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

Glut1 deficiency: when to suspect and how to diagnose?

Impaired glucose transport across the blood-brain barrier results in GLUT1 deficiency syndrome (GLUT1-DS), characterized by infantile seizures, developmental delay, acquired microcephaly, spasticity, ataxia, and hypoglycorrhachia. A part from this classic phenotype, clinical conditions associated with a deficiency of GLUT1 are highly variable and several atypical variants have been described; in particular, patients with movement disorders, but without seizures, with paroxysmal exertion-induced dyskinesia, have been reported. Most patients carry heterozygous de novo mutations in the GLUT1-gene but autosomal dominant and recessive transmission has been identified. Diagnosis is based on low cerebrospinal fluid glucose, in the absence of hypoglycemia, and it is confirmed by molecular analysis of the GLUT1-gene and by glucose uptake studies and immunoreactivity in human erythrocytes. Treatment with a ketogenic diet results in marked improvement of seizures and movement disorders. This review summarizes recent advances in understanding of GLUT1-DS and highlights the diagnostic and therapeutic approach to GLUT1-DS.

Familial clustering of epilepsy and behavioral disorders: evidence for a shared genetic basis

PURPOSE

To examine whether family history of unprovoked seizures is associated with behavioral disorders in epilepsy probands, thereby supporting the hypothesis of shared underlying genetic susceptibility to these disorders.

METHODS

We conducted an analysis of the 308 probands with childhood onset epilepsy from the Connecticut Study of Epilepsy with information on first-degree family history of unprovoked seizures and of febrile seizures whose parents completed the Child Behavior Checklist (CBCL) at the 9-year follow-up. Clinical cutoffs for CBCL problem and Diagnostic and Statistical Manual of Mental Disorders (DSM)-Oriented scales were examined. The association between first-degree family history of unprovoked seizure and behavioral disorders was assessed separately in uncomplicated and complicated epilepsy and separately for first-degree family history of febrile seizures. A subanalysis, accounting for the tendency for behavioral disorders to run in families, was adjusted for siblings with the same disorder as the proband. Prevalence ratios were used to describe the associations.

KEY FINDINGS

In probands with uncomplicated epilepsy, first-degree family history of unprovoked seizure was significantly associated with clinical cutoffs for Total Problems and Internalizing Disorders. Among Internalizing Disorders, clinical cutoffs for Withdrawn/Depressed, and DSM-Oriented scales for Affective Disorder and Anxiety Disorder were significantly associated with family history of unprovoked seizures. Clinical cutoffs for Aggressive Behavior and Delinquent Behavior, and DSM-Oriented scales for Conduct Disorder and Oppositional Defiant Disorder were significantly associated with family history of unprovoked seizure. Adjustment for siblings with the same disorder revealed significant associations for the relationship between first-degree family history of unprovoked seizure and Total Problems and Aggressive Behavior in probands with uncomplicated epilepsy; marginally significant results were seen for Internalizing Disorder, Withdrawn/Depressed, and Anxiety Disorder. There was no association between family history of unprovoked seizure and behavioral problems in probands with complicated epilepsy. First-degree family history of febrile seizure was not associated with behavioral problems in probands with uncomplicated or in those with complicated epilepsy.

SIGNIFICANCE

Increased occurrence of behavioral disorders in probands with uncomplicated epilepsy and first degree family history of unprovoked seizure suggests familial clustering of these disorders. This supports the idea that behavioral disorders may be another manifestation of the underlying pathophysiology involved in epilepsy or closely related to it.

"Electro-clinical syndromes" with onset in paediatric age: the highlights of the clinical-EEG, genetic and therapeutic advances

The genetic causes underlying epilepsy remain largely unknown, and the impact of available genetic data on the nosology of epilepsy is still limited. Thus, at present, classification of epileptic disorders should be mainly based on electroclinical features. Electro-clinical syndrome is a term used to identify a group of clinical entities showing a cluster of electro-clinical characteristics, with signs and symptoms that together define a distinctive, recognizable, clinical disorder. These often become the focus of treatment trials as well as of genetic, neuropsychological, and neuroimaging investigations. They are distinctive disorders identifiable on the basis of a typical age onset, specific EEG characteristics, seizure types, and often other features which, when taken together, permit a specific diagnosis which, in turn, often has implications for treatment, management, and prognosis. Each electro-clinical syndrome can be classified according to age at onset, cognitive and developmental antecedents and consequences, motor and sensory examinations, EEG features, provoking or triggering factors, and patterns of seizure occurrence with respect to sleep. Therefore, according to the age at onset, here we review the more frequently observed paediatric electro-clinical syndrome from their clinical-EEG, genetic and therapeutic point of views.

Mutations in the gene PRRT2 cause paroxysmal kinesigenic dyskinesia with infantile convulsions

Paroxysmal kinesigenic dyskinesia with infantile convulsions (PKD/IC) is an episodic movement disorder with autosomal-dominant inheritance and high penetrance, but the causative genetic mutation is unknown. We have now identified four truncating mutations involving the gene PRRT2 in the vast majority (24/25) of well-characterized families with PKD/IC. PRRT2 truncating mutations were also detected in 28 of 78 additional families. PRRT2 encodes a proline-rich transmembrane protein of unknown function that has been reported to interact with the t-SNARE, SNAP25. PRRT2 localizes to axons but not to dendritic processes in primary neuronal culture, and mutants associated with PKD/IC lead to dramatically reduced PRRT2 levels, leading ultimately to neuronal hyperexcitability that manifests in vivo as PKD/IC.

Stomatin-deficient cryohydrocytosis results from mutations in SLC2A1: a novel form of GLUT1 deficiency syndrome

The hereditary stomatocytoses are a series of dominantly inherited hemolytic anemias in which the permeability of the erythrocyte membrane to monovalent cations is pathologically increased. The causative mutations for some forms of hereditary stomatocytosis have been found in the transporter protein genes, RHAG and SLC4A1. Glucose transporter 1 (glut1) deficiency syndromes (glut1DSs) result from mutations in SLC2A1, encoding glut1. Glut1 is the main glucose transporter in the mammalian blood-brain barrier, and glut1DSs are manifested by an array of neurologic symptoms. We have previously reported 2 cases of stomatin-deficient cryohydrocytosis (sdCHC), a rare form of stomatocytosis associated with a cold-induced cation leak, hemolytic anemia, and hepatosplenomegaly but also with cataracts, seizures, mental retardation, and movement disorder. We now show that sdCHC is associated with mutations in SLC2A1 that cause both loss of glucose transport and a cation leak, as shown by expression studies in Xenopus oocytes. On the basis of a 3-dimensional model of glut1, we propose potential mechanisms underlying the phenotypes of the 2 mutations found. We investigated the loss of stomatin during erythropoiesis and find this occurs during reticulocyte maturation and involves endocytosis. The molecular basis of the glut1DS, paroxysmal exercise-induced dyskinesia, and sdCHC phenotypes are compared and discussed.

Genetic testing in epilepsy: what should you be doing?

With the burgeoning array of molecular tests available in the epilepsies, the clinician needs to know which tests to order for each patient. Epileptic encephalopathies are the most important clinical group for genetic testing with an increasing number of distinctive epilepsy syndromes being recognized. Identification of the causative mutation affects treatment as well as prognostic and genetic counseling.

Differential diagnosis of chorea

Chorea is a common movement disorder that can be caused by a large variety of structural, neurochemical (including pharmacologic), or metabolic disturbances to basal ganglia function, indicating the vulnerability of this brain region. The diagnosis is rarely indicated by the simple phenotypic appearance of chorea, and can be challenging, with many patients remaining undiagnosed. Clues to diagnosis may be found in the patient's family or medical history, on neurologic examination, or upon laboratory testing and neuroimaging. Increasingly, advances in genetic medicine are identifying new disorders and expanding the phenotype of recognized conditions. Although most therapies at present are supportive, correct diagnosis is essential for appropriate genetic counseling, and ultimately, for future molecular therapies.

Paroxysmal dyskinesias

Paroxysmal movement disorders are a relatively rare and heterogenous group of conditions manifesting as episodic dyskinesia lasting a brief duration. Three forms are clearly recognized, namely, paroxysmal kinesigenic (PKD), nonkinisegenic (PNKD), and exercise induced (PED). There have been major advances in the understanding of the pathophysiological mechanisms and the genetics of these disorders, leading to better clinical definitions based on genotype-phenotype correlations in the familial idiopathic forms. PKD is genetically heterogenous, but there is linkage to chromosome 16 in a number of families. PNKD is due to mutations of the MR-1 gene. PED is genetically heterogenous, but a number of familial and sporadic cases may be due to GLUT-1 gene mutations. The GLUT1 gene-related form of PED may respond to a ketogenic diet. Potassium and calcium channel mutations underlie the 2 main forms of episodic ataxia (EA1 and EA2), whereas benign torticollis of infancy may also be a calcium channel disorder.

Milestones in dystonia

The last 25 years have seen remarkable advances in our understanding of the genetic etiologies of dystonia, new approaches into dissecting underlying pathophysiology, and independent progress in identifying effective treatments. In this review we highlight some of these advances, especially the genetic findings that have taken us from phenomenological to molecular-based diagnoses. Twenty DYT loci have been designated and 10 genes identified, all based on linkage analyses in families. Hand in hand with these genetic findings, neurophysiological and imaging techniques have been employed that have helped illuminate the similarities and differences among the various etiological dystonia subtypes. This knowledge is just beginning to yield new approaches to treatment including those based on DYT1 animal models. Despite the lag in identifying genetically based therapies, effective treatments, including impressive benefits from deep brain stimulation and botulinum toxin chemodenervation, have marked the last 25 years. The challenge ahead includes continued advancement into understanding dystonia's many underlying causes and associated pathology and using this knowledge to advance treatment including preventing genetic disease expression.

The genetics of monogenic idiopathic epilepsies and epileptic encephalopathies

The group of idiopathic epilepsies encompasses numerous syndromes without known organic substrate. Genetic anomalies are thought to be responsible for pathogenesis, with a monogenic or polygenic model of inheritance. Over the last two decades, a number of genetic anomalies and encoded proteins have been related to particular idiopathic epilepsies and epileptic encephalopathies. Most of these mutations involve subunits of neuronal ion channels (e.g. potassium, sodium, and chloride channels), and may result in abnormal neuronal hyperexcitability manifesting with seizures. However non-ion channel proteins may also be affected. Correlations between genotype and phenotype are not easy to establish, since genetic and non-genetic factors are likely to play a role in determining the severity of clinical features. The growing number of discoveries on this topic are improving classification, prognosis and counseling of patients and families with these forms of epilepsy, and may lead to targeted therapeutic approaches in the near future. In this article the authors have reviewed the main genetic discoveries in the field of the monogenic idiopathic epilepsies and epileptic encephalopathies, in order to provide epileptologists with a concise and comprehensive summary of clinical and genetic features of these seizure disorders.

Long-term follow-up of the ketogenic diet for refractory epilepsy: multicenter Argentinean experience in 216 pediatric patients

PURPOSE

In this Argentinean retrospective, collaborative, multicenter study, we examine the efficacy and tolerability of the ketogenic diet (KD) for different epilepsy syndromes.

MATERIALS AND METHODS

we evaluated the clinical records of 216 patients started on the KD between March 1, 1990 and December 31, 2010.

RESULTS

One hundred forty of the initial patients (65%) remained on the diet at the end of the study period. Twenty-nine patients (20.5%) became seizure free and 50 children (36%) had a 75-99% decrease in seizures. Thus, 56.5% of the patients had a seizure control of more than 75%. The best results were found in patients with epilepsy with myoclonic-astatic seizures, Lennox-Gastaut syndrome, and West syndrome. Good results were also found in patients with Dravet syndrome, in those with symptomatic focal epilepsy secondary to malformations of cortical development, and in patients with tuberous sclerosis. Seizures were significantly reduced in four patients with fever-induced refractory epileptic encephalopathy in school-age children and in two patients with epileptic encephalopathy with continuous spikes and waves during slow sleep. The median period of follow-up after discontinuation of the diet was 6 years. Twenty patients who had become seizure free discontinued the diet, but seizures recurred in five (25%). Of 40 patients with a seizure reduction of more than 50% who discontinued the diet, 10 presented with recurrent seizures.

CONCLUSION

The ketogenic diet is a good option in the treatment of refractory epilepsy. After discontinuing the diet, seizures recurrence occurred in few patients.

Paroxysmal dyskinesias

Paroxysmal dyskinesias are a rare group of movement disorders affecting both adults and children. Based on the events that precipitate the abnormal movements, they are subdivided into paroxysmal kinesigenic dyskinesia (PKD), precipitated by sudden voluntary movements; paroxysmal nonkinesigenic dyskinesia (PNKD), which occurs at rest; paroxysmal exertion-induced dyskinesia (PED), occurring after prolonged exercise; and paroxysmal hypnogenic dyskinesia (PHD), which occurs in sleep. Paroxysmal dyskinesias can be sporadic, familial (autosomal dominant inheritance), or secondary to other disorders. Recent genetic discoveries may aid us in elucidating the pathophysiology of these disorders. PKD has been linked to the pericentromeric region of chromosome 16, PNKD is associated with mutations in the myofibrillogenesis regulator 1 (MR-1) gene on the long arm of chromosome 2 (2q32-36 locus), and PED is associated with mutations in the glucose transporter gene, GLUT1, responsible for glucose transport across the blood-brain barrier. Lifestyle modification to avoid precipitating factors is important in the management of paroxysmal dyskinesias. Medical therapies have not been examined in controlled trials. Nevertheless, anticonvulsants have been found to be extremely effective in treating PKD and are sometimes useful in other types, suggesting that these disorders may indeed represent forms of channelopathies. Drugs such as acetazolamide, anticholinergics, levodopa, and tetrabenazine have been inconsistently successful. In rare cases with medically refractory symptoms, deep brain stimulation has also been employed. Development of successful treatments for the different paroxysmal dyskinesias rests on elucidating the pathophysiology and targeting therapy to treat the underlying perturbation.

Milder phenotypes of glucose transporter type 1 deficiency syndrome

Glucose transporter type 1 deficiency syndrome (GLUT1DS) is a treatable condition resulting from impaired glucose transport into the brain. The classical presentation is with infantile-onset epilepsy and severe developmental delay. Non-classical phenotypes with movement disorders and early-onset absence epilepsy are increasingly recognized and the clinical spectrum is expanding. The hallmark is hypoglycorrhachia (cerebrospinal fluid [CSF] glucose<2.2 mmol/l) in the presence of normoglycaemia with a CSF/blood glucose ratio of less than 0.4. GLUT1DS is due to a mutation in the solute carrier family 2, member 1 gene (SLC2A1). We present five individuals (four males, one female), all of whom had a mild phenotype, highlighting the importance of considering this diagnosis in unexplained neurological disorders associated with mild learning difficulties, subtle motor delay, early-onset absence epilepsy, fluctuating gait disorders, and/or dystonia. The mean age at diagnosis was 8 years 8 months. This paper also shows phenotypical parallels between GLUT1DS and paroxysmal exertion-induced dyskinesia.

EFNS guidelines on diagnosis and treatment of primary dystonias

OBJECTIVES

to provide a revised version of earlier guidelines published in 2006.

BACKGROUND

primary dystonias are chronic and often disabling conditions with a widespread spectrum mainly in young people.

DIAGNOSIS

primary dystonias are classified as pure dystonia, dystonia plus or paroxysmal dystonia syndromes. Assessment should be performed using a validated rating scale for dystonia. Genetic testing may be performed after establishing the clinical diagnosis. DYT1 testing is recommended for patients with primary dystonia with limb onset before age 30, and in those with an affected relative with early-onset dystonia. DYT6 testing is recommended in early-onset or familial cases with cranio-cervical dystonia or after exclusion of DYT1. Individuals with early-onset myoclonus should be tested for mutations in the DYT11 gene. If direct sequencing of the DYT11 gene is negative, additional gene dosage is required to improve the proportion of mutations detected. A levodopa trial is warranted in every patient with early-onset primary dystonia without an alternative diagnosis. In patients with idiopathic dystonia, neurophysiological tests can help with describing the pathophysiological mechanisms underlying the disorder.

TREATMENT

botulinum toxin (BoNT) type A is the first-line treatment for primary cranial (excluding oromandibular) or cervical dystonia; it is also effective on writing dystonia. BoNT/B is not inferior to BoNT/A in cervical dystonia. Pallidal deep brain stimulation (DBS) is considered a good option, particularly for primary generalized or cervical dystonia, after medication or BoNT have failed. DBS is less effective in secondary dystonia. This treatment requires a specialized expertise and a multidisciplinary team.

Paroxysmal exercise-induced dyskinesia with self-limiting partial epilepsy: a novel GLUT-1 mutation with benign phenotype

Paroxysmal exercise-induced dyskinesia (PED) is a rare form of dystonia induced by prolonged exercise, usually involving lower limbs. PED has been recently described as a possible clinical manifestation of mutations of SLC2A1 gene, encoding for the glucose transport GLUT-1. We report a case of a young woman with a mild form of PED associated with self-limiting partial epilepsy. She carries a novel sporadic heterozygous mutation of the SLC2A1 gene. Diagnostic difficulties and possible treatment with carbamazepine are discussed.

Dystonic tremor caused by mutation of the glucose transporter gene GLUT1

Glucose transporter 1 deficiency syndrome (GLUT1-DS) is due to heterozygous mutation of the glucose transporter type 1 gene (GLUT1/SLC2A1). GLUT1-DS is characterized by movement disorders, including paroxysmal exercise-induced dystonia (PED), as well as seizures, mental retardation and hypoglycorrhachia. Tremor was recently shown to be part of the phenotype, but its clinical and electrophysiological features have not yet been described in detail, and GLUT1 tremor reports are rare. We describe two patients, a young woman and her mother, who were referred to us for tremor. We also systematically review published cases of GLUT1-DS with tremor (14 cases, including ours), focusing on clinical features. In most cases (10/14), the tremor, which involved the limbs and voice, fulfilled clinical criteria for dystonic tremor (DT), occurring in body areas affected by dystonia. Tremor was the only permanent symptom in 2 cases. Recordings, reported here for the first time, showed an irregular 6- to 8.5-Hz tremor compatible with DT in our two patients. These findings show that tremor, and particularly DT, may be a presenting symptom of GLUT1-DS. Patients who present with dystonic tremor, with or without mental retardation, seizures, movement disorders and/or a family history, should therefore be screened for GLUT1-DS.

Engaging neuroscience to advance translational research in brain barrier biology

The delivery of many potentially therapeutic and diagnostic compounds to specific areas of the brain is restricted by brain barriers, of which the most well known are the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier. Recent studies have shown numerous additional roles of these barriers, including an involvement in neurodevelopment, in the control of cerebral blood flow, and--when barrier integrity is impaired--in the pathology of many common CNS disorders such as Alzheimer's disease, Parkinson's disease and stroke.

Characterization and mode of inheritance of a paroxysmal dyskinesia in Chinook dogs

BACKGROUND

Paroxysmal dyskinesias are episodes of abnormal, involuntary movement or muscle tone, distinguished from seizures by the character of the episode and lack of seizure activity on ictal EEG.

HYPOTHESIS

Paroxysmal dyskinesia is an inherited, autosomal recessive disorder in Chinook dogs.

ANIMALS

Families of Chinook dogs with paroxysmal dyskinesia.

METHODS

Pedigrees and medical histories were reviewed for 299 Chinook dogs. A family of 51 dogs was used for analysis. Episodes were classified as seizures, paroxysmal dyskinesia, or unknown, and segregation analysis was performed.

RESULTS

Paroxysmal dyskinesia was identified in 16 of 51 dogs and characterized by an inability to stand or ambulate, head tremors, and involuntary flexion of 1 or multiple limbs, without autonomic signs or loss of consciousness. Episode duration varied from minutes to an hour. Inter-ictal EEGs recorded on 2 dogs with dyskinesia were normal. Three dogs with dyskinesia also had generalized tonic-clonic seizures. One of 51 dogs had episodes of undetermined type. Phenotype was unknown for 6 of 51 dogs, and 28 dogs were unaffected. Segregation was consistent with an autosomal recessive trait.

CONCLUSIONS AND CLINICAL IMPORTANCE

This movement disorder is prevalent in the Chinook breed, and consistent with a partially penetrant autosomal recessive or polygenic trait. Insufficient evidence exists for definitive localization; episodes may be of basal nuclear origin, but atypical seizures and muscle membrane disorders remain possible etiologies. The generalized seizures may be a variant phenotype of the same mutation that results in dyskinesia, or the 2 syndromes may be independent.

Paroxysmal choreodystonic disorders

Paroxysmal choreodystonic disorders or paroxysmal dyskinesias are a heterogeneous group of movement disorders characterized by recurrent attacks of abnormal involuntary movements. They are classified into four categories according to the precipitant, duration of attacks, and etiology: (1) paroxysmal kinesigenic dyskinesia (PKD), in which attacks are brief and induced by sudden voluntary movements; (2) paroxysmal nonkinesigenic dyskinesia (PNKD), in which attacks occur spontaneously; (3) paroxysmal exertion-induced dyskinesia (PED), in which attacks are brought on by prolonged exercise; and (4) paroxysmal hypnogenic dyskinesia (PHD), in which attacks occur during sleep. Among them, PHD is currently known to be a form of mesial frontal-lobe epilepsy, and has been given the term "autosomal-dominant nocturnal frontal lobe epilepsy" (ANDFLE) in some familiar cases with an autosomal-dominant inheritance. The clinical, etiological and pathophysiological features of PKD, PNKD, and PED are reviewed.

Pre- and postprandial electroencephalography in glucose transporter type 1 deficiency syndrome: an illustrative case to discuss the concept of carbohydrate responsiveness

Glucose transporter type 1 deficiency syndrome is an inborn error of glucose transport across the blood-brain barrier with hypoglychorrachia. Patients usually present developmental delay, movement disorders, seizures, and acquired microcephaly, variously associated and leading to different phenotypes. We report a 3-year-old girl affected by glucose transporter type 1 deficiency syndrome with carbohydrate responsiveness. Her history was characterized by worsening of ataxia with an increasing interval to the last food intake, occurrence of seizures in the morning before breakfast, slowing of electroencephalogram (EEG) background activity with the appearance of epileptiform discharges during preprandial recordings, and improvement of the electroclinical picture after food intake. By adding a new case to the pertinent literature, we stress the role of pre- and postprandial EEG recordings for the identification of individuals potentially affected by glucose transporter type 1 deficiency syndrome. We also provide a possible physiopathological interpretation of EEG changes related to food intake.

Treatment of paroxysmal dyskinesias

IMPORTANCE OF THE FIELD

Paroxysmal dyskinesias represent a heterogeneous group of rare diseases sharing characteristics with two important groups of neurological disorders, the movement disorders and the epilepsies. Their common hallmark is the paroxysmal occurrence of dyskinesias including athetosis, ballism, chorea and dystonia. During the last two decades, various genetic abnormalities have been identified thereby providing insight into the underlying pathophysiology and offering therapeutic opportunities for many of these conditions.

AREAS COVERED IN THIS REVIEW

We summarize the diagnostic criteria of idiopathic and symptomatic paroxysmal dyskinesias and describe their therapeutic options. For the preparation of this review article, an extensive literature search was undertaken using PubMed.

WHAT THE READER WILL GAIN

This review provides a practical guide to the diagnosis and treatment of paroxysmal dyskinesias.

TAKE HOME MESSAGE

The mainstay of therapy is carbamazepine for paroxysmal kinesigenic dyskinesias and clonazepam for the nonkinesigenic dyskinesias. In symptomatic paroxysmal dyskinesias, the treatment of the underlying disease will provide best results. The ketogenic diet for patients with paroxysmal exertion-induced dyskinesias is a promising new therapeutic strategy and may not only prevent attacks but also lead to improvement of developmental delay in affected children.

Mild adolescent/adult onset epilepsy and paroxysmal exercise-induced dyskinesia due to GLUT1 deficiency

Paroxysmal exercise-induced dyskinesia (PED) and epilepsy without intellectual disability have recently been recognized as manifestations of deficiency of the glucose transporter GLUT1, due to mutations in the gene SLC2A1. We describe a family with six definitely affected members in two generations. Two had PED, three had epilepsy, and one had both. A missense mutation in SLC2A1 (c.950A>C; p.N317T) was detected in five living affected members, but absent in three nonaffected first-degree members and in one subject believed to be a phenocopy. The clinical picture of mild epilepsy with onset in adolescence or early adulthood plus PED should raise a suspicion of GLUT1 deficiency.

Genetics of primary torsion dystonia

Advances in the genetics of dystonia have further elucidated the pathophysiology of this clinically and etiologically heterogeneous group of movement disorders. Currently, 20 monogenic forms of dystonia, designated by the acronym DYT, are grouped as 1) pure dystonias, 2) dystonia-plus syndromes, and 3) paroxysmal dystonias/dyskinesias. We summarize recently discovered genes and loci, including the 1) detection of two primary dystonia genes (DYT6, DYT16), 2) identification of the DYT17 locus, 3) association of a dystonia/dyskinesia phenotype with a gene previously linked to GLUT1 (glucose transporter of the blood-brain barrier) deficiency syndrome (DYT18), 4) designation of paroxysmal kinesigenic and nonkinesigenic dyskinesia as DYT19 and DYT20, and 5) redefinition of DYT14 as DYT5. Further, we review current knowledge regarding genetic modifiers and susceptibility factors. Because recognizing and diagnosing monogenic dystonias have important implications for patients and their families with regard to counseling, prognosis, and treatment, we highlight clinical "red flags" of individual subtypes and review guidelines for genetic testing.

Movement disorders in neuro-metabolic diseases

Inborn errors of metabolism (IEM) are a group of genetic disorders characterized by dysfunction of an enzyme or other protein involved in cellular metabolism.(1) Most IEMs involve the nervous system (neuro-metabolic diseases or NMD). NMD often present with a complex clinical picture: psychomotor retardation and/or regression, pyramidal signs, ataxia, hypotonia and epilepsy and movement disorders.(1) Movement disorders are more frequently part of this complex picture than a predominant symptom, however in some instances the clinical picture may be summarized in an invalidating movement disorder.(2) On a phenomenology basis, one can distinguish eight main types of movement disorders: dystonia and athetosis, chorea, tremor with or without parkinsonism, ballismus, myoclonus, tics and stereotypies. Most of these abnormal involuntary movements generate from a dysfunction or a lesion in the basal ganglia, excepting myoclonus, the origin of which can vary (cortical, brainstem, basal ganglia, spinal and even peripheral nervous system).(3) Classically the most frequently observed movement disorders in NMD are: dystonia, myoclonus, chorea, tremor and parkinsonism (Fig. 1). The primary goal of this article is, departing from the literature and a large personal series, to describe the types of movement disorders most frequently observed in NMD and to discuss their clinical value in the setting of specific types of NMD.

Paroxysmal exercise-induced dyskinesia, writer's cramp, migraine with aura and absence epilepsy in twin brothers with a novel SLC2A1 missense mutation

We report two monochorionic twins that progressively developed, between ages 5 and 10, a combination of episodic neurological disorders including paroxysmal exercise-induced dyskinesia, migraine without or with aura, absence seizures and writer's cramp. CSF/serum glucose ratio was moderately decreased in both patients. Mutational analysis of SLC2A1 gene identified a de novo heterozygous missense mutation in exon 4. This novel mutation has been previously showed to disrupt glucose transport in vitro. Both patients showed immediate and near-complete response to ketogenic diet. This clinical observation suggests that a high index of suspicion for GLUT1 deficiency syndrome is warranted in evaluating patients with multiple neurological paroxysmal events.

The spectrum of movement disorders in Glut-1 deficiency

To assess the spectrum of movement disorders, we reviewed video recordings and charts of 57 patients with Glut-1 deficiency. Eighty-nine percent of patients with Glut-1 deficiency syndrome had a disturbance of gait. The most frequent gait abnormalities were ataxic-spastic and ataxic. Action limb dystonia was observed in 86% of cases and mild chorea in 75%. Cerebellar action tremor was seen in 70% of patients, myoclonus in 16%, and dyspraxia in 21%. Nonepileptic paroxysmal events occurred in 28% of patients, and included episodes of ataxia, weakness, Parkinsonism and nonkinesogenic dyskinesias. The 40 patients (70%) who were on the ketogenic diet had less severe gait disturbances but more dystonia, chorea, tremor, myoclonus, dyspraxia, and paroxysmal events compared with the 17 patients on a conventional diet. Poor dietary compliance and low ketonuria appear to trigger the paroxysmal events in some patients. Gait disturbances and movement disorders are frequent in patients with Glut-1 deficiency and are signs of chronic and intermittent pyramidal, cerebellar and extrapyramidal circuit dysfunction. These clinical symptoms reflect chronic nutrient deficiency during brain development and may be mitigated by chronic ketosis.

Uncovering microdeletions in patients with severe Glut-1 deficiency syndrome using SNP oligonucleotide microarray analysis

Glut-1 facilitates the diffusion of glucose across the blood-brain barrier and is responsible for glucose entry into the brain. Impaired glucose transport across the blood-brain barrier results in Glut-1 deficiency syndrome (Glut-1 DS, OMIM 606777), characterized in its most severe form by infantile seizures, developmental delay, acquired microcephaly, spasticity, ataxia, and hypoglycorrhachia. Approximately 93% of patients with Glut-1 DS have identifiable mutations by sequence analysis in SLC2A1 which localizes to chromosome 1p34.2. In this report, we describe seven severe cases of Glut-1 DS, including a set of identical twins, caused by microdeletions in the SLC2A1 region. These patients were all mutation negative by molecular sequencing. Microdeletions ranged in size from 45Kb to 4.51Mb, and all were identified using high resolution single nucleotide polymorphism (SNP) oligonucleotide microarray analysis (SOMA). Cases with microdeletions 82Kb were not resolvable by FISH. All patients had severe epilepsy, significant cognitive and motor delay, ataxia, and microcephaly. MRI changes, when present, were of greater severity than are typically associated with missense mutations in SLC2A1.

Familial paroxysmal exercise-induced dystonia: atypical presentation of autosomal dominant GTP-cyclohydrolase 1 deficiency

Paroxysmal exercise-induced dystonia (PED) is one of the rarer forms of paroxysmal dyskinesia, and can occur in sporadic or familial forms. We report a family (male index case, mother and maternal grandfather) with autosomal dominant inheritance of paroxysmal exercise-induced dystonia. The dystonia began in childhood and was only ever induced after many minutes of exercise, and was never present at rest, or on initiation of movements. In addition, family members suffered restless legs syndrome (RLS), depression, and adult-onset Parkinsonism. The index case had low cerebrospinal fluid neurotransmitters and pterins. The PED and RLS stopped on initiation of L-Dopa therapy. Both live family members were found to have a nonsense mutation (p.E84X) in exon 1 of the GTP-cyclohydrolase 1 (GCH-1) gene. We propose that GCH-1 mutations should be considered a genetic cause of familial PED, especially if additional clinical features of monoaminergic deficiency are present in affected individuals.

Glucose transporter-1 deficiency syndrome: the expanding clinical and genetic spectrum of a treatable disorder

Glucose transporter-1 deficiency syndrome is caused by mutations in the SLC2A1 gene in the majority of patients and results in impaired glucose transport into the brain. From 2004-2008, 132 requests for mutational analysis of the SLC2A1 gene were studied by automated Sanger sequencing and multiplex ligation-dependent probe amplification. Mutations in the SLC2A1 gene were detected in 54 patients (41%) and subsequently in three clinically affected family members. In these 57 patients we identified 49 different mutations, including six multiple exon deletions, six known mutations and 37 novel mutations (13 missense, five nonsense, 13 frame shift, four splice site and two translation initiation mutations). Clinical data were retrospectively collected from referring physicians by means of a questionnaire. Three different phenotypes were recognized: (i) the classical phenotype (84%), subdivided into early-onset (<2 years) (65%) and late-onset (18%); (ii) a non-classical phenotype, with mental retardation and movement disorder, without epilepsy (15%); and (iii) one adult case of glucose transporter-1 deficiency syndrome with minimal symptoms. Recognizing glucose transporter-1 deficiency syndrome is important, since a ketogenic diet was effective in most of the patients with epilepsy (86%) and also reduced movement disorders in 48% of the patients with a classical phenotype and 71% of the patients with a non-classical phenotype. The average delay in diagnosing classical glucose transporter-1 deficiency syndrome was 6.6 years (range 1 month-16 years). Cerebrospinal fluid glucose was below 2.5 mmol/l (range 0.9-2.4 mmol/l) in all patients and cerebrospinal fluid : blood glucose ratio was below 0.50 in all but one patient (range 0.19-0.52). Cerebrospinal fluid lactate was low to normal in all patients. Our relatively large series of 57 patients with glucose transporter-1 deficiency syndrome allowed us to identify correlations between genotype, phenotype and biochemical data. Type of mutation was related to the severity of mental retardation and the presence of complex movement disorders. Cerebrospinal fluid : blood glucose ratio was related to type of mutation and phenotype. In conclusion, a substantial number of the patients with glucose transporter-1 deficiency syndrome do not have epilepsy. Our study demonstrates that a lumbar puncture provides the diagnostic clue to glucose transporter-1 deficiency syndrome and can thereby dramatically reduce diagnostic delay to allow early start of the ketogenic diet.

Genetic testing in the epilepsies--report of the ILAE Genetics Commission

In this report, the International League Against Epilepsy (ILAE) Genetics Commission discusses essential issues to be considered with regard to clinical genetic testing in the epilepsies. Genetic research on the epilepsies has led to the identification of more than 20 genes with a major effect on susceptibility to idiopathic epilepsies. The most important potential clinical application of these discoveries is genetic testing: the use of genetic information, either to clarify the diagnosis in people already known or suspected to have epilepsy (diagnostic testing), or to predict onset of epilepsy in people at risk because of a family history (predictive testing). Although genetic testing has many potential benefits, it also has potential harms, and assessment of these potential benefits and harms in particular situations is complex. Moreover, many treating clinicians are unfamiliar with the types of tests available, how to access them, how to decide whether they should be offered, and what measures should be used to maximize benefit and minimize harm to their patients. Because the field is moving rapidly, with new information emerging practically every day, we present a framework for considering the clinical utility of genetic testing that can be applied to many different syndromes and clinical contexts. Given the current state of knowledge, genetic testing has high clinical utility in few clinical contexts, but in some of these it carries implications for daily clinical practice.

GLUT1 gene mutations cause sporadic paroxysmal exercise-induced dyskinesias

Paroxysmal exercise-induced dyskinesias (PED) are involuntary intermittent movements triggered by prolonged physical exertion. Autosomal dominant inheritance may occur. Recently, mutations in the glucose transporter 1 (GLUT1) gene (chr. 1p35-p31.3) have been identified as a cause in some patients with autosomal dominant PED. Mutations in this gene have previously been associated with the GLUT1 deficiency syndrome. We performed mutational analysis in 10 patients with apparently sporadic PED. We identified two novel GLUT1 mutations, at least one likely to be de-novo, in two of our patients. Onset was in early childhood. One of our patients had a predating history of childhood absence epilepsy and a current history of hemiplegic migraine as well as a family history of migraine. The other patient had no other symptoms apart from PED. Brain MRI showed cerebellar atrophy in one case. Mutations in GLUT1 are one cause of apparently sporadic PED. The detection of this has important implications for treatment as ketogenic diet has been reported to be beneficial.

Early-onset absence epilepsy caused by mutations in the glucose transporter GLUT1

Absence epilepsies of childhood are heterogeneous with most cases following complex inheritance. Those cases with onset before 4 years of age represent a poorly studied subset. We screened 34 patients with early-onset absence epilepsy for mutations in SLC2A1, the gene encoding the GLUT1 glucose transporter. Mutations leading to reduced protein function were found in 12% (4/34) of patients. Two mutations arose de novo, and two were familial. These findings suggest GLUT1 deficiency underlies a significant proportion of early-onset absence epilepsy, which has both genetic counseling and treatment implications because the ketogenic diet is effective in GLUT1 deficiency.