Biotinidase biochemical and molecular analyses: Experience at a large reference laboratory

Background: Biotinidase deficiency is caused by absent activity of the biotinidase, encoded by the biotinidase gene (BTD). Affected individuals cannot recycle the biotin, leading to heterogeneous symptoms that are primarily neurological and cutaneous. Early treatment with biotin supplementation can prevent irreversible neurological damage and is recommended for patients with profound deficiency, defined as enzyme activity <10% mean normal (MN). Molecular testing has been utilized along with biochemical analysis for diagnosis and management. In this study, our objective was to correlate biochemical phenotype/enzyme activity to BTD genotype in patients for whom both enzyme and molecular testing were performed at our lab, and to review how the correlations inform on variant severity.

Methods: We analyzed results of biotinidase enzyme analysis and BTD gene sequencing in 407 patients where samples were submitted to our laboratory from 2008 to 2020.

Results: We identified 84 BTD variants; the most common was c.1330G>C, and 19/84 were novel BTD variants. A total of 36 patients had enzyme activity <10% of MN and the most common variant found in this group was c.528G>T. No variant was reported in one patient in the profound deficiency group. The most common variant found in patients with enzyme activity more than 10% MN was c.1330G>C.

Conclusions: Although enzyme activity alone may be adequate for diagnosing profound biotinidase deficiency, molecular testing is necessary for accurate carrier screening and in cases where the enzyme activity falls in the range where partial deficiency and carrier status cannot be discriminated.