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The expanding phenotype of GLUT1-deficiency syndrome
Brain & Development 31 (2009) 545–552 www.elsevier.com/locate/braindev
Review article
The expanding phenotype of GLUT1-deficiency syndrome Knut Brockmann * Department of Pediatrics and Pediatric Neurology, Faculty of Medicine, Georg August University, Robert-Koch-Str. 40, 37075 Go¨ttingen, Germany Received 31 October 2008; accepted 16 February 2009
Abstract Transport of glucose from the bloodstream across the blood–brain barrier to the central nervous system is facilitated by glucose transport protein type 1 (GLUT1), the first member of the solute carrier family 2 (SLC2). Heterozygous mutations in the GLUT1/ SLC2A1 gene, occurring de novo or inherited as an autosomal dominant trait, result in cerebral energy failure and a clinical condition termed GLUT1-deficiency syndrome (GLUT1-DS). Clinical features usually comprise motor and mental developmental delay, seizures with infantile onset, deceleration of head growth often resulting in acquired microcephaly, and a movement disorder with ataxia, dystonia, and spasticity. Subsequent to the delineation of this classic phenotype the variability of signs and symptoms in GLUT1-DS is being recognized. Patients with (i) carbohydrate-responsive symptoms, with (ii) predominant ataxia or dystonia, but without seizures, and with (iii) paroxysmal exertion-induced dyskinesia and seizures have been reported. Common laboratory hallmark in all phenotypes is the reduced glucose level in cerebrospinal fluid with lowered CSF-to-blood glucose ratio. Treatment with a ketogenic diet results in marked improvement of seizures and movement disorders. Ó 2009 Elsevier B.V. All rights reserved. Keywords: Glucose transport protein type 1; Epilepsy; Movement disorders; Paroxysmal dyskinesia; Hypoglycorrhachia; Ketogenic diet
1. Introduction A continuous supply of glucose provides the main energy source for the brain. Given the strict separation of the central nervous system from the bloodstream by the blood–brain barrier it was uncertain for a long time how glucose can cross the lipophilic plasma membranes and pass into the brain in quantities which are sufficient to meet the energy requirements of cerebral tissue. Christian Crone, a physiologist at the University of Copenhagen, Denmark, was obviously the first to postulate a transport system for the facilitated transfer of glucose from blood into brain tissue [1]. He studied the mechanism of glucose transfer in anaesthetized dogs after intracarotid injection of glucose + [14C]glucose with subse*
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quent sampling from the superior sagittal sinus. The fraction of glucose which passed into the cerebral tissue fell with increasing concentration of glucose. He measured an extraction of almost 50% at low concentrations and of 10% at high concentrations. Furthermore, a comparison of the unidirectional passage of [14C]glucose with the passage of fructose showed consistently higher extraction for glucose. These findings provided evidence for a ‘‘special transport mechanism for glucose”, and Crone suggested ‘‘that the endothelial cells in the cerebral capillaries are responsible for the transport of glucose across the blood–brain barrier” [1]. Subsequently four decades of physiological, biochemical, and genetic research have provided a long list of glucose transport proteins [2]. They are assigned to two distinct families of structurally related glucose transporters and mediate the facilitative and sodiumdependent glucose transport processes. Apart from
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GLUT1-deficiency syndrome (GLUT1-DS), clinical conditions associated with a deficiency of one of these transporters have been recognized so far only for few of them, e.g. glucose–galactose malabsorption (SGLT1), renal glycosuria (SGLT2), and Fanconi–Bickel syndrome (GLUT2). 2. Molecular basis Within the family of facilitative glucose transporters comprising 13 proteins (gene symbol SLC2A, protein symbol GLUT), GLUT1 was the first identified member, and in 1985 GLUT1/SLC2A1 was the first gene to be cloned and sequenced [3]. The gene is located on the short arm of chromosome 1 (1p34.2), is 35 kb in length, and consists of 10 exons. GLUT1 is constitutively expressed in most tissues and selectively expressed in erythrocytes, brain microvessels and astroglia. Two distinct molecular forms of GLUT1 with molecular weights of 55 and 45 kDa differ only in the extent of glycosylation. The 55 kDa form is found predominantly in the endothelial cells of brain microvessels and erythrocytes, where it is the principal (if not exclusive) glucose transporter. The 45 kDa form is detected in most cells including astrocytes and may be responsible for basal glucose uptake into these tissues. GLUT1 is a 492amino acid integral membrane protein with intracellular amino and carboxyl termini and 12 transmembrane domains, spanning the plasma membrane as a-helices (H1-H12). Presumably two 6-helical domain halves are separated by the large intercellular loop between helices 6 and 7. The postulated three-dimensional structure is characterized by a central channel across the protein that is essentially formed by helices 2, 4, 5, 7, 8, and 10 and connects the extracellular and intracellular environments [4]. Only one opening is proposed at each end. The domains crucial for transport and pathogenicity are clustered in two groups: one around the central channel, and the other in the long intracellular loop. Aberrant GLUT1 protein may result from truncation due to frameshift mutations, alteration of substrate binding sites due to missense mutations, or absence of protein due to allele deletion. In all cases, the normal allele contributes approximately 50% of functional GLUT1 protein to the plasma membrane. Most SCL2A1 mutations detected occurred de novo, in familial cases the condition is inherited as an autosomal dominant trait. 3. Classic phenotype Recognition of the clinical features associated with GLUT1 deficiency was inaugurated with the seminal discovery made by Darryl C. De Vivo in 1991 [5]. He reported two infants with developmental delay, seizures, mild movement disorder, and acquired micro-
cephaly. Onset of seizures was as early as 2 months in both patients. There were signs of mild ataxia and spasticity. In one patient, a deceleration of head growth was documented resulting in acquired microcephaly at 7 months of age. A cranial MRI suggested a mild delay in myelination. In both infants, multiple lumbar punctures at various times revealed persistent unexplained low glucose concentrations in CSF (hypoglycorrhachia). With normal blood glucose levels, the ratio of CSF glucose to blood glucose was markedly reduced in both patients and ranged from 0.19 to 0.35 (normal, 0.65). Concentrations of lactate in CSF were low as well. As the recognized causes of low glucose concentrations in CSF, such as bacterial meningitis, subarachnoid hemorrhage, and hypoglycemia, were not present, De Vivo considered a defect in the transport of glucose from blood to the brain. This assumption prompted him to treat both patients with a ketogenic diet, providing ketone bodies as an alternative fuel source for the brain. This treatment resulted in cessation of seizures in both patients and in nearly normal neurologic development in one patient who had received ketogenic diet as early as at 7.5 months of age. The hypothesis of a defect in glucose transport from blood into the brain was confirmed by demonstration of reduced binding of cytochalasin B (a fungal product that binds to the glucose transporter proteins) to the erythrocyte membranes of both patients and reduced uptake of 3-Omethyl-D-glucose in red blood cells of one patient. Later the index case was found to have a null allele producing hemizygosity and haploinsufficiency of the GLUT1/SLC2A1 gene [6]. During the following decade more than 40 patients with GLUT1-DS (OMIM #606777) have been identified, mainly in North America, some in Australia. Dominant clinical features comprise infantile seizures, developmental delay, and deceleration of head growth with acquired microcephaly in most patients. Seizures are described as brief, subtle myoclonic limb jerking with staring alternating with eye-rolling, a sudden onset of pallor, a dazed expression, or horizontal roving eye movements, unresponsiveness, and hypotonia as well as head bobbing. Paroxysmal events of possibly non-epileptic nature include intermittent ataxia, periodic confusion, periodic weakness or limb paralysis, recurrent headaches and intermittent sleep disturbances. All patients show to some extent a movement disorder with ataxia and spasticity. Clinical severity varies from mild motor and cognitive dysfunction between epileptic attacks to severe neurological disability, with some patients never achieving language or unsupported walking. In most patients with the classic phenotype neurological symptoms fluctuate unpredictably and are not influenced by fasting or food intake [7].
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4. Carbohydrate-responsive phenotype In 1999 we investigated two brothers aged 8 and 16 years who had moderate to severe delay of motor and mental development, epilepsy with onset of seizures in infancy, movement disorder with ataxia and dystonia, and mild deceleration of head growth that did not result in microcephaly [8,9]. The older boy was more severely affected, he started walking at 3 years and speaking at 4 years of age. His younger brother walked at 14 months and spoke first words in his second year. Seizures were characterized by myoclonic jerking of shoulders and arms, nodding of the head, rolling of the eyes, limpness, and impaired consciousness. There was an impressive correlation between fasting and neurological deterioration including seizure frequency and severity on the one hand, and carbohydrate intake and neurological improvement on the other. Both brothers showed particularly poor motor and mental performance as well as frequent and prolonged seizures in the morning before breakfast, improved a lot after a meal, and steadily worsened again with increasing interval to their last food intake. Their parents provided frequent meals every 2 h and requested the same at school. Their mother was affected in a similar though much milder way. She learned to eat every 2 to 3 h during the day, and to awaken at night to eat some sweets. She avoided drinking coffee as she had noticed that this impaired her cognitive and motor skills. Her father was in charge of a telegraph office in northern Germany. No neurological signs or symptoms were reported, but it was alleged that his wife used to serve him honey at bedside in the morning to improve his alertness and neurological functioning. In both boys a lumbar puncture was performed as part of diagnostic work-up, and an unexplained low concentration of glucose as well as lactate was detected in both boys. The same was found in their mother. Pre- and postprandial EEG recordings demonstrated a striking difference in the extent of epileptiform discharges with marked improvement after breakfast [10]. As video EEG recordings in the fasting state disclosed a loose correlation between myoclonic jerks and epileptiform discharges, the sudden twitches obviously represented a mixture of epileptic seizures and ballistic movements. A heterozygous missense mutation of the GLUT1/SLC2A1 gene was identified in all three patients [9]. These individuals were the first European patients described, and this observation was the first demonstration of autosomal dominant inheritance of GLUT1-DS. Treatment with a ketogenic diet resulted in marked improvement of seizures and motor impairment, whereas the effect on the cognitive performance was less impressive. The mother did not adhere to the diet. Subsequently we have observed a correlation between food intake and motor and mental perfor-
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mance in further GLUT1-DS patients [10,11]. We proposed EEG recording during intravenous infusion of glucose as a simple screening test that might help identifying those patients with a carbohydrate-responsive phenotype. 5. Movement disorder without seizures Seizures have been considered to be an obligatory part of the clinical features in GLUT1-DS patients. However, in 2003 a 9-year-old boy with ataxia and retardation, but without epilepsy, caused by GLUT1deficiency was reported [12]. This patient walked independently at 17 months and spoke first words at 18 months of age. He had mild mental retardation, dysarthric speech, ataxia of both arms and legs, and some dystonia of the arms. His parents reported fluctuations in his motor performance during the day without any relation to food intake. His head circumference was at the 50th percentile. He never had seizures. Several EEGs and cranial MRI were normal. The clue for the diagnosis of GLUT1-DS was hypoglycorrhachia that was consistently found in three CSF investigations with ratios of CSF glucose to blood glucose ranging from 0.31 to 0.38. Glucose uptake studies in red blood cells confirmed the GLUT1 deficiency, and mutation analysis revealed a heterozygous missense mutation. Ketogenic diet proved to be beneficial for his motor performance, but was discontinued due to abdominal discomfort. We studied monozygotic twin girls at 3 years of age with marked ataxia, mild delay in motor development, and acquired microcephaly due to GLUT1 deficiency [11]. They walked freely at 19 months of age, showed marked ataxia and dysarthria, and especially poor motor performance in the morning before breakfast. Seizures had never been observed. The twins had been investigated for evaluation of unclassified signal alterations of subcortical U fibers resembling L-2-hydroxyglutaraciduria, which was ruled out. CSF investigation revealed hypoglycorrhachia with CSF-to-blood glucose ratios as low as 0.36 and 0.39. Glucose uptake in red blood cells was reduced and a heterozygous de novo missense mutation of the GLUT1/SLC2A1 gene was found in both twins [11]. A ketogenic diet proved beneficial for the ataxia and general development. Furthermore, a 3-year-old boy with developmental delay, severe ataxia, and subcortical white matter changes in cranial MRI, but without seizures has been reported [13]. Information on head growth was not provided. At 18 months of age the boy learned to walk freely, and some spasticity appeared. At 40 months diagnosis of GLUT1DS was established based on hypoglycorrhachia and demonstration of a heterozygous truncation mutation of the GLUT1/SLC2A1 gene. A ketogenic diet was started, and clinical as well as neuroradiological improvement was noted at follow-up investigations.
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In a 10-year-old normocephalic boy, a movement disorder comprising ataxia, dystonia, and choreoathetosis constituted the prominent clinical feature of GLUT1DS [14]. This patient showed delayed motor and speech development, severe dysarthria and ataxia, intermittent facial dystonia, mild choreiform movements of the lower extremities, and dystonic posturing of the hands with action. Fine finger movements were slow and dysrhythmic. In addition, he had a longstanding sleep disturbance and aggressive behaviors. CSF analysis revealed hypoglycorrhachia and low lactate. A de novo heterozygous insertion mutation of the GLUT1/SLC2A1 gene was found [14]. A ketogenic diet resulted in striking improvement of the motor abnormalities, as demonstrated in a supplementary video provided with the case report (http://www.interscience.wiley.com/jpages/08853185/suppmat/2006/jws-mds.20660.mpg). 6. Paroxysmal exercise-induced dyskinesia and epilepsy More recently, an additional phenotype of GLUT1DS characterized by familial paroxysmal exertioninduced dyskinesia (PED) and epilepsy has been recognized [15,16]. In one family, four members over three generations had PED that were accompanied by seizures, mild developmental delay, lowered CSF glucose concentrations, mild hemolytic anemia with particular deformation of erythrocytes (echinocytosis), and altered ion levels in red blood cells [15]. PED comprising involuntary dystonic, choreoathetotic, and ballistic movements occurred after prolonged exercise, affecting exclusively the exercised limbs, as demonstrated in a supplementary video (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2350432&blobname =JCI0834438sd1.wmv). The GLUT1/SLC2A1 gene was considered a candidate gene in this condition, as seizures as well as epileptiform discharges in EEG recordings were observed predominantly in the morning before breakfast and a marked improvement of these EEG alterations occurred after intravenous glucose administration, resembling the carbohydrate-responsive phenotype of GLUT1DS. Furthermore, as GLUT1 is the primary glucose transporter in red blood cells, the hemolytic anemia suggested a relation to this protein, although hematological symptoms had never been reported in GLUT1-DS before. CSF analysis revealed mildly reduced glucose concentrations and CSF-to-blood glucose ratios mildy lowered to 0.39–0.55 as well as reduced CSF lactate levels. Glucose uptake in red blood cells was reduced, and a causative deletion of four highly conserved amino acids in the pore region of GLUT1 was demonstrated. Additional functional studies in Xenopus oocytes and human erythrocytes revealed that this mutation caused a cation leak that alters intracellular concentrations of sodium, potassium, and calcium, thus resulting in increased hemolysis due to Ca2+-triggered suicidal cell death of erythro-
cytes. On a molecular level, the cation leak of the mutated GLUT1 molecule was explained by the specific location of the 4–amino acid deletion. The mutation was obviously the first one reported in transmembrane segment 7, which probably plays a crucial role in the transporter’s pore. Moreover, the deleted amino acids were hypothesized to form a bottle neck within the pore of GLUT1. Molecular modelling using the published coordinates of a GLUT1 model suggested a widening of the pore region (central channel) by the deletion, which can explain the observed permanent cation leak [15]. Treatment with a ketogenic diet resulted in marked improvement of seizures and PED, whereas the hemolytic anemia remained unchanged. Screening of four additional families, in which PED was combined with epilepsy, developmental delay, or migraine, but not with hemolysis or echinocytosis, revealed two additional GLUT1 mutations that decreased glucose transport but did not affect cation permeability [15]. In a five generation family with co-occurrence of PED and epilepsy in a dominant inheritance pattern a whole genome linkage analysis indicated a region on chromosome 1p35–p31 including SLC2A1/GLUT1 [16]. Screening of SLC2A1 identified heterozygous missense and frameshift mutations segregating in this and three other nuclear families with a similar phenotype. PED was characterized by choreoathetosis, dystonia or both, affecting mainly the legs. Predominant epileptic seizure types were primary generalized. A median CSF-to-blood glucose ratio of 0.52 in the patients and a reduced glucose uptake by mutated transporters compared with the wild-type as determined in Xenopus oocytes confirmed a pathogenic role of these mutations. Some of these patients were successfully treated with a ketogenic diet [16]. 7. Laboratory features 7.1. CSF investigations The most conclusive laboratory feature pointing to GLUT1-deficiency is lowered glucose concentration in CSF (hypoglycorrhachia) [5,7,17,18]. Reduced glucose levels in CSF are well known in bacterial, fungal, or protozoal meningitis, subarachnoid hemorrhage, and hypoglycemia. If these conditions are excluded hypoglycorrhachia strongly indicates GLUT1-DS. Lumbar puncture should be performed in the fasting state, and the blood specimen for determination of glucose concentration should be obtained immediately before the lumbar puncture to avoid stress-related hyperglycemia [7]. The ratio of CSF glucose to blood glucose is more significant than the absolute value of CSF glucose. Although most GLUT1-DS patients have absolute glucose concentrations in CSF of less than 40 mg/dL and a CSF-to-blood glucose ratio of 0.33–0.37 [7,17], single
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patients with proven GLUT1/SLC2A1 mutations and paroxysmal exercise-induced dyskinesia and epilepsy have been reported with absolute glucose values as high as 64 mg/dL and CSF-to-blood glucose ratios up to 0.59 [14,15]. A second significant CSF finding in GLUT1-DS is the reduced concentration of lactate [5,7,17,18]. Mean CSF lactate values in GLUT1-DS have been reported to be 0.97–1.0 mmol/L [7,17], ranging from 0.5 to 1.4 mmol/ L, which is below the normal mean value of 1.6 mmol/L. 7.2. EEG In a study aiming at characterization of seizure types and EEG features in 20 patients with GLUT1-DS, a normal interictal EEG was the most common EEG finding. When abnormalities occurred, focal epileptiform discharges and slowing were more frequent in the infant. In children aged 2 years or older, a generalized 2.5- to 4Hz spike-wave pattern and slowing were more common [19]. In patients with carbohydrate-responsive phenotype a striking difference between pre- and postprandial EEG recordings with marked improvement of epileptiform discharges after food intake was noted [9,10]. The same observation in a 7-year-old boy with familial paroxysmal exertion-induced dyskinesias and epilepsy prompted investigation of the CSF-to-blood glucose ratio and the GLUT1/SLC2A1 gene in this family [15]. EEG recording during intravenous glucose administration may represent a helpful screening test for identifying a subset of GLUT1-DS patients with carbohydrate-responsive phenotype [10].
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ated at last, this MRI feature probably indicates a mere delay in myelination in GLUT1-DS. 7.3.2. Positron emission tomography (PET) Cerebral fluoro-deoxy-glucose PET in 14 patients with the classic phenotype of GLUT1-DS revealed a global decrease in glucose uptake in the cortex, especially the mesial temporal regions, and the thalami, with relative preservation of basal ganglia. This metabolic pattern was largely constant in all patients regardless of age, seizure history, disease severity, or treatment [21]. Comparable results were obtained in a recent study using PET in familial paroxysmal exercise-induced dyskinesia and epilepsy due to GLUT1 deficiency [16]. 8. Genotype–phenotype correlation Dong Wang, Darryl C. De Vivo, and colleagues outlined the following correlation between genotype and phenotype [17]:
7.3. Brain imaging 7.3.1. Magnetic resonance imaging (MRI) MRI of patients with GLUT1-DS mostly depicts either normal findings or occasionally mild enlargement of inner and outer CSF spaces [17,18]. However, a mild delay in myelination had already been observed in the index patient of the original GLUT1-DS report [5]. More recently a characteristic pattern of white matter abnormalities comprising high T2-signal of subcortical U fibers has been described in single patients [11,13]. This pattern is somewhat similar to the MRI findings in relatively mildly affected patients with L-2-hydroxyglutaraciduria characterized by confluent lesions of subcortical white matter confined to the U fibers [20] and thus may be misleading. However, in L-2-hydroxyglutaraciduria the white matter changes tend to increase, whereas in the few patients with GLUT1-DS reported yet, follow-up MRI investigations revealed an improvement of this subcortical leukoencephalopathy [13] (own unpublished observation). As the U fibers in the periphery of cerebral white matter represent those structures that are being myelin-
Fig. 1. Head growth of a 14 year-old girl with GLUT1-DS before and during treatment with a ketogenic diet (patient #1 from [10]). This girl was born with a head circumference of 35 cm (P. 75, not shown). Head growth decelerated within her 1st year of life, and her head subsequently grew along the 3rd percentile. Ketogenic diet was started at age 4½ years and resulted in a catch-up growth.
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– The classic phenotype is associated with hemizygosity, nonsense mutations, frameshift mutations, and splice-site mutations, resulting in 50% loss of GLUT1 protein. – Heterozygous missense mutations with mild pathogenicity result in 50–75% residual function of GLUT1 and accordingly mild phenotype. – A severe phenotype and preservation of only 25–50% residual function of GLUT1 has been observed in a patient with compound heterozygosity in trans. Wang and colleagues [17] speculate about both ends of the phenotypic spectrum: on one hand there might be minimal phenotypes caused by missense mutations that result in only minimal functional impairment of GLUT1, and on the other homozygous mutations with less than 25% residual function probably proving to be lethal at a very early state of development. However, marked phenotypic heterogeneity implies other genetic and epigenetic modifiers of the primary pathogenic mutation. 9. Treatment A high-fat diet produces ketone bodies that bypass the GLUT1 defect and diffuse across the blood–brain barrier facilitated by a monocarboxylic acid transporter. Ketone bodies serve as an alternative energy source for
brain metabolism. Ketogenic diet markedly improves seizures, movement disorder, and head growth (Fig. 1). The effect on the cognitive impairment is less pronounced. Encouraged by in vitro studies thioctic acid has been proposed to enhance glucose transport, and many patients already receive this drug, although controlled studies are still lacking. A supplementation with L-carnitine has been recommended, as the ketogenic diet is relatively deficient in L-carnitine content [7,17]. Some pharmacological agents impair GLUT1 function and should be avoided, including caffeine, phenobarbital, diazepam, chloral hydrate, and tricyclic antidepressants [17,22,23].
10. Summary Almost 20 years after the first description marked phenotypic variation of GLUT1-DS has been recognized, as summarized in Table 1. The classic phenotype accounts for the vast majority of cases. Most of these patients are sporadic, as they carry de novo mutations and the severe disability in many of them precludes from having children. Patients with carbohydrate-responsive symptoms or predominant movement disorders are comparably rare, but familial cases are not uncommon in these subsets. It is especially concerning these atypical phenotypes that presumably a large number of patients
Table 1 Phenotypes of GLUT1-deficiency syndrome and conclusive laboratory findings. Classic phenotype Mild to severe motor and mental retardation Seizures with infantile onset Deceleration of head growth, resulting in acquired microcephaly in 50% of patients Movement disorder comprising ataxia, dystonia, spasticity Non-epileptical paroxysmal events Carbohydrate-responsive phenotype Correlation of neurological functioning with alimentation: Motor and mental performance as well as seizures worsening with fasting, improving after carbohydrate intake Movement disorder without seizures Prominent ataxia, dystonia, spasticity Motor and mental development mildly, if at all, delayed Head growth normal or decelerated No seizures Paroxysmal exertion-induced dyskinesia and epilepsy Paroxysmal involuntary dystonic, choreoathetotic, ballistic movements after prolonged exercise PED affecting exclusively the exercised limbs or mainly the legs Laboratory hallmark in all phenotypes Reduced glucose concentration in CSF (hypoglycorrhachia) CSF-to-blood glucose ratio reduced to <0.4 in most cases (normal, 0.65) Lumbar puncture to be performed preferably in the morning before breakfast Blood sample for determination of serum glucose level to be collected immediately before lumbar puncture Electroencephalography Screening for carbohydrate-responsive phenotype: intravenous infusion of glucose during EEG recording results in marked improvement of epileptic discharges
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go undiagnosed. With regard to the therapeutic implications of a GLUT1-DS diagnosis it is strongly recommended to perform a CSF investigation with determination of the CSF-to-blood glucose ratio in any unclear neurological disorder. 11. Future perspectives GLUT1-DS is the prototype of a cerebral energy failure condition. However, several other transport proteins are involved in cerebral energy supply. GLUTs deliver glucose from bloodstream to the brain: GLUT1 in the microvascular endothelial cells of the blood–brain barrier and glia; GLUT3 in neurons. Monocarboxylate transporters transfer lactate, the glycolytic product of glucose metabolism, into and out of neural cells: MCT1 in the blood–brain barrier as well as astrocytes and MCT2 in neurons. Glucose has traditionally been considered to be the obligate fuel of the mammalian brain and the only substrate able to fully maintain activity of cerebral neurons. A further longstanding assumption is that the much greater part of cerebral glucose utilization supplies energy for neuronal activity via oxidative metabolism, both in the basal and activated state [2,24]. The proposal of the astrocyte-neuron lactate shuttle hypothesis [25] suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energy demands, especially during activation. This hypothesis is still a matter of debate over whether neuronal activity is fueled primarily by glucose or lactate. In any case, it is highly likely that a deficiency of one of the other transport proteins involved in cerebral energy supply would result in distinct neurological signs and symptoms. However, a clinical condition associated with GLUT3, MCT1 or MCT2 deficiency has not yet been recognized. This remains a challenge for pediatric neurologists.
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Review article
The expanding phenotype of GLUT1-deficiency syndrome Knut Brockmann * Department of Pediatrics and Pediatric Neurology, Faculty of Medicine, Georg August University, Robert-Koch-Str. 40, 37075 Go¨ttingen, Germany Received 31 October 2008; accepted 16 February 2009
Abstract Transport of glucose from the bloodstream across the blood–brain barrier to the central nervous system is facilitated by glucose transport protein type 1 (GLUT1), the first member of the solute carrier family 2 (SLC2). Heterozygous mutations in the GLUT1/ SLC2A1 gene, occurring de novo or inherited as an autosomal dominant trait, result in cerebral energy failure and a clinical condition termed GLUT1-deficiency syndrome (GLUT1-DS). Clinical features usually comprise motor and mental developmental delay, seizures with infantile onset, deceleration of head growth often resulting in acquired microcephaly, and a movement disorder with ataxia, dystonia, and spasticity. Subsequent to the delineation of this classic phenotype the variability of signs and symptoms in GLUT1-DS is being recognized. Patients with (i) carbohydrate-responsive symptoms, with (ii) predominant ataxia or dystonia, but without seizures, and with (iii) paroxysmal exertion-induced dyskinesia and seizures have been reported. Common laboratory hallmark in all phenotypes is the reduced glucose level in cerebrospinal fluid with lowered CSF-to-blood glucose ratio. Treatment with a ketogenic diet results in marked improvement of seizures and movement disorders. Ó 2009 Elsevier B.V. All rights reserved. Keywords: Glucose transport protein type 1; Epilepsy; Movement disorders; Paroxysmal dyskinesia; Hypoglycorrhachia; Ketogenic diet
1. Introduction A continuous supply of glucose provides the main energy source for the brain. Given the strict separation of the central nervous system from the bloodstream by the blood–brain barrier it was uncertain for a long time how glucose can cross the lipophilic plasma membranes and pass into the brain in quantities which are sufficient to meet the energy requirements of cerebral tissue. Christian Crone, a physiologist at the University of Copenhagen, Denmark, was obviously the first to postulate a transport system for the facilitated transfer of glucose from blood into brain tissue [1]. He studied the mechanism of glucose transfer in anaesthetized dogs after intracarotid injection of glucose + [14C]glucose with subse*
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quent sampling from the superior sagittal sinus. The fraction of glucose which passed into the cerebral tissue fell with increasing concentration of glucose. He measured an extraction of almost 50% at low concentrations and of 10% at high concentrations. Furthermore, a comparison of the unidirectional passage of [14C]glucose with the passage of fructose showed consistently higher extraction for glucose. These findings provided evidence for a ‘‘special transport mechanism for glucose”, and Crone suggested ‘‘that the endothelial cells in the cerebral capillaries are responsible for the transport of glucose across the blood–brain barrier” [1]. Subsequently four decades of physiological, biochemical, and genetic research have provided a long list of glucose transport proteins [2]. They are assigned to two distinct families of structurally related glucose transporters and mediate the facilitative and sodiumdependent glucose transport processes. Apart from
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GLUT1-deficiency syndrome (GLUT1-DS), clinical conditions associated with a deficiency of one of these transporters have been recognized so far only for few of them, e.g. glucose–galactose malabsorption (SGLT1), renal glycosuria (SGLT2), and Fanconi–Bickel syndrome (GLUT2). 2. Molecular basis Within the family of facilitative glucose transporters comprising 13 proteins (gene symbol SLC2A, protein symbol GLUT), GLUT1 was the first identified member, and in 1985 GLUT1/SLC2A1 was the first gene to be cloned and sequenced [3]. The gene is located on the short arm of chromosome 1 (1p34.2), is 35 kb in length, and consists of 10 exons. GLUT1 is constitutively expressed in most tissues and selectively expressed in erythrocytes, brain microvessels and astroglia. Two distinct molecular forms of GLUT1 with molecular weights of 55 and 45 kDa differ only in the extent of glycosylation. The 55 kDa form is found predominantly in the endothelial cells of brain microvessels and erythrocytes, where it is the principal (if not exclusive) glucose transporter. The 45 kDa form is detected in most cells including astrocytes and may be responsible for basal glucose uptake into these tissues. GLUT1 is a 492amino acid integral membrane protein with intracellular amino and carboxyl termini and 12 transmembrane domains, spanning the plasma membrane as a-helices (H1-H12). Presumably two 6-helical domain halves are separated by the large intercellular loop between helices 6 and 7. The postulated three-dimensional structure is characterized by a central channel across the protein that is essentially formed by helices 2, 4, 5, 7, 8, and 10 and connects the extracellular and intracellular environments [4]. Only one opening is proposed at each end. The domains crucial for transport and pathogenicity are clustered in two groups: one around the central channel, and the other in the long intracellular loop. Aberrant GLUT1 protein may result from truncation due to frameshift mutations, alteration of substrate binding sites due to missense mutations, or absence of protein due to allele deletion. In all cases, the normal allele contributes approximately 50% of functional GLUT1 protein to the plasma membrane. Most SCL2A1 mutations detected occurred de novo, in familial cases the condition is inherited as an autosomal dominant trait. 3. Classic phenotype Recognition of the clinical features associated with GLUT1 deficiency was inaugurated with the seminal discovery made by Darryl C. De Vivo in 1991 [5]. He reported two infants with developmental delay, seizures, mild movement disorder, and acquired micro-
cephaly. Onset of seizures was as early as 2 months in both patients. There were signs of mild ataxia and spasticity. In one patient, a deceleration of head growth was documented resulting in acquired microcephaly at 7 months of age. A cranial MRI suggested a mild delay in myelination. In both infants, multiple lumbar punctures at various times revealed persistent unexplained low glucose concentrations in CSF (hypoglycorrhachia). With normal blood glucose levels, the ratio of CSF glucose to blood glucose was markedly reduced in both patients and ranged from 0.19 to 0.35 (normal, 0.65). Concentrations of lactate in CSF were low as well. As the recognized causes of low glucose concentrations in CSF, such as bacterial meningitis, subarachnoid hemorrhage, and hypoglycemia, were not present, De Vivo considered a defect in the transport of glucose from blood to the brain. This assumption prompted him to treat both patients with a ketogenic diet, providing ketone bodies as an alternative fuel source for the brain. This treatment resulted in cessation of seizures in both patients and in nearly normal neurologic development in one patient who had received ketogenic diet as early as at 7.5 months of age. The hypothesis of a defect in glucose transport from blood into the brain was confirmed by demonstration of reduced binding of cytochalasin B (a fungal product that binds to the glucose transporter proteins) to the erythrocyte membranes of both patients and reduced uptake of 3-Omethyl-D-glucose in red blood cells of one patient. Later the index case was found to have a null allele producing hemizygosity and haploinsufficiency of the GLUT1/SLC2A1 gene [6]. During the following decade more than 40 patients with GLUT1-DS (OMIM #606777) have been identified, mainly in North America, some in Australia. Dominant clinical features comprise infantile seizures, developmental delay, and deceleration of head growth with acquired microcephaly in most patients. Seizures are described as brief, subtle myoclonic limb jerking with staring alternating with eye-rolling, a sudden onset of pallor, a dazed expression, or horizontal roving eye movements, unresponsiveness, and hypotonia as well as head bobbing. Paroxysmal events of possibly non-epileptic nature include intermittent ataxia, periodic confusion, periodic weakness or limb paralysis, recurrent headaches and intermittent sleep disturbances. All patients show to some extent a movement disorder with ataxia and spasticity. Clinical severity varies from mild motor and cognitive dysfunction between epileptic attacks to severe neurological disability, with some patients never achieving language or unsupported walking. In most patients with the classic phenotype neurological symptoms fluctuate unpredictably and are not influenced by fasting or food intake [7].
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4. Carbohydrate-responsive phenotype In 1999 we investigated two brothers aged 8 and 16 years who had moderate to severe delay of motor and mental development, epilepsy with onset of seizures in infancy, movement disorder with ataxia and dystonia, and mild deceleration of head growth that did not result in microcephaly [8,9]. The older boy was more severely affected, he started walking at 3 years and speaking at 4 years of age. His younger brother walked at 14 months and spoke first words in his second year. Seizures were characterized by myoclonic jerking of shoulders and arms, nodding of the head, rolling of the eyes, limpness, and impaired consciousness. There was an impressive correlation between fasting and neurological deterioration including seizure frequency and severity on the one hand, and carbohydrate intake and neurological improvement on the other. Both brothers showed particularly poor motor and mental performance as well as frequent and prolonged seizures in the morning before breakfast, improved a lot after a meal, and steadily worsened again with increasing interval to their last food intake. Their parents provided frequent meals every 2 h and requested the same at school. Their mother was affected in a similar though much milder way. She learned to eat every 2 to 3 h during the day, and to awaken at night to eat some sweets. She avoided drinking coffee as she had noticed that this impaired her cognitive and motor skills. Her father was in charge of a telegraph office in northern Germany. No neurological signs or symptoms were reported, but it was alleged that his wife used to serve him honey at bedside in the morning to improve his alertness and neurological functioning. In both boys a lumbar puncture was performed as part of diagnostic work-up, and an unexplained low concentration of glucose as well as lactate was detected in both boys. The same was found in their mother. Pre- and postprandial EEG recordings demonstrated a striking difference in the extent of epileptiform discharges with marked improvement after breakfast [10]. As video EEG recordings in the fasting state disclosed a loose correlation between myoclonic jerks and epileptiform discharges, the sudden twitches obviously represented a mixture of epileptic seizures and ballistic movements. A heterozygous missense mutation of the GLUT1/SLC2A1 gene was identified in all three patients [9]. These individuals were the first European patients described, and this observation was the first demonstration of autosomal dominant inheritance of GLUT1-DS. Treatment with a ketogenic diet resulted in marked improvement of seizures and motor impairment, whereas the effect on the cognitive performance was less impressive. The mother did not adhere to the diet. Subsequently we have observed a correlation between food intake and motor and mental perfor-
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mance in further GLUT1-DS patients [10,11]. We proposed EEG recording during intravenous infusion of glucose as a simple screening test that might help identifying those patients with a carbohydrate-responsive phenotype. 5. Movement disorder without seizures Seizures have been considered to be an obligatory part of the clinical features in GLUT1-DS patients. However, in 2003 a 9-year-old boy with ataxia and retardation, but without epilepsy, caused by GLUT1deficiency was reported [12]. This patient walked independently at 17 months and spoke first words at 18 months of age. He had mild mental retardation, dysarthric speech, ataxia of both arms and legs, and some dystonia of the arms. His parents reported fluctuations in his motor performance during the day without any relation to food intake. His head circumference was at the 50th percentile. He never had seizures. Several EEGs and cranial MRI were normal. The clue for the diagnosis of GLUT1-DS was hypoglycorrhachia that was consistently found in three CSF investigations with ratios of CSF glucose to blood glucose ranging from 0.31 to 0.38. Glucose uptake studies in red blood cells confirmed the GLUT1 deficiency, and mutation analysis revealed a heterozygous missense mutation. Ketogenic diet proved to be beneficial for his motor performance, but was discontinued due to abdominal discomfort. We studied monozygotic twin girls at 3 years of age with marked ataxia, mild delay in motor development, and acquired microcephaly due to GLUT1 deficiency [11]. They walked freely at 19 months of age, showed marked ataxia and dysarthria, and especially poor motor performance in the morning before breakfast. Seizures had never been observed. The twins had been investigated for evaluation of unclassified signal alterations of subcortical U fibers resembling L-2-hydroxyglutaraciduria, which was ruled out. CSF investigation revealed hypoglycorrhachia with CSF-to-blood glucose ratios as low as 0.36 and 0.39. Glucose uptake in red blood cells was reduced and a heterozygous de novo missense mutation of the GLUT1/SLC2A1 gene was found in both twins [11]. A ketogenic diet proved beneficial for the ataxia and general development. Furthermore, a 3-year-old boy with developmental delay, severe ataxia, and subcortical white matter changes in cranial MRI, but without seizures has been reported [13]. Information on head growth was not provided. At 18 months of age the boy learned to walk freely, and some spasticity appeared. At 40 months diagnosis of GLUT1DS was established based on hypoglycorrhachia and demonstration of a heterozygous truncation mutation of the GLUT1/SLC2A1 gene. A ketogenic diet was started, and clinical as well as neuroradiological improvement was noted at follow-up investigations.
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In a 10-year-old normocephalic boy, a movement disorder comprising ataxia, dystonia, and choreoathetosis constituted the prominent clinical feature of GLUT1DS [14]. This patient showed delayed motor and speech development, severe dysarthria and ataxia, intermittent facial dystonia, mild choreiform movements of the lower extremities, and dystonic posturing of the hands with action. Fine finger movements were slow and dysrhythmic. In addition, he had a longstanding sleep disturbance and aggressive behaviors. CSF analysis revealed hypoglycorrhachia and low lactate. A de novo heterozygous insertion mutation of the GLUT1/SLC2A1 gene was found [14]. A ketogenic diet resulted in striking improvement of the motor abnormalities, as demonstrated in a supplementary video provided with the case report (http://www.interscience.wiley.com/jpages/08853185/suppmat/2006/jws-mds.20660.mpg). 6. Paroxysmal exercise-induced dyskinesia and epilepsy More recently, an additional phenotype of GLUT1DS characterized by familial paroxysmal exertioninduced dyskinesia (PED) and epilepsy has been recognized [15,16]. In one family, four members over three generations had PED that were accompanied by seizures, mild developmental delay, lowered CSF glucose concentrations, mild hemolytic anemia with particular deformation of erythrocytes (echinocytosis), and altered ion levels in red blood cells [15]. PED comprising involuntary dystonic, choreoathetotic, and ballistic movements occurred after prolonged exercise, affecting exclusively the exercised limbs, as demonstrated in a supplementary video (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2350432&blobname =JCI0834438sd1.wmv). The GLUT1/SLC2A1 gene was considered a candidate gene in this condition, as seizures as well as epileptiform discharges in EEG recordings were observed predominantly in the morning before breakfast and a marked improvement of these EEG alterations occurred after intravenous glucose administration, resembling the carbohydrate-responsive phenotype of GLUT1DS. Furthermore, as GLUT1 is the primary glucose transporter in red blood cells, the hemolytic anemia suggested a relation to this protein, although hematological symptoms had never been reported in GLUT1-DS before. CSF analysis revealed mildly reduced glucose concentrations and CSF-to-blood glucose ratios mildy lowered to 0.39–0.55 as well as reduced CSF lactate levels. Glucose uptake in red blood cells was reduced, and a causative deletion of four highly conserved amino acids in the pore region of GLUT1 was demonstrated. Additional functional studies in Xenopus oocytes and human erythrocytes revealed that this mutation caused a cation leak that alters intracellular concentrations of sodium, potassium, and calcium, thus resulting in increased hemolysis due to Ca2+-triggered suicidal cell death of erythro-
cytes. On a molecular level, the cation leak of the mutated GLUT1 molecule was explained by the specific location of the 4–amino acid deletion. The mutation was obviously the first one reported in transmembrane segment 7, which probably plays a crucial role in the transporter’s pore. Moreover, the deleted amino acids were hypothesized to form a bottle neck within the pore of GLUT1. Molecular modelling using the published coordinates of a GLUT1 model suggested a widening of the pore region (central channel) by the deletion, which can explain the observed permanent cation leak [15]. Treatment with a ketogenic diet resulted in marked improvement of seizures and PED, whereas the hemolytic anemia remained unchanged. Screening of four additional families, in which PED was combined with epilepsy, developmental delay, or migraine, but not with hemolysis or echinocytosis, revealed two additional GLUT1 mutations that decreased glucose transport but did not affect cation permeability [15]. In a five generation family with co-occurrence of PED and epilepsy in a dominant inheritance pattern a whole genome linkage analysis indicated a region on chromosome 1p35–p31 including SLC2A1/GLUT1 [16]. Screening of SLC2A1 identified heterozygous missense and frameshift mutations segregating in this and three other nuclear families with a similar phenotype. PED was characterized by choreoathetosis, dystonia or both, affecting mainly the legs. Predominant epileptic seizure types were primary generalized. A median CSF-to-blood glucose ratio of 0.52 in the patients and a reduced glucose uptake by mutated transporters compared with the wild-type as determined in Xenopus oocytes confirmed a pathogenic role of these mutations. Some of these patients were successfully treated with a ketogenic diet [16]. 7. Laboratory features 7.1. CSF investigations The most conclusive laboratory feature pointing to GLUT1-deficiency is lowered glucose concentration in CSF (hypoglycorrhachia) [5,7,17,18]. Reduced glucose levels in CSF are well known in bacterial, fungal, or protozoal meningitis, subarachnoid hemorrhage, and hypoglycemia. If these conditions are excluded hypoglycorrhachia strongly indicates GLUT1-DS. Lumbar puncture should be performed in the fasting state, and the blood specimen for determination of glucose concentration should be obtained immediately before the lumbar puncture to avoid stress-related hyperglycemia [7]. The ratio of CSF glucose to blood glucose is more significant than the absolute value of CSF glucose. Although most GLUT1-DS patients have absolute glucose concentrations in CSF of less than 40 mg/dL and a CSF-to-blood glucose ratio of 0.33–0.37 [7,17], single
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patients with proven GLUT1/SLC2A1 mutations and paroxysmal exercise-induced dyskinesia and epilepsy have been reported with absolute glucose values as high as 64 mg/dL and CSF-to-blood glucose ratios up to 0.59 [14,15]. A second significant CSF finding in GLUT1-DS is the reduced concentration of lactate [5,7,17,18]. Mean CSF lactate values in GLUT1-DS have been reported to be 0.97–1.0 mmol/L [7,17], ranging from 0.5 to 1.4 mmol/ L, which is below the normal mean value of 1.6 mmol/L. 7.2. EEG In a study aiming at characterization of seizure types and EEG features in 20 patients with GLUT1-DS, a normal interictal EEG was the most common EEG finding. When abnormalities occurred, focal epileptiform discharges and slowing were more frequent in the infant. In children aged 2 years or older, a generalized 2.5- to 4Hz spike-wave pattern and slowing were more common [19]. In patients with carbohydrate-responsive phenotype a striking difference between pre- and postprandial EEG recordings with marked improvement of epileptiform discharges after food intake was noted [9,10]. The same observation in a 7-year-old boy with familial paroxysmal exertion-induced dyskinesias and epilepsy prompted investigation of the CSF-to-blood glucose ratio and the GLUT1/SLC2A1 gene in this family [15]. EEG recording during intravenous glucose administration may represent a helpful screening test for identifying a subset of GLUT1-DS patients with carbohydrate-responsive phenotype [10].
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ated at last, this MRI feature probably indicates a mere delay in myelination in GLUT1-DS. 7.3.2. Positron emission tomography (PET) Cerebral fluoro-deoxy-glucose PET in 14 patients with the classic phenotype of GLUT1-DS revealed a global decrease in glucose uptake in the cortex, especially the mesial temporal regions, and the thalami, with relative preservation of basal ganglia. This metabolic pattern was largely constant in all patients regardless of age, seizure history, disease severity, or treatment [21]. Comparable results were obtained in a recent study using PET in familial paroxysmal exercise-induced dyskinesia and epilepsy due to GLUT1 deficiency [16]. 8. Genotype–phenotype correlation Dong Wang, Darryl C. De Vivo, and colleagues outlined the following correlation between genotype and phenotype [17]:
7.3. Brain imaging 7.3.1. Magnetic resonance imaging (MRI) MRI of patients with GLUT1-DS mostly depicts either normal findings or occasionally mild enlargement of inner and outer CSF spaces [17,18]. However, a mild delay in myelination had already been observed in the index patient of the original GLUT1-DS report [5]. More recently a characteristic pattern of white matter abnormalities comprising high T2-signal of subcortical U fibers has been described in single patients [11,13]. This pattern is somewhat similar to the MRI findings in relatively mildly affected patients with L-2-hydroxyglutaraciduria characterized by confluent lesions of subcortical white matter confined to the U fibers [20] and thus may be misleading. However, in L-2-hydroxyglutaraciduria the white matter changes tend to increase, whereas in the few patients with GLUT1-DS reported yet, follow-up MRI investigations revealed an improvement of this subcortical leukoencephalopathy [13] (own unpublished observation). As the U fibers in the periphery of cerebral white matter represent those structures that are being myelin-
Fig. 1. Head growth of a 14 year-old girl with GLUT1-DS before and during treatment with a ketogenic diet (patient #1 from [10]). This girl was born with a head circumference of 35 cm (P. 75, not shown). Head growth decelerated within her 1st year of life, and her head subsequently grew along the 3rd percentile. Ketogenic diet was started at age 4½ years and resulted in a catch-up growth.
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– The classic phenotype is associated with hemizygosity, nonsense mutations, frameshift mutations, and splice-site mutations, resulting in 50% loss of GLUT1 protein. – Heterozygous missense mutations with mild pathogenicity result in 50–75% residual function of GLUT1 and accordingly mild phenotype. – A severe phenotype and preservation of only 25–50% residual function of GLUT1 has been observed in a patient with compound heterozygosity in trans. Wang and colleagues [17] speculate about both ends of the phenotypic spectrum: on one hand there might be minimal phenotypes caused by missense mutations that result in only minimal functional impairment of GLUT1, and on the other homozygous mutations with less than 25% residual function probably proving to be lethal at a very early state of development. However, marked phenotypic heterogeneity implies other genetic and epigenetic modifiers of the primary pathogenic mutation. 9. Treatment A high-fat diet produces ketone bodies that bypass the GLUT1 defect and diffuse across the blood–brain barrier facilitated by a monocarboxylic acid transporter. Ketone bodies serve as an alternative energy source for
brain metabolism. Ketogenic diet markedly improves seizures, movement disorder, and head growth (Fig. 1). The effect on the cognitive impairment is less pronounced. Encouraged by in vitro studies thioctic acid has been proposed to enhance glucose transport, and many patients already receive this drug, although controlled studies are still lacking. A supplementation with L-carnitine has been recommended, as the ketogenic diet is relatively deficient in L-carnitine content [7,17]. Some pharmacological agents impair GLUT1 function and should be avoided, including caffeine, phenobarbital, diazepam, chloral hydrate, and tricyclic antidepressants [17,22,23].
10. Summary Almost 20 years after the first description marked phenotypic variation of GLUT1-DS has been recognized, as summarized in Table 1. The classic phenotype accounts for the vast majority of cases. Most of these patients are sporadic, as they carry de novo mutations and the severe disability in many of them precludes from having children. Patients with carbohydrate-responsive symptoms or predominant movement disorders are comparably rare, but familial cases are not uncommon in these subsets. It is especially concerning these atypical phenotypes that presumably a large number of patients
Table 1 Phenotypes of GLUT1-deficiency syndrome and conclusive laboratory findings. Classic phenotype Mild to severe motor and mental retardation Seizures with infantile onset Deceleration of head growth, resulting in acquired microcephaly in 50% of patients Movement disorder comprising ataxia, dystonia, spasticity Non-epileptical paroxysmal events Carbohydrate-responsive phenotype Correlation of neurological functioning with alimentation: Motor and mental performance as well as seizures worsening with fasting, improving after carbohydrate intake Movement disorder without seizures Prominent ataxia, dystonia, spasticity Motor and mental development mildly, if at all, delayed Head growth normal or decelerated No seizures Paroxysmal exertion-induced dyskinesia and epilepsy Paroxysmal involuntary dystonic, choreoathetotic, ballistic movements after prolonged exercise PED affecting exclusively the exercised limbs or mainly the legs Laboratory hallmark in all phenotypes Reduced glucose concentration in CSF (hypoglycorrhachia) CSF-to-blood glucose ratio reduced to <0.4 in most cases (normal, 0.65) Lumbar puncture to be performed preferably in the morning before breakfast Blood sample for determination of serum glucose level to be collected immediately before lumbar puncture Electroencephalography Screening for carbohydrate-responsive phenotype: intravenous infusion of glucose during EEG recording results in marked improvement of epileptic discharges
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go undiagnosed. With regard to the therapeutic implications of a GLUT1-DS diagnosis it is strongly recommended to perform a CSF investigation with determination of the CSF-to-blood glucose ratio in any unclear neurological disorder. 11. Future perspectives GLUT1-DS is the prototype of a cerebral energy failure condition. However, several other transport proteins are involved in cerebral energy supply. GLUTs deliver glucose from bloodstream to the brain: GLUT1 in the microvascular endothelial cells of the blood–brain barrier and glia; GLUT3 in neurons. Monocarboxylate transporters transfer lactate, the glycolytic product of glucose metabolism, into and out of neural cells: MCT1 in the blood–brain barrier as well as astrocytes and MCT2 in neurons. Glucose has traditionally been considered to be the obligate fuel of the mammalian brain and the only substrate able to fully maintain activity of cerebral neurons. A further longstanding assumption is that the much greater part of cerebral glucose utilization supplies energy for neuronal activity via oxidative metabolism, both in the basal and activated state [2,24]. The proposal of the astrocyte-neuron lactate shuttle hypothesis [25] suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energy demands, especially during activation. This hypothesis is still a matter of debate over whether neuronal activity is fueled primarily by glucose or lactate. In any case, it is highly likely that a deficiency of one of the other transport proteins involved in cerebral energy supply would result in distinct neurological signs and symptoms. However, a clinical condition associated with GLUT3, MCT1 or MCT2 deficiency has not yet been recognized. This remains a challenge for pediatric neurologists.
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