rs28933978 — OTC R141Q (Arg141Gln)

The most common OTC point mutation, abolishing ornithine transcarbamylase enzyme activity and causing X-linked urea cycle deficiency; hemizygous males face neonatal hyperammonemic crisis while heterozygous females have variable partial deficiency

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OTC R141Q — When the Urea Cycle's Gatekeeper Fails

Every gram of dietary protein you digest produces ammonia — a neurotoxin that must be captured and converted to urea within minutes or it will damage the brain. The enzyme that performs this critical capture step is ornithine transcarbamylase (OTC), which combines ammonia (as carbamoyl phosphate) with ornithine to form citrulline, the first committed step of the urea cycle11 urea cycle
The urea cycle is a five-enzyme sequence in liver cells that converts toxic ammonia into water-soluble urea for urinary excretion. OTC performs the rate-limiting step. OMIM #311250. OTC deficiency is the most common inherited urea cycle disorder, occurring in approximately 1 in 14,000 live births.

The R141Q variant (c.422G>A, NM_000531.6) substitutes glutamine for the arginine at position 141 of the OTC protein. This arginine sits directly in the catalytic active site; its guanidinium side chain makes essential contacts with the carbamoyl phosphate substrate. Replacing it with glutamine's shorter, uncharged amide eliminates all enzymatic activity while leaving the protein structurally intact — the mutant protein folds correctly, is imported into the mitochondrion, and accumulates at normal concentrations, but cannot catalyze the reaction. ClinVar VCV00001098722 ClinVar VCV000010987 classifies it as pathogenic based on multiple submitters with no conflicts.

Because OTC is X-linked, males and females are affected in fundamentally different ways. Males have only one X chromosome, so one mutant allele means zero OTC activity in the liver. Females have two X chromosomes, and random X-inactivation33 X-inactivation
In each cell of a female body, one X chromosome is randomly silenced. Female OTC carriers are therefore mosaics: some liver cells express normal OTC, others express R141Q. The balance between these two populations determines disease severity and can vary dramatically between women in the same family determines what fraction of liver cells retain functional OTC — producing a wide spectrum from fully asymptomatic to severe recurrent crises.

The Mechanism

Arginine 141 contributes two hydrogen bonds to carbamoyl phosphate in the OTC active site. Glutamine's side chain can donate one hydrogen bond but lacks the guanidinium group's ionic character, destroying the precise geometry needed for catalysis. Expression studies by Augustin et al. 200044 Augustin et al. 2000
Augustin L et al. Expression of wild-type and mutant human OTC genes in Chinese hamster ovary cells. Pediatr Res, 2000 confirmed that the R141Q protein accumulates at wild-type levels in transfected cells but shows zero OTC enzyme activity. Critically, when wild-type and R141Q protein were coexpressed together, the mutant showed no dominant negative effect — each R141Q molecule simply contributes nothing to the catalytic pool.

Without OTC activity, carbamoyl phosphate cannot enter the urea cycle and overflows into the pyrimidine synthesis pathway, producing orotic acid. Elevated urinary orotic acid is therefore the biochemical signature of OTC deficiency and is amplified by the allopurinol challenge test — a diagnostic tool that stresses the pyrimidine pathway and unmasks even partial OTC deficiency in female carriers.

The Evidence

In neonatal males with hemizygous R141Q, the clinical course is catastrophic. Infants appear normal at birth but by days 2–3 of life develop rapid ammonia accumulation (typically >200–500 µmol/L, often >1,000 µmol/L in severe cases). Without immediate intervention — nitrogen scavengers, dialysis, protein elimination — cerebral edema and death follow within hours to days. The Tuchman 1998 spectrum review55 Tuchman 1998 spectrum review
Tuchman M et al. The biochemical and molecular spectrum of OTC deficiency. J Inherit Metab Dis, 1998 identifies R141Q as causing "neonatal disease" through active-site disruption, distinguishing it from mutations that produce milder late-onset phenotypes by different mechanisms.

In heterozygous females, the picture is strikingly heterogeneous. Ahrens et al. 199666 Ahrens et al. 1996
Ahrens MJ et al. Clinical and biochemical heterogeneity in females of a large pedigree with OTC deficiency due to the R141Q mutation. Am J Med Genet, 1996 studied 11 females in a large R141Q family and found that 5 of 7 confirmed carriers were symptomatic — a higher rate than traditionally estimated — yet none had elevated plasma ammonia on random testing. Three had markedly elevated plasma glutamine (a sensitive indirect marker of ammonia load); five tested positive for orotic aciduria either spontaneously or following allopurinol challenge. All five symptomatic carriers had remained undiagnosed for years. The Nguyen et al. 202077 Nguyen et al. 2020
Nguyen HH et al. Late-onset OTC deficiency and variable phenotypes in Vietnamese females. Front Pediatr, 2020 series estimated that 15–20% of female carriers present with late-onset symptoms triggered by metabolic stress.

Triggers for acute decompensation in female carriers include: high-protein meals, febrile illness, fasting, surgery, and most critically, the peripartum period. Lamb et al. 201388 Lamb et al. 2013
Lamb S et al. Multidisciplinary management of OTC deficiency in pregnancy. BMJ Case Reports, 2013 reported that pregnancy triggers "potentially fatal hyperammonemic crises" in OTC-deficient women. Successful management through delivery required sodium benzoate 4 g four times daily, sodium phenylbutyrate 2 g four times daily, arginine supplementation, and continuous protein restriction throughout gestation and postpartum.

A Spanish multicenter registry of 104 urea cycle disorder patients (Martín-Hernández et al. 201499 Martín-Hernández et al. 2014
Martín-Hernández E et al. Urea cycle disorders in Spain. Orphanet J Rare Dis, 2014) found that 63% of newly presenting patients experienced hyperammonemic encephalopathy at first crisis, with median peak ammonia of 298 µmol/L; 52.5% subsequently developed neurological sequelae. Early diagnosis — ideally before the first crisis — transforms outcomes.

Practical Actions

For males (hemizygous): Neonatal screening programs (NBS) in many countries include OTC deficiency. Confirmed hemizygous neonates require immediate specialist metabolic care: acute hyperammonemia is managed with IV glucose (to suppress catabolism), IV sodium phenylacetate/benzoate loading, and continuous renal replacement therapy (CVVHD) for severe elevations. Long-term management rests on protein restriction (0.5–1.0 g/kg/day in adults, higher in infants), oral nitrogen scavengers (sodium phenylbutyrate or glycerol phenylbutyrate), and L-arginine supplementation (to replenish the product the cycle cannot make). Liver transplantation, typically before age 6 months in severe neonatal cases, eliminates hyperammonemia risk and allows unrestricted diet — though pre-existing neurological damage is not reversed.

For females (heterozygous carriers): The majority remain asymptomatic under normal conditions but should know their key triggers (protein loading, fasting, fever, surgery, postpartum state). Women with a history of protein intolerance, unexplained migraine-like episodes, episodic confusion, or post-illness encephalopathy should have plasma ammonia and urinary orotic acid measured during or immediately after an episode. The GeneReviews protocol (Lichter-Konecki et al. 20221010 Lichter-Konecki et al. 2022
GeneReviews OTC Deficiency, 2013/2022) recommends metabolic specialist involvement at first diagnosis regardless of current symptom status.

Interactions

OTC deficiency exists within the broader urea cycle pathway. Reduced ornithine availability from lysinuric protein intolerance (SLC7A7 deficiency) can compound OTC dysfunction since ornithine is the substrate that OTC normally processes. In heterozygous females, X-inactivation pattern is the primary modifier of clinical severity — skewed inactivation toward the R141Q-bearing chromosome produces more severe disease, and this is not detectable from standard genotype data alone. There are no documented pharmacogenomic drug interactions specific to OTC R141Q; however, valproate must be strictly avoided in OTC-deficient patients of both sexes, as it inhibits residual OTC activity and precipitates life-threatening hyperammonemia even at therapeutic doses.

Nutrient Interactions

protein altered_metabolism
arginine increased_need
citrulline increased_need

Genotype Interpretations

What each possible genotype means for this variant.

GG Normal / Non-carrier Normal

No OTC R141Q variant — full ornithine transcarbamylase function

You carry two copies of the reference G allele at this position. In females, this means you are not a carrier of the R141Q OTC variant. In males, the single G allele indicates a functional OTC enzyme at this position. Your urea cycle is not impaired by this variant, and you are not at elevated risk for hyperammonemia from this specific mutation.

Note that OTC has many other pathogenic variants beyond R141Q; a negative result here does not exclude other OTC mutations if clinical suspicion exists. However, for this specific well-studied variant, your result is reassuring.

AA Affected (Hemizygous Male / Rare Homozygous Female) High Risk

OTC R141Q — severely impaired urea cycle; requires specialist metabolic care

R141Q abolishes OTC enzymatic activity by destroying the active-site geometry required for carbamoyl phosphate binding, as confirmed in expression studies (Augustin et al. 2000). The mutant protein is produced, processed, and imported into mitochondria normally — so there is no protein degradation signal to amplify; the cell simply contains OTC protein that does nothing.

Neonatal presentation: Affected males are symptom-free at birth (maternal urea cycle clears prenatal ammonia) but develop hyperammonemia by day 2–3 as feeding begins. Presenting symptoms: poor feeding, hypotonia, hypothermia, lethargy progressing to somnolence and coma, with hyperventilation from respiratory compensation. Plasma ammonia typically exceeds 200 µmol/L at presentation and may reach 1,000–3,000 µmol/L. The biochemical signature is elevated plasma glutamine, low citrulline and arginine, markedly elevated urinary orotic acid.

Acute management: Stop all protein intake immediately. Provide IV glucose (8–12 mg/kg/min in neonates) to suppress catabolism. Begin IV sodium phenylacetate/benzoate (Ammonul) as a loading dose over 90–120 minutes, followed by maintenance infusion. For ammonia >500 µmol/L or rapid neurological deterioration, continuous renal replacement therapy (CVVHD) is the most efficient ammonia-clearance method and should be initiated without delay. IV arginine (250 mg/kg loading dose) provides the product the non-functional OTC step cannot produce. Target ammonia to ≤200 µmol/L within 8 hours, ≤80 µmol/L within 24 hours.

Long-term management: Dietary protein restriction (0.5–1.0 g/kg/day in adults; 1.2–2.2 g/kg/day in infants to support growth). Essential amino acid formulas supplement protein quality without excess nitrogen load. Oral nitrogen scavengers: sodium phenylbutyrate 450–600 mg/kg/day (children <25 kg) or glycerol phenylbutyrate equivalent. L-arginine or L-citrulline supplementation replenishes urea cycle intermediates downstream of the block. Branched-chain amino acids (BCAA) may need supplementation because phenylbutyrate depletes plasma BCAAs and low BCAAs predict metabolic crisis.

Neurological prognosis: The duration and severity of the neonatal hyperammonemic episode strongly predicts long-term cognitive outcome. Ammonia exposure exceeding 24–48 hours above 200 µmol/L is associated with permanent brain injury. Neonates rescued within the first 24 hours of crisis onset have better developmental trajectories than those with prolonged elevation.

Liver transplantation: For severe neonatal-onset OTC deficiency, liver transplantation (typically before age 6 months in the current era) eliminates the enzymatic defect, allows unrestricted dietary protein, and removes hyperammonemia risk. Post-transplant neurological outcomes depend on the degree of pre-transplant brain injury. Transplantation does not prevent transmission of the variant to future children.

Valproate and other hazards: Valproate is absolutely contraindicated. Prolonged fasting, high-dose corticosteroids, and haloperidol should be avoided. Any illness causing protein catabolism (sepsis, surgery, burns, prolonged vomiting) requires acute metabolic monitoring and may require temporary increase in nitrogen scavenger therapy.

AG Heterozygous Carrier (Female) Carrier

Carrier of the OTC R141Q variant — partial deficiency risk with key triggers to avoid

Female OTC carriers occupy a wide clinical spectrum that is impossible to predict from genotype alone. X-inactivation determines what fraction of liver OTC activity is retained. Women in the same family carrying the same R141Q mutation can range from completely asymptomatic (normal protein tolerance) to recurrent encephalopathic episodes requiring hospitalization.

Biochemical markers in carriers: Random plasma ammonia is almost always normal. More sensitive markers include: plasma glutamine (elevated in 3/7 symptomatic carriers in the Ahrens 1996 study), urinary orotic acid (elevated in 5/7 R141Q carriers), and the allopurinol challenge test (which stresses the pyrimidine pathway and unmasks partial OTC deficiency in carriers with normal routine labs).

The peripartum danger: Pregnancy increases protein catabolism and metabolic demands on the urea cycle. The postpartum period is particularly high-risk because catabolism spikes sharply after delivery. Women with OTC carrier status who are planning pregnancy need specialist metabolic involvement before conception. Successful managed deliveries have been reported using continuous nitrogen scavengers and protein restriction, but unplanned decompensation postpartum can be fatal if the carrier status is unknown.

Drug warning: Valproate (valproic acid, used for epilepsy and bipolar disorder) is contraindicated in OTC-deficient individuals including symptomatic female carriers. Valproate inhibits residual OTC activity and has caused fatal hyperammonemia in heterozygous OTC females who appeared clinically normal before exposure. Haloperidol and systemic corticosteroids are also listed as agents to avoid in OTC deficiency management guidelines.