Homocystinuria

Mendelian MONDO:0004737 Pathograph 67 Metabolic Disease Inborn Error of Metabolism

Homocystinuria is an autosomal recessive inborn error of sulfur amino acid metabolism, most commonly caused by deficiency of cystathionine beta-synthase (CBS). The metabolic block at the transsulfuration branchpoint of methionine metabolism causes accumulation of homocysteine and methionine with depletion of downstream products (cystathionine, cysteine). Elevated homocysteine drives multisystem disease through oxidative stress, mitochondrial dysfunction, protein modifications via homocysteine thiolactone, endothelial dysfunction with thrombosis, and NMDA receptor-mediated neurotoxicity. Clinical features include ectopia lentis, skeletal abnormalities with marfanoid habitus, thromboembolism, and cognitive impairment. Disease severity is strongly modified by pyridoxine (vitamin B6) responsiveness.

Ask OpenScientist

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

Submitting...

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

⚙

Pathophysiology

CBS molecular function deficiency

Biallelic CBS pathogenic variants reduce cystathionine beta-synthase catalytic activity.

hepatocyte link

CBS link

cystathionine beta-synthase activity link ↓ DECREASED

Downstream

Disrupted transsulfuration and methionine metabolism Reduced CBS activity blocks homocysteine-to-cystathionine flux.

Show evidence (1 reference)

PMID:27778219 SUPPORT Other

"Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine."

Clinical guidelines identify CBS deficiency and impaired cystathionine synthesis as the causal biochemical lesion.

Disrupted transsulfuration and methionine metabolism

Loss of CBS function causes upstream accumulation of homocysteine and methionine with downstream depletion of cystathionine and cysteine, disrupting sulfur amino acid homeostasis.

sulfur amino acid metabolic process link ⚠ ABNORMAL homocysteine metabolic process link ⚠ ABNORMAL transsulfuration link ↓ DECREASED

Downstream

Total homocysteine (tHcy) CBS blockade produces the diagnostic accumulation of total plasma homocysteine.

Hyperhomocystinemia Elevated plasma homocysteine is the clinical phenotype-level expression of the biochemical block.

Methionine The CBS block occurs in methionine catabolism and causes upstream methionine accumulation in classical HCU.

Cysteine Blocked transsulfuration reduces downstream cysteine production.

Hepatomegaly Hepatomegaly is grouped with systemic manifestations of sulfur-amino-acid metabolic disruption in HCU.

Oxidative stress and mitochondrial dysfunction Homocysteine burden and downstream thiol imbalance contribute to oxidative and mitochondrial stress.

Fibrin homocysteinylation and collagen cross-link deficiency Reactive homocysteine species modify plasma proteins, while patient data support a collagen cross-linking defect in bone.

Homocysteic acid neuroexcitotoxicity Homocysteine-derived excitatory metabolites affect neuronal signaling.

Show evidence (1 reference)

PMID:36417581 SUPPORT Other

"The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine, a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications"

Confirms massive homocysteine elevation as the central biochemical defect driving clinical complications.

MTHFR remethylation pathway deficiency

Biallelic MTHFR pathogenic variants reduce methylenetetrahydrofolate reductase activity in the folate-dependent remethylation pathway. This nonclassical homocystinuria mechanism causes elevated plasma and urine homocysteine with low or low-normal methionine, converging on the shared hyperhomocystinemia node while remaining biochemically distinct from CBS-deficient classical homocystinuria.

MTHFR link

one-carbon metabolic process link ⚠ ABNORMAL folic acid metabolic process link ⚠ ABNORMAL homocysteine metabolic process link ⚠ ABNORMAL

methylenetetrahydrofolate reductase [NAD(P)H] activity link ↓ DECREASED

Downstream

Hyperhomocystinemia MTHFR deficiency converges with CBS deficiency on elevated homocysteine as a shared biochemical phenotype.

Total homocysteine (tHcy) Plasma homocysteine surveillance reports disease activity in MTHFR-deficiency homocystinuria.

Show evidence (2 references)

PMID:40440437 SUPPORT Other

"N(5,10)-MTHFR activity is established in a proband with characteristic clinical and laboratory findings (low plasma methionine, increased plasma and urine homocysteine"

GeneReviews supports MTHFR deficiency as a folate-remethylation cause of homocystinuria with high homocysteine and low methionine.

PMID:18658082 SUPPORT Human Clinical

"Severe methylenetetrahydrofolate reductase deficiency is an autosomal recessive metabolic disorder of folate metabolism causing elevated plasma homocysteine levels and homocystinuria (MIM 236250)."

A human case report supports severe MTHFR deficiency as nonclassical homocystinuria through folate-metabolism disruption.

Oxidative stress and mitochondrial dysfunction

Elevated homocysteine is a reactive thiol that generates reactive oxygen species, while cysteine depletion impairs glutathione-dependent antioxidant defenses. CBS-deficient patients show altered NAD redox metrics, elevated mitokines (FGF-21, GDF-15), and increased mitochondrial DAMPs, indicating coupled oxidative damage and mitochondrial dysfunction.

hepatocyte link

cellular response to oxidative stress link mitochondrion organization link

Downstream

GDF-15 (mitokine) GDF-15 is elevated as a mitochondrial stress biomarker and correlates with total homocysteine in patients.

FGF-21 (mitokine) FGF-21 elevation is part of the patient mitochondrial stress signature.

NAD+/NADH ratio NAD redox imbalance is a measurable downstream readout of mitochondrial dysfunction.

Endothelial dysfunction, thrombosis, and vascular injury Oxidative injury contributes to endothelial dysfunction and thrombotic vascular complications.

Show evidence (1 reference)

PMID:39567721 SUPPORT Human Clinical

"The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD."

Directly supports the coupled oxidative stress and mitochondrial dysfunction mechanism in CBS deficiency.

CBS protein misfolding and proteostasis disruption

Many CBS pathogenic variants cause protein misfolding and instability rather than active-site disruption. Misfolded CBS is cleared primarily by proteasomal degradation with contributions from ERAD and lysosomal-autophagic pathways. This conformational disease mechanism suggests proteostasis modulation as a potential therapeutic strategy.

proteasome-mediated ubiquitin-dependent protein catabolic process link endoplasmic reticulum unfolded protein response link

Downstream

CBS molecular function deficiency Misfolding and proteasomal turnover reduce residual CBS activity for many pathogenic missense variants.

Disrupted transsulfuration and methionine metabolism Proteostasis-mediated CBS loss of function reduces transsulfuration flux.

Show evidence (1 reference)

PMID:38051092 SUPPORT In Vitro

"Endoplasmic reticulum stress sensor BiP was found upregulated with CBS I278T variant associated with proteasomes suggesting proteotoxic stress and degradation of misfolded CBS."

Demonstrates ER stress and proteotoxic stress in cells expressing pathogenic CBS variants.

Endothelial dysfunction, thrombosis, and vascular injury

Homocysteine-driven oxidative stress and protein modifications cause endothelial cell dysfunction and platelet activation, promoting thrombus formation and vascular occlusion. Modification of fibrinogen by homocysteine thiolactone increases resistance to fibrinolysis, providing a direct mechanistic link to the high thromboembolism risk in HCU.

endothelial cell link platelet link

blood coagulation link

Downstream

Thromboembolism Endothelial injury, altered coagulation, and impaired fibrinolysis drive the aggregate thromboembolism phenotype.

Arterial thrombosis The vascular-injury mechanism includes arterial thrombotic events.

Venous thrombosis The vascular-injury mechanism includes venous thrombotic events.

Pulmonary embolism Pulmonary embolism is a frequent embolic manifestation of the thrombotic vascular mechanism.

Cerebral ischemia Arterial thrombosis can produce cerebral ischemia and stroke-like presentations.

Stroke Arterial or venous thrombotic vascular events can present clinically as stroke.

Cerebral venous sinus thrombosis Venous thrombosis can involve the cerebral venous sinuses.

Hypertension Vascular remodeling and endothelial dysfunction provide a mechanism for frequent hypertension.

Livedo reticularis Cutaneous livedo reflects microvascular involvement in the vascular phenotype cluster.

Pancreatitis Pancreatitis is grouped with occasional systemic complications that can plausibly reflect vascular injury in severe HCU.

Show evidence (2 references)

PMID:36417581 SUPPORT Other

"a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment"

Identifies thromboembolism as a major clinical complication driven by homocysteine toxicity.

PMID:24659173 SUPPORT In Vitro

"Since these features of the networks have been related to impaired fibrinolysis, the N-homocysteinylation reactions would be involved in the prothrombotic effects associated to hyperhomocysteinemia."

Ex vivo fibrin-network data support impaired fibrinolysis as a prothrombotic mechanism of homocysteine-thiolactone.

Fibrin homocysteinylation and collagen cross-link deficiency

Homocysteine-thiolactone modifies lysine residues on plasma fibrinogen/fibrin and alters fibrin network structure and fibrinolysis. Separately, patient data show reduced collagen type I cross-links in homocystinuria, supporting a collagen cross-linking defect for bone manifestations.

collagen metabolic process link

Downstream

Osteoporosis Reduced collagen cross-linking contributes to the bone fragility and osteoporosis phenotype.

Recurrent fractures Collagen cross-link deficiency provides a mechanism for bone fragility and fracture susceptibility.

Ectopia lentis Connective-tissue matrix dysfunction provides a mechanistic bridge to lens zonule instability and ectopia lentis.

Myopia Myopia is grouped with ocular structural complications downstream of connective-tissue involvement in HCU.

Amblyopia Amblyopia is linked to the ocular structural phenotype cluster, including lens displacement and refractive disturbance.

Glaucoma Glaucoma can occur as part of the ocular complication cluster associated with lens instability.

Retinal detachment Retinal detachment belongs to the ocular complication cluster downstream of connective-tissue and high-myopia effects.

Strabismus Strabismus is grouped with ocular structural and refractive complications in CBS-deficient homocystinuria.

Cataract Cataract is grouped with lens and ocular complications in CBS-deficient homocystinuria.

Marfanoid habitus Collagen cross-link and connective-tissue disruption provide a mechanism for the marfanoid skeletal habitus.

Arachnodactyly Arachnodactyly is part of the marfanoid connective-tissue phenotype cluster.

Dental crowding Dental crowding is grouped with the craniofacial and skeletal connective-tissue phenotype cluster.

Scoliosis Scoliosis is a skeletal deformity downstream of connective-tissue and bone-matrix abnormalities.

Pectus excavatum Pectus excavatum is part of the thoracic skeletal deformity cluster in CBS-deficient homocystinuria.

Pectus carinatum Pectus carinatum is part of the thoracic skeletal deformity cluster in CBS-deficient homocystinuria.

Kyphosis Kyphosis reflects the skeletal deformity and bone-quality phenotype cluster.

Genu valgum Genu valgum is grouped with lower-limb skeletal deformities in CBS-deficient homocystinuria.

Joint stiffness Joint stiffness is part of the connective-tissue and skeletal phenotype cluster.

Pes cavus Pes cavus is grouped with skeletal and lower-limb structural manifestations.

High palate High palate is grouped with craniofacial skeletal manifestations of the connective-tissue phenotype.

Sparse scalp hair Sparse scalp hair is grouped with the connective-tissue and integumentary phenotype spectrum in HCU.

Show evidence (2 references)

PMID:8611653 SUPPORT Human Clinical

"This significant reduction of cross-links in the group with homocystinuria did not correlate with serum homocysteine or homocysteic acid concentrations."

A patient study directly supports reduced collagen type I cross-links in homocystinuria.

PMID:24659173 SUPPORT In Vitro

"HTL reacts with proteins by acylation of free basic amino groups; in particular, the epsilon-amino group of lysine residues forms adducts and induces structural and functional changes in plasma proteins."

Ex vivo plasma data support homocysteine-thiolactone modification of fibrinogen/fibrin as a structural protein mechanism.

Homocysteic acid neuroexcitotoxicity

Homocysteine-derived excitatory metabolites activate NMDA receptor signaling, providing a plausible bridge from the metabolic lesion to developmental delay, cognitive impairment, seizures, and neuropsychiatric manifestations.

neuron link

ionotropic glutamate receptor signaling pathway link chemical synaptic transmission link

Downstream

Intellectual disability Neuroexcitotoxicity provides a plausible contribution to cognitive impairment.

Global developmental delay Early neurotoxicity contributes to delayed neurodevelopment.

Seizures NMDA-receptor activation by homocysteic acid provides a plausible seizure mechanism.

Psychosis Neurochemical disturbance may contribute to occasional psychiatric manifestations.

Depression Depression is grouped with neuropsychiatric manifestations plausibly linked to metabolic neurotoxicity.

Anxiety Anxiety is grouped with neuropsychiatric manifestations plausibly linked to metabolic neurotoxicity.

Dystonia Dystonia is grouped with neurologic manifestations potentially arising from metabolic or vascular brain injury.

Show evidence (2 references)

PMID:1676834 SUPPORT In Vitro

"This effect was prevented by bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist CPP (20 microM)."

Rat hippocampal physiology supports NMDA-receptor mediation of homocysteic-acid synaptic effects.

PMID:36417581 PARTIAL Other

The review supports cognitive impairment as a major clinical complication of homocysteine toxicity.

⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

●

Phenotypes

Blood 1

Thromboembolism FREQUENT Thromboembolism (HP:0001907)

Show evidence (3 references)

PMID:3872065 SUPPORT Human Clinical

"initial clinically detected thromboembolic events (at age 15, chances of having had such an event: 12% and 27%, respectively)"

Classic natural history study quantifies thromboembolic event rates by B6 responsiveness.

PMID:33295057 SUPPORT Human Clinical

"Thromboembolism was the commonest presenting feature in ER patients, whereas it was least likely at presentation in the NR group."

E-HOD registry shows thromboembolism is the commonest presentation in extreme responders.

PMID:38201964 SUPPORT Other

"In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."

Confirms arterial and venous thrombotic events as important clinical manifestations of hyperhomocysteinemia.

Cardiovascular 4

Stroke OCCASIONAL Stroke (HP:0001297)

Show evidence (1 reference)

PMID:38201964 SUPPORT Other

"In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."

Arterial thrombotic vascular events include stroke in the context of severe hyperhomocysteinemia.

Pulmonary embolism FREQUENT Pulmonary embolism (HP:0002204)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0002204 | Pulmonary embolism | Frequent (79-30%)"

Orphanet classifies pulmonary embolism as frequent in CBS-deficient homocystinuria.

PMID:3872065 SUPPORT Human Clinical

"Following 586 surgical procedures, 25 postoperative thromboembolic complications occurred, six of which were fatal."

Classic natural history study documents significant postoperative thromboembolic risk including PE.

Hypertension FREQUENT Hypertension (HP:0000822)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000822 | Hypertension | Frequent (79-30%)"

Orphanet classifies hypertension as frequent in CBS-deficient homocystinuria.

Livedo reticularis OCCASIONAL Livedo reticularis (HP:0033505)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0033505 | Livedo reticularis | Occasional (29-5%)"

Orphanet classifies livedo reticularis as occasional in CBS-deficient homocystinuria.

Digestive 2

Pancreatitis OCCASIONAL Pancreatitis (HP:0001733)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0001733 | Pancreatitis | Occasional (29-5%)"

Orphanet classifies pancreatitis as occasional in CBS-deficient homocystinuria.

Hepatomegaly OCCASIONAL Hepatomegaly (HP:0002240)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0002240 | Hepatomegaly | Occasional (29-5%)"

Orphanet classifies hepatomegaly as occasional in CBS-deficient homocystinuria.

Eye 6

Ectopia lentis VERY_FREQUENT Ectopia lentis (HP:0001083)

Show evidence (3 references)

ORPHA:394 SUPPORT

"HP:0001083 | Ectopia lentis | Very frequent (99-80%)"

Orphanet classifies ectopia lentis as very frequent in CBS-deficient homocystinuria.

PMID:3872065 SUPPORT Human Clinical

"dislocation of optic lenses (at age 10, chances of dislocation: 55% and 82%, respectively)"

Classic natural history study of 629 patients quantifies lens dislocation rates by B6 responsiveness.

PMID:33295057 SUPPORT Human Clinical

"Lens dislocation was common in all groups except ER but the age of dislocation increased with increasing responsiveness."

E-HOD registry confirms lens dislocation is common across all pyridoxine responsiveness groups.

Myopia FREQUENT Myopia (HP:0000545)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000545 | Myopia | Frequent (79-30%)"

Orphanet classifies myopia as frequent in CBS-deficient homocystinuria.

Glaucoma OCCASIONAL Glaucoma (HP:0000501)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000501 | Glaucoma | Occasional (29-5%)"

Orphanet classifies glaucoma as occasional in CBS-deficient homocystinuria.

Retinal detachment OCCASIONAL Retinal detachment (HP:0000541)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000541 | Retinal detachment | Occasional (29-5%)"

Orphanet classifies retinal detachment as occasional in CBS-deficient homocystinuria.

Strabismus OCCASIONAL Strabismus (HP:0000486)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000486 | Strabismus | Occasional (29-5%)"

Orphanet classifies strabismus as occasional in CBS-deficient homocystinuria.

Cataract OCCASIONAL Cataract (HP:0000518)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000518 | Cataract | Occasional (29-5%)"

Orphanet classifies cataract as occasional in CBS-deficient homocystinuria.

Head and Neck 2

Sparse scalp hair FREQUENT Sparse scalp hair (HP:0002209)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0002209 | Sparse scalp hair | Frequent (79-30%)"

Orphanet classifies sparse scalp hair as frequent in CBS-deficient homocystinuria.

High palate OCCASIONAL High palate (HP:0000218)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000218 | High palate | Occasional (29-5%)"

Orphanet classifies high palate as occasional in CBS-deficient homocystinuria.

Limbs 3

Arachnodactyly VERY_FREQUENT Arachnodactyly (HP:0001166)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0001166 | Arachnodactyly | Very frequent (99-80%)"

Orphanet classifies arachnodactyly as very frequent in CBS-deficient homocystinuria.

PMID:33295057 SUPPORT Human Clinical

"neurodevelopmental problems, lens dislocation and marfanoid features in early childhood"

E-HOD registry describes marfanoid features including arachnodactyly as characteristic of CBS deficiency.

Genu valgum FREQUENT Genu valgum (HP:0002857)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0002857 | Genu valgum | Frequent (79-30%)"

Orphanet classifies genu valgum as frequent in CBS-deficient homocystinuria.

Pes cavus FREQUENT Pes cavus (HP:0001761)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0001761 | Pes cavus | Frequent (79-30%)"

Orphanet classifies pes cavus as frequent in CBS-deficient homocystinuria.

Musculoskeletal 7

Osteoporosis VERY_FREQUENT Osteoporosis (HP:0000939)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0000939 | Osteoporosis | Very frequent (99-80%)"

Orphanet classifies osteoporosis as very frequent in CBS-deficient homocystinuria.

PMID:3872065 SUPPORT Human Clinical

"radiologic detection of spinal osteoporosis (at age 15, chances of such osteoporosis having been detected: 36% and 64%, respectively)"

Classic natural history study quantifies spinal osteoporosis rates by B6 responsiveness.

Scoliosis FREQUENT Scoliosis (HP:0002650)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0002650 | Scoliosis | Frequent (79-30%)"

Orphanet classifies scoliosis as frequent in CBS-deficient homocystinuria.

PMID:27778219 SUPPORT Other

"Severely affected patients usually present in childhood with ectopia lentis, learning difficulties and skeletal abnormalities."

Clinical guidelines list skeletal abnormalities including scoliosis as characteristic of severe CBS deficiency.

Recurrent fractures VERY_FREQUENT Recurrent fractures (HP:0002757)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0002757 | Recurrent fractures | Very frequent (99-80%)"

Orphanet classifies recurrent fractures as very frequent in CBS-deficient homocystinuria.

Pectus excavatum FREQUENT Pectus excavatum (HP:0000767)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000767 | Pectus excavatum | Frequent (79-30%)"

Orphanet classifies pectus excavatum as frequent in CBS-deficient homocystinuria.

Pectus carinatum FREQUENT Pectus carinatum (HP:0000768)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000768 | Pectus carinatum | Frequent (79-30%)"

Orphanet classifies pectus carinatum as frequent in CBS-deficient homocystinuria.

Kyphosis FREQUENT Kyphosis (HP:0002808)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0002808 | Kyphosis | Frequent (79-30%)"

Orphanet classifies kyphosis as frequent in CBS-deficient homocystinuria.

Joint stiffness FREQUENT Joint stiffness (HP:0001387)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0001387 | Joint stiffness | Frequent (79-30%)"

Orphanet classifies joint stiffness as frequent in CBS-deficient homocystinuria.

Nervous System 7

Intellectual disability VERY_FREQUENT Intellectual disability (HP:0001249)

Show evidence (3 references)

ORPHA:394 SUPPORT

"HP:0001249 | Intellectual disability | Very frequent (99-80%)"

Orphanet classifies intellectual disability as very frequent in CBS-deficient homocystinuria.

PMID:3872065 SUPPORT Human Clinical

"B6-responsive individuals on the average have significantly better mental capabilities (mean IQ, 79) than do B6-nonresponsive individuals (mean IQ, 57)"

Classic natural history study quantifies cognitive impairment by B6 responsiveness.

PMID:33295057 SUPPORT Human Clinical

"Developmental delay was commonest in the NR group while no ER patient had cognitive impairment."

E-HOD registry confirms cognitive impairment is most common in non-responders.

Seizures OCCASIONAL Seizure (HP:0001250)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0001250 | Seizure | Occasional (29-5%)"

Orphanet classifies seizures as occasional in CBS-deficient homocystinuria.

PMID:3872065 SUPPORT Human Clinical

"Methionine restriction initiated neonatally prevented mental retardation, retarded the rate of lens dislocation, and may have reduced the incidence of seizures."

Classic natural history study documents seizures as a feature and notes treatment may reduce incidence.

Global developmental delay OCCASIONAL Global developmental delay (HP:0001263)

Show evidence (1 reference)

PMID:33295057 SUPPORT Human Clinical

"Developmental delay was commonest in the NR group while no ER patient had cognitive impairment."

E-HOD registry data support developmental delay in the severe non-responder group.

Depression OCCASIONAL Depression (HP:0000716)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000716 | Depression | Occasional (29-5%)"

Orphanet classifies depression as occasional in CBS-deficient homocystinuria.

Anxiety OCCASIONAL Anxiety (HP:0000739)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000739 | Anxiety | Occasional (29-5%)"

Orphanet classifies anxiety as occasional in CBS-deficient homocystinuria.

Psychosis OCCASIONAL Psychosis (HP:0000709)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000709 | Psychosis | Occasional (29-5%)"

Orphanet classifies psychosis as occasional in CBS-deficient homocystinuria.

Dystonia OCCASIONAL Dystonia (HP:0001332)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0001332 | Dystonia | Occasional (29-5%)"

Orphanet classifies dystonia as occasional in CBS-deficient homocystinuria.

Growth 1

Marfanoid habitus VERY_FREQUENT Disproportionate tall stature (HP:0001519)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0001519 | Disproportionate tall stature | Very frequent (99-80%)"

Orphanet classifies disproportionate tall stature as very frequent in CBS-deficient homocystinuria.

PMID:33295057 SUPPORT Human Clinical

"neurodevelopmental problems, lens dislocation and marfanoid features in early childhood"

E-HOD registry describes marfanoid features as characteristic of early-presenting CBS deficiency.

Other 7

Dental crowding VERY_FREQUENT Dental crowding (HP:0000678)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000678 | Dental crowding | Very frequent (99-80%)"

Orphanet classifies dental crowding as very frequent in CBS-deficient homocystinuria.

Hyperhomocystinemia VERY_FREQUENT Hyperhomocystinemia (HP:0002160)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0002160 | Hyperhomocystinemia | Very frequent (99-80%)"

Orphanet classifies hyperhomocystinemia as very frequent in CBS-deficient homocystinuria.

PMID:27778219 SUPPORT Other

"We strongly recommend measuring the plasma total homocysteine concentrations in any patient whose clinical features suggest the diagnosis"

Clinical guidelines recommend tHcy measurement as the key diagnostic test.

Arterial thrombosis FREQUENT Arterial thrombosis (HP:0004420)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0004420 | Arterial thrombosis | Frequent (79-30%)"

Orphanet classifies arterial thrombosis as frequent in CBS-deficient homocystinuria.

PMID:38201964 SUPPORT Other

"In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."

Review confirms arterial thrombotic events as clinical manifestations of severe hyperhomocysteinemia.

Venous thrombosis FREQUENT Venous thrombosis (HP:0004936)

Show evidence (2 references)

ORPHA:394 SUPPORT

"HP:0004936 | Venous thrombosis | Frequent (79-30%)"

Orphanet classifies venous thrombosis as frequent in CBS-deficient homocystinuria.

PMID:38201964 SUPPORT Other

"In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."

Review confirms venous thrombotic events as clinical manifestations of severe hyperhomocysteinemia.

Cerebral ischemia FREQUENT Cerebral ischemia (HP:0002637)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0002637 | Cerebral ischemia | Frequent (79-30%)"

Orphanet classifies cerebral ischemia as frequent in CBS-deficient homocystinuria.

Amblyopia FREQUENT Amblyopia (HP:0000646)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0000646 | Amblyopia | Frequent (79-30%)"

Orphanet classifies amblyopia as frequent in CBS-deficient homocystinuria.

Cerebral venous sinus thrombosis OCCASIONAL Cerebral venous sinus thrombosis (HP:0033724)

Show evidence (1 reference)

ORPHA:394 SUPPORT

"HP:0033724 | Cerebral venous sinus thrombosis | Occasional (29-5%)"

Orphanet classifies cerebral venous sinus thrombosis as occasional in CBS-deficient homocystinuria.

🧬

Genetic Associations

CBS (cystathionine beta-synthase) deficiency

Autosomal recessive

Show evidence (3 references)

ORPHA:394 SUPPORT

"CBS | cystathionine beta-synthase | hgnc:1550 | Disease-causing germline mutation(s) in"

Orphanet confirms CBS as the disease-causing gene for ORPHA:394.

PMID:39041895 SUPPORT In Vitro

"Recent work suggests that missense pathogenic mutations-regardless of their topology-cause instability of the C-terminal regulatory domain, which likely translates into CBS misfolding, impaired assembly, and loss of function."

Directly supports the conformational disorder model for CBS-deficient HCU.

PMID:33295057 SUPPORT Human Clinical

"We developed comprehensive criteria to classify patients into four groups of pyridoxine responsivity: non-responders (NR), partial, full and extreme responders (PR, FR and ER, respectively)."

E-HOD registry provides systematic classification of pyridoxine responsiveness groups.

MTHFR (methylenetetrahydrofolate reductase) deficiency

Autosomal recessive

Show evidence (2 references)

PMID:40440437 SUPPORT Other

"biallelic pathogenic variants in MTHFR identified by molecular genetic testing."

GeneReviews supports biallelic MTHFR variants as the molecular basis of MTHFR-deficiency homocystinuria.

PMID:18658082 SUPPORT Human Clinical

"Severe methylenetetrahydrofolate reductase deficiency is an autosomal recessive metabolic disorder of folate metabolism causing elevated plasma homocysteine levels and homocystinuria (MIM 236250)."

Case-report abstract supports MTHFR deficiency as a nonclassical autosomal recessive cause of homocystinuria.

💊

Treatments

Pyridoxine (vitamin B6) supplementation

Action: Pharmacotherapy NCIT:C15986

Agent: pyridoxine ↗

Pyridoxine is the first-line pharmacological intervention for classical HCU. PLP is a CBS cofactor, and approximately half of patients show some degree of biochemical response (reduced tHcy) to pyridoxine supplementation. Responsiveness stratifies disease severity and guides further treatment decisions. Guidelines recommend keeping tHcy below 100 micromol/L; pyridoxine-responsive patients generally achieve tHcy below 50 micromol/L.

Mechanism Target:

RESTORES CBS molecular function deficiency — Pyridoxine supplies the vitamin B6-derived CBS cofactor in responsive variants, improving residual CBS function and lowering tHcy.

Show evidence (1 reference)

PMID:40095936 SUPPORT Human Clinical

"Pyridoxine-responsive patients generally achieved tHcy concentrations below 50 μmol/L"

Registry data show biochemical response in pyridoxine-responsive patients.

MODULATES Disrupted transsulfuration and methionine metabolism — Improved CBS cofactor availability lowers the homocysteine burden in responsive patients.

Show evidence (1 reference)

PMID:40095936 SUPPORT Human Clinical

"Pyridoxine-responsive patients generally achieved tHcy concentrations below 50 μmol/L"

Registry data connect pyridoxine response to improved tHcy control.

Show evidence (3 references)

PMID:27778219 SUPPORT Other

"we recommend keeping the concentration below 100 μmol/L because levels fluctuate and the complications associated with high levels are so serious."

Clinical guidelines define the tHcy treatment target for CBS-deficient HCU.

PMID:40095936 SUPPORT Human Clinical

"Pyridoxine-responsive patients generally achieved tHcy concentrations below 50 μmol/L"

E-HOD Part II shows pyridoxine-responsive patients generally achieve good biochemical control.

PMID:36417581 SUPPORT Other

"The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine"

Defines pyridoxine responsiveness as a key clinical stratifier.

Methionine-restricted diet

Action: dietary intervention MAXO:0000088

Dietary restriction of methionine intake with cysteine-enriched amino acid mixtures is the cornerstone of management for pyridoxine non-responsive patients. E-HOD Part II data show that treatment can prevent thromboembolism (risk ratio 0.073) and lens dislocation (risk ratio 0.069) in NR patients detected by newborn screening compared to those ascertained clinically.

Mechanism Target:

MODULATES Disrupted transsulfuration and methionine metabolism — Methionine restriction reduces upstream substrate burden across the blocked CBS-dependent pathway.

Show evidence (1 reference)

PMID:40095936 SUPPORT Human Clinical

"Severely affected patients usually present during childhood with learning difficulties, ectopia lentis and skeletal abnormalities; they are pyridoxine non-responders (NR) or partial responders (PR) and require treatment with a low-methionine diet and/or betaine."

Registry study states that severe non-responders/partial responders require low-methionine diet and/or betaine.

INHIBITS Endothelial dysfunction, thrombosis, and vascular injury — Early metabolic control prevents thromboembolic complications in non-responder patients.

Show evidence (1 reference)

PMID:40095936 SUPPORT Human Clinical

"treatment could prevent thromboembolism (risk ratio 0.073) and lens dislocation (risk ratio 0.069)."

Registry time-to-event data show reduced thromboembolism and lens dislocation under early treatment.

Show evidence (3 references)

PMID:40095936 SUPPORT Human Clinical

"treatment could prevent thromboembolism (risk ratio 0.073) and lens dislocation (risk ratio 0.069)."

E-HOD Part II provides quantitative evidence that early treatment dramatically reduces major complications.

PMID:35791106 SUPPORT Other

"The timely recognition of this rare metabolic disorder and prompt methionine-restricted diet are crucial in lessening the systemic consequences."

Directly supports methionine-restricted diet as crucial for reducing systemic consequences.

PMID:36417581 SUPPORT Other

"The current standard of care involves significant dietary interventions that, despite being effective, often adversely affect quality of life of HCU patients, leading to poor adherence to therapy"

Confirms dietary restriction as standard of care despite adherence challenges.

Betaine supplementation

Action: nutritional supplementation MAXO:0000106

Agent: betaine ↗

Betaine (trimethylglycine) serves as an alternative methyl donor supporting homocysteine remethylation via betaine-homocysteine methyltransferase (BHMT) in the liver. Used as adjunctive therapy to lower homocysteine levels, particularly in pyridoxine non-responders.

Mechanism Target:

BYPASSES Disrupted transsulfuration and methionine metabolism — Betaine supports alternative hepatic remethylation of homocysteine, reducing tHcy without requiring CBS flux.

Show evidence (1 reference)

PMID:38201964 SUPPORT Other

"Adequate intake of these vitamins, along with betaine supplementation, supports Hcy remethylation."

The review directly supports betaine as a remethylation-based bypass of homocysteine accumulation.

Show evidence (1 reference)

PMID:38201964 SUPPORT Other

"Adequate intake of these vitamins, along with betaine supplementation, supports Hcy remethylation."

Confirms betaine supplementation as supporting homocysteine remethylation in the management of hyperhomocysteinemia.

Folate and B12 supplementation

Action: nutritional supplementation MAXO:0000106

B-vitamin supplementation with folate and cobalamin supports homocysteine remethylation via methionine synthase, complementing dietary and pharmacological management.

Mechanism Target:

BYPASSES Disrupted transsulfuration and methionine metabolism — Folate and B12 support homocysteine conversion through remethylation pathways.

Show evidence (1 reference)

PMID:38201964 SUPPORT Other

"A nutritional approach to HHcy management involves implementing dietary strategies and targeted supplementation, emphasizing key nutrients like vitamin B6, B12, and folate that are crucial for Hcy conversion."

The review directly supports B-vitamin supplementation for Hcy conversion.

Show evidence (1 reference)

PMID:38201964 SUPPORT Other

Directly supports folate and B12 supplementation as part of nutritional HHcy management.

Enzyme replacement therapy (ERT)

Action: Pharmacotherapy NCIT:C15986

CBS-based enzyme replacement therapy is under development. ERT has shown efficacy in lowering plasma tHcy below 100 micromol/L in mouse models. Efficacy can be enhanced by co-administration of biological reductants (N-acetylcysteine, MESNA, cysteamine) that increase the availability of reduced homocysteine as a CBS substrate.

Mechanism Target:

RESTORES CBS molecular function deficiency — CBS enzyme replacement restores missing CBS catalytic capacity in model systems.

Show evidence (1 reference)

PMID:39366068 SUPPORT Model Organism

"Enzyme replacement therapy (ERT) based on human CBS has been developed and has shown significant efficacy correcting HCU phenotype in several mouse models by bringing plasma total homocysteine below the clinically relevant 100 μM threshold."

Mouse-model ERT data support direct restoration of CBS-dependent homocysteine clearance.

RESTORES Disrupted transsulfuration and methionine metabolism — ERT metabolizes reduced homocysteine and lowers total homocysteine in HCU mouse models.

Show evidence (1 reference)

PMID:39366068 SUPPORT Model Organism

"only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS"

The ERT mechanism paper identifies reduced homocysteine as the CBS substrate that determines enzyme-replacement clearance.

Show evidence (1 reference)

PMID:39366068 SUPPORT Model Organism

Directly supports the quantitative ERT efficacy claim in mouse models.

Thrombosis prevention

Action: Pharmacotherapy NCIT:C15986

Anticoagulation and thromboprophylaxis measures are important for managing vascular risk in HCU. Specific considerations include avoiding oral contraceptives, prophylactic anticoagulation during high-risk periods (surgery, pregnancy), and maintaining optimal metabolic control to reduce thromboembolic risk.

Mechanism Target:

INHIBITS Endothelial dysfunction, thrombosis, and vascular injury — Thromboprophylaxis addresses the final coagulation and embolic risk mechanism while metabolic control is optimized.

Show evidence (1 reference)

PMID:3872065 SUPPORT Human Clinical

"Following 586 surgical procedures, 25 postoperative thromboembolic complications occurred, six of which were fatal."

Natural history data document high-risk thromboembolic contexts that justify thromboprophylaxis.

Show evidence (1 reference)

PMID:36417581 SUPPORT Other

"the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment"

High thromboembolism risk in HCU supports need for thromboprophylaxis strategies.

Supportive care and antioxidant therapy

Action: supportive care MAXO:0000950

Supportive measures including monitoring for multisystem complications and consideration of antioxidant therapy. Evidence of oxidative damage and mitochondrial dysfunction supports potential benefit from antioxidant and mitochondrial support in CBS-deficient patients.

Mechanism Target:

MODULATES Oxidative stress and mitochondrial dysfunction — Antioxidant and mitochondrial support is aimed at the oxidative/mitochondrial stress node.

Show evidence (1 reference)

PMID:39567721 SUPPORT Human Clinical

"Assessment of oxidative damage and addition of anti-oxidant therapy together with mitochondrial support may have additional benefits in reducing long-term morbidity in CBSD patients."

Patient biomarker study directly suggests antioxidant and mitochondrial support for the oxidative damage mechanism.

Show evidence (1 reference)

PMID:39567721 SUPPORT Human Clinical

"Assessment of oxidative damage and addition of anti-oxidant therapy together with mitochondrial support may have additional benefits in reducing long-term morbidity in CBSD patients."

Directly recommends assessment of oxidative damage and consideration of antioxidant therapy with mitochondrial support.

Genetic counseling

Action: genetic counseling MAXO:0000079

Genetic counseling for affected families including discussion of inheritance, carrier detection, prenatal/preimplantation testing options, and recurrence risk.

Mechanism Target:

CBS molecular function deficiency — Genetic counseling is anchored to the familial CBS pathogenic variants and supports carrier and prenatal/preimplantation testing; it does not directly alter metabolism.

Show evidence (1 reference)

PMID:20301697 SUPPORT Other

"Once the CBS pathogenic variants have been identified in an affected family member, carrier testing for at-risk family members and prenatal/preimplantation genetic testing are possible."

CBS GeneReviews links genetic counseling actions to identified familial CBS pathogenic variants.

MTHFR remethylation pathway deficiency — Genetic counseling is anchored to the familial MTHFR pathogenic variants and supports carrier and prenatal/preimplantation testing; it does not directly alter metabolism.

Show evidence (1 reference)

PMID:40440437 SUPPORT Other

"Once the MTHFR pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible."

GeneReviews links genetic counseling actions to the identified familial MTHFR pathogenic variants.

Show evidence (2 references)

PMID:40440437 SUPPORT Other

"Once the MTHFR pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible."

GeneReviews directly supports carrier testing and prenatal/preimplantation genetic testing as genetic counseling content for inherited homocystinuria.

PMID:20301697 SUPPORT Other

"Once the CBS pathogenic variants have been identified in an affected family member, carrier testing for at-risk family members and prenatal/preimplantation genetic testing are possible."

CBS GeneReviews directly supports carrier testing and prenatal/preimplantation genetic testing after familial CBS variants are identified.

🔬

Biochemical Markers

Total homocysteine (tHcy) (INCREASED)

Context: Massively elevated total plasma homocysteine is the biochemical hallmark of classical HCU. Normal tHcy is typically less than 15 micromol/L; tHcy above 100 micromol/L is usually diagnostic in the appropriate clinical context.

Pathograph Readouts

Readout Of Disrupted transsulfuration and methionine metabolism Positive Diagnostic

Elevated total homocysteine reports impaired CBS-dependent homocysteine clearance in classical HCU.

Readout Of MTHFR remethylation pathway deficiency Positive Diagnostic

Elevated total homocysteine also reports impaired folate-dependent remethylation in MTHFR-deficiency homocystinuria.

Show evidence (2 references)

PMID:38962401 SUPPORT Human Clinical

"An algorithm was developed to identify 2 cohorts of patients using broad and strict definitions of HCU."

US prevalence study confirms elevated tHcy as the primary diagnostic criterion for classical HCU.

PMID:39567721 SUPPORT Human Clinical

"Cystathionine beta-synthase deficiency (CBSD) is the most prevalent inherited disorder of homocysteine metabolism in the transsulphuration pathway."

Study documents markedly elevated tHcy in CBS-deficient patients compared to controls.

Methionine (INCREASED)

Context: Elevated plasma methionine results from upstream metabolite accumulation due to the CBS block.

Pathograph Readouts

Readout Of Disrupted transsulfuration and methionine metabolism Positive Diagnostic

Elevated methionine supports classical CBS-deficient HCU and helps distinguish it from remethylation defects with low or normal methionine.

Show evidence (1 reference)

ORPHA:394 SUPPORT

"A rare metabolic disease of methionine catabolism characterized by accumulation of methionine and homocysteine with clinical involvement of the eye, skeletal system, vascular system and central nervous system (CNS)."

Orphanet directly supports methionine accumulation in CBS-deficient homocystinuria.

Cysteine (DECREASED)

Context: Plasma cysteine is depleted downstream of the CBS metabolic block. Cysteine depletion impairs glutathione synthesis and contributes to the oxidative stress observed in HCU patients.

Pathograph Readouts

Readout Of Disrupted transsulfuration and methionine metabolism Negative Diagnostic

Lower cysteine reflects reduced downstream transsulfuration flux from homocysteine through CBS.

Correlates With Oxidative stress and mitochondrial dysfunction Negative Monitoring

Lower cysteine can limit thiol and glutathione homeostasis, aligning with oxidative stress in CBS deficiency.

Show evidence (1 reference)

PMID:39366068 SUPPORT Model Organism

"only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS"

The metabolic block at CBS depletes downstream cysteine, as the transsulfuration pathway to cysteine is interrupted.

GDF-15 (mitokine) (INCREASED)

Context: Growth differentiation factor 15 is elevated in CBS-deficient patients and correlates positively with total homocysteine levels, serving as a biomarker of mitochondrial stress in HCU.

Pathograph Readouts

Readout Of Oxidative stress and mitochondrial dysfunction Positive Monitoring

Elevated GDF-15 reports mitochondrial stress associated with total homocysteine burden in CBS deficiency.

Show evidence (1 reference)

PMID:39567721 SUPPORT Human Clinical

"In patient group, a positive correlation was found between the total homocysteine level and both GDF-15 and NAD+/NADH levels."

Directly supports elevated GDF-15 linked to homocysteine burden in CBS-deficient patients.

FGF-21 (mitokine) (INCREASED)

Context: Fibroblast growth factor 21 is elevated in CBS-deficient patients, reflecting mitochondrial dysfunction and energy stress.

Pathograph Readouts

Readout Of Oxidative stress and mitochondrial dysfunction Positive Monitoring

Altered FGF-21 is modeled as a mitochondrial stress readout in CBS-deficient patients.

Show evidence (1 reference)

PMID:39567721 SUPPORT Human Clinical

"The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD."

Directly supports altered FGF-21 as a mitochondrial stress signal in CBS deficiency.

NAD+/NADH ratio (DECREASED)

Context: The NAD+/NADH ratio is significantly reduced in CBS-deficient patients, reflecting disturbed mitochondrial redox homeostasis.

Pathograph Readouts

Readout Of Oxidative stress and mitochondrial dysfunction Negative Monitoring

Reduced NAD+/NADH ratio reports redox imbalance associated with mitochondrial dysfunction in CBS deficiency.

Show evidence (1 reference)

PMID:39567721 SUPPORT Human Clinical

"The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD."

Directly supports NAD redox-ratio disturbance in CBS-deficient patients.

{ }

Source YAML

click to show

name: Homocystinuria
category: Mendelian
creation_date: '2025-06-12T20:16:27Z'
updated_date: '2026-05-21T18:27:18Z'
synonyms:
- Classical homocystinuria
- CBS-deficient homocystinuria
- Cystathionine beta-synthase deficiency
- HCU
description: 'Homocystinuria is an autosomal recessive inborn error of sulfur amino acid metabolism, most commonly caused by deficiency of cystathionine beta-synthase (CBS). The metabolic block at the transsulfuration branchpoint of methionine metabolism causes accumulation of homocysteine and methionine with depletion of downstream products (cystathionine, cysteine). Elevated homocysteine drives multisystem disease through oxidative stress, mitochondrial dysfunction, protein modifications via homocysteine thiolactone, endothelial dysfunction with thrombosis, and NMDA receptor-mediated neurotoxicity. Clinical features include ectopia lentis, skeletal abnormalities with marfanoid habitus, thromboembolism, and cognitive impairment. Disease severity is strongly modified by pyridoxine (vitamin B6) responsiveness.

  '
disease_term:
  preferred_term: homocystinuria
  term:
    id: MONDO:0004737
    label: homocystinuria
parents:
- Metabolic Disease
- Inborn Error of Metabolism
prevalence:
- population: Global clinically recognized populations
  percentage: 0.82 per 100,000
  notes: >-
    A genetic-database epidemiology study summarized prior meta-analytic data and
    estimated clinically recognized classical homocystinuria prevalence at about
    0.82 per 100,000 worldwide, with somewhat higher newborn-screening-based
    estimates.
  evidence:
  - reference: PMID:32232970
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The worldwide prevalence of HCU is estimated to be 0.82:100,000"
    explanation: This epidemiology study explicitly summarizes global clinically recognized and newborn-screening-based prevalence estimates for classical homocystinuria.
- population: United States claims-based cohort (strict case definition)
  percentage: 1.04 per 100,000
  notes: >-
    A recent US claims analysis produced a similar but slightly higher
    standardized prevalence estimate under a strict identification algorithm,
    again suggesting underdiagnosis in routine clinical practice.
  evidence:
  - reference: PMID:38962401
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The average annual standardized prevalence across 2016-2020 was 5.29 and 1.04 per 100,000 people for the broad and strict cohorts, respectively."
    explanation: This US claims-based study provides a contemporary prevalence estimate for classical homocystinuria using a strict ascertainment algorithm.
- population: Worldwide (newborn screening)
  percentage: 1-9 per 1,000,000
  notes: >-
    Orphanet reports worldwide prevalence at birth of 1-9 per 1,000,000,
    though this likely underestimates true prevalence due to insensitivity
    of methionine-based newborn screening for pyridoxine-responsive forms.
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "1-9 / 1 000 000 | Worldwide | Prevalence at birth | PMID:10328723"
    explanation: Orphanet epidemiology data for worldwide prevalence at birth.
- population: Qatar (newborn screening)
  percentage: 1-5 per 10,000
  notes: >-
    Qatar has one of the highest reported prevalences of CBS-deficient homocystinuria
    in the world, attributed to high consanguinity rates. Direct tHcy measurement
    in newborn screening is used in Qatar.
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "1-5 / 10 000 | Qatar | Prevalence at birth | PMID:19914636"
    explanation: Orphanet epidemiology data showing very high prevalence in Qatar.
pathophysiology:
- name: CBS molecular function deficiency
  description: 'Biallelic CBS pathogenic variants reduce cystathionine beta-synthase catalytic activity.

    '
  genes:
  - preferred_term: CBS
    term:
      id: hgnc:1550
      label: CBS
  molecular_functions:
  - preferred_term: cystathionine beta-synthase activity
    term:
      id: GO:0004122
      label: cystathionine beta-synthase activity
    modifier: DECREASED
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  evidence:
  - reference: PMID:27778219
    reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine."
    explanation: Clinical guidelines identify CBS deficiency and impaired cystathionine synthesis as the causal biochemical lesion.
  downstream:
  - target: Disrupted transsulfuration and methionine metabolism
    description: Reduced CBS activity blocks homocysteine-to-cystathionine flux.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:27778219
      reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine."
      explanation: The guideline directly links impaired CBS-dependent cystathionine synthesis to homocysteine accumulation.
- name: Disrupted transsulfuration and methionine metabolism
  description: 'Loss of CBS function causes upstream accumulation of homocysteine and methionine with downstream depletion of cystathionine and cysteine, disrupting sulfur amino acid homeostasis.

    '
  biological_processes:
  - preferred_term: sulfur amino acid metabolic process
    term:
      id: GO:0000096
      label: sulfur amino acid metabolic process
    modifier: ABNORMAL
  - preferred_term: homocysteine metabolic process
    term:
      id: GO:0050667
      label: homocysteine metabolic process
    modifier: ABNORMAL
  - preferred_term: transsulfuration
    term:
      id: GO:0019346
      label: transsulfuration
    modifier: DECREASED
  chemical_entities:
  - preferred_term: homocysteine
    term:
      id: CHEBI:17230
      label: homocysteine
    modifier: INCREASED
  - preferred_term: methionine
    term:
      id: CHEBI:16811
      label: methionine
    modifier: INCREASED
  - preferred_term: cystathionine
    term:
      id: CHEBI:17755
      label: cystathionine
    modifier: DECREASED
  - preferred_term: cysteine
    term:
      id: CHEBI:15356
      label: cysteine
    modifier: DECREASED
  evidence:
  - reference: PMID:36417581
    reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine, a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications
    explanation: Confirms massive homocysteine elevation as the central biochemical defect driving clinical complications.
  downstream:
  - target: Total homocysteine (tHcy)
    description: CBS blockade produces the diagnostic accumulation of total plasma homocysteine.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:27778219
      reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine."
      explanation: The guideline directly supports homocysteine accumulation downstream of impaired CBS activity.
  - target: Hyperhomocystinemia
    description: Elevated plasma homocysteine is the clinical phenotype-level expression of the biochemical block.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002160 | Hyperhomocystinemia | Very frequent (99-80%)"
      explanation: Orphanet classifies hyperhomocystinemia as very frequent in CBS-deficient homocystinuria.
  - target: Methionine
    description: The CBS block occurs in methionine catabolism and causes upstream methionine accumulation in classical HCU.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: A rare metabolic disease of methionine catabolism characterized by accumulation of methionine and homocysteine with clinical involvement of the eye, skeletal system, vascular system and central nervous system (CNS).
      explanation: Orphanet explicitly describes methionine and homocysteine accumulation in CBS-deficient homocystinuria.
  - target: Cysteine
    description: Blocked transsulfuration reduces downstream cysteine production.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - CBS-dependent cystathionine production precedes cystathionase-mediated cysteine synthesis.
    evidence:
    - reference: PMID:39366068
      reference_title: "Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria."
      supports: PARTIAL
      evidence_source: MODEL_ORGANISM
      snippet: "only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS"
      explanation: The ERT mechanism paper supports the substrate-level CBS flux step; cysteine depletion is inferred from interruption of the downstream transsulfuration pathway.
  - target: Hepatomegaly
    description: Hepatomegaly is grouped with systemic manifestations of sulfur-amino-acid metabolic disruption in HCU.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: PARTIAL
      snippet: "HP:0002240 | Hepatomegaly | Occasional (29-5%)"
      explanation: Orphanet supports hepatomegaly as an occasional phenotype; the metabolic-node assignment reflects hepatic sulfur-amino-acid pathway involvement.
  - target: Oxidative stress and mitochondrial dysfunction
    description: Homocysteine burden and downstream thiol imbalance contribute to oxidative and mitochondrial stress.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:39567721
      reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In patient group, a positive correlation was found between the total homocysteine level and both GDF-15 and NAD+/NADH levels."
      explanation: Patient biomarker data link total homocysteine burden to mitochondrial stress readouts.
  - target: Fibrin homocysteinylation and collagen cross-link deficiency
    description: Reactive homocysteine species modify plasma proteins, while patient data support a collagen cross-linking defect in bone.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:24659173
      reference_title: "Impact of homocysteine-thiolactone on plasma fibrin networks."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "HTL reacts with proteins by acylation of free basic amino groups; in particular, the epsilon-amino group of lysine residues forms adducts and induces structural and functional changes in plasma proteins."
      explanation: This ex vivo plasma study supports a protein-modification mechanism downstream of homocysteine-thiolactone.
  - target: Homocysteic acid neuroexcitotoxicity
    description: Homocysteine-derived excitatory metabolites affect neuronal signaling.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:1676834
      reference_title: "L-homocysteic acid mediates synaptic excitation at NMDA receptors in the hippocampus."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "It is concluded that L-HC is an endogenous NMDA agonist at the Schaffer collateral-CA1 synapse."
      explanation: Rat hippocampal physiology supports homocysteic-acid NMDA agonism as a plausible neurotoxicity mechanism.
- name: MTHFR remethylation pathway deficiency
  description: >
    Biallelic MTHFR pathogenic variants reduce methylenetetrahydrofolate reductase activity in the folate-dependent
    remethylation pathway. This nonclassical homocystinuria mechanism causes elevated plasma and urine homocysteine
    with low or low-normal methionine, converging on the shared hyperhomocystinemia node while remaining biochemically
    distinct from CBS-deficient classical homocystinuria.
  genes:
  - preferred_term: MTHFR
    term:
      id: hgnc:7436
      label: MTHFR
  molecular_functions:
  - preferred_term: methylenetetrahydrofolate reductase [NAD(P)H] activity
    term:
      id: GO:0004489
      label: methylenetetrahydrofolate reductase [NAD(P)H] activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: one-carbon metabolic process
    term:
      id: GO:0006730
      label: one-carbon metabolic process
    modifier: ABNORMAL
  - preferred_term: folic acid metabolic process
    term:
      id: GO:0046655
      label: folic acid metabolic process
    modifier: ABNORMAL
  - preferred_term: homocysteine metabolic process
    term:
      id: GO:0050667
      label: homocysteine metabolic process
    modifier: ABNORMAL
  chemical_entities:
  - preferred_term: homocysteine
    term:
      id: CHEBI:17230
      label: homocysteine
    modifier: INCREASED
  - preferred_term: methionine
    term:
      id: CHEBI:16811
      label: methionine
    modifier: DECREASED
  evidence:
  - reference: PMID:40440437
    reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "N(5,10)-MTHFR activity is established in a proband with characteristic clinical and laboratory findings (low plasma methionine, increased plasma and urine homocysteine"
    explanation: GeneReviews supports MTHFR deficiency as a folate-remethylation cause of homocystinuria with high homocysteine and low methionine.
  - reference: PMID:18658082
    reference_title: "Severe methylenetetrahydrofolate reductase (MTHFR) deficiency: a case report of nonclassical homocystinuria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe methylenetetrahydrofolate reductase deficiency is an autosomal recessive metabolic disorder of folate metabolism causing elevated plasma homocysteine levels and homocystinuria (MIM 236250)."
    explanation: A human case report supports severe MTHFR deficiency as nonclassical homocystinuria through folate-metabolism disruption.
  downstream:
  - target: Hyperhomocystinemia
    description: MTHFR deficiency converges with CBS deficiency on elevated homocysteine as a shared biochemical phenotype.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:40440437
      reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "N(5,10)-MTHFR activity is established in a proband with characteristic clinical and laboratory findings (low plasma methionine, increased plasma and urine homocysteine"
      explanation: GeneReviews directly lists increased plasma and urine homocysteine in MTHFR-deficiency homocystinuria.
  - target: Total homocysteine (tHcy)
    description: Plasma homocysteine surveillance reports disease activity in MTHFR-deficiency homocystinuria.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:40440437
      reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Surveillance: Plasma homocysteine and plasma amino acids to assess methionine concentration every three to four months"
      explanation: GeneReviews identifies plasma homocysteine surveillance as a monitoring readout for MTHFR-deficiency homocystinuria.
- name: Oxidative stress and mitochondrial dysfunction
  description: 'Elevated homocysteine is a reactive thiol that generates reactive oxygen species, while cysteine depletion impairs glutathione-dependent antioxidant defenses. CBS-deficient patients show altered NAD redox metrics, elevated mitokines (FGF-21, GDF-15), and increased mitochondrial DAMPs, indicating coupled oxidative damage and mitochondrial dysfunction.

    '
  biological_processes:
  - preferred_term: cellular response to oxidative stress
    term:
      id: GO:0034599
      label: cellular response to oxidative stress
  - preferred_term: mitochondrion organization
    term:
      id: GO:0007005
      label: mitochondrion organization
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  evidence:
  - reference: PMID:39567721
    reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD.
    explanation: Directly supports the coupled oxidative stress and mitochondrial dysfunction mechanism in CBS deficiency.
  downstream:
  - target: GDF-15 (mitokine)
    description: GDF-15 is elevated as a mitochondrial stress biomarker and correlates with total homocysteine in patients.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:39567721
      reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In patient group, a positive correlation was found between the total homocysteine level and both GDF-15 and NAD+/NADH levels."
      explanation: Patient data link homocysteine burden with the GDF-15 mitokine readout.
  - target: FGF-21 (mitokine)
    description: FGF-21 elevation is part of the patient mitochondrial stress signature.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:39567721
      reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD."
      explanation: Patient data support FGF-21 alteration as a mitochondrial stress marker in CBSD.
  - target: NAD+/NADH ratio
    description: NAD redox imbalance is a measurable downstream readout of mitochondrial dysfunction.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:39567721
      reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD."
      explanation: Patient data directly support altered NAD+/NADH ratio in CBSD.
  - target: Endothelial dysfunction, thrombosis, and vascular injury
    description: Oxidative injury contributes to endothelial dysfunction and thrombotic vascular complications.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38201964
      reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."
      explanation: The review links moderate-to-severe hyperhomocysteinemia with arterial and venous thrombotic vascular events.
- name: CBS protein misfolding and proteostasis disruption
  description: 'Many CBS pathogenic variants cause protein misfolding and instability rather than active-site disruption. Misfolded CBS is cleared primarily by proteasomal degradation with contributions from ERAD and lysosomal-autophagic pathways. This conformational disease mechanism suggests proteostasis modulation as a potential therapeutic strategy.

    '
  biological_processes:
  - preferred_term: proteasome-mediated ubiquitin-dependent protein catabolic process
    term:
      id: GO:0043161
      label: proteasome-mediated ubiquitin-dependent protein catabolic process
  - preferred_term: endoplasmic reticulum unfolded protein response
    term:
      id: GO:0030968
      label: endoplasmic reticulum unfolded protein response
  evidence:
  - reference: PMID:38051092
    reference_title: "Genetic and Pharmacological Modulation of Cellular Proteostasis Leads to Partial Functional Rescue of Homocystinuria-Causing Cystathionine-Beta Synthase Variants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Endoplasmic reticulum stress sensor BiP was found upregulated with CBS I278T variant associated with proteasomes suggesting proteotoxic stress and degradation of misfolded CBS.
    explanation: Demonstrates ER stress and proteotoxic stress in cells expressing pathogenic CBS variants.
  downstream:
  - target: CBS molecular function deficiency
    description: Misfolding and proteasomal turnover reduce residual CBS activity for many pathogenic missense variants.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:39041895
      reference_title: "Cellular turnover and degradation of the most common missense cystathionine beta-synthase variants causing homocystinuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "instability of the C-terminal regulatory domain, which likely translates into CBS misfolding, impaired assembly, and loss of function."
      explanation: Cellular turnover data directly connect variant instability and misfolding to CBS loss of function.
  - target: Disrupted transsulfuration and methionine metabolism
    description: Proteostasis-mediated CBS loss of function reduces transsulfuration flux.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Misfolded CBS degradation reduces functional CBS enzyme activity.
    evidence:
    - reference: PMID:39041895
      reference_title: "Cellular turnover and degradation of the most common missense cystathionine beta-synthase variants causing homocystinuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "instability of the C-terminal regulatory domain, which likely translates into CBS misfolding, impaired assembly, and loss of function."
      explanation: Cellular turnover data connect CBS variant misfolding to loss of function, the proximal intermediate that disrupts transsulfuration.
    - reference: PMID:27778219
      reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine."
      explanation: The guideline links CBS functional deficiency to impaired cystathionine synthesis and homocysteine accumulation.
- name: Endothelial dysfunction, thrombosis, and vascular injury
  description: 'Homocysteine-driven oxidative stress and protein modifications cause endothelial cell dysfunction and platelet activation, promoting thrombus formation and vascular occlusion. Modification of fibrinogen by homocysteine thiolactone increases resistance to fibrinolysis, providing a direct mechanistic link to the high thromboembolism risk in HCU.

    '
  biological_processes:
  - preferred_term: blood coagulation
    term:
      id: GO:0007596
      label: blood coagulation
  cell_types:
  - preferred_term: endothelial cell
    term:
      id: CL:0000115
      label: endothelial cell
  - preferred_term: platelet
    term:
      id: CL:0000233
      label: platelet
  evidence:
  - reference: PMID:36417581
    reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment
    explanation: Identifies thromboembolism as a major clinical complication driven by homocysteine toxicity.
  - reference: PMID:24659173
    reference_title: "Impact of homocysteine-thiolactone on plasma fibrin networks."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Since these features of the networks have been related to impaired fibrinolysis, the N-homocysteinylation reactions would be involved in the prothrombotic effects associated to hyperhomocysteinemia."
    explanation: Ex vivo fibrin-network data support impaired fibrinolysis as a prothrombotic mechanism of homocysteine-thiolactone.
  downstream:
  - target: Thromboembolism
    description: Endothelial injury, altered coagulation, and impaired fibrinolysis drive the aggregate thromboembolism phenotype.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:3872065
      reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "initial clinically detected thromboembolic events (at age 15, chances of having had such an event: 12% and 27%, respectively)"
      explanation: Natural history data quantify thromboembolic events as a major vascular outcome of CBS deficiency.
  - target: Arterial thrombosis
    description: The vascular-injury mechanism includes arterial thrombotic events.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38201964
      reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."
      explanation: The review explicitly links hyperhomocysteinemia with arterial thrombotic vascular events.
  - target: Venous thrombosis
    description: The vascular-injury mechanism includes venous thrombotic events.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38201964
      reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."
      explanation: The review explicitly links hyperhomocysteinemia with venous thrombotic vascular events.
  - target: Pulmonary embolism
    description: Pulmonary embolism is a frequent embolic manifestation of the thrombotic vascular mechanism.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002204 | Pulmonary embolism | Frequent (79-30%)"
      explanation: Orphanet classifies pulmonary embolism as a frequent phenotype in CBS-deficient homocystinuria.
  - target: Cerebral ischemia
    description: Arterial thrombosis can produce cerebral ischemia and stroke-like presentations.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002637 | Cerebral ischemia | Frequent (79-30%)"
      explanation: Orphanet classifies cerebral ischemia as a frequent phenotype in CBS-deficient homocystinuria.
  - target: Stroke
    description: Arterial or venous thrombotic vascular events can present clinically as stroke.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38201964
      reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy."
      explanation: The review links hyperhomocysteinemia with arterial and venous thrombotic vascular events, the mechanism underlying stroke presentations.
  - target: Cerebral venous sinus thrombosis
    description: Venous thrombosis can involve the cerebral venous sinuses.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0033724 | Cerebral venous sinus thrombosis | Occasional (29-5%)"
      explanation: Orphanet classifies cerebral venous sinus thrombosis as an occasional phenotype in CBS-deficient homocystinuria.
  - target: Hypertension
    description: Vascular remodeling and endothelial dysfunction provide a mechanism for frequent hypertension.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000822 | Hypertension | Frequent (79-30%)"
      explanation: Orphanet classifies hypertension as frequent in CBS-deficient homocystinuria.
  - target: Livedo reticularis
    description: Cutaneous livedo reflects microvascular involvement in the vascular phenotype cluster.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0033505 | Livedo reticularis | Occasional (29-5%)"
      explanation: Orphanet classifies livedo reticularis as an occasional phenotype in CBS-deficient homocystinuria.
  - target: Pancreatitis
    description: Pancreatitis is grouped with occasional systemic complications that can plausibly reflect vascular injury in severe HCU.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: PARTIAL
      snippet: "HP:0001733 | Pancreatitis | Occasional (29-5%)"
      explanation: Orphanet supports pancreatitis as an occasional phenotype; vascular-mechanism assignment is a plausible grouping rather than direct proof.
- name: Fibrin homocysteinylation and collagen cross-link deficiency
  description: >
    Homocysteine-thiolactone modifies lysine residues on plasma fibrinogen/fibrin and alters fibrin
    network structure and fibrinolysis. Separately, patient data show reduced collagen type I cross-links
    in homocystinuria, supporting a collagen cross-linking defect for bone manifestations.
  biological_processes:
  - preferred_term: collagen metabolic process
    term:
      id: GO:0032963
      label: collagen metabolic process
  chemical_entities:
  - preferred_term: L-homocysteine thiolactone
    term:
      id: CHEBI:60315
      label: L-homocysteine thiolactone
  evidence:
  - reference: PMID:8611653
    reference_title: "Evidence for McKusick's hypothesis of deficient collagen cross-linking in patients with homocystinuria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This significant reduction of cross-links in the group with homocystinuria did not correlate with serum homocysteine or homocysteic acid concentrations."
    explanation: A patient study directly supports reduced collagen type I cross-links in homocystinuria.
  - reference: PMID:24659173
    reference_title: "Impact of homocysteine-thiolactone on plasma fibrin networks."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "HTL reacts with proteins by acylation of free basic amino groups; in particular, the epsilon-amino group of lysine residues forms adducts and induces structural and functional changes in plasma proteins."
    explanation: Ex vivo plasma data support homocysteine-thiolactone modification of fibrinogen/fibrin as a structural protein mechanism.
  downstream:
  - target: Osteoporosis
    description: Reduced collagen cross-linking contributes to the bone fragility and osteoporosis phenotype.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:8611653
      reference_title: "Evidence for McKusick's hypothesis of deficient collagen cross-linking in patients with homocystinuria."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Our data clearly indicate that the disturbed cross-linking hypothesis still holds and that the bone manifestations of homocystinuria are not due to deficient collagen synthesis."
      explanation: The patient study directly links deficient collagen cross-linking to bone manifestations.
  - target: Recurrent fractures
    description: Collagen cross-link deficiency provides a mechanism for bone fragility and fracture susceptibility.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002757 | Recurrent fractures | Very frequent (99-80%)"
      explanation: Orphanet classifies recurrent fractures as very frequent in CBS-deficient homocystinuria.
  - target: Ectopia lentis
    description: Connective-tissue matrix dysfunction provides a mechanistic bridge to lens zonule instability and ectopia lentis.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:3872065
      reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "dislocation of optic lenses (at age 10, chances of dislocation: 55% and 82%, respectively)"
      explanation: Natural history data support lens dislocation as a major structural complication of CBS deficiency.
  - target: Myopia
    description: Myopia is grouped with ocular structural complications downstream of connective-tissue involvement in HCU.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000545 | Myopia | Frequent (79-30%)"
      explanation: Orphanet classifies myopia as a frequent ocular phenotype in CBS-deficient homocystinuria.
  - target: Amblyopia
    description: Amblyopia is linked to the ocular structural phenotype cluster, including lens displacement and refractive disturbance.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000646 | Amblyopia | Frequent (79-30%)"
      explanation: Orphanet classifies amblyopia as frequent in CBS-deficient homocystinuria.
  - target: Glaucoma
    description: Glaucoma can occur as part of the ocular complication cluster associated with lens instability.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000501 | Glaucoma | Occasional (29-5%)"
      explanation: Orphanet classifies glaucoma as an occasional ocular phenotype in CBS-deficient homocystinuria.
  - target: Retinal detachment
    description: Retinal detachment belongs to the ocular complication cluster downstream of connective-tissue and high-myopia effects.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000541 | Retinal detachment | Occasional (29-5%)"
      explanation: Orphanet classifies retinal detachment as an occasional ocular phenotype in CBS-deficient homocystinuria.
  - target: Strabismus
    description: Strabismus is grouped with ocular structural and refractive complications in CBS-deficient homocystinuria.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000486 | Strabismus | Occasional (29-5%)"
      explanation: Orphanet classifies strabismus as an occasional ocular phenotype in CBS-deficient homocystinuria.
  - target: Cataract
    description: Cataract is grouped with lens and ocular complications in CBS-deficient homocystinuria.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000518 | Cataract | Occasional (29-5%)"
      explanation: Orphanet classifies cataract as an occasional ocular phenotype in CBS-deficient homocystinuria.
  - target: Marfanoid habitus
    description: Collagen cross-link and connective-tissue disruption provide a mechanism for the marfanoid skeletal habitus.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:33295057
      reference_title: "Cystathionine beta-synthase deficiency in the E-HOD registry-part I."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "neurodevelopmental problems, lens dislocation and marfanoid features in early childhood"
      explanation: E-HOD registry data support marfanoid features as part of early classical homocystinuria.
  - target: Arachnodactyly
    description: Arachnodactyly is part of the marfanoid connective-tissue phenotype cluster.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0001166 | Arachnodactyly | Very frequent (99-80%)"
      explanation: Orphanet classifies arachnodactyly as very frequent in CBS-deficient homocystinuria.
  - target: Dental crowding
    description: Dental crowding is grouped with the craniofacial and skeletal connective-tissue phenotype cluster.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000678 | Dental crowding | Very frequent (99-80%)"
      explanation: Orphanet classifies dental crowding as very frequent in CBS-deficient homocystinuria.
  - target: Scoliosis
    description: Scoliosis is a skeletal deformity downstream of connective-tissue and bone-matrix abnormalities.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002650 | Scoliosis | Frequent (79-30%)"
      explanation: Orphanet classifies scoliosis as frequent in CBS-deficient homocystinuria.
  - target: Pectus excavatum
    description: Pectus excavatum is part of the thoracic skeletal deformity cluster in CBS-deficient homocystinuria.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000767 | Pectus excavatum | Frequent (79-30%)"
      explanation: Orphanet classifies pectus excavatum as frequent in CBS-deficient homocystinuria.
  - target: Pectus carinatum
    description: Pectus carinatum is part of the thoracic skeletal deformity cluster in CBS-deficient homocystinuria.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000768 | Pectus carinatum | Frequent (79-30%)"
      explanation: Orphanet classifies pectus carinatum as frequent in CBS-deficient homocystinuria.
  - target: Kyphosis
    description: Kyphosis reflects the skeletal deformity and bone-quality phenotype cluster.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002808 | Kyphosis | Frequent (79-30%)"
      explanation: Orphanet classifies kyphosis as frequent in CBS-deficient homocystinuria.
  - target: Genu valgum
    description: Genu valgum is grouped with lower-limb skeletal deformities in CBS-deficient homocystinuria.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0002857 | Genu valgum | Frequent (79-30%)"
      explanation: Orphanet classifies genu valgum as frequent in CBS-deficient homocystinuria.
  - target: Joint stiffness
    description: Joint stiffness is part of the connective-tissue and skeletal phenotype cluster.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0001387 | Joint stiffness | Frequent (79-30%)"
      explanation: Orphanet classifies joint stiffness as frequent in CBS-deficient homocystinuria.
  - target: Pes cavus
    description: Pes cavus is grouped with skeletal and lower-limb structural manifestations.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0001761 | Pes cavus | Frequent (79-30%)"
      explanation: Orphanet classifies pes cavus as frequent in CBS-deficient homocystinuria.
  - target: High palate
    description: High palate is grouped with craniofacial skeletal manifestations of the connective-tissue phenotype.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000218 | High palate | Occasional (29-5%)"
      explanation: Orphanet classifies high palate as occasional in CBS-deficient homocystinuria.
  - target: Sparse scalp hair
    description: Sparse scalp hair is grouped with the connective-tissue and integumentary phenotype spectrum in HCU.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: PARTIAL
      snippet: "HP:0002209 | Sparse scalp hair | Frequent (79-30%)"
      explanation: Orphanet supports the phenotype association; assignment to the connective-tissue cluster is a mechanistic grouping.
- name: Homocysteic acid neuroexcitotoxicity
  description: >
    Homocysteine-derived excitatory metabolites activate NMDA receptor signaling, providing a
    plausible bridge from the metabolic lesion to developmental delay, cognitive impairment, seizures,
    and neuropsychiatric manifestations.
  biological_processes:
  - preferred_term: ionotropic glutamate receptor signaling pathway
    term:
      id: GO:0035235
      label: ionotropic glutamate receptor signaling pathway
  - preferred_term: chemical synaptic transmission
    term:
      id: GO:0007268
      label: chemical synaptic transmission
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  chemical_entities:
  - preferred_term: homocysteic acid
    term:
      id: CHEBI:90324
      label: homocysteic acid
  evidence:
  - reference: PMID:1676834
    reference_title: "L-homocysteic acid mediates synaptic excitation at NMDA receptors in the hippocampus."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "This effect was prevented by bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist CPP (20 microM)."
    explanation: Rat hippocampal physiology supports NMDA-receptor mediation of homocysteic-acid synaptic effects.
  - reference: PMID:36417581
    reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
    supports: PARTIAL
    evidence_source: OTHER
    snippet: "a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment"
    explanation: The review supports cognitive impairment as a major clinical complication of homocysteine toxicity.
  downstream:
  - target: Intellectual disability
    description: Neuroexcitotoxicity provides a plausible contribution to cognitive impairment.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:3872065
      reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "B6-responsive individuals on the average have significantly better mental capabilities (mean IQ, 79) than do B6-nonresponsive individuals (mean IQ, 57)"
      explanation: Natural history data quantify cognitive impairment severity by biochemical responsiveness group.
  - target: Global developmental delay
    description: Early neurotoxicity contributes to delayed neurodevelopment.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:33295057
      reference_title: "Cystathionine beta-synthase deficiency in the E-HOD registry-part I."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Developmental delay was commonest in the NR group while no ER patient had cognitive impairment."
      explanation: E-HOD registry data support developmental delay in the severe non-responder group.
  - target: Seizures
    description: NMDA-receptor activation by homocysteic acid provides a plausible seizure mechanism.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:1676834
      reference_title: "L-homocysteic acid mediates synaptic excitation at NMDA receptors in the hippocampus."
      supports: PARTIAL
      evidence_source: IN_VITRO
      snippet: "It is concluded that L-HC is an endogenous NMDA agonist at the Schaffer collateral-CA1 synapse."
      explanation: NMDA agonism supports a mechanistic bridge to seizure susceptibility, while the human phenotype is separately supported in the phenotype entry.
  - target: Psychosis
    description: Neurochemical disturbance may contribute to occasional psychiatric manifestations.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000709 | Psychosis | Occasional (29-5%)"
      explanation: Orphanet supports psychosis as an occasional neuropsychiatric manifestation of CBS-deficient homocystinuria.
  - target: Depression
    description: Depression is grouped with neuropsychiatric manifestations plausibly linked to metabolic neurotoxicity.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000716 | Depression | Occasional (29-5%)"
      explanation: Orphanet classifies depression as an occasional phenotype in CBS-deficient homocystinuria.
  - target: Anxiety
    description: Anxiety is grouped with neuropsychiatric manifestations plausibly linked to metabolic neurotoxicity.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0000739 | Anxiety | Occasional (29-5%)"
      explanation: Orphanet classifies anxiety as an occasional phenotype in CBS-deficient homocystinuria.
  - target: Dystonia
    description: Dystonia is grouped with neurologic manifestations potentially arising from metabolic or vascular brain injury.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "HP:0001332 | Dystonia | Occasional (29-5%)"
      explanation: Orphanet classifies dystonia as an occasional phenotype in CBS-deficient homocystinuria.
phenotypes:
- name: Ectopia lentis
  frequency: VERY_FREQUENT
  description: >
    Lens subluxation or dislocation is one of the most characteristic features of classical homocystinuria.
    At age 10, chances of dislocation are 55% in B6-responsive and 82% in B6-nonresponsive patients.
  phenotype_term:
    preferred_term: Ectopia lentis
    term:
      id: HP:0001083
      label: Ectopia lentis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001083 | Ectopia lentis | Very frequent (99-80%)"
    explanation: Orphanet classifies ectopia lentis as very frequent in CBS-deficient homocystinuria.
  - reference: PMID:3872065
    reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "dislocation of optic lenses (at age 10, chances of dislocation: 55% and 82%, respectively)"
    explanation: Classic natural history study of 629 patients quantifies lens dislocation rates by B6 responsiveness.
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Lens dislocation was common in all groups except ER but the age of dislocation increased with increasing responsiveness."
    explanation: E-HOD registry confirms lens dislocation is common across all pyridoxine responsiveness groups.
- name: Myopia
  frequency: FREQUENT
  description: >
    High myopia frequently accompanies or precedes ectopia lentis in homocystinuria patients, related to axial
    elongation and lens changes.
  phenotype_term:
    preferred_term: Myopia
    term:
      id: HP:0000545
      label: Myopia
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000545 | Myopia | Frequent (79-30%)"
    explanation: Orphanet classifies myopia as frequent in CBS-deficient homocystinuria.
- name: Thromboembolism
  frequency: FREQUENT
  description: >
    Arterial and venous thromboembolic events are a major source of morbidity and mortality in untreated HCU.
    At age 15, chances of having had a thromboembolic event are 12% in B6-responsive and 27% in B6-nonresponsive
    patients. Thromboembolism is the commonest presenting feature in extreme pyridoxine responders.
  phenotype_term:
    preferred_term: Thromboembolism
    term:
      id: HP:0001907
      label: Thromboembolism
  evidence:
  - reference: PMID:3872065
    reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "initial clinically detected thromboembolic events (at age 15, chances of having had such an event: 12% and 27%, respectively)"
    explanation: Classic natural history study quantifies thromboembolic event rates by B6 responsiveness.
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thromboembolism was the commonest presenting feature in ER patients, whereas it was least likely at presentation in the NR group."
    explanation: E-HOD registry shows thromboembolism is the commonest presentation in extreme responders.
  - reference: PMID:38201964
    reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy.
    explanation: Confirms arterial and venous thrombotic events as important clinical manifestations of hyperhomocysteinemia.
- name: Intellectual disability
  frequency: VERY_FREQUENT
  description: >
    Cognitive impairment ranging from mild learning difficulties to severe intellectual disability occurs in untreated
    classical HCU, with homocysteic-acid NMDA receptor agonism providing a plausible neurotoxicity mechanism.
    B6-responsive patients have mean IQ of 79 vs 57 in nonresponders. No extreme responder patients had cognitive impairment.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001249 | Intellectual disability | Very frequent (99-80%)"
    explanation: Orphanet classifies intellectual disability as very frequent in CBS-deficient homocystinuria.
  - reference: PMID:3872065
    reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "B6-responsive individuals on the average have significantly better mental capabilities (mean IQ, 79) than do B6-nonresponsive individuals (mean IQ, 57)"
    explanation: Classic natural history study quantifies cognitive impairment by B6 responsiveness.
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Developmental delay was commonest in the NR group while no ER patient had cognitive impairment."
    explanation: E-HOD registry confirms cognitive impairment is most common in non-responders.
- name: Seizures
  frequency: OCCASIONAL
  description: >
    Seizures occur in a subset of HCU patients, mechanistically linked to homocysteic acid acting as a potent
    agonist at glutamatergic NMDA receptors, causing excitotoxicity. Neonatal methionine restriction may reduce
    seizure incidence.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001250 | Seizure | Occasional (29-5%)"
    explanation: Orphanet classifies seizures as occasional in CBS-deficient homocystinuria.
  - reference: PMID:3872065
    reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Methionine restriction initiated neonatally prevented mental retardation, retarded the rate of lens dislocation, and may have reduced the incidence of seizures."
    explanation: Classic natural history study documents seizures as a feature and notes treatment may reduce incidence.
- name: Osteoporosis
  frequency: VERY_FREQUENT
  description: >
    Reduced bone mineral density and osteoporosis are common in HCU, linked to impaired collagen cross-linking
    in patient collagen studies. At age 15, chances of radiologic
    spinal osteoporosis are 36% in B6-responsive and 64% in B6-nonresponsive patients.
  phenotype_term:
    preferred_term: Osteoporosis
    term:
      id: HP:0000939
      label: Osteoporosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000939 | Osteoporosis | Very frequent (99-80%)"
    explanation: Orphanet classifies osteoporosis as very frequent in CBS-deficient homocystinuria.
  - reference: PMID:3872065
    reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "radiologic detection of spinal osteoporosis (at age 15, chances of such osteoporosis having been detected: 36% and 64%, respectively)"
    explanation: Classic natural history study quantifies spinal osteoporosis rates by B6 responsiveness.
- name: Marfanoid habitus
  frequency: VERY_FREQUENT
  description: >
    Tall, thin body habitus with long limbs and arachnodactyly resembling Marfan syndrome, resulting from connective
    tissue and skeletal abnormalities in classical homocystinuria.
  phenotype_term:
    preferred_term: Marfanoid habitus
    term:
      id: HP:0001519
      label: Disproportionate tall stature
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001519 | Disproportionate tall stature | Very frequent (99-80%)"
    explanation: Orphanet classifies disproportionate tall stature as very frequent in CBS-deficient homocystinuria.
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neurodevelopmental problems, lens dislocation and marfanoid features in early childhood"
    explanation: E-HOD registry describes marfanoid features as characteristic of early-presenting CBS deficiency.
- name: Global developmental delay
  frequency: OCCASIONAL
  description: 'Delayed achievement of developmental milestones in childhood, related to early neurotoxic effects of elevated homocysteine on the developing brain.

    '
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Developmental delay was commonest in the NR group while no ER patient had cognitive impairment."
    explanation: E-HOD registry data support developmental delay in the severe non-responder group.
- name: Scoliosis
  frequency: FREQUENT
  description: >
    Spinal deformity reflecting connective tissue abnormalities characteristic of the skeletal phenotype in homocystinuria.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002650 | Scoliosis | Frequent (79-30%)"
    explanation: Orphanet classifies scoliosis as frequent in CBS-deficient homocystinuria.
  - reference: PMID:27778219
    reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Severely affected patients usually present in childhood with ectopia lentis, learning difficulties and skeletal abnormalities."
    explanation: Clinical guidelines list skeletal abnormalities including scoliosis as characteristic of severe CBS deficiency.
- name: Stroke
  frequency: OCCASIONAL
  description: 'Cerebrovascular events can occur in HCU patients due to arterial thrombosis or embolism, particularly in untreated or poorly controlled disease.

    '
  phenotype_term:
    preferred_term: Stroke
    term:
      id: HP:0001297
      label: Stroke
  evidence:
  - reference: PMID:38201964
    reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy.
    explanation: Arterial thrombotic vascular events include stroke in the context of severe hyperhomocysteinemia.
- name: Arachnodactyly
  frequency: VERY_FREQUENT
  description: >
    Long, slender fingers and toes characteristic of the marfanoid skeletal phenotype in homocystinuria,
    resulting from connective tissue abnormalities.
  phenotype_term:
    preferred_term: Arachnodactyly
    term:
      id: HP:0001166
      label: Arachnodactyly
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001166 | Arachnodactyly | Very frequent (99-80%)"
    explanation: Orphanet classifies arachnodactyly as very frequent in CBS-deficient homocystinuria.
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neurodevelopmental problems, lens dislocation and marfanoid features in early childhood"
    explanation: E-HOD registry describes marfanoid features including arachnodactyly as characteristic of CBS deficiency.
- name: Dental crowding
  frequency: VERY_FREQUENT
  description: >
    Crowded dentition is a very frequent skeletal feature of CBS-deficient homocystinuria,
    related to the characteristic facial and skeletal growth pattern.
  phenotype_term:
    preferred_term: Dental crowding
    term:
      id: HP:0000678
      label: Dental crowding
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000678 | Dental crowding | Very frequent (99-80%)"
    explanation: Orphanet classifies dental crowding as very frequent in CBS-deficient homocystinuria.
- name: Recurrent fractures
  frequency: VERY_FREQUENT
  description: >
    Recurrent pathological fractures result from severe osteoporosis and impaired bone quality
    associated with disrupted collagen cross-linking in homocystinuria.
  phenotype_term:
    preferred_term: Recurrent fractures
    term:
      id: HP:0002757
      label: Recurrent fractures
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002757 | Recurrent fractures | Very frequent (99-80%)"
    explanation: Orphanet classifies recurrent fractures as very frequent in CBS-deficient homocystinuria.
- name: Hyperhomocystinemia
  frequency: VERY_FREQUENT
  category: Biochemical
  description: >
    Elevated plasma total homocysteine is the biochemical hallmark of CBS-deficient homocystinuria.
    Concentrations typically exceed 100 micromol/L and may reach over 200 micromol/L in untreated patients.
  phenotype_term:
    preferred_term: Hyperhomocystinemia
    term:
      id: HP:0002160
      label: Hyperhomocystinemia
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002160 | Hyperhomocystinemia | Very frequent (99-80%)"
    explanation: Orphanet classifies hyperhomocystinemia as very frequent in CBS-deficient homocystinuria.
  - reference: PMID:27778219
    reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We strongly recommend measuring the plasma total homocysteine concentrations in any patient whose clinical features suggest the diagnosis"
    explanation: Clinical guidelines recommend tHcy measurement as the key diagnostic test.
- name: Arterial thrombosis
  frequency: FREQUENT
  description: >
    Arterial thrombotic events including cerebral ischemia and peripheral arterial occlusion are
    frequent complications driven by homocysteine-mediated endothelial dysfunction and platelet activation.
  phenotype_term:
    preferred_term: Arterial thrombosis
    term:
      id: HP:0004420
      label: Arterial thrombosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0004420 | Arterial thrombosis | Frequent (79-30%)"
    explanation: Orphanet classifies arterial thrombosis as frequent in CBS-deficient homocystinuria.
  - reference: PMID:38201964
    reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy.
    explanation: Review confirms arterial thrombotic events as clinical manifestations of severe hyperhomocysteinemia.
- name: Venous thrombosis
  frequency: FREQUENT
  description: >
    Deep venous thrombosis and other venous thromboembolic events are frequent in CBS-deficient
    homocystinuria and may be the presenting feature in mildly affected pyridoxine-responsive patients.
  phenotype_term:
    preferred_term: Venous thrombosis
    term:
      id: HP:0004936
      label: Venous thrombosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0004936 | Venous thrombosis | Frequent (79-30%)"
    explanation: Orphanet classifies venous thrombosis as frequent in CBS-deficient homocystinuria.
  - reference: PMID:38201964
    reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: In cases of arterial or venous thrombotic vascular events, particularly with other comorbidities, it is crucial to consider moderate to severe HHcy.
    explanation: Review confirms venous thrombotic events as clinical manifestations of severe hyperhomocysteinemia.
- name: Pulmonary embolism
  frequency: FREQUENT
  description: >
    Pulmonary embolism is a serious thromboembolic complication of CBS-deficient homocystinuria,
    particularly dangerous during surgery and immobilization.
  phenotype_term:
    preferred_term: Pulmonary embolism
    term:
      id: HP:0002204
      label: Pulmonary embolism
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002204 | Pulmonary embolism | Frequent (79-30%)"
    explanation: Orphanet classifies pulmonary embolism as frequent in CBS-deficient homocystinuria.
  - reference: PMID:3872065
    reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Following 586 surgical procedures, 25 postoperative thromboembolic complications occurred, six of which were fatal."
    explanation: Classic natural history study documents significant postoperative thromboembolic risk including PE.
- name: Cerebral ischemia
  frequency: FREQUENT
  description: >
    Cerebral ischemic events due to arterial thrombosis or embolism are a frequent vascular
    complication of CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Cerebral ischemia
    term:
      id: HP:0002637
      label: Cerebral ischemia
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002637 | Cerebral ischemia | Frequent (79-30%)"
    explanation: Orphanet classifies cerebral ischemia as frequent in CBS-deficient homocystinuria.
- name: Pectus excavatum
  frequency: FREQUENT
  description: >
    Concave anterior chest wall deformity is a frequent skeletal feature of the marfanoid habitus
    in CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Pectus excavatum
    term:
      id: HP:0000767
      label: Pectus excavatum
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000767 | Pectus excavatum | Frequent (79-30%)"
    explanation: Orphanet classifies pectus excavatum as frequent in CBS-deficient homocystinuria.
- name: Pectus carinatum
  frequency: FREQUENT
  description: >
    Protruding anterior chest wall deformity is a frequent skeletal feature of the marfanoid habitus
    in CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Pectus carinatum
    term:
      id: HP:0000768
      label: Pectus carinatum
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000768 | Pectus carinatum | Frequent (79-30%)"
    explanation: Orphanet classifies pectus carinatum as frequent in CBS-deficient homocystinuria.
- name: Kyphosis
  frequency: FREQUENT
  description: >
    Exaggerated thoracic kyphosis is a frequent skeletal deformity in CBS-deficient homocystinuria,
    related to vertebral osteoporosis and connective tissue weakness.
  phenotype_term:
    preferred_term: Kyphosis
    term:
      id: HP:0002808
      label: Kyphosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002808 | Kyphosis | Frequent (79-30%)"
    explanation: Orphanet classifies kyphosis as frequent in CBS-deficient homocystinuria.
- name: Genu valgum
  frequency: FREQUENT
  description: >
    Knock-knee deformity is a frequent skeletal feature of CBS-deficient homocystinuria,
    reflecting connective tissue and skeletal abnormalities.
  phenotype_term:
    preferred_term: Genu valgum
    term:
      id: HP:0002857
      label: Genu valgum
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002857 | Genu valgum | Frequent (79-30%)"
    explanation: Orphanet classifies genu valgum as frequent in CBS-deficient homocystinuria.
- name: Joint stiffness
  frequency: FREQUENT
  description: >
    Limited joint mobility is a frequent feature of CBS-deficient homocystinuria, in contrast
    to the joint hypermobility seen in Marfan syndrome.
  phenotype_term:
    preferred_term: Joint stiffness
    term:
      id: HP:0001387
      label: Joint stiffness
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001387 | Joint stiffness | Frequent (79-30%)"
    explanation: Orphanet classifies joint stiffness as frequent in CBS-deficient homocystinuria.
- name: Pes cavus
  frequency: FREQUENT
  description: >
    High-arched feet are a frequent skeletal feature of CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Pes cavus
    term:
      id: HP:0001761
      label: Pes cavus
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001761 | Pes cavus | Frequent (79-30%)"
    explanation: Orphanet classifies pes cavus as frequent in CBS-deficient homocystinuria.
- name: Hypertension
  frequency: FREQUENT
  description: >
    Hypertension is a frequent vascular feature of CBS-deficient homocystinuria, likely
    related to endothelial dysfunction and vascular remodeling driven by elevated homocysteine.
  phenotype_term:
    preferred_term: Hypertension
    term:
      id: HP:0000822
      label: Hypertension
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000822 | Hypertension | Frequent (79-30%)"
    explanation: Orphanet classifies hypertension as frequent in CBS-deficient homocystinuria.
- name: Sparse scalp hair
  frequency: FREQUENT
  description: >
    Thin, sparse scalp hair is a frequent integumentary feature of CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Sparse scalp hair
    term:
      id: HP:0002209
      label: Sparse scalp hair
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002209 | Sparse scalp hair | Frequent (79-30%)"
    explanation: Orphanet classifies sparse scalp hair as frequent in CBS-deficient homocystinuria.
- name: Amblyopia
  frequency: FREQUENT
  description: >
    Reduced visual acuity due to abnormal visual development, frequently associated with
    strabismus and ectopia lentis in CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Amblyopia
    term:
      id: HP:0000646
      label: Amblyopia
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000646 | Amblyopia | Frequent (79-30%)"
    explanation: Orphanet classifies amblyopia as frequent in CBS-deficient homocystinuria.
- name: Glaucoma
  frequency: OCCASIONAL
  description: >
    Glaucoma can develop in HCU patients, often secondary to lens dislocation causing
    pupillary block or damage to the anterior chamber angle.
  phenotype_term:
    preferred_term: Glaucoma
    term:
      id: HP:0000501
      label: Glaucoma
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000501 | Glaucoma | Occasional (29-5%)"
    explanation: Orphanet classifies glaucoma as occasional in CBS-deficient homocystinuria.
- name: Retinal detachment
  frequency: OCCASIONAL
  description: >
    Retinal detachment can occur in HCU patients, potentially related to high myopia
    and connective tissue fragility.
  phenotype_term:
    preferred_term: Retinal detachment
    term:
      id: HP:0000541
      label: Retinal detachment
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000541 | Retinal detachment | Occasional (29-5%)"
    explanation: Orphanet classifies retinal detachment as occasional in CBS-deficient homocystinuria.
- name: Strabismus
  frequency: OCCASIONAL
  description: >
    Ocular misalignment is an occasional ophthalmic feature of CBS-deficient homocystinuria,
    often associated with lens dislocation and refractive errors.
  phenotype_term:
    preferred_term: Strabismus
    term:
      id: HP:0000486
      label: Strabismus
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000486 | Strabismus | Occasional (29-5%)"
    explanation: Orphanet classifies strabismus as occasional in CBS-deficient homocystinuria.
- name: Cataract
  frequency: OCCASIONAL
  description: >
    Lens opacification can occur in HCU patients, potentially secondary to lens subluxation
    or metabolic insults to lens proteins.
  phenotype_term:
    preferred_term: Cataract
    term:
      id: HP:0000518
      label: Cataract
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000518 | Cataract | Occasional (29-5%)"
    explanation: Orphanet classifies cataract as occasional in CBS-deficient homocystinuria.
- name: Depression
  frequency: OCCASIONAL
  description: >
    Depressive symptoms are an occasional psychiatric feature of CBS-deficient homocystinuria,
    but the precise mechanism is not well established.
  phenotype_term:
    preferred_term: Depression
    term:
      id: HP:0000716
      label: Depression
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000716 | Depression | Occasional (29-5%)"
    explanation: Orphanet classifies depression as occasional in CBS-deficient homocystinuria.
- name: Anxiety
  frequency: OCCASIONAL
  description: >
    Anxiety symptoms are an occasional psychiatric feature of CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Anxiety
    term:
      id: HP:0000739
      label: Anxiety
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000739 | Anxiety | Occasional (29-5%)"
    explanation: Orphanet classifies anxiety as occasional in CBS-deficient homocystinuria.
- name: Psychosis
  frequency: OCCASIONAL
  description: >
    Psychotic episodes are an occasional neuropsychiatric feature of CBS-deficient homocystinuria,
    linked to neurotoxic effects of elevated homocysteine.
  phenotype_term:
    preferred_term: Psychosis
    term:
      id: HP:0000709
      label: Psychosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000709 | Psychosis | Occasional (29-5%)"
    explanation: Orphanet classifies psychosis as occasional in CBS-deficient homocystinuria.
- name: Dystonia
  frequency: OCCASIONAL
  description: >
    Dystonia is an occasional extrapyramidal feature of CBS-deficient homocystinuria,
    reflecting basal ganglia involvement from vascular or metabolic insults.
  phenotype_term:
    preferred_term: Dystonia
    term:
      id: HP:0001332
      label: Dystonia
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001332 | Dystonia | Occasional (29-5%)"
    explanation: Orphanet classifies dystonia as occasional in CBS-deficient homocystinuria.
- name: Livedo reticularis
  frequency: OCCASIONAL
  description: >
    Mottled, net-like discoloration of the skin is an occasional cutaneous vascular feature
    of CBS-deficient homocystinuria, reflecting microvascular dysfunction.
  phenotype_term:
    preferred_term: Livedo reticularis
    term:
      id: HP:0033505
      label: Livedo reticularis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0033505 | Livedo reticularis | Occasional (29-5%)"
    explanation: Orphanet classifies livedo reticularis as occasional in CBS-deficient homocystinuria.
- name: High palate
  frequency: OCCASIONAL
  description: >
    High-arched palate is an occasional craniofacial feature of CBS-deficient homocystinuria,
    part of the marfanoid skeletal phenotype.
  phenotype_term:
    preferred_term: High palate
    term:
      id: HP:0000218
      label: High palate
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0000218 | High palate | Occasional (29-5%)"
    explanation: Orphanet classifies high palate as occasional in CBS-deficient homocystinuria.
- name: Pancreatitis
  frequency: OCCASIONAL
  description: >
    Pancreatitis is an occasional gastrointestinal complication of CBS-deficient homocystinuria,
    potentially related to vascular compromise of the pancreatic vasculature.
  phenotype_term:
    preferred_term: Pancreatitis
    term:
      id: HP:0001733
      label: Pancreatitis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0001733 | Pancreatitis | Occasional (29-5%)"
    explanation: Orphanet classifies pancreatitis as occasional in CBS-deficient homocystinuria.
- name: Hepatomegaly
  frequency: OCCASIONAL
  description: >
    Hepatomegaly is an occasional hepatic feature of CBS-deficient homocystinuria,
    potentially related to fatty liver changes from disrupted methionine metabolism.
  phenotype_term:
    preferred_term: Hepatomegaly
    term:
      id: HP:0002240
      label: Hepatomegaly
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0002240 | Hepatomegaly | Occasional (29-5%)"
    explanation: Orphanet classifies hepatomegaly as occasional in CBS-deficient homocystinuria.
- name: Cerebral venous sinus thrombosis
  frequency: OCCASIONAL
  description: >
    Thrombosis of the cerebral venous sinuses is an occasional but serious vascular
    complication of CBS-deficient homocystinuria.
  phenotype_term:
    preferred_term: Cerebral venous sinus thrombosis
    term:
      id: HP:0033724
      label: Cerebral venous sinus thrombosis
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "HP:0033724 | Cerebral venous sinus thrombosis | Occasional (29-5%)"
    explanation: Orphanet classifies cerebral venous sinus thrombosis as occasional in CBS-deficient homocystinuria.
biochemical:
- name: Total homocysteine (tHcy)
  presence: INCREASED
  context: 'Massively elevated total plasma homocysteine is the biochemical hallmark of classical HCU. Normal tHcy is typically less than 15 micromol/L; tHcy above 100 micromol/L is usually diagnostic in the appropriate clinical context.

    '
  biomarker_term:
    preferred_term: homocysteine
    term:
      id: CHEBI:17230
      label: homocysteine
  readouts:
  - target: Disrupted transsulfuration and methionine metabolism
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated total homocysteine reports impaired CBS-dependent homocysteine clearance in classical HCU.
  - target: MTHFR remethylation pathway deficiency
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated total homocysteine also reports impaired folate-dependent remethylation in MTHFR-deficiency homocystinuria.
  evidence:
  - reference: PMID:38962401
    reference_title: "Estimating prevalence of classical homocystinuria in the United States using Optum's de-identified market clarity data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: An algorithm was developed to identify 2 cohorts of patients using broad and strict definitions of HCU.
    explanation: US prevalence study confirms elevated tHcy as the primary diagnostic criterion for classical HCU.
  - reference: PMID:39567721
    reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Cystathionine beta-synthase deficiency (CBSD) is the most prevalent inherited disorder of homocysteine metabolism in the transsulphuration pathway.
    explanation: Study documents markedly elevated tHcy in CBS-deficient patients compared to controls.
- name: Methionine
  presence: INCREASED
  context: 'Elevated plasma methionine results from upstream metabolite accumulation due to the CBS block.

    '
  biomarker_term:
    preferred_term: methionine
    term:
      id: CHEBI:16811
      label: methionine
  readouts:
  - target: Disrupted transsulfuration and methionine metabolism
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated methionine supports classical CBS-deficient HCU and helps distinguish it from remethylation defects with low or normal methionine.
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: A rare metabolic disease of methionine catabolism characterized by accumulation of methionine and homocysteine with clinical involvement of the eye, skeletal system, vascular system and central nervous system (CNS).
    explanation: Orphanet directly supports methionine accumulation in CBS-deficient homocystinuria.
- name: Cysteine
  presence: DECREASED
  context: 'Plasma cysteine is depleted downstream of the CBS metabolic block. Cysteine depletion impairs glutathione synthesis and contributes to the oxidative stress observed in HCU patients.

    '
  biomarker_term:
    preferred_term: cysteine
    term:
      id: CHEBI:15356
      label: cysteine
  readouts:
  - target: Disrupted transsulfuration and methionine metabolism
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Lower cysteine reflects reduced downstream transsulfuration flux from homocysteine through CBS.
  - target: Oxidative stress and mitochondrial dysfunction
    relationship: CORRELATES_WITH
    direction: NEGATIVE
    endpoint_context: MONITORING
    interpretation: Lower cysteine can limit thiol and glutathione homeostasis, aligning with oxidative stress in CBS deficiency.
  evidence:
  - reference: PMID:39366068
    reference_title: "Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS
    explanation: The metabolic block at CBS depletes downstream cysteine, as the transsulfuration pathway to cysteine is interrupted.
- name: GDF-15 (mitokine)
  presence: INCREASED
  context: 'Growth differentiation factor 15 is elevated in CBS-deficient patients and correlates positively with total homocysteine levels, serving as a biomarker of mitochondrial stress in HCU.

    '
  readouts:
  - target: Oxidative stress and mitochondrial dysfunction
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: MONITORING
    interpretation: Elevated GDF-15 reports mitochondrial stress associated with total homocysteine burden in CBS deficiency.
  evidence:
  - reference: PMID:39567721
    reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: In patient group, a positive correlation was found between the total homocysteine level and both GDF-15 and NAD+/NADH levels.
    explanation: Directly supports elevated GDF-15 linked to homocysteine burden in CBS-deficient patients.
- name: FGF-21 (mitokine)
  presence: INCREASED
  context: 'Fibroblast growth factor 21 is elevated in CBS-deficient patients, reflecting mitochondrial dysfunction and energy stress.

    '
  readouts:
  - target: Oxidative stress and mitochondrial dysfunction
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: MONITORING
    interpretation: Altered FGF-21 is modeled as a mitochondrial stress readout in CBS-deficient patients.
  evidence:
  - reference: PMID:39567721
    reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD.
    explanation: Directly supports altered FGF-21 as a mitochondrial stress signal in CBS deficiency.
- name: NAD+/NADH ratio
  presence: DECREASED
  context: 'The NAD+/NADH ratio is significantly reduced in CBS-deficient patients, reflecting disturbed mitochondrial redox homeostasis.

    '
  readouts:
  - target: Oxidative stress and mitochondrial dysfunction
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: MONITORING
    interpretation: Reduced NAD+/NADH ratio reports redox imbalance associated with mitochondrial dysfunction in CBS deficiency.
  evidence:
  - reference: PMID:39567721
    reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The alterations in NAD+, FGF-21, GDF-15 levels, and NAD+/NADH ratio in patients suggest that oxidative damage coexists with mitochondrial dysfunction in CBSD.
    explanation: Directly supports NAD redox-ratio disturbance in CBS-deficient patients.
genetic:
- name: CBS (cystathionine beta-synthase) deficiency
  gene_term:
    preferred_term: CBS
    term:
      id: hgnc:1550
      label: CBS
  inheritance:
  - name: Autosomal recessive
    evidence:
    - reference: ORPHA:394
      supports: SUPPORT
      snippet: "Autosomal recessive"
      explanation: Orphanet confirms autosomal recessive inheritance for CBS-deficient homocystinuria.
    - reference: PMID:36417581
      reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Cystathionine beta-synthase (CBS)-deficient homocystinuria (HCU) is the most common inborn error of sulfur amino acid metabolism.
      explanation: CBS-deficient HCU is an autosomal recessive inborn error of metabolism.
  variants:
  - name: I278T
    description: 'One of the most common CBS pathogenic variants worldwide. Shows reduced protein half-life (approximately 5.9 hours vs 16.6 hours for wild-type) and impaired folding. May respond partially to pyridoxine supplementation.

      '
    evidence:
    - reference: PMID:39041895
      reference_title: "Cellular turnover and degradation of the most common missense cystathionine beta-synthase variants causing homocystinuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Proteasomal inhibition significantly increased the half-life and activity of T191M and I278T CBS mutants.
      explanation: Demonstrates I278T has reduced stability and activity that can be partially rescued by proteasome inhibition.
  - name: T191M
    description: 'A common CBS pathogenic variant showing reduced protein half-life (approximately 7.7 hours) and responsiveness to proteasome inhibition.

      '
    evidence:
    - reference: PMID:39041895
      reference_title: "Cellular turnover and degradation of the most common missense cystathionine beta-synthase variants causing homocystinuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Proteasomal inhibition significantly increased the half-life and activity of T191M and I278T CBS mutants.
      explanation: Demonstrates T191M instability and rescue by proteasome inhibition.
  - name: R125Q
    description: 'A permissive CBS variant that responds to pharmacological proteostasis modulation and chaperone co-expression, showing partial functional rescue.

      '
    evidence:
    - reference: PMID:38051092
      reference_title: "Genetic and Pharmacological Modulation of Cellular Proteostasis Leads to Partial Functional Rescue of Homocystinuria-Causing Cystathionine-Beta Synthase Variants."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Co-expression of the main effector HSP70 or master regulator HSF1 rescued steady-state levels of CBS I278T and R125Q variants with partial functional rescue of the latter.
      explanation: Demonstrates R125Q as a variant permissive to proteostasis-based functional rescue.
  features: >
    CBS-deficient homocystinuria is caused by biallelic pathogenic variants in the CBS gene. The CBS enzyme uses
    pyridoxal-5-phosphate (vitamin B6) as a cofactor and is allosterically activated by S-adenosylmethionine. Over 200 pathogenic
    variants have been described. Many missense variants cause protein misfolding and instability rather than active-site
    disruption, making CBS deficiency a conformational disorder. Pyridoxine responsiveness is a major clinical modifier: non-responders
    present earlier with broader multi-organ disease, while extreme responders may present in adulthood with predominantly
    thromboembolic disease. E-HOD registry data on 328 patients classifies patients into four groups of pyridoxine
    responsiveness (NR, PR, FR, ER) that correlate with clinical severity.
  evidence:
  - reference: ORPHA:394
    supports: SUPPORT
    snippet: "CBS | cystathionine beta-synthase | hgnc:1550 | Disease-causing germline mutation(s) in"
    explanation: Orphanet confirms CBS as the disease-causing gene for ORPHA:394.
  - reference: PMID:39041895
    reference_title: "Cellular turnover and degradation of the most common missense cystathionine beta-synthase variants causing homocystinuria."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Recent work suggests that missense pathogenic mutations-regardless of their topology-cause instability of the C-terminal regulatory domain, which likely translates into CBS misfolding, impaired assembly, and loss of function.
    explanation: Directly supports the conformational disorder model for CBS-deficient HCU.
  - reference: PMID:33295057
    reference_title: "Cystathionine β-synthase deficiency in the E-HOD registry-part I."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We developed comprehensive criteria to classify patients into four groups of pyridoxine responsivity: non-responders (NR), partial, full and extreme responders (PR, FR and ER, respectively)."
    explanation: E-HOD registry provides systematic classification of pyridoxine responsiveness groups.
- name: MTHFR (methylenetetrahydrofolate reductase) deficiency
  gene_term:
    preferred_term: MTHFR
    term:
      id: hgnc:7436
      label: MTHFR
  inheritance:
  - name: Autosomal recessive
    evidence:
    - reference: PMID:40440437
      reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "GENETIC COUNSELING: Homocystinuria due to deficiency of N(5,10)-MTHFR activity is inherited in an autosomal recessive manner."
      explanation: GeneReviews directly supports autosomal recessive inheritance for MTHFR-deficiency homocystinuria.
  features: 'MTHFR deficiency causes homocystinuria through disruption of the remethylation pathway. Unlike CBS deficiency, MTHFR deficiency typically presents with low or normal methionine and may have distinct neurological features. This represents important genetic heterogeneity within the homocystinuria spectrum.

    '
  evidence:
  - reference: PMID:40440437
    reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "biallelic pathogenic variants in MTHFR identified by molecular genetic testing."
    explanation: GeneReviews supports biallelic MTHFR variants as the molecular basis of MTHFR-deficiency homocystinuria.
  - reference: PMID:18658082
    reference_title: "Severe methylenetetrahydrofolate reductase (MTHFR) deficiency: a case report of nonclassical homocystinuria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe methylenetetrahydrofolate reductase deficiency is an autosomal recessive metabolic disorder of folate metabolism causing elevated plasma homocysteine levels and homocystinuria (MIM 236250)."
    explanation: Case-report abstract supports MTHFR deficiency as a nonclassical autosomal recessive cause of homocystinuria.
treatments:
- name: Pyridoxine (vitamin B6) supplementation
  description: >
    Pyridoxine is the first-line pharmacological intervention for classical HCU. PLP is a CBS cofactor, and approximately
    half of patients show some degree of biochemical response (reduced tHcy) to pyridoxine supplementation. Responsiveness
    stratifies disease severity and guides further treatment decisions. Guidelines recommend keeping tHcy below 100 micromol/L;
    pyridoxine-responsive patients generally achieve tHcy below 50 micromol/L.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: pyridoxine
      term:
        id: CHEBI:16709
        label: pyridoxine
  evidence:
  - reference: PMID:27778219
    reference_title: "Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "we recommend keeping the concentration below 100 μmol/L because levels fluctuate and the complications associated with high levels are so serious."
    explanation: Clinical guidelines define the tHcy treatment target for CBS-deficient HCU.
  - reference: PMID:40095936
    reference_title: "Cystathionine β-Synthase Deficiency in the E-HOD Registry-Part II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pyridoxine-responsive patients generally achieved tHcy concentrations below 50 μmol/L"
    explanation: E-HOD Part II shows pyridoxine-responsive patients generally achieve good biochemical control.
  - reference: PMID:36417581
    reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine
    explanation: Defines pyridoxine responsiveness as a key clinical stratifier.
  target_mechanisms:
  - target: CBS molecular function deficiency
    treatment_effect: RESTORES
    description: Pyridoxine supplies the vitamin B6-derived CBS cofactor in responsive variants, improving residual CBS function and lowering tHcy.
    evidence:
    - reference: PMID:40095936
      reference_title: "Cystathionine beta-Synthase Deficiency in the E-HOD Registry-Part II."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Pyridoxine-responsive patients generally achieved tHcy concentrations below 50 μmol/L"
      explanation: Registry data show biochemical response in pyridoxine-responsive patients.
  - target: Disrupted transsulfuration and methionine metabolism
    treatment_effect: MODULATES
    description: Improved CBS cofactor availability lowers the homocysteine burden in responsive patients.
    evidence:
    - reference: PMID:40095936
      reference_title: "Cystathionine beta-Synthase Deficiency in the E-HOD Registry-Part II."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Pyridoxine-responsive patients generally achieved tHcy concentrations below 50 μmol/L"
      explanation: Registry data connect pyridoxine response to improved tHcy control.
- name: Methionine-restricted diet
  description: >
    Dietary restriction of methionine intake with cysteine-enriched amino acid mixtures is the cornerstone of
    management for pyridoxine non-responsive patients. E-HOD Part II data show that treatment can prevent
    thromboembolism (risk ratio 0.073) and lens dislocation (risk ratio 0.069) in NR patients detected by
    newborn screening compared to those ascertained clinically.
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: MAXO:0000088
      label: dietary intervention
  evidence:
  - reference: PMID:40095936
    reference_title: "Cystathionine β-Synthase Deficiency in the E-HOD Registry-Part II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "treatment could prevent thromboembolism (risk ratio 0.073) and lens dislocation (risk ratio 0.069)."
    explanation: E-HOD Part II provides quantitative evidence that early treatment dramatically reduces major complications.
  - reference: PMID:35791106
    reference_title: "Homocystinuria and ocular complications - A review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The timely recognition of this rare metabolic disorder and prompt methionine-restricted diet are crucial in lessening the systemic consequences.
    explanation: Directly supports methionine-restricted diet as crucial for reducing systemic consequences.
  - reference: PMID:36417581
    reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The current standard of care involves significant dietary interventions that, despite being effective, often adversely affect quality of life of HCU patients, leading to poor adherence to therapy
    explanation: Confirms dietary restriction as standard of care despite adherence challenges.
  target_mechanisms:
  - target: Disrupted transsulfuration and methionine metabolism
    treatment_effect: MODULATES
    description: Methionine restriction reduces upstream substrate burden across the blocked CBS-dependent pathway.
    evidence:
    - reference: PMID:40095936
      reference_title: "Cystathionine beta-Synthase Deficiency in the E-HOD Registry-Part II."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Severely affected patients usually present during childhood with learning difficulties, ectopia lentis and skeletal abnormalities; they are pyridoxine non-responders (NR) or partial responders (PR) and require treatment with a low-methionine diet and/or betaine."
      explanation: Registry study states that severe non-responders/partial responders require low-methionine diet and/or betaine.
  - target: Endothelial dysfunction, thrombosis, and vascular injury
    treatment_effect: INHIBITS
    description: Early metabolic control prevents thromboembolic complications in non-responder patients.
    evidence:
    - reference: PMID:40095936
      reference_title: "Cystathionine beta-Synthase Deficiency in the E-HOD Registry-Part II."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "treatment could prevent thromboembolism (risk ratio 0.073) and lens dislocation (risk ratio 0.069)."
      explanation: Registry time-to-event data show reduced thromboembolism and lens dislocation under early treatment.
- name: Betaine supplementation
  description: 'Betaine (trimethylglycine) serves as an alternative methyl donor supporting homocysteine remethylation via betaine-homocysteine methyltransferase (BHMT) in the liver. Used as adjunctive therapy to lower homocysteine levels, particularly in pyridoxine non-responders.

    '
  treatment_term:
    preferred_term: nutritional supplementation
    term:
      id: MAXO:0000106
      label: nutritional supplementation
    therapeutic_agent:
    - preferred_term: betaine
      term:
        id: CHEBI:17750
        label: glycine betaine
  evidence:
  - reference: PMID:38201964
    reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Adequate intake of these vitamins, along with betaine supplementation, supports Hcy remethylation.
    explanation: Confirms betaine supplementation as supporting homocysteine remethylation in the management of hyperhomocysteinemia.
  target_mechanisms:
  - target: Disrupted transsulfuration and methionine metabolism
    treatment_effect: BYPASSES
    description: Betaine supports alternative hepatic remethylation of homocysteine, reducing tHcy without requiring CBS flux.
    evidence:
    - reference: PMID:38201964
      reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Adequate intake of these vitamins, along with betaine supplementation, supports Hcy remethylation."
      explanation: The review directly supports betaine as a remethylation-based bypass of homocysteine accumulation.
- name: Folate and B12 supplementation
  description: 'B-vitamin supplementation with folate and cobalamin supports homocysteine remethylation via methionine synthase, complementing dietary and pharmacological management.

    '
  treatment_term:
    preferred_term: nutritional supplementation
    term:
      id: MAXO:0000106
      label: nutritional supplementation
  evidence:
  - reference: PMID:38201964
    reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: A nutritional approach to HHcy management involves implementing dietary strategies and targeted supplementation, emphasizing key nutrients like vitamin B6, B12, and folate that are crucial for Hcy conversion.
    explanation: Directly supports folate and B12 supplementation as part of nutritional HHcy management.
  target_mechanisms:
  - target: Disrupted transsulfuration and methionine metabolism
    treatment_effect: BYPASSES
    description: Folate and B12 support homocysteine conversion through remethylation pathways.
    evidence:
    - reference: PMID:38201964
      reference_title: "Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "A nutritional approach to HHcy management involves implementing dietary strategies and targeted supplementation, emphasizing key nutrients like vitamin B6, B12, and folate that are crucial for Hcy conversion."
      explanation: The review directly supports B-vitamin supplementation for Hcy conversion.
- name: Enzyme replacement therapy (ERT)
  description: 'CBS-based enzyme replacement therapy is under development. ERT has shown efficacy in lowering plasma tHcy below 100 micromol/L in mouse models. Efficacy can be enhanced by co-administration of biological reductants (N-acetylcysteine, MESNA, cysteamine) that increase the availability of reduced homocysteine as a CBS substrate.

    '
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  evidence:
  - reference: PMID:39366068
    reference_title: "Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Enzyme replacement therapy (ERT) based on human CBS has been developed and has shown significant efficacy correcting HCU phenotype in several mouse models by bringing plasma total homocysteine below the clinically relevant 100 μM threshold.
    explanation: Directly supports the quantitative ERT efficacy claim in mouse models.
  target_mechanisms:
  - target: CBS molecular function deficiency
    treatment_effect: RESTORES
    description: CBS enzyme replacement restores missing CBS catalytic capacity in model systems.
    evidence:
    - reference: PMID:39366068
      reference_title: "Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Enzyme replacement therapy (ERT) based on human CBS has been developed and has shown significant efficacy correcting HCU phenotype in several mouse models by bringing plasma total homocysteine below the clinically relevant 100 μM threshold."
      explanation: Mouse-model ERT data support direct restoration of CBS-dependent homocysteine clearance.
  - target: Disrupted transsulfuration and methionine metabolism
    treatment_effect: RESTORES
    description: ERT metabolizes reduced homocysteine and lowers total homocysteine in HCU mouse models.
    evidence:
    - reference: PMID:39366068
      reference_title: "Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS"
      explanation: The ERT mechanism paper identifies reduced homocysteine as the CBS substrate that determines enzyme-replacement clearance.
- name: Thrombosis prevention
  description: 'Anticoagulation and thromboprophylaxis measures are important for managing vascular risk in HCU. Specific considerations include avoiding oral contraceptives, prophylactic anticoagulation during high-risk periods (surgery, pregnancy), and maintaining optimal metabolic control to reduce thromboembolic risk.

    '
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  evidence:
  - reference: PMID:36417581
    reference_title: "Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment
    explanation: High thromboembolism risk in HCU supports need for thromboprophylaxis strategies.
  target_mechanisms:
  - target: Endothelial dysfunction, thrombosis, and vascular injury
    treatment_effect: INHIBITS
    description: Thromboprophylaxis addresses the final coagulation and embolic risk mechanism while metabolic control is optimized.
    evidence:
    - reference: PMID:3872065
      reference_title: "The natural history of homocystinuria due to cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Following 586 surgical procedures, 25 postoperative thromboembolic complications occurred, six of which were fatal."
      explanation: Natural history data document high-risk thromboembolic contexts that justify thromboprophylaxis.
- name: Supportive care and antioxidant therapy
  description: 'Supportive measures including monitoring for multisystem complications and consideration of antioxidant therapy. Evidence of oxidative damage and mitochondrial dysfunction supports potential benefit from antioxidant and mitochondrial support in CBS-deficient patients.

    '
  treatment_term:
    preferred_term: supportive care
    term:
      id: MAXO:0000950
      label: supportive care
  evidence:
  - reference: PMID:39567721
    reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Assessment of oxidative damage and addition of anti-oxidant therapy together with mitochondrial support may have additional benefits in reducing long-term morbidity in CBSD patients.
    explanation: Directly recommends assessment of oxidative damage and consideration of antioxidant therapy with mitochondrial support.
  target_mechanisms:
  - target: Oxidative stress and mitochondrial dysfunction
    treatment_effect: MODULATES
    description: Antioxidant and mitochondrial support is aimed at the oxidative/mitochondrial stress node.
    evidence:
    - reference: PMID:39567721
      reference_title: "Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Assessment of oxidative damage and addition of anti-oxidant therapy together with mitochondrial support may have additional benefits in reducing long-term morbidity in CBSD patients."
      explanation: Patient biomarker study directly suggests antioxidant and mitochondrial support for the oxidative damage mechanism.
- name: Genetic counseling
  description: 'Genetic counseling for affected families including discussion of inheritance, carrier detection, prenatal/preimplantation testing options, and recurrence risk.

    '
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: MAXO:0000079
      label: genetic counseling
  evidence:
  - reference: PMID:40440437
    reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Once the MTHFR pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible."
    explanation: GeneReviews directly supports carrier testing and prenatal/preimplantation genetic testing as genetic counseling content for inherited homocystinuria.
  - reference: PMID:20301697
    reference_title: "Homocystinuria due to Cystathionine Beta-Synthase Deficiency."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Once the CBS pathogenic variants have been identified in an affected family member, carrier testing for at-risk family members and prenatal/preimplantation genetic testing are possible."
    explanation: CBS GeneReviews directly supports carrier testing and prenatal/preimplantation genetic testing after familial CBS variants are identified.
  target_mechanisms:
  - target: CBS molecular function deficiency
    description: Genetic counseling is anchored to the familial CBS pathogenic variants and supports carrier and prenatal/preimplantation testing; it does not directly alter metabolism.
    evidence:
    - reference: PMID:20301697
      reference_title: "Homocystinuria due to Cystathionine Beta-Synthase Deficiency."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Once the CBS pathogenic variants have been identified in an affected family member, carrier testing for at-risk family members and prenatal/preimplantation genetic testing are possible."
      explanation: CBS GeneReviews links genetic counseling actions to identified familial CBS pathogenic variants.
  - target: MTHFR remethylation pathway deficiency
    description: Genetic counseling is anchored to the familial MTHFR pathogenic variants and supports carrier and prenatal/preimplantation testing; it does not directly alter metabolism.
    evidence:
    - reference: PMID:40440437
      reference_title: "Homocystinuria due to Deficiency of N(5,10)-Methylenetetrahydrofolate Reductase Activity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Once the MTHFR pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible."
      explanation: GeneReviews links genetic counseling actions to the identified familial MTHFR pathogenic variants.
references:
- reference: PMID:20301697
  title: Homocystinuria due to Cystathionine Beta-Synthase Deficiency.
  tags:
  - GeneReviews
  findings: []
notes: 'Classical CBS-deficient homocystinuria shows marked clinical heterogeneity largely driven by pyridoxine responsiveness. Non-responsive patients typically present in childhood with multisystem disease including lens dislocation, skeletal abnormalities, intellectual disability, and early thrombosis. Pyridoxine-responsive patients may present in adulthood, sometimes solely with thromboembolic events. Newborn screening via methionine on tandem MS has known sensitivity limitations, particularly for milder pyridoxine-responsive forms. Direct tHcy measurement in NBS is used in some programs (e.g., Qatar) but is not widely implemented. NBS-based global incidence estimates for classical HCU are approximately 1 in 200,000 to 1 in 340,000, though underestimation is likely. A 2024 US claims-based study estimated average annual standardized prevalence at 5.29 per 100,000 (broad definition) and 1.04 per 100,000 (strict definition). Recent advances in understanding CBS protein misfolding and proteostasis (2023-2024) have opened potential new therapeutic avenues including proteostasis modulation and chaperone-based rescue strategies. Enzyme replacement therapy with CBS is in preclinical development
  with encouraging results in mouse models.

  '

📚

References & Deep Research

References

PMID:20301697

Homocystinuria due to Cystathionine Beta-Synthase Deficiency.

No top-level findings curated for this source.

Deep Research

Falcon ▸

Disease Pathophysiology Research Template

Edison Scientific Literature 30 citations 2026-02-23T23:18:07.194890

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Disease Pathophysiology Research Template

Target Disease

Disease Name: Homocystinuria
MONDO ID: (if available)
Category: Genetic

Research Objectives

Please provide a comprehensive research report on the pathophysiology of Homocystinuria. Focus on the molecular and cellular mechanisms underlying disease progression.

Required Information

1. Core Pathophysiology

What are the primary pathophysiological mechanisms?
What molecular pathways are dysregulated?
What cellular processes are affected?

2. Key Molecular Players

Genes/Proteins: Which genes are causally involved or implicated?
Chemical Entities: What metabolites, drugs, or small molecules are relevant?
Cell Types: Which cell types are primarily affected?
Anatomical Locations: Which tissues/organs are involved?

3. Biological Processes (for GO annotation)

What biological processes (GO terms) are disrupted?
Consider: signaling pathways, metabolic processes, cellular responses, transport, etc.

4. Cellular Components

Where in the cell do key processes occur?
Consider: organelles, membrane compartments, extracellular space, etc.

5. Disease Progression

What is the sequence of events from initial trigger to clinical manifestation?
Are there distinct stages or phases?

6. Phenotypic Manifestations

What are the key clinical phenotypes?
How do they relate to the underlying mechanisms?

Citation Requirements

Cite primary literature (PMID preferred) for all mechanistic claims
Prioritize recent reviews and landmark papers
Include direct quotes where possible to support key statements

Output Format

Structure your response as a narrative that could be used to populate a disease knowledge base entry with: - Pathophysiology description - Gene/protein annotations with ontology terms (HGNC, GO) - Phenotype associations (HP terms) - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Evidence items with PMIDs

Title: Pathophysiology of Homocystinuria (Genetic) — Molecular and Cellular Mechanisms

Target disease and ontology mapping - Disease: Homocystinuria (genetic heterogeneous group) - MONDO ID: MONDO_0004737 (homocystinuria) (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4) - Key subtype emphasized here: Classical homocystinuria due to cystathionine β-synthase (CBS) deficiency (also “CBS-deficient homocystinuria”). (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4)

Executive summary (current understanding) Homocystinuria comprises inherited disorders that elevate homocysteine (Hcy) by blocking either (i) Hcy transsulfuration (classical CBS deficiency) or (ii) Hcy remethylation (e.g., MTHFR/MTR/MTRR and cobalamin-processing defects). In classical CBS deficiency, a metabolic block at the methionine-cycle/transsulfuration branchpoint causes accumulation of Hcy and upstream one-carbon metabolites (methionine, SAM, SAH) with depletion of cystathionine/cysteine, and these biochemical perturbations drive multisystem disease through redox stress/mitochondrial dysfunction, toxic protein modifications (notably via Hcy thiolactone), ER stress/proteostasis disruption, endothelial dysfunction/platelet activation with thrombosis, and neuro-excitotoxicity via homocysteic acid/NMDA signaling. (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4)

Key concepts and definitions (mechanism-centered)

1.1 Definitions - Hyperhomocysteinemia (HHcy): blood Hcy >15 µmol/L. Severity categories often used clinically: mild 16–30 µmol/L, moderate 31–100 µmol/L, severe >100 µmol/L. (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4) - Classical homocystinuria (CBS deficiency): an autosomal recessive inborn error of sulfur amino acid metabolism; the biochemical hallmark is markedly elevated total homocysteine (tHcy), typically with elevated methionine and reduced cysteine/cystathionine. (collard2023geneticandpharmacological pages 1-2, ziegler2023inbornerrorsof pages 7-8) - Biochemical diagnostic heuristics used in practice: normal tHcy is typically <15 µM and tHcy >100 µM is usually diagnostic/highly suggestive for classical HCU in the right biochemical context. (jain2024estimatingprevalenceof pages 1-2, ziegler2023inbornerrorsof pages 7-8)

1.2 Core biochemical lesion in CBS deficiency CBS “lies at the branch point where the fate of Hcy is decided” between remethylation back to methionine and irreversible commitment to cysteine synthesis via transsulfuration. (philipp2024mechanismofaction pages 1-2) - CBS reaction: condensation of homocysteine (Hcy) + serine → cystathionine; downstream steps produce cysteine and support glutathione/redox homeostasis. (philipp2024mechanismofaction pages 1-2) - Cofactor/regulatory biology: CBS uses PLP (pyridoxal-5′-phosphate; vitamin B6-derived) and a heme cofactor, and is allosterically activated by S-adenosylmethionine (SAM). (philipp2024mechanismofaction pages 1-2) - Loss of CBS function leads to a characteristic metabolite pattern: upstream accumulation (Hcy, methionine, SAM, SAH) and downstream depletion (cystathionine, cysteine). (philipp2024mechanismofaction pages 1-2, ziegler2023inbornerrorsof pages 7-8)

Core pathophysiology (molecular and cellular mechanisms)

2.1 Redox stress, oxidative damage, and mitochondrial dysfunction A central current model is that massively elevated Hcy (a reactive thiol) and depletion of cysteine/cysteine-derived antioxidants jointly drive oxidative stress and mitochondrial injury. - Review-level mechanism: Hcy is described as “highly reactive, redox active” and implicated in “formation of reactive oxygen species” and ER stress; decreased cysteine “contribute[s] to oxidative stress”. (majtan2023recenttherapeuticapproaches pages 3-4) - Primary 2024 human biomarker evidence (CBS-deficient cohort): Balci et al. (Scientific Reports; publication month Nov 2024; https://doi.org/10.1038/s41598-024-80273-w) quantified mitochondrial/redox stress markers in 23 CBS-deficient patients vs controls. Patients had markedly elevated tHcy (102±63 µmol/L vs 8.9±1.7 µmol/L; p=0.000) with altered NAD redox metrics (NAD+ 16±1.2 vs 29.0±0.6 pmol/µL; NAD+/NADH ratio 0.66±0.10 vs 1.89±0.06; all p=0.000) and elevated mitokines FGF-21 (median 446 pg/mL vs 30 pg/mL) and GDF-15 (median 153 pg/mL vs 78 pg/mL). Mitochondrial DAMPs were ~2-fold elevated (e.g., MT-COX1/GAPDH 2.4±0.4 vs 1.0±0.1). (balci2024oxidativedamageand pages 2-3) - Correlation structure linking biochemistry to mitochondrial stress: total homocysteine correlated positively with GDF-15 (r=0.524, p<0.05) and with NAD+/NADH (r=0.499, p<0.05), and negatively with total NAD (r=-0.446, p<0.05) and NADH (r=-0.512, p<0.05). (balci2024oxidativedamageand pages 4-5) Interpretation: these data support a disease-relevant coupling between Hcy burden and mitochondrial stress signaling/redox imbalance, consistent with oxidative damage coexisting with mitochondrial dysfunction in CBS deficiency. (balci2024oxidativedamageand pages 1-2, balci2024oxidativedamageand pages 2-3)

2.2 Proteome damage and proteostasis/ER stress (conformational disease biology) A major contemporary mechanistic theme (2023–2024) is that classical HCU is not only a metabolite-toxicity disorder but also a “conformational disorder” at the protein level, because many CBS pathogenic variants misfold and are cleared by cellular quality-control systems. - Review/primary evidence: “HCU should be viewed as a protein conformational disorder with protein misfolding and instability as the main mechanism resulting in CBS deficiency.” (Protein Science; Jul 2024; https://doi.org/10.1002/pro.5123) (mijatovic2024cellularturnoverand pages 1-2) - Cellular degradation pathways: Mijatovic et al. (2024) found “proteasomal degradation is the major pathway for CBS disposal, with a minor involvement of lysosomal-autophagic and endoplasmic reticulum-associated degradation (ERAD) pathways for HCU mutants”; proteasome inhibition increased half-life and activity of T191M and I278T, and ERAD inhibition also rescued activity. (mijatovic2024cellularturnoverand pages 1-2) - Variant stability kinetics (quantitative): WT CBS half-life ~16.6 h; common pathogenic variants show shorter half-lives, e.g., I278T 5.9 h and T191M 7.7 h. (mijatovic2024cellularturnoverand pages 8-10) - ER stress signaling and chaperone biology (2023): Collard & Majtan (Molecular and Cellular Biology; Dec 2023; https://doi.org/10.1080/10985549.2023.2284147) describe upregulation of ER stress sensor BiP and association with proteasomes for I278T, implicating proteotoxic stress and proteasomal degradation; they also show partial rescue of permissive variants via HSP70/HSF1 co-expression and pharmacologic proteostasis modulation. (collard2023geneticandpharmacological pages 1-2, collard2023geneticandpharmacological pages 2-4) Mechanistic implication: at the cellular level, ER proteostasis pathways (UPR/ISR), ubiquitin–proteasome system (UPS), ERAD, and autophagy contribute to the degree of residual CBS function and may be leveraged therapeutically. (mijatovic2024cellularturnoverand pages 1-2, collard2023geneticandpharmacological pages 1-2)

2.3 Toxic homocysteine chemistry: homocysteic acid, Hcy thiolactone, and (N/S)-homocysteinylation Several chemically defined Hcy-derived species link the metabolic lesion to specific molecular damage. - Homocysteic acid: described as “excitotoxic” and a “potent agonist of glutamatergic NMDA receptors,” supporting a mechanistic bridge to seizures and neurocognitive pathology. (majtan2023recenttherapeuticapproaches pages 3-4) - Hcy thiolactone and irreversible N-homocysteinylation: Hcy thiolactone “reacts with free amino groups of protein lysine residues causing irreversible N-homocysteinylation,” leading to “structural changes, protein aggregation and loss of function.” (majtan2023recenttherapeuticapproaches pages 3-4) - Pro-thrombotic protein modification: “Modification of fibrinogen by Hcy thiolactone increases resistance to fibrinolysis” (mechanistic basis for thrombosis). (majtan2023recenttherapeuticapproaches pages 3-4) - Connective tissue mechanism: N-homocysteinylation “impairs collagen cross-linking” in mouse HCU models, supporting skeletal/connective-tissue abnormalities. (majtan2023recenttherapeuticapproaches pages 3-4)

2.4 Endothelial dysfunction, platelet activation, and thrombosis Vascular complications are among the most clinically consequential outcomes, and current models connect them to Hcy redox chemistry, protein modifications, and methylation imbalance. - Mechanistic chain: Hcy-driven ROS/protein stress contributes to “endothelial cell dysfunction and platelet activation,” progressing to “thrombus formation and vascular occlusion.” (majtan2023recenttherapeuticapproaches pages 3-4) - SAH and methylation: “Endothelial dysfunction was found to associate with increased levels of SAH” in CBS-deficient mice; increased SAH can inhibit methylation reactions, contributing to cognitive impairment and potentially vascular dysfunction. (majtan2023recenttherapeuticapproaches pages 3-4) - Natural history risk statistic: thromboembolic risk in untreated classical HCU can rise to ~50% by age 30, and risk appears proportional to degree/duration of Hcy elevation. (ziegler2023inbornerrorsof pages 7-8)

Key molecular players (structured)

3.1 Genes/proteins (HGNC symbols) Causal/central - CBS (cystathionine beta-synthase): loss-of-function causes classical homocystinuria; CBS is PLP-dependent, heme-containing, SAM-activated enzyme at the remethylation/transsulfuration branchpoint. (philipp2024mechanismofaction pages 1-2) Remethylation and cobalamin/folate-related (important for disease heterogeneity and differential diagnosis) - MTHFR, MTR, MTRR: remethylation pathway components; defects can cause HHcy/homocystinuria phenotypes, often with hypomethioninemia. (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4) - MMACHC/MMADHC and other cobalamin-processing genes: implicated in methylmalonic acidemia with homocystinuria (cblC/cblD spectrum). (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4) Proteostasis modifiers (mechanistic context) - HSPA1A/HSP70, HSF1, BiP (HSPA5), HSPB3, HSPB8, HSP40 family: implicated in CBS mutant folding/ER stress and partial functional rescue in cellular models. (collard2023geneticandpharmacological pages 1-2)

3.2 Chemical entities/metabolites (CHEBI) Central metabolites - Homocysteine (Hcy) (CHEBI:17230); total homocysteine (tHcy) used clinically as key biomarker. (jain2024estimatingprevalenceof pages 1-2) - Methionine (Met) (CHEBI:16811); elevated in classical HCU and used in newborn screening. (ziegler2023inbornerrorsof pages 6-7) - S-adenosylmethionine (SAM) (CHEBI:15414), S-adenosylhomocysteine (SAH) (CHEBI:16680): accumulate upstream of CBS block; SAH inhibits methylation reactions. (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4) - Cystathionine (Cth) (CHEBI:15611), cysteine (Cys) (CHEBI:15356): depleted downstream of CBS; cysteine depletion contributes to oxidative stress. (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4) Reactive/toxic derivatives and redox-linked species - Homocysteic acid (CHEBI:17241): NMDA receptor agonist implicated in excitotoxicity/seizures. (majtan2023recenttherapeuticapproaches pages 3-4) - Homocysteine thiolactone: reactive acylating intermediate driving N-homocysteinylation. (majtan2023recenttherapeuticapproaches pages 3-4) - NAD+/NADH (CHEBI:13389 / CHEBI:57945): redox coenzymes altered in patients; linked to mitochondrial dysfunction. (balci2024oxidativedamageand pages 2-3) - GDF-15, FGF-21: mitokines elevated in CBS deficiency; correlate with disease biochemistry (GDF-15 particularly). (balci2024oxidativedamageand pages 2-3, balci2024oxidativedamageand pages 4-5) Therapeutic chemicals - Pyridoxine / PLP (vitamin B6): cofactor and potential pharmacological chaperone for some variants. (majtan2023recenttherapeuticapproaches pages 4-6) - Betaine (trimethylglycine): methyl donor supporting remethylation in liver via BHMT. (majtan2023recenttherapeuticapproaches pages 4-6) - Biological reductants: N-acetylcysteine (NAC), MESNA, cysteamine can increase reduced Hcy availability (substrate form for CBS), improving efficacy of CBS-based enzyme replacement therapy in models. (philipp2024mechanismofaction pages 1-2)

3.3 Cell types (CL) and tissues/anatomy (UBERON) most implicated - Hepatocytes / liver (UBERON:0002107): main site of transsulfuration/remethylation flux; therapies target restoring hepatic CBS activity; betaine remethylation via liver BHMT. (majtan2023recenttherapeuticapproaches pages 4-6) - Vascular endothelial cells (CL:0000115) and platelets (CL:0000233): key in thrombosis phenotype; Hcy-associated ROS and protein modifications contribute to endothelial dysfunction and platelet activation. (majtan2023recenttherapeuticapproaches pages 3-4) - Neurons (CL:0000540) / brain (UBERON:0000955): NMDA receptor-mediated excitotoxicity implicated; cognitive impairment/seizures common in untreated patients. (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 6-7) - Ocular lens zonular fibers / eye (UBERON:0000970): ectopia lentis is common and tied to connective tissue/ECM disruption. (ziegler2023inbornerrorsof pages 6-7) - Bone/connective tissue (UBERON:0002384 / UBERON:0002385): collagen cross-link impairment and ECM alterations; osteoporosis and marfanoid habitus. (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 6-7)

Biological processes disrupted (GO-oriented; suggested annotations) The following GO biological-process themes are supported by the mechanistic evidence in recent literature:
Sulfur amino acid metabolic process / homocysteine metabolic process (block at CBS) (philipp2024mechanismofaction pages 1-2)
Transsulfuration pathway / cysteine biosynthetic process (depletion of Cth/Cys) (philipp2024mechanismofaction pages 1-2)
Cellular response to oxidative stress / reactive oxygen species metabolic process (Hcy-driven ROS; cysteine depletion) (majtan2023recenttherapeuticapproaches pages 3-4, balci2024oxidativedamageand pages 2-3)
Mitochondrial dysfunction pathways (mitochondrial DAMP release; altered NAD redox; mitokines) (balci2024oxidativedamageand pages 2-3)
Protein folding / protein refolding / proteasome-mediated ubiquitin-dependent protein catabolic process (CBS variant misfolding and UPS disposal; rescue by proteasome inhibition/chaperones) (mijatovic2024cellularturnoverand pages 1-2, collard2023geneticandpharmacological pages 1-2)
Endoplasmic reticulum stress / unfolded protein response (UPR) (BiP upregulation; ER stress in CBS variants; Hcy-induced ER stress) (majtan2023recenttherapeuticapproaches pages 3-4, collard2023geneticandpharmacological pages 1-2)
Blood coagulation / thrombosis-related processes (endothelial dysfunction, platelet activation, fibrinolysis resistance via fibrinogen modification) (majtan2023recenttherapeuticapproaches pages 3-4)
Regulation of methylation / transmethylation (SAH inhibition of methylation; DNA methylation effects) (majtan2023recenttherapeuticapproaches pages 3-4, gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4)
Glutamatergic synaptic signaling / NMDA receptor signaling (homocysteic acid; seizure mechanisms) (majtan2023recenttherapeuticapproaches pages 3-4)
Cellular components (where processes occur)
Cytosol and enzyme complexes of methionine cycle/transsulfuration (CBS enzymatic function) (philipp2024mechanismofaction pages 1-2)
Endoplasmic reticulum (ER) quality control, ERAD interfaces and associated proteostasis signaling (BiP/ER stress; ERAD involvement) (mijatovic2024cellularturnoverand pages 1-2, collard2023geneticandpharmacological pages 1-2)
Proteasome complex (UPS disposal of misfolded CBS variants) (mijatovic2024cellularturnoverand pages 1-2)
Mitochondria (redox imbalance, DAMP release, mitokines; mtDNA-linked markers) (balci2024oxidativedamageand pages 2-3)
Extracellular space/plasma (tHcy biomarker; reduced vs protein-bound homocysteine fractions; fibrinogen modification) (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4)
Disease progression model (sequence of events and phases)

6.1 Trigger and early biochemical phase - Primary trigger: biallelic pathogenic variants in CBS causing reduced/absent enzyme activity, frequently via misfolding/instability rather than active-site disruption. (mijatovic2024cellularturnoverand pages 1-2, collard2023geneticandpharmacological pages 1-2) - Immediate biochemical consequences: accumulation of Hcy and upstream metabolites (Met, SAM, SAH) with depletion of downstream products (cystathionine, cysteine). (philipp2024mechanismofaction pages 1-2, ziegler2023inbornerrorsof pages 7-8)

6.2 Cellular injury phase (parallel mechanisms) A. Oxidative/mitochondrial arm - ROS generation and redox imbalance; in patients, NAD metrics and mitochondrial stress markers (GDF-15, FGF-21, mtDAMPs) are elevated and correlate with tHcy. (balci2024oxidativedamageand pages 2-3, balci2024oxidativedamageand pages 4-5) B. Proteostasis/ER stress arm - Hcy is linked to unfolded protein response/ER stress at a general level, while CBS variants specifically trigger ER stress and are cleared primarily by the proteasome with contributions from ERAD/autophagy. (majtan2023recenttherapeuticapproaches pages 3-4, mijatovic2024cellularturnoverand pages 1-2) C. Protein modification/ECM and coagulation arm - Hcy thiolactone causes irreversible N-homocysteinylation; fibrinogen modification increases resistance to fibrinolysis (pro-thrombotic), and collagen cross-link impairment contributes to skeletal/connective tissue defects. (majtan2023recenttherapeuticapproaches pages 3-4) D. Neuro-excitotoxic arm - Homocysteic acid is a potent NMDA receptor agonist; NMDA-related mechanisms are invoked to explain seizures and neurocognitive injury. (majtan2023recenttherapeuticapproaches pages 3-4)

6.3 Organ-level clinical manifestation phase (temporal patterns) - Clinical variability and staging: individuals may appear normal at birth; manifestations can appear at any age. (ziegler2023inbornerrorsof pages 6-7) - Ocular: ectopia lentis is common by age 5–10 and becomes very frequent by later adulthood if untreated. (ziegler2023inbornerrorsof pages 6-7) - Neurologic: developmental delay/cognitive impairment (~60% untreated) and seizures (~50% untreated) are reported. (ziegler2023inbornerrorsof pages 6-7) - Vascular: thromboembolism often emerges in young adulthood; untreated risk may approach ~50% by age 30 and correlates with degree/duration of Hcy elevation. (ziegler2023inbornerrorsof pages 7-8)

6.4 Major modifier: pyridoxine responsiveness - Pyridoxine responsiveness stratifies phenotype: non-responders often manifest in early childhood with multi-system features; “extreme responders” may present in (late) adulthood predominantly with thromboembolism. (majtan2023recenttherapeuticapproaches pages 3-4)

Phenotypic manifestations (HP terms; selected) Mechanistically linked phenotypes highlighted in recent sources include:
Thromboembolism / stroke (vascular occlusion; endothelial dysfunction and platelet activation) (majtan2023recenttherapeuticapproaches pages 3-4)
Ectopia lentis / lens dislocation / myopia (connective tissue/ECM changes) (ziegler2023inbornerrorsof pages 6-7, philipp2024mechanismofaction pages 1-2)
Osteoporosis / osteopenia and marfanoid habitus (collagen cross-linking impairment, ECM alterations) (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 6-7)
Cognitive impairment / developmental delay; seizures (neurotoxicity, NMDA mechanisms, methylation disruption) (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 6-7)
Recent developments and latest research (prioritizing 2023–2024)

8.1 Mitochondrial dysfunction biomarkers (2024) Balci et al. (Nov 2024) provide recent patient-based evidence connecting tHcy to altered NAD metrics, elevated mitokines (GDF-15/FGF-21) and increased mitochondrial DAMPs, supporting mitochondria-centered pathogenic hypotheses and potentially actionable monitoring biomarkers. (balci2024oxidativedamageand pages 2-3, balci2024oxidativedamageand pages 4-5)

8.2 CBS proteostasis as a therapeutic target (2023–2024) - Collard & Majtan (Dec 2023) propose and experimentally support proteostasis modulation (chaperone induction, ISR modulation, proteasome inhibition) as variant-dependent functional rescue of misfolded CBS variants (e.g., R125Q permissive; I278T refractory for activity rescue in their system). (collard2023geneticandpharmacological pages 1-2, collard2023geneticandpharmacological pages 2-4) - Mijatovic et al. (Jul 2024) refine the degradation-pathway map for common CBS mutants and quantify half-life defects, reinforcing UPS/ERAD as mechanistically central and druggable. (mijatovic2024cellularturnoverand pages 1-2, mijatovic2024cellularturnoverand pages 8-10)

8.3 Enzyme replacement therapy (ERT) mechanism refinement (2024) Philipp et al. (Redox Biology; accepted Oct 2024; https://doi.org/10.1016/j.redox.2024.103383) clarify an important mechanistic detail for ERT: “only a reduced homocysteine serves as a substrate for CBS,” and biological reductants (NAC, MESNA, cysteamine) can increase reduced Hcy and improve ERT efficacy, lowering plasma tHcy below a “clinically relevant 100 µM threshold” in mouse models. (philipp2024mechanismofaction pages 1-2)

Current applications and real-world implementations

9.1 Newborn screening (NBS) - Current implementation: most NBS programs screen using methionine (tandem MS). A positive screening methionine range cited is ~200–1500 µM (reference 10–40 µM). (ziegler2023inbornerrorsof pages 6-7) - Known limitation: pyridoxine-responsive/milder cases may be missed because methionine may not reach the screening threshold at sampling time. (ziegler2023inbornerrorsof pages 6-7, majtan2023recenttherapeuticapproaches pages 3-4) - Alternative approach: Qatar includes direct Hcy measurement in NBS; most programs do not, because total Hcy measurement requires chemical reduction. (ziegler2023inbornerrorsof pages 6-7, ziegler2023inbornerrorsof pages 7-8)

9.2 Standard-of-care biochemical targets and treatments - Guideline-based biochemical targets summarized in recent sources: tHcy <100 µM for pyridoxine non-/partial responders; <50 µM for full/extreme responders; free Hcy <11 µM is also cited as a goal. (majtan2023recenttherapeuticapproaches pages 4-6, ziegler2023inbornerrorsof pages 7-8) - Pyridoxine testing and dosing framework: standardized pyridoxine loading tests guide stratification; responsiveness categories correlate with severity. (majtan2023recenttherapeuticapproaches pages 4-6) - Dietary methionine/protein restriction: key intervention for non-responders and some partial responders; requires Met-free, cysteine-enriched amino acid mixtures to maintain nitrogen balance. (majtan2023recenttherapeuticapproaches pages 4-6) - Betaine: adjunct remethylation strategy; recommended dosing in recent review: 50 mg/kg/day (children, split twice daily) and ~3 g/day (adults), with methionine monitoring to keep plasma methionine <1,000 µM due to rare cerebral edema risk. (majtan2023recenttherapeuticapproaches pages 6-7)

9.3 Thrombosis prevention considerations (implementation) Because thromboembolism is a major morbidity driver: - Practical prevention guidance highlighted in 2023 literature includes avoiding oral contraceptives, considering prophylactic anticoagulation in the third trimester and postpartum, and surgical caution given high thrombotic risk. (ziegler2023inbornerrorsof pages 6-7)

Epidemiology and statistics (recent/real-world data)
Worldwide prevalence estimates vary widely; in a 2024 US claims/EHR algorithm study (Jain et al.; Sep 2024; https://doi.org/10.1016/j.ymgmr.2024.101101), average annual standardized prevalence across 2016–2020 was estimated at 5.29 per 100,000 (broad definition) and 1.04 per 100,000 (strict definition). (jain2024estimatingprevalenceof pages 1-2)
The same study restates widely cited diagnostic anchors: normal tHcy <15 µM and tHcy >100 µM usually diagnostic; it also notes that tHcy >30 µM may indicate HCU in some undiagnosed individuals. (jain2024estimatingprevalenceof pages 1-2)
NBS-based global incidence estimates for classic HCU are ~1 in 200,000 to 1 in 340,000, but underestimation is likely because of NBS detection limitations and ascertainment bias. (ziegler2023inbornerrorsof pages 6-7)
Expert opinions and analysis (authoritative sources; with direct quotes)

Selected direct mechanistic quotes from 2023–2024 sources: - CBS branchpoint and metabolite directionality: “CBS lies at the branch point where the fate of Hcy is decided…” and CBS deficiency leads to upstream metabolite accumulation with downstream depletion. (philipp2024mechanismofaction pages 1-2) - Hcy chemical reactivity and ER stress: “Hcy with its free sulfhydryl group is a highly reactive, redox active compound with potential pathological mechanisms involving formation of reactive oxygen species, protein modification, induction of unfolded protein response and endoplasmic reticulum stress.” (majtan2023recenttherapeuticapproaches pages 3-4) - Conformational disorder framing: “HCU should be viewed as a protein conformational disorder with protein misfolding and instability as the main mechanism resulting in CBS deficiency.” (mijatovic2024cellularturnoverand pages 1-2) - ERT substrate form: “only a reduced homocysteine serves as a substrate for CBS.” (philipp2024mechanismofaction pages 1-2)

Knowledge-base ready annotation blocks

12.1 Pathophysiology description (narrative) Classical homocystinuria results from biallelic CBS deficiency at the branchpoint of methionine metabolism, producing accumulation of homocysteine and upstream one-carbon metabolites (methionine, SAM, SAH) with depletion of cystathionine/cysteine. The elevated, reactive thiol homocysteine drives oxidative stress and mitochondrial dysfunction (as shown by altered NAD redox, mitokine elevation, and mitochondrial DAMP release in patients), induces ER stress/UPR, and generates reactive derivatives including homocysteic acid (NMDA agonist) and homocysteine thiolactone. Homocysteine thiolactone mediates irreversible N-homocysteinylation of proteins such as fibrinogen (increasing resistance to fibrinolysis) and collagen (impairing cross-linking), mechanistically linking the biochemical lesion to thrombosis and connective tissue/bone phenotypes. Endothelial dysfunction and platelet activation promote vascular occlusion and thromboembolism, while NMDA-linked excitotoxicity and methylation inhibition (via SAH) contribute to seizures and cognitive impairment. Disease timing and severity are strongly modified by pyridoxine responsiveness, with non-responders typically presenting earlier and with broader multi-organ involvement. (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4, balci2024oxidativedamageand pages 2-3)

12.2 Gene/protein annotations (HGNC; evidence items) - CBS (HGNC:1550): causal for classical homocystinuria; mechanistic roles include transsulfuration flux control and H2S generation; pathogenic variants frequently misfold and are cleared mainly by UPS/proteasome with ERAD contributions. Evidence: Philipp et al. 2024; Mijatovic et al. 2024; Collard & Majtan 2023. (philipp2024mechanismofaction pages 1-2, mijatovic2024cellularturnoverand pages 1-2, collard2023geneticandpharmacological pages 1-2) - MTHFR, MTR, MTRR: remethylation pathway genes causing HHcy/homocystinuria phenotypes when defective (disease heterogeneity). (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4)

12.3 Suggested GO biological process terms (evidence) - Homocysteine metabolic process / sulfur amino acid metabolic process (philipp2024mechanismofaction pages 1-2) - Cellular response to oxidative stress; mitochondrial dysfunction-associated processes (balci2024oxidativedamageand pages 2-3) - Protein folding; proteasome-mediated ubiquitin-dependent catabolic process; ERAD/UPR (mijatovic2024cellularturnoverand pages 1-2, collard2023geneticandpharmacological pages 1-2) - Blood coagulation/fibrinolysis regulation (via fibrinogen modification) (majtan2023recenttherapeuticapproaches pages 3-4) - NMDA receptor signaling / excitotoxicity-associated processes (majtan2023recenttherapeuticapproaches pages 3-4)

12.4 Phenotype associations (HP; evidence) - Thromboembolism/stroke (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 7-8) - Ectopia lentis / myopia (ziegler2023inbornerrorsof pages 6-7, philipp2024mechanismofaction pages 1-2) - Osteoporosis / skeletal abnormalities / marfanoid habitus (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 6-7) - Cognitive impairment / seizures (majtan2023recenttherapeuticapproaches pages 3-4, ziegler2023inbornerrorsof pages 6-7)

12.5 Cell type involvement (CL; evidence) - Endothelial cells, platelets (vascular pathology) (majtan2023recenttherapeuticapproaches pages 3-4) - Hepatocyte/liver metabolism (BHMT/betaine remethylation; CBS target tissue) (majtan2023recenttherapeuticapproaches pages 4-6) - Neurons (NMDA-mediated excitotoxicity; neurotoxicity) (majtan2023recenttherapeuticapproaches pages 3-4)

12.6 Anatomical locations (UBERON; evidence) - Liver (metabolic control; therapeutic target) (majtan2023recenttherapeuticapproaches pages 4-6) - Vasculature (thrombosis phenotype) (majtan2023recenttherapeuticapproaches pages 3-4) - Eye/lens zonules (ectopia lentis) (ziegler2023inbornerrorsof pages 6-7) - Bone/connective tissue (osteoporosis, marfanoid features) (majtan2023recenttherapeuticapproaches pages 3-4)

12.7 Chemical entities (CHEBI; evidence) - Homocysteine; methionine; cystathionine; cysteine; SAM; SAH; homocysteic acid; homocysteine thiolactone; NAD+/NADH; betaine; pyridoxine/PLP; N-acetylcysteine. (philipp2024mechanismofaction pages 1-2, majtan2023recenttherapeuticapproaches pages 3-4, balci2024oxidativedamageand pages 2-3)

12.8 Evidence items (PMIDs and notes) The current tool-accessible excerpts did not reliably expose PMIDs for all cited sources. Where PMID information was visible in retrieved metadata, it was limited (e.g., OpenTargets evidence for MTHFR included PMID: 36567323 in association data). (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4) Primary citations with DOIs and publication dates (URLs included): - Majtan T, Kožich V, Kruger WD. “Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria.” British Journal of Pharmacology. Dec 2023. https://doi.org/10.1111/bph.15991 (majtan2023recenttherapeuticapproaches pages 3-4) - Collard R, Majtan T. “Genetic and Pharmacological Modulation of Cellular Proteostasis…” Molecular and Cellular Biology. Dec 2023. https://doi.org/10.1080/10985549.2023.2284147 (collard2023geneticandpharmacological pages 1-2) - González-Lamuño D, et al. “Hyperhomocysteinemia in Adult Patients: A Treatable Metabolic Condition.” Nutrients. Dec 2023. https://doi.org/10.3390/nu16010135 (gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4) - Mijatovic E, et al. “Cellular turnover and degradation of the most common missense CBS variants…” Protein Science. Jul 2024. https://doi.org/10.1002/pro.5123 (mijatovic2024cellularturnoverand pages 1-2) - Jain M, et al. “Estimating prevalence of classical homocystinuria in the United States…” Molecular Genetics and Metabolism Reports. Sep 2024. https://doi.org/10.1016/j.ymgmr.2024.101101 (jain2024estimatingprevalenceof pages 1-2) - Philipp TM, et al. “Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria.” Redox Biology. Available online Oct 2024; issue/month Nov 2024. https://doi.org/10.1016/j.redox.2024.103383 (philipp2024mechanismofaction pages 1-2) - Balci MC, et al. “Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency.” Scientific Reports. Nov 2024. https://doi.org/10.1038/s41598-024-80273-w (balci2024oxidativedamageand pages 2-3)

Visual evidence
Figure: Methionine cycle and transsulfuration pathway with therapeutic intervention points for CBS-deficient homocystinuria (Majtan et al., 2023). (majtan2023recenttherapeuticapproaches media c1dbb0d7)

Limitations - PMID fields were not consistently available in the retrieved full-text excerpts. Mechanistic claims above are supported by the cited 2023–2024 primary and review literature via DOI-linked sources, but a comprehensive PMID list would require direct PubMed lookups beyond the currently retrieved excerpts. (majtan2023recenttherapeuticapproaches pages 3-4, balci2024oxidativedamageand pages 2-3)

References

(gonzalezlamuno2023hyperhomocysteinemiainadult pages 2-4): Domingo González-Lamuño, Francisco Jesús Arrieta-Blanco, Elena Dios Fuentes, María Teresa Forga-Visa, Monstserrat Morales-Conejo, Luis Peña-Quintana, and Isidro Vitoria-Miñana. Hyperhomocysteinemia in adult patients: a treatable metabolic condition. Nutrients, 16:135, Dec 2023. URL: https://doi.org/10.3390/nu16010135, doi:10.3390/nu16010135. This article has 70 citations.
(philipp2024mechanismofaction pages 1-2): Thilo Magnus Philipp, Teodoro Bottiglieri, Wilmelenne Clapper, Kai Liu, Steve Rodems, Csaba Szabo, and Tomas Majtan. Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria. Nov 2024. URL: https://doi.org/10.1016/j.redox.2024.103383, doi:10.1016/j.redox.2024.103383. This article has 5 citations and is from a domain leading peer-reviewed journal.
(majtan2023recenttherapeuticapproaches pages 3-4): Tomas Majtan, Viktor Kožich, and Warren D. Kruger. Recent therapeutic approaches to cystathionine beta‐synthase‐deficient homocystinuria. Dec 2023. URL: https://doi.org/10.1111/bph.15991, doi:10.1111/bph.15991. This article has 27 citations and is from a highest quality peer-reviewed journal.
(collard2023geneticandpharmacological pages 1-2): Renata Collard and Tomas Majtan. Genetic and pharmacological modulation of cellular proteostasis leads to partial functional rescue of homocystinuria-causing cystathionine-beta synthase variants. Molecular and Cellular Biology, 43:664-674, Dec 2023. URL: https://doi.org/10.1080/10985549.2023.2284147, doi:10.1080/10985549.2023.2284147. This article has 8 citations and is from a domain leading peer-reviewed journal.
(ziegler2023inbornerrorsof pages 7-8): SG Ziegler, J Kim, and JT Ehmsen. Inborn errors of amino acid metabolism–from underlying pathophysiology to therapeutic advances. Unknown journal, 2023.
(jain2024estimatingprevalenceof pages 1-2): Mahim Jain, Mehul Shah, Kamlesh M. Thakker, Andrew Rava, Agness Pelts Block, Colette Ndiba-Markey, and Lionel Pinto. Estimating prevalence of classical homocystinuria in the united states using optum's de-identified market clarity data. Sep 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101101, doi:10.1016/j.ymgmr.2024.101101. This article has 2 citations.
(balci2024oxidativedamageand pages 2-3): Mehmet Cihan Balci, Asuman Gedikbasi, Sukru Anil Dogan, Sevde Kahraman, Suzin Tatoryan, Sebnem Tekin Neijmann, Meryem Karaca, Fatmahan Atalar, and Gulden Gokcay. Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency. Scientific Reports, Nov 2024. URL: https://doi.org/10.1038/s41598-024-80273-w, doi:10.1038/s41598-024-80273-w. This article has 1 citations and is from a peer-reviewed journal.
(balci2024oxidativedamageand pages 4-5): Mehmet Cihan Balci, Asuman Gedikbasi, Sukru Anil Dogan, Sevde Kahraman, Suzin Tatoryan, Sebnem Tekin Neijmann, Meryem Karaca, Fatmahan Atalar, and Gulden Gokcay. Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency. Scientific Reports, Nov 2024. URL: https://doi.org/10.1038/s41598-024-80273-w, doi:10.1038/s41598-024-80273-w. This article has 1 citations and is from a peer-reviewed journal.
(balci2024oxidativedamageand pages 1-2): Mehmet Cihan Balci, Asuman Gedikbasi, Sukru Anil Dogan, Sevde Kahraman, Suzin Tatoryan, Sebnem Tekin Neijmann, Meryem Karaca, Fatmahan Atalar, and Gulden Gokcay. Oxidative damage and mitochondrial dysfunction in cystathionine beta-synthase deficiency. Scientific Reports, Nov 2024. URL: https://doi.org/10.1038/s41598-024-80273-w, doi:10.1038/s41598-024-80273-w. This article has 1 citations and is from a peer-reviewed journal.
(mijatovic2024cellularturnoverand pages 1-2): Ela Mijatovic, Kelly Ascenção, Csaba Szabo, and Tomas Majtan. Cellular turnover and degradation of the most common missense cystathionine beta‐synthase variants causing homocystinuria. Protein Science : A Publication of the Protein Society, Jul 2024. URL: https://doi.org/10.1002/pro.5123, doi:10.1002/pro.5123. This article has 9 citations.
(mijatovic2024cellularturnoverand pages 8-10): Ela Mijatovic, Kelly Ascenção, Csaba Szabo, and Tomas Majtan. Cellular turnover and degradation of the most common missense cystathionine beta‐synthase variants causing homocystinuria. Protein Science : A Publication of the Protein Society, Jul 2024. URL: https://doi.org/10.1002/pro.5123, doi:10.1002/pro.5123. This article has 9 citations.
(collard2023geneticandpharmacological pages 2-4): Renata Collard and Tomas Majtan. Genetic and pharmacological modulation of cellular proteostasis leads to partial functional rescue of homocystinuria-causing cystathionine-beta synthase variants. Molecular and Cellular Biology, 43:664-674, Dec 2023. URL: https://doi.org/10.1080/10985549.2023.2284147, doi:10.1080/10985549.2023.2284147. This article has 8 citations and is from a domain leading peer-reviewed journal.
(ziegler2023inbornerrorsof pages 6-7): SG Ziegler, J Kim, and JT Ehmsen. Inborn errors of amino acid metabolism–from underlying pathophysiology to therapeutic advances. Unknown journal, 2023.
(majtan2023recenttherapeuticapproaches pages 4-6): Tomas Majtan, Viktor Kožich, and Warren D. Kruger. Recent therapeutic approaches to cystathionine beta‐synthase‐deficient homocystinuria. Dec 2023. URL: https://doi.org/10.1111/bph.15991, doi:10.1111/bph.15991. This article has 27 citations and is from a highest quality peer-reviewed journal.
(majtan2023recenttherapeuticapproaches pages 6-7): Tomas Majtan, Viktor Kožich, and Warren D. Kruger. Recent therapeutic approaches to cystathionine beta‐synthase‐deficient homocystinuria. Dec 2023. URL: https://doi.org/10.1111/bph.15991, doi:10.1111/bph.15991. This article has 27 citations and is from a highest quality peer-reviewed journal.
(majtan2023recenttherapeuticapproaches media c1dbb0d7): Tomas Majtan, Viktor Kožich, and Warren D. Kruger. Recent therapeutic approaches to cystathionine beta‐synthase‐deficient homocystinuria. Dec 2023. URL: https://doi.org/10.1111/bph.15991, doi:10.1111/bph.15991. This article has 27 citations and is from a highest quality peer-reviewed journal.

Ask OpenScientist

OpenScientist Report

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Treatments

Biochemical Markers

Source YAML

References & Deep Research

References

Deep Research

Disease Pathophysiology Research Template

Target Disease

Research Objectives

Required Information

1. Core Pathophysiology

2. Key Molecular Players

3. Biological Processes (for GO annotation)

4. Cellular Components

5. Disease Progression

6. Phenotypic Manifestations

Citation Requirements

Output Format