Glut1 Deficiency Syndrome (Glut1DS): State of the art in 2020 and recommendations of the international Glut1DS study group

3.1 Clinical features in Glut1DS

Epilepsy: Pharmaco-resistant seizures are frequently the first sign of Glut1DS. Any type of seizure can be observed.⁹ Generalized seizures are more frequent than focal seizures.^{10, 11} Early-onset absence epilepsy (onset prior to age 4 years)^{12, 13} and epilepsy with myoclonic-atonic seizures (Doose syndrome) have been associated with SLC2A1 pathogenic variants. Any epilepsy associated with movement disorders should suggest Glut1DS. In contrast, catastrophic epilepsy syndromes such as Lennox-Gastaut syndrome have not been reported. The initiation of KDT is highly effective in controlling seizures. Dietary treatment can achieve seizure freedom within a couple of days with normalization of EEG changes, frequently allowing for the withdrawal of any antiseizure medicines that might have been started before the correct diagnosis of Glut1DS had been established. Epilepsy tends to be the major clinical problem in infants and young children with Glut1DS, whereas seizures tend to decline or disappear in later childhood, adolescence, and adulthood.

Movement Disorders: In early infancy, distinctive paroxysmal eye-head movements are the second most common initial sign of Glut1DS, after seizures.^14-16 Episodes are typically involuntary and brief. The eye movements are repeated, multidirectional, saccadic, usually conjugate, and often accompanied by a head movement in the same direction. Later in childhood, other paroxysmal events emerge but the severity is highly variable. Presentation often involves some type of motor disturbance such as involuntary movements, ataxia, or weakness/paralysis. Awareness is generally preserved. Gradual clinical improvement, decreased frequency, and decreased severity of paroxysmal events are typical in adult life.^17-30 Paroxysmal nonmotor episodes include migraines, behavioral disturbances, cyclical vomiting, and sleep episodes.^{19, 22, 24, 30}

In general, movement disorders are characteristic of Glut1DS. Severity ranges from minimal to severe. The movements can be persistent or paroxysmal, classically present preprandially, and are mitigated by meals. Persistent movement disorders include spasticity, ataxia, and dystonia often producing disturbances of gait, followed by chorea and tremor.^{19, 24, 25} Ataxia is increasingly evident in late infancy as the child stands and starts to walk.²⁵ Ataxia is more truncal than appendicular. Chorea is often mild and involves the face and the distal upper limbs. A terminal intention tremor is frequent and often associated with other signs of cerebellar dysfunction. Myoclonus is generally epileptic; nonepileptic myoclonus is less common and includes startle myoclonus, action, and postural myoclonus.^{17, 19, 27} Dyspraxia is under-recognized and includes oculomotor dyspraxia and oro-buccal dyspraxia.^{19, 28} Paroxysmal movement disorders affect approximately 75% of patients and include paroxysmal eye-head movements, paroxysmal exercise-induced dyskinesia (PED), paroxysmal events manifested by major motor dysfunction, and paroxysmal events with complex neurological symptoms.³¹ These movement abnormalities are frequent and often precipitated by fasting and exercise.^{18-21, 26, 27} Potential triggers are emotional stress, fever, fatigue, insufficient ketosis, sleep deprivation, temperature changes, and drugs.^{17-22, 26-29}

Microcephaly may be acquired during infancy and is of varying degree likely correlating with clinical severity. ³²

Development and cognitive function: Glut1DS is associated with mild-to-severe intellectual disability, with the degree of severity being proportional to the overall disease severity.^{20, 26, 28, 33} Dysarthria, with varying degrees of speech impairment, is observed in all affected individuals. Social adaptive behavior is an exceptional strength.^{23, 25} Performance skills usually are more affected than verbal skills, with prominent deficiencies in visuospatial and visuomotor abilities. Timing of KDT introduction is a predictive factor for cognitive outcome.^{29, 34} Early dietary treatment correlates with better intellectual and social adaptive skills.²⁵

Atypical manifestations: Rare features described in Glut1DS include writer's cramp, intermittent ataxia, total body paralysis, Parkinsonism, and nocturnal painful muscle cramps in the legs.³² Alternating hemiplegia of childhood, hemiplegic migraine, cyclic vomiting, and stroke-like episodes with paroxysmal hemiparesis, dysarthria, or aphasia have been described in individual patients.^{23, 35, 36} Other rare features include hemolytic anemia associated with PED, hepatosplenomegaly, periventricular calcifications, brain atrophy, pseudohyperkalemia, cataracts, and retinal dysfunction.^{18, 37}

Adult Glut1DS: Data on adult Glut1DS are just emerging. Long-term prognosis and long-term adverse effects of KDT largely remain unknown. Changes in symptomatology over time include a shift from infantile-childhood onset epilepsy to adolescent-adult onset movement disorders including PED.^{24, 25, 31} Evaluation and treatment of adult Glut1DS also differ from pediatric Glut1DS. An extended metabolic evaluation may not be necessary, and for women of childbearing age, a pregnancy test should be considered prior to a KDT given that the risks of teratogenicity are unknown. Initiation of a KDT in adults is controversial given the side effects described in drug-resistant childhood epilepsy including osteopenia, osteoporosis,³⁸ potential cardiovascular risks,^{39, 40} and unclear effects on pregnancy. Also, energy and nutritional requirements in the mature brain are less than in the developing brain. In general, the modified Atkins diet (MAD) is considered a reasonable alternative for adolescents and adults^{41, 42} when treating poorly controlled seizures and paroxysmal movement disorders.

3.2 Diagnosis of Glut1DS

EEG: In Glut1DS of all ages, an interictal EEG is often normal. Abnormalities appear more common at certain ages: In infants, focal slowing and epileptiform discharges are more prevalent, whereas in children ages two years or older 2.5- to 4-Hz generalized spike-wave pattern is observed.⁴³ An intriguing feature, when present, is preprandial EEG abnormality that improves with feeding as glucose is restored to a starving brain.⁴⁴

Lumbar puncture: Low CSF glucose values in the setting of normoglycemia, termed hypoglycorrhachia, represent the metabolic hallmark of Glut1DS.⁴⁵ Other causes of hypoglycorrhachia such as hypoglycemia, meningitis, subarachnoid hemorrhage, or ventriculoperitoneal shunt systems need to be excluded.^46-48 In Glut1DS, CSF lactate levels are always low-normal or abnormally low, separating this condition from other diseases causing impaired brain energy metabolism, most notably mitochondrial diseases. The lumbar puncture should be performed in a postabsorptive state following a four- to six-hour fast. Blood glucose measurements should be determined immediately prior to performing the lumbar puncture to avoid stress-related hyperglycemia. Reference ranges are gender-independent, but age-specific.⁴⁹ Hypoglycorrhachia in typical Glut1DS was originally defined as a cutoff value of 2.2 mmol/L (40 mg/dL).^{2, 46} In a retrospective analysis of 147 patients, CSF glucose levels ranged from 0.9 to 2.8 mmol/L (16.2 to 50.5 mg/dL) and CSF to blood glucose ratios ranged from 0.19 to 0.59.⁴⁵ It is now clear that milder phenotypes may have CSF values of 2.2 to 2.9 mmol/L (41-52 mg/dL), but never normal.⁴⁶

Neuroimaging: Cranial magnetic resonance imaging (MRI) is useful to exclude structural and neurometabolic epileptic encephalopathies. One of four patients has abnormal nonspecific cranial MRI findings: hyperintensity of subcortical U-fibers, prominence of perivascular Virchow spaces, and delayed myelination for age.^{32, 50, 51} 18F-deoxyglucose-positron emission tomography (¹⁸F-FDG-PET) can be a useful additional diagnostic tool. A diminished FDG signal emanating from the cerebellum, thalamus, and cerebral cortex, and an apparent increase in glucose accumulation in the striatum, particularly in the caudate nucleus, is the imaging signature of Glut1DS. The thalami display hypometabolism comparable to the degree of cortical depression. Within the cerebral cortex, a more pronounced uptake deficit is observed in the mesial temporal lobe.^{29, 52, 53} Magnetic resonance spectroscopy imaging has been applied in single cases of Glut1DS for diagnostic evaluation of brain energy metabolism but controlled studies are needed to assess diagnostic sensitivity and specificity.^{54, 55}

Genetic analysis: Sequence analysis identifies heterozygous de pathogenic variants (or rarely, biallelic pathogenic variants) in SLC2A1 in 81%-89% of patients.³² Another 11%-14% of patients are confirmed by deletion/duplication analysis.³² However, the absence of SLC2A1 pathogenic variants does not always exclude Glut1DS. Pathogenic mechanisms may involve noncoding RNA genes as well as downstream defects in Glut1 translation, transcription, processing, activating, and trafficking (see Research section). SLC2A1-negative patients can be diagnosed on the basis of hypoglycorrhachia and distinctive clinical features (Table 2), especially when responding favorably to KDT. Several specific de novo pathogenic SLC2A1 variants affecting functional domains of the glut1 transporter have been identified. Also, pathogenic variants causing familial autosomal dominant and autosomal recessive inheritance have been reported in individual families.^56-58 The type of genetic mutation often correlates with phenotypic severity: missense variants (mild and moderate severity); splice site and nonsense variants and insertions, deletions, and exon deletions (moderate and severe severity); and complete gene microdeletions (severe severity).

Table 3. Consensus recommendation for Glut1DS diagnosis generated from survey results and discussions based on three key diagnostic criteria: characteristic clinical features, definite hypoglycorrhachia, and pathogenic <em>SLC2A1</em> variants

Laboratory studies: In the absence of pathogenic SLC2A1 variants, an impaired 3-O-methyl-D-glucose uptake in erythrocytes between 35% and 74% of controls is diagnostic (for details see³²). Western blot and oocyte expression analysis are additional tools to confirm pathogenicity. Recently, the measure of Glut1 present on the surface of the circulating red blood cells, as determined by flow cytometry (METAglut1^TM test), has been reported to be of diagnostic value for Glut1DS.⁵⁹

3.3 Treatment of Glut1DS

Ketogenic Diets: In Glut1DS, dietary therapy should be initiated as early as possible, providing the developing brain with a supplemental supply of metabolic fuel. Seizure control when managed with a classical KDT or MAD is good—one survey of Glut1DS families described 80% of patients with >90% seizure reduction including 64% of patients who no longer required antiseizure drugs.⁶⁰ Ineffective seizure control in Glut1DS despite KDT has been reported.^{29, 61} Movement disorders and cognitive issues also improve with dietary KDT.^{25, 29, 34, 47} The general management of KDT for drug-resistant childhood epilepsy and for Glut1DS is described in the revised international consensus.⁸ Specific aspects for KDT in Glut1DS are shown in Table 3. All patients with Glut1DS should monitor their ketosis by determination of beta-hydroxybutyrate in capillary or venous blood. Urine ketones are a qualitative measure of acetoacetate and as such are not as adequate to quantitatively monitor ketonemia in Glut1DS patients.^{32, 47} Classic KDT typically provide higher levels of ketosis and may be preferred in younger children, especially those under age three years.^{62, 63} In adolescents and adults, the MAD may be more feasible for quality of life and compliance.⁶⁴ The low glycemic index treatment (LGIT) provides very low ketones, has no evidence of benefit for Glut1DS, and is not recommended.^{47, 65} More effective KDT should be used in Glut1DS. If KDT are intolerable, increasing carbohydrates in regular diets has been described as an option.^{66, 67} Carnitine levels need to be monitored as patients may become secondarily carnitine-deficient over time.⁶⁸ Although most children with epilepsy on KDT can be transitioned off this treatment after two years, this plan is not appropriate for a child with Glut1DS. The benefits of KDT have no clear “end-date” and are thought to extend into adulthood. ⁶⁹

Concurrent antiseizure drugs: Treating Glut1DS with antiseizure medications does not address the underlying metabolic defect. As a result, no individual drug can be recommended. In addition to being therapeutically ineffective, medications may do harm. Several examples were discussed. For example, carbonic anhydrase inhibitors such as acetazolamide, topiramate, and sulthiame may aggravate acidosis.⁷⁰ Topiramate, acetazolamide, and zonisamide may increase the risk of urolithiasis, especially in combination with KDT.⁷¹ Decreased efficacy of KDT has been reported with the concomitant use of lamotrigine in childhood drug-resistant epilepsy.⁷² Barbiturates, diazepam, sodium valproate, chloralhydrate, methylxanthine, and ethanol are nonspecific inhibitors of Glut1 function in vitro. No inhibitory effects in vitro were observed for carbamazepine and phenytoin.^73-76 Prolonged exposure to phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin was shown to stimulate glucose transport in vitro by up to 30%-60%.⁷⁶ However, the implication of these in vitro data for the clinical setting remains unclear.

Ketone salts and ketoesters: Ketone salts and ketoesters may serve as a supplemental fuel for the brain without dietary restriction. Both compounds can be administered orally and can achieve ketone plasma levels equivalent to KDT.⁷⁷ Sodium 3-hydroxybutyrate has been used individually in metabolic diseases such as multiple acyl-CoA-dehydrogenase deficiency and specific glycogen storage diseases.⁷⁸ Oral ketone salts are freely available, but daily volumes up to 400 grams taste unpleasant and may cause sodium overload. Triheptanoin provides 5-carbon ketone body ketosis and an additional anaplerotic effect.^{79, 80} Despite promising initial reports,^{55, 81} a phase 3 study of triheptanoin (UX007) for the treatment of Glut1DS (ClinicalTrials.gov Identifier: NCT01993186) failed to meet its primary and secondary endpoints. A similar phase 3, randomized, double-blind, placebo-controlled, crossover study to assess the efficacy and safety of triheptanoin (UX007) in the treatment of movement disorders associated with Glut1DS (ClinicalTrials.gov Identifier: NCT02960217) was halted prematurely due to lack of efficacy (https://clinicaltrials.gov/ct2/show/ NCT02960217). Peer-reviewed National Institutes of Health funded studies using standard clinical trial criteria also are currently underway (NCT03041363, NCT03181399, NCT03301532). Novel approaches are ongoing, targeting small molecules and other biologics to enhance Glut1 expression and function.⁸² Anecdotal reports have described individual benefits of acetazolamide and L-DOPA as treatment for paroxysmal movement disorders in Glut1DS.^83-85

3.4 Glut1DS management and research

Follow-up. All patients with Glut1DS should be seen at regular intervals to monitor KDT, to address individual issues, and to share novel developments in the field.^{8, 32, 47} Follow-up needs to be age-specific as symptoms change from primarily seizures in infancy-early childhood to movement disorders such as dystonia and paroxysmal exertional dyskinesias in adolescence-adulthood.²⁵ Cognitive dysfunction persists throughout life but there is no evidence for progressive worsening.³⁴ Long-term adverse effects of KDT such as growth impairment, nephrolithiasis, and cardiovascular risks need to be monitored at regular intervals.⁸

With patients approaching adulthood, it is critical to develop a transitional plan for diet therapy and for medical care to transition from a pediatric to an adult subspecialist.⁴¹

Research. A family of hexose and MCT transporters provides metabolic fuel for the brain. At the cellular level, Glut1 molecules function as tetramers⁸⁶ to facilitate glucose transport across tissue barriers. The impact of Glut1DS on these systems is complex and the subject of ongoing research. SLC2A1 variants could destabilize GLUT1 native interactions, generate novel interactions, trigger protein misfolding, and enhance protein aggregation.⁸⁷ Glut1DS could be influenced by noncoding RNA genes^{88, 89} as well as downstream defects in Glut1 translation, transcription, processing, activating, and trafficking.^{90, 91} Research into disease mechanisms has identified novel targets for therapy by focusing (a) on the link between the fundamental metabolic defect (ie, brain glucose depletion or neuroglycopenia) and its metabolic and neurophysiological consequences, (b) on the mechanisms of transporter dysfunction,^{79, 92, 93} and (c) on the MRI and PET-based investigation of human brain metabolism.⁹⁴ These ongoing studies are enabling the development of supplemental metabolites to compensate for the neuroglycopenia as therapeutic strategies.⁸⁰ Glut1 haploinsufficiency arrests brain angiogenesis during brain development, and the diminished brain capillary network further aggravates neuroglycopenia.⁹⁵ Approaches to restore Glut1 protein content and function include upregulation of the normal SLC2A1 allele using small molecule strategies or gene transfer strategies whereby a normal SLC2A1 gene is delivered in a suitable viral vector to the Glut1-deficient patient.⁸² Preclinical experiments in Glut1DS model mice using AAV9 vectors have demonstrated that gene replacement is effective and durable: Brain Glut1 expression and CSF glucose concentrations increase, brain growth and volume are maintained during development, motor performance is preserved, seizure activity is controlled, and brain angiogenesis proceeds normally.^{82, 95, 96} In contrast to successful presymptomatic or early symptomatic treatment, gene replacement in adult mice failed to improve symptoms, suggesting a therapeutic window of opportunity. This observation in Glut1DS and other neurodevelopmental disorders emphasizes the fundamental importance of newborn screening for such genetically determined conditions to facilitate early postnatal diagnosis and proactive treatment before the onset of symptoms and irreversible damage to the developing brain.