Alternative pathway therapy for urea cycle disorders: twenty years later.

Alternative pathway therapy is currently an accepted treatment approach for inborn errors of the urea cycle. This involves the long-term use of oral sodium phenylbutyrate, arginine supplements, or both, depending on the specific enzyme deficiency, and treatment of acute hyperammonemic crises with intravenous sodium benzoate/sodium phenylacetate plus arginine. A review of 20 years of experience with this approach illustrates the strengths and limitations of this treatment. It has clearly decreased the mortality and morbidity from these disorders, but they remain unacceptably high. The medications are generally well tolerated, but severe accidental overdosage has been reported because of the infrequent use of the medication. There is also a difference in their metabolism between newborns and older children that must be addressed in determining dosage. To avoid these complications it is recommended that drug levels in blood be monitored routinely and that very specific treatment protocols and oversight be followed to avoid overdoses. Finally, it must be acknowledged that alternative pathway therapy has limited effectiveness in preventing hyperammonemia and must be combined with effective dietary management. Therefore in children with neonatal-onset disease or in those with very poor metabolic control, liver transplantation should be considered. There should also be the continued search for innovative therapies that may offer a more permanent and complete correction, such as gene therapy.

Psychoeducational profile of the 22q11.2 microdeletion: A complex pattern.

OBJECTIVES: To examine the psychoeducational profile associated with the chromosome 22q11.2 microdeletion (DiGeorge/velocardiofacial syndrome). STUDY DESIGN: Thirty-three patients (aged 6 to 27 years) with a 22q11.2 microdeletion underwent psychoeducational testing as part of a comprehensive evaluation. Nonparametric statistics were used to compare verbal and performance IQ, academic achievement scores, and receptive versus expressive language scores. Post hoc comparisons were made of IQ subtest scores and of language versus verbal IQ. RESULTS: Full-scale IQ ranged from the normal to the moderately retarded range. Mean verbal IQ was significantly higher than mean performance IQ. In a similar manner, mean reading and spelling scores were superior to the mean mathematics score, although achievement scores typically were in the range of verbal IQ. In addition, many children showed clinically significant language impairments, with mean language scores lower than mean verbal IQ. CONCLUSIONS: The IQ and academic profiles are reminiscent of a "nonverbal learning disability," although achievement was not discrepant from IQ. The coincidence of language impairment with a relative strength in reading belies a unique neuropsychologic profile. Educational programming for these children must address both verbal and nonverbal deficits.

Inborn errors of urea synthesis.

Inborn errors of urea synthesis can present in the newborn period as a catastrophic illness or later in childhood or adulthood with an indolent course punctuated by hyperammonemic episodes. Because symptoms mimic other neuropsychiatric disorders, it is common for there to be a delay in diagnosis, often with dire consequences. Diagnosis relies on the combination of clinical suspicion and the measurement of ammonium, lactate, and amino acids in plasma and organic acids and orotic acid in urine. Treatment involves nitrogen restriction combined with the stimulation of alternate pathways of waste nitrogen excretion. More recently liver transplantation has been performed as enzyme replacement therapy. The outcome is poor in children who survive prolonged neonatal hyperammonemic coma, with most manifesting developmental disabilities. The etiology of neuronal injury in this disorder is unclear but may involve some combination of ammonia/amino acid accumulation, neurotransmitter alterations, and excitotoxic injury. Gene therapy holds the promise of improved treatment in the future.

Prenatal diagnosis and heterozygote detection by DNA analysis in ornithine transcarbamylase deficiency.

This report summarizes our experience with DNA analysis using a complementary DNA probe for ornithine transcarbamylase in 24 individuals or families with deficiency of this enzyme. In four cases, including three reported elsewhere, a Taql restriction site alteration directly detected the mutation. In 10 additional cases, only an affected male was available, and results of DNA analysis using the Taql enzyme were normal. In 10 cases, family studies were performed with the use of restriction fragment length polymorphisms. Prenatal diagnostic studies were performed for three informative pregnancies, and two affected male fetuses were identified. Analysis of two restriction fragment length polymorphisms, Mspla and BamHl, was informative in 14 of 19 (74%) known carrier females and in 21 of 35 (60%) females (the total number studied). One female previously predicted to be a noncarrier by protein-loading test was determined to be a carrier by analysis of restriction fragment length polymorphisms. The frequency of Taql site alterations was 4 of 24 families (17%). These data illustrate the importance of DNA analysis, pedigree analysis, and biochemical testing in families with ornithine transcarbamylase deficiency to detect carriers and establish the diagnosis prenatally.

Therapeutic approaches to cobalamin-C methylmalonic acidemia and homocystinuria.

The use of hydroxocobalamin (OH-B12), betaine, carnitine, and folinic acid were studied in two children with the cobalamin C form of methylmalonic acidemia and homocystinuria. When daily injections of 1 mg OH-B12 were discontinued for 3 weeks, there was no significant change in total plasma homocysteine or methionine levels and only a modest increase in methylmalonate. Orally administered OH-B12 1 mg/d in one patient was associated with an increase in plasma homocystine and a decrease in methionine within 1 month. Withdrawal of betaine 250 mg/kg/d was also associated with a rise in plasma homocystine and a fall in methionine levels. Carnitine 100 mg/kg/d lead to an increase in urinary excretion of propionylcarnitine, but did not affect plasma methylmalonate levels. No beneficial biochemical effect of folinic acid could be documented at a dose of 25 mg/d. Our results suggest that daily injections of OH-B12 are not necessary to maintain metabolic control and that orally administered OH-B12 is unlikely to be effective. Betaine appears to act synergistically with OH-B12 and should be part of the treatment regimen. Although there are theoretical reasons for using L-carnitine and folinic acid, we could not document their effectiveness in these two patients.

Anorexia and altered serotonin metabolism in a patient with argininosuccinic aciduria.

We studied serotonin metabolism in a metabolically stable 7-year-old girl with argininosuccinic aciduria who had severe anorexia. The CSF concentration of 5-hydroxyindoleacetic acid (HIAA), the metabolite of serotonin, was markedly elevated at 79 ng/ml (normal 33 +/- 11 ng/ml). Altered serotonin metabolism was also reflected in the sleep EEG, which showed decreased REM sleep. Reducing her intake of tryptophan, the precursor of serotonin, from 35 mg/kg/day to 7 mg/kg/day resulted in a decrease in CSF concentration of HIAA to 20 ng/ml and the onset of spontaneous eating for the first time in 4 1/2 years. REM sleep increased from 3% to 9.5% of total sleep time. Two days after increasing tryptophan intake to 25 mg/kg/day, spontaneous feeding stopped, associated with a rise in CSF HIAA to 45 ng/ml. Caloric/carbohydrate intake was found to affect CSF HIAA levels and food intake in an additive manner with tryptophan intake. These observations suggest that altered serotonin metabolism affected feeding behavior in this child, and that this effect could be modified by changing tryptophan or carbohydrate intake.

Neurologic outcome in premature infants with transient asymptomatic hyperammonemia.

We studied the short-term and long-term effects of transient asymptomatic neonatal hyperammonemia on neurologic function in 21 preterm infants with normal ammonium levels and 25 with hyperammonemia (range 40 to 72 mumol/L) during the first weeks of life. The hyperammonemic infants were prospectively randomized to treatment with orally administered arginine free base 1 to 2 mmol/kg/day for 2 months (n = 13) or to a no-treatment control group (n = 12). Cortical function was assessed by auditory response and habituation during the first month of life. An auditory response was shown by 64% of the hyperammonemic infants and 43% of the normoammonemic infants (P not significant). Plasma ammonium levels at the time of examination bore no consistent relationship to whether an infant responded to an auditory stimulus. Number of trials to reach auditory habituation was also not different, and plasma ammonium level did not correlate with the presence or absence of habituation. IQ testing at 6, 12, 18, and 30 months showed no significant differences between groups. Early plasma ammonium levels did not have an effect on 30-month IQ scores. These findings suggest that transient asymptomatic hyperammonemia in premature infants is not associated with short-term or long-term neurologic deficits through 30 months of age. This study does not support the need for treatment of transient asymptomatic hyperammonemia in the premature infant.

Risk of serious illness in heterozygotes for ornithine transcarbamylase deficiency.

Ornithine transcarbamylase (OTC) deficiency is an X-linked disorder associated with hyperammonemia. Heterozygous females have variable clinical expression, ranging from asymptomatic illness to recurrent episodes of hyperammonemic coma. We studied 17 OTC-deficient kindreds containing 114 women at risk for heterozygosity. Sixty-one of these women were designated heterozygotes by pedigree analysis, history of protein intolerance, protein tolerance tests, or DNA probe studies. Eleven (18%) of the 61 heterozygotes had experienced encephalopathic episodes; nine (82%) girls died during these episodes. Our findings indicate that there is a significant risk of symptomatic hyperammonemia in females heterozygous for OTC deficiency. We suggest that, within OTC-deficient kindreds, females at risk should be identified early, by means of protein tolerance tests and DNA probe studies. Those who develop significant hyperammonemia after a protein load should be considered for long-term alternate pathway therapy and should receive aggressive therapy during hyperammonemic episodes.

Arginine-responsive asymptomatic hyperammonemia in the premature infant.

We found that more than 50% of premature infants have elevated plasma ammonium levels during the first 2 months of life. Ammonium levels were twice normal and were unaccompanied by clinical symptoms of vomiting or lethargy. Ten of these infants were given supplements of arginine (1 to 2 mmol/kg/day PO) for 1 to 2 weeks preceded and followed by control periods. In each infant, plasma ammonium levels fell significantly within 2 days of start of arginine supplementation, and increased once arginine was discontinued. We studied 59 additional premature infants, of whom 26 had normal ammonium levels and 33 were hyperammonemic. Plasma arginine and ornithine levels were significantly lower in the hyperammonemic group, but there was no difference in urinary excretion of arginine or ornithine between groups. Half of the hyperammonemic infants received arginine supplementation between 2 and 8 weeks of age. Plasma ammonium levels in the arginine group was 33 + 1 mumol/L., compared to 45 + 2 mumol/L in the untreated group. Follow-up at 18 months of age showed similar IQ scores in all groups, suggesting that significant neurologic deficits do not result from this transient metabolic defect. The mechanism of the hyperammonemia is unclear.

Treatment of hyperammonemic coma caused by inborn errors of urea synthesis.

The relative effectiveness of exchange transfusion, peritoneal dialysis, arginine, and sodium benzoate was evaluated during 44 episodes of hyperammonemic coma in 31 patients with congenital urea cycle enzymopathies. The overall survival rate was 56%. In 15 episodes treated with EXT the fall in ammonium was 19 +/- 24%, P > 0.05. In 30 episodes treated with PD, the fall in ammonium was 60 +/- 9%, P < 0.001. Ten times more nitrogen was removed as glutamine than as ammonium during dialysis, suggesting that the effectiveness of PD resides in the removal of glutamine, glutamate, and alanine as well as ammonium. Prior to therapy all patients had hypoargininemia (18 +/- 2 microM); they responded to arginine supplementation with a rise in plasma arginine concentration to normal. In patients with AL deficiency, arginine supplementation (4 mmol/kg/day) was associated with a fall in ammonium level from 917 +/- 62 to 103 +/- 18 microM within 24 hours. When sodium benzoate (250 mg/kg/day) was used during eight episodes of coma, six patients responded with a significant decrease in plasma ammonium.