Biotinidase Deficiency: BTD Gene Deletion/Duplication

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Condition Description

Biotinidase deficiency (BTD) is an autosomal recessive inborn error of biotin metabolism [1]. It is clinically and biochemically similar to the disorder early-onset or holocarboxylase synthetase deficiency which is caused by mutations in the holocarboxylase synthetase gene (HLCS) (refer to the holocarboxylase synthetase deficiency test for more information) [2]. Biotin is an essential water-soluble vitamin that serves as a coenzyme for four carboxylases in humans (acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and b-methylcrotonyl-CoA carboxylase) [3]. Its serum level depends on dietary biotin intake and the recycling of endogenous biotin. Biotinidase is the enzyme that catalyzes the cleavage of biotin from biocytin or biotinylpeptides, which are the products of carboxylase degradation. BTD is classified as either profound or partial based on the serum biotinidase activity (0-10% and 10-30%, respectively). Profound BTD can present between the ages of one week and ten years, with a mean age of three and one-half months [4]. In an untreated state it is usually characterized initially by seizures, hypotonia, ataxia, developmental delay, vision problems, hearing loss, and cutaneous abnormalities such as alopecia, skin rash, and candidiasis. With age, motor limb weakness, spastic paresis, and decreased visual acuity occur. Individuals with partial BTD may have hypotonia, skin rash, and hair loss, particularly during times of stress. Once vision problems, hearing loss, and developmental delay occur, they are usually irreversible even with biotin therapy. Early recognition and biotin supplementation results in rapid clinical improvement. Newborn screening allows early presymptomatic treatment that can prevent neurological delays [5]. The age of onset is one of the distinguishing factors, with BTD typically presenting after 3 months of age and holocarboxylase synthetase deficiency typically presenting before 3 months. The symptoms in these disorders are similar and clinical differentiation is often difficult. Organic acid abnormalities are similar in BTD and holocarboxylase synthetase deficiency and may be reported as consistent with multiple carboxylase deficiency on tandem mass spectrometry utilized in neonatal screening. Definitive enzyme determinations are required to distinguish between the two disorders [6]. Biotinidase activity is normal in serum of individuals with holocarboxylase synthetase deficiency; therefore, the enzymatic assay of biotinidase activity used in newborn screening is specific for biotinidase deficiency and does not identify children with holocarboxylase synthetase deficiency. Both biotinidase deficiency and holocarboxylase synthetase deficiency are characterized by deficient activities of the three mitochondrial carboxylases in peripheral blood leukocytes prior to biotin treatment. In both disorders, these activities increase to near-normal or normal after biotin treatment. BTD is caused by mutations in the BTD gene (3p25) and about 100 mutations have been described worldwide to date [7-8]. Genotype-phenotype correlations are limited [9-10]. Gene sequence analysis is available to test for mutations in the BTD gene (test code JI).

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References:
1. Wolf B. Disorders of Biotin Metabolism, in: C.R. Scriver, A.L. Beaudet, W. Sly, D. Valle (Eds.), The Metabolic and Molecular Bases of Inherited Disease, McGraw-Hill, New York, 2001, pp. 3947-3956.
2. Sweetman L. Two forms of biotin-responsive multiple carboxylase deficiency. J Inherit Metab Dis 1981, 4:53-54.
3. Hart et al. Biochemical and immunological characterization of serum biotinidase in profound biotinidase deficiency. Am J Hum Genet 1992, 50:126-136.
4. Wolf et al. Biotinidase deficiency: initial clinical features and rapid diagnosis. Ann Neurol 1985, 18:614-617.
5. Heard et al. Neonatal screening for biotinidase deficiency: results of a 1-year pilot Study. J Pediatr 1986, 108:40-46.
6. Bartlett et al. Enzyme studies in combined carboxylase deficiency. Ann NY Acad Sci 1985, 447:235-51.
7. Milankovics et al. Mutations causing biotinidase deficiency in children ascertained by newborn screening in Western Hungary. Mol Genet Metab 2007, 90:345-348.
8. Hymes et al. Mutations in BTD causing biotinidase deficiency. Hum Mutat 2001, 200:375-381.
9. Norrgard et al. Double mutation [A171T and D444H] is a common cause of profound biotinidase deficiency in children ascertained by newborn screening in the United States. Hum Mutat 1998, 11:410.
10. Wolf et al. Profound biotinidase deficiency in two asymptomatic adults. Am J Med Genet 1997, 73:5-9.
11. Wolf, et al. Biotinidase Deficiency: Novel Mutations and Their Biochemical and Clinical Correlates. Hum Mutat 2005, Mutation in Brief #800, Online.
12. Wolf B. Worldwide survey of neonatal screening for biotinidase deficiency. J Inherit Metab Dis 1991, 14:923-7.

Genes (1)

Indications

This test is indicated for:
  • Confirmation of a clinical/biochemical diagnosis of biotinidase deficiency
  • Carrier testing in adults with a family history of biotinidase deficiency

Methodology

DNA isolated from peripheral blood is hybridized to a CGH array to detect deletions and duplications. The targeted CGH array has overlapping probes which cover the entire genomic region.

Please note that a "backbone" of probes across the entire genome are included on the array for analytical and quality control purposes. Rarely, off-target copy number variants causative of disease may be identified that may or may not be related to the patient's phenotype. Only known pathogenic off-target copy number variants will be reported. Off-target copy number variants of unknown clinical significance will not be reported.

Detection

Detection is limited to duplications and deletions. Array CGH will not detect point mutations or intronic mutations. Results of molecular analysis must be interpreted in the context of the patient's clinical and/or biochemical phenotype.

Specimen Requirements

Submit only 1 of the following specimen types

Preferred specimen type: Whole Blood

Type: Whole Blood

Specimen Requirements:

In EDTA (purple top) or ACD (yellow top) tube:
Infants (<2 years): 2-3 ml
Children (>2 years): 3-5 ml
Older Children & Adults: 5-10 ml

Specimen Collection and Shipping: Refrigerate until time of shipment. Ship sample within 5 days of collection at room temperature with overnight delivery.

Type: Saliva

Specimen Requirements:

OrageneTM Saliva Collection kit (available through EGL) used according to manufacturer instructions.

Specimen Collection and Shipping: Store sample at room temperature. Ship sample within 5 days of collection at room temperature with overnight delivery.

Special Instructions

Submit copies of diagnostic biochemical test results with the sample. Sequence analysis is required before deletion/duplication analysis by targeted CGH array. If sequencing is performed outside of Emory Genetics Laboratory, please submit a copy of the sequencing report with the test requisition.
  • Biotinidase enzyme assay (BX), urine organic acid analysis (OA) and plasma acylcarnitine analysis (AR) are used in the diagnosis of a patient with BTD.
  • Custom diagnostic mutation analysis (KM) is available to family members if mutations are identified by sequencing.
  • Prenatal testing is available for known familial mutations only. Please call the Laboratory Genetic Counselor before collecting a fetal sample.

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