W. L. Nyhan

3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency: review of 18 reported patients.

Abstract3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency (HMG-CoA lyase) is an inborn error of leucine catabolism which often leads to life-threatening illness in the neonatal period. The cardinal clinical features include severe infantile hypoglycemia, metabolic acidosis, hepatomegaly, lethargy or coma and apnea. Hyperammonemia is variable. There is a characteristic absence of ketosis. Considerable heterogeneity has been observed in clinical and biochemical presentation. Acute episodes of illness have been mistaken for Reye syndrome. The pattern of organic acids in the urine includes large amounts of 3-hydroxy-3-methylglutaric, 3-methylglutaconic, 3-methylglutaric and 3-hydroxyisovaleric acids. Smaller, but appreciable levels of glutaric, adipic and other dicarboxylic acids may also be excreted in the urine. Lactic acid may be present in sizable amounts at times of acute illness. The primary defect is a deficiency of 3-hydroxy-3-methylglutaryl-coenzyme A lyase, a key enzyme in the cycle of ketogenesis.

The recognition of Lesch-Nyhan syndrome as an inborn error of purine metabolism

Lesch-Nyhan syndrome was first described over thirty years ago. The original patient was a 4-year-old boy with neurological abnormalities as well as haematuria. Crystals in his urine were identified and confirmed to be uric acid. The massive excretion of this purine led to metabolic studies using isotopically labelled uric acid to study turnover rates. Clues to the site of the enzyme defect resulted from studies with the immunosuppressive agent azathioprine, which normally causes uric acid concentrations to fall in blood and urine but was without effect in a Lesch-Nyhan patient. A deficiency of hypoxanthine phosphoribosyltransferase (HPRT) activity explained this observation in Lesch-Nyhan patients. Subsequent studies have indicated that the degree of HPRT deficiency appears to determine the severity of the disease. Molecular studies have shown that most families carry a unique mutation. Attempts are being made to correlate the type and site of a specific mutation with a particular phenotype.

Neurological manifestations of organic acid disorders

Neurological manifestations are very common and can be the leading and/or presenting feature in organic acid disorders, sometimes in the absence of metabolic derangement. Review of the time course and presentation of neurological disease in organic acid disorders reveals characteristic clinical findings of ataxia, myoclonus, extrapyramidal symptoms, metabolic stroke and megalencephaly. A group of organic acid disorders presents exclusively with neurological symptoms. These include glutaryl-CoA dehydrogenase deficiency (glutaric aciduria type I), succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria), mevalonic aciduria,n-acetylaspartic aciduria (Canavan disease) andl-2-hydroxyglutaric aciduria. As a group these “cerebral” orgamic acid disorders appear to remain often undiagnosed and their true incidence is much less wellknown than that of the “classical” organic acid disorders. Unfortunately, stringent guidelines for a clinical preselection of neuropaediatric patients to be investigated for organic acid disorders cannot be provided. Today, screening for neurometabolic disorders should be as comprehensive as possible and include determinations of amino acids, purines and pyrimidines and markers of peroxisomal function in addition to organic acid analysis.

Stable Isotope Dilution Analysis of 3-Hydroxyisovaleric Acid in Amniotic Fluid: Contribution to the Prenatal Diagnosis of Inherited Disorders of Leucine Catabolism

A quantitative assay for 3-hydroxyisovaleric acid in amniotic fluid was developed using D6-3-hydroxyisovaleric acid as an internal standard. 3-Hydroxyisovaleric acid was isolated by liquid partition chromatography and the amount determined by selected ion monitoring, ammonia chemical ionization gas chromatography-mass spectrometry of the trimethylsilyl derivatives. The concentration of 3-hydroxyisovaleric acid in ten normal amniotic fluids was 4.52±1.73 µmol/l. The level was elevated eight-fold in the amniotic fluid from a pregnancy resulting in the birth of a child with biotin-responsive multiple carboxylase deficiency. The stable isotope dilution assay of 3-hydroxyisovaleric acid in amniotic fluid is a rapid, sensitive and accurate method for the prenatal diagnosis of this disorder, and may be of value in the prenatal diagnosis of other inherited disorders of leucine catabolism.

Organic acids in cerebrospinal fluid and plasma of patients with L-2-hydroxyglutaric aciduria.

L-2-Hydroxyglutaric aciduria is a distinct inherited neurometabolic disorder leading to mental regression, seizures and severe cerebellar dysfunction, as well as pyramidal and extrapyramidal symptom_s (Barth et al 1991, 1993). On neuroimaging, all patients show a characteristic pattern with severe progressive loss of myelinated arcuate fibres, cerebellar atrophy, and signal changes in the basal ganglia. The relationship of c-2hydroxyglutarate to human metabolic pathways is so far unknown, and no other abnormalities are found on urinary organic acid analysis. In the search for the underlying metabolic defect a number of loading and fasting studies have been conducted in patients with L-2-hydroxyglutaric aciduria, with inconclusive results (Barth et al 1992, 1993). A refined method for the quantitative determination of organic acids in CSF and plasma was used in combination with stable isotope dilution assays to study samples from patients with L-2-hydroxyglutaric aciduria and to provide information on specific disturbances of cerebral metabolic pathways.

3-Methylglutaconic aciduria: a phenotype in which activity of 3-methylglutaconyl-coenzyme A hydratase is normal

Abstract3-Methylglutaconic aciduria has been found in two distinct syndromes. In one there is deficient activity of 3-methylglutaconyl coenzyme A hydratase, and the only clinical manifestation observed has been retardation of speech development. In the other, which includes a majority of the patients studied, we document that the activity of this enzyme in fibroblast extracts is normal. The phenotype of this disorder is one of profound neurological impairment with retarded psychomotor development, hypotonicity and/or spasticity, convulsions or EEG abnormalities, and sensorineural changes in the eye and ear.

Demonstration of 4-Aminobutyric acid aminotransferase deficiency in lymphocytes and lymphoblasts

Lysates of lymphocytes, isolated from whole blood, and Epstein-Barr virus transformed cultured lymphoblasts catalysed the transamination of 4-aminobutyric acid with 2-oxoglutaric acid as co-substrate. 4-Aminobutyric acid aminotransferase activity in lymphocyte and lymphoblast sonicates derived from 12 unrelated control individuals (6 each) was 39 ± 19 pmol min−1 (mg protein−1) (mean ± 1 SD). Activities in lysates of both types of cell derived from a Flemish patient were less than 3% of control. 4-Aminobutyric acid aminotransferase activity in sonicates derived from the parents and a healthy sibling were 15–37% of the control mean for lymphocytes and 13–20% of the control mean in lymphoblasts, respectively.Km values in a control lymphoblast sonicate were 0.63 and 0.08 mmol L−1 for 4-aminobutyric and 2-oxoglutaric acids, respectively. These data indicate that the parents and healthy sibling are heterozygous and the patient is homozygous for a defective gene responsible for 4-aminobutyric acid aminotransferase deficiency, and that inheritance is autosomal recessive.

Excretion of hippuric acid during sodium benzoate therapy in patients with hyperglycinaemia or hyperammonaemia

SummaryIn patients with non-ketotic hyperglycinaemia and two patients with urea cycle disorders treated with varying doses of sodium benzoate there was a linear correlation between intake of benzoate and excretion of hippurate. Patients with non-ketotic hyperglycinaemia excreted significantly more benzoate in the form of hippurate than patients with urea cycle disorders (74 ± 7.0vs 41 ± 3.6%). The plasma concentration of glycine decreased following benzoate treatment only in the patients with non-ketotic hyperglycinaemia. The observed difference between the two groups in the excretion of hippurate seems to support the concept that glycine availability may be limiting in benzoate therapy for some patients.

Carnitine reduces fasting ketogenesis in patients with disorders of propionate metabolism.

Patients with disorders of propionate metabolism have low plasma levels of free carnitine and excrete higher than normal quantities of esterified carnitine. The response to a 19 h fast was assessed as a physiological index of carnitine deficiency. In patients with propionic acidaemia and methylmalonic acidaemia a substantial ketogenesis developed in response to fasting. Supplementation with L-carnitine significantly reduced this ketogenic response.

Urinary excretion of 2-methylacetoacetate, 2-methyl-3-hydroxybutyrate and tiglylglycine after isoleucine loading in the diagnosis of 2-methylacetoacetyl-CoA thiolase deficiency

SummaryThe concentrations of 2-methylacetoacetate, 2-methyl-3-hydroxybutyrate and tiglylglycine were determined by gas chromatography-mass spectrometry in urine collected before and for 8 h after loading with 100mg of isoleucine per kg of body weight. The sum of 2-methylacetoacetate and 2-butanone, a decarboxylation product, was determined as the 2-butanone dinitrophenylhydrazone derivative. Substantial increases in each compound were encountered in a patient with a documented defect of 2-methylacetoacetyl-CoA thiolase. Increased quantities of 2-methyl-3-hydroxybutyrate and tiglylglycine were also found in four children with clinical symptoms similar to those associated with 2-methylacetoacetyl-CoA thiolase deficiency but in whom the activity of the enzyme was found to be normal. The concentration of 2-methylacetoacetate plus 2-butanone in the urine increased after an isoleucine load only in the patient with 2-methylacetoacetyl-CoA thiolase deficiency.