The Biotinidase Gene Variants Registry: A Paradigm Public Database

Biallelic loss-of-function variations in BTD cause profound biotinidase deficiency in an Indian patient

BACKGROUND

Biotinidase deficiency (BD) is a rare, autosomal recessive metabolic disorder characterized by neurocutaneous symptoms. This study investigates a case of profound BD in an Indian infant and the underlying genetic basis.

METHODS

A 10-month-old male presenting with seizures, hypotonia, ataxia, visual impairments, and developmental delay underwent biochemical and genetic analysis. Biotinidase activity was measured using an ELISA kit. Sanger sequencing of the biotinidase (BTD) gene was performed to identify genetic variations. In silico analysis was employed to assess the potential impact of the identified variants.

RESULTS

The infant biotinidase activity was undetectable and its suggest profound biotinidase deficiency. Novel biallelic loss-of-function variations (c.903G > A and c.946 C > T) in the BTD gene were identified, leading to premature stop codons and truncated, non-functional protein fragments.

CONCLUSION

This case expands our knowledge of BD genetic diversity and underscores the critical role of early diagnosis and newborn screening programs in managing this treatable condition.

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.

Two novel BTD mutations causing profound biotinidase deficiency in a Chinese patient

Background

Biotinidase deficiency (OMIM 253260) is an autosomal recessively inherited disorder affecting about 1/60,000 people worldwide. The absence or deficiency of biotinidase impairs free biotin recycling and affects biotin‐dependent carboxylase functions.

Methods

A Chinese patient with spontaneous recurrent epilepsy, an eczema‐like rash, hair loss, hypotonia, and hearing loss began at three months of age. Her biotinidase activity was 1.0 nmol/ml/min, 9.5% of the mean control activity, which confirmed profound biotinidase deficiency.

Results

Compound heterozygous for c.250‐1G > C and c.878dupT variants in the BTD gene were identified in this patient. These two variants were novel and absent in the population matched controls and any databases.

Conclusions

This study expanded the mutation spectrum of alterations of the BTD gene. Our patient also emphasized the critical role of biotinidase activity measurement combined with mutation analysis in early diagnosis of biotinidase deficiency.

Biotinidase deficiency is a rare, potentially treatable cause of peripheral neuropathy with or without optic neuropathy in adults

Background

We describe two adult brothers with lower limb neuropathy and one with progressive optic neuropathy. One brother was found to have profound biotinidase deficiency by identifying biallelic pathogenic variants of the BTD gene by whole exome sequencing, which was confirmed by markedly decreased serum biotinidase activity.

Case report and methods

The first brother had progressive optic atrophy and vision loss over 10 years and progressive peripheral neuropathy with weakness, pain, and fatigue for 20 years. Profound biotinidase deficiency was also identified in an older brother, who exhibited peripheral neuropathy since four years of age, but had no vision loss.

Results

The first brother's vision loss and neuropathy improved markedly with biotin in six months. However, the neuropathy of the other brother did not improve with 16 months of biotin therapy.

Conclusions

The first brother's neurological issues partially reversed with biotin. However, the longer-term symptoms of the other brother were irreversible. These cases emphasize the importance of considering biotinidase deficiency in the differential diagnosis of adolescents and adults with peripheral neuropathy with or without optic neuropathy/atrophy before symptoms become irreversible. Although WES initially identified the disorder in this family, measuring serum biotinidase activity was a necessary confirmatory step after WES and is less expensive than performing whole exome sequencing.

FREQMAX provides an alternative approach for determining high-resolution allele frequency thresholds in carrier screening

As whole-genome data become available for increasing numbers of individuals across diverse populations, the list of genomic variants of unknown significance (VOUS) continues to grow. One powerful tool in VOUS interpretation is determining whether an allele is too common to be considered pathogenic. As genetic and epidemiological parameters vary across disease models, so too does the pathogenic allele frequency threshold for each disease gene. One threshold-setting approach is the maximum credible allele frequency (MCAF) method. However, estimating some of the input values MCAF requires, especially those involving heterogeneity, can present nontrivial statistical challenges. Here, we introduce FREQMAX, our alternative approach for determining allele frequency thresholds in carrier screening. FREQMAX makes efficient use of the data available for well-studied traits and exhibits flexibility for traits where information may be less complete. For cystic fibrosis, more alleles are excluded as benign by FREQMAX than by MCAF. For less-comprehensively characterized traits like ciliary dyskinesia and Smith-Lemli-Opitz syndrome, FREQMAX is able to set the allele frequency threshold without requiring a priori estimates of maximum genetic and allelic contributions. Furthermore, though we describe FREQMAX in the context of carrier screening, its classical population genetics framework also provides context for adaptation to other trait models.

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.

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%.

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.

Biochemical Hyperthyroidism in a Newborn Baby Caused by Assay Interaction from Biotin Intake

We describe a case of biochemical neonatal thyrotoxicosis caused by biotin supplementation. Biotin may interact with thyroid function testing to imitate thyrotoxicosis with low thyroid-stimulating hormone and elevated triiodothyronine and thyroxine levels.

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.

Clinical, Biochemical and Outcome Profile of Biotinidase Deficient Patients from Tertiary Centre in Northern India

INTRODUCTION

Biotinidase deficiency is an inherited metabolic disorder with estimated birth incidence of 1 in 61,000 for profound and partial deficiency. Estimated incidence of profound and partial biotinidase deficiency is 1 in 1, 37,000 and 1 in 1, 10,000 respectively. The carrier frequency in general population is 1 in 120. We attempt to study clinical, biochemical and outcome from 10 Biotinidase deficient patients.

MATERIALS AND METHODS

A retrospective case record study was conducted to record Clinical, biochemical and outcome profile from genetic records. Biotinidase level was measured using spectrophotometric method.

RESULTS

Study group comprised of 8 males and 2 females with median age of presentation 6 (2-45.75) months. Median (interquartile range) Biotinidase level in study group 0.3 (0.08-1.5) nmol/ml/min. Study group was further divided in to early onset group (< 12 months, n-6) and late onset group (> 12 months, n-4). Seizure, alopecia and hearing loss were predominant phenotypes in study group. The other rare presentations were: hypotonia, ataxia, skin rash, seborrhoea. The most common seizure type was focal seizure. Control of seizure activity was important immediate outcome measured in study group. Median duration (interquartile range) of seizure control in early onset group was 3 (2-4)days against 13.5 (12.25-14.75) days in late onset group.

CONCLUSION

This study highlights the need of early diagnosis for favourable outcome for a potentially treatable inherited metabolic disorder.

Inborn Errors of Metabolism in the United Arab Emirates: Disorders Detected by Newborn Screening (2011-2014)

This study reports on the inborn errors of metabolism (IEM) detected by our national newborn screening between 2011 and 2014. One hundred fourteen patients (55 UAE citizens and 59 residents) were diagnosed during this period. The program was most comprehensive (tested 29 IEM) and universally applied in 2013, giving an incidence of 1 in 1,787 citizens. This relatively high prevalence resulted from the frequent consanguineous marriages (81.5%) among affected families. The following eight disorders accounted for 80% of the entities: biotinidase deficiency (14 of 55), phenylketonuria (11 of 55), 3-methylcrotonyl glycinuria (9 of 55), medium-chain acyl-CoA dehydrogenase deficiency (4 of 55), argininosuccinic aciduria, glutaric aciduria type 1, glutaric aciduria type 2, and methylmalonyl-CoA mutase deficiency (2 of 55 each). Mutation analysis was performed in 48 (87%) of the 55 patients, and 33 distinct mutations were identified. Twenty-nine (88%) mutations were clinically significant and, thus, could be included in our premarital screening. Most mutations were homozygous, except for the biotinidase deficiency. The BTD mutations c.1207T>G (found in citizens) and c.424C>A (found in Somalians) were associated with undetectable biotinidase activity. Thus, the high prevalence of IEM in our region is amenable to newborn and premarital screening, which is expected to halt most of these diseases.

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 mimicking neuromyelitis optica: Initially exhibiting symptoms in adulthood

Background:

Children with untreated biotinidase deficiency can experience variable symptoms depending on their age of presentation. Older children and adolescents can exhibit predominant neurological deficits including para- or tetraparesis and vision loss.

Methods:

We report the first case of delayed-onset biotinidase deficiency in a young adult.

Results:

A 22-year-old man presented with a disabling extensive myelopathy and bilateral optic neuropathy which mimicked the findings of a (seronegative) neuromyelitis optica. Imaging investigations were characterized by an MRI T2 hyper-intensity involving the spinal cord, the optic nerves, the fornix and the mammillar bodies, together with an increased 18F-FDG uptake on positron emission tomography. He was ultimately shown to have profound biotinidase deficiency due to a novel missense mutation and was partly improved by oral biotin therapy.

Conclusion:

This individual exemplifies the need to include biotinidase deficiency in the differential diagnosis of patients with extensive myelopathy and/or bilateral optic neuropathy and argues for newborn screening for the disorder.

Mutations in BTD gene causing biotinidase deficiency: a regional report

Biotinidase deficiency is an autosomal recessive inborn error of biotin metabolism. Children with biotinidase deficiency cannot cleave biocytin and, therefore, cannot recycle biotin. Untreated individuals become secondarily biotin deficient, which in turn results in decreased activities of the biotin-dependent carboxylases and the subsequent accumulation of toxic metabolites causing clinical symptoms. Biotinidase deficiency is characterized by neurological, cutaneous manifestations and metabolic abnormalities. The worldwide incidence of profound biotinidase deficiency has been estimated at 1:112,271. The human biotinidase gene is located on chromosome 3p25 and consists of four exons with a total length of 1629 base pairs. To date, more than 100 mutations in the biotinidase gene known to cause biotinidase deficiency have been reported. The vast majority of mutations are homozygous or compound heterozygous. Finding known mutations can be correlated with the biochemical enzymatic results. This report summarizes the demographic features of patients identified as biotinidase deficient from August of 2012 through August of 2013 and mutation analysis results for 20 cases in the southeast region of Turkey.

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.