Functional studies of threonine 310 mutations in Glut1: T310I is pathogenic, causing Glut1 deficiency

Comprehensive Exonic Sequencing of Hemiplegic Migraine-Related Genes in a Cohort of Suspected Probands Identifies Known and Potential Pathogenic Variants

Hemiplegic migraine (HM) is a rare migraine disorder with aura subtype including temporary weakness and visual, sensory, and/or speech symptoms. To date, three main genes—CACNA1A, ATP1A2, and SCN1A—have been found to cause HM. These encode ion channels or transporters, important for regulating neuronal ion balance and synaptic transmission, leading to HM being described as a channelopathy. However, <20% of HM cases referred for genetic testing have mutations in these genes and other genes with roles in ion and solute transport, and neurotransmission has also been implicated in some HM cases. In this study, we performed whole exome sequencing for 187 suspected HM probands referred for genetic testing, but found to be negative for CACNA1A, ATP1A2, and SCN1A mutations, and applied targeted analysis of whole exome sequencing data for rare missense or potential protein-altering variants in the PRRT2, PNKD, SLC1A3, SLC2A1, SLC4A4, ATP1A3, and ATP1A4 genes. We identified known mutations and some potentially pathogenic variants in each of these genes in specific cases, suggesting that their screening improves molecular diagnosis for the disorder. However, the majority of HM patients were found not to have candidate mutations in any of the previously reported HM genes, suggesting that additional genetic factors contributing to the disorder are yet to be identified.

Mechanistic Insights into Protein Stability and Self-aggregation in GLUT1 Genetic Variants Causing GLUT1-Deficiency Syndrome

Human sodium-independent glucose cotransporter 1 (hGLUT1) has been studied for its tetramerization and multimerization at the cell surface. Homozygous or compound heterozygous mutations in hGLUT1 elicit GLUT1-deficiency syndrome (GLUT1-DS), a metabolic disorder, which results in impaired glucose transport into the brain. The reduced cell surface expression or loss of function have been shown for some GLUT1 mutants. However, the mechanism by which deleterious mutations affect protein structure, conformational stability and GLUT1 oligomerization is not known and require investigation. In this review, we combined previous knowledge of GLUT1 mutations with hGLUT1 crystal structure to analyze native interactions and several natural single-point mutations. The modeling of native hGLUT1 structure confirmed the roles of native residues in forming a range of side-chain interactions. Interestingly, the modeled mutants pointed to the formation of a variety of non-native novel interactions, altering interaction networks and potentially eliciting protein misfolding. Self-aggregation of the last part of hGLUT1 was predicted using protein aggregation prediction tool. Furthermore, an increase in aggregation potential in the aggregation-prone regions was estimated for several mutants suggesting increased aggregation of misfolded protein. Protein stability change analysis predicted that GLUT1 mutant proteins are unstable. Combining GLUT1 oligomerization behavior with our modeling, aggregation prediction, and protein stability analyses, this work provides state-of-the-art view of GLUT1 genetic mutations that could destabilize native interactions, generate novel interactions, trigger protein misfolding, and enhance protein aggregation in a disease state.

Vibrational Imaging of Glucose Uptake Activity in Live Cells and Tissues by Stimulated Raman Scattering

Glucose is a ubiquitous energy source for most living organisms. Its uptake activity closely reflects cellular metabolic demand in various physiopathological conditions. Extensive efforts have been made to specifically image glucose uptake, such as with positron emission tomography, magnetic resonance imaging, and fluorescence microscopy, but all have limitations. A new platform to visualize glucose uptake activity in live cells and tissues is presented that involves performing stimulated Raman scattering on a novel glucose analogue labeled with a small alkyne moiety. Cancer cells with differing metabolic activities can be distinguished. Heterogeneous uptake patterns are observed with clear cell-cell variations in tumor xenograft tissues, neuronal culture, and mouse brain tissues. By offering the distinct advantage of optical resolution but without the undesirable influence of fluorophores, this method will facilitate the study of energy demands of living systems with subcellular resolution.

Implications of aberrant temperature-sensitive glucose transport via the glucose transporter deficiency mutant (GLUT1DS) T295M for the alternate-access and fixed-site transport models

In silico glucose docking to the transporter GLUT1 templated to the crystal structure of Escherichia coli XylE, a bacterial homolog of GLUT1-4 (4GBZ.pdb), reveals multiple docking sites. One site in the external vestibule in the exofacial linker between TM7 and -8 is adjacent to a missense T295M and a 4-mer insertion mutation. Glucose docking to the adjacent site is occluded in these mutants. These mutants cause an atypical form of glucose transport deficiency syndrome (GLUT1DS), where transport into the brain is deficient, although unusually transport into erythrocytes at 4 °C appears normal. A model in which glucose traverses the transporter via a network of saturable fixed sites simulates the temperature sensitivity of normal and mutant glucose influx and the mutation-dependent alterations of influx and efflux asymmetry when expressed in Xenopus oocytes at 37 °C. The explanation for the temperature sensitivity is that at 4 °C glucose influx between the external and internal vestibules is slow and causes glucose to accumulate in the external vestibule. This retards net glucose uptake from the external solution via two parallel sites into the external vestibule, consequently masking any transport defect at either one of these sites. At 37 °C glucose transit between the external and internal vestibules is rapid, with no significant glucose buildup in the external vestibule, and thereby unmasks any transport defect at one of the parallel input sites. Monitoring glucose transport in patients' erythrocytes at higher temperatures may improve the diagnostic accuracy of the functional test of GLUT1DS.

Glucose transporter 1 deficiency in the idiopathic generalized epilepsies

OBJECTIVE

We examined whether glucose transporter 1 (GLUT1) deficiency causes common idiopathic generalized epilepsies (IGEs).

METHODS

The IGEs are common, heritable epilepsies that usually follow complex inheritance; currently little is known about their genetic architecture. Previously considered rare, GLUT1 deficiency, due to mutations in SLC2A1, leads to failure of glucose transport across the blood-brain barrier and inadequate glucose for brain metabolism. GLUT1 deficiency was first associated with an encephalopathy and more recently found in rare dominant families with epilepsy and paroxysmal exertional dyskinesia (PED). Five hundred four probands with IGEs and 470 controls underwent SLC2A1 sequencing. Glucose transport was assayed following expression of SLC2A1 variants in Xenopus oocytes. All available relatives were phenotyped, and SLC2A1 was sequenced.

RESULTS

Functionally validated mutations in SLC2A1 were present in 7 of 504 (1.4%) probands and 0 of 470 controls. PED, undiagnosed prior to study, occurred in 1 proband and 3 of 13 relatives with mutations. The IGEs in probands and relatives were indistinguishable from typical IGE. Three cases (0.6%) had mutations of large functional effect and showed autosomal dominant inheritance or were de novo. Four (0.8%) cases had a subtle functional effect; 2 showed possible dominant inheritance, and 2 did not. These alleles leading to subtle functional impairment may contribute to complex, polygenic inheritance of IGE.

INTERPRETATION

SLC2A1 mutations contribute to approximately 1% of IGE both as a dominant gene and as a susceptibility allele in complex inheritance. Diagnosis of GLUT1 deficiency has important treatment (ketogenic diet) and genetic counseling implications. The mechanism of restricted glucose delivery differs from the current focus on IGEs as ion channel disorders.

Glut1 deficiency: inheritance pattern determined by haploinsufficiency

Two families manifesting Glut1 deficiency syndrome (DS) as an autosomal recessive trait are described. In 1 family, a severely affected boy inherited a mutated allele from his asymptomatic heterozygous mother. A de novo mutation developed in the paternal allele, producing compound heterozygosity. In another family, 2 mildly affected sisters inherited mutations from their asymptomatic heterozygous consanguineous parents. Red blood cell glucose uptake residual activity, a surrogate of haploinsufficiency, correlated with the clinical severity. These cases demonstrate that Glut1 DS may present as an autosomal recessive trait. The clinical pattern of inheritance is determined by the relative pathogenicity of the mutation and the resulting degree of haploinsufficiency.

Akt2 modulates glucose availability and downstream apoptotic pathways during development

Glucose is the primary energy substrate for eukaryotic cells and the predominant substrate for the brain. Studies suggest that glucose serves an additional role in the regulation of cellular functions, including viability. Zebrafish is a tractable system for defining the cellular and molecular mechanisms perturbed by impaired glucose transport and metabolism. Previously, we demonstrated a critical role for the facilitative glucose transporter, Glut1, in the regulation of embryonic brain development. In this study, we aim to identify mediators in this Glut1-sensitive process by investigating the role of the antiapoptotic kinase, Akt2. Results show that abrogating expression of akt2 causes a phenotype strikingly similar to that observed when glut1 expression is inhibited. akt2-deficient embryos exhibit increased neuronal apoptosis, impaired glucose uptake, and death by 72 h postfertilization. Similar to what was observed in the glut1 morphants, inhibiting the expression of the proapoptotic protein, bad, in the context of impaired akt2 expression results in the inhibition of apoptosis and rescue of the morphant embryos. Intriguingly, overexpression of glut1 in the akt2 morphants was also able to rescue these embryos. Quantitative reverse transcription-PCR analysis revealed decreased glut1 transcript expression in akt2 morphant embryos. Taken together, these data suggest that Akt2 modulates glucose availability by regulating Glut1 expression at the transcript level. These data support a role for akt2 in an integrative pathway directly linking glucose, Glut1 expression, and activation of apoptosis and demonstrate the dependence of akt2 on glucose availability for the maintenance of cellular viability, particularly in the central nervous system.

Functional studies of the T295M mutation causing Glut1 deficiency: glucose efflux preferentially affected by T295M

Glucose transporter type 1 (Glut1) deficiency syndrome (Glut1 DS, OMIM: #606777) is characterized by infantile seizures, acquired microcephaly, developmental delay, hypoglycorrhachia (CSF glucose <40 mg/dL), and decreased erythrocyte glucose uptake (56.1 +/- 17% of control). Previously, we reported two patients with a mild Glut1 deficiency phenotype associated with a heterozygous GLUT1 T295M mutation and normal erythrocyte glucose uptake. We assessed the pathogenicity of T295M in the Xenopus laevis oocyte expression system. Under zero-trans influx conditions, the T295M Vmax (590 pmol/min/oocyte) was 79% of the WT value and the Km (14.3 mM) was increased compared with WT (9.6 mM). Under zero-trans efflux conditions, both the Vmax (1216 pmol/min/oocyte) and Km (8.8 mM) in T295M mutant Glut1 were markedly decreased in comparison to the WT values (7443 pmol/min/oocyte and 90.8 mM). Western blot analysis and confocal studies confirmed incorporation of the T295M mutant protein into the plasma membrane. The side chain of M295 is predicted to block the extracellular "gate" for glucose efflux in our Glut-1 molecular model. We conclude that the T295M mutation specifically alters Glut1 conformation and asymmetrically affects glucose flux across the cell by perturbing efflux more than influx. These findings explain the seemingly paradoxical findings of Glut1 DS with hypoglycorrhachia and "normal" erythrocyte glucose uptake.

Paroxysmal exercise-induced dyskinesia and epilepsy is due to mutations in SLC2A1, encoding the glucose transporter GLUT1

Paroxysmal exercise-induced dyskinesia (PED) can occur in isolation or in association with epilepsy, but the genetic causes and pathophysiological mechanisms are still poorly understood. We performed a clinical evaluation and genetic analysis in a five-generation family with co-occurrence of PED and epilepsy (n = 39), suggesting that this combination represents a clinical entity. Based on a whole genome linkage analysis we screened SLC2A1, encoding the glucose transporter of the blood-brain-barrier, GLUT1 and 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/blood glucose ratio of 0.52 (normal >0.60) 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. Functional imaging studies implicated alterations in glucose metabolism in the corticostriate pathways in the pathophysiology of PED and in the frontal lobe cortex in the pathophysiology of epileptic seizures. Three patients were successfully treated with a ketogenic diet. In conclusion, co-occurring PED and epilepsy can be due to autosomal dominant heterozygous SLC2A1 mutations, expanding the phenotypic spectrum associated with GLUT1 deficiency and providing a potential new treatment option for this clinical syndrome.

Structural signatures and membrane helix 4 in GLUT1: inferences from human blood-brain glucose transport mutants

Exon IV of SLC2A1, a multiple facilitator superfamily (MFS) transporter gene, is particularly susceptible to mutations that cause GLUT1 deficiency syndrome, a human encephalopathy that results from decreased glucose flux through the blood-brain barrier. Genotyping of 100 patients revealed that in a third of them who harbor missense mutations in the GLUT1 transporter, transmembrane domain 4 (TM4), encoded by SLC2A1 exon IV, contains mutant residues that have the periodicity of one face of a kinked alpha-helix. Arg-126, located at the amino terminus of TM4, is the locus for most of the mutations followed by other arginine and glycine residues located elsewhere in the transporter but conserved among MFS proteins. The Arg-126 mutants were constructed and assayed for protein expression, targeting, and transport capacity in Xenopus oocytes. The role of charge at position 126, as well as its accessibility, was investigated in R126H by determining its activity as a function of extracellular pH. The results indicate that intracellular charges at the MFS TM2-3 and TM8-9 signature loops and flanking TMs 3, 5, and 6 are critical for the structure of GLUT1 as are TM glycines and that TM4, located at the catalytic core of MFS proteins, forms a helix that surfaces into the extracellular solution where another proton facilitates transport.

GLUT1 deficiency syndrome--2007 update

GLUT1 deficiency syndrome (GLUT1DS, OMIM 606777) is a treatable epileptic encephalopathy resulting from impaired glucose transport into the brain. The essential biochemical finding is a low glucose concentration in the cerebrospinal fluid (CSF; hypoglycorrhachia; mean 1.7 [SD 0.3mmol/L]) in the setting of normoglycaemia. CSF lactate is normal. Patients present with an early-onset epilepsy resistant to anticonvulsants, developmental delay, and a complex movement disorder. Hypotonic, ataxic, and dystonic features are most prominent. Speech is often severely affected. Some patients develop spasticity and secondary microcephaly. The phenotype is highly variable ranging from severe impairment to children without seizures. Electroencephalography (EEG) may show 2.5-4Hz spike-waves improving on food intake. Neuroimaging is uninformative. Most patients carry heterozygous de novo mutations in the GLUT1 gene (OMIM 138140, gene map locus 1p35-31.3). Autosomal dominant transmission and several mutational hot spots have been identified, but phenotype-genotype correlations are not yet apparent. Homozygous GLUT1 mutations presumably are lethal. The ketogenic diet is the treatment of choice as it provides an alternative fuel to the brain. It should be introduced early and maintained into puberty. Seizures are effectively controlled with the onset of ketosis, but might recur and require comedication. The effect on neurodevelopment appears less impressive. The increasing number of patients, molecular and biochemical analysis, recent research into ketogenic diet mechanisms, and the development of animal models for GLUT1DS have brought substantial insights in disease manifestations and mechanisms. This review summarizes data on 84 published cases and highlights recent advances in understanding this entity.

T3 strongly regulates GLUT1 and GLUT3 mRNA in cerebral cortex of hypothyroid rat neonates

Experimental hypothyroidism alters the expression of the GLUT1 and GLUT4 glucose transporters in brown adipose tissue, skeletal muscle and heart. Congenital hypothyroidism disrupts the development and function of the CNS, and the importance of GLUT1 for proper brain function has been dramatically evidenced in the cases of GLUT1 deficiency syndrome. Because of this, we hypothesised that the expression of GLUT1 and GLUT3, glucose transporters expressed in brain cortex, may be altered in congenital hypothyroidism. GLUT3 mRNA was induced during postnatal development whereas GLUT1 mRNA was initially repressed and further induced; both processes were essentially similar in control and hypothyroid animals. Under these conditions GLUT1 protein expression was reduced in cerebral cortex from 15-day-old hypothyroid neonates, which suggests the existence of post-transcriptional alterations. The most striking differences were observed when hypothyroid animals at different developmental stages were treated acutely with T(3). GLUT1 and GLUT3 mRNA expression behaved in opposite ways in response to treatment with the hormone. Furthermore, the behaviour of each glucose transporter isoform against T(3) was not uniform but changed alongside development. In all, our data show that the regulation of GLUT1 and GLUT3 in cerebral cortex is regulated by T(3) in a complex way and suggest that alterations in the expression of glucose transporters induced by hypothyroidism might have a functional impact on brain glucose uptake.

Impaired glucose transport into the brain: the expanding spectrum of glucose transporter type 1 deficiency syndrome

PURPOSE OF REVIEW

Glucose transporter type 1 deficiency syndrome (OMIM 606777) is a treatable epileptic encephalopathy resulting from impaired glucose transport into the brain. In recent years, the increasing number of patients has generated substantial insights into the manifestations and mechanisms of this disease. Current understanding of this novel disorder is reviewed, and recent advances in diagnosis and treatment are highlighted.

RECENT FINDINGS

The syndrome is now understood to be a complex neurological disorder. The clinical spectrum has recently been extended by infants with 'benign' transient hypoglycorrhachia, glucose transporter type 1 deficiency syndrome without seizures, and by adult cases. Other key findings in the last couple of years include (1) the description of electroencephalogram abnormalities, (2) a characteristic cerebral metabolic footprint in positron emission tomography imaging, and (3) the definition of molecular mechanisms and functional domains within the glucose transporter type 1 protein by in-vitro mutagenesis. The disease has also shed a new light on the mechanisms and the effectiveness of the ketogenic diet for seizure control.

SUMMARY

The syndrome is now well characterized in children and should be considered in any patient with intractable epilepsy. An effective therapy is available. The clinical spectrum and the molecular basis of the disease are increasingly heterogeneous and indicate complex pathogenic mechanisms that will ultimately lead to a classification on clinical, biochemical, and molecular grounds.