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BTD
biotinidase
The BTD gene provides instructions for the biotinidase enzyme, which is crucial for recycling the B vitamin biotin, enabling its reuse in various metabolic processes. The BTD gene encodes the biotinidase enzyme, a key player in biotin metabolism.
BTD is located on the short (p) arm of chromosome 3, at band 3p25.1. Arm ratio per GRCh38 - banding schematic.
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Overview
The BTD gene provides the genetic blueprint for producing the biotinidase enzyme. This enzyme plays a fundamental role in the body's utilisation and recycling of biotin, a vital B vitamin. Biotinidase ensures that biotin, obtained from food and released from cellular processes, remains in its active form to support numerous metabolic pathways.
Biotin is essential for the function of several carboxylase enzymes that are involved in the breakdown of proteins, fats, and carbohydrates. By facilitating biotin's availability, the biotinidase enzyme indirectly contributes to energy production and cellular health. Dysregulation of biotinidase activity due to genetic changes in BTD can have significant health implications, particularly affecting metabolic functions.
What the gene does
The biotinidase enzyme, encoded by the BTD gene, performs critical functions related to biotin metabolism within the human body. Its primary role involves the recycling of biotin, a B vitamin indispensable for various metabolic activities. The enzyme removes biotin from proteins to which it is bound, both from dietary sources and from endogenous compounds.
Specifically, biotinidase acts on biotin that is attached to proteins consumed in the diet, releasing free biotin for absorption and utilisation. Furthermore, it recycles biotin within the body. When biotin-dependent carboxylase enzymes are broken down, they release biocytin, a complex composed of biotin linked to the amino acid lysine. Biotinidase cleaves this biocytin complex, liberating free biotin so it can be reused by other carboxylase enzymes.
Beyond its well-established role in biotin recycling, researchers suspect biotinidase may have additional functions. This enzyme may transport free biotin through the bloodstream. It might also have the ability to attach biotin to certain proteins through a process called biotinylation. Within the nucleus, biotinylation of DNA-associated proteins called histones may help determine whether certain genes are turned on or off; however, it is unclear whether biotinidase plays a role in regulating gene activity.
The biotinidase enzyme's catalytic activity is concentrated within its CN hydrolase domain, which enables the hydrolysis of amide bonds to release biotin. This enzymatic action is fundamental to activating biotin-dependent carboxylases, which in turn are crucial for the catabolism of fatty acids, amino acids, and glucose, thereby impacting overall energy metabolism.
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Chromosome location
The BTD gene is situated on chromosome 3, specifically at position 3p25.1. This location refers to the short (p) arm of chromosome 3, within band 25, sub-band 1. The gene consists of a number of exons, which are the coding regions that provide the instructions for assembling the biotinidase enzyme. Its precise position on the chromosome ensures its organised expression and proper function within the human genome.
Protein structure
The biotinidase enzyme comprises 523 amino acid residues. A key functional region of this protein is the CN hydrolase domain, which spans amino acids 52 to 331. This domain is responsible for the enzyme's catalytic activity, particularly its ability to hydrolyse amide bonds, a crucial step in releasing biotin from biocytin and biotinylated proteins. The structural integrity and proper folding of this domain are essential for the biotinidase enzyme to perform its role in biotin recycling effectively.
Key variants
Variants within the BTD gene encompass a range of genetic changes that can influence the function of the biotinidase enzyme. These variations can include single nucleotide changes, deletions, or insertions within the gene's sequence. Over 150 different pathogenic variants have been identified that are associated with biotinidase deficiency. These genetic alterations can lead to a reduction or complete loss of enzyme activity, thereby impairing the body's ability to recycle biotin. Understanding these variants is crucial for diagnosing and managing conditions linked to BTD.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.1330G>C | - | Pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1030dup | p.Ile344fs | Pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1036_1037dup | p.Gly347fs | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.1098G>A | p.Trp366Ter | Pathogenic | ★★☆☆ | Biotinidase deficiency |
c.111T>G | p.Tyr37Ter | Pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1167_1181delinsTTCCAATGGCC | p.Trp389fs | Pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1181_1192del | p.Tyr394_Val397del | Pathogenic/Likely pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1192T>C | p.Cys398Arg | Pathogenic/Likely pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1193G>C | p.Cys398Ser | Pathogenic/Likely pathogenic | ★★☆☆ | Biotinidase deficiency |
c.1204dup | p.Leu402fs | Pathogenic | ★★☆☆ | Biotinidase deficiency |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Pathogenic variants in the BTD gene are primarily associated with Biotinidase deficiency. This is an inherited metabolic disorder that, if left untreated, can have widespread effects on the body, including developmental delays and neurological issues. The severity of the condition often correlates with the residual activity of the biotinidase enzyme.
Inheritance pattern
Conditions caused by pathogenic BTD variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
The BTD gene is recognised within several NHS Genomic Medicine Service clinical panels, indicating its importance in diagnostic testing within the UK. It is listed on the DDG2P panel and is included in panels for Early onset or syndromic epilepsy, Foetal anomalies (R21), Intellectual disability, Ketotic hypoglycaemia, Likely inborn error of metabolism (R98), Mitochondrial disorders, Optic neuropathy (R41), Possible mitochondrial disorder, nuclear genes (R63), and Undiagnosed metabolic disorders. This widespread inclusion underscores the clinical significance of BTD variants in various paediatric and metabolic presentations.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Maintaining a balanced diet generally supports overall metabolic health. For individuals with BTD-related conditions, particularly biotinidase deficiency, dietary considerations often involve ensuring adequate biotin availability, though this is typically managed through specific medical interventions rather than general dietary changes. Generally, lifestyle factors such as a nutritious diet, regular physical activity, and avoiding smoking and excessive alcohol consumption contribute to general wellbeing. However, for inherited metabolic disorders such as biotinidase deficiency, these general lifestyle recommendations do not replace specific medical management and treatment prescribed by healthcare professionals.
Supplement considerations
While biotin is a vital B vitamin, there is no conclusive evidence that biotin supplements can prevent biotinidase deficiency or reverse its effects without specific medical guidance. For individuals diagnosed with biotinidase deficiency, treatment typically involves prescribed biotin supplementation under the supervision of a healthcare provider. Taking any supplement, including biotin, should always be discussed with a doctor or a qualified healthcare professional, especially for those with underlying health conditions or who are taking other medications. Self-prescribing supplements is not recommended, as excessive intake can also have adverse effects.
Frequently asked questions
What does the BTD gene do?
The BTD gene provides instructions for making the biotinidase enzyme, which is essential for recycling the B vitamin biotin. This recycling process ensures biotin is available for other enzymes involved in breaking down fats, proteins, and carbohydrates.
What is biotinidase deficiency?
Biotinidase deficiency is an inherited metabolic disorder caused by pathogenic variants in the BTD gene. It impairs the body's ability to recycle biotin, leading to insufficient biotin for crucial metabolic functions, which can result in developmental and neurological issues if untreated.
How is biotinidase deficiency inherited?
Biotinidase deficiency is inherited in an autosomal recessive pattern. This means an individual must inherit two copies of a pathogenic BTD variant, one from each parent, to develop the condition.
Can diet affect biotinidase deficiency?
For individuals with biotinidase deficiency, diet alone cannot manage the condition. While biotin is obtained from food, the core issue is the body's inability to recycle it. Treatment typically involves medical-grade biotin supplementation prescribed by a healthcare provider.
Is the BTD gene included in UK genetic testing panels?
Yes, the BTD gene is included in several NHS Genomic Medicine Service clinical panels in the UK, including those for early-onset epilepsy, intellectual disability, and various inherited metabolic disorders. This indicates its importance in clinical diagnostics.
What is the function of the CN hydrolase domain in the biotinidase enzyme?
The CN hydrolase domain, located between amino acids 52 and 331 of the biotinidase enzyme, is the key region responsible for its catalytic activity. It enables the enzyme to hydrolyse amide bonds, which is crucial for releasing free biotin from biocytin and other biotin-bound proteins.
References
- Wolf B. Biotinidase deficiency: "if you have to have an inherited metabolic disease, this is the one to have". Genetics in medicine : official journal of the American College of Medical Genetics. 2012. PMID: 22241090
- Wolf B, Jensen KP, Barshop B. Biotinidase deficiency: novel mutations and their biochemical and clinical correlates. Human mutation. 2005. PMID: 15776412
- Wolf B. Biotinidase: its role in biotinidase deficiency and biotin metabolism. The Journal of nutritional biochemistry. 2005. PMID: 15992688
- Dobrowolski SF, Angeletti J, Banas RA. Real time PCR assays to detect common mutations in the biotinidase gene and application of mutational analysis to newborn screening for biotinidase deficiency. Molecular genetics and metabolism. 2003. PMID: 12618081
- McMahon RJ. Biotin in metabolism and molecular biology. Annual review of nutrition. 2002. PMID: 12055344
- Hymes J, Stanley CM, Wolf B. Mutations in BTD causing biotinidase deficiency. Human mutation. 2001. PMID: 11668630