Multiple Carboxylase Deficiency in Children

Multiple Carboxylase Deficiency (MCD) is a rare autosomal recessive metabolic disorder characterized by deficient activity of multiple biotin-dependent carboxylases. It encompasses two distinct but related conditions:

Holocarboxylase synthetase (HCS) deficiency: Also known as early-onset MCD or neonatal MCD
Biotinidase deficiency: Also known as late-onset MCD or juvenile MCD

Incidence rates:

HCS deficiency: Extremely rare, with fewer than 1 in 200,000 live births
Biotinidase deficiency: Approximately 1 in 60,000 live births

Both forms result in impaired metabolism of proteins, fats, and carbohydrates, leading to various clinical manifestations. The severity of the condition can range from mild to life-threatening, depending on the extent of enzyme deficiency and the time of diagnosis and treatment initiation.

Pathophysiology

MCD affects the function of four biotin-dependent carboxylases, which are critical enzymes in various metabolic pathways:

Pyruvate carboxylase (PC):
- Function: Catalyzes the carboxylation of pyruvate to oxaloacetate
- Role: Critical in gluconeogenesis and anaplerosis of the citric acid cycle
- Deficiency effects: Impaired glucose homeostasis, lactic acidosis
Propionyl-CoA carboxylase (PCC):
- Function: Catalyzes the carboxylation of propionyl-CoA to D-methylmalonyl-CoA
- Role: Essential in the catabolism of odd-chain fatty acids and certain amino acids
- Deficiency effects: Accumulation of propionic acid, methylcitric acid, and other organic acids
3-methylcrotonyl-CoA carboxylase (MCC):
- Function: Catalyzes the carboxylation of 3-methylcrotonyl-CoA to 3-methylglutaconyl-CoA
- Role: Involved in leucine catabolism
- Deficiency effects: Accumulation of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine
Acetyl-CoA carboxylase (ACC):
- Function: Catalyzes the carboxylation of acetyl-CoA to malonyl-CoA
- Role: Rate-limiting step in fatty acid synthesis
- Deficiency effects: Impaired fatty acid synthesis, which can affect myelin formation

Pathophysiological mechanisms:

HCS deficiency:
- Caused by mutations in the HLCS gene
- Results in impaired attachment of biotin to apocarboxylases
- Leads to reduced activity of all four biotin-dependent carboxylases
Biotinidase deficiency:
- Caused by mutations in the BTD gene
- Results in failure to recycle biotin from degraded carboxylases and dietary protein-bound biotin
- Leads to biotin deficiency and subsequent reduction in carboxylase activities

Both conditions result in:

Accumulation of organic acids (e.g., lactic acid, propionic acid, 3-hydroxyisovaleric acid)
Impaired energy metabolism
Dysfunction in gluconeogenesis, amino acid metabolism, and fatty acid synthesis
Secondary carnitine deficiency due to increased urinary excretion of acylcarnitines

Clinical Presentation

The clinical presentation of MCD can vary widely, depending on the specific enzyme deficiency and its severity. Symptoms typically manifest in two distinct patterns:

Holocarboxylase Synthetase (HCS) Deficiency (Early-onset MCD)

Onset: Typically within hours to weeks after birth

Neurological symptoms:
- Seizures (often refractory)
- Hypotonia
- Lethargy progressing to coma
- Developmental delay
Respiratory issues:
- Tachypnea
- Respiratory distress
- Apnea
Dermatological manifestations:
- Erythematous, scaly rash (often perioral, periorbital, or genital)
- Alopecia
Gastrointestinal symptoms:
- Vomiting
- Feeding difficulties
Metabolic derangements:
- Severe metabolic acidosis
- Hyperammonemia
- Organic aciduria
- Ketosis
- Lactic acidosis

Biotinidase Deficiency (Late-onset MCD)

Onset: Usually between 2 months and 2 years of age, but can occur later in childhood or even adulthood

Neurological symptoms:
- Seizures
- Hypotonia
- Ataxia
- Developmental delay or regression
- Sensorineural hearing loss
- Optic atrophy
Dermatological manifestations:
- Alopecia
- Perioral and perianal dermatitis
- Seborrheic dermatitis
- Conjunctivitis
Respiratory issues:
- Chronic respiratory problems
- Hyperventilation
Immunological issues:
- Recurrent fungal and bacterial infections
- Chronic candidiasis
Metabolic derangements:
- Metabolic acidosis (usually less severe than in HCS deficiency)
- Organic aciduria

Note: The clinical presentation can be highly variable, and some patients may present with only a subset of these symptoms. Additionally, partial biotinidase deficiency may result in a milder clinical course or may only become symptomatic during periods of metabolic stress (e.g., infections, prolonged fasting).

Diagnosis

The diagnosis of Multiple Carboxylase Deficiency requires a combination of clinical suspicion, biochemical testing, and genetic analysis. The diagnostic approach may differ slightly between HCS deficiency and biotinidase deficiency.

1. Newborn Screening

Biotinidase deficiency is included in newborn screening programs in many countries
Screening typically involves measuring biotinidase enzyme activity in dried blood spots
HCS deficiency is not routinely screened for in most newborn screening programs

2. Biochemical Testing

Urine Organic Acid Analysis:

Elevated levels of:
- 3-hydroxyisovaleric acid
- 3-methylcrotonylglycine
- 3-hydroxypropionic acid
- Methylcitric acid
- Lactic acid

Plasma Acylcarnitine Profile:

Elevated levels of:
- C5-OH (3-hydroxyisovalerylcarnitine)
- C3 (propionylcarnitine)

Blood Tests:

Elevated lactate and ammonia levels
Metabolic acidosis (decreased pH and bicarbonate)
Hypoglycemia (in some cases)

3. Enzyme Assays

Biotinidase deficiency:
- Measurement of biotinidase activity in serum
- Profound deficiency: <10% of mean normal activity
- Partial deficiency: 10-30% of mean normal activity
HCS deficiency:
- Measurement of holocarboxylase synthetase activity in cultured fibroblasts
- Carboxylase activities can also be measured in lymphocytes or cultured fibroblasts

4. Genetic Testing

Biotinidase deficiency:
- Sequencing of the BTD gene
- Over 150 pathogenic variants identified
HCS deficiency:
- Sequencing of the HLCS gene
- Over 80 pathogenic variants identified

5. Additional Diagnostic Considerations

Brain MRI: May show cerebral atrophy, delayed myelination, or other nonspecific changes
Electroencephalogram (EEG): May show abnormal patterns in patients with seizures
Audiometry: To assess for hearing loss, particularly in biotinidase deficiency
Ophthalmological examination: To evaluate for optic atrophy

Diagnostic Challenges:

The clinical and biochemical features of MCD can mimic other organic acidemias and mitochondrial disorders
Partial biotinidase deficiency may present with milder or intermittent symptoms
Some patients may be asymptomatic at the time of diagnosis (especially if detected through newborn screening)

Treatment

The cornerstone of treatment for Multiple Carboxylase Deficiency is biotin supplementation. However, management often requires a multidisciplinary approach to address various aspects of the condition.

1. Biotin Therapy

Dosage:
- Biotinidase deficiency: 5-20 mg/day
- HCS deficiency: Often requires higher doses, ranging from 10-40 mg/day, sometimes up to 200 mg/day
Administration: Oral or nasogastric if the patient cannot take oral medications
Monitoring: Regular assessment of symptoms and biochemical markers to adjust dosage
Duration: Lifelong treatment is necessary
Response:
- Biotinidase deficiency: Often shows rapid improvement
- HCS deficiency: Response may be slower and more variable

2. Acute Management

Metabolic decompensation:
- Intravenous fluids with glucose to prevent catabolism
- Correction of metabolic acidosis with sodium bicarbonate
- Management of hyperammonemia (if present) with ammonia scavengers
Seizure control: Anticonvulsants as needed, with caution as some may interfere with biotin metabolism

3. Nutritional Management

Dietary modifications:
- Protein restriction may be necessary during acute decompensation
- Avoid prolonged fasting
Supplementation:
- L-carnitine: 50-100 mg/kg/day to address secondary carnitine deficiency
- Essential fatty acids: If deficiency is detected

4. Supportive Care

Dermatological management: Topical treatments for skin manifestations
Respiratory support: As needed for respiratory complications
Developmental support: Early intervention with physical, occupational, and speech therapy
Hearing and vision: Regular audiological and ophthalmological evaluations

5. Long-term Management

Regular monitoring:
- Clinical evaluations: Every 3-6 months or as needed
- Biochemical monitoring: Urine organic acids, plasma amino acids, and acylcarnitine profile
- Nutritional status: Including micronutrient levels
Emergency protocol: Provide families with a written plan for managing intercurrent illnesses and potential metabolic decompensation

6. Genetic Counseling

Offer genetic counseling to affected families
Discuss recurrence risk (25% for each pregnancy) and available prenatal testing options

Note: Treatment response can vary significantly between individuals and between the two forms of MCD. Close monitoring and individualized adjustments to the treatment plan are essential for optimal outcomes.

Prognosis

The prognosis for Multiple Carboxylase Deficiency varies depending on several factors:

Factors Influencing Prognosis

Type of deficiency (HCS vs. Biotinidase)
Age at diagnosis and initiation of treatment
Severity of enzyme deficiency
Compliance with biotin therapy
Frequency and severity of metabolic decompensations

Biotinidase Deficiency

Early diagnosis and treatment:
- Excellent prognosis with normal development
- Prevention of most clinical manifestations
Delayed diagnosis:
- Risk of irreversible neurological damage
- Potential for residual hearing and vision problems
Partial deficiency: Generally good prognosis, even with delayed diagnosis

Holocarboxylase Synthetase Deficiency

Generally more severe than biotinidase deficiency
Prognosis can be good with early diagnosis and appropriate biotin therapy
Higher risk of neurological sequelae, even with treatment
Some patients may require very high biotin doses for optimal control

Long-term Outcomes

Neurodevelopmental: Range from normal development to severe intellectual disability
Seizures: Often well-controlled with biotin therapy
Hearing loss: May be irreversible if present before treatment initiation
Vision problems: Optic atrophy may persist despite treatment
Skin manifestations: Generally resolve with biotin therapy

Importance of lifelong treatment: Discontinuation of biotin therapy can lead to rapid deterioration, even in previously asymptomatic individuals.

Genetic Aspects

Inheritance Pattern

Both forms of Multiple Carboxylase Deficiency are inherited in an autosomal recessive manner.

Biotinidase Deficiency

Gene: BTD (biotinidase gene)
Location: Chromosome 3p25
Mutations:
- Over 150 pathogenic variants identified
- Most common mutation: c.98_104del7ins3
- Genotype often correlates with enzyme activity levels

Holocarboxylase Synthetase Deficiency

Gene: HLCS (holocarboxylase synthetase gene)
Location: Chromosome 21q22.13
Mutations:
- Over 80 pathogenic variants identified
- Mutations in the biotin-binding region often result in more severe phenotypes

Genotype-Phenotype Correlations

Biotinidase deficiency:
- Profound deficiency: <10% enzyme activity
- Partial deficiency: 10-30% enzyme activity
- Some correlation between genotype and biochemical phenotype, but clinical phenotype can vary
HCS deficiency:
- Limited genotype-phenotype correlations established
- Mutations affecting biotin binding tend to cause more severe phenotypes

Genetic Testing and Counseling

Diagnostic testing: Sequencing of BTD or HLCS gene
Carrier testing: Offered to at-risk relatives
Prenatal testing: Available for pregnancies at increased risk
Preimplantation genetic testing: Option for families with known mutations

Genetic counseling considerations:

25% risk of affected offspring in each pregnancy for carrier parents
Importance of cascade screening in families with identified cases
Discussion of reproductive options and prenatal/preimplantation genetic testing

Differential Diagnosis

The clinical presentation of Multiple Carboxylase Deficiency can overlap with several other metabolic and neurological disorders. Key differential diagnoses include:

1. Other Organic Acidemias

Propionic acidemia
Methylmalonic acidemia
Isovaleric acidemia
3-Methylcrotonyl-CoA carboxylase deficiency

2. Mitochondrial Disorders

Leigh syndrome
Pyruvate dehydrogenase deficiency
Respiratory chain defects

3. Urea Cycle Disorders

Particularly in cases presenting with hyperammonemia

4. Neurocutaneous Syndromes

Acrodermatitis enteropathica (zinc deficiency)
Essential fatty acid deficiency

5. Neurological Disorders

Epileptic encephalopathies
Neurodegenerative disorders

6. Other Vitamin-Responsive Disorders

Thiamine-responsive disorders
Pyridoxine-dependent epilepsy

Key Distinguishing Features

Biochemical profile: Characteristic pattern of organic acids and acylcarnitines
Rapid response to biotin: Particularly in biotinidase deficiency
Combination of symptoms: Neurological, dermatological, and metabolic features

Note: Definitive diagnosis often requires specific enzyme assays and/or genetic testing.

Multiple Carboxylase Deficiency in Children