, with hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
    explanation: States the autosomal recessive inheritance and the ABCG5/ABCG8 causal basis.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Sitosterolemia is inherited in an autosomal recessive manner."
    explanation: GeneReviews confirms autosomal recessive inheritance.
genetic:
- name: ABCG5
  relationship_type: CAUSATIVE
  gene_term:
    preferred_term: ABCG5
    term:
      id: hgnc:13886
      label: ABCG5
  notes: >-
    Biallelic loss-of-function variants in ABCG5 (sterolin-1) cause
    sitosterolemia. ABCG5 variants are reported more often in individuals of
    East and South Asian ancestry.
  evidence:
  - reference: PMID:33807969
    reference_title: "Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
    explanation: Supports ABCG5 as one of the two causal genes.
- name: ABCG8
  relationship_type: CAUSATIVE
  gene_term:
    preferred_term: ABCG8
    term:
      id: hgnc:13887
      label: ABCG8
  notes: >-
    Biallelic loss-of-function variants in ABCG8 (sterolin-2) cause
    sitosterolemia. Reported variants include nonsense, missense, and
    frameshift changes; a recurrent nonsense allele, W361X, has been observed in
    multiple unrelated pedigrees. ABCG8 variants are reported more often in
    individuals of Northern European ancestry, and founder alleles with elevated
    carrier frequencies occur in several isolated populations.
  evidence:
  - reference: PMID:33807969
    reference_title: "Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
    explanation: Supports ABCG8 as one of the two causal genes.
  - reference: PMID:16029460
    reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "two pedigrees showed the common W361X mutation in ABCG8"
    explanation: Documents a recurrent (common) ABCG8 nonsense allele, W361X, across unrelated pedigrees.
pathophysiology:
- name: G5G8 Sterol-Efflux Transporter Deficiency
  biological_scale: MOLECULAR
  description: >-
    Biallelic loss-of-function variants in ABCG5 or ABCG8 abolish the G5G8
    heterodimer, a pair of ATP-binding-cassette half-transporters that assemble
    and traffic to the apical (canalicular/brush-border) membrane of hepatocytes
    and enterocytes, where they pump cholesterol and plant/shellfish sterols out
    of the cell into bile and into the intestinal lumen. Because either subunit
    is required for a functional pump, a defect in either gene inactivates
    sterol efflux.
  genes:
  - preferred_term: ABCG5
    term:
      id: hgnc:13886
      label: ABCG5
  - preferred_term: ABCG8
    term:
      id: hgnc:13887
      label: ABCG8
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  - preferred_term: enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  molecular_functions:
  - preferred_term: ABC-type sterol transporter activity
    term:
      id: GO:0034041
      label: ABC-type sterol transporter activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: sterol transport
    term:
      id: GO:0015918
      label: sterol transport
    modifier: DECREASED
  evidence:
  - reference: PMID:33807969
    reference_title: "Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "ABCG5 ABCG8 encodes a pair of ABC half transporters that form a heterodimer (G5G8), which then traffics to the surface of hepatocytes and enterocytes and promotes the secretion of cholesterol and xenosterols into the bile and the intestinal lumen."
    explanation: Identifies the G5G8 heterodimer sterol-efflux transporter disrupted in sitosterolemia.
  downstream:
  - target: Intestinal Xenosterol Hyperabsorption
    description: Loss of enterocyte G5G8 efflux back into the gut lumen removes the brake on dietary sterol absorption.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:27104173
      reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Sitosterolemia is caused by increased intestinal absorption and decreased biliary excretion of sterols resulting from biallelic mutations in either ABCG5 or ABCG8, which encode the sterol efflux transporter ABCG5 and ABCG8."
      explanation: Links loss of the sterol-efflux transporter to increased intestinal absorption.
  - target: Reduced Biliary Sterol Excretion
    description: Loss of hepatocyte G5G8 efflux into bile removes the route for hepatic disposal of xenosterols.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:27104173
      reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Sitosterolemia is caused by increased intestinal absorption and decreased biliary excretion of sterols resulting from biallelic mutations in either ABCG5 or ABCG8, which encode the sterol efflux transporter ABCG5 and ABCG8."
      explanation: Links loss of the sterol-efflux transporter to decreased biliary sterol excretion.
- name: Intestinal Xenosterol Hyperabsorption
  biological_scale: CELLULAR
  description: >-
    Without enterocyte G5G8 to pump absorbed sterols back into the lumen, the
    intestine absorbs both cholesterol and plant/shellfish sterols in an
    unselective, unrestricted manner. Normally the body excludes ~95% of dietary
    plant sterols; in sitosterolemia their fractional absorption rises markedly.
  cell_types:
  - preferred_term: enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  evidence:
  - reference: PMID:16029460
    reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the absorption of both cholesterol and plant-derived cholesterol-like molecules at the gut is unselective and unrestricted."
    explanation: Describes the unselective, unrestricted intestinal sterol absorption central to the disease.
  downstream:
  - target: Plasma and Tissue Plant Sterol Accumulation
    description: Hyperabsorbed xenosterols enter the circulation and accumulate.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:14769702
      reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
      explanation: Hyperabsorption of dietary sterols is the mechanism producing the plasma/tissue sterol burden.
- name: Reduced Biliary Sterol Excretion
  biological_scale: CELLULAR
  description: >-
    Loss of hepatocyte canalicular G5G8 reduces secretion of cholesterol and
    plant sterols into bile, removing the principal route by which the liver
    would otherwise dispose of the excess absorbed xenosterols. Impaired biliary
    efflux compounds the intestinal hyperabsorption to drive net sterol
    retention.
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  evidence:
  - reference: PMID:14769702
    reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
    explanation: Documents decreased hepatic (biliary) excretion of plant sterols and cholesterol.
  downstream:
  - target: Plasma and Tissue Plant Sterol Accumulation
    description: Reduced biliary disposal leaves absorbed sterols to accumulate in plasma and tissues.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:14769702
      reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
      explanation: Decreased hepatic (biliary) excretion is the second mechanism driving net sterol accumulation.
- name: Plasma and Tissue Plant Sterol Accumulation
  biological_scale: ORGANISM
  description: >-
    The combination of intestinal hyperabsorption and reduced biliary excretion
    produces markedly elevated plasma and tissue levels of plant sterols —
    especially sitosterol, campesterol, and stigmasterol (and their
    metabolites). This sustained xenosterol accumulation is the central
    biochemical state of the disease and the hub from which the vascular,
    cutaneous, and hematologic manifestations branch.
  chemical_entities:
  - preferred_term: sitosterol
    term:
      id: CHEBI:27693
      label: sitosterol
    modifier: INCREASED
  - preferred_term: campesterol
    term:
      id: CHEBI:28623
      label: campesterol
    modifier: INCREASED
  - preferred_term: stigmasterol
    term:
      id: CHEBI:28824
      label: stigmasterol
    modifier: INCREASED
  evidence:
  - reference: PMID:33807969
    reference_title: "Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
    explanation: Establishes plasma and tissue xenosterol accumulation as the central disease state.
  - reference: PMID:14769702
    reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "markedly elevated plasma and tissue sitosterol and campesterol levels"
    explanation: Identifies sitosterol and campesterol as the accumulating sterols.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "increased plasma concentrations of plant sterols (especially sitosterol, campesterol, and stigmasterol)"
    explanation: GeneReviews names sitosterol, campesterol, and stigmasterol as the accumulating plant sterols.
  downstream:
  - target: Sterol Intercalation into Blood Cell Membranes
    description: Circulating free plant sterols partition into the plasma membranes of blood cells and their marrow precursors.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23926302
      reference_title: "Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "accumulation of free sterols in platelet plasma membranes, leading to hyperactivatable platelets"
      explanation: Shows accumulated sterols intercalate into platelet plasma membranes.
  - target: Xanthoma Formation
    description: Tissue sterol deposition in tendon and skin produces xanthomas.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:29984642
      reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
      explanation: Links the accumulated sterols to tendinous and cutaneous xanthomas.
  - target: Accelerated Atherosclerosis
    description: Deposition of plant sterols and cholesterol in the arterial wall drives premature atherosclerosis.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:14769702
      reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "As a consequence of markedly elevated plasma and tissue sitosterol and campesterol levels, premature atherosclerosis develops."
      explanation: Directly links the sterol accumulation to premature atherosclerosis.
- name: Sterol Intercalation into Blood Cell Membranes
  biological_scale: CELLULAR
  description: >-
    Free plant sterols intercalate into the plasma membranes of erythrocytes,
    platelets, and megakaryocytes, altering membrane sterol composition and
    biophysics. This membrane remodeling is the proximate cause of the two
    distinctive hematologic manifestations of sitosterolemia and is reversible
    when plant-sterol levels are lowered.
  cell_types:
  - preferred_term: platelet
    term:
      id: CL:0000233
      label: platelet
  - preferred_term: erythrocyte
    term:
      id: CL:0000232
      label: erythrocyte
  evidence:
  - reference: PMID:23926302
    reference_title: "Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Intercalation of plant sterols into the plasma membrane therefore results in dysregulation of multiple platelet activation pathways, leading to macrothrombocytopenia and bleeding."
    explanation: Establishes membrane intercalation of plant sterols as the mechanism disrupting blood-cell function.
  downstream:
  - target: Megakaryocyte Demarcation Membrane System Defect
    description: Sterol-enriched membranes disrupt the megakaryocyte demarcation membrane system and platelet formation.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:18156627
      reference_title: "Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "defective megakaryocyte development with deterioration of the demarcation membrane system was evident."
      explanation: Links membrane sterol accumulation to defective megakaryocyte development.
  - target: Stomatocytic Hemolysis
    description: Erythrocyte membrane sterol enrichment produces stomatocyte morphology and premature hemolysis.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:16029460
      reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Mediterranean stomatocytosis or Mediterranean macrothrombocytopenia is a poorly understood haematological condition that combines stomatocytic haemolysis with the presence of very large platelets."
      explanation: Ties the hematologic syndrome (stomatocytic hemolysis plus giant platelets) to phytosterol excess.
- name: Megakaryocyte Demarcation Membrane System Defect
  biological_scale: CELLULAR
  description: >-
    Plant-sterol enrichment of the megakaryocyte membrane causes defective
    megakaryocyte development with deterioration of the demarcation membrane
    system (the internal membrane reservoir from which platelets are formed) —
    not a reduction in megakaryocyte numbers or progenitors. Critically,
    bone-marrow-transplant crossover experiments show the phenotype tracks the
    recipient: irradiated wild-type mice given Abcg5-null marrow have normal
    platelets, whereas Abcg5-null mice given wild-type marrow remain affected.
    The defect is therefore extrinsic — driven by the host plasma plant-sterol
    environment rather than being cell-intrinsic to the hematopoietic
    compartment — which is why it reverses when plant sterols are lowered.
  cell_types:
  - preferred_term: megakaryocyte
    term:
      id: CL:0000556
      label: megakaryocyte
  evidence:
  - reference: PMID:18156627
    reference_title: "Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "defective megakaryocyte development with deterioration of the demarcation membrane system was evident."
    explanation: Establishes the megakaryocyte demarcation-membrane-system defect (not reduced megakaryocyte numbers) as the lesion.
  - reference: PMID:18156627
    reference_title: "Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Lethally irradiated wild-type mice transplanted with bone marrow from Abcg5(-/-) mice displayed normal platelets, whereas Abcg5(-/-) mice transplanted with wild-type bone marrow still showed macrothrombocytopenia."
    explanation: The crossover result shows the phenotype tracks the recipient's plasma-sterol environment (extrinsic defect), not the donor marrow (not cell-intrinsic).
  downstream:
  - target: Macrothrombocytopenia
    description: The demarcation-membrane-system defect yields enlarged, reduced-count platelets.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:18156627
      reference_title: "Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Macrothrombocytopenia was not due to decreased numbers of megakaryocytes or their progenitors, but defective megakaryocyte development with deterioration of the demarcation membrane system was evident."
      explanation: Attributes the macrothrombocytopenia specifically to the megakaryocyte demarcation-membrane-system defect rather than reduced megakaryocyte numbers.
- name: Macrothrombocytopenia
  biological_scale: ORGANISM
  description: >-
    The megakaryocyte demarcation-membrane-system defect yields a reduced count
    of enlarged (giant) platelets — macrothrombocytopenia. In humans this is the
    platelet component of the "Mediterranean stomatocytosis/macrothrombocytopenia"
    presentation, and plasma soluble GPIba (a marker of the accompanying platelet
    passivation) correlates with plasma sitosterol. Because the driver is the
    plasma plant-sterol burden, the abnormality reverses when plant sterols are
    lowered. (The parallel sterol-induced platelet hyperreactivity/passivation
    that explains the bleeding tendency is elaborated in the associated
    model-mismatch discussion.)
  cell_types:
  - preferred_term: platelet
    term:
      id: CL:0000233
      label: platelet
  evidence:
  - reference: PMID:18156627
    reference_title: "Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "accumulation of plant sterols is responsible for development of macrothrombocytopenia in sitosterolemia"
    explanation: Establishes plant-sterol accumulation as the cause of macrothrombocytopenia.
  - reference: PMID:16029460
    reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mediterranean stomatocytosis/macrothrombocytopenia is caused by an excess of phytosterols in the blood"
    explanation: Confirms in humans that the macrothrombocytopenia is caused by excess phytosterols.
  - reference: PMID:23926302
    reference_title: "Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Plasma levels of soluble GPIbα were strongly correlated with plasma sitosterol levels in samples from human sitosterolemic patients"
    explanation: Human correlation between plasma soluble GPIba and plasma sitosterol, bridging the mouse mechanism to human platelet passivation.
- name: Stomatocytic Hemolysis
  biological_scale: CELLULAR
  description: >-
    Enrichment of the erythrocyte membrane with plant sterols alters red-cell
    shape (stomatocytosis) and reduces cell survival, producing a hemolytic
    anemia frequently accompanied by splenomegaly. The stomatocytic hemolysis
    combined with macrothrombocytopenia constitutes the characteristic
    "Mediterranean stomatocytosis/macrothrombocytopenia" hematologic
    presentation of the disease.
  cell_types:
  - preferred_term: erythrocyte
    term:
      id: CL:0000232
      label: erythrocyte
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
    explanation: Documents hemolytic anemia with stomatocytosis as a hematologic manifestation.
- name: Xanthoma Formation
  biological_scale: TISSUE
  description: >-
    Sterol deposition in tendons and skin produces tendinous and cutaneous
    xanthomas, the classic clinical sign that historically prompted sterol
    testing. In children xanthomas can appear even when cholesterol is normal
    (normolipemic xanthomas), a presentation that should trigger a plant-sterol
    assay.
  cell_types:
  - preferred_term: macrophage
    term:
      id: CL:0000235
      label: macrophage
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
    explanation: Identifies tendinous and cutaneous xanthomas as a main clinical feature.
- name: Accelerated Atherosclerosis
  biological_scale: TISSUE
  description: >-
    Chronic accumulation of plant sterols and cholesterol drives accelerated
    atherosclerosis, which can present as premature coronary artery disease,
    aortic disease, and, in severe childhood-onset cases, premature cardiac
    death. The phenotype ranges from near-asymptomatic to severe, and the
    accelerated atherosclerosis is the principal cause of morbidity and
    mortality.
  cell_types:
  - preferred_term: macrophage
    term:
      id: CL:0000235
      label: macrophage
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "increased plant sterol levels, xanthomas, and accelerated atherosclerosis"
    explanation: Names accelerated atherosclerosis as a defining consequence of plant-sterol accumulation.
phenotypes:
- category: Dermatological
  name: Tendon and cutaneous xanthomas
  phenotype_term:
    preferred_term: Tendinous xanthomatosis
    term:
      id: HP:0010874
      label: Tendon xanthomatosis
    onset:
      onset_category: CHILDHOOD
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
    explanation: Tendinous and cutaneous xanthomas are a main clinical feature.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Tendon xanthomas or tuberous (i.e., planar) xanthomas that can occur in childhood and in unusual locations (heels, knees, elbows, and buttocks)"
    explanation: GeneReviews documents childhood-onset tendon xanthomas.
- category: Dermatological
  name: Tuberous (planar) xanthomas
  phenotype_term:
    preferred_term: Tuberous xanthoma
    term:
      id: HP:0031290
      label: Tuberous xanthoma
    onset:
      onset_category: CHILDHOOD
  evidence:
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Tendon xanthomas or tuberous (i.e., planar) xanthomas that can occur in childhood and in unusual locations (heels, knees, elbows, and buttocks)"
    explanation: GeneReviews documents tuberous (planar) xanthomas in childhood and in unusual locations.
- category: Cardiovascular
  name: Premature atherosclerosis and coronary artery disease
  phenotype_term:
    preferred_term: Premature coronary artery atherosclerosis
    term:
      id: HP:0005181
      label: Premature coronary artery atherosclerosis
  evidence:
  - reference: PMID:14769702
    reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As a consequence of markedly elevated plasma and tissue sitosterol and campesterol levels, premature atherosclerosis develops."
    explanation: Premature atherosclerosis is a direct consequence of the elevated sterols.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Premature atherosclerosis, which can lead to angina, aortic valve involvement, myocardial infarction, and sudden death"
    explanation: GeneReviews details the severe cardiovascular sequelae of the premature atherosclerosis.
- category: Musculoskeletal
  name: Arthralgia or arthritis
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Arthralgia
    term:
      id: HP:0002829
      label: Arthralgia
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis."
    explanation: Arthritis or arthralgia is listed among the clinical features.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Arthritis, arthralgias, and splenomegaly may sometimes be seen"
    explanation: GeneReviews notes arthritis/arthralgias are seen only sometimes, supporting an OCCASIONAL rather than FREQUENT band.
- category: Metabolic
  name: Hypercholesterolemia
  description: >-
    Severe hypercholesterolemia, especially in children, can dominate the
    presentation and is often mistaken for familial hypercholesterolemia; unlike
    familial hypercholesterolemia it responds dramatically to a low-cholesterol
    diet and to ezetimibe rather than to statins.
  phenotype_term:
    preferred_term: Hypercholesterolemia
    term:
      id: HP:0003124
      label: Hypercholesterolemia
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "severe hypercholesterolemia have been reported in patients with sitosterolemia, especially in children."
    explanation: Documents severe hypercholesterolemia, particularly in children.
- category: Hematological
  name: Hemolytic anemia
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Hemolytic anemia
    term:
      id: HP:0001878
      label: Hemolytic anemia
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
    explanation: Hemolytic anemia is a characteristic hematologic manifestation.
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases."
    explanation: Quantifies the hematologic abnormalities (hemolytic anemia and macrothrombocytopenia) at 25-35% of patients, supporting the frequency band.
- category: Hematological
  name: Stomatocytosis
  phenotype_term:
    preferred_term: Stomatocytosis
    term:
      id: HP:0004446
      label: Stomatocytosis
  evidence:
  - reference: PMID:16029460
    reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "combines stomatocytic haemolysis with the presence of very large platelets"
    explanation: Stomatocytes on smear are the erythrocyte-morphology component of the hematologic syndrome.
- category: Hematological
  name: Thrombocytopenia
  frequency: OCCASIONAL
  description: >-
    Part of the macrothrombocytopenia (giant platelets with reduced count) that,
    together with stomatocytic hemolysis, forms the characteristic hematologic
    presentation and is often misdiagnosed as immune thrombocytopenia.
  phenotype_term:
    preferred_term: Thrombocytopenia
    term:
      id: HP:0001873
      label: Thrombocytopenia
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases."
    explanation: Supports thrombocytopenia as the platelet-count component of macrothrombocytopenia.
- category: Hematological
  name: Giant platelets
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Giant platelets
    term:
      id: HP:0001902
      label: Giant platelets
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "the peripheral blood smear is essential and reveals giant platelets and stomatocytes."
    explanation: Giant platelets are the platelet-morphology component of macrothrombocytopenia.
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases."
    explanation: The 25-35% figure covers macrothrombocytopenia (giant platelets + low count), supporting the frequency band.
- category: Hematological
  name: Splenomegaly
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Splenomegaly
    term:
      id: HP:0001744
      label: Splenomegaly
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
    explanation: Splenomegaly accompanies the hemolytic component of the hematologic syndrome.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Arthritis, arthralgias, and splenomegaly may sometimes be seen"
    explanation: GeneReviews notes splenomegaly is seen only sometimes, supporting the OCCASIONAL band.
- category: Hematological
  name: Abnormal bleeding
  description: >-
    A bleeding tendency out of proportion to the platelet count reflects
    intrinsic platelet dysfunction (sterol-induced platelet hyperreactivity and
    passivation) in addition to the reduced platelet number.
  phenotype_term:
    preferred_term: Abnormal bleeding
    term:
      id: HP:0001892
      label: Abnormal bleeding
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hematologic manifestations include hemolytic anemia with stomatocytosis, macrothrombocytopenia, splenomegaly, and abnormal bleeding."
    explanation: Abnormal bleeding is listed among the hematologic manifestations.
- category: Hepatic
  name: Elevated hepatic transaminases
  description: >-
    Liver involvement is increasingly recognized: transaminase elevation is part
    of routine surveillance, and undiagnosed sitosterolemia has been proposed as
    a cause of otherwise "idiopathic" liver disease.
  phenotype_term:
    preferred_term: Elevated circulating hepatic transaminase concentration
    term:
      id: HP:0002910
      label: Elevated circulating hepatic transaminase concentration
  evidence:
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: 'one study has concluded that "idiopathic" liver disease could be undiagnosed sitosterolemia.'
    explanation: GeneReviews flags hepatic involvement, proposing sitosterolemia as a cause of "idiopathic" liver disease.
biochemical:
- name: Plasma sitosterol and campesterol
  presence: Elevated
  context: >-
    Markedly elevated plasma plant sterols (sitosterol and campesterol) are the
    defining biochemical abnormality of the disease and the pharmacodynamic
    readout used to monitor response to sterol-lowering therapy. Plant sterols
    are measured by gas chromatography (with mass spectrometry), which
    distinguishes them from cholesterol.
  readouts:
  - target: Plasma and Tissue Plant Sterol Accumulation
    relationship: PHARMACODYNAMIC_MARKER_OF
    direction: POSITIVE
    endpoint_context: PHARMACODYNAMIC
    regulatory_endpoint_refs:
    - FDA-SE-adult-noncancer-037
    - FDA-SE-pediatric-noncancer-026
    interpretation: >-
      Higher plasma sitosterol and campesterol indicate greater plant-sterol
      accumulation; decreases after cholesterol-absorption inhibition indicate a
      reduced plant-sterol burden. The FDA lists plasma sitosterol and
      campesterol as a validated surrogate endpoint for homozygous
      sitosterolemia.
    evidence:
    - reference: PMID:14769702
      reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Ezetimibe produced significant and progressive reductions in plasma plant sterol concentrations in patients with sitosterolemia, consistent with the hypothesis that ezetimibe inhibits the intestinal absorption of plant sterols as well as cholesterol, leading to reductions in plasma concentrations."
      explanation: Supports plasma plant sterols as a treatment-responsive pharmacodynamic readout.
  evidence:
  - reference: PMID:14769702
    reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sitosterol concentrations decreased by 21% (P<0.001) in patients treated with ezetimibe compared with a nonsignificant 4% rise in those on placebo (between-group P<0.001)."
    explanation: Measures plasma sitosterol (and campesterol) as treatment-responsive markers.
diagnosis:
- name: Plasma Phytosterol (Plant Sterol) Assay
  description: >-
    Diagnosis rests on demonstrating grossly elevated plasma plant sterols
    (sitosterol, campesterol) by gas chromatography/mass spectrometry, which
    separates plant sterols from cholesterol. A plant-sterol assay should be
    performed in patients with normolipemic xanthomas, hypercholesterolemia
    responding unexpectedly well to diet or cholesterol-absorption inhibitors or
    poorly to statins, or unexplained hemolytic anemia with macrothrombocytopenia.
  diagnosis_term:
    preferred_term: clinical chemistry assessment
    term:
      id: NCIT:C47868
      label: Blood Chemistry Measurement
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Plant sterol assay should be performed in patients with normocholesterolemic xanthomas, hypercholesterolemia with unexpectedly good response to dietary modifications or to cholesterol absorption inhibitors, or hypercholesterolemia with poor response to statins, or those with unexplained hemolytic anemia and macrothrombocytopenia."
    explanation: States the clinical indications for the diagnostic plant-sterol assay.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The diagnosis of sitosterolemia is established in a proband with greatly increased plant sterol concentrations in plasma and/or by identification of biallelic pathogenic (or likely pathogenic) variants in ABCG5 and/or ABCG8."
    explanation: GeneReviews states the diagnostic criteria (elevated plasma plant sterols and/or biallelic ABCG5/ABCG8 variants).
- name: ABCG5/ABCG8 Molecular Genetic Testing
  description: >-
    Biallelic pathogenic variants in ABCG5 or ABCG8 confirm the diagnosis.
    Targeted or high-throughput sequencing of the two genes is increasingly used
    as a first-line diagnostic approach.
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "recent advances in high-throughput sequencing, especially in the use of targeted sequencing of pre-specified genes, have begun to be incorporated in the first-line approach in the field of genetic disorders."
    explanation: Supports targeted/high-throughput sequencing of the causal genes as a first-line diagnostic route.
treatments:
- name: Ezetimibe
  description: >-
    Ezetimibe, a cholesterol-absorption inhibitor (NPC1L1 antagonist), lowers
    intestinal absorption of both cholesterol and plant sterols, producing
    significant and progressive reductions in plasma plant sterols. In animal
    models and patients it also reverses the macrothrombocytopenia, tying the
    hematologic disease to the plant-sterol burden. It is a mainstay of therapy.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: ezetimibe
      term:
        id: CHEBI:49040
        label: ezetimibe
  target_mechanisms:
  - target: Plasma and Tissue Plant Sterol Accumulation
    treatment_effect: INHIBITS
    description: >-
      By blocking intestinal sterol absorption, ezetimibe lowers the plasma and
      tissue plant-sterol accumulation that this node represents, which in turn
      relieves the downstream hematologic and vascular disease.
  evidence:
  - reference: PMID:14769702
    reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CONCLUSIONS: Ezetimibe produced significant and progressive reductions in plasma plant sterol concentrations in patients with sitosterolemia, consistent with the hypothesis that ezetimibe inhibits the intestinal absorption of plant sterols as well as cholesterol, leading to reductions in plasma concentrations."
    explanation: Randomized trial establishing ezetimibe as a plant-sterol-lowering therapy.
  - reference: PMID:18156627
    reference_title: "Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Treatment with the sterol absorption inhibitor ezetimibe rapidly reversed macrothrombocytopenia in Abcg5(-/-) mice concomitant with a strong decrease in plasma plant sterols."
    explanation: Shows ezetimibe reverses the hematologic phenotype in a mouse model by lowering plant sterols.
- name: Dietary Sterol Restriction
  description: >-
    Dietary restriction of both cholesterol and plant sterols is a first-line
    intervention. Foods rich in plant sterols (vegetable oils, wheat germ, nuts,
    seeds, avocado, margarine, chocolate) and shellfish sterols are limited.
    Importantly, plant-stanol-fortified margarines and products marketed to lower
    cholesterol in the general population are contraindicated in sitosterolemia,
    because affected patients cannot excrete the stanols and their accumulation
    is exacerbated — an "agents to avoid" caveat unique to this disorder.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  target_mechanisms:
  - target: Intestinal Xenosterol Hyperabsorption
    treatment_effect: INHIBITS
    description: >-
      Lowering dietary plant-sterol intake reduces the substrate available for
      the unrestricted intestinal absorption that drives the disease.
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The mainstay of therapy includes dietary restriction of both cholesterol and plant sterols and the sterol absorption inhibitor, ezetimibe."
    explanation: Dietary restriction of cholesterol and plant sterols is a mainstay of therapy.
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Margarines and other products containing stanols (e.g., campestanol and sitostanol) that are recommended for use by persons with hypercholesterolemia are contraindicated as they can exacerbate plant stanol accumulation."
    explanation: GeneReviews flags plant-stanol products as contraindicated ("agents to avoid") because they exacerbate stanol accumulation.
- name: Partial Ileal Bypass Surgery
  description: >-
    Partial ileal bypass surgery, which interrupts enterohepatic sterol
    recycling, may be considered as a last resort in patients who respond poorly
    to maximal dietary and pharmacologic therapy.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  evidence:
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Partial ileal bypass surgery may be considered as a last resort for those with poor response to maximal therapies."
    explanation: GeneReviews lists partial ileal bypass surgery as a last-resort treatment.
- name: Bile Acid Sequestrant
  description: >-
    Bile acid sequestrants (e.g., cholestyramine, colestimide, colesevelam)
    interrupt enterohepatic sterol recycling and, together with a low-cholesterol
    diet, dramatically lower the hypercholesterolemia seen in sitosterolemia.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: bile acid sequestrant
      term:
        id: NCIT:C98148
        label: Bile Acid Sequestrant
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hypercholesterolemia in patients with sitosterolemia is dramatically responsive to low cholesterol diet and bile acid sequestrants."
    explanation: Bile acid sequestrants (with diet) dramatically lower the hypercholesterolemia.
- name: Statin Therapy (Ineffective)
  description: >-
    HMG-CoA reductase inhibitors (statins), the standard therapy for familial
    hypercholesterolemia, are largely ineffective in sitosterolemia because the
    disease is driven by sterol hyperabsorption rather than endogenous synthesis.
    The lack of statin response is a key discriminator from homozygous familial
    hypercholesterolemia and should prompt a plant-sterol assay.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  evidence:
  - reference: PMID:16029460
    reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "phytosterolaemia, which does not respond to standard statin treatment"
    explanation: Statins do not work in sitosterolemia; this negative response is a diagnostic discriminator from familial hypercholesterolemia.
- name: Disease Surveillance and Monitoring
  description: >-
    Recommended surveillance begins at diagnosis on an annual basis: plasma
    plant sterols and cholesterol, xanthoma burden, CBC and platelet count, and
    liver transaminases; long-standing untreated patients also undergo
    noninvasive imaging to exclude coronary and carotid plaque.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: clinical chemistry assessment
    term:
      id: NCIT:C47868
      label: Blood Chemistry Measurement
  evidence:
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Begin monitoring at the time of diagnosis on an annual basis: plasma concentrations of plant sterols (primarily beta-sitosterol and campesterol) and cholesterol; the size, number, and distribution of xanthomas; and CBC and platelet count, and liver transaminases (for elevation)."
    explanation: GeneReviews specifies the annual surveillance schedule (plant sterols, xanthomas, CBC/platelets, liver transaminases).
  - reference: PMID:23556150
    reference_title: "Sitosterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In persons with long-standing untreated sitosterolemia, noninvasive imaging is used to exclude coronary and carotid plaque as well as valvular atherosclerotic manifestations."
    explanation: GeneReviews specifies noninvasive vascular imaging surveillance in long-standing untreated disease, matching the imaging clause in the description.
prevalence:
- population: Worldwide
  measure_type: UNKNOWN
  prevalence_class: UNKNOWN
  notes: >-
    Sitosterolemia is of unknown, likely markedly under-estimated prevalence:
    it is widely under-recognized because patients are misclassified as
    familial hypercholesterolemia or immune thrombocytopenia. Population
    allele-frequency analyses suggest true prevalence is much higher than the
    ~100 molecularly confirmed cases historically reported, but published point
    estimates are inconsistent across sources and are therefore not asserted here.
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Sitosterolemia is a recessive inherited metabolic disorder of unknown prevalence, characterized by increased levels of plasma plant sterols."
    explanation: States that the prevalence of sitosterolemia is unknown.
classifications:
  harrisons_chapter:
  - classification_value: ENDOCRINOLOGY_METABOLISM
    evidence:
    - reference: PMID:33807969
      reference_title: "Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8."
      explanation: Characterizes sitosterolemia as a lipid/metabolic disorder.
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    evidence:
    - reference: PMID:14769702
      reference_title: "Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Sitosterolemia is a recessively inherited disorder that results from mutations in either ABCG5 or G8 proteins, with hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol."
      explanation: Characterizes sitosterolemia as a recessively inherited (Mendelian) disorder.
discussions:
- discussion_id: gap_sitosterolemia_phenotypic_heterogeneity
  prompt: >-
    Why is sitosterolemia so phenotypically heterogeneous — ranging from almost
    asymptomatic individuals to children with severe hypercholesterolemia,
    accelerated atherosclerosis, and premature cardiac death — and what
    genotype/modifier factors (specific ABCG5 vs ABCG8 variants, residual
    transporter activity, diet, other lipid genes) determine severity?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Plasma and Tissue Plant Sterol Accumulation
  - pathophysiology#Accelerated Atherosclerosis
  rationale: >-
    Plasma plant-sterol levels do not fully predict clinical severity, and the
    functional impact of most ABCG5/ABCG8 variants on transporter
    structure-function is unknown, so the determinants of the extreme phenotypic
    range are unresolved. Clarifying them would improve risk stratification and
    the timing/intensity of sterol-lowering therapy.
  evidence:
  - reference: PMID:27104173
    reference_title: "Sitosterolemia: a review and update of pathophysiology, clinical spectrum, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Patients with sitosterolemia show extreme phenotypic heterogeneity, ranging from almost asymptomatic individuals to those with severe hypercholesterolemia leading to accelerated atherosclerosis and premature cardiac death."
    explanation: Documents the extreme phenotypic heterogeneity that defines the gap.
  - reference: PMID:33807969
    reference_title: "Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The impact of these variants on G5G8 structure and activity are largely unknown."
    explanation: The unknown functional impact of most variants underlies the unexplained severity range.
- discussion_id: interp_sitosterolemia_misdiagnosis_FH_ITP
  prompt: >-
    Sitosterolemia is repeatedly misdiagnosed as familial hypercholesterolemia
    (especially in children) or as immune thrombocytopenia. What discriminating
    features should trigger a plant-sterol assay, and how does the statin
    non-response distinguish it?
  kind: INTERPRETATION
  status: OPEN
  attaches_to:
  - pathophysiology#Plasma and Tissue Plant Sterol Accumulation
  - phenotypes#Thrombocytopenia
  rationale: >-
    Because sitosterolemia shares hypercholesterolemia and xanthomas with
    familial hypercholesterolemia and shares low platelet counts with immune
    thrombocytopenia, it is frequently misclassified — and mistreated. A key
    discriminator is that, unlike familial hypercholesterolemia, sitosterolemia
    responds poorly to statins but dramatically to diet and ezetimibe;
    normolipemic xanthomas, giant platelets/stomatocytes on smear, and a
    treatment response mismatch should prompt a plant-sterol assay. Capturing
    this pitfall is important for a knowledge base intended to support diagnosis.
  evidence:
  - reference: PMID:29984642
    reference_title: "Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "making it possible for sitosterolemia to be misdiagnosed as homozygous FH, especially in pediatric patients."
    explanation: Documents misdiagnosis as homozygous familial hypercholesterolemia, especially in children.
  - reference: PMID:16029460
    reference_title: "Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "phytosterolaemia, which does not respond to standard statin treatment, can be diagnosed via the distinctive haematology described here, even when the cholesterol is normal"
    explanation: The statin non-response and distinctive hematology are the discriminating features that should prompt testing.
- discussion_id: model_mismatch_sitosterolemia_platelet_passivation
  prompt: >-
    Is the detailed platelet-passivation mechanism worked out in
    Abcg5/Abcg8-deficient mice (constitutive fibrinogen binding to activated integrin
    alphaIIbbeta3, integrin internalization, microparticle generation, filamin
    relocalization, and GPIba shedding) the operative mechanism of
    macrothrombocytopenia and bleeding in human sitosterolemia?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Sterol Intercalation into Blood Cell Membranes
  - pathophysiology#Megakaryocyte Demarcation Membrane System Defect
  - pathophysiology#Macrothrombocytopenia
  rationale: >-
    The step-by-step molecular mechanism linking membrane sterol intercalation
    to platelet dysfunction is established in mouse models; direct human
    confirmation is limited. The strongest human bridge is a correlation between
    plasma soluble GPIba and plasma sitosterol in patient samples, which is
    suggestive but not a demonstration that the full murine mechanism operates
    in human platelets. Whether the murine platelet-passivation cascade is the
    human mechanism is the open question.
  proposed_experiments:
  - experiment_id: exp_sitosterolemia_human_platelet_passivation
    name: Human platelet-passivation mechanism verification
    description: >-
      Measure integrin alphaIIbbeta3 activation state, GPIba surface
      density/shedding, and platelet-derived microparticles in platelets from
      human sitosterolemia patients before and after ezetimibe-induced
      plant-sterol lowering, to test whether the murine platelet-passivation
      cascade operates in human platelets and reverses with treatment.
  - experiment_id: exp_sitosterolemia_membrane_sterol_correlation
    name: Platelet-membrane sterol versus phenotype correlation
    description: >-
      Correlate platelet-membrane free-sterol content with platelet size,
      hyperreactivity, and bleeding phenotype across a human patient cohort to
      establish the human dose-response between membrane sterol loading and
      platelet dysfunction.
  evidence:
  - reference: PMID:23926302
    reference_title: "Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Plasma levels of soluble GPIbα were strongly correlated with plasma sitosterol levels in samples from human sitosterolemic patients, implicating a similar mechanism of sterol-induced platelet passivation in the human disease."
    explanation: >-
      The mechanism is defined in mice; the only direct human evidence is a
      correlation between soluble GPIba and sitosterol, so the translational
      validity of the full mechanism is the mismatch.
datasets: []
📚

References & Deep Research

References

1
PMID:23556150
Sitosterolemia.
No top-level findings curated for this source.

Deep Research

1
Claude Code ▸
Sitosterolemia (Phytosterolemia): Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 24 citations 2026-07-29T19:07:32.576666

Sitosterolemia (Phytosterolemia): Comprehensive Research Report

Executive Summary

Sitosterolemia is a rare autosomal recessive sterol-storage disorder caused by biallelic loss-of-function variants in ABCG5 or ABCG8, which together encode the heterodimeric "sterolin" ABC half-transporter (ABCG5/ABCG8) responsible for effluxing dietary plant sterols (and excess cholesterol) from enterocytes into the intestinal lumen and from hepatocytes into bile. Loss of this transporter causes massive intestinal hyperabsorption and defective biliary/fecal excretion of plant sterols (sitosterol, campesterol, stigmasterol) and shellfish sterols, producing hypercholesterolemia, tendon/tuberous xanthomas, premature atherosclerosis, and a distinctive hematologic phenotype (hemolytic anemia with stomatocytes, macrothrombocytopenia, splenomegaly). It is a key mimic of familial hypercholesterolemia (FH) that is treated very differently — first-line therapy is dietary sterol restriction plus ezetimibe, not statins.

1. Disease Information

Overview: Sitosterolemia (also called phytosterolemia) is an inborn error of sterol metabolism in which plant-derived (xeno)sterols accumulate in plasma and tissues owing to failure of the intestinal/hepatic sterol efflux pump. It was first molecularly characterized in 2000–2001 when mutations in the adjacent, divergently transcribed genes ABCG5 and ABCG8 on chromosome 2p21 were identified as causative (Berge et al., Science 2000, PMID:11060012; Lee et al., Nat Genet 2001, PMID:11138003; Lu et al., Am J Hum Genet 2001, PMID:11499225).

Key identifiers: - OMIM: Sitosterolemia 1 (STSL1), #210250 — caused by ABCG8 (gene 605460); Sitosterolemia 2 (STSL2), #618666 — caused by ABCG5 (gene 605459) (OMIM #210250, OMIM #618666) - MONDO: MONDO:0020748 (per current MONDO lookups) — note: the prompt's suggested MONDO:0008863 should be independently reconciled against the live MONDO release before KB entry, since search-based lookup returned MONDO:0020748 for "sitosterolemia" as the parent grouping term; both STSL1/STSL2-specific and umbrella MONDO IDs should be checked with runoak -i sqlite:obo:mondo before committing. - Orphanet: ORPHA:65282 (Sitosterolemia) - ICD-10: E75.6 (Lipidosis, unspecified) — no dedicated sitosterolemia code exists; ICD-11 similarly lacks a specific code, falling under disorders of lipoprotein metabolism - MeSH: Sitosterolemia (D032169) - Gene: ABCG5 (HGNC:13886), ABCG8 (HGNC:13887)

Synonyms: Phytosterolemia; β-sitosterolemia; sitosterolemia with xanthomatosis; STSL

Data provenance: Nearly all published information derives from aggregated case reports/series and small cohort natural-history studies (GeneReviews estimates ~110 molecularly confirmed cases reported worldwide as of its most recent update, though this is acknowledged to be an underestimate — GeneReviews, NBK131810), rather than large EHR-derived cohorts, reflecting the rarity and under-recognition of the disease.

2. Etiology

Disease causal factors: Sitosterolemia is a monogenic, purely genetic disorder — biallelic (homozygous or compound heterozygous) pathogenic variants in ABCG5 or ABCG8 are necessary and sufficient to cause disease; there is no known environmental or infectious primary cause. However, dietary plant sterol intake is the essential environmental trigger that converts the genetic lesion into the clinical phenotype — sterol accumulation and its sequelae are directly proportional to dietary exposure.

Genetic risk factors: - Loss-of-function missense, nonsense, frameshift, and splice-site variants in ABCG5 or ABCG8 disrupting formation or trafficking of the obligate ABCG5/ABCG8 heterodimer. - No deletions/duplications of these genes have been reported as pathogenic mechanisms in sitosterolemia (GeneReviews). - Founder variants raise local carrier frequency in specific populations (see §9). - Emerging evidence that monoallelic (heterozygous) variants may confer a milder, incompletely penetrant phenotype with modestly elevated plant sterols — an active area of genotype-phenotype research (2026 Frontiers in Nutrition review, "Molecular genetic basis and clinical heterogeneity of sitosterolemia").

Environmental risk factors: - High dietary intake of plant sterol-rich foods: vegetable oils, margarine/spreads fortified with plant sterols or stanols, nuts, seeds, avocado, chocolate. - Shellfish consumption (shellfish sterols are structurally similar xenosterols also normally excluded by ABCG5/ABCG8). - Parenteral nutrition containing plant sterol-derived lipid emulsions can precipitate/unmask hypersterolemia, particularly in infants. - Formula feeding in infants — immature ABCG5/G8 expression in neonates can produce transient plant-sterol elevation that is a diagnostic confounder (false positive), distinct from the fixed genetic disease.

Protective factors: - No genetic protective variants specific to sitosterolemia have been described (unlike, e.g., PCSK9 loss-of-function variants in general hypercholesterolemia). - Environmentally, strict avoidance of dietary plant sterols/stanols and shellfish is the principal modifiable protective factor and is the mainstay of lifelong management. - Plant stanol-containing products (campestanol, sitostanol), often recommended for general hypercholesterolemia, are specifically contraindicated and exacerbate sterol accumulation in sitosterolemia patients — an important clinical counter-intuitive point (GeneReviews).

Gene-environment interactions: This is the paradigm case of a "hyperabsorber" gene-diet interaction: in unaffected individuals, dietary plant sterols are absorbed at <5% and rapidly re-excreted by ABCG5/G8; in biallelic-variant carriers, absorption efficiency rises toward that of cholesterol (~40–60%) and biliary/fecal re-excretion is essentially abolished, so that identical dietary sterol intake produces a 30- to 100-fold difference in plasma sterol concentration between affected and unaffected individuals. Intrafamilial phenotypic variability among individuals with identical genotypes is attributed largely to differences in dietary sterol exposure (GeneReviews).

3. Phenotypes

Clinical signs and symptoms

Phenotype Type Onset Frequency/Notes Suggested HPO term
Tendon/tuberous xanthomas Physical sign Childhood (atypical distribution: heels, knees, elbows, buttocks) Common presenting feature, often the first sign in children (PMC9563796) HP:0100678 (Tendon xanthomatosis) / HP:0010741 (Tuberous xanthomas)
Hypercholesterolemia Laboratory abnormality Childhood Marked, but paradoxically responds to diet/bile-acid sequestrants, poorly/not to statins HP:0003124 (Hypercholesterolemia)
Premature/accelerated atherosclerosis Clinical sign Childhood through young adulthood (documented onset ages 5–33y) Risk of angina, MI, sudden cardiac death if untreated HP:0100762 (Coronary artery atherosclerosis) / HP:0001677 (Coronary artery disease)
Hemolytic anemia (with stomatocytes) Laboratory/clinical Any age; can be presenting feature Chronic or episodic HP:0004870 (Chronic hemolytic anemia) / HP:0004446 (Stomatocytosis)
Macrothrombocytopenia Laboratory abnormality Any age; may be isolated presenting finding Giant platelets on smear, surrounded by vacuole halo HP:0001902 (Giant platelets) / thrombocytopenia HP:0001873
Impaired platelet aggregation / bleeding tendency Clinical/lab Variable Paradoxical bleeding despite membrane sterol-driven "hyperreactivity" HP:0003540 (Impaired platelet aggregation)
Splenomegaly Clinical sign Variable Associated with hemolysis HP:0001744
Arthralgia/arthritis Symptom Variable, often childhood Reported in a substantial minority HP:0002829 (Arthralgia) / HP:0001369 (Arthritis)
Abnormal liver function tests Laboratory Variable Can be occult; some hepatic steatosis/impairment reported HP:0002910 (Elevated hepatic transaminase)
Neonatal/infantile presentation Variable Neonatal-infantile Can mimic other pediatric hypercholesterolemia/hemolytic syndromes —

Phenotype characteristics: - Age of onset: Predominantly childhood (xanthomas and hypercholesterolemia often detected in the first decade); hematologic presentation can occur at any age and is sometimes the sole presenting feature in adults (e.g., PMC10951126: a child presenting with hemolytic anemia/thrombocytopenia before other stigmata appeared). - Severity/progression: Highly variable even within families with identical genotype — attributed to dietary sterol intake differences. Untreated disease is progressive with respect to atherosclerosis and xanthoma burden; treatment leads to xanthoma regression and normalization/improvement of hematologic indices. - Frequency among affected individuals: Xanthomas and hypercholesterolemia are near-universal in classically ascertained cases; hematologic abnormalities (stomatocytic hemolysis, macrothrombocytopenia) are common but not universal, and some patients present with hematologic findings alone, without xanthomas or overt hypercholesterolemia (per GeneReviews and multiple case reports, e.g., PMID:24166850). - Quality of life impact: Chronic hemolytic anemia and bleeding tendency, disfiguring xanthomas, arthralgias, and anxiety around premature cardiovascular events affect daily functioning; no disease-specific QoL instrument data were identified in the literature searched, though ASCVD risk-related morbidity (per general dyslipidemia QoL literature) applies.

4. Genetic/Molecular Information

Causal genes: - ABCG8 (chr 2p21; HGNC:13887; OMIM 605460) — biallelic variants cause Sitosterolemia 1 (STSL1, OMIM #210250). - ABCG5 (chr 2p21, immediately adjacent to ABCG8, transcribed in a head-to-head/divergent orientation; HGNC:13886; OMIM 605459) — biallelic variants cause Sitosterolemia 2 (STSL2, OMIM #618666). - The two genes together account for essentially all molecularly confirmed cases; published case-series estimates of the relative proportion of ABCG5 vs ABCG8 cases vary by cohort/ancestry (a 2026 review reports population-dependent skew; see population section).

Pathogenic variants: - Variant types: missense (most common), nonsense, frameshift, splice-site; sequence analysis detects >95% of pathogenic alleles; no causative large deletions/duplications reported (GeneReviews). - Classification: ACMG/AMP pathogenic/likely pathogenic variants are catalogued in ClinVar for both genes (e.g., ClinVar RCV000005255 ABCG8 c.1083G>A p.Trp361Ter; RCV000269126 ABCG8 c.55G>C p.Asp19His). - Functional consequence: Loss-of-function — disrupted heterodimer assembly/trafficking to the apical (canalicular/brush-border) membrane, or loss of ATPase/transport activity, abolishing selective sterol efflux. - Founder alleles (see §9) reach carrier frequencies of several percent in isolated populations, making compound heterozygosity and homozygosity locally more frequent than the global rarity of the disease would suggest. - Germline vs somatic: Exclusively germline; no somatic mosaicism or cancer-related somatic mutation relevance reported.

Modifier genes: No formally validated modifier genes are established; NPC1L1 (Niemann-Pick C1-Like 1), which mediates apical intestinal cholesterol/sitosterol uptake and is the molecular target of ezetimibe (Altmann et al., Science 2004, PMID:15044802), is mechanistically upstream/complementary rather than a genetic modifier per se, but its pharmacologic inhibition is the basis of first-line therapy.

Epigenetic information: No disease-specific epigenetic (DNA methylation/histone) mechanism has been reported; sitosterolemia is a straightforward loss-of-function Mendelian disorder.

Chromosomal abnormalities: None reported; disease is due to intragenic point/small-indel variants, not large chromosomal rearrangements.

Suggested ontology terms: hgnc:13886 (ABCG5), hgnc:13887 (ABCG8); GO molecular function GO:0034632 (retinol transmembrane transporter activity — analog class) is not exact; more precisely GO:1901664 (sterol transmembrane transporter activity, ABC-type) / GO:0034041 (sterol-transporting ATPase activity) should be verified via OAK before curation.

5. Environmental Information

  • Environmental factors: Not a toxin/pollution-mediated disease; the sole relevant "environmental" input is dietary sterol load (see §2).
  • Lifestyle factors: Diet composition (plant-sterol-rich vegetable oils, margarines, nuts, seeds, avocado, chocolate, shellfish) is the dominant modifiable lifestyle determinant of phenotype severity. No smoking/alcohol-specific interactions were identified in the literature reviewed, though these would be expected to compound general atherosclerotic risk as in any dyslipidemia.
  • Infectious agents: Not applicable — sitosterolemia has no infectious etiology or trigger.

6. Mechanism / Pathophysiology

Causal chain (upstream → downstream):

  1. Molecular lesion: Biallelic loss-of-function variants in ABCG5 or ABCG8 prevent formation of a stable, functional ABCG5/ABCG8 heterodimeric ATP-binding cassette (ABC) half-transporter ("sterolin-1"/"sterolin-2") (PMC7961684, 20-year review of ABCG5/G8 function).
  2. Transporter dysfunction: ABCG5/G8 normally localizes to the apical membrane of enterocytes and the canalicular membrane of hepatocytes, where it acts as the primary, ATP-dependent, bile-salt-facilitated efflux pump exporting cholesterol and (preferentially) non-cholesterol/plant sterols into the intestinal lumen and bile, respectively. Structural work has defined a "transmembrane polar relay" that allosterically couples ATP hydrolysis to sterol translocation and confers plant-sterol-over-cholesterol substrate preference (PMC7699580).
  3. Loss of selective sterol efflux: With the transporter absent/non-functional, dietary plant sterols (sitosterol, campesterol, stigmasterol) and shellfish sterols — which are normally absorbed at <5% efficiency and aggressively re-excreted — are instead retained and absorbed at cholesterol-like efficiency (~40–60%), while biliary/fecal excretion of both cholesterol and plant sterols is markedly reduced.
  4. Systemic and tissue accumulation: Untreated patients show 30- to 100-fold elevation of plasma sitosterol (versus healthy controls), with sterol deposition in plasma membranes, tendon, skin (xanthomas), red cells, and platelets.
  5. Downstream cellular/tissue consequences (parallel branches):
  6. Vascular/atherogenic branch: Elevated total/LDL cholesterol plus phytosterol incorporation into lipoprotein particles and vascular tissue drives accelerated foam-cell formation and premature atherosclerotic plaque development (relevant to the dismech atherogenesis module pattern) — Current Atherosclerosis Reports 2023 review ("Update on Sitosterolemia and Atherosclerosis").
  7. Erythrocyte branch: Incorporation of excess sterol into the red-cell membrane alters membrane fluidity/shape, producing stomatocytic red cells and chronic hemolysis with reticulocytosis and splenomegaly.
  8. Platelet branch: Plant sterol accumulation in the platelet membrane induces platelet hyperreactivity; a murine model demonstrated internalization of the αIIbβ3 (GPIIb/IIIa) integrin complex and filamin A degradation, driving macrothrombocytopenia and a paradoxical bleeding phenotype despite baseline hyperreactivity (Blood 2013, "Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia"). A 2024 platelet proteomics study (Blood Advances 8(10):2466) found the platelet proteome is longitudinally stable pre/post treatment and concluded thrombocytopenia is "driven by lipid disorder and not [primary] platelet aberrations" — supporting a lipid-membrane-mediated rather than intrinsic megakaryocytic mechanism.
  9. Hepatic branch: Reduced biliary cholesterol/sterol secretion causes hepatic sterol retention; mouse knockout studies show "failure to secrete biliary cholesterol" as a direct consequence of Abcg8 loss (PMC394351), and human patients can show occult hepatic transaminase elevation.

Cell types involved: enterocyte (CL:0000584, absorptive cell of intestinal epithelium), hepatocyte (CL:0000182), erythrocyte (CL:0000232), platelet/thrombocyte (CL:0000233), macrophage/foam cell (CL:0000235 / foam cell context), vascular smooth muscle cell.

Suggested GO biological process terms: GO:0032367 (intracellular cholesterol transport), GO:0033344 (cholesterol efflux), GO:0010875 (positive regulation of cholesterol efflux — inverse applies as loss), GO:0034381 (plasma lipoprotein particle clearance), GO:0006febris — verify exact plant-sterol transport GO term via OAK (candidate: GO:0034191, apolipoprotein-mediated... not exact; recommend runoak -i sqlite:obo:go search "sterol transport" during curation).

Molecular profiling / advanced technologies: The 2024 platelet proteomics paper (Blood Advances) is the most direct "omics" dataset identified; no transcriptomic/spatial/single-cell atlas specific to sitosterolemia tissue was found in this search — an evidence gap worth flagging in a KB discussions/KNOWLEDGE_GAP entry.

7. Anatomical Structures Affected

  • Organ level:
  • Primary: small intestine (enterocyte apical membrane), liver (hepatocyte canalicular membrane) — the two sites of normal ABCG5/G8 expression and action.
  • Secondary/complication organs: cardiovascular system (coronary arteries — atherosclerosis), skin/tendons (xanthomas), spleen (splenomegaly from hemolysis), joints (arthritis/arthralgia), bone marrow (reactive/compensatory changes to hemolysis).
  • Body systems: digestive, cardiovascular, hematologic/lymphoid, musculoskeletal, integumentary (dermatologic).
  • Tissue/cell level: intestinal absorptive epithelium; hepatobiliary canalicular epithelium; vascular endothelium/intima (atheroma); erythrocyte membrane; platelet/megakaryocyte membrane; xanthoma histiocytes/foam cells in tendon and skin.
  • Subcellular level: plasma membrane (apical enterocyte membrane, canalicular hepatocyte membrane, erythrocyte/platelet membrane) is the principal subcellular site of ABCG5/G8 localization and sterol accumulation (GO Cellular Component candidate: GO:0016324 apical plasma membrane; GO:0016323 basolateral plasma membrane for contrast; GO:0005903 brush border).
  • Localization (UBERON): small intestine (UBERON:0002108), liver (UBERON:0002107), coronary artery (UBERON:0001621), tendon (UBERON:0000043), spleen (UBERON:0002106).
  • Lateralization: Not applicable — systemic/bilateral disease process, no laterality preference.

8. Temporal Development

  • Onset: Typically pediatric — xanthomas and hypercholesterolemia usually detected in childhood; hematologic presentation can occur from infancy through adulthood. Documented atherosclerotic onset ages range from 5 to 33 years in untreated patients (GeneReviews).
  • Onset pattern: Insidious/chronic for the sterol-accumulation and xanthoma phenotype; the hemolytic anemia can be chronic or episodic (HP:0004802, Episodic hemolysis).
  • Progression: Untreated disease is progressive with respect to xanthoma burden and atherosclerotic disease; treated disease shows xanthoma regression and stabilization/improvement of lipid and hematologic parameters.
  • Disease course pattern: Chronic lifelong metabolic disorder requiring ongoing dietary/pharmacologic management; not self-limited.
  • Remission patterns: No spontaneous remission; treatment-induced improvement (xanthoma regression, normalization of platelet count/hemolysis markers) is well documented with dietary restriction plus ezetimibe.
  • Critical periods: Early diagnosis and treatment initiation (ideally before significant atherosclerotic burden accrues) is emphasized as critical to prognosis; treatment experience in children <2 years is limited (GeneReviews notes this explicitly as a management caveat).

9. Inheritance and Population

Epidemiology: - Historically reported as exceedingly rare, with wide-ranging prevalence estimates: sources cite figures from ~1 in 200,000 to 1 in a million; a more recent gnomAD-based allele-frequency analysis suggested population studies imply a range of roughly 1/384 to 1/48,076 carriers/cases (95% CI) once under-ascertainment is accounted for, and a separate estimate placed global prevalence at "at least 1 in 2.6 million" for an ABCG5 mutation and "1 in 360,000" for an ABCG8 mutation (search-sourced NORD/derivative summary) — these figures are inconsistent across sources and should be treated cautiously/flagged as an evidence gap; the recurring theme across essentially all recent literature is that the disease is substantially under-recognized and under-diagnosed, particularly in hypercholesterolemic children misclassified as FH (PMC7449458: "High prevalence of increased sitosterol levels in hypercholesterolemic children suggest underestimation of sitosterolemia incidence"). - ~110 molecularly confirmed cases reported worldwide per GeneReviews, though this substantially understates true prevalence.

Inheritance pattern: Autosomal recessive. Parents of an affected individual are obligate heterozygous carriers (typically asymptomatic or only mildly biochemically affected); siblings of an affected individual have a 25% chance of being affected, 50% chance of being an unaffected carrier, and 25% chance of being unaffected/non-carrier.

Penetrance: Biallelic pathogenic variants are considered highly (though not perfectly) penetrant for biochemical hypersterolemia; clinical penetrance (xanthomas, hematologic disease, atherosclerosis) is modulated by diet, producing variable expressivity even at fixed genotype.

Expressivity: Variable — intrafamilial phenotypic variability with identical genotypes is well documented and attributed chiefly to differing dietary sterol exposure (GeneReviews); some patients present solely with hematologic abnormalities without xanthomas/hypercholesterolemia.

Genetic anticipation / germline mosaicism: Not reported as a feature of this disorder (it is not a repeat-expansion disease).

Founder effects: - ABCG8 p.Ser107Ter — Hutterite population, carrier frequency ~8%. - ABCG8 p.Gly574Arg — Old Order Amish, carrier frequency ~4%. - High prevalence also reported in inhabitants of Kosrae (Micronesia), with a founder-variant carrier frequency cited around 13%, and in some South African and Finnish population groups (per multiple case-series/GeneReviews summaries). - Ancestry-related gene skew: Northern European/white individuals more frequently carry ABCG8 variants, while Chinese, Japanese, and Indian individuals more frequently carry ABCG5 variants (GeneReviews; corroborated by multiple East/South Asian case series).

Consanguinity: As an autosomal recessive disorder, consanguineous unions elevate risk, consistent with reported case clusters in populations with high intra-community marriage rates (Amish, Hutterite, certain Middle Eastern/South Asian cohorts).

Sex ratio / age distribution: No strong sex predilection is reported in the literature reviewed; age at diagnosis spans neonatal/infantile through adult, with pediatric ascertainment (via xanthomas or incidental hypercholesterolemia/hematologic workup) predominating in published series.

10. Diagnostics

Clinical/laboratory tests: - Plasma phytosterol quantification (sitosterol, campesterol, stigmasterol) via gas chromatography, GC/MS, HPLC, or LC-MS/MS — the essential first-line biochemical test, since routine cholesterol panels do not distinguish plant sterols from cholesterol. Untreated sitosterolemia patients show sitosterol concentrations 30- to 100-fold above normal (roughly 10–65 mg/dL vs. a normal reference of ~0.21 ± 0.7 mg/dL); >1 mg/dL is generally considered diagnostic (GeneReviews). - Complete blood count and peripheral blood smear: macrothrombocytopenia (giant platelets with a vacuolated halo) and stomatocytic red cells are characteristic morphologic clues. - Liver function tests (transaminases) as part of surveillance. - Coronary artery calcium scoring/angiography in long-standing untreated cases to assess atherosclerotic burden.

Genetic testing: - Multigene panel testing including ABCG5, ABCG8, and differential-diagnosis genes (e.g., LDLR, APOB, PCSK9 for FH; CYP27A1 for cerebrotendinous xanthomatosis; ABCA1 for Tangier disease; LCAT) is the recommended first-tier approach when clinical suspicion is high. - Exome/genome sequencing when the phenotype is atypical or sitosterolemia was not initially suspected. - Sequence analysis (Sanger/NGS) detects >95% of pathogenic variants; no deletion/duplication (CNV) analysis is typically needed given no reported pathogenic CNVs. - Diagnosis is formally established by the combination of markedly elevated plasma plant sterols PLUS biallelic pathogenic/likely pathogenic ABCG5/ABCG8 variants.

Diagnostic pitfalls: - False-positive mild elevation in formula-fed infants (immature transporter expression). - Confounding from parenteral nutrition (plant-sterol-containing lipid emulsions). - Heterozygous carriers may show mildly elevated sitosterol, complicating interpretation without genetic confirmation.

Differential diagnosis (critical clinical distinction):

Disorder Overlap with Sitosterolemia Key Distinguishing Feature
Heterozygous FH Childhood xanthomas, hypercholesterolemia LDL-C typically >190 mg/dL in adults; no macrothrombocytopenia/hemolysis; responds well to statins
Homozygous FH Severe xanthomatosis, marked hypercholesterolemia LDL-C often >500 mg/dL; both parents typically hypercholesterolemic; statin-responsive (with PCSK9i/lomitapide adjuncts)
Cerebrotendinous xanthomatosis Tendon xanthomas from childhood Elevated cholestanol, chronic diarrhea, cataracts, progressive neurologic disease; caused by CYP27A1
Tangier disease Stomatocytosis Extreme HDL-C reduction (<1–2 mg/dL); caused by ABCA1
LCAT deficiency Stomatocytosis Extreme HDL-C reduction (<10 mg/dL), elevated VLDL/triglycerides, corneal opacities

Misdiagnosis of sitosterolemia as FH (and vice versa) is a recurring, well-documented clinical pitfall with real patient-safety implications, since statins — the standard FH therapy — are typically ineffective in sitosterolemia and the two conditions require different management (multiple 2024–2026 case reports: PMID:38707657 "Two Cases of Sitosterolemia Falsely Diagnosed as Familial Hypercholesterolemia: Could Digging Deeper Have Avoided Harm?"; additional case reports of sitosterolemia misdiagnosed as Evans syndrome plus FH, and as homozygous FH).

Screening: No formal population-based newborn screening program for sitosterolemia was identified; case-finding is currently opportunistic (via pediatric hypercholesterolemia or hematology workups). Cascade family testing is recommended once a proband's variants are known; a familial hypercholesterolemia cascade-screening program has been used as a vehicle to also screen for ABCG5/ABCG8 variants (Circ Genom Precis Med, AHA journals).

11. Outcome/Prognosis

  • Survival/mortality: No population-level survival statistics (5-/10-year survival) specific to sitosterolemia were identified in a rare-disease registry format; the dominant mortality risk described in case literature is premature cardiovascular death from accelerated atherosclerosis (documented sudden cardiac death cases; atherosclerotic disease onset as early as age 5–33 years in untreated individuals per GeneReviews).
  • Morbidity: Chronic hemolytic anemia, bleeding tendency from platelet dysfunction, arthritis/arthralgia, disfiguring xanthomas, and premature coronary artery disease constitute the principal morbidity burden.
  • Prognostic factors: Age at diagnosis/treatment initiation, degree of dietary sterol restriction adherence, and pre-treatment atherosclerotic burden are the main prognostic modifiers described qualitatively in the literature; no formal validated prognostic scoring system specific to sitosterolemia was found.
  • Recovery potential: With early diagnosis and sustained dietary/pharmacologic treatment, xanthomas regress, hematologic parameters normalize or improve substantially, and cardiovascular risk trajectory is markedly reduced — supporting a generally favorable prognosis when treated, contrasted with a poor, potentially lethal prognosis if the condition remains undiagnosed/untreated or is mismanaged (e.g., treated as FH with statins alone, which do not address the underlying sterol-absorption defect).

12. Treatment

Pharmacotherapy (first line): - Ezetimibe (10 mg/day in adults) — a cholesterol/sterol absorption inhibitor acting on the intestinal NPC1L1 transporter; FDA-approved specifically for sitosterolemia since October 2002, and is the current treatment of choice. It reduces gastrointestinal absorption of both cholesterol and plant sterols, lowering plasma phytosterol concentrations, and has also been reported to increase platelet count and decrease mean platelet volume (potentially reducing bleeding risk) and to improve VLDL/HDL subfraction distribution. Suggested MAXO/NCIT annotation: treatment_term NCIT:C15986 (Pharmacotherapy) + therapeutic_agent CHEBI (ezetimibe) — verify exact CHEBI ID via OAK. - Bile acid sequestrants (cholestyramine 8–15 g/day, colestipol, colesevelam) as second-line/add-on therapy for incomplete ezetimibe response; typically reduce plant sterol levels by ~30% and can decrease xanthoma size. Combined dietary + pharmacologic treatment achieves an overall 10–50% reduction in plasma cholesterol and sitosterol concentrations. - Statins are generally ineffective as monotherapy in sitosterolemia (a key point differentiating management from FH) since the primary defect is one of absorption/excretion, not endogenous cholesterol synthesis. - Contraindicated: plant stanol-fortified products (campestanol, sitostanol margarines/spreads), which paradoxically worsen sterol accumulation despite being generally beneficial in ordinary hypercholesterolemia.

Dietary intervention (foundational): Restriction of plant sterol-rich foods (vegetable oils, margarine, nuts, seeds, avocados, chocolate) and shellfish. This is described as the foundational management approach and is typically combined with pharmacotherapy.

Surgical/interventional: Partial ileal bypass surgery has been used as a last-resort option in maximal-therapy failures, achieving >50% sterol reduction, but is reserved given its invasiveness.

Supportive care: Arthritis, anemia, thrombocytopenia, and splenomegaly are generally managed by treating the underlying sterol excess (which improves all these secondary manifestations) rather than by disease-specific symptomatic therapy.

Emerging/experimental therapies: No sitosterolemia-specific gene therapy, RNA-based therapy, or targeted biologic was identified as approved or in late-stage trials in this search. Broader cholesterol-lowering pipeline developments (oral PCSK9 inhibitors achieving ~65% LDL-C reduction, bempedoic acid, PPRH-based PCSK9 gene-suppression approaches) are advancing rapidly in general dyslipidemia/FH but were not identified as being specifically studied in or indicated for sitosterolemia; this represents a potential future-therapy knowledge gap worth flagging rather than asserting.

Treatment outcomes: A published case series with clinical, genetic, and therapeutic data reported favorable outcomes with varying combinations of dietary treatment and ezetimibe across 55 children and 5 adults with sitosterolemia (cited via search aggregation; original primary-literature citation should be independently verified/fetched before KB use — likely Tada et al. or a related Japanese cohort study, to be confirmed with just fetch-reference).

Pregnancy: No adequate controlled studies of ezetimibe in pregnancy exist; use only if benefit outweighs fetal risk, with close monitoring (e.g., via MotherToBaby) recommended (GeneReviews).

Suggested MAXO/NCIT terms: MAXO:0000088 (dietary intervention) for the sterol-restricted diet; NCIT:C15986 (Pharmacotherapy) + therapeutic_agent for ezetimibe and bile acid sequestrants; MAXO:0000004/NCIT:C15329 (surgical procedure) for partial ileal bypass.

13. Prevention

  • Primary prevention: Not applicable in the classic sense (this is a genetic disorder present from conception), but dietary avoidance of plant sterols/stanols and shellfish from infancy in genetically confirmed individuals functions as primary prevention of the clinical phenotype (xanthomas, atherosclerosis, hematologic disease) even though it cannot prevent the genotype itself.
  • Secondary prevention: Early biochemical/genetic diagnosis in at-risk families (siblings of an index case, or children/relatives in high-founder-frequency populations) enables treatment before significant atherosclerotic or hematologic morbidity accrues. Cascade genetic testing following identification of a proband's variants is recommended.
  • Tertiary prevention: Ongoing surveillance (annual plasma sterol panels, CBC/platelet counts, liver enzymes, xanthoma assessment, and periodic non-invasive cardiac imaging in previously untreated/long-standing cases) aims to detect and mitigate complications (progressive atherosclerosis, worsening cytopenias) in individuals with established disease.
  • Genetic counseling: Recommended for families of affected individuals — informing them of the 25%/50%/25% recurrence risk pattern for future pregnancies, discussing prenatal/preimplantation genetic testing options once family-specific variants are known (GeneReviews notes differing family perspectives on utilizing this option), and offering carrier testing in high-prevalence populations (e.g., Hutterite, Amish communities) for founder variants.
  • Screening programs: No dedicated national/international sitosterolemia newborn screening program was identified; screening currently relies on clinical suspicion (pediatric hypercholesterolemia, unexplained hemolytic anemia/macrothrombocytopenia) triggering targeted biochemical/genetic testing, and FH cascade-screening programs opportunistically capturing sitosterolemia cases.
  • Public health/environmental interventions: Not applicable in the traditional sanitation/vector-control sense; the relevant "environmental" intervention is individualized dietary counseling.

14. Other Species / Natural Disease

  • No naturally occurring veterinary/companion-animal sitosterolemia analog was identified in this search (no OMIA entry surfaced). This disease is primarily studied via engineered rodent models rather than naturally occurring animal disease.
  • Orthologous genes: Mouse Abcg5/Abcg8 (NCBI Gene) are the direct orthologs used in essentially all animal modeling.
  • Comparative biology: The ABCG5/ABCG8 heterodimer and its sterol-selectivity mechanism are evolutionarily conserved across mammals, underpinning the validity of mouse knockout models for mechanistic study (see §15).

15. Model Organisms

Genetic models: - Abcg5⁻/⁻, Abcg8⁻/⁻, and combined Abcg5/Abcg8⁻/⁻ knockout mice are the principal disease models. - Yu et al., PNAS 2002 ("Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion") established that these transporters are essential for biliary cholesterol/sterol secretion. - A related model paper ("A mouse model of sitosterolemia: absence of Abcg8/sterolin-2 results in failure to secrete biliary cholesterol," PMC394351) confirmed loss of biliary cholesterol secretion in Abcg8-null mice. - Under standard chow, Abcg5/Abcg8-null mice show no overt gross phenotype but have markedly increased plasma phytosterol concentrations and very low biliary cholesterol content compared to wild-type. - Phytosterol Feeding Causes Toxicity in ABCG5/G8 Knockout Mice (PMID:23380580, Am J Pathol 2013): a high-phytosterol diet is well tolerated by wild-type mice but produces severe toxicity (premature death, hepatic abnormalities, severe cardiac lesions) in knockout mice — directly modeling the gene-diet interaction central to human disease and supporting dietary sterol restriction as mechanistically causal, not merely correlative, in disease severity. - Platelet hyperreactivity model: Blood 2013 study using the murine sitosterolemia model demonstrated that plant sterol incorporation into the platelet membrane drives platelet hyperreactivity, αIIbβ3 integrin internalization, and filamin A degradation, mechanistically explaining the human macrothrombocytopenia/bleeding phenotype.

Phenotype recapitulation: The mouse models successfully recapitulate the core biochemical lesion (hypersterolemia, defective biliary sterol secretion) and, under phytosterol challenge, the cardiac/hepatic toxicity and platelet abnormalities seen in humans. They do not spontaneously recapitulate human xanthoma formation or overt atherosclerotic plaque under standard chow without additional dietary/genetic sensitization (e.g., combination with atherogenic diets or apoE-null backgrounds would likely be needed — not confirmed in this search and should be flagged as a HUMAN_MODEL_MISMATCH-type consideration if used in KB curation).

Model limitations: Baseline (non-phytosterol-challenged) knockout mice lack an overt spontaneous phenotype, meaning the "natural" mouse phenotype under-represents human disease severity unless a high-phytosterol diet is imposed experimentally — an important translational caveat for any curated mechanistic_hypotheses/evidence_source annotation (evidence_source: MODEL_ORGANISM should be used, and the diet-dependency explicitly noted).

Applications: These models have been used to establish the causal transporter mechanism, define the gene-diet interaction, and dissect the platelet/hematologic mechanism — but not, per this search, to test emerging pharmacologic candidates beyond ezetimibe-class NPC1L1 inhibition.

Resources: MGI (Mouse Genome Informatics) for Abcg5/Abcg8 allele records; no zebrafish, Drosophila, or C. elegans models were identified in this search.

Key Evidence Gaps Identified (for KB discussions/KNOWLEDGE_GAP consideration)

  1. Prevalence estimates are inconsistent across sources (ranging over several orders of magnitude) and systematically confounded by under-diagnosis — a KNOWLEDGE_GAP rather than a settled Prevalence figure.
  2. MONDO ID discrepancy: the research-template-suggested MONDO:0008863 vs. the MONDO:0020748 surfaced by search should be reconciled with a direct OAK lookup before curation.
  3. Genotype-phenotype correlation is explicitly absent (per GeneReviews) — variable expressivity is attributed to diet, not genotype, which is itself a citable, curatable claim.
  4. Monoallelic/heterozygous carrier phenotype is an emerging, not yet fully established, area (2026 Frontiers review) — a candidate for a MECHANISTIC_HYPOTHESIS-flagged definition/claim rather than an ESTABLISHED_CRITERIA one.
  5. Mouse model baseline phenotype requires phytosterol dietary challenge to manifest overt pathology — any evidence item citing the knockout mouse model should specify this diet-dependency to avoid overstating spontaneous phenotype recapitulation (a HUMAN_MODEL_MISMATCH candidate).
  6. No dedicated newborn screening or formal prognostic/survival registry data were located — treatment-outcome quotes (e.g., the 55-children/5-adults cohort) should be traced to primary literature and PMID-confirmed via just fetch-reference before use, since this report surfaced it only via secondary aggregation.

Sources