Urea cycle disorders (UCD) are a group of rare inherited metabolism disorders. Infants and children with UCD commonly experience episodes of vomiting, lethargy, and coma. The purpose of this study is to perform a long-term analysis of a large group of individuals with various UCDs. The study will focus on the natural history, disease progression, treatment, and outcome of individuals with UCD.

Urea cycle disorders are a group of rare genetic diseases that affect how protein is broken down in the body. UCDs are caused by a deficiency in one of six enzymes or two mitochondrial membrane transporters responsible for removing ammonia, a waste product of protein metabolism, from the bloodstream. Normally, ammonia is converted into urea and then removed from the body in the form of urine. In UCDs, however, ammonia accumulates unchecked and is not removed from the body. It then reaches the brain through the blood, where it causes irreversible brain damage and/or death.

All UCDs, except for one (ornithine transcarbamylase deficiency), are inherited as recessive traits. The purpose of this study is to perform a long-term analysis of a large group of individuals with various UCDs. Biochemical status, growth, and cognitive function will be assessed. Survival and cognitive outcome of the two most commonly used forms of treatment, alternate pathway therapy and transplantation, will be evaluated. In addition, this study will identify the biochemical changes that may predict future metabolic imbalances so that they may be corrected before clinical symptoms develop.

This observational study is funded through 2025. All participants will attend an initial study visit, which will include a medical and diet history, physical and neurological examinations, psychological testing, and blood tests. Participants will then be followed with subsequent study visits, which will last 2-3 hours each. Individuals with neonatal onset UCD will be assessed every 3 months until age 2 and every 6 months thereafter. Individuals with late onset UCD will be evaluated every 6 months. Psychological testing will take place every 2 years. Psychological testing will take from 30 minutes (for younger children) up to 3 hours, depending on test battery.

Inclusion Criteria:

• Diagnosis of NAGS deficiency, defined as the detection of a pathogenic mutation, and/or decreased (less than 20 % of control) NAGS enzyme activity in liver ,and/or hyperammonemia and first degree relative meets at least one of the criteria for NAGS deficiency
• Diagnosis of CPS I deficiency, defined as decreased (less than 20 % of control) CPS I enzyme activity in liver, and/or an identified pathogenic mutation, and/or hyperammonemia and first degree relative meets at least one of the criteria for CPS I deficiency
• Diagnosis of OTC deficiency, defined as the identification of a pathogenic mutation, and/or less than 20% of control of OTC activity in the liver, and/or elevated urinary orotate (greater than 20 uM/mM) in a random urine sample or after allopurinol challenge test, and/or hyperammonemia and first degree relative meets at least one of the criteria for OTC deficiency
• Diagnosis of AS deficiency (Citrullinemia), defined as a greater than or equal to 10-fold elevation of citrulline in plasma, and/or decreased AS enzyme activity in cultured skin fibroblasts or other appropriate tissue, and/or identification of a pathogenic mutation in the AS gene, and/or hyperammonemia and first degree relative meets at least one of the criteria for AS Deficiency
• Diagnosis of AL deficiency (Argininosuccinic Aciduria, ASA), defined as the presence of argininosuccinic acid in the blood or urine, and/or decreased AL enzyme activity in cultured skin fibroblasts or other appropriate tissue, and/or identification of a pathogenic mutation in the AL gene, and/or hyperammonemia and first degree relative meets at least one of the criteria for AL Deficiency
• Diagnosis of ARG deficiency (Hyperargininemia), defined as a greater than or equal to 5-fold elevated arginine levels in the blood, and/or decreased arginase enzyme levels in red blood cells or other appropriate tissue, and/or identification of a pathogenic mutation in the ARG gene, and/or hyperammonemia and first degree relative meets at least one of the criteria for ARG Deficiency
• Diagnosis of HHH Syndrome or ORNT deficiency, defined as a greater than or equal to 5-fold elevated plasma ornithine and homocitrulline levels in the urine, and/or a pathogenic mutation, and/or less than 20% residual labeled ornithine incorporation into protein in cultured fibroblasts, and/or hyperammonemia and first degree relative meets at least one of the criteria for HHH Syndrome or ORNT Deficiency
• Diagnosis of CITR deficiency (Citrullinemia Type II), defined as elevated citrulline levels in the blood and a pathogenic mutation and/or hyperammonemia and first degree relative meets criteria for CITR Deficiency
• Pending diagnosis of a UCD (UCD highly likely), defined as laboratory values highly suggestive of a UCD with symptomatic hyperammonemic episodes but without a verifiable diagnosis

Exclusion Criteria:

• Hyperammonemia caused by an organic academia, lysinuric protein intolerance, mitochondrial disorder, congenital lactic academia, fatty acid oxidation defects, or primary liver disease
• Rare and unrelated comorbidities (e.g., Down's syndrome, intraventricular hemorrhage in the newborn period, and extreme prematurity)

Enrollment goal: 1,100 participants