PROJECT SUMMARY Ornithine transcarbamylase (OTC) deficiency and N-acetylglutamate synthetase (NAGS) deficiency are inborn errors of metabolism that are caused by genetic defects in the urea cycle genes OTC and NAGS. Genetic defects in these genes can cause hyperammonemia, brain edema, and neurological injury that ranges from mild executive functioning deficits to profound intellectual and developmental disabilities and even death. OTC deficiency is the most common and the only X-linked urea cycle disorder (UCD) whereas NAGS deficiency is the only UCD that can be effectively treated with drug monotherapy. Because of the broad spectrum of neuropsychological sequelae associated with OTC and NAGS deficiencies, understanding their molecular basis and improving their early diagnosis are among NICHD high research priorities. OTC and NAGS deficiencies are most often diagnosed by DNA testing because biochemical testing may not always sufficiently differentiate between different UCDs. DNA sequence variants identified in these genes are not always deleterious, and the current ACMG (American College of Medical Genetics and Genomics) and AMP (Association for Molecular Pathology) standardized framework for the interpretation and classification of DNA variants relies on published or publicly available computational, functional, segregation, population, allelic and clinical data in order to classify a variant. Because such publicly available data is limited, the majority of known sequence variants in the urea cycle genes OTC and NAGS would currently be classified as variants of uncertain significance. Correct classification of such variants is necessary 1) to accurately diagnose and treat these disorders 2) for reproductive counseling of affected patients and their relatives and 3) to inform the design of diagnostic eligibility criteria for clinical trials of these disorders. To permit the correct assignment of pathogenicity of known OTC and NAGS variants by clinical diagnostic laboratories and by the NIH-supported ClinGen UCD Variant Curation Expert Panel, this project seeks to leverage the combined data from 3 sources: 1) the largest natural history study of UCDs conducted by the NICHD-funded UCD Consortium, 2) the largest US clinical UCD expert center, Children’s National Hospital and 3) the largest private OTC and NAGS dataset from the legacy Tuchman lab. The Tuchman lab was an academic laboratory that prior to 2012 served as the US reference center for clinical OTC and NAGS sequencing. The legacy database of this lab contains over 20 years of variant and unpublished clinical data from US patients with OTC and NAGS and their relatives. We propose to combine and disambiguate anonymized data from these 3 complementary datasets, and publish the data that fulfill segregation, population, allelic and clinical variant curation criteria. For each OTC and NAGS variant, we intend to submit these data into ClinVar, a publicly accessible database that aggregates inform...