Seventeen novel mutations that cause profound biotinidase deficiency

A Rare Biotinidase Deficiency in the Pediatrics Population: Genotype-Phenotype Analysis

Biotinidase (BTD) deficiency is a rare autosomal recessive metabolic disorder caused by insufficient biotin metabolism, where it cannot recycle the vitamin biotin. When this deficiency is not treated with supplements, it can lead to severe neurological conditions. Approximately 1 in 60,000 newborns are affected by BTD deficiency. The BTD deficiency causes late-onset biotin-responsive multiple carboxylase deficiency, which leads to acidosis or lactic acidosis, hypoglycemia, and abnormal catabolism. BTD deficiency is of two types based on the amount of BTD Enzyme present in the serum. A wide range of pathogenic mutations in the BTD gene are reported worldwide. Mutations in the BTD gene lead to profound and partial BTD deficiency. Profound BTD deficiency results in a severe pathogenic condition. A high frequency of newborns are affected with the partial deficiency worldwide. They are mostly asymptomatic, but symptoms may appear during stressful conditions such as fasting or viral infections. Several pathogenic mutations are significantly associated with neurological, ophthalmological, and skin problems along with several other clinical features. This review discusses the BTD gene mutation in multiple populations detected with phenotypic features. The molecular-based biomarker screening is necessary for the disease during pregnancy, as it could be helpful for the early identification of BTD deficiency, providing a better treatment strategy. Moreover, implementing newborn screening for the BTD deficiency helps patients prevent several diseases.

Molecular Background and Disease Prevalence of Biotinidase Deficiency in a Polish Population—Data Based on the National Newborn Screening Programme

Biotinidase deficiency (BD) is a rare autosomal recessive metabolic disease. Previously the disease was identified only by clinical signs and symptoms, and since recently, it has been included in newborn screening programs (NBS) worldwide, though not commonly. In Europe, BD prevalence varies highly among different countries, e.g., from 1:7 116 in Turkey to 1:75 842 in Switzerland. This paper aimed to present the molecular spectrum of BD (profound and partial forms) in Polish patients diagnosed within the national NBS of 1,071,463 newborns. The initial suspicion of BD was based on an abnormal biotinidase activity result determined in a dry blood spot (DBS) by colorimetric and by fluorimetric methods while biochemical verification was determined by serum biotinidase activity (as quantitative analysis). The final diagnosis of BD was established by serum enzyme activity and the BTD gene direct sequencing. The obtained results allowed for the estimation of disease prevalence (1:66,966 births, while 1:178,577 for profound and 1:107,146 for partial forms), and gave novel data on the molecular etiology of BD.

Newborn Screening for Biotinidase Deficiency. The Experience of a Regional Center in Italy

Introduction: Biotinidase deficiency (BD) is an autosomal recessive disease causing a defect in the biotin-releasing enzyme. Newborn screening (NBS) allows early diagnosis and treatment, ensuring excellent prognosis. The aim of this study was to describe our experience in the diagnosis, treatment, and follow-up showing key strategies and unsolved questions of the management of BD patients. Methods: We analyzed data of patients identified by the Regional Centre for Newborn Screening of Verona and followed by the Inherited Metabolic Disease Unit of Verona and Neonatal Intensive Care Unit of Bolzano, Italy, from 2014 to 2020. Results: Thirty-seven patients were diagnosed by NBS (five profound and 32 partial BD), with a total incidence of 1:5,996. All were started on biotin at diagnosis and presented no symptoms at follow-up. Analysis of parents and siblings led to identification of five asymptomatic patients with partial BD: one asymptomatic parent and four young siblings. Genetic analysis of the BTD gene identified 17 different genotypes and one mutation not previously known. Discussion: Our data confirm that NBS introduction had a dramatic impact on BD diagnosis, and the incidence has increased significantly compared to other areas. Partial defects are more common than profound and have a distinctive genotype. Partial BD treatment is still controversial even at what dose of biotin and for how long. At the end, BD treatment is very easy and inexpensive and prevents severe neurological damage. Sharing experiences is essential to achieving guidelines for treatment and follow-up and a better genotype-phenotype correlation.

Identification and Characterization of BTD Gene Mutations in Jordanian Children with Biotinidase Deficiency

Biotinidase deficiency is an autosomal recessive metabolic disorder whose diagnosis currently depends on clinical symptoms and a biotinidase enzyme assay. This study aimed to investigate the mutational status and enzymatic activity of biotinidase deficiency in seven unrelated Jordanian families including 10 patients and 17 healthy family members. Amplified DNA was analyzed by the automated Sanger sequencing method, and the enzymatic assay was performed using a colorimetric assessment. Biotinidase level was significantly lower (p < 0.001) in BTD children compare to their non-affected family members. Genetic sequencing revealed six different mutations in Jordanian patients. One mutation was novel and located in exon 4, which could be a prevalent mutation for biotinidase deficiency in the Jordanian population. Identification of these common mutations and combing the enzymatic activity with genotypic data will help clinicians with regard to better genetic counseling and management through implementing prevention programs in the future.

Frequency of biotinidase gene variants and incidence of biotinidase deficiency in the Newborn Screening Program in Minas Gerais, Brazil

OBJECTIVE

The prevalence of biotinidase deficiency and the frequency of biotinidase gene variants in Brazil are not documented. We aimed to determine the incidence of partial and profound biotinidase deficiency in the state of Minas Gerais, Brazil, and to calculate the frequency of biotinidase gene variants in the newborn screening program of Minas Gerais.

METHODS

Neonates (1,168,385) were screened from May 2013 to June 2018. Those detected with abnormal biotinidase activity based on semi-quantitative assays underwent confirmatory serum tests. The biotinidase gene was sequenced in all confirmed cases.

RESULTS

The combined incidence of partial and profound biotinidase deficiency was estimated at 1:13,909 live births (95% confidence limit 1:11,235-1:17,217), much higher than the incidence rates reported in other populations worldwide. The most frequent biotinidase gene variants were p.D444H (allele frequency, 0.016), haplotype c.1330G>C;c.511G>A (p.D444H;A171T), p.D543E, c.310-15delT (intronic), p.V199M, and p.H485Q. Together these accounted for 74.6% of the alleles analysed.

CONCLUSION

Newborn screening for biotinidase deficiency, which revealed a higher incidence in Minas Gerais, is feasible and plays a critical role in the early identification of affected neonates and prevention of symptoms and irreversible sequelae. Biotinidase gene sequencing is a useful tool to confirm the diagnosis, and also provides valuable information about genetic variability among different populations.

Clinical features, BTD gene mutations, and their functional studies of eight symptomatic patients with biotinidase deficiency from Southern China

Biotinidase (BTD) deficiency is a rare autosomal recessive metabolic disease, which develops neurological and cutaneous symptoms because of the impaired biotin recycling. Pathogenic mutations on BTD gene cause BTD deficiency. Clinical features and mutation analysis of Chinese children with BTD deficiency were rarely described. Herein, for the first time, we reported the clinical features, BTD gene mutations and their functional studies of eight symptomatic children with BTD deficiency from southern China. Fatigue, hypotonia, proximal muscular weakness, hearing deficits, rash and respiratory problems are common clinical phenotype of our patients. Seizures are observed only in patients with profound BTD deficiency. Five novel mutations were detected, among which c.637delC (H213TfsTer51) was found in 50% of our patients and might be considered as a common mutation. In vitro studies confirmed three mild mutations c.1368A>C (Q456H), c.1613G>A (R538H), and c.644T>A (L215H) which retained 10-30% of wild type enzyme activity, and six severe mutations c.235C>T (R79C), c.1271G>C (C424S), c.1412G>A (C471Y), c.637delC (H213TfsTer51), c.395T>G (M132W), c.464T>C (L155P), and c.1493dupT (L498FfsTer13) which retained <10% of wild type enzyme activity. c.1330G>C (D444H) decreased the protein expression but not activity of BTD enzyme, and H213TfsTer51 was structurally damaging while L498FfsTer13 was functionally damaging. These results will be helpful in establishing the definitive diagnosis of BTD deficiency at the gene level, offering appropriate genetic counseling, and providing clues to structure/function relationships of the enzyme.

Carvalho MR, S. Fonseca PA, S. Camelo J, M. Ribeiro E, F. V. de Medeiros P, M. Lourenço C, F. M. de Souza C, Boy R, Félix TM, M. Bittar C, L. C. Pinto L, C. Neto E, J. Blom H, D. Schwartz IV. 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.

Neonatal screening for profound biotinidase deficiency in the Netherlands: consequences and considerations

Biotinidase deficiency is a rare inherited metabolic disorder that can cause severe neurological symptoms. To prevent severe clinical presentations, it was included in the Dutch neonatal screening programme in 2007. Since then the number of cases detected has been high. This study set out to describe the incidence of the disease, the clinical and demographic characteristics of the neonates identified and the type of mutations found. In the south-western Netherlands, 304 982 neonates were screened between 2007 and 2012; and 92 were identified for further testing. Confirmatory testing revealed 6 (7%) with a profound biotinidase deficiency (<10% enzyme activity), 44 (48%) with a partial deficiency (10-30%) and 42 (46%) with normal activity (>30%). All six patients whose profound deficiency was confirmed had enzyme activities below 15% on neonatal screening. Mutation analysis was performed in 61 neonates: 5 'profound', 35 'partial' and 21 'normal'. All five 'profound' cases had two severe mutations. Comparison with the northern Netherlands showed that the frequency and types of mutation were representative for the Netherlands as a whole. The most common mutation detected was c.[1330G>C] (p.(Asp444His); 34%), which is considered to be mild, followed by three severe mutations c.[1368A>C], c.[1595C>T] and c.[1330G>C;511G>A]. Seven new mutations were identified. We conclude that neonatal screening for profound biotinidase produces a high number of false positives. Biotinidase deficiency was profound in less than 10% of cases identified. As biotinidase activity lay below 15% on neonatal screening in all such cases, the screening threshold might be reduced to 15%.

Outcomes of oral biotin treatment in patients with biotinidase deficiency - Twenty years follow-up

INTRODUCTION

Biotinidase deficiency (BTD) is an inborn error of biotin metabolism inherited as an autosomal recessive trait. Due to the, biotinidase deficiency, biotin is not recycled. Individuals with BTD usually exhibit neurological and cutaneous abnormalities unless treated with biotin. Supplementation with biotin may either ameliorate or if early introduced even prevent symptoms when introduced presymptomatically.

PATIENTS AND METHODS

Since 1991, 22 Polish patients from 19 families have been diagnosed with BTD. In 16 children the diagnosis had been suspected on the basis of clinical signs: skin lesions, hyperventilation, seizures, spasticity, and laboratory investigation (elevated lactate and metabolites on urine organic acids profile). The defect was enzymatically (serum biotinidase activity measurement) and genetically (tested for mutations in the BTD gene) confirmed afterwards. All patients were treated with biotin. Urine organic acids analysis (GC/MS) for 3-hydroxizovaleric acid was used for patients' monitoring. Neurological, audiological and ophthalmological evaluation has been conducted once a year.

RESULTS

In 5 symptomatic patients a progressive optic nerve atrophy had already been noted at the time of treatment initiation. In these patients sensorineural hearing loss has also been diagnosed despite biotin supplementation. Asymptomatic patients treated with biotin supplementation presented no signs or symptoms of BTD. Supplementation with biotin slows the progression of BTD in symptomatic patients, but does not reverse nerve atrophy. Nonetheless, introduction of the treatment with biotin during presymptomatic stage of the disease prevents the onset of symptoms including optic atrophy and hearing loss. Homozygosity for the p.Leu215Phe mutation in BTD gene seems to be frequent in patients from the North-Eastern region of Poland and is connected with the hearing loss.

CONCLUSION

Since the prognosis for individuals diagnosed with BTD is good, provided they are treated before symptoms occur, it is justified to add this metabolic disorder to the panel of conditions screened under the national newborn screening programme in Poland.

Biotinidase deficiency: Spectrum of molecular, enzymatic and clinical information from newborn screening Ontario, Canada (2007-2014)

Untreated profound biotinidase deficiency results in a wide range of clinical features, including optic atrophy, cutaneous abnormalities, hearing loss and developmental delay. Ontario, Canada incorporated this treatable deficiency in newborn screening over the past 8years. This study elucidates the molecular, biochemical, and clinical findings from the pilot project. Information from initial screens, serum biotinidase activity level assays, molecular testing, and family history for 246 positive newborns screens were analyzed. A mutation spectrum was created for the province of Ontario, including common mutations such as D444H, D444H/A171T, Q456H, C33fs, and R157H. Individuals with partial deficiency were separated into 3 groups: D444H homozygotes (Group 1); compound heterozygotes for D444H with another profound allele (Group 2); compound heterozygotes with two non-D444H alleles (Group 3). Biochemical phenotype-genotype associations in partial deficiency showed a significant difference in serum biotinidase activity in between any given two groups. Three children with partial deficiency discontinued biotin for varied lengths of time. Two of whom became symptomatic with abnormal gait, alopecia, skin rashes and developmental delay. A need for more congruency in diagnostic, treatment and educational practices was highlighted across the province. Heterogeneity and variation in clinical presentations and management was observed in patients with the partial deficiency.

Detection of biotinidase gene mutations in Turkish patients ascertained by newborn and family screening

The incidence of biotinidase deficiency in Turkey is currently one of the highest in the world. To expand upon the information about the biotinidase gene (BTD) variations in Turkish patients, we conducted a mutation screening in a large series (n = 210) of probands with biotinidase deficiency, using denaturing high-performance liquid chromatography and direct DNA sequencing. The putative effects of novel mutations were predicted by computational program. Twenty-six mutations, including six novels (p.C143F, p.T244I, c.1212-1222del11, c.1320delG, p.V457L, p.G480R) were identified. Nine of the patients were symptomatic at the initial clinical assessment with presentations of seizures, encephalopathy, and lactic acidemia. The most common mutation in this group of symptomatic patients was c.98-104 del7ins3. Among the screened patients, 72 have partial and 134 have profound biotinidase deficiency (BD) of which 106 are homozygous for BTD mutations. The common mutations (p.R157H, p.D444H, c.98-104del7ins3, p.T532M) cumulatively accounted for 72.3% of all the mutant alleles in the Turkish population.

CONCLUSION

The identification of common mutations and hot spot regions of the BTD gene in Turkish patients is important for mutation screening in the Turkish population and helps to ascertain carriers, may have impact on genetic counseling and implementing prevention programs.

Biotinidase deficiency: clinical and genetic studies of 38 Brazilian patients

Background

Biotinidase deficiency (BD) is an inborn error of metabolism in which some genetic variants correlate with the level of enzyme activity. Biotinidase activity, however, may be artifactually low due to enzyme lability, premature birth, and jaundice; this hinders both phenotypic classification and the decision to implement therapy. This study sought to characterize the clinical and genetic profile of a sample of Brazilian patients exhibiting reduced biotinidase activity.

Methods

This observational, multicenter study used a convenience sampling strategy, with sequencing of exons 2, 3, and 4 of the BTD gene.

Results

The sample comprised 38 individuals with biochemical phenotypes defined a priori on the basis of biotinidase activity in serum/plasma (2 with profound deficiency, 9 with partial deficiency, 15 heterozygous, 1 borderline between partial deficiency and heterozygosity, 2 borderline between heterozygous and normal) or dried blood spot sample (n = 9, all with unspecified deficiency). Most patients were from Southern Brazil (n = 29/38) and were identified by neonatal screening (n = 33/38). Parental consanguinity was reported in two cases. The most commonly found genetic variants were c.1330G > C (p.D444H), c.755A > G (p.D252G), and c.[511G > A;1330G > C] (p.[A171T;D444H]), with allele frequencies of 50%, 9.4%, and 5.4% respectively. Three novel pathogenic variants were identified (c.119 T > C or p.L40P, c.479G > A or p.C160Y, and c.664G > A or p.D222N). Twenty-nine patients had two pathogenic variants detected (with cis/trans status ascertained in 26/29), six had only one variant, and three had no pathogenic variants detected. Genotyping confirmed the original phenotypic classification based on enzyme activity in 16/26 cases. Three polymorphic variants were identified in control individuals, of which two were nonpathogenic (c.1171C > T or p.P391S and c.1413 T > C or p.C471C, with a frequency of 1.5% and 5.5% respectively) and one pathogenic (c.1330G > C, frequency 4%).

Conclusions

Our findings suggest that partial BD is the most common form of BD in Brazil, and expand current knowledge on the allelic heterogeneity of this condition.

Analysis of mutations causing biotinidase deficiency

Biotinidase deficiency is an inherited disorder in which the vitamin, biotin, is not recycled. Individuals with biotinidase deficiency can develop neurological and cutaneous symptoms if they are not treated with biotin. Biotinidase deficiency screening has been incorporated into essentially all newborn screening programs in the United States and in many countries. We now report 140 known mutations in the biotinidase gene (BTD) that cause biotinidase deficiency. All types of mutations have been found to cause biotinidase deficiency. Variants have been identified throughout the coding sequence. Essentially all the variants result in enzymatic activities with less than 10% of mean normal enzyme activity (profound biotinidase deficiency) with the exception of the c.1330G>C (p.D444H) mutation, which results in an enzyme having 50% of mean normal serum activity. The putative three-dimensional structure of biotinidase has been predicted by homology to that of nitrilases/amidases. The effect of the various missense mutations can be predicted to affect various important sites within the structure of the enzyme. This compilation of variants causing biotinidase deficiency will be useful to clinical laboratories that are performing mutation analysis for confirmational testing when the enzymatic results are equivocal for children identified through newborn screening.

Profound biotinidase deficiency: a rare disease among native Swedes

Biotinidase deficiency is an autosomal recessive metabolic disorder included in many newborn screening programmes. Prior to the introduction of screening for biotinidase deficiency in Sweden in 2002, the disorder was almost unknown, with only one case diagnosed clinically. Biotinidase activity was measured in dried blood spots with a semiquantitative method using biotin-6-amidoquinoline as substrate. The cutoff value was set at 25% (later lowered to 20%) of the mean activity of all samples measured on that day. The disorder was confirmed by quantitative determination of biotinidase activity in plasma and DNA analyses. Over a period of 6 years, 13 patients were identified among 637,452 screened newborns and 5,068 adoptive/immigrant children. None of the patients had clinical symptoms at the time of diagnosis. Six patients had profound biotinidase deficiency, with an activity of 0-5% of normal in plasma. Four of these patients were born to parents who were first cousins of Middle Eastern or African origin. Eighteen gene alterations were identified, nine of which have not previously been described: seven mutations p.L83S (c.248T > C), p.R148H (c.443G > A), p.N202I (c.605A > T), p.I255T (c.764T > C), p.N402S (c.1205A > G), p.L405P (c.1214T > C), p.G445R (c.1333G > A) and two silent mutations p.L71L (c.211C > T) and p.L215L (c.645C > T). The predicted severity of the novel mutations was analyzed by sorting intolerant from tolerant (SIFT) and polymorphism phenotyping (PolyPhen), predicting p.L83S, p.L405P and p.G445R as severe mutations. Due to the high rate of immigrants since 1990 from non-Nordic countries, the incidence of biotinidase deficiency is similar to that found in many other Western countries.

The identification of novel mutations in the biotinidase gene using denaturing high pressure liquid chromatography (dHPLC)

Biotinidase deficiency (BD) is an autosomal recessive disorder of biotin metabolism that causes incomplete recycling of free biotin. The resulting depletion of intracellular biotin leads to impaired activities of biotin-dependent carboxylases. The ensuing clinical phenotype includes progressive neurologic deterioration with epileptic seizures, muscular hypotonia as well as skin eczema. BD may be readily diagnosed by analysing enzyme activity in dried blood spots during newborn screening but typically requires molecular confirmation. More than 100 different mutations in the biotinidase gene have been reported to date. To simplify molecular testing we have developed a rapid and accurate denaturing high pressure liquid chromatography (dHPLC) method of the promoter, 3'UTR, all exons including exon/intron boundaries as a first line screen followed by direct sequencing of the respective PCR products. To validate this method we used DNA from 23 different, newly diagnosed patients with biochemically proven BD from Austria, India, Morocco and Spain. A total of 11 mutations, missense 7, frameshift 3 and 1 nonsense, were screened. Six mutations were novel to this study. All mutations revealed distinct dHPLC pattern thus enabling their accurate detection. This study revealed that dHPLC method is robust, automated, economical and above all highly sensitive for the molecular analysis of biotinidase gene and should be used as a pre-analytical tool followed by sequencing of aberrant heteroduplex forming amplicons.

Biotin and biotinidase deficiency

Biotin is a water-soluble vitamin that serves as an essential coenzyme for five carboxylases in mammals. Biotin-dependent carboxylases catalyze the fixation of bicarbonate in organic acids and play crucial roles in the metabolism of fatty acids, amino acids and glucose. Carboxylase activities decrease substantially in response to biotin deficiency. Biotin is also covalently attached to histones; biotinylated histones are enriched in repeat regions in the human genome and appear to play a role in transcriptional repression of genes and genome stability. Biotin deficiency may be caused by insufficient dietary uptake of biotin, drug-vitamin interactions and, perhaps, by increased biotin catabolism during pregnancy and in smokers. Biotin deficiency can also be precipitated by decreased activities of the following proteins that play critical roles in biotin homeostasis: the vitamin transporters sodium-dependent multivitamin transporter and monocarboxylate transporter 1, which mediate biotin transport in the intestine, liver and peripheral tissues, and renal reabsorption; holocarboxylase synthetase, which mediates the binding of biotin to carboxylases and histones; and biotinidase, which plays a central role in the intestinal absorption of biotin, the transport of biotin in plasma and the regulation of histone biotinylation. Symptoms of biotin deficiency include seizures, hypotonia, ataxia, dermatitis, hair loss, mental retardation, ketolactic acidosis, organic aciduria and also fetal malformations. This review focuses on the deficiencies of both biotin and biotinidase, and the medical management of such cases.

Use of synthetic peptides for identifying biotinylation sites in human histones

Posttranslational modifications of histones play an important role in the regulation of chromatin structure and, hence, gene regulation. Recently, we have identified a novel modification of histones: binding of the vitamin biotin to lysine residues in histones H2A, H3, and H4. Here, we describe a procedure to identify those amino acids that are targets for biotinylation in histones. Briefly, the following analytical sequence is used to identify biotinylation sites: (i) short peptides (<20 amino acids in length) are synthesized chemically; amino acid sequences in the peptides are based on the sequence in a given region of a given histone; (ii) peptides are incubated with biotinidase or holocarboxylase synthetase to conduct enzymatic biotinylation; and (iii) biotin in peptides are probed using streptavidin peroxidase. Amino acid substitutions (e.g., lysine-to-alanine substitutions) in synthetic peptides can be used to corroborate identification of biotinylation sites.

Three dimensional structure of human biotinidase: computer modeling and functional correlations

Untreated individuals with deficient activity of biotinidase, the enzyme responsible for recycling the vitamin biotin, usually exhibit neurological and cutaneous findings. To better understand the variability in expression of the disorder it is important to understand the structure of the enzyme and the putative effects of various mutations on its activity. Past attempts to express and purify sufficient quantities of the enzyme by us and others have failed. Therefore, we have resorted to computer modeling using homologous related, crystallized nitrilases/amidases to predict the 3-dimensional structure of biotinidase. The resultant structure is a two domain protein with the catalytic triad consisting of glutamate, lysine and cysteine, within the larger domain. The model predicts multiple glycosylation sites at the surface of the enzyme and multiple disulfide bonds. The precise location of the biotin-binding site could not be determined. Characteristics of 45 missense mutations known to cause profound and partial biotinidase deficiency were examined, including their location, their distance from the catalytic triad, and their potential effect on the structure of the enzyme. Although there are obviously short-comings in predicting the 3-dimensional structure of a protein without crystallographic data, because of the marked homology between biotinidase and specific crystallized amidases/nitrilases, the predicted 3-dimensional structure of biotinidase is probable and should be useful providing clues to structure-function relationships and ultimately the effect of mutations on altering the enzyme's hydrolase and transferase activities.

Novel mutation causing partial biotinidase deficiency in a Syrian boy with infantile spasms and retardation

We report a case of partial biotinidase deficiency (plasma biotinidase levels: 1.30 nm/minute/mL) in a 7-month-old boy who presented with evidence of perinatal distress followed by developmental delay, hypotonia, seizures, and infantile spasms without alopecia or dermatitis. His neurologic symptoms improved markedly on biotin supplementation and antiepileptic drug therapy. DNA mutational analysis revealed that the patient was homozygous for a novel E64K mutation and his parents were heterozygous for the same mutation. Whereas preexisting perinatal distress probably contributed to the severity of the patient's symptoms, the described mutation is novel and is possibly responsible for at least some of his clinical manifestations.

K4, K9 and K18 in human histone H3 are targets for biotinylation by biotinidase

Histones are modified post-translationally, e.g. by methylation of lysine and arginine residues, and by phosphorylation of serine residues. These modifications regulate processes such as gene expression, DNA repair, and mitosis and meiosis. Recently, evidence has been provided that histones are also modified by covalent binding of the vitamin biotin. The aims of this study were to identify biotinylation sites in histone H3, and to investigate the crosstalk among histone biotinylation, methylation and phosphorylation. Synthetic peptides based on the sequence of human histone H3 were used as substrates for enzymatic biotinylation by biotinidase; biotin in peptides was probed using streptavidin peroxidase. These studies provided evidence that K4, K9 and K18 in histone H3 are good targets for biotinylation; K14 and K23 are relatively poor targets. Antibodies were generated to histone H3, biotinylated either at K4, K9 or K18. These antibodies localized to nuclei in human placental cells in immunocytochemistry and immunoblotting experiments, suggesting that lysines in histone H3 are biotinylated in vivo. Dimethylation of R2, R8 and R17 increased biotinylation of K4, K9 and K18, respectively, by biotinidase; phosphorylation of S10 abolished biotinylation of K9. These observations are consistent with crosstalk between biotinylation of histones and other known modifications of histones. We speculate that this crosstalk provides a link to known roles for biotin in gene expression and cell proliferation.

Biotinidase: its role in biotinidase deficiency and biotin metabolism

Renewed interest in biotinidase, the enzyme responsible for recycling the vitamin biotin, initially came from the discovery of biotinidase deficiency in 1982. Since then, the elucidation of other activities of the enzyme, alternative splicing of the biotinidase gene and differential subcellular localization of the enzyme have prompted speculation and investigations of its other possible functions. The results of these studies have implicated biotinidase in aspects of biotin metabolism, specifically the biotinylation of various proteins, such as histones. Biotinidase may have an important regulatory role(s) in chromatin/DNA function.

Biotinidase Deficiency: New Directions and Practical Concerns

Biotinidase deficiency is a readily treatable inherited disorder. Discovery of the enzyme deficiency as the cause for late-onset multiple carboxylase deficiency initially seemed to answer almost all of the questions about the disorder. However, as is the case for most inborn errors of metabolism, finding the enzyme that causes the disorder, cloning the gene, and determining the spectrum of clinical features of the disease only opens a Pandora's box. As researchers have found, there are still many important and interesting questions about this disorder that must be addressed and answered. However, when compared with other inherited metabolic diseases, biotinidase deficiency is still one of the most readily treatable. If a child must have an inborn error of metabolism, let it be biotinidase deficiency and let it be identified by newborn screening.

Neonatal screening for biotinidase deficiency in Hungary: clinical, biochemical and molecular studies

From 1989 to 2001, 1,336,145 newborns were screened for biotinidase deficiency in Hungary. Fifty-eight children with the disorder were identified as enzyme-deficient. We have characterized the clinical and biochemical features and mutations of 20 of these children. Eleven children had profound biotinidase deficiency, 7 had partial biotinidase deficiency, and 2 were found to be heterozygous for profound deficiency by mutation analysis. Seventeen different mutations were identified in this population including seven novel mutations. Six of these new mutations are missense, 245C>A, 334G>A, 652G>C, 832C>G, 1253G>C, 1511T>A, and one is a unique allelic double mutation [212T>C;236G>A]. Of five Romanian Gypsies, four were homozygous for the 1595C>T mutation and one was heterozygous for this mutation. Most of the children with profound deficiency have been asymptomatic on therapy; however, four exhibited minimal brain abnormalities, motor delay and abnormal blood chemistries. Compliance with therapy must be questioned in these cases. Of clinical importance, all of the children with partial deficiency exhibited mild symptoms at the time of diagnosis, at several weeks to months of age. These symptoms resolved following biotin therapy. This is in contrast to the experience in the United States, where the children with partial deficiency have been asymptomatic at the time of diagnosis. This finding further indicates that children with partial deficiency should be treated. The incidence of biotinidase deficiency in Hungary is more than twice that observed in a worldwide survey. These results indicate that newborn screening in Hungary is effective and warranted.