Deficiency in carbomoyl phosphate synthetase 1 leading to Hyperammonemia

Carbamoyl Phosphate Synthetase I Deficiency: Causes, Pathophysiology, and Hyperammonaemia Carbamoyl phosphate synthetase I (CPS1) deficiency is a rare inherited metabolic disorder that causes a severe defect in the urea cycle, resulting in the accumulation of ammonia in the blood, a condition known as hyperammonaemia. The urea cycle is essential for converting toxic ammonia, produced during protein metabolism, into urea, which is then excreted by the kidneys. Causes and Genetics CPS1 deficiency is caused by pathogenic variants in the CPS1 gene and is inherited in an autosomal recessive pattern. These genetic changes reduce or eliminate the activity of carbamoyl phosphate synthetase I, a mitochondrial enzyme found primarily in liver cells. Normal Function of CPS1 Under normal conditions, CPS1 catalyses the first committed step of the urea cycle. It combines ammonia, bicarbonate, and ATP to produce carbamoyl phosphate. This reaction requires N-acetylglutamate as an essential activator. Carbamoyl phosphate subsequently combines with ornithine to form citrulline, allowing nitrogen to proceed through the urea cycle for eventual conversion into urea. Pathophysiology of Hyperammonaemia When CPS1 activity is deficient, the liver cannot efficiently incorporate ammonia into the urea cycle. Consequently, ammonia accumulates in the bloodstream and may cross the blood-brain barrier. In the brain, ammonia is incorporated into glutamine within astrocytes. Excessive glutamine accumulation increases osmotic stress, which can cause astrocyte swelling and cerebral oedema. Ammonia also disrupts neurotransmission and cellular energy metabolism, potentially resulting in severe neurological injury. Unlike ornithine transcarbamylase deficiency, CPS1 deficiency typically does not cause increased urinary orotic acid because insufficient carbamoyl phosphate is produced for excess diversion into pyrimidine synthesis. Clinical Features Severe cases may present in newborns with: Poor feeding and vomiting. Lethargy and irritability. Rapid breathing or respiratory alkalosis. Reduced muscle tone. Seizures and altered consciousness. Cerebral oedema and coma in severe cases. Partial enzyme deficiency may cause later-onset episodes, particularly during infections, fasting, surgery, or excessive protein intake. Diagnosis Diagnosis involves measuring plasma ammonia, plasma amino acids, urine orotic acid, and other metabolic markers. Low citrulline and low or normal urinary orotic acid may support CPS1 deficiency. Genetic testing can confirm the diagnosis. Markedly elevated ammonia in a newborn is a medical emergency. Treatment and Management Acute hyperammonaemia requires urgent specialist treatment. Management may include temporarily stopping protein intake under medical supervision, providing adequate intravenous calories, administering nitrogen-scavenging medicines, and using haemodialysis or another extracorporeal technique when rapid ammonia removal is necessary. Long-term management may involve controlled protein intake, essential amino acid supplementation, nitrogen scavengers, and specialist metabolic monitoring. Complications Untreated hyperammonaemia can cause cerebral oedema, permanent neurological damage, developmental impairment, seizures, coma, and death. CPS1 deficiency is an important topic in medical biochemistry and human physiology because it demonstrates the essential role of the urea cycle in ammonia detoxification and nitrogen balance. #CPS1Deficiency #Hyperammonaemia #UreaCycle #CarbamoylPhosphateSynthetase #AmmoniaMetabolism #MedicalBiochemistry #HumanPhysiology #MetabolicDisorders #GeneticDiseases #MedicalEducation