Genotypic and phenotypic spectrum of cytosolic phosphoenolpyruvate carboxykinase deficiency.

OBJECTIVES: Pathogenic biallelic variants in PCK1 coding for the cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) cause PEPCK-C deficiency, a rare disorder of gluconeogenesis presenting with hypoglycemia, lactic acidosis, and hepatopathy. To date, there has been no systematic analysis of its phenotypic, biochemical, and genetic spectrum. METHODS: All currently published individuals and a novel patient with genetically confirmed PEPCK-C deficiency were included. Clinical, biochemical, and genetic findings were analyzed. Protein and in-silico prediction score modeling was applied to analyze potential variant effects. RESULTS: Thirty-two individuals from 25 families were found, including one previously unreported patient. The typical biochemical pattern was hypoglycemia triggered by catabolic situations, elevated urinary concentrations of tricarboxylic acid cycle metabolites, mildly elevated alanine and aspartate aminotransferase and elevated lactate concentrations in serum. Plasma glutamine concentrations were elevated in some patients and may be a suitable marker for newborn screening. With adequate treatment, biochemical abnormalities usually normalized following a hypoglycemic episode. Symptom onset usually occurred in infancy with a broad range from neonatal age to adulthood. Regardless of the genotype, different phenotypes with a broad clinical spectrum were found. To date, eight genotypes with nine different PCK1 variants were identified, of which alleles with the recurrent variant c.925G > A; p.(Gly309Arg) are predominant and appear to be endemic in the Finnish population. Protein modeling suggests altered manganese- and substrate-binding as superordinate pathomechanisms. CONCLUSIONS: Environmental factors appear to be the main determinant for the phenotype in patients with biallelic variants in PCK1. Based on the biochemical pattern, PEPCK-C deficiency is a recognizable cause of childhood hypoglycemia. It is a treatable disease and early diagnosis is important to prevent metabolic derailment and morbidity. Newborn screening can identify at least a sub-cohort of affected individuals through elevated glutamine concentrations in dry blood.

Trimethylamine-N-oxide is elevated in the acute phase after ischaemic stroke and decreases within the first days.

BACKGROUND AND PURPOSE: Trimethylamine-N-oxide (TMAO) is a biomarker of the gut microbiome and correlates with the risk of cardiovascular diseases. However, conflicting data exist on the specific role of TMAO in ischaemic stroke patients. We aimed to analyze the time course of TMAO levels in stroke patients compared with controls. METHODS: In this prospective, case-control study, patients suffering from ischaemic stroke (onset <24 h) and control patients with less than two cardiovascular risk factors were enrolled. Plasma TMAO levels were analyzed on admission, after 48 h and after 3 months. The primary endpoint was the difference in TMAO levels on admission between stroke patients and controls. RESULTS: A total of 196 patients with ischaemic stroke and 100 controls were included between February 2018 and April 2019. Plasma TMAO levels on admission were significantly higher in stroke patients than in controls [median value 4.09 (2.87-6.49) vs. 3.16 (2.08-5.16) µmol/L, P = 0.001]. There was a significant decrease in TMAO levels in stroke patients after 48 h [median at 48 h, 3.49 (2.30-5.39) µmol/L, P = 0.027]. TMAO levels increased again 3 months after stroke [median 4.23 (2.92-8.13) µmol/L, P = 0.047]. In controls, TMAO levels did not change between admission and after 48 h [median at 48 h, 3.14 (1.63-4.61) µmol/L, P = 0.11]. An inverse correlation between TMAO values and kidney function was found (Spearman rho -0.334, P < 0.001). CONCLUSIONS: Our study emphasizes the importance of the time course of TMAO levels after ischaemic stroke. Future studies should define the time point of TMAO analysis, preferably in the acute phase (<24 h).

The branched-chain amino acid transaminase 1 sustains growth of antiestrogen-resistant and ERα-negative breast cancer.

Antiestrogen-resistant and triple-negative breast tumors pose a serious clinical challenge because of limited treatment options. We assessed global gene expression changes in antiestrogen-sensitive compared with antiestrogen-resistant (two tamoxifen resistant and two fulvestrant resistant) MCF-7 breast cancer cell lines. The branched-chain amino acid transaminase 1 (BCAT1), which catalyzes the first step in the breakdown of branched-chain amino acids, was among the most upregulated transcripts in antiestrogen-resistant cells. Elevated BCAT1 expression was confirmed in relapsed tamoxifen-resistant breast tumor specimens. High intratumoral BCAT1 levels were associated with a reduced relapse-free survival in adjuvant tamoxifen-treated patients and overall survival in unselected patients. On a tissue microarray (n=1421), BCAT1 expression was detectable in 58% of unselected primary breast carcinomas and linked to a higher Ki-67 proliferation index, as well as histological grade. Interestingly, BCAT1 was predominantly expressed in estrogen receptor-α-negative/human epidermal growth factor receptor-2-positive (ERα-negative/HER-2-positive) and triple-negative breast cancers in independent patient cohorts. The inverse relationship between BCAT1 and ERα was corroborated in various breast cancer cell lines and pharmacological long-term depletion of ERα induced BCAT1 expression in vitro. Mechanistically, BCAT1 indirectly controlled expression of the cell cycle inhibitor p27Kip1 thereby affecting pRB. Correspondingly, phenotypic analyses using a lentiviral-mediated BCAT1 short hairpin RNA knockdown revealed that BCAT1 sustains proliferation in addition to migration and invasion and that its overexpression enhanced the capacity of antiestrogen-sensitive cells to grow in the presence of antiestrogens. Importantly, silencing of BCAT1 in an orthotopic triple-negative xenograft model resulted in a massive reduction of tumor volume in vivo, supporting our findings that BCAT1 is necessary for the growth of hormone-independent breast tumors.

Pretreatment d-2-hydroxyglutarate serum levels negatively impact on outcome in IDH1-mutated acute myeloid leukemia.

Mutations in isocitrate dehydrogenases (IDHs) 1 and 2 frequently occur in acute myeloid leukemia (AML) and result in the production of the oncometabolite d-2-hydroxyglutarate (D2HG). D2HG has been shown to promote leukemogenesis even in the absence of mutated IDH, but the prognostic significance of pretreatment serum D2HG levels in patients with IDH-mutated AML is unclear. We measured D2HG serum levels in 84 patients with IDH-mutated AML treated in the prospective, randomized multicenter AML2003 trial of the German Study Alliance Leukemia. Multivariate Cox regression showed D2HG levels to negatively impact on event-free survival (EFS) as a continuous variable in the entire IDH(mut) cohort (P=0.04), with no effect on overall survival (OS). In a subgroup analysis, the negative impact of D2HG on EFS was found to be restricted to patients with mutations in IDH1 (P=0.003), adjusted for age, leukocyte count, serum lactate dehydrogenase and European LeukemiaNet risk score. We thus conclude that pretreatment D2HG serum levels may yield prognostic information in patients with IDH1-mutated, but not in IDH2-mutated AML, possibly due to different subcellular localizations of IDH1 and IDH2.

Molecular and biochemical alterations in tubular epithelial cells of patients with isolated methylmalonic aciduria.

Methylmalonic acidurias (MMAurias) are a group of inherited disorders in the catabolism of branched-chain amino acids, odd-chain fatty acids and cholesterol caused by complete or partial deficiency of methylmalonyl-CoA mutase (mut(0) and mut(-) subtype respectively) and by defects in the metabolism of its cofactor 5'-deoxyadenosylcobalamin (cblA, cblB or cblD variant 2 type). A long-term complication found in patients with mut(0) and cblB variant is chronic tubulointerstitial nephritis. The underlying pathomechanism has remained unknown. We established an in vitro model of tubular epithelial cells from patient urine (hTEC; 9 controls, 5 mut(0), 1 cblB). In all human tubular epithelial cell (hTEC) lines we found specific tubular markers (AQP1, UMOD, AQP2). Patient cells showed disturbance of energy metabolism in glycolysis, mitochondrial respiratory chain and Krebs cycle in concert with increased reactive oxygen species (ROS) formation. Electron micrographs indicated increased autophagosome production and endoplasmic reticulum stress, which was supported by positive acridine orange staining and elevated levels of LC3 II, P62 and pIRE1. Screening mTOR signaling revealed a release of inhibition of autophagy. Patient hTEC produced and secreted elevated amounts of the pro-inflammatory cytokine IL8, which was highly correlated with the acridine orange staining. Summarizing, hTEC of MMAuria patients are characterized by disturbed energy metabolism and ROS production that lead to increased autophagy and IL8 secretion.

Inborn oxidative phosphorylation defect as risk factor for propofol infusion syndrome.

Propofol is an anesthetic agent widely used for induction and maintenance of anesthesia, and sedation in children. Although generally considered as reliable and safe, administration of propofol can occasionally induce a potentially fatal complication known as propofol infusion syndrome (PRIS). Mitochondrial dysfunction has been implicated in the pathogenesis of PRIS. We report on an adult patient with Leber hereditary optic neuropathy (LHON) who developed PRIS. He was a carrier of the m.3460G>A mutation, one of the major three pathogenic point mutations associated with LHON. The propositus was blind and underwent propofol sedation after severe head injury. Five days after start of propofol infusion, the patient died. The activity of complex I of the oxidative phosphorylation (OXPHOS) system was severely deficient in skeletal muscle. Our observation indicates that fulminate PRIS can occur in an adult patient with an inborn OXPHOS defect and corroborates the hypothesis that PRIS is caused by inhibition of the OXPHOS system.

Long-term exposure of human proximal tubule cells to hydroxycobalamin[c-lactam] as a possible model to study renal disease in methylmalonic acidurias.

Dysfunction of proximal tubules resulting in tubulointerstitial nephritis and chronic renal failure is a frequent long-term complication of methylmalonic acidurias. However, the underlying pathomechanisms have not yet been extensively studied owing to the lack of suitable in vitro and in vivo models. Application of hydroxycobalamin[c-lactam] has been shown to inhibit the metabolism of hydroxycobalamin and, thereby, to induce methylmalonic aciduria in rats, oligodendrocytes, and rat hepatocytes. Our study characterizes the biochemical and bioenergetic effects of long-term exposure of human proximal tubule cells to hydroxycobalamin[c-lactam], aiming to establish a novel in vitro model for the renal pathogenesis of methylmalonic acidurias. Incubation of human proximal tubule cells with hydroxycobalamin[c-lactam] and propionic acid resulted in a strong, time-dependent intra- and extracellular accumulation of methylmalonic acid. Bioenergetic studies of respiratory chain enzyme complexes revealed an increase of complex II-IV activity after 2 weeks and an increase of complex I and IV activity as well as a decrease of complex II and III activity after 3 weeks of incubation. In addition, human proximal tubule cells displayed reduced glutathione content after the exposure to hydroxycobalamin[c-lactam] and propionic acid.

Coenzyme Q(10) is decreased in fibroblasts of patients with methylmalonic aciduria but not in mevalonic aciduria.

The content of coenzyme Q(10) (CoQ(10)) was examined in skin fibroblasts of 10 patients with mevalonic aciduria (MVA) and of 22 patients with methylmalonic aciduria (MMA). Patients with these inborn errors of metabolism are thought to be at risk for CoQ(10) depletion either by direct inhibition of the proximal pathway of CoQ(10) synthesis (MVA) or indirectly by inhibition of mitochondrial energy metabolism (MMA). We demonstrated that CoQ(10) concentrations were not significantly different from controls in MVA patients, suggesting that there may be upregulatory effects. On the other hand the CoQ(10) content in fibroblasts of patients with MMA was significantly reduced.

Pathogenesis of CNS involvement in disorders of amino and organic acid metabolism.

Inherited disorders of amino and organic acid metabolism have a high cumulative frequency, and despite heterogeneous aetiology and varying clinical presentation, the manifestation of neurological disease is common. It has been demonstrated for some of these diseases that accumulating pathological metabolites are directly involved in the manifestation of neurological disease. Various pathomechanisms have been suggested in different in vitro and in vivo models including an impairment of brain energy metabolism, an imbalance of excitatory and inhibitory neurotransmission, altered transport across the blood-brain barrier and between glial cells and neurons, impairment of myelination and disturbed neuronal efflux of metabolic water. This review summarizes recent knowledge on pathomechanisms involved in phenylketonuria, glutaric aciduria type I, succinic semialdehyde dehydrogenase deficiency and aspartoacylase deficiency with examples, highlighting general as well as disease-specific concepts and their putative impact on treatment.

Neurodegeneration and chronic renal failure in methylmalonic aciduria--a pathophysiological approach.

In the last decades the survival of patients with methylmalonic aciduria has been improved. However, the overall outcome of affected patients remains disappointing. The disease course is often complicated by acute life-threatening metabolic crises, which can result in multiple organ failure or even death, resembling primary defects of mitochondrial energy metabolism. Biochemical abnormalities during metabolic derangement, such as metabolic acidosis, ketonaemia/ketonuria, lactic acidosis, hypoglycaemia and hyperammonaemia, suggest mitochondrial dysfunction. In addition, long-term complications such as chronic renal failure and neurological disease are frequently found. Neuropathophysiological studies have focused on various effects caused by accumulation of putatively toxic organic acids, the so-called 'toxic metabolite' hypothesis. In previous studies, methylmalonate (MMA) has been considered as the major neurotoxin in methylmalonic aciduria, whereas more recent studies have highlighted a synergistic inhibition of mitochondrial energy metabolism (pyruvate dehydrogenase complex, tricarboxylic acid cycle, respiratory chain, mitochondrial salvage pathway of deoxyribonucleoside triphosphate (dNTP)) induced by propionyl-CoA, 2-methylcitrate and MMA as the key pathomechanism of inherited disorders of propionate metabolism. Intracerebral accumulation of toxic metabolites ('trapping' hypothesis') is considered a biochemical risk factor for neurodegeneration. Secondary effects of mitochondrial dysfunction, such as oxidative stress and impaired mtDNA homeostasis, contribute to pathogenesis of these disorders. The underlying pathomechanisms of chronic renal insufficiency in methylmalonic acidurias are not yet understood. We hypothesize that renal and cerebral pathomechanisms share some similarities, such as an involvement of dicarboxylic acid transport. This review aims to give a comprehensive overview on recent pathomechanistic concepts for methylmalonic acidurias.

Enzymatic and metabolic evidence for a region specific mitochondrial dysfunction in brains of murine succinic semialdehyde dehydrogenase deficiency (Aldh5a1-/- mice).

Succinic semialdehyde dehydrogenase deficiency, a rare inherited defect of gamma-aminobutyrate (GABA) catabolism, presents with characteristic biochemical abnormalities in the central nervous system (CNS). These include elevated concentrations of GABA, gamma-hydroxybutyrate (GHB), succinic semialdehyde (SSA), 4,5-dihydroxyhexanoic acid (DHHA) and alanine as well as decreased concentrations of glutamine. GABA degradation is coupled to Krebs cycle function in mammalian CNS ("GABA shunt") through succinate and alpha-ketoglutarate. Accordingly, we hypothesized that disruption of Krebs cycle and respiratory chain function in the CNS is involved in the neuropathogenesis of this disease. For this purpose, we investigated cerebral activities of Krebs cycle and respiratory chain enzymes as well as the glutathione content in Aldh5a1(-/-) mice, a recently generated mouse model for this disease. In CNS tissue of Aldh5a1(-/-) mice, we found a significantly decreased glutathione content (hippocampus, cortex) and decreased activities of complexes I-IV (hippocampus) suggesting increased oxidative stress and mitochondrial dysfunction. However, specific activities of Krebs cycle and respiratory chain were not affected by GABA, GHB, SSA, or DHHA (up to 1 mmol/L). Although our results suggest hippocampal and cortical dysfunction in Aldh5a1(-/-) brain, we found no evidence that accumulating key metabolites of SSADH deficiency directly induce impairment of energy metabolism.

Guideline for the diagnosis and management of glutaryl-CoA dehydrogenase deficiency (glutaric aciduria type I).

Glutaryl-CoA dehydrogenase (GCDH) deficiency is an autosomal recessive disease with an estimated overall prevalence of 1 in 100 000 newborns. Biochemically, the disease is characterized by accumulation of glutaric acid, 3-hydroxyglutaric acid, glutaconic acid, and glutarylcarnitine, which can be detected by gas chromatography-mass spectrometry of organic acids or tandem mass spectrometry of acylcarnitines. Clinically, the disease course is usually determined by acute encephalopathic crises precipitated by infectious diseases, immunizations, and surgery during infancy or childhood. The characteristic neurological sequel is acute striatal injury and, subsequently, dystonia. During the last three decades attempts have been made to establish and optimize therapy for GCDH deficiency. Maintenance treatment consisting of a diet combined with oral supplementation of L: -carnitine, and an intensified emergency treatment during acute episodes of intercurrent illness have been applied to the majority of patients. This treatment strategy has significantly reduced the frequency of acute encephalopathic crises in early-diagnosed patients. Therefore, GCDH deficiency is now considered to be a treatable condition. However, significant differences exist in the diagnostic procedure and management of affected patients so that there is a wide variation of the outcome, in particular of pre-symptomatically diagnosed patients. At this time of rapid expansion of neonatal screening for GCDH deficiency, the major aim of this guideline is to re-assess the common practice and to formulate recommendations for diagnosis and management of GCDH deficiency based on the best available evidence.

Abnormal sterol metabolism in holoprosencephaly: studies in cultured lymphoblasts.

BACKGROUND: Holoprosencephaly (HPE) is the most common structural malformation of the developing forebrain in humans. The aetiology is heterogeneous and remains unexplained in approximately 75% of patients. OBJECTIVE: To examine cholesterol biosynthesis in lymphoblastoid cell lines of 228 patients with HPE, since perturbations of cholesterol homeostasis are an important model system to study HPE pathogenesis in animals. METHODS: An in vitro loading test that clearly identifies abnormal increase of C27 sterols in lymphoblast-derived cells was developed using [2-(14)C] acetate as substrate. RESULTS: 22 (9.6%) HPE cell lines had abnormal sterol pattern in the in vitro loading test. In one previously reported patient, Smith-Lemli-Opitz syndrome was diagnosed, whereas others also had clearly reduced cholesterol biosynthesis of uncertain cause. The mean (SD) cholesterol levels were 57% (15.3%) and 82% (4.7%) of total sterols in these cell lines and controls, respectively. The pattern of accumulating sterols was different from known defects of cholesterol biosynthesis. In six patients with abnormal lymphoblast cholesterol metabolism, additional mutations in genes known to be associated with HPE or chromosomal abnormalities were observed. CONCLUSIONS: Impaired cholesterol biosynthesis may be a contributing factor in the cause of HPE and should be considered in the evaluation of causes of HPE, even if mutations in HPE-associated genes have already been found.

Massive insulin secretion in response to anaerobic exercise in exercise-induced hyperinsulinism.

Exercise-induced hyperinsulinism (EIHI) is a recently described entity characterised by recurrent episodes of hypoglycaemia induced by physical exercise. The index patient for this disorder and a matched control were subjected to aerobic and anaerobic exercise tests on a cycle ergometer. Aerobic exercise was performed at an intensity of 60% of the respective 4 mmol/l lactate threshold (40 min). Anaerobic exercise with an intensity corresponding to 130% VO2max lead to exertion within 2-3 min and elicited comparable maximal lactate levels in both subjects (10-11 mmol/l). The patient experienced a massive increase in insulin from 34 to 649 mU/l after the anaerobic test, and a lower increase in insulin from 27 to 79 mU/l during the aerobic test. Insulin concentration remained unchanged during both tests in the control. Epinephrine increased in the EIHI patient, which was probably a counterregulatory response to hypoglycaemia. The activity of lactate dehydrogenase of the index patient in isolated leukocytes as well as the response to inhibition of oxamate was normal. The hypothesis of abnormal transport or metabolism of lactate/pyruvate in the beta-cells of patients with EIHI was further supported by the parallel increase of lactate and insulin in this study elicited in particular by anaerobic exercise.

Measurement of bile acid CoA esters by high-performance liquid chromatography-electrospray ionisation tandem mass spectrometry (HPLC-ESI-MS/MS).

The novel and rapid assay presented here combines high-performance liquid chromatography and electrospray ionisation tandem mass spectrometry (HPLC-ESI-MS/MS) to directly measure and quantify the CoA esters of 3alpha,7alpha,12alpha-trihydroxy- and 3alpha,7alpha-dihydroxy-5beta-cholestan-26-oic acid (THCA and DHCA). The latter are converted inside peroxisomes to the primary bile acids, cholic and chenodeoxycholic acids, respectively. Prior to MS/MS, esters were separated by reversed-phase HPLC on a C(18) column using an isocratic mobile phase (acetonitrile/water/2-propanol) and subsequently detected by multiple reaction monitoring. For quantification, the CoA ester of deuterium-labelled 3alpha,7alpha,12alpha-trihydroxy-5beta-cholan-24-oic acid (d(4)-CA) was used as internal standard. To complete an assay took less than 8 min. To verify the validity of the assay, the effect of peroxisomal proteins on the efficacy of extraction of the CoA esters was tested. To this end, variable amounts of the CoA esters were spiked with a fixed amount of either intact peroxisomes or peroxisomal matrix proteins and then extracted using a solid-phase extraction system. The CoA esters could be reproducibly recovered in the range of 0.1-4 micromol l(-1) (linear correlation coefficient R(2) > 0.99), with a detection limit of 0.1 micromol l(-1). In summary, electrospray ionization tandem mass spectrometry combined with HPLC as described here proved to be a rapid and versatile technique for the determination of bile acid CoA esters in a mixture with peroxisomal proteins. This suggests this technique to become a valuable tool in studies dealing with the multi-step biosynthesis of bile acids and its disturbances in disorders like the Zellweger syndrome.

Methylmalonic acid--an endogenous toxin?

Methylmalonic acid was previously considered as major neurotoxin in methylmalonic acidurias. In contrast, recent studies support the notion that other metabolites deriving from propionyl-coenzyme A, inducing synergistic inhibition of mitochondrial energy metabolism, are more important than methylmalonic acid to understand the neuropathogenesis of this disease. However, it is not yet known whether methylmalonic acid is involved in the induction of other organ manifestations in this disease, such as chronic renal failure.

S-Acetylglutathione normalizes intracellular glutathione content in cultured fibroblasts from patients with glutathione synthetase deficiency.

Glutathione synthetase deficiency is an autosomal recessive inherited metabolic defect in the gamma-glutamyl cycle. Decreased intracellular glutathione levels are one of the characteristic biochemical features. In this study we show that addition of S-acetylglutathione to the medium raised intracellular glutathione content in cultured fibroblasts from patients with glutathione synthetase deficiency. This has implications for the treatment of patients with this inborn error of metabolism.

Animal models for glutaryl-CoA dehydrogenase deficiency.

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.

Excitotoxicity and bioenergetics in glutaryl-CoA dehydrogenase deficiency.

Glutaryl-CoA dehydrogenase deficiency is an inherited organic acid disorder with predominantly neurological presentation. The biochemical hallmark of this disease is an accumulation and enhanced urinary excretion of two key organic acids, glutaric acid and 3-hydroxyglutaric acid. If untreated, acute striatal damage is often precipitated by febrile illnesses during a vulnerable period of brain development in infancy or early childhood, resulting in a dystonic dyskinetic movement disorder. 3-hydroxyglutaric and glutaric acids are structurally similar to glutamate, the main excitatory amino acid of the human brain, and are considered to play an important role in the pathophysiology of this disease. 3-hydroxyglutaric acid induces excitotoxic cell damage specifically via activation of N-methyl-D-aspartate receptors. It has also been suggested that secondary amplification loops potentiate the neurotoxic properties of these organic acids. Probable mechanisms for this effect include cytokine-stimulated NO production, a decrease in energy metabolism, and reduction of cellular creatine phosphate levels. Finally, maturation-dependent changes in the expression of neuronal glutamate receptors may affect the vulnerability of the immature brain to excitotoxic cell damage in this disease.

Challenges for basic research in glutaryl-CoA dehydrogenase deficiency.

During the last decades, efforts have been made to elucidate the complex mechanisms underlying neuronal damage in glutaryl-CoA dehydrogenase deficiency. A combination of in vitro and in vivo investigations have facilitated the development of several hypotheses, including the probable pathogenic role of accumulating glutaric acid and 3-hydroxyglutaric acid. However, there are still many shortcomings that limit an evidence-based approach to treating this inborn error of metabolism. Major future goals should include generation of a suitable animal model for acute striatal necrosis, investigation of the formation, distribution and exact intra- and extracellular concentrations of accumulating metabolites, a deeper understanding of striatal vulnerability, and systematic investigation of effects on cerebral gene expression during development and of the modulatory role of inflammatory cytokines.