The Role of Preclinical Models in Creatine Transporter Deficiency: Neurobiological Mechanisms, Biomarkers and Therapeutic Development

Creatine (Cr) Transporter Deficiency (CTD) is an X-linked metabolic disorder, mostly caused by missense mutations in the SLC6A8 gene and presenting with intellectual disability, autistic behavior, and epilepsy. There is no effective treatment for CTD and patients need lifelong assistance. Thus, the research of novel intervention strategies is a major scientific challenge. Animal models are an excellent tool to dissect the disease pathogenetic mechanisms and drive the preclinical development of therapeutics. This review illustrates the current knowledge about Cr metabolism and CTD clinical aspects, with a focus on mainstay diagnostic and therapeutic options. Then, we discuss the rodent models of CTD characterized in the last decade, comparing the phenotypes expressed within clinically relevant domains and the timeline of symptom development. This analysis highlights that animals with the ubiquitous deletion/mutation of SLC6A8 genes well recapitulate the early onset and the complex pathological phenotype of the human condition. Thus, they should represent the preferred model for preclinical efficacy studies. On the other hand, brain- and cell-specific conditional mutants are ideal for understanding the basis of CTD at a cellular and molecular level. Finally, we explain how CTD models might provide novel insight about the pathogenesis of other disorders, including cancer.

Sphingolipids as Regulators of Neuro-Inflammation and NADPH Oxidase 2

Neuro-inflammation accompanies numerous neurological disorders and conditions where it can be associated with a progressive neurodegenerative pathology. In a similar manner, alterations in sphingolipid metabolism often accompany or are causative features in degenerative neurological conditions. These include dementias, motor disorders, autoimmune conditions, inherited metabolic disorders, viral infection, traumatic brain and spinal cord injury, psychiatric conditions, and more. Sphingolipids are major regulators of cellular fate and function in addition to being important structural components of membranes. Their metabolism and signaling pathways can also be regulated by inflammatory mediators. Therefore, as certain sphingolipids exert distinct and opposing cellular roles, alterations in their metabolism can have major consequences. Recently, regulation of bioactive sphingolipids by neuro-inflammatory mediators has been shown to activate a neuronal NADPH oxidase 2 (NOX2) that can provoke damaging oxidation. Therefore, the sphingolipid-regulated neuronal NOX2 serves as a mechanistic link between neuro-inflammation and neurodegeneration. Moreover, therapeutics directed at sphingolipid metabolism or the sphingolipid-regulated NOX2 have the potential to alleviate neurodegeneration arising out of neuro-inflammation.

Organic acidurias: Major gaps, new challenges, and a yet unfulfilled promise

Organic acidurias (OADs) comprise a biochemically defined group of inherited metabolic diseases. Increasing awareness, reliable diagnostic work-up, newborn screening programs for some OADs, optimized neonatal and intensive care, and the development of evidence-based recommendations have improved neonatal survival and short-term outcome of affected individuals. However, chronic progression of organ dysfunction in an aging patient population cannot be reliably prevented with traditional therapeutic measures. Evidence is increasing that disease progression might be best explained by mitochondrial dysfunction. Previous studies have demonstrated that some toxic metabolites target mitochondrial proteins inducing synergistic bioenergetic impairment. Although these potentially reversible mechanisms help to understand the development of acute metabolic decompensations during catabolic state, they currently cannot completely explain disease progression with age. Recent studies identified unbalanced autophagy as a novel mechanism in the renal pathology of methylmalonic aciduria, resulting in impaired quality control of organelles, mitochondrial aging and, subsequently, progressive organ dysfunction. In addition, the discovery of post-translational short-chain lysine acylation of histones and mitochondrial enzymes helps to understand how intracellular key metabolites modulate gene expression and enzyme function. While acylation is considered an important mechanism for metabolic adaptation, the chronic accumulation of potential substrates of short-chain lysine acylation in inherited metabolic diseases might exert the opposite effect, in the long run. Recently, changed glutarylation patterns of mitochondrial proteins have been demonstrated in glutaric aciduria type 1. These new insights might bridge the gap between natural history and pathophysiology in OADs, and their exploitation for the development of targeted therapies seems promising.

Epilepsy in inherited neurotransmitter disorders: Spotlights on pathophysiology and clinical management

Inborn errors of neurotransmitter metabolism are ultrarare disorders affecting neurotransmitter biosynthesis, breakdown or transport or their essential cofactors. Neurotransmitter dysfunctions could also result from the impairment of neuronal receptors, intracellular signaling, vesicle release or other synaptic abnormalities. Epilepsy is the main clinical hallmark in some of these diseases (e.g. disorders of GABA metabolism, glycine encephalopathy) while it is infrequent in others (e.g. all the disorders of monoamine metabolism in exception for dihydropteridine reductase deficiency). This review analyzes the epileptogenic mechanisms, the epilepsy phenotypes and the principle for the clinical management of epilepsy in primary and secondary inherited disorders of neurotransmitter metabolism (disorders of GABA, serine and glycine metabolism, disorders of neurotransmitter receptors and secondary neurotransmitter diseases).

Inborn errors of metabolite repair

It is traditionally assumed that enzymes of intermediary metabolism are extremely specific and that this is sufficient to prevent the production of useless and/or toxic side-products. Recent work indicates that this statement is not entirely correct. In reality, enzymes are not strictly specific, they often display weak side activities on intracellular metabolites (substrate promiscuity) that resemble their physiological substrate or slowly catalyse abnormal reactions on their physiological substrate (catalytic promiscuity). They thereby produce non-classical metabolites that are not efficiently metabolised by conventional enzymes. In an increasing number of cases, metabolite repair enzymes are being discovered that serve to eliminate these non-classical metabolites and prevent their accumulation. Metabolite repair enzymes also eliminate non-classical metabolites that are formed through spontaneous (ie, not enzyme-catalysed) reactions. Importantly, genetic deficiencies in several metabolite repair enzymes lead to 'inborn errors of metabolite repair', such as L-2-hydroxyglutaric aciduria, D-2-hydroxyglutaric aciduria, 'ubiquitous glucose-6-phosphatase' (G6PC3) deficiency, the neutropenia present in Glycogen Storage Disease type Ib or defects in the enzymes that repair the hydrated forms of NADH or NADPH. Metabolite repair defects may be difficult to identify as such, because the mutated enzymes are non-classical enzymes that act on non-classical metabolites, which in some cases accumulate only inside the cells, and at rather low, yet toxic, concentrations. It is therefore likely that many additional metabolite repair enzymes remain to be discovered and that many diseases of metabolite repair still await elucidation.

Phosphocyclocreatine is the dominant form of cyclocreatine in control and creatine transporter deficiency patient fibroblasts

Creatine transporter deficiency (CTD) is a metabolic disorder resulting in cognitive, motor, and behavioral deficits. Cyclocreatine (cCr), a creatine analog, has been explored as a therapeutic strategy for the treatment of CTD. We developed a rapid, selective, and accurate HILIC ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method to simultaneously quantify the intracellular concentrations of cCr, creatine (Cr), creatine-d3 (Cr-d3), phosphocyclocreatine (pcCr), and phosphocreatine (pCr). Using HILIC-UPLC-MS/MS, we measured cCr and Cr-d3 uptake and their conversion to the phosphorylated forms in primary human control and CTD fibroblasts. Altogether, the data demonstrate that cCr enters cells and its dominant intracellular form is pcCr in both control and CTD patient cells. Therefore, cCr may replace creatine as a therapeutic strategy for the treatment of CTD.

Oxidative stress among L-2-hydroxyglutaric aciduria disease patients: evaluation of dynamic thiol/disulfide homeostasis

L-2-hydroxyglutaric aciduria (L2HGA) is an autosomal recessive disorder that is caused by deficiency of 2-hydroxyglutarate dehydrogenase. Pathophysiology of brain damage is poorly understood. In recent years, it was proposed that oxidative stress was elevated and led to brain injury. Aim of this study is to evaluate thiol/disulphide homeostasis as an indicator of oxidative stress in L2HGA patients who have been receiving antioxidant treatment. Sixteen L2HGA patients and 16 healthy individuals were included in the study. All the L2HGA patients were regularly followed up and presented neurological dysfunction at different grades. Fourteen patients had been receiving antioxidant treatment. Serum native thiol (-SH), total thiol (-SH + -S-S-) and disulphide (-S-S) levels were measured. Disulphide/native thiol, disulphide/total thiol and native thiol/total thiol ratios were calculated from these values. No significant difference was observed in -SH, -SH + -S-S-, -S-S levels between two groups. In addition to that, no increase of disulphide/native thiol and disulphide/total thiol ratios was detected. Thiol/disulphide homeostasis parameters were also compared between patients who had been receiving and not receiving antioxidant therapy; and between different types of antioxidant therapy and the results did not point to any significant difference. This is the first study that evaluates dynamic thiol/disulphide homeostasis as an indicator of oxidative stress in L2HGA and it has one of the largest sample sizes among previous studies. In our study we suggest that antioxidant therapy should be effective in preventing oxidative stress in L2HGA patients, which has been reported in previous studies and should be a part of standard therapy.

Synaptic metabolism: a new approach to inborn errors of neurotransmission

To date, inborn errors of neurotransmitters have been defined based on the classic concept of inborn error of metabolism (IEM), and they include defects in synthesis, catabolism, and transport pathways. However, the omics era is bringing insights into new diseases and is leading to an extended definition of IEM including new categories and mechanisms. Neurotransmission takes place at the synapse, the most specialized tight junction in the brain. The concept of "synaptic metabolism" would point to the specific chemical composition and metabolic functions of the synapse. Based on these specialized functions, we aim to provide a tentative overview about the major categories of IEM susceptible to affect neurotransmission. Small molecule defects (biogenic amines and amino acids) and energy defects are amongst the most prevalent diseases reported to disturb the concentration of CSF neurotransmitters. In these IEM, the neurological phenotypes have been largely described. Disorders of complex molecules are not typically considered as diseases affecting neurotransmission. However, most of them have been recently discovered and are involved in intracellular vesiculation, trafficking, processing, and quality control mechanisms. In this large group, neurotransmission is affected in disorders of chaperones and autophagy, disorders of the synaptic vesicle, and diseases affecting pre-synaptic membranes (synthesis and remodeling of complex lipids, defects of glycosylation). Disorders of the vesicle pools, receptor trafficking, and the chronobiology of neurotransmission are potentially emerging new categories. Finally, although not considered as IEM, channelopathies are a large group of diseases disturbing neurotransmitter homeostasis. New CSF biomarkers will probably contribute to improve the diagnosis of these disorders and find new therapeutic targets.

Functional assessment of creatine transporter in control and X-linked SLC6A8-deficient fibroblasts

Creatine transporter is currently the focus of renewed interest with emerging roles in brain neurotransmission and physiology, and the bioenergetics of cancer metastases. We here report on amendments of a standard creatine uptake assay which might help clinical chemistry laboratories to extend their current range of measurements of creatine and metabolites in body fluids to functional enzyme explorations. In this respect, short incubation times and the use of a stable-isotope-labeled substrate (D₃-creatine) preceded by a creatine wash-out step from cultured fibroblast cells by removal of fetal bovine serum (rich in creatine) from the incubation medium are recommended. Together, these measures decreased, by a first order of magnitude, creatine concentrations in the incubation medium at the start of creatine-uptake studies and allowed to functionally discriminate between 4 hemizygous male and 4 heterozygous female patients with X-linked SLC6A8 deficiency, and between this cohort of eight patients and controls. The functional assay corroborated genetic diagnosis of SLC6A8 deficiency. Gene anomalies in our small cohort included splicing site (c.912G > A [p.Ile260_Gln304del], c.778-2A > G and c.1495 + 2 T > G), substitution (c.407C > T) [p.Ala136Val] and deletion (c.635_636delAG [p.Glu212Valfs*84] and c.1324delC [p.Gln442Lysfs*21]) variants with reduced creatine transporter function validating their pathogenicity, including that of a previously unreported c.1324delC variant. The present assay adaptations provide an easy, reliable and discriminative manner for exploring creatine transporter activity and disease variations. It might apply to drug testing or other evaluations in the genetic and metabolic horizons covered by the emerging functions of creatine and its transporter, in a way, however, requiring and completed by additional studies on female patients and blood-brain barrier permeability properties of selected compounds. As a whole, the proposed assay of creatine transporter positively adds to currently existing measurements of this transporter activity, and determining on a large scale the extent of its exact suitability to detect female patients should condition in the future its transfer in clinical practice.

An overview of combined D-2- and L-2-hydroxyglutaric aciduria: functional analysis of CIC variants

Combined D-2- and L-2-hydroxyglutaric aciduria (D/L-2-HGA) is a devastating neurometabolic disorder, usually lethal in the first years of life. Autosomal recessive mutations in the SLC25A1 gene, which encodes the mitochondrial citrate carrier (CIC), were previously detected in patients affected with combined D/L-2-HGA. We showed that transfection of deficient fibroblasts with wild-type SLC25A1 restored citrate efflux and decreased intracellular 2-hydroxyglutarate levels, confirming that deficient CIC is the cause of D/L-2-HGA. We developed and implemented a functional assay and applied it to all 17 missense variants detected in a total of 26 CIC-deficient patients, including eight novel cases, showing reduced activities of varying degrees. In addition, we analyzed the importance of residues affected by these missense variants using our existing scoring system. This allowed not only a clinical and biochemical overview of the D/L-2-HGA patients but also phenotype-genotype correlation studies.

[Metabolic approach in epileptic encephalopathies in infants]

INTRODUCTION

Although the overall incidence of inborn errors of metabolism is low, their early diagnosis is essential, since some of them have a specific treatment.

DEVELOPMENT

We review the main treatable inborn errors of metabolism that can present as early-onset epileptic encephalopathies, together with their biochemical markers and their treatment.

CONCLUSIONS

It is important to think about the possibility of an inborn error of metabolism with a specific therapy, since it is crucial for this to be started as soon as possible in order to prevent permanent neurological damage.

Successful treatment of a patient with ethylmalonic encephalopathy by intravenous N-acetylcysteine

Ethylmalonic encephalopathy (EE) is an autosomal recessive devastating metabolic disorder affecting the brain, gastrointestinal tract, peripheral vessels and rarely the other vascular organs. We report a 10-month-old girl who presented as a meningococcemia clinic but later diagnosed ethylmalonic encephalopathy. Molecular analyses revealed a homozygous c.554 T > G; p. L185R mutation in ETHE1 gene. She was only partially benefited from riboflavine, coenzyme Q10, metronidazole, N-acetylcysteine and symptomatic treatment and discharged from hospital with the sequela of oxygene dependance and developmental delay. We observed N-acetylcysteine 100 mg/kg/day intravenous infusion theraphy may be the most important drug especially in comatous EE patients.

Impact of age at onset and newborn screening on outcome in organic acidurias

BACKGROUND AND AIM

To describe current diagnostic and therapeutic strategies in organic acidurias (OADs) and to evaluate their impact on the disease course allowing harmonisation.

METHODS

Datasets of 567 OAD patients from the E-IMD registry were analysed. The sample includes patients with methylmalonic (MMA, n = 164), propionic (PA, n = 144) and isovaleric aciduria (IVA, n = 83), and glutaric aciduria type 1 (GA1, n = 176). Statistical analysis included description and recursive partitioning of diagnostic and therapeutic strategies, and odds ratios (OR) for health outcome parameters. For some analyses, symptomatic patients were divided into those presenting with first symptoms during (i.e. early onset, EO) or after the newborn period (i.e. late onset, LO).

RESULTS

Patients identified by newborn screening (NBS) had a significantly lower median age of diagnosis (8 days) compared to the LO group (363 days, p < 0.001], but not compared to the EO group. Of all OAD patients 71 % remained asymptomatic until day 8. Patients with cobalamin-nonresponsive MMA (MMA-Cbl(-)) and GA1 identified by NBS were less likely to have movement disorders than those diagnosed by selective screening (MMA-Cbl(-): 10 % versus 39 %, p = 0.002; GA1: 26 % versus 73 %, p < 0.001). For other OADs, the clinical benefit of NBS was less clear. Reported age-adjusted intake of natural protein and calories was significantly higher in LO patients than in EO patients reflecting different disease severities. Variable drug combinations, ranging from 12 in MMA-Cbl(-) to two in isovaleric aciduria, were used for maintenance treatment. The effects of specific metabolic treatment strategies on the health outcomes remain unclear because of the strong influences of age at onset (EO versus LO), diagnostic mode (NBS versus selective screening), and the various treatment combinations used.

CONCLUSIONS

NBS is an effective intervention to reduce time until diagnosis especially for LO patients and to prevent irreversible cerebral damage in GA1 and MMA-Cbl(-). Huge diversity of therapeutic interventions hampers our understanding of optimal treatment.

Creatine synthesis and exchanges between brain cells: What can be learned from human creatine deficiencies and various experimental models?

While it has long been thought that most of cerebral creatine is of peripheral origin, the last 20 years has provided evidence that the creatine synthetic pathway (AGAT and GAMT enzymes) is expressed in the brain together with the creatine transporter (SLC6A8). It has also been shown that SLC6A8 is expressed by microcapillary endothelial cells at the blood-brain barrier, but is absent from surrounding astrocytes, raising the concept that the blood-brain barrier has a limited permeability for peripheral creatine. The first creatine deficiency syndrome in humans was also discovered 20 years ago (GAMT deficiency), followed later by AGAT and SLC6A8 deficiencies, all three diseases being characterized by creatine deficiency in the CNS and essentially affecting the brain. By reviewing the numerous and latest experimental studies addressing creatine transport and synthesis in the CNS, as well as the clinical and biochemical characteristics of creatine-deficient patients, our aim was to delineate a clearer view of the roles of the blood-brain and blood-cerebrospinal fluid barriers in the transport of creatine and guanidinoacetate between periphery and CNS, and on the intracerebral synthesis and transport of creatine. This review also addresses the question of guanidinoacetate toxicity for brain cells, as probably found under GAMT deficiency.

A mouse model of L-2-hydroxyglutaric aciduria, a disorder of metabolite repair

The purpose of the present work was to progress in our understanding of the pathophysiology of L-2-hydroxyglutaric aciduria, due to a defect in L-2-hydroxyglutarate dehydrogenase, by creating and studying a mouse model of this disease. L-2-hydroxyglutarate dehydrogenase-deficient mice (l2hgdh-/-) accumulated L-2-hydroxyglutarate in tissues, most particularly in brain and testis, where the concentration reached ≈ 3.5 μmol/g. Male mice showed a 30% higher excretion of L-2-hydroxyglutarate compared to female mice, supporting that this dicarboxylic acid is partially made in males by lactate dehydrogenase C, a poorly specific form of this enzyme exclusively expressed in testes. Involvement of mitochondrial malate dehydrogenase in the formation of L-2-hydroxyglutarate was supported by the commensurate decrease in the formation of this dicarboxylic acid when down-regulating this enzyme in mouse l2hgdh-/- embryonic fibroblasts. The concentration of lysine and arginine was markedly increased in the brain of l2hgdh-/- adult mice. Saccharopine was depleted and glutamine was decreased by ≈ 40%. Lysine-α-ketoglutarate reductase, which converts lysine to saccharopine, was inhibited by L-2-hydroxyglutarate with a Ki of ≈ 0.8 mM. As low but significant activities of the bifunctional enzyme lysine-α-ketoglutarate reductase/saccharopine dehydrogenase were found in brain, these findings suggest that the classical lysine degradation pathway also operates in brain and is inhibited by the high concentrations of L-2-hydroxyglutarate found in l2hgdh-/- mice. Pathological analysis of the brain showed significant spongiosis. The vacuolar lesions mostly affected oligodendrocytes and myelin sheats, as in other dicarboxylic acidurias, suggesting that the pathophysiology of this model of leukodystrophy may involve irreversible pumping of a dicarboxylate in oligodendrocytes. Neurobehavioral testing indicated that the mice mostly suffered from a deficit in learning capacity. In conclusion, the findings support the concept that L-2-hydroxyglutaric aciduria is a disorder of metabolite repair. The accumulation of L-2-hydroxyglutarate exerts toxic effects through various means including enzyme inhibition and glial cell swelling.

Successive distinct high-grade gliomas in L-2-hydroxyglutaric aciduria

Patients with L-2-hydroxyglutaric aciduria are at risk for developing cerebral neoplasms, particularly gliomas, as one of the optical isomers of the known oncometabolite, 2-hydroxyglutarate is produced in L-2-hydroxyglutaric aciduria. To illustrate the concept of sustained oncogenic potential in permanent exposure to L-2-hydroxyglutarate in brain tissue, we present the medical history of a patient with L-2-hydroxyglutaric aciduria who underwent surgery to remove a right temporal anaplastic astrocytoma and developed an anatomically distinct, but histopathologically similar, tumor in the left frontal region 40 months later. This is the first reported case of successive distinct gliomas in a patient with L-2-hydroxyglutaric aciduria. While this implies a significant, cumulative lifetime risk for cerebral neoplasms in patients with this rare organic aciduria, it also allows further insight into a unique mechanism of tumorigenesis in the brain.

Combined D2-/L2-hydroxyglutaric aciduria (SLC25A1 deficiency): clinical course and effects of citrate treatment

Combined D,L-2-hydroxyglutaric aciduria (DL-2HGA; OMIM #615182) is a rare neurometabolic disorder clinically characterized by muscular hypotonia, severe neurodevelopmental dysfunction, and intractable seizures associated with respiratory distress. Biochemically, DL-2HGA patients excrete increased amounts of D- and L-2-hydroxyglutarate (D2HG and L2HG, respectively), with predominance of D2HG, and α-ketoglutarate, and show a decrease in urinary citrate. Impaired function of the mitochondrial citrate carrier (CIC) due to pathogenic mutations within the SLC25A1 gene has been identified as the underlying molecular cause of the disease. CIC mediates efflux of the mitochondrial tricarboxylic acid (TCA) cycle intermediates citrate and isocitrate in exchange for cytosolic malate. Thus, depletion of cytosolic citrate as well as accumulation of citrate inside mitochondria have been considered to play a role in the pathophysiology of DL-2HGA. Here, we report for the first time on a patient with a genetically confirmed diagnosis of DL-2HGA and treatment with either malate or citrate. During malate treatment, urinary malate concentration increased, but beyond that, neither biochemical nor clinical alterations were observed. In contrast, treatment with citrate led to an increased urinary excretion of TCA cycle intermediates malate and succinate, and by trend to an increased concentration of urinary citrate. Furthermore, excretion of D2HG and L2HG was reduced during citrate treatment. Clinically, the patient showed stabilization with regard to frequency and severity of seizures. Treating DL-2HGA with citrate should be considered in other DL-2HGA patients, and its effects should be studied systematically.

Congenital MTHFR deficiency causing early-onset cerebral stroke in a case homozygous for MTHFR thermolabile variant

Hyperhomocysteinemia is a risk factor for early-onset venous thrombosis. It can be caused by genetic defects in methionine-homocysteine metabolism. The thermolabile variant of methylene-tetrahydrofolate reductase (MTHFR), c.677C>T, is one of the most common genetic condition, which has been associated with mild to moderate hyperhomocysteinemia, and carriers of this variant are at increased risk of an early-onset stroke-like episode. However, congenital MTHFR deficiency is a rare inborn error of folate metabolism, causing marked hyperhomocysteinemia, and its combination with the thermolabile variant is rarely reported. In this report, we describe a young adult with cerebral infarction. The patient was homozygous for the MTHFR thermolabile variant, but markedly elevated hyperhomocysteinemia led us to investigate the whole MTHFR gene, which revealed two novel MTHFR mutations. This is the first report of MTHFR deficiency in a Korean patient, and one of only a few cases reported in East Asian countries. Despite its rarity, our report underlines the importance of its identification in hyperhomocysteinemia for patient prognosis with appropriate management.

Current concepts in organic acidurias: understanding intra- and extracerebral disease manifestation

This review focuses on the pathophysiology of organic acidurias (OADs), in particular, OADs caused by deficient amino acid metabolism. OADs are termed classical if patients present with acute metabolic decompensation and multiorgan dysfunction or cerebral if patients predominantly present with neurological symptoms but without metabolic crises. In both groups, however, the brain is the major target. The high energy demand of the brain, the gate-keeping function of the blood-brain barrier, a high lipid content, vulnerable neuronal subpopulations, and glutamatergic neurotransmission all make the brain particularly vulnerable against mitochondrial dysfunction, oxidative stress, and excitotoxicity. In fact, toxic metabolites in OADs are thought to cause secondary impairment of energy metabolism; some of these toxic metabolites are trapped in the brain. In contrast to cerebral OADs, patients with classical OADs have an increased risk of multiorgan dysfunction. The lack of the anaplerotic propionate pathway, synergistic inhibition of energy metabolism by toxic metabolites, and multiple oxidative phosphorylation (OXPHOS) deficiency may best explain the involvement of organs with a high energy demand. Intriguingly, late-onset organ dysfunction may manifest even under metabolically stable conditions. This might be explained by chronic mitochondrial DNA depletion, increased production of reactive oxygen species, and altered gene expression due to histone modification. In conclusion, pathomechanisms underlying the acute disease manifestation in OADs, with a particular focus on the brain, are partially understood. More work is required to predict the risk and to elucidate the mechanism of late-onset organ dysfunction, extracerebral disease manifestation, and tumorigenesis.

Gastrointestinal and hepatic manifestations of mitochondrial disorders

Inherited defects of oxidative phosphorylation lead to heterogeneous, often multisystem, mitochondrial diseases. This review highlights those mitochondrial syndromes with prominent gastrointestinal and hepatic symptoms, categorised according to underlying disease mechanism. Mitochondrial encephalopathies with major gastrointestinal involvement include mitochondrial neurogastrointestinal encephalopathy and ethylmalonic encephalopathy, which are each associated with highly specific clinical and metabolic profiles. Mitochondrial hepatopathies are most frequently caused by defects of mitochondrial DNA maintenance and expression. Although mitochondrial disorders are notorious for extreme clinical, biochemical and genetic heterogeneity, there are some pathognomonic clinical and metabolic clues that suggest a specific diagnosis, and these are highlighted. An approach to diagnosis of these complex disorders is presented, together with a genetic classification, including mitochondrial DNA disorders and nuclear-encoded defects of mitochondrial DNA maintenance and translation, OXPHOS complex assembly and mitochondrial membrane lipids. Finally, supportive and experimental therapeutic options for these currently incurable diseases are reviewed, including liver transplantation, allogeneic haematopoietic stem cell transplantation and gene therapy.

Biochemical, molecular, and clinical diagnoses of patients with cerebral creatine deficiency syndromes

Cerebral creatine deficiency syndromes (CCDS) are a group of inborn errors of creatine metabolism that involve AGAT and GAMT for creatine biosynthesis disorders and SLC6A8 for creatine transporter (CT1) deficiency. Deficiencies in the three enzymes can be distinguished by intermediate metabolite levels, and a definitive diagnosis relies on the presence of deleterious mutations in the causative genes. Mutations and unclassified variants were identified in 41 unrelated patients, and 22 of these mutations were novel. Correlation of sequencing and biochemical data reveals that using plasma guanidinoacetate (GAA) as a biomarker has 100% specificity for both AGAT and GAMT deficiencies, but AGAT deficiency has decreased sensitivity in this assay. Furthermore, the urine creatine:creatinine ratio is an effective screening test with 100% specificity in males suspected of having creatine transporter deficiency. This test has a high false-positive rate due to dietary factors or dilute urine samples and lacks sensitivity in females. We conclude that biochemical screening for plasma GAA and measuring of the urine creatine:creatinine ratio should be performed for suspected CCDS patients prior to sequencing. Also, based on the results of this study, we feel that sequencing should only be considered if a patient has abnormal biochemical results on repeat testing.

Metabolic stroke in a late-onset form of isolated sulfite oxidase deficiency

We report the first case of late-onset isolated sulfite oxidase deficiency (ISOD) presenting with a stroke-like episode. Clinical, biochemical and neuroradiological features at diagnosis and during follow-up after dietary treatment intervention are described. Furthermore, pathogenic mechanisms possibly leading to stroke in ISOD are discussed.

Ethylmalonic encephalopathy associated with crescentic glomerulonephritis

Ethylmalonic encephalopathy (EE) is a rare autosomal recessive disorder caused by mutations in the ETHE1 gene and characterized by chronic diarrhea, encephalopathy, relapsing petechiae and acrocyanosis. Nephrotic syndrome has been described in an infant with EE but the renal histology findings were not described in previous reports. We report a Palestinian girl with EE who presented with chronic diarrhea, encephalopathy, petechial rash and acrocyanosis. Subsequently, she developed progressive deterioration of renal function caused by rapidly progressive glomerulonephritis resulting in death within few days. This is, to our knowledge, the first reported occurrence of rapidly progressive glomerulonephritis in a child with ethylmalonic encephalopathy. Its presence is a serious complication associated with poor prognosis and may be explained by the diffuse vascular damage.

Progress in understanding 2-hydroxyglutaric acidurias

The organic acidurias D: -2-hydroxyglutaric aciduria (D-2-HGA), L-2-hydroxyglutaric aciduria (L-2-HGA), and combined D,L-2-hydroxyglutaric aciduria (D,L-2-HGA) cause neurological impairment at young age. Accumulation of D-2-hydroxyglutarate (D-2-HG) and/or L-2-hydroxyglutarate (L-2-HG) in body fluids are the biochemical hallmarks of these disorders. The current review describes the knowledge gathered on 2-hydroxyglutaric acidurias (2-HGA), since the description of the first patients in 1980. We report on the clinical, genetic, enzymatic and metabolic characterization of D-2-HGA type I, D-2-HGA type II, L-2-HGA and D,L-2-HGA, whereas for D-2-HGA type I and type II novel clinical information is presented which was derived from questionnaires.

The basis of inborn errors of metabolism for neuroradiologists

Although relatively common together, inborn errors of metabolism (IEM) are individually rare, which makes difficult for most neuroradiologists to accumulate significant experience on these conditions. To facilitate the diagnostic approach, several schemes based on clinical, biochemical, genetic, and neuroimaging criteria have been proposed. Neuroimaging techniques have a great potential to improve diagnostic accuracy in as well as follow-up and management of the patients with an IEM. This article aimed to provide a nonexhaustive review of some basic information related to genetic conditions such as pattern of inheritance, penetrance, variable expressivity, genotype, phenotype and to provide information on how to classify, investigate, diagnose, and treat IEM besides highlighting some findings that would aid neuroradiologists to improve the diagnostic accuracy of imaging techniques, particularly magnetic resonance imaging.

Encephalopathy caused by ablation of very long acyl chain ceramide synthesis may be largely due to reduced galactosylceramide levels

Sphingolipids (SLs) act as signaling molecules and as structural components in both neuronal cells and myelin. We now characterize the biochemical, histological, and behavioral abnormalities in the brain of a mouse lacking very long acyl (C22-C24) chain SLs. This mouse, which is defective in the ability to synthesize C22-C24-SLs due to ablation of ceramide synthase 2, has reduced levels of galactosylceramide (GalCer), a major component of myelin, and in particular reduced levels of non-hydroxy-C22-C24-GalCer and 2-hydroxy-C22-C24- GalCer. Noteworthy brain lesions develop with a time course consistent with a vital role for C22-C24-GalCer in myelin stability. Myelin degeneration and detachment was observed as was abnormal motor behavior originating from a subcortical region. Additional abnormalities included bilateral and symmetrical vacuolization and gliosis in specific brain areas, which corresponded to some extent to the pattern of ceramide synthase 2 expression, with astrogliosis considerably more pronounced than microglial activation. Unexpectedly, unidentified storage materials were detected in lysosomes of astrocytes, reminiscent of the accumulation that occurs in lysosomal storage disorders. Together, our data demonstrate a key role in the brain for SLs containing very long acyl chains and in particular GalCer with a reduction in their levels leading to distinctive morphological abnormalities in defined brain regions.

Acute administration of 5-oxoproline induces oxidative damage to lipids and proteins and impairs antioxidant defenses in cerebral cortex and cerebellum of young rats

5-Oxoproline accumulates in glutathione synthetase deficiency, an autossomic recessive inherited disorder clinically characterized by hemolytic anemia, metabolic acidosis, and severe neurological symptoms whose mechanisms are poorly known. In the present study we investigated the effects of acute subcutaneous administration of 5-oxoproline to verify whether oxidative stress is elicited by this metabolite in vivo in cerebral cortex and cerebellum of 14-day-old rats. Our results showed that the acute administration of 5-oxoproline is able to promote both lipid and protein oxidation, to impair brain antioxidant defenses, to alter SH/SS ratio and to enhance hydrogen peroxide content, thus promoting oxidative stress in vivo, a mechanism that may be involved in the neuropathology of gluthatione synthetase deficiency.

PNPO deficiency: an under diagnosed inborn error of pyridoxine metabolism

The rare autosomal recessive disorder pyridoxine 5'-phosphate oxidase (PNPO) deficiency is a recently described cause of neonatal and infantile seizures. Clinical evaluation, and biochemical and genetic testing, were performed on a neonate with intractable seizures who did not respond to anticonvulsant drugs and pyridoxine. Sequencing of the PNPO gene revealed a novel homozygous c.284G>A transition in exon 3, resulting in arginine to histidine substitution and reduced activity of the PNPO mutant to 18% relative to the wild type. This finding enabled molecular prenatal diagnosis in a subsequent pregnancy, accurate genetic counseling in the large inbred family, and population screening.

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.

Hyperornithinemia–hyperammonemia–homocitrullinuria syndrome with stroke-like imaging presentation: Clinical, biochemical and molecular analysis

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is an autosomal recessive disorder caused by mutations in ORNT1 gene that encodes a mitochondrial ornithine transporter. It has variable clinical presentations with episodic hyperammonemia, liver dysfunction, and chronic neurological manifestations. In this work, we report the findings of HHH syndrome in 3 Saudi siblings. The 4-year-old proband presented with recurrent Reye-like episodes, hypotonia, and multiple stroke-like lesions on brain MRI. Biochemical and molecular analysis confirmed that she had HHH syndrome. She significantly improved on protein restriction and sodium benzoate. Her two older siblings have milder phenotypes with protein intolerance and learning problems. In comparison to their sister, their homocitrulline and orotic acid were only mildly elevated even before treatment. The three patients were homozygous for a novel mutation in ORNT1 with a Gly220Arg change. In view of the CNS lesions, which initially were felt to be suggestive of MELAS, we sequenced the entire mtDNA genome and no potential pathogenic mutations were detected. Analysis of ORNT2 did not provide explanation of the clinical and biochemical variability. This work presents a yet unreported CNS involvement pattern, notably multiple supratentorial stroke-like lesions in association with HHH syndrome. Moreover, it illustrates considerable clinical/biochemical correlation, and describes a novel mutation. We suggest including HHH syndrome in the differential diagnosis of patients found to have stroke-like lesions on brain MRI.

Neuronal and glial accumulation of alpha- and beta-synucleins in human lipidoses

A number of the lysosomal storage diseases that have now been characterized are associated with intra-lysosomal accumulation of lipids, caused by defective lysosomal enzymes. We have previously reported neuronal accumulation of both alpha- and beta-synucleins in brain tissue of a GM2 gangliosidosis mouse model. Although alpha-synuclein has been implicated in several neurodegenerative disorders including Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, its functions remain largely unclear. In our present study, we have examined a cohort of human lipidosis cases, including Sandhoff disease, Tay-Sachs disease, metachromatic leukodystrophy, beta-galactosialidosis and adrenoleukodystrophy, for the expression of alpha- and beta-synucleins and the associated lipid storage levels. The accumulation of alpha-synuclein was found in brain tissue in not only cases of lysosomal storage diseases, but also in instances of adrenoleukodystrophy, which is a peroxisomal disease. alpha-synuclein was detected in both neurons and glial cells of patients with these two disorders, although its distribution was found to be disease-dependent. In addition, alpha-synuclein-positive neurons were also found to be NeuN-positive, whereas NeuN-negative neurons did not show any accumulation of this protein. By comparison, the accumulation of beta-synuclein was detectable only in the pons of Sandhoff disease cases. This differential accumulation of alpha- and beta-synucleins in human lipidoses may be related to functional differences between these two proteins. In addition, the accumulation of alpha-synuclein may also be a condition that is common to lysosomal storage diseases and adrenoleukodystrophies that show an enhanced expression of this protein upon the elevation of stored lipids.

Evidence for oxidative stress in tissues derived from succinate semialdehyde dehydrogenase-deficient mice

Animal models of inborn errors of metabolism are useful for investigating the pathogenesis associated with the corresponding human disease. Since the mechanisms involved in the pathophysiology of succinate semialdehyde dehydrogenase (SSADH) deficiency (Aldh5a1; OMIM 271980) are still not established, in the present study we evaluated the tissue antioxidant defences and lipid peroxidation in various cerebral structures (cortex, cerebellum, thalamus and hippocampus) and in the liver of SSADH-deficient mice. The parameters analysed were total radical-trapping antioxidant potential (TRAP) and glutathione (GSH) levels, the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), as well as thiobarbituric acid-reactive substances (TBARS). We first observed that the tissue nonenzymatic antioxidant defences were significantly reduced in the SSADH-deficient animals, particularly in the liver (decreased TRAP and GSH) and in the cerebral cortex (decreased GSH), as compared to the wild-type mice. Furthermore, SOD activity was significantly increased in the liver and cerebellum, whereas the activity of CAT was significantly higher in the thalamus. In contrast, GPx activity was significantly diminished in the hippocampus. Finally, we observed that lipid peroxidation (TBARS levels) was markedly increased in the liver and cerebral cortex, reflecting a high lipid oxidative damage in these tissues. Our data showing an imbalance between tissue antioxidant defences and oxidative attack strongly indicate that oxidative stress is involved in the pathophysiology of SSADH deficiency in mice, and likely the corresponding human disorder.

HIF activation and VEGF overexpression are coupled with ZO-1 up-phosphorylation in the brain of dystrophic mdx mouse

In Duchenne muscular dystrophy (DMD) metabolic and structural alterations of the central nervous system are described. Here, we investigated in the brain of 10 mdx mice and in five control ones, the expression of hypoxia inducible factor-1alpha (HIF-1alpha) and we correlated it with the expression of vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor-2 (VEGFR-2) and of the endothelial tight junction proteins zonula occludens-1 (ZO-1) and claudin-1. Results showed an activation of mRNA HIF-1alpha by reverse transcription polymerase chain reaction (RT-PCR) and a strong HIF1-alpha labeling of perivascular glial cells and cortical neurons by immunohistochemistry, in mdx mouse. Moreover, overexpression of VEGF and VEGFR-2, respectively, in neurons and in endothelial cells coupled with changes to endothelial ZO-1 and claudin-1 expression in the latter were detected by immunoblotting and immunohistochemistry, in the mdx brain. Furthermore, by immunoprecipitation, an up-phosphorylation of ZO-1 was demonstrated in mdx endothelial cells in parallel with the reduction in ZO-1 protein content. These data suggest that the activation of HIF-1alpha in the brain of dystrophic mice coupled with VEGF and VEGFR-2 up-regulation and ZO-1 and claudin-1 rearrangement might contribute to both blood-brain barrier opening and increased angiogenesis.

Autistic disorder in 2 children with mitochondrial disorders

Autistic disorder is a heterogeneous disorder. The majority of the cases are idiopathic, and only a small number of the autistic children have associated secondary diagnosis. This article reports 2 children with mitochondrial disorders associated with autistic disorder fulfilling the diagnostic criteria of the American Psychiatric Association Manual of Psychiatric Diseases, 4th edition, and briefly reviews the literature on autistic disorder associated with mitochondrial disorders.

Hyperammonemia-induced toxicity for the developing central nervous system

In pediatric patients, hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle deficiencies or organic acidemias. The brain is much more susceptible to the deleterious effects of ammonium during development than in adulthood. Hyperammonemia can provoke irreversible damages to the developing central nervous system that lead to cortical atrophy, ventricular enlargement and demyelination, responsible for cognitive impairment, seizures and cerebral palsy. Until recently, the mechanisms leading to these irreversible cerebral damages were poorly understood. Using experimental models allowing the analysis of the neurotoxic effects of ammonium on the developing brain, these last years have seen the emergence of new clues showing that ammonium exposure alters several amino acid pathways and neurotransmitter systems, as well as cerebral energy metabolism, nitric oxide synthesis, oxidative stress, mitochondrial permeability transition and signal transduction pathways. Those alterations may explain neuronal loss and impairment of axonal and dendritic growth observed in the different models of congenital hyperammonemia. Some neuroprotective strategies such as the potential use of NMDA receptor antagonists, nitric oxide inhibitors, creatine and acetyl-l-carnitine have been suggested to counteract these toxic effects. Unraveling the molecular mechanisms involved in the chain of events leading to neuronal dysfunction under hyperammonemia may be useful to develop new potential strategies for neuroprotection.

Molybdenum cofactor deficiency: clinical features in a Turkish patient

The molybdenum cofactor is essential for the function of sulphite oxidase, xanthine dehydrogenase, and aldehyde oxidase enzymes. Molybdenum cofactor deficiency (MoCD) is a fatal disease resulting in severe neurological damage and death in early childhood. MoCD is an autosomal recessive condition which may mimic ischaemic encephalopathy. Although milder cases with later onset and less severe symptoms have been identified, the classic presentation involves neonatal seizures, progressive encephalopathy and death at an early age. There is currently no effective therapy, and the prognosis is poor. The disorder should be considered in all cases of intractable seizures in the newborn period and infants with clinical and radiological features of ischaemic encephalopathy, especially when no obvious lesion is detected. Blood uric acid measurement should be included in the battery of tests to be performed in all neonates' refractory seizures. We reported here an infant with MoCD who presented with hypoxic ischaemic encephalopathy and identified a novel mutation, c.130C>T in cDNA of the MOCS2 gene from the infant.

3-Hydroxy-3-methylglutaryl coenzyme a lyase deficiency with reversible white matter changes after treatment

We report the case of an 8-month-old infant with 3-hydroxy-3-methylglutaryl coenzyme A deficiency (OMIM 246450), an inborn error of leucine catabolism and ketogenesis, who presented with nonketotic hypoglycemia and seizures. He demonstrated reversible white matter changes on serial brain magnetic resonance imaging, together with clinical normalization, after initiation of a leucine-restricted diet.

Impaired complex III assembly associated with BCS1L gene mutations in isolated mitochondrial encephalopathy

We investigated two unrelated children with an isolated defect of mitochondrial complex III activity. The clinical picture was characterized by a progressive encephalopathy featuring early-onset developmental delay, spasticity, seizures, lactic acidosis, brain atrophy and MRI signal changes in the basal ganglia. Both children were compound heterozygotes for novel mutations in the human bc1 synthesis like (BCS1L) gene, which encodes an AAA mitochondrial protein putatively involved in both iron homeostasis and complex III assembly. The pathogenic role of the mutations was confirmed by complementation assays, using a DeltaBcs1 strain of Saccharomyces cerevisiae. By investigating complex III assembly and the structural features of the BCS1L gene product in skeletal muscle, cultured fibroblasts and lymphoblastoid cell lines from our patients, we have demonstrated, for the first time in a mammalian system, that a major function of BCS1L is to promote the maturation of complex III and, more specifically, the incorporation of the Rieske iron-sulfur protein into the nascent complex. Defective BCS1L leads to the formation of a catalytically inactive, structurally unstable complex III. We have also shown that BCS1L is contained within a high-molecular-weight supramolecular complex which is clearly distinct from complex III intermediates.

Ethylmalonic encephalopathy: clinical and biochemical observations

Ethylmalonic encephalopathy (EE) is a rare, recently defined inborn error of metabolism which affects the brain, gastrointestinal system and peripheral blood vessels and is characterized by a unique constellation of clinical and biochemical features. A 7-month-old male, who presented with psychomotor retardation, chronic diarrhea and relapsing petechiae is described with the objective of highlighting the biochemical and neuroradiological features of this disorder as well as the effect of high-dose riboflavin therapy. Urinary organic acid analysis revealed markedly increased excretion of ethylmalonic acid, isobutyrylglycine, 2-methylbutyrylglycine and isovalerylglycine. Acylcarnitine analysis in dried blood spots showed increased butyrylcarnitine. Short-chain acyl-CoA dehydrogenase (SCAD) activity in muscle was normal as were mitochondrial OXPHOS enzyme activities in cultured skin fibroblasts. In skeletal muscle the catalytic activity of complex II was decreased. Brain MRI revealed bilateral and symmetrical atrophy in the fronto-temporal areas, massive enlargement of the subarachnoid spaces and hyperdensities on T (2) sequences of the basal ganglia. Mutation analysis of the ETHE1 gene demonstrated homozygosity for the Arg163Gly mutation, confirming the diagnosis of EE at a molecular level. On repeat MRI, a significant deterioration was seen, correlating well with the clinical deterioration of the patient.

Glutaric aciduria type 1: clinical, biochemical and molecular findings in patients from Israel

Glutaric aciduria type 1 (GA1) is a rare cerebral organic aciduria which typically manifests as an acute encephalopathic crisis followed by profound long-term neurological handicap. We report the diagnosis of 12 new patients from a single laboratory in Israel during a 5-year period. Eleven of the 12 were of Palestinian origin, and only two were related. One patient was asymptomatic whilst one was mildly, one moderately and nine severely affected, two of whom had unusual MRI findings. Two patients had normal glutaric acid excretion and normal blood glutarylcarnitine levels yet glutarylcarnitine excretion was increased, indicating its utility as a diagnostic marker. Four novel GCDH mutations (Thr193_Arg194insHis, Asn329Ser, Thr341Pro, Met405Val) and five previously reported mutations (Ser119Leu, Leu283Pro, Ala293Thr, Gly390Arg and Thr416Ile) were identified. Severely and mildly affected or even asymptomatic patients shared the same genotypes (Thr416Ile/Thre416Ile and Aal293Thr/Thr193_Arg194insHis). Knowledge of the responsible mutation enabled successful prenatal diagnosis on chorionic villous DNA in three families. In conclusion, GA1 is genetically heterogeneous and has a relatively high incidence in the Palestinian population, reflecting the historical tradition of marriages within extended kindreds, particularly in isolated villages. Additional genetic and/or environmental factors must account for the phenotypic heterogeneity in patients with the same genotype. The diagnosis was not suspected in the majority of cases despite typical clinical and/or neuroimaging features, suggesting that glutaric aciduria may be under-diagnosed. Greater awareness of glutaric aciduria amongst pediatricians, neonatologists and radiologists is the key to identifying the disorder in the presymptomatic phase and preventing its catastrophic consequences.

Xeroderma pigmentosum, trichothiodystrophy and Cockayne syndrome: a complex genotype-phenotype relationship

Patients with the rare genetic disorders, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS) have defects in DNA nucleotide excision repair (NER). The NER pathway involves at least 28 genes. Three NER genes are also part of the basal transcription factor, TFIIH. Mutations in 11 NER genes have been associated with clinical diseases with at least eight overlapping phenotypes. The clinical features of these patients have some similarities but also have marked differences. NER is involved in protection against sunlight-induced DNA damage. While XP patients have 1000-fold increase in susceptibility to skin cancer, TTD and CS patients have normal skin cancer risk. Several of the genes involved in NER also affect somatic growth and development. Some patients have short stature and immature sexual development. TTD patients have sulfur deficient brittle hair. Progressive sensorineural deafness is an early feature of XP and CS. Many of these clinical diseases are associated with developmental delay and progressive neurological degeneration. The main neuropathology of XP is a primary neuronal degeneration. In contrast, CS and TTD patients have reduced myelination of the brain. These complex neurological abnormalities are not related to sunlight exposure but may be caused by developmental defects as well as faulty repair of DNA damage to neuronal cells induced by oxidative metabolism or other endogenous processes.

Human coenzyme Q10 deficiency

Ubiquinone (coenzyme Q₁₀ or CoQ₁₀) is a lipid-soluble component of virtually all cell membranes and has multiple metabolic functions. Deficiency of CoQ₁₀ (MIM 607426) has been associated with five different clinical presentations that suggest genetic heterogeneity, which may be related to the multiple steps in CoQ₁₀ biosynthesis. Patients with all forms of CoQ₁₀ deficiency have shown clinical improvements after initiating oral CoQ₁₀ supplementation. Thus, early diagnosis is of critical importance in the management of these patients. This year, the first molecular defect causing the infantile form of primary human CoQ₁₀ deficiency has been reported. The availability of genetic testing will allow for a better understanding of the pathogenesis of this disease and early initiation of therapy (even presymptomatically in siblings of patients) in this otherwise life-threatening infantile encephalomyopathy.

Infant mice with glutaric acidaemia type I have increased vulnerability to 3-nitropropionic acid toxicity

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.

Is Huntington's a glutamine storage disease?

Huntington's disease is a neurological disorder caused by the expansion of a polyglutamine tract in the protein huntingtin. Several other neurological diseases also result from the expansion of polyglutamine regions in different proteins. Despite intense efforts, no definitive biochemical or physiological role for huntingtin has been described, nor has a function been assigned to the polyglutamine region in unaffected individuals. This article presents the hypothesis that polyglutamine expansions within huntingtin and other polyglutamine proteins provide a function in and of themselves. Incorporating multiple glutamine residues into a protein during synthesis, and releasing them during protein turnover, may represent a means of minimizing interruptions in brain levels of glutamine and glutamate during periods of malnutrition. The number and variety of different proteins containing polyglutamine expansions can be interpreted as a series of evolutionary "experiments" toward a nontoxic form for glutamine storage.

Glutaric acidemia type 1

Glutaric acidemias comprise different disorders resulting in an increased urinary excretion of glutaric acid. Glutaric acidemia type 1 (GA-1) is an autosomal recessive disorder of lysine, hydroxylysine, and tryptophan metabolism caused by deficiency of glutaryl-CoA dehydrogenase. It results in the accumulation of 3-hydroxyglutaric and glutaric acid. Affected patients can present with brain atrophy and macrocephaly and with acute dystonia secondary to striatal degeneration in most cases triggered by an intercurrent childhood infection with fever between 6 and 18 months of age. This disorder can be identified by increased glutaryl (C5DC) carnitine on newborn screening. Urine organic acid analysis indicates the presence of excess 3-OH-glutaric acid, and urine acylcarnitine profile shows glutaryl carnitine as the major peak. Therapy consists in carnitine supplementation to remove glutaric acid, a diet restricted in amino acids capable of producing glutaric acid, and prompt treatment of intercurrent illnesses. Early diagnosis and therapy reduce the risk of acute dystonia in patients with GA-1.

Creatine prevents behavioral alterations caused by methylmalonic acid administration into the hippocampus of rats in the open field task

Although a variable degree of psychomotor delay/mental retardation is found in a considerable number of patients affected by methylmalonic acidemia, the mechanisms underlying the neuropathology of this disorder are still poorly defined. The present study investigated the effect of acute intrahippocampal administration of methylmalonic acid (MMA), the biochemical hallmark of this disease, on rat behavior in the open field task. Cannulated 60-day-old male Wistar rats received bilateral intrahippocampal injection of MMA (0.1-1.0 micromol) 10 min before training. Controls received 0.1-1.0 micromol NaCl. Testing session was performed 24 h later. We observed that rats administered with 1.0 micromol MMA, but not with lower doses, did not habituate in the open field task, reflecting a deficit of performance. Motor activity, assessed by the number of crossing responses, was the same at training for the groups infused with MMA or NaCl. The effect of MK-801 (15 nmol) and succinate (1.5 micromol) administered 30 min before MMA injection, and of creatine (50 mg/kg, i.p.) administered twice a day for 3 days on the behavioral alterations provoked by MMA in the open field task revealed that only the energetic substrate creatine prevented these effects, reflecting a possible compromise of brain energy production by MMA. The results indicate that high intrahippocampal concentrations of the major metabolite accumulating in methylmalonic acidemia compromises brain functioning, causing deficit of performance in the open field task that may be related to the psychomotor delay/mental retardation observed in the affected patients.