In vivo-stable bis-iminobiotin for targeted radionuclide delivery with the mutant streptavidin.

Targeted delivery of radionuclides to tumors is significant in theranostics applications for precision medicine. Pre-targeting, in which a tumor-targeting vehicle and a radionuclide-loaded effector small molecule are administered separately, holds promise since it can reduce unnecessary internal radiation exposure of healthy cells and can minimize radiation decay. The success of the pre-targeting delivery requires an in vivo-stable tumor-targeting vehicle selectively binding to tumor antigens and an in vivo-stable small molecule effector selectively binding to the vehicle accumulated on the tumor. We previously reported a drug delivery system composed of a low-immunogenic streptavidin with weakened affinity to endogenous biotin and a bis-iminobiotin with high affinity to the engineered streptavidin. It was, however, unknown whether the bis-iminobiotin is stable in vivo when administered alone for the pre-targeting applications. Here we report a new in vivo-stable bis-iminobiotin derivative. The keys to success were the identification of the degradation site of the original bis-iminobiotin treated with mouse plasma and the structural modification of the degradation site. We disclosed the successful pre-targeting delivery of astatine-211 (211At), α-particle emitter, to the CEACAM5-positive tumor in xenograft mouse models.

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.

Evaluation of clinical, laboratory, and molecular genetic features of patients with biotinidase deficiency.

Biotinidase deficiency (BD) is an autosomal recessive inherited metabolic disorder which results from the inability of biotin-dependent carboxylase enzymes to function due to the release and absorption of biotin, leading to neurological and cutaneous findings. In the present study, evaluation of demographic characteristics, clinical findings, laboratory results, molecular genetic characteristics, and genotype-phenotype correlations of cases with BD. Two hundred forty-seven cases were included in the study who were admitted to the Department of Pediatric Metabolism of Ankara Bilkent City Hospital after being identified with potential BD through the Newborn Screening Program (NBS), during family screening or based on suspicious clinical findings, or following the detection of a pathogenic variant in a BTD genetic analysis during the period of October 2020 and February 2022. The medical files of the cases were reviewed retrospectively. An analysis of the admission routes of all cases to our clinic revealed 89.5% NBS, 5.7% family screening, and 4.9% suspicious clinical findings suggestive of BD. Complete enzyme deficiency was identified in 19.8%, partial enzyme deficiency in 55.1%, and heterogenous enzyme deficiency in 9.7%. The most common pathogenic variants were c.1270G > C (p.Asp424His), c.410G > A (p.Arg137His), and c.38_44delGCGCTGinsTCC (p.Cys13Phefs*36) in BTD gene. The c.1270G > C variant was most common in patients with cutaneous symptoms. The c.410G > A and c.38_44delGCGCTGinsTCC variants were more common in the patients with neurological symptoms. The mean activity level in patients with the c.1270G > C homozygous variant was statistically significantly higher than the mean activity level in the c.1270G > C compound heterozygous patients and the activity level of patients without the c.1270G > C variant. The mean activity level in c.410G > A homozygous patients was statistically significantly lower than the mean activity level of the c.410G > A compound heterozygous patients and the activity level of patients without the c.410G > A variant. In the course of our study, four new pathogenic variants were detected, namely: c.190G > A (p.Glu64Lys), c.249 + 5G > T, c.228delA (p.Val77*), and c.682A > G (p.Ile228Val).     Conclusions: The present study has determined the clinical and genetic spectrum of a large group of patients with BD in a single center. The frequent mutations in our study were similar to those reported in literature, and four novel variants were also described. What is Known: • Biotinidase deficiency is an autosomal recessive, treatable inborn error of metabolism. Two hundred ninety-four pathogenic variants in the BTD gene have been identified and the c.1270G > C variant is the most frequent BTD gene mutation in both Turkey and around the world. What is New: • Four new pathogenic variants (c.190G > A, p.Glu64Lys; c.249 + 5G > T; c.228delA, p.Val77*; and c.682A > G, p.Ile228Val) have been identified. It is believed that the c.38_44delGCGGCTGinsTCC variant is more commonly seen in individuals with ocular issues; however, further genotype-phenotype correlations are needed.

A retrospective study on biotinidase deficiency: analysis of the Eastern Anatolia region patient cohort.

ABTRACTThis study retrospectively reviews individuals diagnosed with biotinidase deficiency in Eastern Anatolia to analyze the genetic variants and their relationship with biotinidase activity levels. The research focuses on determining the percentage impact of different variants on enzyme activity. The study included 357 patients who presented to Erzurum City Hospital with symptoms of biotinidase deficiency between 2018 and 2023 and were referred to the medical genetics department. Biotinidase enzyme levels were determined using spectrophotometric and colorimetric techniques, while Sanger sequencing analyzed the four exons and intron boundaries of the BTD gene. In the analysis of 357 patients (181 boys, 176 girls), the most frequent variant was c.1270G > C | p.Asp424His. Biotinidase enzyme activity was above 30% in 97.3% of patients with a homozygous p.D424His mutation. The mutations that caused the most significant decrease in enzyme activity were c.410G > A p.Arg137His, c.38_delinsTCC p.Cys13phefs*36, and c.1535C > T p.Thr512Met. Hearing loss (4 patients) and optic atrophy (1 patient) were mainly observed in patients with the c.38_delinsTCC mutation (homozygous or heterozygous). Most patients were asymptomatic, and mild symptoms were effectively prevented with biotin treatment. This study provides a detailed analysis of genetic diversity and clinical presentation in biotinidase deficiency cases in Eastern Anatolia, demonstrating the efficacy of biotin treatment. It highlights the significant role of genetic variants in phenotypic diversity and the need for personalized treatment, calling for further genetic research to enhance understanding of variant diversity and its impact on enzyme activity.

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.

Biotin Homeostasis and Human Disorders: Recent Findings and Perspectives.

Biotin (vitamin B7, or vitamin H) is a water-soluble B-vitamin that functions as a cofactor for carboxylases, i.e., enzymes involved in the cellular metabolism of fatty acids and amino acids and in gluconeogenesis; moreover, as reported, biotin may be involved in gene regulation. Biotin is not synthesized by human cells, but it is found in food and is also produced by intestinal bacteria. Biotin status/homeostasis in human individuals depends on several factors, including efficiency/deficiency of the enzymes involved in biotin recycling within the human organism (biotinidase, holocarboxylase synthetase), and/or effectiveness of intestinal uptake, which is mainly accomplished through the sodium-dependent multivitamin transporter. In the last years, administration of biotin at high/"pharmacological" doses has been proposed to treat specific defects/deficiencies and human disorders, exhibiting mainly neurological and/or dermatological symptoms and including biotinidase deficiency, holocarboxylase synthetase deficiency, and biotin-thiamine-responsive basal ganglia disease. On the other hand, according to warnings of the Food and Drug Administration, USA, high biotin levels can affect clinical biotin-(strept)avidin assays and thus lead to false results during quantification of critical biomarkers. In this review article, recent findings/advancements that may offer new insight in the abovementioned research fields concerning biotin will be presented and briefly discussed.

Identification and characterization of the largest deletion in the PCCA gene causing severe acute early-onset form of propionic acidemia.

Whole-exome sequencing (WES) is an excellent method for the diagnosis of diseases of uncertain or heterogeneous genetic origin. However, it has limitations for detecting structural variations such as InDels, which the bioinformatics analyzers must be aware of. This study aimed at using WES to evaluate the genetic cause of the metabolic crisis in a 3-day-old neonate admitted to the neonatal intensive care unit (NICU) and deceased after a few days. Tandem mass spectrometry (MS/MS) showed a significant increase in propionyl carnitine (C3), proposing methylmalonic acidemia (MMA) or propionic acidemia (PA). WES demonstrated a homozygous missense variant in exon 4 of the BTD gene (NM_000060.4(BTD):c.1330G > C), responsible for partial biotinidase deficiency. Segregation analysis of the BTD variant revealed the homozygous status of the asymptomatic mother. Furthermore, observation of the bam file, around genes responsible for PA or MMA, by Integrative Genomics Viewer (IGV) software displayed a homozygous large deletion in the PCCA gene. Comprehensive confirmatory studies identified and segregated a novel outframe deletion of 217,877 bp length, "NG_008768.1:g.185211_403087delinsTA", extended from intron 11 to 21 of the PCCA, inducing a premature termination codon and activation of nonsense-mediated mRNA decay (NMD). Homology modeling of the mutant PCCA demonstrated eliminating the protein's active site and critical functional domains. Thereupon, this novel variant is suggested as the largest deletion in the PCCA gene, causing an acute early-onset PA. These results could expand the PCCA variants spectrum, and improve the existing knowledge on the molecular basis of PA, as well as provide new evidence of pathogenicity of the variant (NM_000060.4(BTD):c.1330G > C.

Biotinidase deficiency: What have we learned in forty years?

BACKGROUND: Biotinidase deficiency (BD) is an autosomal recessively inherited disorder that was first described in 1982. Forty years after its first description, we compiled available clinical data on BD with the aim of generating a more comprehensive picture of this condition. METHODS: A systematic search strategy was performed in relevant databases without limits for publication date or languages. We screened 3966 records and included 144 articles reporting individuals with BD and their clinical presentation as well as the outcomes, when available. RESULTS: This study included 1113 individuals with BD. More than half (51.5%) of these individuals were diagnosed by newborn screening, 43.3% in presence of clinical symptoms and 5.2% due to family screening. We grouped symptomatic individuals into four main clinical presentations: neonatal-onset (<1 month; 7.9%), early childhood-onset (<2 years; 59.2%), juvenile-onset (2-16 years; 25.1%) and adult-onset (>16 years; 7.7%). BD affected five main organ systems: nervous system (67.2%), skin (53.7%), eye (34.4%), auditory (26.9%) and respiratory system (17.8%). Involvement was mainly multisystemic (82.2%) of individuals, whereas isolated system presentation was seen in only 17.2% of individuals. When reported, metabolic acidosis was present in 42.4% of symptomatic individuals and characteristic abnormal organic acid metabolites were found in 57.1%. Biotin treatment led to clinical stability or improvement in 89.2% of individuals. 1.6% of reported individuals with BD died due to non-availability of treatment or late diagnosis. CONCLUSION: Newborn screening has had a major positive impact on the outcome of many individuals with BD. However, undiagnosed and non-treated BD remains a health concern. Given the risk of mortality or complications associated with late or missed diagnosis if newborn screening is not available, a trial of biotin should be considered in undiagnosed infants and adults exhibiting suspected clinical signs. Enzymatic activity and/or analysis of genetic variants can readily confirm the diagnosis of BD.

Delayed Biotin Therapy in a Child with Atypical Profound Biotinidase Deficiency: Late Arrival of the Truth and a Lesson Worth Thinking.

Biotinidase (BTD) deficiency (OMIM 253260) is an autosomal recessively inherited metabolic disorder resulting from deficient activity of the BTD enzyme, which can cleave and release biotin from a variety of biotin-dependent carboxylases, and is therefore recognized as a tool to recycle biotin. Being a condition caused by variations on BTD gene with a consequence of free biotin shortage, BTD deficiency may impair the activity of biotin-dependent carboxylases, and thus bring about a buildup of potentially toxic compounds in the body, primarily 3-hydroxyisovaleryl-carnitine in plasma as well as 3-hydroxyisovaleric acid in urine. The phenotype of BTD deficiency may vary dramatically, from asymptomatic adults to severe neurological anomalies, even death in infancy. In the present study, we reported on a 5-month-old boy, whose parents sought for medical consultation in our clinic for their son due to his loss of consciousness, repeated tetany, and motor retardation. Detailed clinical features included severe psychomotor retardation, hypotonia, as well as failure to thrive. The brain MRI at 12 months showed cerebellar hypoplasia and multiple foci of leukodystrophy. The result of antiepileptic therapy was not satisfying. During hospitalization, BTD deficiency was suggested by elevated concentration of 3-hydroxyisovaleryl-carnitine in the blood spots and 3-hydroxyisovaleric acid in the urine. The child was then diagnosed with profound BTD deficiency based on the above findings and low BTD enzyme activity. Subsequent mutational analysis revealed a novel homozygous variant, c.637_637delC (p.H213Tfs*51) in exon 4 of BTD gene in the proband, which was recognized as a further support to the diagnosis. Therefore, biotin treatment was started immediately, eventually with satisfactory outcomes achieved in terms of prevention of epileptic seizure, performance in deep tendon reflexes, and improvement of muscular hypotonia, but unfortunately, the therapy failed to show any evident effects on poor feeding and intellectual disability. This painful lesson suggests that newborn screening for inherited metabolic diseases is essential for early identification and treatment, which should have been performed in this case to avoid this tragedy.

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A boy, aged 16 months, attended the hospital due to head and facial erythema for 15 months and vulva erythema for 10 months with aggravation for 5 days. The boy developed perioral and periocular erythema in the neonatal period and had erythema and papules with desquamation and erosion in the neck, armpit, and trigone of vulva in infancy. Blood gas analysis showed metabolic acidosis; the analysis of amino acid and acylcarnitine profiles for inherited metabolic diseases and the analysis of organic acid in urine suggested multiple carboxylase deficiency; genetic testing showed a homozygous mutation of c.1522C>T(p.R508W) in the HLCS gene. Finally the boy was diagnosed with holocarboxylase synthetase deficiency and achieved a good clinical outcome after oral biotin treatment. This article analyzes the clinical data of a child with holocarboxylase synthetase deficiency and summarizes the etiology, diagnosis, and treatment of this child, so as to provide ideas for clinicians to diagnose this rare disease.

Revisiting the administration of biotin to children with biotin-responsive disorders.

There continues to be questions and misconceptions about the administration of the vitamin, biotin, to children with the inherited biotin-responsive disorder, especially infants. Therefore, this commentary is intended to address the issues of biotin administration for healthcare workers, parents of children with the biotin-responsive disorders and the individuals with the disorders.

Childhood-onset hereditary spastic paraplegia and its treatable mimics.

Early-onset forms of hereditary spastic paraplegia and inborn errors of metabolism that present with spastic diplegia are among the most common "mimics" of cerebral palsy. Early detection of these heterogenous genetic disorders can inform genetic counseling, anticipatory guidance, and improve outcomes, particularly where specific treatments exist. The diagnosis relies on clinical pattern recognition, biochemical testing, neuroimaging, and increasingly next-generation sequencing-based molecular testing. In this short review, we summarize the clinical and molecular understanding of: 1) childhood-onset and complex forms of hereditary spastic paraplegia (SPG5, SPG7, SPG11, SPG15, SPG35, SPG47, SPG48, SPG50, SPG51, SPG52) and, 2) the most common inborn errors of metabolism that present with phenotypes that resemble hereditary spastic paraplegia.

Recovery of enzyme activity in biotinidase deficient individuals during early childhood.

Deficiency of the biotinidase (BTD) enzyme is an inborn error of biotin metabolism caused by biallelic pathogenic variants in the BTD gene. There are two forms, partial and profound BTD deficiency, which both can be successfully treated with pharmacological doses of biotin, justifying the inclusion of this disorder in the newborn screening in numerous countries. We investigated the BTD deficiency cohort (N = 87) in our metabolic center, as it was detected upon newborn screening since 2005, and aimed to better understand the long-term course of BTD enzyme activity and how it may relate to the patients' genetic background. We observed that individuals with partial BTD deficiency display an elevation of BTD enzyme activity with increasing age in 48% of cases-a recovery which allowed adjustment or stop of biotin supplementation in 20% of all individuals. In addition, we were able to recruit 56 patients (64%) for genetic testing, revealing 19 different variants (2 novel), and constituting 22 different genotypes. Genotype-phenotype correlations revealed that the most abundant allele in our cohort p.(Asp444His) was also the most common variant in patients displaying recovery of BTD enzyme activity. Based on our results, we recommend to retest all patients with partial BTD deficiency at the age of 5 years, as this may result in an impact on therapy. Moreover, genetic testing of BTD deficient individuals can allow prediction of the severity of BTD deficiency and of the likelihood of BTD enzyme activity recovery with age.

Establishing biotinidase reference interval: A foundation stone for newborn screening of biotinidase deficiency in Pakistan.

OBJECTIVE: To determine the reference interval of biotinidase activity in healthy neonates. METHODS: The cross-sectional study was conducted at the Department of Chemical Pathology and Endocrinology, Armed Forces Institute of Pathology, Rawalpindi, Pakistan, from May to November 2019, and comprised blood samples collected from healthy neonates aged 2-6 days. The samples were collected on filter paper and analysed on genetic screening processor based on dissociation-enhanced lanthanide flouroimmunoassay. Data was analysed using SPSS 21. RESULTS: Of the 120 dried blood spot specimens, 81(67.5%) were from male babies and 39(32.5%) from female babies. Reference interval for biotinidase activity, based on 2.5th and 97.5th percentiles, was from 3.0 to 11.0 nmol/ml/min. CONCLUSIONS: Screening of newborns for biotinidase deficiency is crucial to prevent irreversible neurological damage.

Perspective on newborn screening (NBS): Evidence sharing on conditions to be included in NBS in Pakistan.

Newborn screening aims at detecting treatable disorders early so that the treatment can be initiated to prevent mortality and morbidity. Such programmes are well established in most developed countries, and all newborns are screened for selected metabolic, endocrine and other disorders based on disease epidemiology, testing and treatment availability, efficiency and cost-effectiveness. Even in developing countries, such screening programmes are initiated using heel prick capillary blood collected on filter paper. The current narrative review was planned to provide a perspective with evidence in favour of starting newborn screening for different disorders. The programme project should be initiated nationwide, taking one disorder, congenital hypothyroidism, as the prototype and a newborn screening panel can then be extended to include other disorders. A task force should be set up to recommend disorders to be included in the panel, develop the national plan policies, and define procedures to strengthen the testing.

Expert consensus on screening, diagnosis and treatment of multiple carboxylase deficiency.

Multiple carboxylase deficiency (MCD) includes autosomal recessive holocarboxylase synthetase (HLCS) deficiency and biotinidase (BTD) deficiency, which are caused by and gene mutations respectively. Neonatal screening for HLCS deficiency is based on 3-hydroxyisovaleryl carnitine in dry blood filter paper, and BTD deficiency is based on BTD activity determination. HLCS deficiency and BTD deficiency are characterized by neurocutaneous syndrome and organic aciduria, however, they are different in onset age, neurological symptoms and metabolic decompensation, which needed to be differentiated from acquired biotin deficiency or other genetic metabolic diseases. The diagnosis of the disease requires a combination of biochemical characteristics of hematuria, enzyme activity determination and genetic test. Routine biotin doses are effective for most MCD patients. This consensus is intended to benefit early screening and diagnosis of MCD.

Immunophenotypic analysis of lymphocyte subsets in newborns with biotinidase deficiency.

BACKGROUND/AIM: Biotin is a vital micronutrient that plays a role in metabolic homeostasis and the regulation of innate and adaptive immune system functions. Biotinidase deficiency (BTD) leads to impairment in biotin-dependent immune functions. This study focused on immunophenotypic analysis of lymphocyte subsets in newborns with BTD. PATIENTS AND METHODS: A total of 181 (95 female and 86 male; 114 had BTD and 67 were healthy) newborns underwent biotinidase enzyme activity, molecular and lymphocyte immunophenotyping analyses. BTD is classified into four biochemical phenotypes: profound, partial, heterozygous and normal. The following lymphocyte subsets were studied in all participants: total B lymphocyte (CD19), total T lymphocyte (CD3), helper T lymphocyte (CD3/CD4), cytotoxic T lymphocyte (CTL) (CD3/CD8), natural killer T lymphocyte (CD16/56) and a T-lymphocyte activation marker (HLA-DR). RESULTS: The percentages of lymphocyte subsets were similar in newborns with and without BTD. In all newborns with BTD, the mean CD3/CD4 levels were higher in females, while the CD3/CD8 levels were higher in males (P < .001 for each). In female and male newborns, the CD3/CD4 levels were 53.83 ± 9.46 and 16.82 ± 5.19, respectively, and the CD3/CD8 levels were 48.80 ± 8.65 and 21.48 ± 6.02, respectively. A moderate negative correlation was found between CD3/CD4 and CD3/CD8 in female and male newborns (rfemale  = -0.488, rmale  = -0.574, P < .001). CONCLUSION: This study showed that although there were no differences in the lymphocyte subsets in newborns with BTD, the CD3/CD4 levels were higher in females, and the CD3/CD8 levels were higher in males. In addition, there was a negative correlation between the CD3/CD4 and CD3/CD8 levels in both genders. Although these results indicate sexual dimorphism between CD3/CD4 and CD3/CD8 levels, whether this dissociation is unique to BTD in newborns is not fully clear.

Developmental and behavioral outcomes of preschool-aged children with biotinidase deficiency identified by newborn screening.

Biotinidase deficiency (BD) may cause neurological symptoms and developmental problems. However, newborn screening of BD and early biotin treatment prevent the manifestation of the majority of symptoms. This study intended to examine the developmental and behavioral outcomes as well as maternal anxiety and depressive symptoms of preschool-aged children with BD and to compare these with the outcomes of healthy preschool-aged children. In total, 49 children with BD and 23 healthy children are included. All children were screened for developmental and behavioral problems. Moreover anxiety and depressive symptomatology of their mothers were evaluated. Despite the high percentage of developmental delay in BD group, the numbers of children screened positive for a developmental delay were statistically similar in children with BD and healthy children. Among patients with BD, children with risk of developmental delay had more unfavorable socio-demographic features compared to typically developing ones. Behavioral problem scores, maternal anxiety, and depressive symptoms scores of children with BD were not higher than the healthy children.Conclusion: Children with BD were not different from their healthy peers in terms of developmental and behavioral outcomes. Developmental problems of children with BD may be related to the unfavorable socio-demographic features, not the BD itself. What is known: • Biotinidase deficiency (BD) may result in neurological symptoms and developmental problems. • Newborn screening and early biotin supplementation prevent the manifestation of the majority of symptoms. What is new: • Preschool-aged children with BD identified by newborn screening are not different from their healthy peers in terms of developmental and behavioral outcomes. • Maternal anxiety and depressive symptoms scores of children with BD are similar to scores of healthy children.

The novel homozygous p.Asn197_Ser201del mutation in BTD gene is associated with profound biotinidase deficiency in an Iranian consanguineous family.

BACKGROUND: Biotinidase deficiency is an autosomal recessive inherited inborn error of biotin metabolism. Biotin as a water-soluble vitamin is the prosthetic group of biotin-dependent carboxylase enzymes, and by enhancing their function plays a key role in amino acid catabolism, fatty acid synthesis, and gluconeogenesis. Beyond its prosthetic group role, it has been recognized that biotin regulates the level of gene transcription in the eukaryotic cells, therefore any defect in these pathways causes a multisystem metabolic disorder characterized by neurological and cutaneous symptoms. METHODS AND RESULTS: We report the identification of a novel pathogenic variant in the BTD gene, c.528_542del15 (p.Asn197_Ser201del, UniProt P43251-1) in an Iranian consanguineous family with a severe form of the disease. The segregation analysis in the family was consistent with phenotype and the identified variant was predicated as a pathogenic mutation by the in-silico prediction tools. Computer structural modeling suggests the deleted amino acid residues are located near the biotinidase active site and disrupt the special conformations which are critical for the enzyme activity, and also N-glycosylation. CONCLUSIONS: This study further expands the mutation spectrum of the BTD gene underlying cause of profound biotinidase deficiency.

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.