Medical Genetics & Metabolism & Pediatrics & Internal Medicine
Homocystinuria | Clinique Omicron Quebec
Classic homocystinuria is a rare inherited metabolic disorder with autosomal recessive inheritance; in its most common form, it is caused by a deficiency of cystathionine beta-synthase (CBS—an enzyme that catalyzes the condensation of homocysteine with serine to form cystathionine in the transsulfuration pathway). This enzyme deficiency leads to a massive accumulation of homocysteine and methionine in the plasma, urine, and tissues. The global incidence ranges from 1 in 200,000 to 1 in 335,000 births, with significant geographical variations (particularly high prevalence in Ireland—1 in 65,000—due to a founder effect). The term «homocystinuria» strictly refers to the presence of homocystine in the urine (the dimeric oxidized form of homocysteine), which reflects a very high plasma concentration of total homocysteine (>100–400 µmol/L). The clinical picture—when the diagnosis is not made early through neonatal screening—is dominated by four systems: ocular (lens dislocation—ectopia lentis), skeletal (Marfanoid phenotype—tall stature + arachnodactyly + kyphoscoliosis + pectus excavatum + early-onset osteoporosis), neurovascular (early arterial and venous thromboembolism—stroke + myocardial infarction + pulmonary thromboembolism as early as adolescence), and neurological (intellectual disability + psychiatric disorders). Treatment is based on two complementary strategies: in patients who respond to pyridoxine (vitamin B6 — approximately 50% of CBS patients), high-dose pyridoxine normalizes or drastically reduces plasma homocysteine levels; in non-responders, a methionine-restricted diet with supplementation of cysteine and betaine (trimethylglycine—an alternative methyl donor that stimulates remethylation via BHMT) forms the basis of treatment.
Biochemistry, genetics, and clinical manifestations
- Biochemistry of CBS Deficiency and Causes of Homocystinuria - Etiological Classification: Transsulfuration pathway and CBS deficiency: methionine → SAM (S-adenosylmethionine) → SAH (S-adenosylhomocysteine) → homocysteine → CBS → cystathionine → cysteine → glutathione + taurine → sulfates → in case of CBS deficiency: homocysteine not metabolized by transsulfuration → massive accumulation + the backup remethylation pathway (methionine synthase + MTHFR + betaine BHMT) is overwhelmed → total plasma homocysteine: 100–400 µmol/L (normal range: 5–15 µmol/L) + very high methioninemia → massive urinary homocystine (positive sodium nitroprusside test — Brandl reaction) → low cysteine (lack of substrate for glutathione synthesis → oxidative stress); genetics of CBS deficiency: CBS gene located on chromosome 21q22.3 → >200 pathogenic mutations described → predominantly missense mutations → most common mutations in the European population: c.833T>C (p.I278T) and c.919G>A (p.G307S) → c.833T>C (I278T): often associated with response to pyridoxine + milder presentation → c.919G>A (G307S): often non-responsive + more severe phenotype → Fowler 1997 — Journal of Inherited Metabolic Disease: genotype-phenotype correlation in CBS deficiency + autosomal recessive inheritance → heterozygous carriers: moderately elevated homocysteine (10–25 µmol/L) → slightly increased cardiovascular risk → no classic homocystinuria → two subtypes based on response to pyridoxine: pyridoxine responders (B6-responsive): ≈50% of patients → high-dose pyridoxine normalizes or drastically reduces total homocysteine → milder phenotype → pyridoxine non-responders (B6-non-responsive): ≈50% of patients → pyridoxine does not significantly alter homocysteine levels → often more severe phenotype → more complex treatment required; other causes of biochemical homocystinuria (in the broad sense — non-CBS): cobalamin metabolism defects (cbIC + cbID + cbIE + cbIF + cbIG — methylmalonic acidemias with homocystinuria): mutations in the MMACHC + MMADHC + MTR + MTRR genes → elevated homocysteine + homocystinuria + low methionine levels (unlike CBS, where methionine is elevated) + sometimes methylmalonic acidemia + megaloblastic anemia → severe MTHFR deficiency (homozygous mutation): homocystinuria + hyperhomocysteinemia + low methionine levels + no methylmalonic acid → methionine synthase (MTR) deficiency + methionine synthase reductase (MTRR) deficiency: rare forms → biochemical distinction: measurement of methioninemia + urinary/plasma methylmalonic acid + acylcarnitine profile → CBS homocystinuria vs. cobalamin deficiencies → CBS: elevated methionine + no methylmalonate + CBL: low methionine + elevated methylmalonate (except CBL-E/G); neonatal screening: screening by tandem mass spectrometry (MS/MS) on the Guthrie test (neonatal screening card): measurement of methionine + methionine/phenylalanine ratio → cutoff value: methionine >50–100 µmol/L → suspected → confirmation by measurement of total plasma homocysteine + plasma amino acid chromatography → in case of CBS deficiency: elevated methionine + very high homocysteine + low cysteine → cobalamin deficiencies are not always detected by methionine alone → some screening programs add the Met/Phe ratio or measure homocysteine directly → in Quebec: expanded newborn screening (Quebec Newborn Screening Program—PQDN) has included screening for classic homocystinuria (CBS) since its expansion → confirmation at the CHU (Center for Medical Genetics)
- Clinical manifestations — the 4 affected systems: Ocular involvement — lens ectopia (dislocation of the lens): the most characteristic manifestation of untreated CBS homocystinuria → direction of dislocation: downward and inward (unlike Marfan syndrome, where dislocation occurs upward and outward) → mechanism: accumulation of homocysteine → alters the disulfide bonds of fibryllin-1 in the zonules (lens suspensory ligaments) → progressive rupture of the zonules → dislocation → Mudd 1985 — American Journal of Human Genetics (Natural History Study): lens ectopia in 90% of untreated patients by age 10 → ocular complications: severe axial myopia + glaucoma + cataract + retinal detachment → possible blindness if untreated + even in treated patients: annual ophthalmological follow-up is mandatory + skeletal involvement — Marfanoid phenotype: tall stature + long limbs (arachnodactyly + dolichostenomelia) + arm span > height + pectus excavatum or carinatum + kyphoscoliosis + valgus deformity of the knees + early-onset osteoporosis → mechanism: homocysteine → inhibits collagen and elastin cross-linking (inhibition of lysyl oxidase) → weakness of the extracellular matrix + connective tissue damage → osteoporosis: common as early as the second decade of life (very low bone mineral density) → possible pathological fractures → spinal X-ray: biconcave vertebrae (codfish vertebrae) + risk of progressive scoliosis → neurovascular involvement — early arterial and venous thromboembolism: the most serious and potentially fatal complication → affects vessels of all sizes (arteries + veins + capillaries) → mechanism: homocysteine → endothelial dysfunction + oxidative stress + platelet activation (increased TXA2) + inhibition of thrombomodulin + activation of factor V + activation of factor X + resistance to activated protein C → severe prothrombotic state → Mudd 1985: thromboembolism in 50% of untreated patients before age 30 → 30% mortality rate in the third decade → locations: stroke (cerebral arteries) + early myocardial infarction (coronary arteries) + pulmonary embolism (deep veins) + mesenteric thrombosis + vena cava thrombosis → increased risk in situations with a high risk of thrombosis: dehydration + surgery + immobilization + pregnancy → thrombosis may be the first telltale sign of homocystinuria in an undiagnosed young adult + neurological and psychiatric involvement: intellectual disability: variable → more frequent and severe in forms unresponsive to B6 + absent or mild in responders treated early → seizure disorder (epilepsy) + frequent psychiatric disorders: depression + anxiety + behavioral disorders + autism spectrum disorders → strokes may contribute to neurological deficits + Mudd 1985 — AJHG: Natural History of Homocystinuria due to CBS deficiency → reference series of 629 patients → follow-up data on all manifestations + complications
Diagnosis, treatment and follow-up
| Domain | Data, methods and criteria | Key studies and recommendations |
|---|---|---|
| Diagnostic assessment and biochemical confirmation Plasma homocysteine — methionineemia — amino acid chromatography — urinary homocystine — nitroprusside test — pyridoxine test — CBS activity — genotyping |
Confirmatory biochemical test: total plasma homocysteine (tHcy): reference method — HPLC or immunoassay → in an untreated CBS patient: 100–400 µmol/L (normal range: 5–15 µmol/L) → sample must be collected in the fasting state + in an EDTA tube and centrifuged immediately (otherwise false positives) + plasma amino acid chromatography (HPLC or tandem MS): very high methionine (often 100–400 µmol/L — normal 10–40 µmol/L in CBS) + low or undetectable cysteine + homocystine present (oxidized dimeric form of homocysteine) + comparison with cobalamin deficiencies: low methionine + elevated methylmalonate (except cblE/G) + elevated homocysteine + urinary homocystine: urinary amino acid chromatography → presence of homocystine → sodium nitroprusside test (cyanide-nitroprusside — Brandl reaction): rapid semi-quantitative test on fresh urine → presence of homocystine + cystine → red-purple colorimetric reaction → positive in CBS + cystinuria (red stain as well) → limited specificity → useful as a rapid screening test but must be confirmed by quantitative methods → plasma or urinary methylmalonic acid (MMA): elevated in cobalamin deficiencies (cbIB + cbIC + cbID) + normal in isolated CBS → differentiates the two major categories of homocystinuria → plasma acylcarnitine profile: C2 propionylcarnitine in methylmalonic acidemias → measurement of CBS activity: on leukocyte pellets or skin biopsy (fibroblasts) → very low activity (<1–2 % of normal in classic forms) + higher residual activity in partial forms or B6 responders → Kraus 1999 — Human Mutation: correlation between CBS activity and phenotype + CBS gene genotyping: genomic DNA sequencing → identification of the 2 pathogenic mutations (screening for all coding variants + regulatory regions) → essential for genetic counseling + prenatal screening + genotype-phenotype correlation → Fowler 1997 — Journal of Inherited Metabolic Disease: CBS mutation database + correlations; pyridoxine (B6) response test — essential before starting treatment: standard protocol: baseline status (homocysteine + methionine) → pyridoxine 100–500 mg/day PO × 2–4 weeks → homocysteine + methionine measurement on Day 14 and Day 28 → positive response: reduction in total homocysteine to <50 µmol/L (and ideally <15 µmol/L with optimal treatment) + reduction in methionine levels → SSIEM (Society for the Study of Inborn Errors of Metabolism) response criteria: reduction of at least 50% in total homocysteine → partial responders may benefit from pyridoxine but also require other treatments → interpretation: response BEFORE initiating betaine therapy, as betaine may skew the test results + prenatal screening and antenatal diagnosis: homocysteine measurement + CBS activity on amniotic cells or chorionic villi → genotyping if mutations are identified in the parents → allows for management starting at birth + clinical differential diagnosis: Marfan syndrome → FBN1 → upward lens dislocation + no elevated homocysteine + no early thrombosis + no cognitive impairment → homocysteine measurement + FBN1 genotyping → CBS deficiency vs. Marfan syndrome → differential biochemical workup | Key diagnostic data: Mudd 1985 — American Journal of Human Genetics (n=629 patients CBS): Natural History Study → baseline data on natural history, clinical spectrum, complications, and treatment effects → foundational series Fowler 1997 — Journal of Inherited Metabolic Disease: CBS mutations → genotype-phenotype correlations + c.833T>C (I278T) and c.919G>A (G307S) mutations → different predispositions to B6 response Kraus 1999 — Human Mutation: CBS activity in fibroblasts → correlates with phenotype Levy 2009 — JIMD: SSIEM recommendations for diagnosis and treatment of CBS homocystinuria Yap 2010 — Journal of Inherited Metabolic Disease: Revised SSIEM guidelines → pyridoxine challenge protocol + therapeutic targets + monitoring Brenton 2000 — Neurology: Early treatment → impact on neurological outcome and complications SSIEM (Society for the Study of Inborn Errors of Metabolism) + Quebec Neonatal Screening Program (PQDN): Inclusion of CBS homocystinuria in expanded screening by MS/MS |
| Treatment — B6 responders, non-responders, and specific treatments Pyridoxine — betaine — methionine-restricted diet — cysteine — folates — vitamin B12 — anticoagulation — antiplatelets — therapeutic goals — monitoring |
Treatment of patients responsive to pyridoxine (≈50% of patients with CBS): high-dose pyridoxine (vitamin B6): 100–500 mg/day in adults (dose adjusted according to weight in children: 10–15 mg/kg/day) → mechanism: pyridoxine → pyridoxal-5-phosphate (PLP — active cofactor of CBS) → increase in residual activity of the mutant CBS + stabilization of the mutant protein → in responders: normalization or major reduction in homocysteine within 2–4 weeks → therapeutic targets (SSIEM guidelines — Yap 2010): fasting total homocysteine <50 µmol/L + ideally <15 µmol/L → normal methionine levels → long-term outcomes in responders treated early: near-normalization of the natural history → Mudd 1985: treated responders → no lens ectopia + near absence of thrombosis + normal intellectual development → associated supplementation: folates (5 mg/day) to optimize remethylation + vitamin B12 if deficient + high-dose pyridoxine is generally well tolerated → but: peripheral neuropathy if doses >200 mg/day for years → monitor sensory function + EMG if high doses are used long-term → maximum dose: 500 mg/day in adults (reduce gradually if neuropathy occurs); treatment of pyridoxine non-responders — multimodal approach: betaine (trimethylglycine — Cystadane): orphan drug → 6–9 g/day in adults in 2–3 doses → mechanism: betaine = methyl donor → activates betaine-homocysteine methyltransferase (BHMT) in the liver → alternative remethylation of homocysteine to methionine → reduces homocysteine by 50–80% → but: methionine increases (because it is remethylated) → if methionine is already very high → risk of further increase → target methionine level: <1,000 µmol/L → if methionine >1,000 µmol/L on betaine → reduce betaine → risk of neurological complications (cerebral edema + myelin damage) if methionine too high → Wilcken 1985 — New England Journal of Medicine: betaine + CBS non-responders → reduction in homocysteine + reduction in thrombotic events → Wilcken 1997 — Journal of Pediatrics: series of non-responding CBS patients on betaine × 10 years → 90% reduction in thrombotic events → methionine-restricted diet: reduction in dietary methionine intake (meat + eggs + dairy products + legumes) → maintaining adequate protein intake with low-methionine protein substitutes (methionine-free amino acid mixtures) → very difficult to maintain long-term + restrictive → most effective in young children (better compliance) → cysteine supplementation: cysteine becomes an essential amino acid (because the transsulfuration pathway is blocked) → L-cysteine supplements + protein preparations enriched with cysteine + folates (5–10 mg/day) + vitamin B12 (if deficient) → support for the remethylation pathway; concomitant treatment — thrombosis prevention: antiplatelet agents: aspirin 75–150 mg/day or dipyridamole → reduces the risk of arterial thrombosis → Mudd 1985: aspirin → reduction in arterial thrombotic events + prophylactic anticoagulation: to be discussed based on the context → high-risk situations: surgery + general anesthesia + prolonged immobilization + pregnancy → preventive heparin therapy in these contexts → as soon as possible after documented thrombosis → biochemical treatment goals: total fasting homocysteine: <50 µmol/L (minimum target) + <15 µmol/L (optimal target — maximum risk reduction) → methionine: within normal limits if B6 responder + <1,000 µmol/L if betaine → cysteine: within normal limits → laboratory monitoring: total homocysteine + methionine + cysteine + amino acid chromatography: every 3–6 months → CBC (macrocytosis if folate or B12 deficiency) → bone densitometry (DXA) annually (frequent osteoporosis) → annual ophthalmology (lens ectopia + glaucoma + retinal detachment) → neuropsychological evaluation → spinal radiology → cardiovascular evaluation | Key treatment data: Mudd 1985 — AJHG (Natural History Study): B6 responders treated early → virtually no complications vs. untreated patients → major impact of early treatment on prognosis → Wilcken 1985 — NEJM: betaine + non-responders to CBS → reduction in homocysteine + reduction in thrombosis → seminal publication on betaine + Wilcken 1997 — Journal of Pediatrics: long-term betaine series × 10 years → −90 % thrombotic events → confirmation of long-term efficacy + Yap 2010 — Journal of Inherited Metabolic Disease: revised SSIEM guidelines → comprehensive management protocol → biochemical targets + monitoring → current international reference + Brenton 2000 — Neurology: early treatment → impact on neurological outcome + Smith 2006 — Journal of Inherited Metabolic Disease: long-term outcomes based on B6 responder status + timing of treatment initiation + Levy 2009 — JIMD: European SSIEM recommendations + Blom 2011 — Journal of Inherited Metabolic Disease: review of outcomes under treatment → responders treated upon screening → near-normal development + Picker 2014 — Journal of Inherited Metabolic Disease: intensive treatment of CBS → reduction in complications if initiated early |
| Complications, Special Situations, and Genetic Counseling Pregnancy — surgery — anesthesia — acute thrombosis — osteoporosis — prenatal screening — genetic counseling — undiagnosed adult |
Pregnancy and CBS homocystinuria: significant thrombotic risk during pregnancy (naturally prothrombotic state) and the postpartum period → very high-risk pregnancy → multidisciplinary management: metabolism + obstetrics + hematology → maximum optimization of treatment prior to conception: homocysteine <15 µmol/L → systematic preventive anticoagulation (LMWH) from the 1st trimester through 6 weeks postpartum → betaine may be continued → pyridoxine continued if responsive → adapted diet → monthly laboratory monitoring → fetal risk: pyridoxine is generally safe during pregnancy → betaine: limited data but used in many pregnancies with favorable outcomes → methionine restriction is difficult to maintain during pregnancy (adequate protein intake required) → specialized nutritional counseling mandatory + Yap 2014 — Journal of Inherited Metabolic Disease: series of pregnancies in CBS patients → favorable outcomes with optimal treatment → complications if homocysteine is uncontrolled + surgery and anesthesia in the presence of homocystinuria: maximal perioperative thrombotic risk → specific surgical protocol: continue treatment until the morning of the procedure → systematic IV hydration before, during, and after the procedure → avoid dehydration → prophylactic heparin therapy if major surgery → reduced duration ofimmobilization → early rehydration + resume treatment as soon as possible postoperatively → correction of homocysteine levels before elective surgery if possible + prior consultation with a metabolism specialist; acute thrombosis in homocystinuria: standard therapeutic anticoagulation (LMWH → VKAs or DOACs depending on the context) → immediate optimization of metabolic therapy (increased pyridoxine + betaine) → aggressive rehydration → recurrence is very common if metabolic therapy is not optimized → additional thrombophilia workup (FV Leiden + prothrombin mutation + proteins C and S) to identify cumulative risk factors → patients with CBS + additional thrombotic risk factor: very high cumulative risk → long-term anticoagulation to be discussed; Osteoporosis and osteopenia in homocystinuria: very common → pathophysiology: inhibition of lysyl oxidase by homocysteine → impaired cross-linking of bone collagen → weakening of the bone matrix + cysteine deficiency → reduced glutathione synthesis → bone oxidative stress → treatment: calcium 1,000–1,200 mg/day + vitamin D 800–1,000 IU/day + bisphosphonates if T-score <−2.5 → extensive osteoporosis + fractures → alendronate or risedronate → early treatment → reduced fracture risk → annual DXA scan; young adult with stroke or thrombosis + marfanoid phenotype + severe myopia + diagnosis not made at birth: consider CBS homocystinuria even in adults → measure total homocysteine on an empty stomach + methioninemia + amino acid chromatography + methylmalonic acid → if homocysteine >100 µmol/L + elevated methionine → diagnosis virtually certain → urgent consultation with a metabolic disease specialist + genetics center → initiate treatment (B6 test + ± betaine + ± diet) → genetic counseling for relatives + several adults diagnosed late following an early stroke or thrombosis; genetic counseling: autosomal recessive inheritance → 25% recurrence risk for each pregnancy of a couple of heterozygous carriers → carrier status screening of relatives (homocysteine measurement after methionine loading + genotyping) → prenatal diagnosis possible: homocysteine measurement + CBS activity on chorionic villi at 10–12 weeks of gestation or amniotic cells at 15–18 weeks of gestation → genotyping if known familial mutations → PGD (preimplantation genetic diagnosis): possible in certain centers if familial mutation(s) have been identified → psychological support for families + care at a reference center for inherited metabolic disorders (Quebec: Medical Genetics Clinic at the CHU de Québec or the CHU Sainte-Justine) | Data on specific conditions and prognosis: Mudd 1985 — AJHG: Natural History Study → 629 patients → thrombosis in 50 % of untreated patients before age 30 + 30 % mortality in the third decade → demonstration of the severity and life-threatening impact of the condition → Wilcken 1997 — Journal of Pediatrics: long-term betaine → −90 % thromboses → vital impact demonstrated + Yap 2014 — JIMD: pregnancy + CBS → favorable outcomes if homocysteine is controlled → recommendations for periconceptional management + Blom 2011 — JIMD: neonatal screening + early treatment → near-normal development in B6 responders → screening = most effective public health measure + Picker 2014 — JIMD: early vs. late treatment → major differences in outcomes + Smith 2006 — JIMD: long-term CBS outcomes → early-treated responders = near-normal + treated non-responders = better outcome than untreated but persistent complications → Levy 2009 — JIMD + Yap 2010 — JIMD: SSIEM guidelines → current standards of care → biochemical targets + monitoring protocols + Fowler 1997 — JIMD: CBS mutations + genotype-phenotype correlations → allows prediction of B6 response prior to testing → Brenton 2000 — Neurology: neurological impact of early vs. late treatment → early = normal development + late = sequelae |
ℹ️
Classic homocystinuria is one of the only hereditary metabolic diseases where treatment can completely normalize the natural history: In patients who respond to pyridoxine, diagnosed by newborn screening and treated in the first few weeks of life, development is practically normal—without lentis ectopia, thrombosis, or intellectual deficiency. Therefore, newborn screening is a major public health measure for this disease. In undiagnosed adults, stroke or arterial thrombosis before age 40 with a Marfanoid habitus and high myopia should always warrant a fasting homocysteine and methionine level test.
Situations requiring urgent or emergency consultation
Ischemic stroke or arterial thrombosis (MI + mesenteric thrombosis + cerebral embolism) in a young adult (<40 years old) with tall stature + arachnodactyly + severe myopia + lens dislocation → suspect an undiagnosed CBS homocystinuria → urgently measure fasting total homocysteine + methionine → if homocysteine >100 µmol/L + elevated methionine → urgently contact a metabolic disease specialist → initiate therapeutic anticoagulation + hydrate vigorously + do not wait for genetic results to treat.
Patient known to have CBS homocystinuria scheduled for surgery or general anesthesia. → Maximal perioperative thrombotic risk → Urgent preoperative consultation with the metabolic specialist → Treatment optimization + systematic IV hydration + preventive heparin therapy + resume metabolic treatment as soon as possible postoperatively → Never operate on a CBS patient without a pre-established protocol with the metabolic team.
Infant with abnormal newborn screening result (elevated methionine on Guthrie card) pending confirmation → urgent contact with the newborn screening program (PQDN in Quebec) + genetics center → homocysteine + methionine levels on plasma → if confirmed → immediately start diet + treatment + avoid any delay in management → every week counts for neurological development.
Pregnant woman with known CBS homocystinuria developing thrombotic signs (limb pain + swelling + dyspnea + chest pain) → Suspicion of MTEV on homocystinuria in pregnancy → obstetric emergencies → venous echo-Doppler + angio-CT if embolism suspected → immediate therapeutic LMWH → contact with the metabolic specialist and obstetrician → urgent optimization of metabolic treatment.
Consult at Clinique Omicron
At Clinique Omicron, physicians evaluate adults with suspected homocystinuria (early stroke + Marfanoid habitus + ectopia lentis + unexplained thrombosis), prescribe tests for total homocysteine and methionine levels, and immediately refer them to specialists in medical genetics and inherited metabolic diseases for diagnostic confirmation and treatment. Consultations are available at several service points in Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
The content of this page is provided for informational purposes only and does not replace the advice of a doctor specializing in inherited metabolic diseases or medical genetics. Homocystinuria is a rare disease requiring management in a specialized center.
Omicron Clinic
Need to consult a doctor?
Treatment within 24-48 hours. In-clinic or telemedicine, anywhere in Quebec.
Insurance receipts. 7j/7. No family doctor required.