Test code: 3229

Biotinidase Deficiency: BTD Gene Sequencing
Test code: 3229
Turnaround time: 5 weeks

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 3229). For patients with mutations not identified by full gene sequencing, a separate deletion/duplication assay is available using a targeted CGH array (Test code 1903).

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
BTD

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

PCR amplification of 4 exons contained in the BTD gene is performed on patient genomic DNA. Direct sequencing of amplification products is performed in both the forward and reverse directions using automated fluorescence dideoxy sequencing methods. Patient gene sequences are compared to a normal reference sequence. Sequence variations are then classified as mutations, benign variants unrelated to disease or variations of unknown clinical significance. Variants of unknown clinical significance may require further studies of the patient and/or family members. This assay does not interrogate the promoter region, deep intronic regions or other regulatory elements. Large deletions are not detected by this analysis.

DETECTION
The majority of patients with clinical and biochemical diagnosis of BTD will have an abnormal DNA test.
Clinical Sensitivity: 97/98 mutations identified in 49 patients [8], 26/26 mutations identified in 13 patients [12].
Analytical Sensitivity: ~99%

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 CEN4GEN) 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 by another third party provider, please submit a copy of the sequencing report with the test requisition.

RELATED TESTS

  • Custom diagnostic mutation analysis (Test code: 6875) is available to family members if mutations are identified by sequencing.
  • A deletion/duplication assay is available separately for individuals where mutations are not identified by sequence analysis. Refer to the test requisition or contact the laboratory for more information.