Analysis of mutations causing biotinidase deficiency.

Biotinidase deficiency is an inherited disorder in which the vitamin, biotin, is not recycled. Individuals with biotinidase deficiency can develop neurological and cutaneous symptoms if they are not treated with biotin. Biotinidase deficiency screening has been incorporated into essentially all newborn screening programs in the United States and in many countries. We now report 140 known mutations in the biotinidase gene (BTD) that cause biotinidase deficiency. All types of mutations have been found to cause biotinidase deficiency. Variants have been identified throughout the coding sequence. Essentially all the variants result in enzymatic activities with less than 10% of mean normal enzyme activity (profound biotinidase deficiency) with the exception of the c.1330G>C (p.D444H) mutation, which results in an enzyme having 50% of mean normal serum activity. The putative three-dimensional structure of biotinidase has been predicted by homology to that of nitrilases/amidases. The effect of the various missense mutations can be predicted to affect various important sites within the structure of the enzyme. This compilation of variants causing biotinidase deficiency will be useful to clinical laboratories that are performing mutation analysis for confirmational testing when the enzymatic results are equivocal for children identified through newborn screening.