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OTC
ornithine transcarbamylase
The *OTC* gene provides instructions for making ornithine transcarbamylase, an enzyme vital for the body's urea cycle, which processes excess nitrogen and prevents ammonia accumulation. The *OTC* gene encodes the ornithine transcarbamylase enzyme, a key component of the urea cycle.
OTC is located on the short (p) arm of chromosome X, at band Xp11.4. Arm ratio per GRCh38 - banding schematic.
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Overview
The *OTC* gene, or ornithine transcarbamylase gene, is fundamental for human metabolic health. It contains the genetic blueprint for producing the ornithine transcarbamylase enzyme. This enzyme plays a critical role within the urea cycle, a biochemical pathway primarily active in the liver.
Within hepatic tissue, this cycle breaks down nitrogenous compounds that arise during amino acid catabolism. By converting toxic ammonia into less harmful urea, the *OTC* gene and its encoded enzyme are crucial for preventing severe metabolic complications.
What the gene does
The ornithine transcarbamylase enzyme, specified by the *OTC* gene, is an integral part of the urea cycle. This metabolic pathway operates within liver cells and is responsible for detoxifying ammonia, a byproduct of protein metabolism. Ammonia is particularly harmful to the nervous system, and its accumulation can have serious health consequences.
The enzyme's catalytic activity produces citrulline from its substrates ornithine and carbamoyl phosphate. This step is a key intermediate in the urea cycle, ensuring the efficient conversion of nitrogenous waste into urea. Urea is then excreted by the kidneys, thus preventing the build-up of toxic ammonia in the bloodstream. Proper functioning of this enzyme is therefore essential for maintaining metabolic balance and neurological health.
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Chromosome location
The *OTC* gene is located on the short arm of the X chromosome, specifically at position Xp11.4. The gene's coding sequence provides instructions for a protein that is 354 amino acids long. As an X-linked gene, its inheritance pattern differs between males and females.
Protein structure
The protein produced from the *OTC* gene consists of 354 amino acids [UniProt:P00480]. It functions as part of a mitochondrial enzyme system within the urea cycle. The enzyme facilitates a crucial step in nitrogen metabolism.
Key variants
Variants within the *OTC* gene can alter the function or production of the ornithine transcarbamylase enzyme. Several hundred pathogenic variants across the *OTC* coding sequence have been catalogued in clinical databases. These changes can range from single base-pair alterations to larger deletions or insertions. Such variants typically impact the enzyme's ability to participate effectively in the urea cycle. The severity of the resulting condition often depends on the specific variant and its effect on enzyme activity.
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.1019C>T | p.Ser340Phe | Pathogenic/Likely pathogenic | ★★☆☆ | Ornithine carbamoyltransferase deficiency |
c.-106C>A | - | Pathogenic | ★★☆☆ | OTC-related disorder |
c.118C>T | p.Arg40Cys | Pathogenic/Likely pathogenic | ★★☆☆ | Ornithine carbamoyltransferase deficiency |
c.119G>A | p.Arg40His | Pathogenic/Likely pathogenic | ★★☆☆ | Ornithine carbamoyltransferase deficiency |
c.158T>C | p.Ile53Thr | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.205C>T | p.Gln69Ter | Pathogenic | ★★☆☆ | not provided |
c.238A>G | p.Lys80Glu | Pathogenic | ★★☆☆ | not provided |
c.241T>C | p.Ser81Pro | Pathogenic/Likely pathogenic | ★★☆☆ | Ornithine carbamoyltransferase deficiency |
c.274C>T | p.Arg92Ter | Pathogenic | ★★☆☆ | not provided |
c.275G>A | p.Arg92Gln | Pathogenic | ★★☆☆ | not provided |
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 *OTC* gene are primarily associated with Ornithine transcarbamylase deficiency. This inherited disorder leads to an accumulation of ammonia in the blood, which is toxic, particularly to the nervous system. The condition is inherited in an X-linked recessive manner, meaning it predominantly affects males.
Inheritance pattern
Conditions caused by pathogenic OTC variants typically follow x-linked inheritance.
X-linked recessive: sons of a carrier mother have a 50% chance of being affected. Daughters have a 50% chance of being carriers.
UK clinical status
The *OTC* gene is recognised within the UK's genomic medicine framework, featuring on several NHS Genomic Medicine Service national panels. It is listed as 'green' on the DDG2P, Hyperammonaemia, Intellectual disability, Likely inborn error of metabolism, and Undiagnosed metabolic disorders panels, indicating that there is strong evidence for its association with these conditions and it is routinely assessed in diagnostic testing.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Maintaining a healthy lifestyle, including a balanced diet, is generally beneficial for overall metabolic health. For individuals with conditions affecting ammonia metabolism, dietary protein intake often requires careful management under medical supervision. General advice includes staying hydrated and avoiding excessive protein consumption without professional guidance, as this could impact the metabolic pathways involved in nitrogen excretion.
Supplement considerations
There is no conclusive evidence that specific supplements can prevent or cure conditions associated with *OTC* gene variants. Any dietary or nutritional supplements should only be taken after consultation with a healthcare professional, especially for individuals with diagnosed metabolic disorders, due to potential interactions or adverse effects on ammonia levels.
Frequently asked questions
What is the primary role of the *OTC* gene?
The *OTC* gene provides instructions for the ornithine transcarbamylase enzyme, which is essential for the urea cycle. This cycle helps convert toxic ammonia into urea, a less harmful substance that can be excreted from the body.
What happens if the *OTC* gene doesn't function correctly?
If the *OTC* gene is not functioning properly, it can lead to Ornithine transcarbamylase deficiency. This condition results in the build-up of ammonia in the blood, which can be toxic, particularly to the brain and nervous system.
How is Ornithine transcarbamylase deficiency inherited?
Ornithine transcarbamylase deficiency is an X-linked inherited disorder. This means that the gene is located on the X chromosome, and the condition typically affects males more severely than females.
Is the *OTC* gene routinely checked in the UK?
Yes, the *OTC* gene is included in several NHS Genomic Medicine Service national panels, such as those for hyperammonaemia and undiagnosed metabolic disorders, indicating its clinical significance for diagnosis in the UK.
Can diet help manage conditions related to the *OTC* gene?
For individuals with *OTC*-related conditions, dietary management, particularly controlling protein intake, is often a crucial part of treatment. However, any dietary changes should be made under the strict guidance of a healthcare professional.
References
- Brassier A, Gobin S, Arnoux JB. Long-term outcomes in Ornithine Transcarbamylase deficiency: a series of 90 patients. Orphanet journal of rare diseases. 2015. PMID: 25958381
- Choi JH, Lee BH, Kim JH. Clinical outcomes and the mutation spectrum of the OTC gene in patients with ornithine transcarbamylase deficiency. Journal of human genetics. 2015. PMID: 25994866
- Caldovic L, Abdikarim I, Narain S. Genotype-Phenotype Correlations in Ornithine Transcarbamylase Deficiency: A Mutation Update. Journal of genetics and genomics = Yi chuan xue bao. 2015. PMID: 26059767
- Helman G, Pacheco-Colón I, Gropman AL. The urea cycle disorders. Seminars in neurology. 2014. PMID: 25192511
- Adam MP, Bick S, Mirzaa GM. Urea Cycle Disorders Overview. 1993. PMID: 20301396