Learning Objective: Differentiate between Homocystinuria and Marfan syndrome based on clinical presentation, inheritance, and pathophysiology, and identify the metabolic defect responsible for Homocystinuria.
Overview
Homocystinuria is a metabolic disorder characterized by elevated homocysteine levels due to enzyme deficiency, most commonly cystathionine β-synthase deficiency. It shares several phenotypic features with Marfan syndrome, such as tall stature and skeletal abnormalities, but differs in pathogenesis, inheritance, and neurologic involvement.
Pathophysiology
Homocysteine is normally converted to cystathionine (via cystathionine synthase) or methionine (via methionine synthase).
A deficiency of cystathionine β-synthase or cofactors (e.g., vitamin B₆, B₁₂, folate) leads to accumulation of homocysteine, causing endothelial injury and thrombosis.
Causes of Homocystinuria
| Defective Enzyme or Cofactor | Mechanism | Treatment |
|---|---|---|
| Cystathionine β-synthase deficiency (most common) | Homocysteine → Cystathionine step blocked | ↓ Methionine diet, ↑ Cysteine, B₆ supplement |
| Decreased affinity of cystathionine synthase for pyridoxal phosphate | Poor B₆ utilization | High-dose pyridoxine |
| Methionine synthase or methylcobalamin deficiency | Homocysteine → Methionine step blocked | Supplement B₁₂, folate |
Clinical Features
| Feature | Homocystinuria | Marfan Syndrome |
|---|---|---|
| Inheritance | Autosomal recessive | Autosomal dominant |
| Intellect | ↓ (Intellectual disability is common) | Normal |
| Vascular Complications | Thrombosis (stroke, MI, DVT, PE) | Aortic root dilation → aneurysm/dissection |
| Lens Dislocation | Downward and nasal | Upward and temporal (“Marfan fans out”) |
| Skeletal Findings | Tall stature, arachnodactyly, scoliosis, pectus deformity | Similar |
| Skin & Joints | Hyperlaxity, hyperelasticity | Similar |
| Complexion | Fair (due to decreased melanin synthesis) | Normal |
Key Step 1 Takeaways
- Homocystinuria = Autosomal recessive disorder with vascular thrombosis and intellectual impairment.
- Marfan syndrome = Autosomal dominant connective tissue disorder with aortic dilation and normal intellect.
- Lens direction mnemonic:
- Marfan → fans out (up and temporal)
- Homocystinuria → falls (down and nasal)
- Treatment: Pyridoxine (B₆), folate, B₁₂ supplementation, and restriction of methionine.
