Biotinidase deficiency: Genotype-biochemical phenotype association in Brazilian patients.

INTRODUCTION: The association between the BTD genotype and biochemical phenotype [profound biotinidase deficiency (BD), partial BD or heterozygous activity] is not always consistent. This study aimed to investigate the genotype-biochemical phenotype association in patients with low biotinidase activity. METHODS: All exons, the 5'UTR and the promoter of the BTD gene were sequenced in 72 Brazilian individuals who exhibited low biotinidase activity. For each patient, the expected biochemical phenotype based on the known genotype was compared with the observed biochemical phenotype. Additional non-genetic factors that could affect the biotinidase activity were also analysed. RESULTS: Most individuals were identified by neonatal screening (n = 66/72). When consecutive results for the same patient were compared, age, prematurity and neonatal jaundice appeared to affect the level of biotinidase activity. The biochemical phenotype at the time of the second blood collection changed in 11/22 patients compared to results from the first sample. Three novel variants were found: c.1337T>C (p.L446P), c.1466A>G (p.N489S) and c.962G>A (p.W321*). Some patients with the same genotype presented different biochemical phenotypes. The expected and observed biochemical phenotypes agreed in 68.5% of cases (concordant patients). The non-coding variants c.-183G>A, c.-315A>G and c.-514C>T were present in heterozygosis in 5/17 discordant patients. In addition, c.-183G>A and c.-514C>T were also present in 10/37 concordant patients. CONCLUSIONS: The variants found in the promoter region do not appear to have a strong impact on biotinidase activity. Since there is a disparity between the BTD genotype and biochemical phenotype, and biotinidase activity may be affected by both genetic and non-genetic factors, we suggest that the diagnosis of BD should be based on more than one measurement of plasma biotinidase activity. DNA analysis can be of additional relevance to differentiate between partial BD and heterozygosity.

Hampered Vitamin B12 Metabolism in Gaucher Disease?

Abstract Untreated vitamin B12 deficiency manifests clinically with hematological abnormalities and combined degeneration of the spinal cord and polyneuropathy and biochemically with elevated homocysteine (Hcy) and methylmalonic acid (MMA). Vitamin B12 metabolism involves various cellular compartments including the lysosome, and a disruption in the lysosomal and endocytic pathways induces functional deficiency of this micronutrient. Gaucher disease (GD) is characterized by dysfunctional lysosomal metabolism brought about by mutations in the enzyme beta-glucocerebrosidase (Online Mendelian Inheritance in Man (OMIM): 606463; Enzyme Commission (EC) 3.2.1.45, gene: GBA1). In this study, we collected and examined available literature on the associations between GD, the second most prevalent lysosomal storage disorder in humans, and hampered vitamin B12 metabolism. Results from independent cohorts of patients show elevated circulating holotranscobalamin without changes in vitamin B12 levels in serum. Gaucher disease patients under enzyme replacement therapy present normal levels of Hcy and MMA. Although within the normal range, a significant increase in Hcy and MMA with normal serum vitamin B12 was documented in treated GD patients with polyneuropathy versus treated GD patients without polyneuropathy. Thus, a functional deficiency of vitamin B12 caused by disrupted lysosomal metabolism in GD is a plausible mechanism, contributing to the neurological form of the disorder but this awaits confirmation. Observational studies suggest that an assessment of vitamin B12 status prior to the initiation of enzyme replacement therapy may shed light on the role of vitamin B12 in the pathogenesis and progression of GD.