Caracterização bioquímica e genética da deficiência de biotinidase no Programa de Triagem Neonatal de Minas Gerais: estudo prospectivo de cinco anos

Abstract

Biotinidase deficiency (BD) is an autosomal recessive metabolic disorder caused by mutations in the BTD gene, located on chromosome 3. Diverse phenotypic expression is due to variable deficiency of biotinidase enzyme activity. If not diagnosed early in life, BD may cause mental retardation and even death. Preventive treatment is simple and inexpensive, consisting of administration of free biotin at pharmacological doses throughout life. This is a population-based study aiming to confirm the incidence of profound and partial BD in newborns (NB) screened by the PTN-MG, to establish the frequency of mutations identified in BTD gene, to estimate the frequency of p.D444H variant in the screened population, and to correlate levels of biotinidase enzymatic activity with the genotype. All biochemical and molecular tests were performed at Nupad-UFMG laboratories. During the five-year study, 1,168,385 newborns were screened and 634 had abnormal results in the filter-paper screening. Serum biotinidase activity was determined in 620 newborns, and BD was confirmed in 84 NB (6 with profound and 78 with partial BD); 52 NB were suspected of having the disease (upper borderline range). The combined incidence of BD was 1:13,909 (95%CI; 1:11,235 to 1:17,217). BTD sequencing in the 136 NB identified 36 mutations, 9 of which had not yet been registered in a public database. The most frequent variants were p.D444H, p.[A171T;D444H], p.D543E, intronic (c.310-15delT), p.V199M and p.H485Q. The frequency of the p.D444H allele was estimated at 0.016 and for heterozygous individuals, 0.031. Biochemical phenotype and genotype correlation has not been always consistent given some variability of enzymatic activity both between patients with the same genotype and in the same patient in consecutive dosages. In the great majority of patients with partial BD, the double heterozygosis of p.D444H was identified with another variant, being observed a continuum of values between 15% and 33% of the reference enzymatic activity. When the other variant was known to be “severely” pathogenic, the variation always occurred within the range for partial BD. When the second variant was due to a milder enzyme defect, the values approached or slightly exceeded the upper cutoff point for partial BD. In conclusion, the combined incidence of BD in MG is among the highest in the world and, therefore, neonatal screening plays a crucial role in the early identification of the disease, providing preventive treatment of symptoms and avoiding sequelae. The large genotypic variability observed in patients reflects the multiethnic origin of the state of MG. The serum determination of enzymatic activity is undoubtedly the most important test to confirm the diagnosis of BD. The BTD gene sequencing, especially in cases with doubtful biochemical classification, plays a relevant role in defining patient status and the need for biotin supplementation. This study demonstrates that DB screening program is feasible, useful, and probably cost-effective in Minas Gerais.