A Short Indel is Causing Biotinidase Deficiency in a 4-Month-Old Boy from Jammu and Kashmir

Background: Biotinidase is an enzyme recycling endogenous biotin by hydrolyzing ε- N -biotinyl lysine (biocytin) to lysine and biotin. Biotin acts as a coenzyme in various carboxylation reactions. Biotinidase deficiency is rare, with an incidence of 1 per 112,271 individuals. Objectives: Genetic characterization of a 4-month-old boy born out of a consanguineous marriage presented with cutaneous manifestations, alopecia, developmental delay, hypotonia with absent neck holding, respiratory problems, and biotinidase deficiency. Methods: Biochemical characterization was performed using tandem mass spectrometry and immunofluorescence enzyme assays. Pathogenic genetic variant was identified by Sanger sequencing of the BTD (biotinidase) gene in a patient and his parents. Results: Biochemical characterization indicated elevated methylmalonylcarnitine/hydroxyisovalerylcarnitine and decreased biotinidase, citrulline, glycine, and ornithine levels, suggesting a profound biotinidase deficiency. Sequencing of BTD gene indicates the presence of a 7-bp deletion and a 3-bp insertion (98-104del7ins3) in a homozygous state, resulting in a change of amino acid from cystine to phenylalanine at the 33rd position, causing a frameshift and premature amino acid termination at the 68th position. Conclusions: A 7-bp deletion and a 3-bp insertion (98-104del7ins3) homozygous variation in the BTD gene segregating in a homozygous recessive manner in the family, causing truncated, nonfunctional biotinidase deficiency in the proband. This is a recurrent mutation and has the potential to be used as a biomarker for screening patients with biotinidase deficiency.