Citrullinaemia with rapidly fatal neonatal course

Insights into Gene Therapy for Urea Cycle Defects by Mathematical Modeling

Metabolic liver diseases are attractive gene therapy targets that necessitate reconstitution of enzymatic activity in functionally complex biochemical pathways. The levels of enzyme activity required in individual hepatocytes and the proportion of the hepatic cell mass that must be gene corrected for therapeutic benefit vary in a disease-dependent manner that is difficult to predict. While empirical evaluation is inevitably required, useful insights can nevertheless be gained from knowledge of disease pathophysiology and theoretical approaches such as mathematical modeling. Urea cycle defects provide an excellent example. Building on a previously described one-compartment model of the urea cycle, we have constructed a two-compartment model that can simulate liver-targeted gene therapy interventions using the computational program Mathematica. The model predicts that therapeutically effective reconstitution of ureagenesis will correlate most strongly with the proportion of the hepatic cell mass transduced rather than the level of enzyme-encoding transgene expression achieved in individual hepatocytes. Importantly, these predictions are supported by experimental data in mice and human genotype/phenotype correlations. The most notable example of the latter is ornithine transcarbamylase deficiency (X-linked) where impairment of ureagenesis in male and female patients is closely simulated by the one- and two-compartment models, respectively. Collectively, these observations support the practical value of mathematical modeling in evaluation of the disease-specific gene transfer challenges posed by complex metabolic phenotypes.

Electroencephalographic findings in ornithine transcarbamylase deficiency

A 3-day-old infant presented with anorexia, irritability, hypotonia, and seizures. Blood ammonia was 2115 micromol/L and amino and organic acid analyses were consistent with ornithine transcarbamylase deficiency. Liver biopsy confirmed only 1% enzyme activity. The patient was treated with hemodialysis. An electroencephalogram (EEG) revealed multifocal independent spike-and-sharp-wave discharges. After initial stabilization he was placed on a low-protein diet with citrulline and phenylbutyrate. Conjugating agents (arginine, sodium benzoate, and sodium phenylacetate) have been added during periods of metabolic decompensation. Although developmentally delayed, the patient has shown signs of clinical improvement and EEG activity has likewise improved with only mild background slowing and no evidence of epileptogenic activity at 4 years of age. A second infant presented at 3 days of age with a similar history, blood ammonia of 1382 micromol/L, and metabolic studies indicative of ornithine transcarbamylase deficiency. EEG showed multifocal independent ictal and interictal discharges. Electrographic abnormalities persisted despite lowering of blood ammonia with hemodialysis and conjugating agents. The patient continued to decline clinically and died on the 7th hospital day. EEG changes parallel the clinical course of ornithine transcarbamylase deficiency and may serve as an objective marker of the effectiveness of therapeutic interventions.

A case of citrullinemia with abnormal messenger RNA for argininosuccinate synthetase

A male neonate, thus far healthy and fed with breast milk, developed rapidly increasing apathy on the third day of life. Sucking became poor and hyperhidrosis was present. Hyperammonemia (3,305 micrograms/dl) was noted. He became comatose and died on the fourth day. There was a profound derangement of amino acid concentration in the body fluids, with highly elevated citrulline levels (4.70 mumols/ml in serum and 8.47 mumols/ml in urine). Autopsy showed diffuse pulmonary bleeding, as the only noteworthy pathological finding. The liver contained no detectable argininosuccinate synthetase (ASS) activity. The defect of ASS in the present case was not due to a reduced amount of ASSmRNA, but its structure was found to be abnormal; it was approximately 1.57 kb in length due to a defect of about 0.1 kb near the 3' end of the coding region.

EEG changes during recovery from acute severe neonatal citrullinemia

We report our observations of serial clinical and EEG examinations in 3 neonates during recovery from acute severe encephalopathy due to citrullinemia. Their electroclinical picture closely resembles the clinical stages of experimental models of hyperammonemia in monkeys. The length of the EEG interburst interval, a quantitative measure of EEG background abnormality, correlated with elevated serum levels of ammonia and suggests that hyperammonemia itself is a key figure in the genesis of encephalopathy in this condition. Finally, the manner in which the EEG normalizes during recovery from hyperammonemia in this setting suggests that burst-suppression resembles an exaggerated regression to the discontinuity of the very premature infant.

Citrullinemia presenting as uncontrollable epilepsy

We report the case of a 16-year-old girl with a variant form of citrullinemia who had been treated with anticonvulsants for uncontrolled epilepsy during the last 4 years. The diagnosis of citrullinemia was made because she had elevated values for serum citrulline (about 10 times control levels), elevated blood ammonia (over 400 micrograms/dl) and reduced activity of argininosuccinate synthetase in the biopsied liver tissue. Her EEG showed high voltage slow activity, but not triphasic waves, when she had high concentrations of blood ammonia. Treatment with a low-protein diet and sodium benzoate resulted in a normalized blood ammonia level, but her plasma citrulline levels remained unchanged. After the therapy she had neither convulsions nor seizure discharges on EEG, even when all anticonvulsant drug therapy was stopped. Thus it is suggested that hyperammonemia may account for the observed abnormal EEG findings, and triphasic waves on EEG are not always recorded in cases of hyperammonemia.

Kinetic analysis of argininosuccinate synthetase in a variant form of citrullinaemia

Argininosuccinate (ASA) synthetase in the liver from a patient with a variant form of citrullinaemia was analysed. Serum citrulline level was approximately 40 times higher than the control level, and the ASA synthetase activity was approximately 10% that of the control. Other urea cycle enzymes were within normal ranges. The specific activity (enzyme activity/enzyme protein, measured by immunochemical method), pH optimum, Michaelis constants, and thermal denaturation in the presence or absence of ASA were similar to those in enzymes obtained from the normal and the patient's liver cells. No in vitro effect of the patient's liver homogenate on ASA synthetase in normal liver was observed. Serum citrulline levels of the patient's parents and young brother were at the maximal level of control at fasting and definitely elevated 4 h after citrulline loading (100 mg/kg).

Citrullinemia

The pathological findings in two cases of neonatal citrullinemia are reported. One patient survived for eight months with treatment using alpha keto-analogues of essential amino acids. The untreated patient expired at eight days of age. Necropsy findings in these two cases are compared. The major histopathological changes were present in the brain and liver. They were much less prominent in the treated patient. Changes in a section of rib from the untreated patient were consistent with growth arrest and suggest than damage may occur in utero in neonatal citrullinemia.

Neonatal citrllinemia: treatment with keto-analogues of essential amino acids

A patient with neonatal onset citrullinemia survived to 8 months of age when treated with a mixture of essential amino acids and their keto-analogues. The initial plasma citrulline concentration was 2.7 mM; the blood ammonia concentration was greater than 500 muM. During the first week of therapy, the blood ammonia concentration became normal and that of plasma citrulline was reduced by almost 50%. It was possible to institute progressive increases in dietary calories and protein; growth and developmetn with resolution of almost all clinical signs of disease ensued. The patient died at 8 months of age after an episode of diarrhea and dehydration, probably of viral origin.

Variants of citrullinaemia

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