Biotinidase Deficiency (Late onset Multiple Carboxylase Deficiency)

1. DEFINITION AND ETIOLOGY

• Definition: An autosomal recessive disorder of biotin recycling caused by the deficiency of the enzyme Biotinidase.
• Synonym: Late-onset Multiple Carboxylase Deficiency (MCD).
  • Note: Early-onset MCD is caused by Holocarboxylase Synthetase deficiency.
• Epidemiology: Estimated incidence 1:60,000.
• Genetics:
  • Gene: BTD gene located on chromosome 3p25.
  • Inheritance: Autosomal Recessive.
  • Classification:
    • Profound Deficiency: <10% enzyme activity.
    • Partial Deficiency: 10–30% enzyme activity (often asymptomatic unless stressed).

2. PATHOPHYSIOLOGY

• Normal Physiology: Biotin is a coenzyme required for 4 carboxylase enzymes (Pyruvate, Propionyl-CoA, Acetyl-CoA, and 3-Methylcrotonyl-CoA carboxylases). Biotinidase recycles endogenous biotin from Biocytin (biotin-lysine) and releases dietary protein-bound biotin.
• Defect: Deficiency of Biotinidase prevents biotin recycling.
• Consequence: Secondary depletion of free biotin leads to dysfunction of all four biotin-dependent carboxylases (Multiple Carboxylase Deficiency).
• Metabolic Effects: Accumulation of lactate, pyruvate, 3-hydroxyisovaleric acid, and organic acids.

3. CLINICAL FEATURES

• Onset: Typically between 3 to 6 months of age (can range from 1 week to 10 years).
• Neurologic (Dominant Feature):
  • Seizures: Myoclonic seizures, often refractory to anticonvulsants.
  • Hypotonia: Generalized "floppy infant".
  • Ataxia and developmental delay.
  • Sensory Deficits:
    • Sensorineural Hearing Loss: (75% of untreated cases).
    • Optic Atrophy: Visual impairment.
• Dermatologic (Cutaneous):
  • Alopecia: Diffuse hair thinning to total alopecia (involves scalp, eyebrows, lashes).
  • Skin Rash: Eczematous or scaly, erythematous rash, often periorificial (around eyes, nose, mouth) and perineal. similar to Acrodermatitis Enteropathica.
• Immunologic:
  • Recurrent infections (fungal/Candida) due to T-cell and B-cell dysfunction.
• Respiratory:
  • Stridor, apnea, or hyperventilation (due to metabolic acidosis).

4. INVESTIGATIONS

• Biochemical Screening:
  • Metabolic Acidosis: High anion gap.
  • Hyperammonemia: Mild to moderate.
  • Lactate: Elevated (Lactic acidosis).
  • Urine Organic Acids: Elevated 3-Hydroxyisovaleric acid, Lactate, 3-Methylcrotonylglycine, Methylcitrate.
• Confirmatory Test (Gold Standard):
  • Enzyme Assay: Absent or reduced Serum Biotinidase Activity.
  • Note: Unlike other metabolic enzymes often requiring tissue, biotinidase is abundant in serum.
• Newborn Screening (NBS):
  • Many programs screen for this using colorimetric or fluorometric enzyme assays on dried blood spots.

5. DIAGNOSIS DIFFERENTIAL

• Holocarboxylase Synthetase Deficiency: Presents earlier (neonatal), severe acidosis, normal biotinidase activity.
• Zinc Deficiency (Acrodermatitis Enteropathica): Similar rash and alopecia.
• Essential Fatty Acid Deficiency.

6. MANAGEMENT

"One of the most treatable metabolic disorders."

• Pharmacotherapy:
  • Free Biotin (Vitamin B7): Oral supplementation.
  • Dose: 5–20 mg/day (pharmacologic dose, much higher than nutritional requirement).
  • Duration: Lifelong therapy is required.
• Monitoring:
  • Annual vision and hearing assessment.
  • Regular developmental checks.

7. PROGNOSIS

• Excellent: If treated early (asymptomatic patients detected by NBS remain asymptomatic).
• Symptomatic Patients:
  • Reversible: Cutaneous symptoms (rash, alopecia), seizures, ataxia, and metabolic abnormalities resolve rapidly with biotin.
  • Irreversible: Sensorineural hearing loss and Optic atrophy usually do not reverse once established.
• Mortality: Can be fatal if untreated due to severe acidosis and infection.