Structure of the human biotinidase gene

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.

Evaluation of 700 patients referred with a preliminary diagnosis of biotinidase deficiency by the national newborn metabolic screening program: a single-center experience

OBJECTIVES

This study aimed to investigate the clinical, demographic and laboratory characteristics of the patients referred with a preliminary diagnosis of biotinidase deficiency through the national newborn metabolic screening program. We also attempted to determine the cut-off level of the fluorometric method used for screening biotinidase deficiency by the Ministry of Health.

METHODS

A total of 700 subjects who were referred to the Pediatric Metabolism Outpatient Clinic with a preliminary diagnosis of biotinidase deficiency through the national newborn metabolic screening program were retrospectively evaluated. Patients detected by family screening were excluded. Biotinidase enzyme activity was assessed and BTD gene analysis was performed in all patients.

RESULTS

Of 700 subjects who were referred by the screening program, 284 (40.5 %) had biotinidase deficiency (BD). The enzyme activity was 0-10, 10-30 and >30 % in 39 (5.5 %), 245 (35 %) and 416 (59.5 %) patients, respectively. The BD was partial in majority of patients (86.2 %). The cut-off level was 59.5 MRU for partial BD and 50.5 MRU for profound BD. The most common mutation detected was p.Arg157His (c.470G>A) among patients with profound BD, and p.D444H (c.1330G>C) among patients with partial BD.

CONCLUSIONS

Treatment should be initiated promptly in patients who are referred by the newborn screening program. Any mean activity under 59.5 MRU should be considered partial BD, while less than 50.5 MRU should be considered profound BD. It should be kept in mind that clinical manifestations may develop both in profound and partial BD.

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.

Clinical, biochemical and genotypical characteristics in biotinidase deficiency

OBJECTIVES

Hypotonia, lethargy, eczema, alopecia, conjunctivitis, ataxia, hearing loss, optic atrophy, cognitive retardation, and seizures can occur in patients with biotinidase deficiency, and it is inherited as autosomal recessive. The aim of this study was to evaluate the cases followed up with the diagnosis of biotinidase deficiency in our unit, in terms of clinical, biochemical and genetic analyses.

METHODS

A total of 112 cases followed up in our centre with the diagnosis of biotinidase deficiency between August 2018-September 2020 were included in the study. Data were collected retrospectively.

RESULTS

A total of 112 cases (55.4% male, mean age: 2.2 ± 2.8 years) diagnosed with biotinidase deficiency were evaluated. Diagnoses were made by newborn screening in 90.2% of the cases, by family screening in 4.5%, and by investigating symptoms in 5.4%. The most frequently (27.5%) detected mutations were c.1330G>C (p.D444H)/c.1330G>C (p.D444H) homozygous mutation, followed by (13.0%) c.1330G>C (p.D444H)/c.470G>A (p.R157H) compound heterozygous mutation, and (13.0%) c.470G>A (p.R157H)/c.470G>A (p.R157H) homozygous mutation. Biotinidase enzyme levels were found to be higher in patients with the p.D444H homozygous mutation than patients with other mutations. Biotin treatment was started in all patients with enzyme deficiency.

CONCLUSIONS

Since the treatment is inexpensive and easily available, it is vital to detect this disease before symptom onset, especially findings related to the central nervous system, hearing and vision loss. In patients diagnosed with enzyme deficiency, the diagnosis should be definitively confirmed by genetic analysis.

Biotinidase Deficiency: Prevalence, Impact And Management Strategies

Biotinidase deficiency is an autosomal recessive inherited neurocutaneous disorder. Clinically untreated patients with BD can present with variable neurological and dermatological signs, such as seizures, hypotonia, feeding problems, developmental delay, hearing loss, optic atrophy ataxia, alopecia, and skin rash. Clinical findings of patients with partial BD reported in the literature show that it can occur from infancy to adulthood. Outcomes of newborn screening programs support the fact that biotin treatment started after birth prevents patients with biotinidase deficiency from developing symptoms. Presence of late-onset cases with different clinical findings indicates that there is still much to learn about BD.

Clinical, biochemical and mutational findings in biotinidase deficiency among Malaysian population

Introduction

Biotinidase deficiency (BD) is an autosomal recessively inherited disorder characterized by developmental delay, seizures, hypotonia, ataxia, skin rash/eczema, alopecia, conjunctivitis/visual problem/optic atrophy and metabolic acidosis. Delayed diagnosis may lead to irreversible neurological damage.

Methodology

Clinically suspected patients were screened for biotinidase level by a fluorometry method. Profound BD patients were confirmed by mutation analysis of BTD gene.

Results

9 patients had biotinidase activity of less than 77 U. 3 patients (33%) had profound BD while 6 patients (67%) had partial BD. Compound heterozygous mutations were detected at c.98_104delinsTCC p.(Cys33Phefs*36) in Exon 2 and c.833T>C p.(Leu278Pro) in Exon 4 in two patients and a homozygous mutation at c.98_104delinsTCC p.(Cys33Phefs*36) in Exon 2 in another patient.

Conclusion

Correct diagnosis lead to early treatment and accurate management of patient. Biochemical screening of BD in symptomatic child is prerequisite to determine enzyme status however molecular confirmation is vital in differentiating individuals with profound biotinidase deficiency from partial biotinidase deficiency and also individuals' carriers.

Effect of BTD gene variants on in vitro biotinidase activity

INTRODUCTION

Biotinidase deficiency (BD), an autosomal recessive disease, is classified into profound (activity <10%) or partial BD (activity 10-30%). The most frequent variant in patients worldwide is c.1330G > C (p.Asp444His), which is associated with partial BD. In vivo studies indicate that this variant reduces the biotinidase activity by 50%. The objective of this study was to evaluate the in vitro effect of p.Asp444His and of five novel variants identified among Brazilian individuals showing low activity of biotinidase in serum.

METHODS

The variants c.119 T > C (p.Leu40Pro), c.479G > A (p.Cys160Tyr), c.664G > A (p.Asp222Asn), c.1330G > C (p.Asp444His), c.1337 T > C (p.Leu446Pro), c.1466A > G (p.Asn489Ser) and the wild type (wt) BTD gene were expressed in HEK 293 cells. Biotinidase activity was quantified by colorimetric method in cells homogenates and culture medium. The wtBTD activity was considered 100%.

RESULTS

The p.Leu40Pro, p.Cys160Tyr and p.Leu446Pro variants were associated to impaired biotinidase activity (activity in cells: 33%, 14%, 0%, respectively; activity in medium: 7%, 0.3%, 2%, respectively) and undetectable amount of protein in intra and extracellular space. The p.Asn489Ser variant had these effects restricted to the extracellular space (activity in medium: 43%), and the p.Asp222Asn variant showed normal activity. The expression of p.Asp444His variant resulted in detectable protein and slightly reduced activity only in cells (activity in cells: 46%; activity in medium: 115%).

CONCLUSION

Our findings suggest that the p.Leu40Pro, p.Cys160Tyr and p.Leu446Pro variants are deleterious; the p.Asn489Ser is probably related to a mild biochemical phenotype; and p.Asp222Asn variant is probably not deleterious. The p.Asp444His variant seems to code for a protein with variable activity.

Single center experience of biotinidase deficiency: 259 patients and six novel mutations

Background Biotinidase deficiency (BD) is an autosomal recessively inherited disorder of biotin recycling. It is classified into two levels based on the biotinidase enzyme activity: partial deficiency (10%-30% enzyme activity) and profound deficiency (0%-10% enzyme activity). The aims of this study were to evaluate our patients with BD, identify the spectrum of biotinidase (BTD) gene mutations in Turkish patients and to determine the clinical and laboratory findings of our patients and their follow-up period. Methods A total of 259 patients who were diagnosed with BD were enrolled in the study. One hundred and forty-eight patients were male (57.1%), and 111 patients were female (42.9%). Results The number of patients detected by newborn screening was 221 (85.3%). By family screening, 31 (12%) patients were diagnosed with BD. Seven patients (2.7%) had different initial complaints and were diagnosed with BD. Partial BD was detected in 186 (71.8%) patients, and the profound deficiency was detected in 73 (28.2%) patients. Most of our patients were asymptomatic. The most commonly found variants were p.D444H, p.R157H, c.98_104delinsTCC. The novel mutations which were detected in this study are p.D401N(c.1201G>A), p.A82G (c.245C>G), p.F128S(c.383T>C), c617_619del/TTG (p.Val207del), p.A287T(c.859G>A), p.S491H(c.1471A>G). The most common mutation was p.R157H in profound BD and p.D444H in partial BD. All diagnosed patients were treated with biotin. Conclusions The diagnosis of BD should be based on plasma biotinidase activity and molecular analysis. We determined the clinical and genetic spectra of a large group of patients with BD from Western Turkey. The frequent mutations in our study were similar to the literature. In this study, six novel mutations were described.

Twenty-seven mutations with three novel pathologenic variants causing biotinidase deficiency: a report of 203 patients from the southeastern part of Turkey

BACKGROUND

Biotinidase deficiency (BD) is an autosomal recessive inborn error of metabolism characterized by neurologic and cutaneous symptoms and can be detected by newborn screening. Newborn screening for BD was implemented in Turkey at the end of 2008.

METHODS

In total, 203 patients who were identified among the infants detected by the newborn screening were later confirmed to have BD through measurement of serum biotinidase activity. We also performed BTD mutation analysis to characterize the genetic profile.

RESULTS

Twenty-seven mutations were identified. The most commonly found variants were c.1330G>C (p.D444H), c.1595C>T (p.T532M), c.470G>A (p.R157H), and c.198_104delGCGGCTGinsTCC (p.C33Ffs ) with allele frequencies of 0.387, 0.175, 0.165 and 0.049, respectively. Three novel pathogenic and likely pathogenic variants were identified: p.W140* (c.419G>A), p.S319F (c.956C>T) and p.L69Hfs*24 (c.192_193insCATC). We also identified three mutations reported in just one patient in the past (p.V442Sfs*59 [c.1324delG], p.H447R [c.1340A>G] and p.198delV [c.592_594delGTC]). Although all of the patients were asymptomatic under the treatment of biotin, only one patient, who had the novel c.419G>A homozygous mutation became symptomatic during an episode of acute gastroenteritis with a presentation of ketosis and metabolic acidosis. Among the screened patients, 156 had partial and 47 had profound BD.

CONCLUSIONS

We determined the mutation spectra of BD from the southeastern part of Turkey. The results of this study add three more mutations to the total number of mutations described as causing BD.

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.

Laboratory diagnosis of biotinidase deficiency, 2017 update: a technical standard and guideline of the American College of Medical Genetics and Genomics

Disclaimer: These ACMG Standards and Guidelines are intended as an educational resource for clinical laboratory geneticists to help them provide quality clinical laboratory genetic services. Adherence to these Standards and Guidelines is voluntary and does not necessarily assure a successful medical outcome. These Standards and Guidelines should not be considered inclusive of all proper procedures and tests or exclusive of others that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, clinical laboratory geneticists should apply their professional judgment to the specific circumstances presented by the patient or specimen. Clinical laboratory scientists and geneticists are encouraged to document in the patient's record the rationale for the use of a particular procedure or test, whether or not it is in conformance with these Standards and Guidelines. They also are advised to take notice of the date any particular guideline was adopted, and to consider other relevant medical and scientific information that becomes available after that date. It also would be prudent to consider whether intellectual property interests may restrict the performance of certain tests and other procedures.Biotinidase deficiency is an autosomal recessively inherited disorder of biotin recycling that is associated with neurologic and cutaneous consequences if untreated. Fortunately, the clinical features of the disorder can be ameliorated or prevented by administering pharmacological doses of the vitamin biotin. Newborn screening and confirmatory diagnosis of biotinidase deficiency encompasses both enzymatic and molecular testing approaches. These guidelines were developed to define and standardize laboratory procedures for enzymatic biotinidase testing, to delineate situations for which follow-up molecular testing is warranted, and to characterize variables that can influence test performance and interpretation of results.

Differential gene expression during early development in brains of wildtype and biotinidase-deficient mice

Biotinidase deficiency is an autosomal recessively inherited disorder characterized by neurological and cutaneous abnormalities. Untreated individuals with biotinidase deficiency cannot recycle biotin from biocytin (N-biotinyl-ϵ-lysine), the proteolytic digestion product of protein-bound biotin. Biotin therapy can markedly resolve symptoms, or can prevent the development of symptoms if initiated early. To understand better the pathogenesis of the neurological problems in the disorder in humans, we have compared gene transcription changes during the first week post-birth in the brains of biotinidase-deficient, transgenic, knock-out mice at days 1 and 8 and compared to changes in wildtype mice at the same times. The knockout pups that were not supplemented with unconjugated biotin became symptomatic by day 8 and exhibiting failure to thrive. Wildtype pups remained asymptomatic under the same experimental conditions. We compared all four possible combinations and noted the most significant up- and down-regulated genes in the knockout animals at Day 8 compared to those at Day 1, reflecting the changes in gene expression over the first week of development. These alterations involved neurological development and immunological function pathways and provide some clues to avenues for further research. At this time, these preliminary analyses provide us with limited, but new information. However, with the development of new algorithms and programs examining various mechanisms and pathways in the central nervous system, these analyses may help us to understand better the role of biotinidase and the pathogenesis of biotinidase deficiency.

Successful outcomes of older adolescents and adults with profound biotinidase deficiency identified by newborn screening

PURPOSE

We began screening newborns for biotinidase deficiency disorder in 1984, and now all states in the United States and many countries perform this screening. The purpose of this study was to determine the outcomes of older adolescent and adult individuals with the disorder identified by newborn screening.

SUBJECTS AND METHODS

We located and surveyed, by questionnaire and telephone interviews, 44 individuals with profound biotinidase deficiency identified by newborn screening with a mean age of 23.1 years.

RESULTS

All individuals had successfully completed high school, and many were attending or had completed college or graduate school. Compliance in using biotin has been excellent. Several individuals developed a variety of symptoms when they discontinued biotin for days or weeks. These features readily resolved when biotin was resumed. In addition, five treated women had nine uneventful pregnancies and deliveries.

CONCLUSIONS

Newborn screening for profound biotinidase deficiency and early treatment with biotin result in excellent outcomes for older adolescents and adults with the disorder. In addition, mothers with profound biotinidase deficiency who were treated with biotin had pregnancies with good outcomes. These outcome results indicate that newborn screening for biotinidase deficiency is one of the most successful newborn screening programs.Genet Med 19 4, 396-402.

Forty-eight novel mutations causing biotinidase deficiency

Biotinidase deficiency is an autosomal recessively inherited disorder that results in the inability to recycle the vitamin biotin and is characterized by neurological and cutaneous symptoms. The symptoms can be ameliorated or prevented by administering pharmacological doses of biotin. Since 2008, approximately 300 samples have been submitted to ARUP's Molecular Sequencing Laboratory for biotinidase mutation analysis. Of these, 48 novel alterations in the biotinidase gene have been identified. Correlating the individual's serum enzymatic activity with the genotype, we have been able to determine the effect of the novel alteration on enzyme activity and, thereby, determine its likelihood of being pathogenic in 44 of these individuals. The novel mutations and uncertain alterations have been added to the database established by ARUP (http://arup.utah.edu/database/BTD/BTD_welcome.phps) to help clinicians make decisions about management and to better counsel their patients based on their genotypes.

First microdeletion involving only the biotinidase gene that can cause biotinidase deficiency: A lesson for clinical practice

We report the first microdeletion (26 kb) of the biotinidase gene (BTD) that involves three of the four exons of the gene. This deletion further exemplifies the importance of performing microarray analysis or other methodologies for a deletion of the BTD gene when the enzymatic activity indicates lower activity than can be attributed to the mutations identified by DNA sequencing.

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.

First contiguous gene deletion causing biotinidase deficiency: The enzyme deficiency in three Sri Lankan children

We report three symptomatic children with profound biotinidase deficiency from Sri Lanka. All three children presented with typical clinical features of the disorder. The first is homozygous for a missense mutation in the BTD gene (c.98_104 del7insTCC; p.Cys33PhefsX36) that is commonly seen in the western countries, the second is homozygous for a novel missense mutation (p.Ala439Asp), and the third is the first reported instance of a contiguous gene deletion causing the enzyme deficiency. In addition, this latter finding exemplifies the importance of considering a deletion within the BTD gene for reconciling enzymatic activity with genotype, which can occur in asymptomatic children who are identified by newborn screening.

Novel mutations causing biotinidase deficiency in individuals identified by newborn screening in Michigan including an unique intronic mutation that alters mRNA expression of the biotinidase gene

Biotinidase deficiency (BD) is an autosomal recessive disorder resulting in the inability to recycle the vitamin biotin. Individuals with biotinidase deficiency can develop neurological and cutaneous symptoms if they are not treated with biotin. To date, more than 165 mutations in the biotinidase gene (BTD) have been reported. Essentially all the mutations result in enzymatic activities with less than 10% of mean normal serum 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 and causes partial biotinidase deficiency (10-30% of mean normal serum biotinidase activity) if there is a mutation for profound biotinidase deficiency on the second allele. We now reported eight novel mutations in ten children identified by newborn screening in Michigan from 1988 to the end of 2012. Interestingly, one intronic mutation, c.310-15delT, results in an approximately two-fold down-regulation of BTD mRNA expression by Quantitative real-time reverse-transcription PCR (qRT-PCR). This is the first report of an intronic mutation in the BTD gene with demonstration of its effect on enzymatic activity by altering mRNA expression. This study identified three other mutations likely to cause partial biotinidase deficiency. These results emphasize the importance of full gene sequencing of BTD on patients with biotinidase deficiency to better understand the genotype and phenotype correlation in the future.

Biotinidase deficiency: novel mutations in Algerian patients

Biotinidase deficiency is an autosomal recessive disorder of biotin metabolism leading to varying degrees of neurologic and cutaneous symptoms when untreated. In the present study, we report the clinical features and the molecular investigation of biotinidase deficiency in four unrelated consanguineous Algerian families including five patients with profound biotinidase deficiency and one child characterized as partial biotinidase deficiency. Mutation analysis revealed three novel mutations, c.del631C and c.1557T>G within exon 4 and c.324-325insTA in exon 3. Since newborn screening is not available in Algeria, cascade screening in affected families would be very helpful to identify at risk individuals.

Neurological deficits in mice with profound biotinidase deficiency are associated with demylination and axonal degeneration

Biotinidase deficiency is an autosomal recessively inherited disorder characterized by neurological and cutaneous abnormalities. We have developed a transgenic knock-out mouse with biotinidase deficiency to better understand aspects of pathophysiology and natural history of the disorder in humans. Neurological deficits observed in symptomatic mice with biotinidase deficiency are similar to those seen in symptomatic children with the disorder. Using a battery of functional neurological assessment tests, the symptomatic mice performed poorly compared to wild-type mice. Demyelination, axonal degeneration, ventriculomegaly, and corpus callosum compression were found in the brains of untreated, symptomatic enzyme-deficient mice. With biotin treatment, the symptomatic mice improved neurologically and the white matter abnormalities resolved. These functional and anatomical findings and their reversal with biotin therapy are similar to those observed in untreated, symptomatic and treated individuals with biotinidase deficiency. The mouse with biotinidase deficiency appears to be an appropriate animal model in which to study the neurological abnormalities and the effects of treatment of the disorder.

Development and characterization of a mouse with profound biotinidase deficiency: a biotin-responsive neurocutaneous disorder

Biotinidase deficiency is the primary enzymatic defect in biotin-responsive, late-onset multiple carboxylase deficiency. Untreated children with profound biotinidase deficiency usually exhibit neurological symptoms including lethargy, hypotonia, seizures, developmental delay, sensorineural hearing loss and optic atrophy; and cutaneous symptoms including skin rash, conjunctivitis and alopecia. Although the clinical features of the disorder markedly improve or are prevented with biotin supplementation, some symptoms, once they occur, such as developmental delay, hearing loss and optic atrophy, are usually irreversible. To prevent development of symptoms, the disorder is screened for in the newborn period in essentially all states and in many countries. In order to better understand many aspects of the pathophysiology of the disorder, we have developed a transgenic biotinidase-deficient mouse. The mouse has a null mutation that results in no detectable serum biotinidase activity or cross-reacting material to antibody prepared against biotinidase. When fed a biotin-deficient diet these mice develop neurological and cutaneous symptoms, carboxylase deficiency, mild hyperammonemia, and exhibit increased urinary excretion of 3-hydroxyisovaleric acid and biotin and biotin metabolites. The clinical features are reversed with biotin supplementation. This biotinidase-deficient animal can be used to study systematically many aspects of the disorder and the role of biotinidase, biotin and biocytin in normal and in enzyme-deficient states.

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.

High frequencies of biotinidase (BTD) gene mutations in the Hungarian population

Biotinidase deficiency, an autosomal recessively inherited disorder, is characterized by neurologic and cutaneous symptoms and can be detected by newborn screening. In Hungary the national screening programme was launched in 1989 with two screening centres. Over 1,070,000 neonates from western Hungary were screened for biotinidase deficiency in the Budapest Screening Centre between 1989 and December 2008. In this period, 57 patients with profound or partial biotinidase deficiency from 50 families were identified through routine newborn screening. The incidence of the disorder in western Hungary is 1 in 18,700, which is about three times the worldwide incidence. Twenty-four different mutations were identified in patients including the c.406delC novel mutation in exon 3, which is a frameshift mutation. To better understand the background of the unusually high disease incidence, 100 healthy subjects from the Hungarian population were screened by PCR and RFLP for the frequencies of p.D444H, p.Q456H and p.A171T;p.D444H, the three most common BTD mutations. The frequencies were found to be 5.5, 0.5 and 0%, respectively. The results demonstrate that the frequencies of two of the most common biotinidase variant alleles are higher in the Hungarian population than in other Caucasian populations. This and the presence of a unique Hungarian mutation may explain the high incidence of biotinidase deficiency in Hungary.

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.

Evaluation of the biotinidase activity in hepatic glycogen storage disease patients. Undescribed genetic finding associated with atypical enzymatic behavior: an outlook

Repeated evaluation of biotinidase (BTD) activity was carried out for a long-term follow-up in patients with hepatic glycogen storage diseases (GSDs). The results indicated inter-intra variability among the GSD-Ia, GSD-III and GSD-IX patients. In addition, a c.1330G>C transversion in the BTD gene, resulting in a p.Asp444His substitution was detected in one allele of a GSD-Ia patient with sustained normal enzyme activity. Thus far, it is necessary to be cautious in the interpretation of the results of BTD activity as a presumptive GSD diagnostic element. It is not known why plasma BTD activity increases in GSDs patients, or the clinical importance of the increment. When viewed from a global perspective, there are some lines of biotin biology that could indicate a relationship between BTD´s behavior and GSDs.

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.

Diagnosis, treatment, follow-up and gene mutation analysis in four Chinese children with biotinidase deficiency

OBJECTIVE

To report the clinical course and explore the gene mutation spectrum of four Chinese children with biotinidase deficiency.

METHODS

Four Chinese patients aged 4 months to 8 years were referred to this study. Tandem mass spectrometry, gas chromatography-mass spectrometry and the determination of biotinidase activities were performed for selective screening of biotinidase deficiency. Four patients with biotinidase deficiency were diagnosed, treated with biotin and followed.

RESULTS

(1) Four patients with biotinidase deficiency were diagnosed by characteristic metabolites, such as elevated blood levels of 3-hydroxyisovalerylcarnitine (6.22 +/- 3.1 mumol/L), elevated 3-methylcrontonylglycine, methylcitrate and 3-hydroxypropionate in urine and very low biotinidase activities. (2) These patients have been treated with biotin for 1-8 years; two of them still have mental retardation, and two have irreversible hearing or vision disability. (3) In the four patients, six different mutations in the biotinidase gene were identified: c.98G:del7ins3, c.1369G>A (p. V457M), c.1384delA, c.1493_1494insT, c.1284C>A (p.Y428X) and c.1157G>A (p.W386X). The latter four mutations are novel variations. Seven out of eight mutations are located on exon 4 of the biotinidase gene.

CONCLUSIONS

Early recognition of biotinidase deficiency is crucial to avoid permanent damage. Determination of biotinidase activity should be included in neonatal screening in China. Exon 4 may be a hot-spot for biotinidase gene mutations in Chinese patients. Four novel gene variations may be disease-causing mutations and should be confirmed by expression studies.

Biotinyl-methyl 4-(amidomethyl)benzoate is a competitive inhibitor of human biotinidase

Posttranslational modification of histones by biotinylation can be catalyzed by both biotinidase (BTD) and holocarboxylase synthetase. Biotinylation of histones is an important epigenetic mechanism to regulate gene expression, DNA repair, and chromatin remodeling. The role of BTD in histone biotinylation is somewhat ambiguous, given that BTD also catalyzes removal of the biotin tag from histones. Here, we sought to develop BTD inhibitors for future studies of the role of BTD in altering chromatin structure. We adopted an existing colorimetric BTD assay for use in a novel 96-well plate format to permit high-throughput screening of potential inhibitors. Biotin analogs were chemically synthesized and tested for their ability to inhibit human BTD. Seven of these compounds inhibited BTD by 26-80%. Biotinyl-methyl 4-(amidomethyl)benzoate had the largest effect on BTD, causing an 80% inhibition at 1 mM concentration. Enzyme kinetics studies were conducted to determine V(max), K(m) and K(i) for the seven inhibitors; kinetics were consistent with the hypothesis that biotinyl-methyl 4-(amidomethyl)benzoate and the other compounds acted by competitive inhibition of BTD. Finally, biotinyl-methyl 4-(amidomethyl)benzoate did not affect biotin transport in human cells, suggesting specificity in regard to biotin-related processes.

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.

Impaired biotinidase activity disrupts holocarboxylase synthetase expression in late onset multiple carboxylase deficiency

Biotinidase catalyzes the hydrolysis of the vitamin biotin from proteolytically degraded biotin-dependent carboxylases. This key reaction makes the biotin available for reutilization in the biotinylation of newly synthesized apocarboxylases. This latter reaction is catalyzed by holocarboxylase synthetase (HCS) via synthesis of 5'-biotinyl-AMP (B-AMP) from biotin and ATP, followed by transfer of the biotin to a specific lysine residue of the apocarboxylase substrate. In addition to carboxylase activation, B-AMP is also a key regulatory molecule in the transcription of genes encoding apocarboxylases and HCS itself. In humans, genetic deficiency of HCS or biotinidase results in the life-threatening disorder biotin-responsive multiple carboxylase deficiency, characterized by a reduction in the activities of all biotin-dependent carboxylases. Although the clinical manifestations of both disorders are similar, they differ in some unique neurological characteristics whose origin is not fully understood. In this study, we show that biotinidase deficiency not only reduces net carboxylase biotinylation, but it also impairs the expression of carboxylases and HCS by interfering with the B-AMP-dependent mechanism of transcription control. We propose that biotinidase-deficient patients may develop a secondary HCS deficiency disrupting the altruistic tissue-specific biotin allocation mechanism that protects brain metabolism during biotin starvation.

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.

Synthetic ecosystems based on airborne inter- and intrakingdom communication

Intercellular communication within an organism, between populations, or across species and kingdoms forms the basis of many ecosystems in which organisms coexist through symbiotic, parasitic, or predator-prey relationships. Using multistep airborne communication and signal transduction, we present synthetic ecosystems within a mammalian cell population, in mice, or across species and kingdoms. Inter- and intrakingdom communication was enabled by using sender cells that produce volatile aldehydes, small vitamin-derived molecules, or antibiotics that diffuse, by gas or liquid phase, to receiver cells and induce the expression of specific target genes. Intercellular and cross-kingdom communication was shown to enable quorum sensing between and among mammalian cells, bacteria, yeast, and plants, resulting in precise spatiotemporal control of IFN-beta production. Interconnection of bacterial, yeast, and mammalian cell signaling enabled the construction of multistep signal transduction and processing networks as well as the design of synthetic ecosystems that mimic fundamental coexistence patterns in nature, including symbiosis, parasitism, and oscillating predator-prey interactions.

Uptake, localization, and noncarboxylase roles of biotin

Evidence is emerging that biotin participates in processes other than classical carboxylation reactions. Specifically, novel roles for biotin in cell signaling, gene expression, and chromatin structure have been identified in recent years. Human cells accumulate biotin by using both the sodium-dependent multivitamin transporter and monocarboxylate transporter 1. These transporters and other biotin-binding proteins partition biotin to compartments involved in biotin signaling: cytoplasm, mitochondria, and nuclei. The activity of cell signals such as biotinyl-AMP, Sp1 and Sp3, nuclear factor (NF)-kappaB, and receptor tyrosine kinases depends on biotin supply. Consistent with a role for biotin and its catabolites in modulating these cell signals, greater than 2000 biotin-dependent genes have been identified in various human tissues. Many biotin-dependent gene products play roles in signal transduction and localize to the cell nucleus, consistent with a role for biotin in cell signaling. Posttranscriptional events related to ribosomal activity and protein folding may further contribute to effects of biotin on gene expression. Finally, research has shown that biotinidase and holocarboxylase synthetase mediate covalent binding of biotin to histones (DNA-binding proteins), affecting chromatin structure; at least seven biotinylation sites have been identified in human histones. Biotinylation of histones appears to play a role in cell proliferation, gene silencing, and the cellular response to DNA repair. Roles for biotin in cell signaling and chromatin structure are consistent with the notion that biotin has a unique significance in cell biology.

Biological functions of biotinylated histones

Histones H1, H2A, H2B, H3 and H4 are DNA-binding proteins that mediate the folding of DNA into chromatin. Various posttranslational modifications of histones regulate processes such as transcription, replication and repair of DNA. Recently, a novel posttranslational modification has been identified: covalent binding of the vitamin biotin to lysine residues in histones, mediated by biotinidase and holocarboxylase synthetase. Here we describe a novel peptide-based technique, which was used to identify eight distinct biotinylation sites in histones H2A, H3 and H4. Biotinylation site-specific antibodies were generated to investigate biological functions of histone biotinylation. Evidence was provided that biotinylation of histones plays a role in cell proliferation, gene silencing and cellular response to DNA damage.

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.

Identification of alternatively spliced human biotinidase mRNAs and putative localization of endogenous biotinidase

Biotinidase is essential for recycling the vitamin biotin and for transferring biotin to proteins, such as histones, suggesting that the enzyme localizes to various cellular and extracellular sites. To better understand the functions of the enzyme, we examined its gene structure and subcellular localization. Using RACE-PCR and a BLAST search, we extended the 5' sequence of the biotinidase gene. Three novel, alternatively spliced variants of biotinidase, 1a, 1b, and 1c, were identified in multiple human tissues. Exon 1c is present only in testes. The sequence of the 5' splice variants, 1a and 1b, suggest that biotinidase localizes to the mitochondria and/or ER, respectively. Using indirect immunofluorescence studies, biotinidase localizes to organelles in the cytoplasm, but not nucleus, of human fibroblasts and Hep G2 cells. Endogenous expression was examined by isopycnic gradient centrifugation of rat liver organelles, which identified an 85kDa biotinidase protein with biotinyl-hydrolase and transferase activities in microsomes and possibly lysosomes. A 48kDa protein, which also reacts with anti-biotinidase, localizes to mitochondria. The 48kDa protein is not N-glycosylated but is biotinylated, is in the inner mitochondrial matrix, but has no biotinyl-hydrolase or transferase activities. The function and validation of the mitochondrial species remains to be determined. The 5' splice variants and organelle fractionation studies indicate that biotinidase is directed to the secretory pathway and perhaps mitochondria.

Molecular characterisation and neuropsychological outcome of 21 patients with profound biotinidase deficiency detected by newborn screening and family studies

Early recognition by newborn screening and oral biotin supplementation may prevent clinical and neurological deficits in profound biotinidase deficiency (residual plasma biotinidase activity <10%). In order to evaluate possible correlations of molecular characteristics, onset and continuation of treatment and clinical outcome, we investigated 21 patients detected by newborn screening and consecutive family investigations. In 18 patients found by newborn screening, the range of biotinidase activities was 0%-9% residual activity. Application of a sensitive HPLC assay enabled us to discriminate five patients with residual biotinidase activities <1%. Two patients with zero activities were homozygous for the G98:d7i3 mutation and three patients with activities <1% carried mutations G98:d7i3, R157H, and Q456H. The mutation spectrum of the remaining patients included T532M, A171T+D444H, V62M,C432W, and D444H. Evaluation of clinical and neuropsychological outcome showed that only patients with biotinidase activities <1% exhibited characteristic clinical symptoms within the first weeks of life whereas five patients with residual activities of 1.2%-4.6% did not develop clinical symptoms even when not treated until 3.5-21 years. In all patients treated with biotin within the first weeks of life, neuropsychological outcome was normal whereas abnormal in three out of five patients tested for IQ and treated after the age of 3.5 years.

CONCLUSION

The clinical and molecular spectrum of profound biotinidase deficiency is heterogeneous. Early onset of symptoms is predicted by the presence of zero residual activity as measured by sensitive assays and by homozygosity for the G98:d7i3 mutation. In patients with higher residual activities and variable mutational spectrum, correlation with the onset and severity of symptoms cannot be made.

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.

Biotin in metabolism and molecular biology

Biotin is a water-soluble vitamin required by all organisms by virtue of its essential role in carboxylation reactions. Although the metabolism and role of biotin in intermediary metabolism are well established, biotin remains one of the most poorly understood water-soluble vitamins in terms of nutritional requirements and responsiveness to physiological and pharmacological states. Significant advances in the understanding of biotin nutriture have been recently accomplished through the description of the kinetics and regulation of biotin transport and improved methods for biotin status assessment. Additionally, the potential role of biotin in the regulation of gene expression has been strengthened through description of altered gene expression during biotin deficiency and through newly described enzymatic activities of the enzyme biotinidase. Given mounting evidence of suboptimum biotin status, a more complete understanding of these aspects of biotin should lead to a greater appreciation of the ways in which biotin aids in the maintenance of health.

Seventeen novel mutations that cause profound biotinidase deficiency

We report 17 novel mutations that cause profound biotinidase deficiency. Six of the mutations are due to deletions, whereas the remaining 11 mutations are missense mutations located throughout the gene and encode amino acids that are conserved in mammals. Our results increase the total number of different mutations that cause biotinidase deficiency to 79. These additional mutations will undoubtedly be helpful in identifying structure/function relationships once the three-dimensional structure of biotinidase is determined.

Mutations in BTD causing biotinidase deficiency

Biotinidase (BTD) is the only enzyme that can cleave biocytin, a product of the proteolytic digestion of holocarboxylases. Profound BTD deficiency (less than 10% mean normal activity in serum) is an autosomal recessive disorder that can result in neurological and cutaneous abnormalities. Both the cDNA and the genomic DNA of normal BTD gene have been isolated and characterized. The BTD gene is localized to chromosome 3p25. Thus far 61 mutations in three of the four exons of the BTD and one mutation in an intron gene that cause profound BTD deficiency have been reported. Mutations occur at different frequencies in symptomatic children than they do in children ascertained by newborn screening. Two mutations, 98-104del7ins3 and R538C, were present in 52% or 31 of 60 alleles found in symptomatic patients. Three other mutations, A755G, Q456H, and 511 G>A; 1330G>C (double mutation), accounted for 52% of the alleles detected by newborn screening in the United States. Two asymptomatic adults, parents of children with profound BTD deficiency detected by newborn screening, have been described. Additional different mutations have been found in Turkish, Saudi Arabian, and Japanese children with profound BTD deficiency. Partial BTD deficiency (10-30% of mean normal serum activity) is predominantly caused by the single 1330G>C mutation that results in D444H on one allele in combination with one of the mutations causing profound deficiency on the other allele. Four intragenic polymorphisms, three neutral and one amino acid change, have also been found. Although a preponderance of mutations causing the production of truncated BTD protein occurs in symptomatic children with profound deficiency, preliminary studies fail to demonstrate clear genotype-phenotype correlations.

Novel mutations cause biotinidase deficiency in Turkish children

Mutation analysis was performed on DNA from 31 Turkish children with profound biotinidase deficiency who were symptomatic or ascertained by newborn screening. The 98G:del7ins3 mutation is common in clinically ascertained children in both the United States and Turkish populations, but a unique common mutation, R79C, is found only in the Turkish children identified both clinically and by newborn screening. Another frequently occurring mutation, T532M, is only observed in the Turkish newborn screening group. There are four other less frequent novel mutations identified in the Turkish population. Interestingly, the Q456H and the A171T:D444H double mutation, which are the most common mutations found in the US newborn screening population and have not been observed in symptomatic children, do occur in clinically ascertained children in the Turkish population, although the double mutation may be associated with milder and/or later-onset symptoms.

Fine mapping of the human biotinidase gene and haplotype analysis of five common mutations

Biotinidase deficiency is an autosomal recessive defect in the recycling of biotin that can lead to a variety of neurologic and cutaneous symptoms. The disease can be prevented or effectively treated with exogenous biotin. The biotinidase locus (BTD) has been maped to 3p25 by in situ hybridization. The gene has been cloned, the coding region sequenced, the genomic organization determined, and a spectrum of mutations has been characterized in more than 90 individuals with profound or partial biotinidase deficiency. We have conducted haplotype analysis of 10 consanguineous and 39 nonconsanguineous probands from the United States and 8 consanguineous probands from Turkey to localize BTD with respect to polymorphic markers on 3p and to investigate the origins of five common mutations. The inbred probands were homozygous for overlapping regions of 3p ranging in size from 1.1 to 80 cM which were flanked most narrowly by D3S1259 and D3S1293. Radiation hybrids and haplotype analysis of markers within this region suggest that BTD is located within a 0.1-cM region flanked by D3S3510 and D3S1286. The radiation hybrid data suggest that the BTD gene is oriented 5' to 3' between the centromere and the 3p telomere. Association studies indicate that the gene is closer to a third locus D3S3613 than D3S3510, two markers which cannot be resolved by existing linkage data. The BTD locus and D3S3613 must therefore lie between D3S3510 and D3S1286. Comparison of haplotypes reveals evidence for possible founder effects for four of the five common mutations.

Mutations causing profound biotinidase deficiency in children ascertained by newborn screening in the United States occur at different frequencies than in symptomatic children

Biotinidase deficiency is an autosomal recessive disorder of biotin metabolism that can lead to varying degrees of neurologic and cutaneous symptoms when untreated. Because this disorder meets the criteria for newborn screening, many states and countries perform this testing. Because newborn screening should result in complete ascertainment of mutations causing profound biotinidase deficiency (less than 10% of mean normal serum activity), we compared the mutations in a group of 59 children with profound biotinidase deficiency who were identified by newborn screening in the United States with 33 children ascertained by exhibiting symptoms. Of the 40 total mutations identified among the two populations, four mutations comprise 59% of the disease alleles studied. Two of these mutations occur in both populations, but in the symptomatic group at a significantly greater frequency. The other two common mutations occur only in the newborn screening group. Because two common mutations do not occur in the symptomatic population, it is possible that individuals with these mutations either develop mild or no symptoms if left untreated. However, inasmuch as biotin treatment is inexpensive and innocuous, it is still recommended that all children with profound biotinidase deficiency be treated.