rs397507174 — BTD BTD Tyr190Cys

BTD Tyr190Cys — A Pathogenic Variant in the Biotin Recycling Enzyme

Every time a biotin-dependent enzyme finishes its job — carboxylating pyruvate, acetyl-CoA, propionyl-CoA, or 3-methylcrotonyl-CoA — the biotin cofactor is covalently attached to the enzyme and must be liberated before it can be reused. Biotinidase¹¹ Biotinidase
Encoded by the BTD gene on chromosome 3p25; a serum enzyme that cleaves biocytin (biotinyl-lysine) to free biotin for re-use across the four biotin-dependent carboxylases that drive fat, protein, and carbohydrate metabolism is the enzyme responsible for this recycling step. When both copies of BTD are non-functional, free biotin becomes depleted, all four carboxylases fail, and a characteristic neurocutaneous syndrome²² neurocutaneous syndrome
Biotinidase deficiency (OMIM #253260): autosomal recessive disorder presenting with seizures, hypotonia, ataxia, dermatitis, alopecia, and sensorineural hearing loss — reversible with biotin supplementation if treated early follows within weeks to years.

The rs397507174 variant (c.569A>G on the BTD coding sequence, plus strand) replaces tyrosine at position 190 of the biotinidase protein with cysteine (p.Tyr190Cys). The substitution introduces a free thiol group at a position that normally anchors a bulkier aromatic residue within the carbon-nitrogen hydrolase domain³³ carbon-nitrogen hydrolase domain
The catalytic core of biotinidase; contains the active-site cysteine nucleophile (Cys153) that attacks the amide bond of biocytin. Tyr190 is a conserved residue in the substrate-binding pocket adjacent to this active site. ClinVar classifies this variant as Pathogenic/Likely Pathogenic (VCV000046830, two-star review status, no conflicting interpretations) for biotinidase deficiency.

The Mechanism

Biotinidase catalyses the hydrolysis of biocytin through a two-step mechanism: a nucleophilic attack by the active-site Cys153 forms a biotinyl-enzyme intermediate, followed by transfer of biotin to an acceptor (free amino group or water). Conserved residues in the substrate-binding pocket position biocytin for this reaction. Tyr190 is predicted to contribute to substrate orientation — its replacement with cysteine alters the geometry of the binding pocket and impairs catalytic turnover. Bioinformatic tools (SIFT, PolyPhen) flag this substitution as damaging; the position is conserved across vertebrate biotinidase orthologues.

When biotinidase activity falls to less than 10% of mean normal serum activity, free biotin cannot be recovered efficiently from biocytin generated by protein turnover and dietary intake. The biotin pool depletes, and all four biotin- dependent carboxylases — pyruvate carboxylase (gluconeogenesis)⁴⁴ pyruvate carboxylase (gluconeogenesis)
PC deficiency causes lactic acidosis and hypoglycaemia, acetyl-CoA carboxylase (fatty acid synthesis)⁵⁵ acetyl-CoA carboxylase (fatty acid synthesis)
ACC1/ACC2 deficiency impairs fatty acid synthesis and beta-oxidation regulation, propionyl-CoA carboxylase (odd-chain fatty acid and amino acid catabolism)⁶⁶ propionyl-CoA carboxylase (odd-chain fatty acid and amino acid catabolism)
PCC deficiency causes propionate accumulation and metabolic acidosis, and 3-methylcrotonyl-CoA carboxylase (leucine catabolism) — lose function in concert, producing the organic aciduria and metabolic crisis characteristic of profound biotinidase deficiency.

The Evidence

The defining clinical study is Pomponio et al. (1997)⁷⁷ Pomponio et al. (1997)
Pomponio RJ et al. Mutations in the human biotinidase gene that cause profound biotinidase deficiency in symptomatic children: molecular, biochemical, and clinical analysis. Pediatr Res, 1997, which characterized 21 distinct BTD mutations in 37 symptomatic children with profound deficiency. The Tyr190Cys variant (rs397507174) was among the mutations submitted to ClinVar from this and related studies, and is classified pathogenic by Baylor Genetics, LabCorp, and Counsyl in the ClinVar record (RCV000021949).

The broader disease framework is well established: Wolf (2012)⁸⁸ Wolf (2012)
Wolf B. Biotinidase deficiency: "if you have to have an inherited metabolic disease, this is the one to have." Genet Med, 2012 reviewed the >150 BTD mutations known at that time, all of which produce profound deficiency (<10% activity) except D444H, which retains ~50% activity and causes partial deficiency. Tyr190Cys falls into the profound-deficiency category based on its predicted complete disruption of the substrate-binding pocket.

Newborn screening using a colorimetric biotinidase activity assay on dried blood spots has been universal in the United States since 1984 and is now standard in most countries. Norrgard et al. (1999)⁹⁹ Norrgard et al. (1999)
Norrgard KJ et al. Mutations causing profound biotinidase deficiency in children ascertained by newborn screening in the United States occur at different frequencies than in symptomatic children. Pediatr Res, 1999 showed that newborn-detected children have better outcomes than symptom-detected children, confirming the benefit of early treatment. Untreated profound deficiency causes seizures, hypotonia, developmental delay, sensorineural hearing loss (in ~76% of untreated patients), optic atrophy, and eventually coma or death. With timely biotin supplementation, all metabolic abnormalities reverse rapidly and development is typically normal.

Practical Actions

For carriers (one copy of Tyr190Cys): biotinidase activity is reduced to an intermediate level (typically 30–65% of normal) but this is sufficient for normal biotin homeostasis under most conditions. Carriers do not develop biotinidase deficiency. The primary relevance is reproductive: if both partners carry a pathogenic BTD variant, each pregnancy has a 25% chance of a child with profound deficiency. Carrier couple screening is indicated.

For individuals with biallelic Tyr190Cys, or compound heterozygotes (one Tyr190Cys allele plus any other pathogenic BTD allele): this is a medical diagnosis. The treatment is pharmacological free biotin by mouth — 5–10 mg/day for profound deficiency — which completely bypasses the recycling defect by providing exogenous free biotin directly. Clinical response is rapid: seizures resolve within days to weeks, cutaneous features within weeks, and metabolic parameters normalise. Neurological deficits (hearing loss, optic atrophy) may not fully reverse once established, underscoring the importance of early detection and treatment.

Interactions

Biotinidase deficiency requires biallelic loss of BTD function. Compound heterozygosity — one Tyr190Cys allele on one chromosome plus any other pathogenic BTD variant on the other — produces the same clinical picture as homozygous Tyr190Cys. The most common pathogenic BTD allele globally is D444H (c.1330G>C, p.Asp444His, rs13078881), which causes only partial deficiency when homozygous but can cause profound deficiency when combined with a severe allele like Tyr190Cys. Carriers of Tyr190Cys who have a partner of similar ancestry should consider BTD sequencing of the partner to assess compound-heterozygote risk for offspring.