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SummaryObjectives Glutaric acidemia type I (GA-I) is an inherited neurometabolic disorder caused by deficiency of glutaryl-CoA dehydrogenase (GCDH) and characterized by increased levels of glutaric, 3-OH-glutaric, and glutaconic acids in the brain parenchyma. The increment of these organic acids inhibits glutamate decarboxylase (GAD) and consequently lowers the γ-aminobutyric acid (GABA) synthesis. Untreated patients exhibit severe neurologic deficits during development, including epilepsy, especially following an acute encephalopathy outbreak. In this work, we evaluated the role of the GABAergic system on epileptogenesis in GA-I using the Gcdh−/− mice exposed to a high lysine diet (Gcdh−/−-Lys). Methods Spontaneous recurrent seizures (SRS), seizure susceptibility, and changes in brain oscillations were evaluated by video–electroencephalography (EEG). Cortical GABAergic synaptic transmission was evaluated using electrophysiologic and neurochemical approaches. Results SRS were observed in 72% of Gcdh−/−-Lys mice, whereas no seizures were detected in age-matched controls (Gcdh+/+ or Gcdh−/− receiving normal diet). The severity and number of PTZ-induced seizures were higher in Gcdh−/−-Lys mice. EEG spectral analysis showed a significant decrease in theta and gamma oscillations and predominant delta waves in Gcdh−/−-Lys mice, associated with increased EEG left index. Analysis of cortical synaptosomes revealed a significantly increased percentage of glutamate release and decreased GABA release in Gcdh−/−-Lys mice that were associated with a decrease in cortical GAD immunocontent and activity and confirmed by reduced frequency of inhibitory events in cortical pyramidal cells. Significance Using an experimental model with a phenotype similar to that of GA-I in humans—the Gcdh−/− mice under high lysine diet (Gcdh−/−-Lys)—we provide evidence that a reduction in cortical inhibition of Gcdh−/−-Lys mice, probably induced by GAD dysfunction, leads to hyperexcitability and increased slow oscillations associated with neurologic abnormalities in GA-I. Our findings offer a new perspective on the pathophysiology of brain damage in GA-I.

Human fibroblasts, cultured on glass microscope slides, were examined autoradiographically for their ability to incorporate radioactivity from Na-propionate-l- 14C into TCA-insoluble cell material. Cells from patients with propionic acidemia (PA) and methylmalonic acidemia (MMA), both B12-sensitive and B12-insensitive, incorporate little or no radioactivity and thus can be readily distinguished from cells of a number of individuals with no defects in this pathway. Incorporation in cells from non-MMA individuals is significantly enhanced in a basal medium by high levels of glucose. This procedure should be readily adaptable for use in screening for disorders of propionate metabolism, particularly before birth.

Glutaric acidaemia type I (GA I) is an inborn error of metabolism caused by a deficiency of glutaryl-CoA dehydrogenase (GCDH) and is characterized clinically by striatal degeneration that almost always occurs in early childhood. A murine knockout model of GA I has the organic aciduria seen in the human disorder, but this model does not develop striatal degeneration spontaneously. 3-Nitropropionic acid (3NP), a succinic dehydrogenase inhibitor with specificity for the striatum, was investigated as a potential initiator of striatal degeneration in GCDH-deficient mice. This study shows that GCDH-deficient mouse pups are more susceptible to 3NP than their wild-type littermates, and that all mouse pups are more sensitive to 3NP as infants than as adolescents and adults. Increased sensitivity to 3NP early in life may model the developmental window for the striatal damage observed in human GA I.

Glutaric acid (GA) and 3-hydroxyglutaric acids (3-OH-GA) are key metabolites in glutaryl co-enzyme A dehydrogenase (GCDH) deficiency and are both considered to be potential neurotoxins. As cerebral concentrations of GA and 3-OH-GA have not yet been studied systematically, we investigated the tissue-specific distribution of these organic acids and glutarylcarnitine in brain, liver, skeletal and heart muscle of Gcdh-deficient mice as well as in hepatic Gcdh–/– mice and in C57Bl/6 mice following intraperitoneal loading. Furthermore, we determined the flux of GA and 3-OH-GA across the blood–brain barrier (BBB) using porcine brain microvessel endothelial cells. Concentrations of GA, 3-OH-GA and glutarylcarnitine were significantly elevated in all tissues of Gcdh–/– mice. Strikingly, cerebral concentrations of GA and 3-OH-GA were unexpectedly high, reaching similar concentrations as those found in liver. In contrast, cerebral concentrations of these organic acids remained low in hepatic Gcdh–/– mice and after intraperitoneal injection of GA and 3-OH-GA. These results suggest limited flux of GA and 3-OH-GA across the BBB, which was supported in cultured porcine brain capillary endothelial cells. In conclusion, we propose that an intracerebral de novo synthesis and subsequent trapping of GA and 3-OH-GA should be considered as a biochemical risk factor for neurodegeneration in GCDH deficiency.

Summary:In vitro studies suggest that excitotoxic cell damage is an underlying mechanism for the acute striatal damage in glutaryl-CoA dehydrogenase (GCDH) deficiency. It is believed to result from an imbalance of glutamatergic and GABAergic neurotransmission induced by the accumulating organic acids 3-hydroxyglutaric acid (3-OH-GA) and to a lesser extent glutaric acid (GA). Stereotaxic administration of 3-OH-GA and GA into the rat striatum have confirmed these results, but may not truly represent the effect of chronic exposure to these compounds. In an attempt to better understand the pathophysiology of GCDH deficiency in vivo, two animal models have been utilized. A mouse that lacks GCDH activity in all tissues was generated by gene targeting in embryonic stem cells. These animals develop the characteristic biochemical phenotype of the human disease. Pathologically, these mice have a diffuse spongiform myelinopathy similar to that in human patients; however, there is no evidence for acute striatal damage or sensitivity to acute encephalopathy induced by catabolism or inflammatory cytokines. A naturally occurring animal model, the fruit-eating bat Rousettus aegypticus, lacks hepatic and renal GCDH activity, but retains cerebral enzyme activity. Like the mouse, these bats develop the characteristic biochemical phenotype of glutaryl-CoA dehydrogenase deficiency, but lack overt neurological symptoms such as dystonia. It is not known whether they also develop the spongiform myelinopathy seen in the Gcdh-deficient mice. Otherwise, these constellations would suggest that cerebral GCDH deficiency is responsible for the development of neuronal damage. The lack of striatal damage in these two rodent models may also be related to species differences. However, they also highlight our lack of a comprehensive understanding of additional factors that might modulate the susceptibiliy of neurons to accumulating 3-OH-GA and GA in GCDH deficiency. Unravelling these mechanisms may be the key to understanding the pathophysiology of this unique disease and to the development of neuroprotective strategies.

Glutaric aciduria type I (GA1) is a preventable cause of acute brain damage in early childhood, leading to a severe dystonic-dyskinetic disorder that is similar to cerebral palsy and ranges from extreme hypotonia to choreoathetosis to rigidity with spasticity. Degeneration of the putamen and caudate typically occurs between 6 and 18 months of age and is probably linked to changes in metabolic demand caused by normal maturational changes and superimposed catabolic stress. Recognition of this biochemical disorder before the brain has been injured is essential to outcome. Diagnosis depends upon the recognition of relatively non-specific physical findings such as hypotonia, irritability and macrocephaly, and on performance of urine organic acid quantification by gas chromatography--mass spectrometry or selective searches of urine or blood specimens by tandem mass spectrometry for glutarylcarnitine. The diagnosis may also be suggested by characteristic findings on neuroimaging. In selected patients diagnosis can only be reached by enzyme assay. Specific current management by the authors of this paper includes pharmacological doses of L-carnitine, as well as dietary protein restriction. Metabolic decompensation must be treated aggressively to avoid permanent brain damage. Multicentre studies are needed to establish best methods of diagnosis and optimal therapy of this disorder.

Investigation of cultured skin fibroblasts in a patient with atypical riboflavin-responsive glutaric aciduria revealed a marked deficiency of peroxisomal glutaryl-CoA oxidase. This is the first patient to be reported with glutaric aciduria caused by a peroxisomal rather than a mitochondrial dysfunction. This enzyme appears to be specific for glutaryl-CoA, as lauryl-CoA and dodecanedioyl-CoA oxidase activities in the fibroblasts were both normal. The urinary excretion of glutaric acid (0.5 mmol mmol creatinine-1) suggests that the flux through this pathway is considerably less than the mitochondrial flux through glutaryl-CoA dehydrogenase. The elevated glutaric acid excretion (to 0.8 mmol mmol creatinine-1) in response to lysine loading suggests that lysine is a precursor.

This unit describes methods for the preparation of samples for analysis of physiological amino acids and organic acids. Amino acids are analyzed by ion-exchange chromatography using an automated system. Organic acids are analyzed by gas-chromatography/mass spectrometry (GC-MS). Analysis of amino and organic acids is necessary to detect and monitor the treatment of many inborn errors of metabolism.

Two brothers with a recently described inborn error of metabolism characterized by glyceroluria, hyperglycerolemia, and generalized glycerol kinase deficiency had moderate psychomotor retardation, spasticity, growth failure, a nonspecific myopathy, osteoporosis, and adrenal insufficiency. Glycerol kinase activity in leukocytes and cultured fibroblasts was less than 5% of control values. Hepatic and renal tissue obtained at autopsy in one patient had similarly low enzyme activity. Thus the deficiency of glycerol kinase in these patients appears to be generalized and heritable, though the relationship of the clinical phenotype to the enzymatic defect is not yet established.

At least eight inborn errors of mitochondrial fatty acid β-oxidation are currently known. In the span of the last few years, almost 200 patients suffering from one of these disorders have been diagnosed (Vianey-Liaud et al., 1987), and some of them were initially mistaken as Reye’s or sudden infant death syndrome. Since the prognosis of these patients can be greatly improved by treatment with available therapeutic measures, fast and accurate diagnosis of this group of diseases is crucial. In reality, however, the diagnosis has been difficult for many of these disorders (Editorial in Lancet, 1986).

Galactosaemia due to galactose-1-phosphate uridyltransferase deficiency (McKusick 23040) is associated, in the untreated patient, with gastrointestinal symptoms, hepatomegaly, hepatic dysfunction, cataracts, septicaemia and, eventually, mental retardation. Neonates with classical galactosaemia may also present with increased intracranial pressure, which is reported to resolve on a galactose-free diet (Huttenlocher et al., 1970; Belman et al., 1986). We describe a newborn with galactosaemia who died at 19 days of age with cerebral oedema unresponsive to medical management