A 6-month-old male infant presents with developmental delay, hypotonia, and recurrent seizures. Metabolic workup reveals elevated plasma methionine (450 µmol/L, normal <30), elevated homocysteine (85 µmol/L, normal <15), and normal methylmalonic acid. Urine organic acid analysis shows elevated homocysteine metabolites. Vitamin B₁₂ levels are normal. The infant's parents are consanguineous. Which of the following enzyme defects is most likely responsible for this clinical presentation?

Explanation

## Diagnosis: Methionine Synthase Deficiency ### Clinical Presentation Analysis The infant presents with a triad of: 1. **Neurological features**: developmental delay, hypotonia, seizures (suggestive of CNS involvement) 2. **Elevated plasma methionine** (450 µmol/L) 3. **Elevated homocysteine** (85 µmol/L) despite **normal vitamin B₁₂ levels** 4. **Normal methylmalonic acid** — this is the critical discriminator ### Biochemical Pathway Context ```mermaid flowchart TD A[Homocysteine]:::outcome --> B{Metabolic fate?}:::decision B -->|Remethylation| C[Methionine synthase<br/>requires B12 + methylfolate]:::action B -->|Transsulfuration| D[Cystathionine β-synthase<br/>requires B6]:::action C --> E[Methionine]:::outcome D --> F[Cysteine]:::outcome E --> G[S-adenosylmethionine<br/>SAM]:::outcome A -->|Accumulates if remethylation blocked| H[Homocystinuria]:::urgent ``` ### Why Methionine Synthase Deficiency? **Key Point:** Methionine synthase catalyzes the **remethylation of homocysteine to methionine** using 5-methyltetrahydrofolate as the methyl donor and cobalamin (B₁₂) as a cofactor. **High-Yield:** The critical clue is the **combination of elevated methionine AND elevated homocysteine WITH normal B₁₂ levels**. - In methionine synthase deficiency, the enzyme itself is defective, not the B₁₂ availability. - Homocysteine cannot be remethylated → accumulates (homocystinuria). - Methionine synthesis is blocked → paradoxically, methionine also accumulates (due to shunting via alternative pathways and impaired feedback regulation). - **Normal methylmalonic acid** rules out B₁₂ malabsorption or methylmalonyl-CoA mutase defects. - **Normal B₁₂ levels** rule out dietary B₁₂ deficiency or cobalamin metabolism disorders (cblC, cblD, cblE, cblG). ### Neurological Sequelae Methionine synthase deficiency causes: - **Impaired myelin synthesis** (reduced SAM-dependent methylation reactions). - **Accumulation of homocysteine** (neurotoxic, pro-thrombotic, endothelial damage). - **Megaloblastic anemia** (impaired DNA synthesis from reduced 5,10-methylenetetrahydrofolate recycling). - **Subacute combined degeneration** (spinal cord and peripheral nerve demyelination). **Clinical Pearl:** Unlike CBS deficiency (which presents with lens dislocation, marfanoid habitus, thrombosis), methionine synthase deficiency presents with **pure neuropsychiatric and hematologic features** — no skeletal or ocular stigmata. ### Differential Diagnosis Table | Feature | Methionine Synthase | CBS Deficiency | MMA Mutase | Phe Hydroxylase | |---------|-------------------|----------------|-----------|------------------| | Elevated methionine | Yes | No (low) | No | No | | Elevated homocysteine | Yes | Yes | No | No | | Methylmalonic acid | Normal | Normal | Elevated | Normal | | B₁₂ level | Normal | Normal | Low/normal | Normal | | Lens dislocation | No | Yes (ectopia lentis) | No | No | | Marfanoid features | No | Yes | No | No | | Seizures/neurodegeneration | Yes | No (thrombosis) | Yes | Yes | | Consanguinity | Yes (AR) | AR or AD | AR | AR | **Tip:** The **normal methylmalonic acid** is the single most important discriminator — it excludes all B₁₂-dependent enzyme defects (methylmalonyl-CoA mutase, methionine synthase with secondary B₁₂ malabsorption). ### Management 1. **Vitamin B₁₂ supplementation** (parenteral, high-dose) — may provide partial benefit if residual enzyme activity exists. 2. **Folinic acid** (5-formyl-THF) — bypasses the methionine synthase block and supports nucleotide synthesis. 3. **Betaine** — alternative methyl donor for homocysteine remethylation (via betaine-homocysteine methyltransferase). 4. **Dietary methionine restriction** — reduces accumulation. 5. **Seizure prophylaxis** and supportive neurodevelopmental care. **High-Yield:** Early diagnosis and aggressive treatment with folinic acid + betaine can halt or slow neurological progression if initiated before irreversible CNS damage occurs.