rs72552297 — OTC Asn10fs (c.29_32del)

OTC Asn10fs — A Frameshift Deletion That Silences the Urea Cycle's Rate-Limiting Enzyme

Every gram of dietary protein that you digest releases nitrogen — and nitrogen, left to accumulate, becomes ammonia, one of the brain's most potent toxins. The liver normally converts this ammonia into harmless urea through the urea cycle, a six-step reaction whose gatekeeper is ornithine transcarbamylase (OTC). Nestled in the inner mitochondrial matrix of liver cells, OTC catalyses the condensation of carbamoyl phosphate (the ammonia-derived intermediate) with ornithine to form citrulline — the committed step that keeps nitrogen flowing safely out of the body.

rs72552297 is a 4-base-pair deletion in exon 1 of OTC (c.29_32del), arising early in the protein-coding sequence at codon 10. The deletion shifts the reading frame immediately after the leader peptide region, producing a garbled amino acid sequence that is rapidly degraded by cellular quality-control machinery. The result: no functional OTC protein. OTC deficiency is the most common urea cycle disorder¹¹ OTC deficiency is the most common urea cycle disorder
Estimated at 1 in 62,000 to 1 in 77,000 live births; NCBI Genetics Review 2023, and because OTC is encoded on the X chromosome, the clinical picture splits sharply by sex.

The Mechanism

The OTC gene sits on Xp11.4 and encodes a 354-amino-acid precursor that is imported into the mitochondrial matrix and cleaved to produce the mature enzyme. The mature enzyme assembles as a homotrimer with three active sites at subunit interfaces. The c.29_32del frameshift strikes within the first 10 codons of the mature enzyme sequence — p.Asn10fs²² p.Asn10fs
Asparagine at position 10 of the mature protein is replaced by an aberrant reading frame that terminates prematurely — long before any of the catalytic residues (Arg141, His168) or ornithine-binding loops are synthesized. The truncated peptide cannot fold into a functional trimer. With no OTC activity, carbamoyl phosphate accumulates in mitochondria and spills into the cytoplasm, where it is shunted to orotic acid production — explaining the diagnostic hallmark of OTC deficiency: markedly elevated urinary orotic acid.

Sex-linked severity: Because OTC is X-linked, hemizygous males have no backup copy. When the deletion allele is the only OTC gene present, ammonia cannot be metabolized at all. Heterozygous females carry one normal X chromosome that can sustain partial OTC activity through the liver cells that preferentially express the wild-type allele (a process called lyonization³³ lyonization
Random inactivation of one X chromosome per cell during embryogenesis. In liver, the proportion of cells expressing the normal OTC allele determines residual enzyme activity and clinical severity.). Females with skewed inactivation favouring the deletion-carrying X can be severely symptomatic; those with favourable inactivation may remain asymptomatic for decades.

The Evidence

Complete loss-of-function variants (frameshifts, nonsense mutations, splice-site disruptions) are among the 149 mutations associated exclusively with neonatal-onset hyperammonemia⁴⁴ 149 mutations associated exclusively with neonatal-onset hyperammonemia
McCullough et al. 2000, Am J Med Genet; analysis of 157 OTC families showing 60% of mutations correlate with neonatal-onset disease in hemizygous males. Neonatal OTC deficiency follows a predictable trajectory: normal appearance at birth, then progressive lethargy, poor feeding, and hyperventilation within the first 2–5 days of life as protein catabolism from feeds drives ammonia accumulation. Without rapid intervention⁵⁵ Without rapid intervention
ammonia >200 μmol/L causes cerebral oedema and irreversible neurological injury; >300 μmol/L without treatment is frequently fatal. Plasma ammonia in untreated neonatal OTC crisis can exceed 1,000 μmol/L (normal: <50 μmol/L).

In heterozygous females, the clinical range is broad. Females may first present in adulthood with protein avoidance, recurrent vomiting, or psychiatric symptoms masking the underlying metabolic disorder. Kido et al. 2022⁶⁶ Kido et al. 2022
Front Genet, nationwide Japanese study of 523 OTC variants including 55 frameshift mutations confirmed that the degree of enzyme deactivation correlates strongly with specific OTC variants, and that skewed lyonization — the variable X-inactivation pattern in individual liver cells — explains why some heterozygous females are profoundly symptomatic while others are discovered only through family cascade testing. Peng et al. 2020⁷⁷ Peng et al. 2020
Clin Biochem, 35-patient cohort documented that six hemizygous males died at disease onset, while female patients showed lower ammonia but persistent biochemical abnormalities (glutamine accumulation, citrulline and carnitine depletion across all groups).

Long-term management with oral nitrogen scavengers can normalize biochemistry in affected females: Andrews et al. 2022⁸⁸ Andrews et al. 2022
Mol Genet Metab Reports described a 19-month-old female carrier whose ammonia and glutamine normalized within 1 month of starting oral nitrogen scavenger + citrulline + protein restriction, with near-complete resolution of MRI white matter changes at 3 months.

Practical Actions

The metabolic management of OTC deficiency rests on three pillars: reducing nitrogen load (protein restriction), redirecting waste nitrogen out of the body (nitrogen scavengers), and replenishing the urea cycle intermediates lost when citrulline production is blocked (citrulline or arginine supplementation).

Nitrogen scavengers work by providing an alternative nitrogen excretion pathway: sodium phenylbutyrate (or its prodrug glycerol phenylbutyrate) is converted to phenylacetate, which conjugates glutamine (the primary ammonia store) for renal excretion. Sodium benzoate conjugates glycine. These drugs effectively create a secondary route to dispose of nitrogen that would otherwise become ammonia. Standard oral dosing is 450–600 mg/kg/day sodium phenylbutyrate plus 170 mg/kg/day L-citrulline for children under 25 kg, titrated by plasma ammonia and glutamine levels. For acute hyperammonemic crisis, intravenous sodium phenylacetate + sodium benzoate (Ammonul) is given with high-calorie IV nutrition to suppress catabolism.

Valproate, haloperidol, and systemic corticosteroids are contraindicated in OTC deficiency: valproate inhibits residual urea cycle function directly, while corticosteroids promote protein catabolism and can trigger hyperammonemic crises. Any surgery, infection, or prolonged fast in a known OTC patient requires pre-emptive metabolic stabilization.

Liver transplantation corrects the enzymatic defect and is performed in severe cases, typically by age 6 months in affected males who survive the neonatal period with good neurological baseline.

Interactions

OTC deficiency is a single-gene disorder; disease severity is determined by the specific OTC mutation combined with X-inactivation pattern in females. In heterozygous females, the proportion of hepatocytes expressing the normal vs deletion allele (i.e., the X-inactivation pattern) is the dominant modifier of clinical expression — this is not a separate gene variant but a stochastic epigenetic event. Metabolic stressors (infection, surgery, pregnancy, high-protein intake) that increase protein catabolism acutely unmask latent OTC deficiency in heterozygous females who are otherwise asymptomatic. Pregnancy and the postpartum period are recognized high-risk windows for first presentation or decompensation in carrier females.