OTC deficiency is caused by pathogenic variants in the OTC gene, which encodes the enzyme ornithine transcarbamylase — the second step of the urea cycle, responsible for combining ornithine and carbamoyl phosphate to produce citrulline. The OTC gene is located on the X chromosome (Xp21.1), and OTC deficiency follows X-linked inheritance. The OTC gene–disease relationship has been classified as DEFINITIVE by ClinGen, indicating the strongest level of evidence for causation. Affected hemizygous males with little or no residual enzyme activity typically have the most severe disease. In females, who carry two copies of the X chromosome, one normal and one altered OTC allele, the degree of clinical involvement depends substantially on the pattern of X-chromosome inactivation (lyonization) in liver cells. Females in whom a greater proportion of hepatocytes express the affected allele will have greater enzyme deficiency and more pronounced symptoms. There is no single variant that universally predicts severity; the clinical course within a given variant can vary considerably, particularly in the context of additional metabolic stressors.

Diagnosis of OTC deficiency is established through a combination of biochemical and genetic testing. Biochemical markers include elevated plasma ammonia, elevated plasma glutamine, elevated urinary orotic acid, and low plasma citrulline — a pattern distinguishing OTC deficiency from other urea cycle disorders such as carbamoyl phosphate synthetase I deficiency, which shows similar hyperammonemia but without orotic aciduria. Plasma amino acid analysis and urine organic acid testing are important components of the evaluation. Molecular genetic testing identifying a pathogenic variant in OTC confirms the diagnosis and enables accurate family counseling. In female carriers, biochemical markers may be within normal limits between episodes, making molecular testing especially important. Newborn screening with expanded metabolic panels may identify low citrulline as a signal warranting further evaluation, though OTC deficiency is not uniformly detected through routine newborn screening programs.

Management of OTC deficiency is best coordinated by a metabolic physician or biochemical geneticist working alongside a metabolic dietitian. The goals of treatment are to prevent hyperammonemic crises, minimize ammonia accumulation, and support adequate nutrition for growth and development. The foundational approach involves a protein-restricted diet combined with essential amino acid supplementation to reduce the nitrogen load on the urea cycle, while ensuring that protein restriction does not impair growth or development. Nitrogen scavenger medications — agents that provide alternative pathways for waste nitrogen excretion — are a cornerstone of long-term management. These include sodium benzoate and sodium phenylbutyrate (or its prodrug glycerol phenylbutyrate), which divert ammonia away from the impaired urea cycle.

Several substances and circumstances must be avoided in OTC deficiency. Valproate and haloperidol can precipitate hyperammonemia and should not be used. Prolonged fasting, physical stress, and systemic corticosteroids (which promote catabolism) are also significant precipitants of hyperammonemic crisis and should be managed proactively. Patients and families should have emergency protocols in place for illness management.

Liver transplantation corrects the underlying enzymatic defect and prevents hyperammonemic crises, though it does not reverse pre-existing neurological damage. Gene therapy represents an investigational frontier: several clinical trials are evaluating AAV-based OTC gene delivery and mRNA-lipid nanoparticle approaches, including a Phase 3 study of an AAV-mediated gene transfer (DTX301) in OTC-deficient individuals, a Phase 1 study in male infants with neonatal-onset disease (ECUR-506), and additional gene delivery programs. A Phase 1 clinical trial is also evaluating a small molecule RNA modulator (CMP-CPS-001) in OTC deficiency.

Prognosis in OTC deficiency is closely tied to the severity of initial presentation and the prevention of hyperammonemic episodes. Neonatal-onset males who experience severe, prolonged hyperammonemia before diagnosis or treatment initiation are at high risk for permanent neurological injury, including intellectual disability, learning difficulties, and in the most severe cases, profound developmental impairment. With early diagnosis — including through expanded newborn screening — and prompt metabolic management, outcomes can be substantially improved. Late-onset disease in males and carrier females tends to follow a more variable course; some individuals have few or no episodes with careful dietary management, while others experience recurrent crises despite treatment. The long-term neurological outcome depends heavily on the cumulative burden of hyperammonemic events and the adequacy of metabolic control. Liver transplantation, when performed, generally stabilizes the condition and prevents further ammonia crises, though it is not without procedural risk. Emerging gene therapy approaches aim to provide a more durable correction of the underlying defect.

OTC deficiency has one of the most active gene therapy research pipelines among urea cycle disorders. Multiple ongoing clinical trials — spanning Phase 1 through Phase 3 — are evaluating AAV-mediated gene delivery, mRNA-based therapeutics, and RNA splicing modulators as strategies to restore OTC enzyme function. A Phase 3 study (DTX301, Ultragenyx Pharmaceutical) is underway in adults with OTC deficiency, and a Phase 1 study (ECUR-506, iECURE) is enrolling male infants under 9 months of age with neonatal-onset disease. Additional programs at UCL (HORACE study) and Arcturus Therapeutics (ARCT-810 mRNA therapy) are in Phase 1 and Phase 2, respectively. Long-term follow-up studies are being conducted to assess the durability and safety of gene delivery over time. Published research includes case reports and case series detailing the clinical spectrum of OTC deficiency, with biomarker studies and gene therapy outcome data forming an increasingly prominent component of the literature. These advances hold promise for transforming the management of a condition that currently relies heavily on lifelong dietary restriction and nitrogen scavenger medications.