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Holocarboxylase synthetase deficiency (HLCSD)

Holocarboxylase synthetase deficiency (HLCSD)



Overview

Holocarboxylase synthetase deficiency (HLCSD) is a rare, inherited metabolic disorder in which the body cannot use vitamin B7 (biotin) effectively, leading to a condition known as multiple carboxylase deficiency.

Classified as a multiple carboxylase deficiency, the condition impairs the activity of biotin-dependent enzymes (carboxylases) needed to process proteins, fats, and carbohydrates.

Symptoms

Symptoms typically begin in infancy—often within the first few weeks or months of life—though onset can occasionally vary.

• Feeding & Digestive Issues: Difficulty feeding, poor appetite, vomiting (emesis), and food refusal.

• Skin & Hair Changes: Distinctive skin rashes (dermatitis/peeling rash) and hair loss (alopecia).

• Neurological & Muscular Signs: Lethargy or extreme tiredness, low muscle tone (hypotonia), irritability, developmental delay, and seizures (convulsions).

• Respiratory Problems: Rapid breathing (tachypnea), hyperventilation, or general respiratory distress.

• Metabolic Crisis: If left untreated, the condition can escalate to severe metabolic acidosis, ketolactic acidosis, hyperammonemia, cerebral edema, coma, and life-threatening complications.

Causes

• Gene Mutations: The HLCS gene provides instructions to make the holocarboxylase synthetase enzyme. Pathogenic variants or mutations in this gene impair the structure and function of the enzyme.

• Impaired Biotin Attachment: Holocarboxylase synthetase normally attaches biotin (vitamin B7) to vital carboxylase enzymes (propionyl-CoA carboxylase, pyruvate carboxylase, 3-methylcrotonyl-CoA carboxylase, and acetyl-CoA carboxylase). When the enzyme is defective, it cannot activate these carboxylases.

• Disrupted Metabolism: Without functioning carboxylases, the body cannot properly process proteins, fats, and carbohydrates, leading to a harmful buildup of organic acids and toxic metabolites (such as 3-hydroxyisovaleric acid and methylcitric acid).

Diagnosis

• Newborn Screening (NBS): Identified via tandem mass spectrometry on dried blood spots, typically flagged by elevated C5-OH (3-hydroxyisovaleroylcarnitine) levels.

• Biochemical & Metabolite Testing:

• Urine Organic Acid Analysis: Shows characteristic elevations of 3-hydroxyisovaleric, 3-hydroxypropionic, lactic, methylcitric, and 3-methylcrotonylglycine acids.

• Blood Acylcarnitine Profile: Confirms elevated levels of specific acylcarnitines.

• Differentiation (Ruling Out Biotinidase Deficiency): Because symptoms and chemical markers overlap with biotinidase deficiency, clinicians measure biotinidase enzyme activity in the blood to distinguish between the two conditions.

• Enzyme Assay (Optional/Specialized): Demonstrates deficient activity of biotin-dependent carboxylases in cultured skin fibroblasts or peripheral blood leukocytes that normalizes with biotin addition.

• Genetic Testing: Confirmed by molecular analysis showing biallelic pathogenic variants in the HLCS gene 

Treatment

• Biotin Supplementation: Lifelong daily oral biotin is utilized, with the specific amount tailored by a metabolic specialist depending on the genetic mutation and individual metabolic response. Depending on the specific mutation, requirements can vary significantly among individuals.

• Supportive & Acute Management: During periods of illness, vomiting, or metabolic decompensation, emergency care is required to prevent life-threatening metabolic acidosis or hyperammonemia. This includes avoiding prolonged fasting, managing skin or hair complications, and providing physical or developmental therapy if delays occur.

• Regular Monitoring: Patients require ongoing follow-up with a metabolic specialist or pediatrician to track biochemical markers, growth, and neurological development, adjusting the management plan as needed.

Type of Doctor Department : Metabolic Specialist / Geneticist


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