Biotinidase deficiency is an autosomal recessive disorder of
biotin metabolism caused by defects in the
biotinidase gene. Symptoms of
biotinidase deficiency are resolved or prevented with oral
biotin supplementation and as such newborn screening is performed to prospectively identify affected individuals prior to the onset of symptoms.
Biotinidase deficiency is detected by determining the activity of the
biotinidase enzyme utilizing the newborn dried blood spot and colorimetric end point analysis. While newborn screening by
enzyme analysis is effective, external factors may compromise results of the
enzyme analysis and difficulty is encountered in distinguishing between complete and partial
enzyme deficiencies. In the United States, the four mutations most commonly associated with complete
biotinidase deficiency are c98:d7i3, Q456H, R538C, and the double mutation D444H:A171T. Partial
biotinidase deficiency is almost universally attributed to the D444H mutation. To more effectively distinguish between profound and partial
biotinidase deficiency, a panel of assays utilizing real time PCR and melting curve analysis using Light Cycler technology was developed. Employing
DNA extracted from the original dried blood specimens from newborns identified through prospective newborn screening as presumptive positive for
biotinidase deficiency, the specimens were analyzed for the presence of the five common mutations. Using this approach it was possible to separate newborns with partial and complete deficiency from each other as well as from many of those with false positive results. In most cases it was also possible to correlate the genotype with the degree of residual
enzyme activity present. In newborn screening for
biotinidase deficiency, we have shown that the analysis of common mutations is useful in distinguishing between partial and complete
enzyme deficiency as well as improving specificity. Combining
biotinidase enzyme analysis with genotypic data also increases the sensitivity of screening for
biotinidase deficiency and provides information useful to clinicians earlier than would otherwise be possible.