YEBEH-04309; No of Pages 7 Epilepsy & Behavior xxx (2015) xxx–xxx

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Genetic mutations associated with status epilepticus M. Bhatnagar, S. Shorvon ⁎ UCL Institute of Neurology, University College London, UK

a r t i c l e

i n f o

Article history: Revised 6 April 2015 Accepted 7 April 2015 Available online xxxx Keywords: Status epilepticus Genetics Cerebral dysplasia Inborn errors of metabolism Mitochondrial disease Epileptic encephalopathy

a b s t r a c t This paper reports the results of a preliminary search of the literature aimed at identifying the genetic mutations reported to be strongly associated with status epilepticus. Genetic mutations were selected for inclusion if status epilepticus was specifically mentioned as a consequence of the mutation in standard genetic databases or in a case report or review article. Mutations in 122 genes were identified. The genetic mutations identified were found in only rare conditions (sometimes vanishingly rare) and mostly in infants and young children with multiple other handicaps. Most of the genetic mutations can be subdivided into those associated with cortical dysplasias, inborn errors of metabolism, mitochondrial disease, or epileptic encephalopathies and childhood syndromes. There are no identified ‘pure status epilepticus genes’. The range of genes underpinning status epilepticus differs in many ways from the range of genes underpinning epilepsy, which suggests that the processes underpinning status epilepticus differ from those underpinning epilepsy. It has been frequently postulated that status epilepticus is the result of a failure of ‘seizure termination mechanisms’, but the wide variety of genes affecting very diverse biochemical pathways identified in this survey makes any unitary cause unlikely. The genetic influences in status epilepticus are likely to involve a wide range of mechanisms, some related to development, some to cerebral energy production, some to diverse altered biochemical pathways, some to transmitter and membrane function, and some to defects in networks or systems. The fact that many of the identified genes are involved with cerebral development suggests that status epilepticus might often be a system or network phenomenon. To date, there are very few genes identified which are associated with adult-onset status epilepticus (except in those with preexisting neurological damage), and this is disappointing as the cause of many adult-onset status epilepticus cases remains obscure. It has been suggested that idiopathic adult-onset status epilepticus might often have an immunological cause but no gene mutations which relate to immunological mechanisms were identified. Overall, the clinical utility of what is currently known about the genetics of status epilepticus is slight and the findings have had little impact on clinical treatment despite what has been a very large investment in money and time. New genetic technologies may result in the identification of further genes, but if the identified genetic defects confer only minor susceptibility, this is unlikely to influence therapy. It is also important to recognize that genetics has social implications in a way that other areas of science do not. This article is part of a Special Issue entitled “Status Epilepticus”. © 2015 Published by Elsevier Inc.

1. Introduction In the field of epilepsy, new genetic technologies have the potential to enhance greatly our understanding of the causes and mechanisms of epilepsy. To date, there have been numerous studies exploring the role of genetics in ordinary epilepsy, for instance, in various epilepsy syndromes, rare Mendelian ‘pure’ epilepsies, and epileptic encephalopathies. However, there has been no study focusing synoptically on the genetics of status epilepticus.

⁎ Corresponding author at: UCL Institute of Neurology, Box 5, National Hospital for Neurology and Neurosurgery, Queen Square, London WC1N 3BG, UK. Tel.: +44 203 448 3782; fax: +44 203 448 8994. E-mail address: [email protected] (S. Shorvon).

Status epilepticus, however, is potentially an important target for genetic studies. The range of known causes of epilepsy which characteristically result in status epilepticus differs considerably from the range of genes that cause ‘ordinary’ epilepsy (i.e., epilepsy with nonstatus epilepticus seizures). Furthermore, in lesional status epilepticus, the features of the causative lesions (nature, location, size, etc.) differ in relative terms from those in ordinary lesional epilepsy. Status epilepticus can thus be considered not just a more severe form of an ‘ordinary’ seizure but also one that involves different networks, pathways, biochemical or physiological processes, and mechanisms. It has been postulated that the mechanisms of status epilepticus include a failure of the normal processes of ‘seizure termination', but this seems likely to be an oversimplification. A study of the genetic causes of those epilepsies characteristically associated with status epilepticus might yield clues to the nature or form

http://dx.doi.org/10.1016/j.yebeh.2015.04.013 1525-5050/© 2015 Published by Elsevier Inc.

Please cite this article as: Bhatnagar M, Shorvon S, Genetic mutations associated with status epilepticus, Epilepsy Behav (2015), http://dx.doi.org/ 10.1016/j.yebeh.2015.04.013

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of these mechanisms. It has been hoped that this will lead to new approaches to treatment. It was for these reasons that this preliminary survey was undertaken, and this paper presents the preliminary findings of an ongoing analysis. 2. Aims and methods The survey takes the form of a review of the published literature undertaken with the following aims: (1) To identify genes in which mutations are are strongly associated with status epilepticus (2) To contrast these genes with genes in which mutations are known to be associated with ‘pure’ epilepsy. For the purposes of this survey, the ‘gene mutations strongly associated with status epilepticus’ are defined as mutations in a gene (a) in which status epilepticus is specifically listed as a symptom in standard genetic databases or (b) in which status epilepticus is specifically mentioned in a case report or review article. A literature search of the OMIM, GeneCard, NCBI Gene, DisGeNet, Medline, and PubMed databases [1–4] and also bibliographic searches of the reference lists from published literature reviews of the past 5 years [5–48] were conducted for (1). The search terms were ‘genetics’, ‘gene’, and ‘status epilepticus’. This was not a systematic search, and a degree of subjective evaluation was made by the authors: (a) in focusing on definitive and authoritative reviews only in the bibliographic searches which was therefore selective and (b) in the exclusion of genes on the grounds that status epilepticus was considered a minor or infrequent association. The list for (2) was taken from Zara [49]. This is a preliminary list from a first-pass search of the literature, and further work is necessary to refine the list. 3. Results Mutations in 122 genes were identified as being strongly associated with status epilepticus (as defined above). In some of these genes, a variety of different mutations were reported in the same gene. It proved possible to subdivide these genes into the following categories: (a) Genes with mutations causing malformations of cortical development (cortical dysplasia) or other gross cerebral structural changes (Table 1); (b) Genes with mutations causing inborn errors of metabolism and other congenital conditions (Table 2); (c) Mitochondrial genetic defects (Table 3); (d) Genes associated with early childhood epileptic encephalopathies and idiopathic forms of other childhood epilepsy syndromes (Table 4); and (e) Genes with mutations in cases occurring without gross structural cerebral disorders, inborn errors of metabolism, early childhood epileptic encephalopathy, or epilepsy syndromes (Table 5). Some of the genetic defects feature in several different categories, and the categories are not mutually exclusive. The genes underpinning ‘pure epilepsy’ are shown in Table 6. Table 1 Nuclear genes causing cortical dysplasia and other cerebral structural abnormalities reported to be strongly associated with status epilepticus. ADAR, AKT3, ARX, FOLR1, FOXG1, GRIN2A, KCTD7, LYK5, MLC1, OCLN, PAFAH1B1, PDYN, PNKP, PRKCZ, QARS, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, SCN1A, SCN2A, SPTAN1, SRPX2, STXBP1, ST3GAL5, TBC1D24, TREX1, TSC1, TSC2

Table 2 Genes causing inborn errors of metabolism and other congenital conditions reported to be strongly associated with status epilepticus. Disorders of amino acid metabolism D- and L-2-hydroxyglutaric aciduria D-glyceric academia Disorder of citric acid metabolism (Krebs cycle) Fumaric aciduria Disorder of copper metabolism Wilson disease Disorder of creatine metabolism Guanidinoacetate methyltransferase deficiency Disorder of cytosolic protein synthesis Cytosolic glutaminyl-tRNA synthetase Disorder of fatty acid oxidation Combined oxidative phosphorylation deficiency 23 Disorder of cerebral folate transport Folate transporter deficiency Disorder of GABA metabolism GABA transaminase deficiency Disorder of glucose transport Glucose transporter 1 deficiency Disorder of glycosylation Asparagine-linked glycosylation defect Disorders of lipid storage (NCL — see below) Gaucher disease type 3 GM2 gangliosidosis (Tay–Sachs) GM2 gangliosidosis (Sandhoff) Metachromatic leukodystrophy Disorders of purine and pyrimidine metabolism Adenylosuccinase deficiency Beta-ureidopropionase deficiency Disorders of pyridoxine metabolism Pyridoxine-dependent epilepsy Pyridoxal-5′-phosphate dependent epilepsies Disorder of serine synthesis Phosphoserine aminotransferase deficiency Urea cycle defects Ornithine transcarbamylase deficiency Citrullinemia type 11 Other disorders (IEMs and other congenital conditions) Aicardi–Goutières syndrome 6 Alexander disease Angelman syndrome Angelman-like syndrome GM3 synthase deficiency Menkes disease 3-Methylcrotonyl CoA carboxylase 2 deficiency Molybdenum cofactor deficiency (leading to sulfite oxidase deficiency) Mucopolysaccharidosis type II (MPS2: Hunter syndrome) 1-Phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-1 defects Neuronal ceroid lipofuscinoses types 3 and 6 Nonketotic hyperglycinemia Rett syndrome Sialidosis types 1 and 2

D2HGDH, L2HGDH GLYCTK FH ATP7B GAMT QARS GTPBP3 FOLR1 ABAT SCL2A1 ALG13 GBA HEXA HEXB ARSA ADSL UPB1 ALDH7A1 PNPO PSAT1 OTC SLC25A13 ADAR GFAP UBE3A, MECP2 CDKL5 ST3GAL5 ATP7A MCCC2 MOCS1, MOCS1, MOCS3 IDS PLCB1 CLN3, CLN6 AMT, GLDC, GCSH FOXG1, MECP2 NEU1, PPGB

4. Discussion This literature review has shown that it is possible to identify genes in which mutations are strongly associated with status epilepticus (i.e., in which status epilepticus is a characteristic, specific, or common feature). Many different types of mutations are found, including Table 3 Mitochondrial defects (both mitochondrial and nuclear genes) reported to be strongly associated with status epilepticus. Mitochondrial genetic defects strongly associated with status epilepticus a. Nuclear gene defects b. Mitochondrial gene defects

ADCK3, COX10, GTPBP3, PDHX, PDSS2, PEO1, POLG, RRM2B, SLC25A13, SLC25A22 MTCO1, MTND4, MTND4, MTTF, MTTK, MTTH, MTTL1, MTTS1, MTTS2

Progressive myoclonic epilepsies due to mitochondrial disease are discussed below (under syndromes).

Please cite this article as: Bhatnagar M, Shorvon S, Genetic mutations associated with status epilepticus, Epilepsy Behav (2015), http://dx.doi.org/ 10.1016/j.yebeh.2015.04.013

M. Bhatnagar, S. Shorvon / Epilepsy & Behavior xxx (2015) xxx–xxx Table 4 Epileptic encephalopathies and syndromes reported to be strongly associated with status epilepticus. a. Early childhood epileptic encephalopathies

b. Epilepsy syndromes Dravet syndrome Lennox–Gastaut syndrome (idiopathic forms) and West syndrome Malignant migrating partial seizures of infancy c. Progressive myoclonic epilepsies Unverricht–Lundborg disease Lafora disease Progressive myoclonic epilepsy type 3 Progressive myoclonic epilepsy type 4 Progressive myoclonic epilepsy type 5 Progressive myoclonic epilepsy type 6 Progressive myoclonic epilepsy type 7 Myoclonus epilepsy and ragged-red fibers (MERRF) Sialidosis — type 1 Sialidosis — type 2 Neuronal ceroid lipofuscinoses (Jansky–Bielschowsky disease; CLN2) Neuronal ceroid lipofuscinoses (CLN3) Neuronal ceroid lipofuscinoses (Kufs disease; CLN6)

ALG13, ARX, ARHGEF9, CACNA1A, CDKL5, CHD2, DNM1, FLNA, GABRA1, GABRB3, GRIN2B, HCN1, HNRNPU, IQSEC2, KCNQ2, KCNT1, MAG12, MTOR, NEDD4L, PCDH19, PLCB1, PNKP, SCN1A, SCN2A, SCN2B, SCN8A, SLC25A22, STK9, SPTAN1, STXBP1, STYXL1, SYNGAP1 CHD2, GABRG2, PCDH19, SCN1A, SCN1B, SCN2A ARX, CDKL5, CHD2, DNM1, FOXG1, GABRB3, PLCB1, PNKP, SCN8A, SPTAN1, STXBP1 KCNT1, PLCB1

CSTB EPM2A, EPM2B KCTD7 SCARB2 PRICKLE2 GOSR2 KCNC1 MTTF, MTTK, MTTH, MTTL1, MTTS1, MTTS2 NEU1 PPGB TPP1 CLN3 CLN6

missense, nonsense, and null mutations; truncation and premature termination; rearrangements; alternative splice sites; insertions; deletions; expansions; and copy number variations. One hundred twenty-two genes were identified in this preliminary search, and it is possible that others will be found after further investigation. Our findings, in various categories, are listed in Tables 1–5. 4.1. Categories of genetic disorders Malformations of cortical development (cortical dysplasia) are developmental abnormalities of the cerebral cortex and are often associated with epilepsy in infants, children, adolescents, and adults. We identified those genes with mutations causing cortical dysplasia that are categorized as strongly associated with status epilepticus (Table 1). This selection is to some extent arbitrary as other common gene defects causing cortical dysplasia can result in status epilepticus if the dysplasia has a severe phenotype or in the presence of concomitant provoking factors. In this situation, status epilepticus may not be listed as a characteristic feature and may be omitted from the listing. Nevertheless, it is clear that status epilepticus is more characteristic of dysplasias occurring with some gene defects rather than others, and these are the ones listed. In the case of lissencephaly, for instance, the cases related to ARX gene mutations are reportedly more likely to develop status epilepticus than those related to mutations of the DCX or TUBA1A gene, and the defects associated with PAFAH1B1 gene mutations fall somewhere in the middle.

Table 5 Nuclear genes reported to be strongly associated with status epilepticus in cases without gross structural cerebral disorders or inborn errors and which are not part of a syndrome or early-onset encephalopathy. Status epilepticus in patients without gross structural change and without known inborn errors of metabolism or encephalopathy

DNM1, FASN, GABBR2, MAPK10, RYR3

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Table 6 Mendelian ‘pure epilepsies’ with a strong genetic component but without a typical status epilepticus phenotype. Benign familial neonatal seizures Benign familial neonatal–infantile seizures Benign familial infantile seizures Generalized epilepsy and febrile seizures plus (GEFS+) syndrome Autosomal dominant partial epilepsy with auditory features Autosomal dominant nocturnal frontal lobe epilepsy Familial partial epilepsy with variable foci (FPEVF) Autosomal dominant partial epilepsy with auditory features Familial partial lobe epilepsy with variable foci Idiopathic generalized epilepsy (rare families) Rolandic epilepsy (rare families)

KCNQ2, KCNQ3 KCNQ2, SCN2A KCNQ2, SCN2A, PRRT2 SCN1A, SCN1B, GABRG2 LG1 CHRNA4, CHRNA2, CHRNB2, KCNT1 DEPDC5 LG1 DEPDC5 EFHC1, GABRA1, GABRB3, SLC2A1 GRIN2A

This table was derived from Zara [49] and is a list of the pure epilepsies in which causative genes have been clearly identified. SE is not a characteristic feature of the phenotype of any of these conditions (although may occur in certain situations and in severe cases).

The same applies to the other disorders. The phenotype can be varied, and agenesis of the corpus callosum, for instance, is in some genetic defects associated with severe epilepsy (and status epilepticus) and in others with no epilepsy at all. The extent of the dysplastic lesion and its cerebral position may also influence the risk of status epilepticus, and it may be the different distribution of lesions in different genetic disorders, rather than some inherent property of the gene product, that results in status epilepticus being more or less common. The situation is further complicated by the fact that some of the genes associated with cortical dysplasia can cause status epilepticus in some patients even in the absence of visible cortical dysplasia (see below). Similarly, some genes are sometimes associated with an epileptic encephalopathy, although the encephalopathy can also occur with normal MRI (e.g., TBC1D24 and SCN2A). Other genes are associated with not only subcortical changes but also status epilepticus, indicating cortical involvement even if this involvement is not evident on MRI (e.g., changes associated with mutations in the MLC1 gene). Some malformations too can also have an acquired, structural toxic, or metabolic cause. There are various genetic inborn errors of metabolism strongly associated with status epilepticus, and these are listed in Table 2. These come from a much larger list of inborn errors of metabolism associated with epilepsy (at least 250 such conditions exist). We also list other congenital conditions which, although not inborn errors and in which the pathophysiological defects are not fully understood, are wellestablished entities in which status epilepticus is a strongly associated symptom. Some of the genes also cause cortical dysplasia or encephalopathy, and these thus may appear in more than one table in this paper. A number of striking features can be observed. First, there are many different pathways and systems affected among the genetic conditions associated with status epilepticus, and no one system or feature stands out. This implies that status epilepticus can have various different underlying mechanisms. Initially, we had hoped that the inborn errors of metabolism might point to biochemical mechanisms underpinning status epilepticus, but the inborn errors in our list have little in common with each other and the list provides no clue to any common mechanisms except that most of the conditions are involved with aspects of cerebral development. Most of these conditions are associated with usually severe intellectual impairment and neurological disorders, in none of these is epilepsy (or status epilepticus) the sole feature, most have their onset in infancy or childhood, and some are associated with epileptic encephalopathies (see below) or with cerebral malformations. All are serious diseases, although for several, early diagnosis is essential as restorative therapy is available which will (or in some cases might) prevent ongoing deterioration. The potentially treatable conditions include GLUT 1 deficiency; disorders of creatine biosynthesis; disorders

Please cite this article as: Bhatnagar M, Shorvon S, Genetic mutations associated with status epilepticus, Epilepsy Behav (2015), http://dx.doi.org/ 10.1016/j.yebeh.2015.04.013

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of serine biosynthesis; disorders of pyridoxine metabolism; disorders of folate metabolism and cerebral folate transporter deficiency; nonketotic hyperglycinemia; molybdenum cofactor deficiency; Menkes disease; and Wilson disease. Mitochondrial gene defects are particularly frequently associated with status epilepticus. There are 31 mitochondrial genes encoding 13 proteins (the oxidative phosphorylation system) and 24 RNA molecules necessary for their synthesis. There are also a much larger number of nuclear genes which are involved in mitochondrial function (and comprise the mitochondrial proteome), and mutations (or other genetic abnormalities) in these genes cause mitochondrial disease. Epilepsy occurs in about half of individuals with biochemically confirmed mitochondrial disease [32]. However, only a few of these genes are specifically associated with status epilepticus — and these are shown in Table 3. How mitochondrial disease causes status epilepticus is unclear. As most of the defects are associated with mitochondrial energy failure or reduction, it seems surprising that the consequence is status epilepticus, a high-energy activity. Additional factors complicating analysis are the findings of mosaicism and mitochondrial heteroplasmy. Although the genes directly causing complex 1 deficiency are often associated with seizures, status epilepticus is listed as a feature in mutations of only a few genes (e.g., MTND4). Furthermore, the phenotype of defects of MTND4 (as with other genes) can be very variable, at times producing MELAS, Leber's optic atrophy, or other cerebral disorders not associated with status epilepticus or even epilepsy. Similarly, although complex IV and V deficiencies are often associated with epilepsy, for instance, as part of Leigh syndrome, only in mutations of COX10 is status epilepticus specifically mentioned. Seizures and status epilepticus are rare in complex II and III disorders. Status epilepticus in mitochondrial disease is most commonly associated with mutations in nuclear genes causing multiple respiratory chain defects. Examples are the disorders of mitochondrial DNA maintenance (depletion and translation and coenzyme Q10 deficiency). The most common gene defects associated with mitochondrial DNA depletion syndromes are mutations in the POLG gene, the cause of Alpers syndrome, for example, but other examples are deletions in the PEO1 and RRM2B genes. Mitochondrial translation disorders also cause multiple enzyme deficiencies and status epilepticus, for instance, 3243ANG in the MTTL1 gene causing MELAS. Coenzyme Q10 deficiency also causes combined deficiency of the respiratory chain enzymes and mutations in the kinase gene ADCK3 typically cause epilepsia partialis continua (a form of focal status epilepticus). Some mitochondrial diseases can present as an epileptic encephalopathy, and for instance, disorders of the mitochondrial glutamate carrier caused by mutations in SLC25A22 are one of the genetic causes of Ohtahara syndrome (see below). Epileptic encephalopathies are examples of disorders with profound epilepsy, with prolonged periods of electrographic status epilepticus in most individuals. Most are due to injury, lesions, or other nongenetic mechanisms, but a few are due to genetic mutations. There are a number of susceptibility and causative genes which have been identified. Interestingly, mutations in some of these can also cause cerebral malformations, but in many cases, even when mutations in these genes do underpin an encephalopathy, no visible brain abnormalities are present. Furthermore, mutations in many of these genes can cause other forms of epilepsy. There is thus, for some of these genes, a marked variability in the phenotype. The ARX gene, for instance, is associated with conditions without epilepsy, with epileptic encephalopathies, with milder types of epilepsy, and with or without brain malformations (lissencephaly, hydranencephaly, agenesis of the corpus callosum). This variability is presumably partly due to epistatic and epigenetic factors or possibly even ‘chance’ or temporal factors. SCN1A and PCDH19 are other genes associated with very variable epilepsy phenotypes. However, mutations in some genes, for example, SPTAN1, are more specific for encephalopathy and are not found in other conditions. CDKL5 is another interesting gene, which is found not only in some cases of severe

infantile epileptic encephalopathy but also in atypical cases of Rett syndrome. CDKL5 has kinase activity and is able to autophosphorylate and to mediate MeCP2 phosphorylation, and it is possible that CDKL5 and MeCP2 belong to the same molecular pathway. The overlapping nature of the genetic consequences in early childhood encephalopathies is well demonstrated by Ohtahara syndrome and, of course, raises questions about whether classification into syndromes is ‘meaningful’. Ohtahara syndrome is an encephalopathy with a more distinctive EEG ‘biomarker’ than other syndromes, the pattern of burst suppression, and yet, etiologically, is very heterogeneous: (a) it can be due to mutations in a variety of different genes — including ARX, CLDK5, SLC35A22, STXBP1, SCN2A, SCN8A, PLCB1, and others depending on definitions; (b) sometimes, there is an underlying cortical maldevelopment, and reported cases include megalencephaly (and hemimegalencephaly), microcephaly, cerebral atrophy (generalized or focal), or pachygyria, agyria, polymicrogyria, and focal cortical dysplasia, or hindbrain abnormalities such as dentate–olivary dysplasia or colliculus dysgeneses; (c) sometimes, the dysplasia occurs with identifiable genetic mutations and sometimes not; and (d) the condition can evolve into other encephalopathies or syndromes. Status epilepticus is also a common feature of some other epilepsy syndromes. These syndromes often can be due to a variety of different etiologies or identifiable causes. Some of these are acquired, and some are symptomatic with a genetic cause (i.e., are caused by defined inborn errors of metabolism or cortical malformations). However, others are ‘idiopathic’ in which specific genes have been identified in at least some cases. These include KCNT1 and PLCB1 genes in malignant migrating partial seizures of infancy and ARX, CDKL5, CHD2, DNM1, GABRB3, SCN8A, SPTAN1, and STXBP1 genes (or deletions such as the 1p36 deletion or trisomies such as trisomy 21) in Lennox–Gastaut and West syndromes (which can also be caused by a mitochondrial disorder or inborn errors of metabolism). DNM1, GABRB3, SCN8A, and STXBP1 mutations sometimes cause West syndrome evolving into Lennox–Gastaut syndrome and sometimes Lennox–Gastaut syndrome without prior West syndrome. Dravet syndrome is, in about 85% of cases, due to mutations in the SCN1A gene, but cases with other mutations are also described (including the CHD2, GABRG2, PCDH19, SCN1A, SCN1B, and SCN2A genes). Genetic mutations are also a common cause of the progressive myoclonic epilepsy syndrome (see Table 4) in which mitochondrial gene defects are prominent. There are a small number of genes which seem to be associated with status epilepticus but which do not have a clear association with either structural disease, or the defined epilepsy syndromes/encephalopathies, or known inborn errors of metabolism (Table 5). The gene functions vary. DNM1 was identified as a susceptibility gene in the largest international collaborative study of 356 patients with severe epilepsy [8] and encodes a member of the dynamin subfamily of GTP-binding proteins. Mutations in FASN, GABBR2, and RYR2 were also found in this study. The enzyme product of FASN has many functions, including catalyzing the synthesis of palmitate from acetyl-CoA and malonyl-CoA, in the presence of NADPH, into long-chain saturated fatty acids. The protein encoded by MAPK10 is one of the members of the MAP kinase family, which are involved in multiple biochemical signals and in a wide variety of cellular processes such as proliferation, differentiation, transcription regulation, and development.

4.2. Other genetic mechanisms Status epilepticus can also be caused by other genetic mechanisms, for instance, chromosomal abnormalities such as a ring chromosome (e.g., of 20); a trisomy or tetrasomy (e.g., of 21); deletions (e.g., 1p3, 7q11.23, 1p36, and inv dup chromosome 15), especially large deletions causing contiguity syndromes; or copy number variations. These are important but beyond the scope of this article and not considered further here.

Please cite this article as: Bhatnagar M, Shorvon S, Genetic mutations associated with status epilepticus, Epilepsy Behav (2015), http://dx.doi.org/ 10.1016/j.yebeh.2015.04.013

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In some genes, for instance, SCN1A, various different types of mutation occur, including missense, nonsense, and splice-site mutations; small deletions; and insertions. Large deletions, affecting continuous genes, also produce varying phenotypes. Furthermore, the type of mutation or gene defect does not necessarily correlate very precisely with the phenotype, implying that other factors must be involved. Some genetic mutations are present in mosaics, with the severity of the epilepsy depending on the proportion of cells involved. Compound heterozygosity is found in some of the variants of conditions causing status epilepticus (for instance, in GM2 gangliosidoses: Tay–Sachs and Sandhoff syndromes). X-linked recessive (e.g., in PCDH19) and X-linked dominant (e.g., in Rett syndrome) inheritance can occur and be difficult to identify. The complexity of the inheritance patterns of some genes is exhibited by the mutations in the PCDH19 gene [see 20]. Sixty different pathogenic mutations have been demonstrated, some missense, nonsense, small nucleotide deletions and insertions, involving splice sites, and whole gene deletions. There are also other variants and polymorphisms of unknown significance. Many of the mutations are associated with epilepsy, of a widely varying phenotype, some mild and some severe, and some take the form of Dravet syndrome or epileptic encephalopathy and some with mild epilepsy. Other cases have ataxia, language difficulties, and cognitive impairment, and some are normal neurologically. The genotype–phenotype correlation is not at all clear-cut, sometimes because of mosaicism and X inactivation, but twins with widely differing phenotypes have been reported, showing the importance of nongenetic factors. Modes of inheritance are complex, and although situated on the X chromosome, males and females can be affected. It is postulated that this is because of the phenomenon of cellular interference in mosaic males. Similarly, in the ARX gene, 44 different pathogenic mutations have been associated, in about 100 families, with highly variable phenotypes including some with cortical dysplasias such as X-linked lissencephaly, agenesis of the corpus callosum, or hydranencephaly; some without dysplasias but with encephalopathy such as Ohtahara and West syndromes; some with nonsyndromic epilepsy; some without epilepsy; and some with somatic anomalies (abnormal genitalia) or movement disorders [see 21,22]. 4.3. Other theoretical considerations It is of course quite possible that some instances of status epilepticus are due to the same mechanisms as ordinary seizures but are just ‘severe’ examples of these mechanisms. Indeed, status epilepticus can occur occasionally in ‘ordinary epilepsies’, especially under certain conditions (i.e., drug withdrawal, severe lesions, sudden acute lesions, etc.), even if status epilepticus is not a characteristic feature of the epilepsy. Because of this, the above listing of genes has a degree of subjectivity dependent on the vagaries of the clinical descriptions in the genetic databases. There is a gray area here, and for instance, it could have been argued that some conditions should be included — examples are biotinidase deficiency due to mutations in the BTD gene or peroxisome biogenesis disorders (for instance Zellweger disease) due to PEX gene mutations. The concept of ‘cause’ itself in epilepsy is not straightforward for several reasons ([50]; see Table 7). First, in any individual, the condition is likely to be multifactorial and there may well be other genetic influences which confer a ‘susceptibility’ only and perhaps sometimes a rather modest susceptibility. Distinguishing between a ‘remote’ and a ‘proximate’ cause is also an important issue (an issue first raised by Hughlings Jackson). For instance, a cortical dysplasia or other brain structural defects can have many downstream effects, and deciding which is the ‘cause’ is to an extent arbitrary. Another example is the variety of disparate genes causing disorders of GABAergic function at the interneuron level (interneuronopathy) including TSC1, ARX, and SCN1A. There are also almost certainly many epistatic or epigenetic influences which change the degree of susceptibility, as well as the effects

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Table 7 Complications of assigning a causative role to genetic factors in status epilepticus. Derived from [50]. Multifactorial nature of epilepsy Difference between remote and proximate causes (difference between cause and mechanism) Influence of epigenetic and epistatic factors, temporal aspects (cerebral development), and chance Provoking factors Epilepsy is a dynamic condition, and seizures/status epilepticus influences brain processes

of time (development) and even chance on the risk of status epilepticus. Status epilepticus can be provoked by environmental factors (for instance, drug reduction and intercurrent illness). Finally, epilepsy itself is a dynamic process, and the risk of status epilepticus might itself be altered by the progression of the condition, the occurrence of previous seizures, or the occurrence of episodes of status epilepticus.

4.4. Genetic methodological considerations The yield of findings of genetic influences in idiopathic ‘ordinary’ epilepsy has been generally very disappointing, and although some susceptibility genes have been discovered, none account for more than a slight effect. Larger studies are underway, on the basis that these newer more powerful methodologies have the potential to increase understanding, although this seems currently doubtful. The approaches to gene discovery have evolved. Initially, family linkage analysis of large pedigrees was used. This approach is suitable for uncovering single gene disorders in which mutations of a single gene have a large effect size, and has identified the genes underpinning most of the inborn errors of metabolism for instance. It does seem clear though that epilepsy is a Mendelian ‘single gene’ disorder in only rare cases. Association studies in non-familial epilepsies were then used to identify common variants in candidate genes and some progress was made, although it has become clear that there is significant `missing hereditability' using this method. With increasingly powerful technologies, whole genome and especially exome sequencing have become feasible, but results so far have been generally disappointing. It is possible that, using new technologies, more genes conferring susceptibility will be identified. However, it should be recognised that the uncovering of genetic variants resulting in only minor degrees of susceptibility (small effect size) has little practical utility. CGH array to identify copy number variants is another technology recently employed. Interpreting genetic data from powerful new technologies is increasingly problematic, and carries the particular risk of drawing false positive conclusions. When comparing two genotypes of healthy individuals from a similar population, for instance, there are said to be typically more than 3.5million SNPs, 20,000 insertions/deletions and 1000 copy number variants. There is thus a significant risk of attributing pathological significance to harmless variation. Larger variations such as abnormal ploidy, translocations, or large amplifications, however, identified using simpler methodologies, are usually pathogenic. A promising powerful method is to use ‘gene-regulatory network analysis’ to uncover the underlying molecular processes and pathways. This has been recently shown in a study of the hippocampus from 129 patients with temporal lobe epilepsy in which a gene regulatory network was identified to be involved with the transcriptional module encoding proconvulsive cytokines and Toll-like receptor signaling genes, in particular the SESN3 gene. In vivo experiments have confirmed its importance [51]. A similar approach based on groups of genes uncovered in the current study might be worth exploring.

Please cite this article as: Bhatnagar M, Shorvon S, Genetic mutations associated with status epilepticus, Epilepsy Behav (2015), http://dx.doi.org/ 10.1016/j.yebeh.2015.04.013

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5. Conclusions It is thus quite clear from the above that the genetics of status epilepticus (like the genetics of epilepsy) is complex. For all these reasons, it would be naïve to expect a simple direct relationship between a ‘genetic’ influence and the occurrence of a symptom such as status epilepticus, and indeed, it seems clear that in the vast majority of cases, there is none. Nevertheless, despite the preliminary nature of this survey and its conceptual limitations, a few broad conclusions can be drawn from this study. 1. The range of genes underpinning status epilepticus differs in many ways from the range of genes underpinning epilepsy. There are no Mendelian ‘pure status epilepticus genes’ in the way in which there are a number of ‘pure epilepsy genes’ (Table 6). Most of the pure epilepsy genes code for neuronal ion channels whereas the genes coding for status epilepticus are invariably associated with multiple other clinical features, and a relatively small number of the genes are channel genes. This in itself suggests strongly that the processes underpinning status epilepticus differ from those underpinning epilepsy. It is of interest in this regard to note also that the range of causes of symptomatic nongenetic lesional epilepsies differs significantly from the range of causes of ‘ordinary epilepsy’, both in their nature and in the anatomical location of the causal lesions (for instance, the predilection of acute lesions or lesions situated in the frontal lobes to cause status epilepticus). These findings lend circumstantial support to the concept that status epilepticus is categorically distinct, and not simply a ‘severe’ form of an ordinary epileptic seizure. 2. It has been frequently postulated that status epilepticus is the result of a failure of ‘seizure termination mechanisms’. However, the wide variety of genes affecting very diverse pathways identified in this survey makes any unitary cause unlikely. The genetic influences in status epilepticus thus are likely to involve a much broader range of mechanisms, some resulting in altered biochemistry and in changes in networks and systems, and some affecting physiological functions and network or system functioning (differing in convulsive and nonconvulsive status epilepticus). 3. Many of the implicated genes are involved in cerebral development, and on this basis, one can speculate that in most cases, the propensity for status epilepticus is a network or system phenomenon rather than a defect at the single cell level. 4. Another important set of genes is involved in cerebral energy production and mitochondrial function, although how energy failure results in the high energy-consuming phenomenon of status epilepticus is unclear. Other genes influence GABAergic function especially at the level of the interneuron. There are also genes causing a wide variety of inborn errors of metabolism, but no common pattern of defects emerges from scrutiny of this list of disorders. 5. Interestingly, no genes associated with inflammation or immunological mechanisms were uncovered in our survey, apart from the genes causing Aicardi–Goutières syndrome, despite the fact that status epilepticus is a characteristic feature of some inflammatory or autoimmune diseases. It is possible that such genes exist but have not yet been uncovered. 6. One common source of ‘status epilepticus genes’ are the early childhood epileptic encephalopathies. Status epilepticus is often profound in these cases, and the genetic influences might provide a pointer to potential pathophysiological mechanisms. However, most cases of epileptic encephalopathy are due to brain injury or lesions and are nongenetic in origin, and the genetic causes are often extremely rare conditions. It is not at all clear to what extent the genetic findings in these conditions can be extrapolated to the more common types of status epilepticus. This is research for the future. 7. Almost all the genes uncovered so far, which are associated with

status epilepticus, are in infantile- or childhood-onset status epilepticus cases and are associated with intellectual impairment and oftenwidespread neurological disturbance. There are few to date which have been found in adult-onset status epilepticus, except in those with preexisting neurological damage. This is disappointing as the cause of many adult-onset status epilepticus cases remains obscure. It seems likely that some have immunological causes. To date, no genetic influences in such cases have been uncovered in status epilepticus, and this should be a focus for further research. 8. What is known currently about the genetics of status epilepticus has no impact on the treatment, except in relation: (a) to counselling and prognostication, which are both important clinically; and (b) therapy in some rare inborn errors of metabolism. The relative lack of clinical utility of such gene hunting is disappointing. Indeed, very large amounts of money and time have been spent on uncovering the genetics of epilepsy in general, and yet, this has yielded little of value to its treatment except for the minor advantage of being able to predict a rash from exposure to carbamazepine. It could be argued that the current research focus should turn again more towards molecular physiology and chemistry than pure genetics. 9. The newer genetic technologies offer the promise of identifying other genes. Hopefully, useful discovery will be made, but if the identified genes confer only minor susceptibility, in our view, this is a wasted effort. At one level, everything we do or are has some genetic influence. Minor degrees of susceptibility (or effect size) have not only little clinical utility but also form a genetic landscape where 'pathology' merges into 'normal human variation'. If effect size is small, even if statistically significant in large cohorts, the distinction between the two is blurred. Furthermore, genetics has social implications in a way that other areas of science do not. The unhappy history of eugenics in the late nineteenth and twentieth centuries shows that focusing on minor genetic variation can be devastating for individuals, races, and society. In our view, there is inadequate concern about, or discourse on, these points in contemporary epileptology.

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Please cite this article as: Bhatnagar M, Shorvon S, Genetic mutations associated with status epilepticus, Epilepsy Behav (2015), http://dx.doi.org/ 10.1016/j.yebeh.2015.04.013