Urea Cycle Enzyme Defects Research Articles

Diffusion Tensor Imaging Parameters in Children with Acute Hyperammonemic Encephalopathy due to Urea Cycle Enzyme Defects and Organic Acidemia

Diffusion Tensor Imaging Parameters in Children with Acute Hyperammonemic Encephalopathy due to Urea Cycle Enzyme Defects and Organic Acidemia

Encéphalopathie hyperammoniémique au valproate de sodium

Valproate is a drug commonly prescribed in neurology and psychiatry. Hyperammonemic encephalopathy due to valproate is a severe complication. A 44-year-old man with a past medical history of partial symptomatic epilepsy and alcoholic cirrhosis was admitted for drowsiness. He was receiving valproate and gabapentine. A valproate-induced hyperammonemic encephalopathy was diagnosed on physical examination, generalized slow waves on electroencephalogram and hyperammonemia. Valproate withdrawal led to a progressive recovery of the consciousness, with a rapid normalisation of electroencephalogram and ammonium level. The pathogenesis of this encephalopathy is not clearly established. No correlation has been shown between the severity of encephalopathy, the plasma ammonium level, the valproate dose and its plasma concentration. Additional factors have been pointed out, such as carnitine deficiency or urea cycle enzyme defects. Furthermore, our case suggests an enhancing role of the liver disease in this encephalopathy.

Evaluation of endogenous nitric oxide synthesis in congenital urea cycle enzyme defects

Nitric oxide (NO) is synthesized from arginine and O 2 by nitric oxide synthase (NOS). Citrulline, which is formed as a by-product of the NOS reaction, can be recycled to arginine by the 2 enzymes acting in the urea cycle: argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL). Although the complete urea cycle is expressed only in the liver, ASS and ASL are expressed in other organs including the kidney and vascular endothelium. To examine possible alterations of the NO pathway in urea cycle defects, we measured plasma concentrations of arginine and citrulline and serum concentrations of nitrite/nitrate (NOx −, stable NO metabolites) and asymmetric dimethylarginine (ADMA, an endogenous NOS inhibitor) in patients with congenital urea cycle disorders of 3 types: ornithine transcarbamylase (OTC) deficiency, ASS deficiency, and ASL deficiency. All were receiving oral arginine replacement at the time of this study. The same parameters were also measured in healthy subjects, who participated as controls. The OTC-deficient patients had significantly high NOx − and nonsignificantly high ADMA concentrations. Their NOx − was significantly positively correlated with arginine. The ASS-deficient patients had significantly low NOx − and significantly high ADMA concentrations. The ASL-deficient patients had normal NOx − and nonsignificantly high ADMA concentrations. In ASS-deficient and ASL-deficient patients, the NOx − was significantly inversely correlated with citrulline. These results suggest that NO synthesis is enhanced in OTC-deficient patients while receiving arginine but that NO synthesis remains low in ASS-deficient patients despite receiving arginine. They also suggest that endogenous NO synthesis is negatively affected by citrulline and ADMA in ASS-deficient and ASL-deficient patients. Although the molecular mechanisms remain poorly understood, we infer that the NO pathway might play a role in the pathophysiology related to congenital urea cycle disorders.

Survival after Treatment with Phenylacetate and Benzoate for Urea-Cycle Disorders

The combination of intravenous sodium phenylacetate and sodium benzoate has been shown to lower plasma ammonium levels and improve survival in small cohorts of patients with historically lethal urea-cycle enzyme defects. We report the results of a 25-year, open-label, uncontrolled study of sodium phenylacetate and sodium benzoate therapy (Ammonul, Ucyclyd Pharma) in 299 patients with urea-cycle disorders in whom there were 1181 episodes of acute hyperammonemia. Overall survival was 84% (250 of 299 patients). Ninety-six percent of the patients survived episodes of hyperammonemia (1132 of 1181 episodes). Patients over 30 days of age were more likely than neonates to survive an episode (98% vs. 73%, P<0.001). Patients 12 or more years of age (93 patients), who had 437 episodes, were more likely than all younger patients to survive (99%, P<0.001). Eighty-one percent of patients who were comatose at admission survived. Patients less than 30 days of age with a peak ammonium level above 1000 micromol per liter (1804 microg per deciliter) were least likely to survive a hyperammonemic episode (38%, P<0.001). Dialysis was also used in 56 neonates during 60% of episodes and in 80 patients 30 days of age or older during 7% of episodes. Prompt recognition of a urea-cycle disorder and treatment with both sodium phenylacetate and sodium benzoate, in conjunction with other therapies, such as intravenous arginine hydrochloride and the provision of adequate calories to prevent catabolism, effectively lower plasma ammonium levels and result in survival in the majority of patients. Hemodialysis may also be needed to control hyperammonemia, especially in neonates and older patients who do not have a response to intravenous sodium phenylacetate and sodium benzoate.

Nutritional therapy improves growth and protein status of children with a urea cycle enzyme defect

Background Poor growth has been described in patients with urea cycle enzyme defects treated with protein-restricted diets, while protein status is seldom reported. Objective To assess the effects of nutritional therapy with a medical food on growth and protein status of patients with a urea cycle enzyme defect. Methods A 6-mo multicenter outpatient study was conducted with infants and toddlers managed by nutrition therapy with Cyclinex-1 Amino Acid-Modified Medical Food with Iron (Ross Products Division, Abbott Laboratories, Columbus, OH). Main outcome variables were anthropometrics and plasma amino acids (selected), albumin, and transthyretin concentrations. Results Seventeen patients completed the study. Mean (±SE) baseline age was 11.30 ± 3.20 months (median 4.40 months; range 0.22–38.84 months). Length and weight z-scores increased significantly during the 6-month study. Head circumference increased, but not significantly. Three patients were stunted and two were wasted (−2.0 z-score) at baseline while at study end, only one patient was both stunted and wasted. The majority of patients increased in length, head circumference, and weight z-scores during study. Mean (±SE) plasma albumin concentration increased from 34 ± 2 g/L at baseline to 38 ± 1 g/L at study end. Plasma transthyretin increased from a mean (±SE) of 177 ± 13 mg/L at baseline to 231 ± 15 mg/L at study end. No correlation was found between plasma NH 3 concentrations and medical food intake. Plasma NH 3 concentration was positively correlated with the percentage of Food and Agriculture Organization/World Health Organization/United Nations recommended protein ingested. Conclusions Intakes of adequate protein and energy for age result in anabolism and linear growth without increasing plasma NH 3 concentrations. Medical food intakes did not correlate with plasma NH 3 concentrations.

Hepatocyte Glycogen Accumulation in Patients Undergoing Dietary Management of Urea Cycle Defects Mimics Storage Disease

Anecdotal reports have described excess hepatocyte glycogen in patients with urea cycle enzyme defects. Retrospectively, the authors evaluated the prevalence and possible cause of liver glycogen accumulation in such patients. The authors searched the files of the Division of Pathology at Cincinnati Children's Hospital from 1975 and 2004 for cases of urea cycle enzyme defects and identified 11 patients who had had liver biopsy performed and/or liver transplantation. All patients were on diets containing essential amino acids as the protein source before liver biopsy and/or transplantation. All but one patient had focal or diffuse glycogen accumulation in hepatocytes in at least one specimen by light microscopic examination. Two young infants also had cholestasis. Electron microscopy performed on six patients showed diffuse or focal glycogen excess in the cytoplasm of individual hepatocytes. Biochemical studies of three patients revealed two with hepatic glycogen content in the upper normal range and one that was abnormally high. Glycolytic enzyme activities were normal in two patients, and one patient had low phosphorylase activity. Hepatocyte glycogen accumulation in urea cycle enzyme defects resembles that seen in glycogen storage disease but can be distinguished in most cases by non-uniformity of distribution and/or the absence of sinusoidal compression by expanded hepatocytes. We speculate that therapeutic modification of dietary protein content by restriction to essential amino acids, including leucine, may promote glycogen accumulation by increasing insulin secretion.

A syndrome of congenital hyperinsulinism and hyperammonemia

This report describes two patients from unrelated families with an unusual syndrome of hyperinsulinism plus hyperammonemia. The diagnosis of hyperinsulinism was based on the demonstration of fasting hypoglycemia with inappropriately elevated insulin levels, inappropriately low β-hydroxybutyrate and free fatty acid levels, and inappropriately large glycemic response to the administration of glucagon. In both patients, plasma ammonium levels were persistently elevated and unaffected by protein feeding, protein restriction, or benzoate therapy. Plasma and urinary amino acids, urinary organic acids, and urinary orotic acid levels were not consistent with any of the urea cycle enzyme defects or other hyperammonemic disorders. These two patients appear to represent a unique form of congenital hyperinsulinism distinct from the previously described autosomal dominant and autosomal recessive variants. We speculate that the underlying defect involves a site that is common to the amino acid regulation of both insulin secretion in pancreatic β-cells and urea synthesis in the liver. (J Pediatr 1997;130:661-4)

Differentiation of transient hyperammonemia of the newborn and urea cycle enzyme defects by clinical presentation

We reviewed clinical data in 33 patients with transient hyperammonemia of the newborn (THAN): six previously unreported cases and 27 from the literature. Thirteen neonates with urea cycle enzyme deficiencies (UCED) served for comparison. No differences were found in the incidence of perinatal complications, route of delivery, Apgar scores, sex, or incidence or time of onset of seizures. On the other hand, neonates with THAN had significantly lower birth weights (mean +/- SEM 2282 +/- 78 gm vs 3336 +/- 222 gm, P less than 0.001) and gestational ages (35.1 +/- 0.5 weeks vs 39.6 +/- 0.5 weeks, P less than 0.001). Mean time of onset of respiratory distress (3.9 +/- 1.4 hours vs 71.5 +/- 26.1 hours, P less than 0.001), ventilatory support (P less than 0.001), lethargy (P less than 0.005), and coma (P less than 0.005) occurred earlier in THAN. Distinctive laboratory findings in patients with THAN included abnormal chest radiographic findings and plasma ammonium concentrations that were higher (1871 +/- 209 microM vs 973 +/- 169 microM, P less than 0.02) at an earlier age. Respiratory distress occurred in all but one patient with THAN before 24 hours; in contrast, only 62% of infants with UCED had respiratory symptoms, and none before 30 hours. In this retrospective study, the clinical presentation alone differentiated THAN from UCED.

Hyperammonemia and Orotic Aciduria in Portacaval-Shunted Rats

The effect of a portacaval shunt-induced alteration in liver function on nitrogen metabolism was studied in rats. Within a few days after surgery, portacaval-shunted rats grew with an average daily gain in body weight equal to shamoperated control rats. Within 1 week after surgery, portacaval-shunted rats excreted 20% more orotic acid in their urine compared to control rats. The difference increased to 37% after 3 weeks. Plasma ammonia levels were elevated by 78% in portacaval-shunted rats compared to control rats after 2 weeks. Portacaval-shunted rats injected with a challenging load of ammonium chloride (5 mmol/kg) excreted half as much orotic acid in their urine over a 24-hour period as similarly injected controls. The simultaneous injection of 1.5 mmol/kg of arginine prevented the ammonia-induced increase in orotic acid excretion in both shunted and control rats. However, feeding rats diets supplemented with 1% arginine did not prevent the chronic hyperammonemia and orotic aciduria produced by the construction of portacaval shunts. Similar experiments with diets supplemented with 1% sodium benzoate to induce alternative pathways for nitrogen excretion were also without effect. These results are in contrast to recent clinical studies reporting the effectiveness of sodium benzoate in treating hyperammonemia in patients with urea cycle enzyme defects.

Neonatal Hyperammonemia

Neonatal hyperammonemia is a life-threatening complication of several conditions seen in the newborn infant (Table 1). Some of these disorders are potentially reversible; others are amenable to long-term therapy if early diagnosis and aggressive management in the neonatal period can prevent the catastrophic neurologic deterioration that accompanies massive hyperammonemia. Normal plasma ammonia concentrations for both term and preterm infants vary according to different laboratories and methods, from 50 to 150 µg/dl,1 but values greater than 75 µg/dl in the term infant and 150 µg/dl in the preterm infant are considered abnormal and should be investigated. Most of the neonatal hyperammonemic conditions produce ammonia concentrations that are several-fold greater than normal, so small interlaboratory differences are less important. INCIDENCE The true incidence of hyperammonemic conditions in the general population is not known. Many infants probably die before a diagnosis is made. Ornithine transcarbamylase deficiency and argininosuccinate lyase deficiency are the two most common of the five urea cycle defects, with the frequency of argininosuccinate aciduria estimated to be 1/70,000.2 When all of the disorders are taken together, the incidence approaches 1/30,000 births. The increased awareness of transient neonatal hyperammonemia in recent years has led to an increase in diagnosis, and, in our experience, it is more common than the inherited urea cycle enzyme defects.

Neonatal Hyperammonemia

Neonatal hyperammonemia is a life-threatening complication of several conditions seen in the newborn infant (Table 1). Some of these disorders are potentially reversible; others are amenable to long-term therapy if early diagnosis and aggressive management in the neonatal period can prevent the catastrophic neurologic deterioration that accompanies massive hyperammonemia.Normal plasma ammonia concentrations for both term and preterm infants vary according to different laboratories and methods, from 50 to 150 µg/dl,1 but values greater than 75 µg/dl in the term infant and 150 µg/dl in the preterm infant are considered abnormal and should be investigated. Most of the neonatal hyperammonemic conditions produce ammonia concentrations that are several-fold greater than normal, so small interlaboratory differences are less important.INCIDENCEThe true incidence of hyperammonemic conditions in the general population is not known. Many infants probably die before a diagnosis is made. Ornithine transcarbamylase deficiency and argininosuccinate lyase deficiency are the two most common of the five urea cycle defects, with the frequency of argininosuccinate aciduria estimated to be 1/70,000.2 When all of the disorders are taken together, the incidence approaches 1/30,000 births. The increased awareness of transient neonatal hyperammonemia in recent years has led to an increase in diagnosis, and, in our experience, it is more common than the inherited urea cycle enzyme defects.

Leukocyte urea cycle enzymes in hyperammonemia.

All enzymes of the urea cycle are demonstrable in circulating leukocytes. They show the same relative activities as those in liver except for argininosuccinate synthetase + lyase (combined) which seems to be disproportionately active. To see whether leukocytes reflect liver activity, blood from patients with three hepatic urea cycle disorders was tested. In each case, the leukocytes showed the same enzyme deficiency as was apparent from a liver biopsy (Table 4). Leukocyte assays appear to be reliable indicators of the enzyme lesions in inherited urea cycle enzyme defects and therefore may obviate the need for liver biopsy.

CITRULLINEMIA WITH DEFECTIVE UREA PRODUCTION

The second case of citrullinemia is described in a 21-month-old female. In contrast to previous patients with urea cycle enzyme defects, the present child fails to produce normal amounts of urea. Physical growth has been adequate, except for microcephaly. Unfortunately, her psychomotor milestones are slow. Triiodothyronine, pyridoxine, and neomycin were given with no beneficial effects. Nitrogen balances in addition to protein and arginine loading studies suggest alternate pathways for the degradation of arginine.