Ornithine Transcarbamylase Deficiency: OTC Gene Deletion/Duplication

Ornithine Transcarbamylase (OTC) deficiency is the most common inherited urea cycle disorder, and is transmitted in an X-linked pattern [1]. The clinical phenotype in affected males as well as heterozygous females shows a spectrum of severity ranging from neonatal hyperammonemic coma to asymptomatic adults. Clinical presentation is complex because male hemizygotes usually present in infancy, while female heterozygotes may be asymptomatic or develop usually milder disease due to skewed X-inactivation[2].

OTC deficiency results in the accumulation of ammonia and other precursor metabolites during the first few days of life. Because no effective secondary clearance system for ammonia exists, disruption of the urea cycle results in a rapid development of catabolism which may cause cerebral edema, lethargy, anorexia, hyper-/hypoventilation, hypothermia, seizures, neurologic posturing, coma and death, if untreated. Pharmacologic management with sodium benzoate/phenylacetate (Buphenyl) and protein restriction in diet may prevent or alleviate primary complications [3].

OTC deficiency involves an impairment of the reaction that leads to condensation of carbamyl phosphate and ornithine to form citrulline [1]. This impairment leads to reduced ammonia incorporation, which causes symptomatic hyperammonemia and increased urinary excretion of orotic acid. The OTC enzyme is encoded by the OTC gene (Xp21) which is normally expressed in the liver. Heterogeneous mutations have been reported in the OTC gene in individuals with OTC deficiency [4-5]. There is also some evidence for genotype-phenotype correlation [6]. Gene sequence analysis is available to test for mutations in the OTC gene (HU).

References:
1. Brusilow and Horwich. Urea Cycle Enzymes, in: C.R. Scriver, A.L. Beaudet, W. Sly, D. Valle (Eds.), The Metabolic and Molecular Bases of Inherited Disease, McGraw-Hill, New York, 2001, pp. 1916-1925.
2. Smith W. et al. Urea Cycle Disorders: Clinical Presentation Outside the Newborn Period Crit Care Clin 2005, 21:S9-S17.
3. Singh R.H. et al. Nutritional Management of Urea Cycle Disorders. Crit Care Clin 2005, 21:S27-S35.
4. Yamaguchi et al. Mutations and Polymorphisms in the Human Ornithine Transcarbamylase (OTC) Gene. Hum Mutat 2006, 27(7),626-632.
5. Arranz et al. Estimation of the total number of disease-causing mutations in ornithine transcarbamylase (OTC) deficiency. Value of the OTC structure in predicting a mutation pathogenic potential. J Inherit Metab Dis 2007, 30:217-226.
6. McCullough et al. Genotype spectrum of ornithine transcarbamylase deficiency: correlation with the clinical and biochemical phenotype. Am J Med Genet 2000, 93: 313-319.
7. Brusilow and Maestri. Urea cycle disorders: diagnosis, pathophysiology and therapy. Adv Pediatr 1996, 43:127-170.

• Confirmation of a clinical/biochemical diagnosis of OTC deficiency
• Carrier testing in adults with a family history of OTC deficiency

• Plasma Amino Acid (AA) Analysis, Urine Organic Acids (OA) including urine orotic acid (OT), and plasma ammonia levels are used in the diagnosis of a patient with OTC deficiency
• Custom Diagnostic Mutation Analysis (KM) is available to family members if mutations are identified by sequencing.
• Prenatal testing is available for known familial mutations only. Please call the Laboratory Genetic Counselor before collecting a fetal sample.

Requisition Forms

• Comprehensive Molecular Test Requisition Form
• Metabolic/Newborn Screening Follow-up Test Requisition Form