Biotinidase Deficiency (BTD) Sequencing

Molecular DNA test to confirm a diagnosis of BTD when biotinidase enzymatic activity is low

Related Tests

Initial biotinidase enzyme test to diagnose or rule out BTD

Recommended test for a known familial sequence variant previously identified in a family member

Biotinidase deficiency (BTD), a disorder that affects approximately 1 in 60,000 individuals, is caused by biallelic pathogenic variants in the BTD gene.  Specific variants are associated with the degree of deficiency, either partial or profound. Molecular testing of the BTD gene may be useful if enzymatic testing suggests BTD.

Deficiency in biotinidase enzymatic activity impairs the body’s ability to recycle and reuse the vitamin biotin, resulting primarily in neurologic and dermatologic manifestations.   Timely diagnosis is important. Early identification and treatment of BTD can prevent and even reverse some symptoms, whereas untreated BTD may result in permanent neurologic, visual, and hearing impairment.   Refer to the ARUP Consult Biotinidase Deficiency topic for additional information about screening and laboratory testing for this condition.

Disease Overview

Incidence

  • Carrier frequency: 1/120 
  • Variants confer :
    • Profound BTD in 1/~137,000
    • Partial BTD in 1/~110,000
    • Profound and partial BTD (combined incidence) in 1/~61,000

Symptoms

  • Profound BTD (<10% of normal biotinidase activity) :
    • Seizures
    • Developmental delay
    • Hypotonia
    • Ataxia
    • Vision problems
    • Hearing loss
    • Alopecia
    • Rashes
  • Partial BTD (10-30% of normal biotinidase activity) 
    • Mild forms of symptoms associated with profound BTD may manifest under stress (eg, surgery or infection). 

Screening

  • Newborn screening for BTD is performed across the United States. 
  • Confirmatory testing following an abnormal newborn screen includes evaluation of enzyme activity in serum and may include molecular testing of the BTD gene. 

Refer to the American College of Medical Genetics and Genomics Biotinidase Deficiency algorithm for more information. 

Genetics

Gene

BTD (NM_001370658)

Inheritance

Autosomal recessive

Variants

More than 200 different variants have been identified in the BTD gene. 

Test Interpretation

Clinical Sensitivity

99%

Analytical Sensitivity

For massively parallel sequencing:

Variant Class Analytical Sensitivity (PPA) Estimatea (%) Analytical Sensitivity (PPA) 95% Credibility Regiona (%)

SNVs

99.2

96.9-99.4

Deletions, 1-10 bp

93.8

84.3-98.2

Deletions, 11-44 bp

99.9

87.8-100

Insertions, 1-10 bp

94.8

86.8-98.5

Insertions, 11-23 bp

99.9

62.1-100

aGene included in this test is a subset of a larger methods-based validation from which the PPA values are derived.

bp, base pairs; PPA, positive percent agreement; SNVs, single nucleotide variants

Results

Variant(s) Detected Clinical Significance

2 pathogenic BTD gene variants identified on opposite chromosomes

Predicts a diagnosis of BTD

1 severe and 1 mild BTD gene variant identified

Predicts partial BTD

1 copy of a pathogenic BTD gene variant identified

Predicts that individual is at least a carrier of BTD

No pathogenic gene variants detected by sequencing

Likelihood is reduced that the individual is a carrier of or affected by BTD

Limitations

  • Diagnostic errors may occur due to rare sequence variations.
  • Large deletions and duplications are not detected.
  • Deep intronic and promoter variants will not be detected.
  • A negative result does not exclude a diagnosis of BTD.
  • The following exon is not sequenced due to technical limitations of the assay:
    • BTD (NM_000060) exon 1

References