Biotinidase deficiency is a late-onset form of multiple carboxylase deficiency, an inborn error of metabolism that, if untreated, is characterized by seizures, difficulty breathing, hypotonia, skin rashes, hair loss, hearing loss and developmental delay.
Biotinidase deficiency is caused by mutations in the BTD (3p25) gene resulting in a reduced or absent activity BTD protein. This enzyme recycles free biotin, needed for multiple dependent metabolic processes of biotin. BTD deficiency is inherited as an autosomal recessive trait and there are more than 150 mutations described in the BTD gene.
The prevalence of biotinidase deficiency is estimated at 1/61,000. The frequency of carriers in the general population is approximately 1/120. Symptoms usually appear in the first months of life, but has also been described a later appearance.
Genetic counseling for parents of affected children is recommended. Parents are necessarily heterozygotes asymptomatic carriers. Siblings of patients with BTD deficits should be tested for this deficit even if they do not show symptoms.
Individuals with untreated profound deficiency (less than 10 % of mean normal serum biotinidase activity) have variable clinical findings including seizures, hypotonia, eczematoid rash, alopecia, ataxia, hearing loss, fungal infections, and developmental delay. Metabolically, untreated children can exhibit ketolactic acidosis, organic acidemia and mild hyperammonemia. Individuals with untreated partial BTD deficiency (10% to 30% of mean normal BTD activity) may be asymptomatic, but during periods of stress, such as illness, fever or fasting, may develop symptoms similar to those of individuals with profound BTD deficiency.
The disorder is detected through newborn screening when available. Other cases are diagnosed by clinical signs and symptoms and confirmed by demonstration of deficient serum BTD activity. Molecular mutation analysis of the BTD gene is also possible.