Recovery of enzyme activity in biotinidase deficient individuals during early childhood

Abstract

Deficiency of the biotinidase (BTD) enzyme is an inborn error of biotin metabolism caused by biallelic pathogenic variants in the BTD gene. There are two forms, partial and profound BTD deficiency, which both can be successfully treated with pharmacological doses of biotin, justifying the inclusion of this disorder in the newborn screening in numerous countries. We investigated the BTD deficiency cohort (N = 87) in our metabolic center, as it was detected upon newborn screening since 2005, and aimed to better understand the long-term course of BTD enzyme activity and how it may relate to the patients' genetic background. We observed that individuals with partial BTD deficiency display an elevation of BTD enzyme activity with increasing age in 48% of cases-a recovery which allowed adjustment or stop of biotin supplementation in 20% of all individuals. In addition, we were able to recruit 56 patients (64%) for genetic testing, revealing 19 different variants (2 novel), and constituting 22 different genotypes. Genotype-phenotype correlations revealed that the most abundant allele in our cohort p.(Asp444His) was also the most common variant in patients displaying recovery of BTD enzyme activity. Based on our results, we recommend to retest all patients with partial BTD deficiency at the age of 5 years, as this may result in an impact on therapy. Moreover, genetic testing of BTD deficient individuals can allow prediction of the severity of BTD deficiency and of the likelihood of BTD enzyme activity recovery with age.

Keywords: BTD variants; biotin therapy; biotinidase deficiency; biotinidase enzyme activity; newborn screening.

FIGURE 1

Clinical and enzymatic cohort characterization. (A) Schematic depiction of the biotin cycle, adapted from Zschocke and Hoffmann. (B) Distribution of dates of birth of each individual in the cohort. (C) Bar chart indicating proportion of individuals with partial and profound BTD deficiency. (D) Distribution of age at biotinidase (BTD) enzyme activity measurements of the full study cohort, subdivided in the two BTD deficiency types. (E) Comparison of all BTD enzyme activity measurements during the study course. (F) Gender comparison per BTD deficiency category. Comparisons indicate p values

FIGURE 2

Recovery of biotinidase (BTD) activity with increasing age. (A) BTD enzyme activity data plotted according to age at measurement; linear regressions are calculated per test group. (B) Arrows indicate the chronological relationship between minimal and maximal BTD enzyme activity levels in each individual with partial BTD deficiency; the gray segment indicates the critical difference margin beyond which the change is regarded significant. (C) Bar chart summarizing the changes of BTD enzyme activity with increasing age. (D) Minimal and maximal BTD enzyme activities of the individuals with partial BTD deficiency and significant activity increase with age. (E) Density plot depicting the distribution of ages at minimal and maximal BTD enzyme activity; dotted line indicates the time point at which a majority of patients show recovery of BTD enzyme activity; the solid vertical line indicates the proposed age for retesting BTD enzyme activity in patients with BTD deficiency

FIGURE 3

Landscape of BTD variants. (A) Pie chart of all variants detected in the cohort and their relative occurrence. (B) Bar chart depicting the frequency of all different variants. (C) Lollipop plot indicating the location and frequency of all variants detected in the cohort. Active site and dimer interface residues are putative based on the reference sequence NP_001357587.1 on the NCBI database, https://www.ncbi.nlm.nih.gov/protein/NP_001357587.1, accessed December 2, 2021); to match the old reference used in our study (and to be in accordance with the nomenclature of variants in the literature) 20 amino acids were added to each residue position. Amino acids 1–41 indicate a signaling peptide

FIGURE 4

Functional impact of variants. (A) Comparisons of variant classes and their abundance, and (B) different allele combinations with regards to biotinidase (BTD) enzyme activity. (C) Bar chart of all allele combinations which showed BTD enzyme activity recovery in at least one patient. (D + E) Boxplots and proportional bar plots illustrating the impact of a specific mutant allele on BTD enzyme activity and the alteration of said activity with increasing age. Comparisons indicate p values