OTC<unk>  is caused by mutations in the OTC gene, which is located on the X chromosome. OTC codes for the mitochondrial enzyme<unk><unk> , which is expressed only in liver. The functional enzyme consists of three identical subunits. OTC is the last enzyme in the proximal portion of the urea cycle, which consists of the reactions that take place in the mitochondria. The substrates of the reaction catalyzed by<unk><unk>  are<unk>  and carbamyl phosphate, while the product is citrulline. There are no common mutations that cause disease, however 10 - 15% of disease causing mutations are deletions. It is inherited in an X-linked recessive manner, meaning males are more commonly affected than females. Females who carry a defective copy of the gene can be severely affected or asymptomatic, largely depending on the random nature of X-inactivation. There is some degree of genotype - phenotype correlation with OTC<unk> , but this depends on a number of situations. Individuals with milder mutations, often associated with late onset disease can still present with severe illness when exposed to sufficient metabolic stress. Correlations are more difficult to ascertain in females, since the residual activity of OTC in the liver is impacted not only by the nature of the mutation, but also by the random pattern of X-inactivation. OTC<unk>  is estimated to be the most common urea cycle disorder. An exact incidence is difficult to calculate, due to the varying clinical presentations of later onset forms of the disease. Early estimates of the incidence were as high as 1:14,000 live births, however later studies have decreased these estimates to approximately 1:60,000 - 1:72,000.
Ornithine transcarbamylase deficiency
Ornithine aminotransferase deficiency
Arginine:glycine amidinotransferase deficiency
Glutaminase deficiency
3-Methylcrotonyl-CoA carboxylase deficiency
Galactose-1-phosphate uridylyltransferase deficiency
Malonyl-CoA decarboxylase deficiency
2-Methylbutyryl-CoA dehydrogenase deficiency