rs397507173 — BTD

BTD Pro167Ser — A Rare Pathogenic Allele in the Biotin Recycling Enzyme

Biotin (vitamin B7) is indispensable for four carboxylase enzymes¹¹ carboxylase enzymes
pyruvate carboxylase, propionyl-CoA carboxylase, 3-methylcrotonyl-CoA carboxylase, and acetyl-CoA carboxylase — all require biotin as a covalently attached cofactor to function that power fat synthesis, amino acid metabolism, and gluconeogenesis. Unlike most vitamins, biotin is largely recovered from food proteins rather than absorbed free — the digestive breakdown of biotin-dependent enzymes in the diet releases biocytin²² biocytin
biotin-ε-lysine, the product of proteolytic digestion of holocarboxylases and other biotinylated proteins, which must then be cleaved back to free biotin before the body can reuse it. The enzyme responsible for this cleavage is biotinidase, encoded by the BTD gene on chromosome 3p25.1.

The rs397507173 variant introduces a c.499C>T nucleotide change that substitutes serine for proline at position 167 of the biotinidase protein (p.Pro167Ser). ClinVar records two "Likely pathogenic" submissions for this allele in the context of biotinidase deficiency, with a single "Uncertain significance" submission also on record (VCV003339734; conflicting interpretations status). The variant is absent from more than 250,000 control chromosomes in population databases, consistent with a rare disease allele. It was first documented in patients with biochemically confirmed biotinidase deficiency by Iqbal et al.³³ Iqbal et al.
Iqbal F et al. The identification of novel mutations in the biotinidase gene using denaturing high pressure liquid chromatography (dHPLC). Mol Genet Metab, 2010.

The Mechanism

Biotinidase is a member of the nitrilase superfamily. It cleaves the amide bond between biotin and lysine in biocytin, regenerating free biotin for re-attachment to newly synthesized apo-carboxylases. Without functional biotinidase, biocytin accumulates in urine (biotinuria), free biotin concentrations fall, and all four biotin-dependent carboxylases progressively lose activity. The proline at position 167 lies within the enzyme's catalytic domain; replacement with serine is predicted to disrupt local protein folding and reduce or abolish catalytic activity, consistent with ClinVar's pathogenicity assessment and in-silico tools predicting a damaging effect.

Biotinidase deficiency is classified by residual serum enzyme activity: profound deficiency (<10% of mean normal activity) causes severe neurological disease if untreated; partial deficiency (10–30% of mean normal) produces milder or stress-triggered symptoms. Heterozygous carriers typically retain ~50% of normal activity — sufficient for health under ordinary conditions but relevant for compound heterozygosity risk assessment.

The Evidence

The clinical significance of biotinidase deficiency is well established⁴⁴ well established
Wolf B. Biotinidase Deficiency. GeneReviews, 2000 (updated 2026): untreated profound deficiency causes seizures, hypotonia, developmental delay, optic atrophy, hearing loss, and alopecia, typically presenting in the first weeks to years of life. Biotin supplementation is curative if started before irreversible neurological damage occurs; some deficits (optic atrophy, sensorineural hearing loss) may not fully reverse even with treatment. Partial deficiency can remain asymptomatic for years but cause symptoms under physiological stress — intercurrent illness, pregnancy, or periods of increased biotin turnover.

The rs397507173 Pro167Ser allele itself has been reported in individuals with confirmed biotinidase deficiency, including at least one homozygous case (ClinVar record). Its rarity — absent in 251,442 control chromosomes across gnomAD datasets — and the in-silico evidence of functional disruption support the "likely pathogenic" classification. The conflicting "uncertain significance" submission likely reflects the limited published functional data specific to this single variant, rather than any evidence of benignity.

Newborn screening programs that test biotinidase activity on dried blood spots identify profound deficiency at approximately 1 in 137,000 births and combined (profound + partial) deficiency at 1 in 61,000 births⁵⁵ 1 in 137,000 births and combined (profound + partial) deficiency at 1 in 61,000 births
Wolf B, GeneReviews 2026. BTD is a required target on expanded newborn screening panels in the United States and many European countries precisely because early biotin supplementation is inexpensive, safe, and prevents irreversible harm.

Practical Actions

Heterozygous carriers (CT genotype) have sufficient biotinidase activity for normal health and do not require supplementation. The primary clinical relevance is reproductive: if both parents carry a pathogenic BTD allele, each pregnancy has a 25% probability of inheriting biallelic pathogenic variants and presenting with biotinidase deficiency. Carrier couples should receive genetic counseling before or during pregnancy.

Individuals who carry Pro167Ser in compound heterozygosity with a second pathogenic BTD variant (particularly the common profound-deficiency alleles) should have biotinidase enzyme activity measured to determine their actual activity level and need for supplementation.

For homozygous Pro167Ser (TT) individuals — an exceptionally rare genotype not yet documented in population databases — the expected clinical scenario is profound biotinidase deficiency requiring immediate and lifelong biotin supplementation, as for all confirmed profound cases.

Interactions

The key interactions for BTD pathogenic alleles involve compound heterozygosity: carrying one Pro167Ser allele together with a second BTD pathogenic variant on the other chromosome results in biallelic loss of function, producing partial or profound deficiency depending on the combined residual activity. The most common co-occurring allele in many populations is p.Asp444His (associated with partial deficiency), and the most common severe allele is the c.98_104delinsTCC frameshift. When Pro167Ser is paired with a severe loss-of-function allele, the phenotype is expected to be profound deficiency; paired with the milder p.Asp444His, the combined activity typically falls in the partial deficiency range (approximately 10–25% of normal). Any individual found to carry Pro167Ser should have their partner screened for BTD pathogenic variants before or during pregnancy, and any child of two carriers should receive biotinidase activity testing as part of newborn screening.