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Symmetrical epileptogenic foci in monkey cerebral cortex . Arch Neurol 1968 ; 19 : 99 \200\223 116 . 88. Steriade M , Contreras D . Spike-wave complexes and fast components of cortically gener ated seizures. I. Role of neocortex and thalamus . J Neurophysiol 1998 ; 80 : 1439 \200 \223 55 . 66. Igelmund P , Zhao YQ , Heinemann U . Effects of T-type, L-type, N-type, P-type, and Q-type calcium channel blockers on stimulus- induced pre- and postsynap tic calcium \202 uxes in rat hippocampal slices . Exp Brain Res 1996 ; 109 : 22 \200\223 32 . 89. Pinault D , Leresche N , Charpier S , Deniau JM , Marescaux C , Vergnes M , et al . Intracellular recordings in thalamic neurones during spontaneous spike and wave discharges in rats with absence epilepsy . J Physiol (Lond) 1998 ; 509 : 449 \200\223 56 . 67. Heinemann U , Pumain R . Effects of tetrodotoxin on changes in extracellular free calcium induced by repetitive electrical stimulation and iontophoretic a pplication of excitatory amino acids in the sensorimotor cortex of cats . Neurosci Lett 198 1 ; 21 : 87 \200\223 91 . 90. Mintz IM , Sabatini BL , Regehr WG . Calcium control of transmitter release at a cerebellar synapse . Neuron 1995 ; 15 : 675 \200\223 88 . 68. Bollmann JH , Helmchen F , Borst JG , Sakmann B . Postsynaptic Ca 2 + 91. Qian J , Colmers WF , Saggau P . Inhibition of synaptic transmission by neuropeptide Y in rat hippocampal area CA1: modulation of presynaptic Ca 2 + entry . J Neurosci 1997 ; 17 : 8169 \200\223 77 . in \202 ux mediated by three different pathways during synaptic transmission at a calyx-ty pe synapse . J Neurosci 1998 ; 18 : 10409 \200\223 19 . CHAPTER 2 Neurogenetics of Epilepsy Renzo Guerrini and Elena Parrini molecular diagnosis is now available for different forms of epilepsy. However, ethical prob lems may arise especially in asymptomatic mutation carriers or in individuals and families in which although mutations of speci \201 c genes have been identi \201 ed, severity of t he associated phenotype is unpredictable. Epilepsy can be caused by genetic or acquired factors, although, often, both contribute to its determinism. Epilepsies of unknown cause are classically divided into two main aetiolog ical categories: \200\230presumed symptomatic \200\231 epilepsies are those whose cause is sus- pected to be induced by a pathology that is below the limit of detec- tion of the ava ilable diagnostic tests; \200\230idiopathic \200\231 epilepsies are instead thought to be caused by a genetic predisposition. Symptomatic epilepsies are caused by an obvious brain a bnormal- ity, which can in turn be genetically determined or by external fac- tors acting p renatally or after birth. De \201 ning as \200\230symptomatic \200\231 a genetically dete rmined epilepsy is easy, for example, in front of a neuronal migration disorder, or a neuro cutaneous disease such as tuberous sclerosis or a syndromic form of mental retardation. The same applies to many forms of progressive myoclonus epilepsy, whose clinical expression is relatively homogeneous in relation to the genetically determined degenerative or metabolic causes. The task is much more complex when the genetic cause is hidden and will only be re cognizable or hypothesizable later during the course, when a given electroclinical pattern or syndrome will become obvi- ous. This is, for example, the case of several early-onset ep ileptic encephalopathies such as those caused by alterations of the CDKL5 , STXBP1 , or ARX genes. For this reason, the category of \200\230genetic epilepsy \200\231 which has recen tly been proposed is controversial and, perhaps, somewhat uncertain as what is presumed to be sympto- matic today, may become genetic after a molecular screening, or remain as such i f screening is unavailable. Genetic epilepsy syndromes without structural brain abnormalities About 30 % of all epileps ies are idiopathic ( 3 ). The idiopathic epi- lepsies are characterized by age-related onse t, normal neurological and cognitive development, and absence of brain damage. The study of electroencephalogram (EEG) anomalies while awake and during sleep is useful for characteri zing the different forms. An underlying genetic defect has been identi \201 ed for some fo rms with Mendelian inheritance. Most idiopathic epilepsies, however, do not follow a simple Mendelian pattern of inheritance. Even in fam- ilies where involvement of a single gene is suspected, a high degree of complexity can actually be present. The phenotypic variability in some families has been ascribed to genetic modi \201 ers (polymor- phisms) or environm ental factors that influence phenotypic expression ( 4 ). The genetic alterations identi \201 ed so far are responsible for rare forms of idiopathic epilepsies with a dominant pattern of inherit- ance, which occur with repeated seizures in the neonatal or early infantile period, and with febrile, generalized, or partial seizures that persist into adulthood. These forms are observed in a limited number of patients, but they are of great interest as they are caused by alterations in genes that encode voltage- gated ion channel subu- nits or receptor subunits. The identi \201 ed mutations are locate d in the neuronal nicotinic acetylcholine receptor in familial forms of frontal epilepsy, t he K + channels in benign familial neonatal sei- zures, the Na + channels in a particular f orm of generalized epilepsy with febrile seizures, and the gamma aminobutyric acid (GABA) r eceptor in a variant of juvenile myoclonic epilepsy ( 5 ). For the most common forms of idi opathic epilepsy, however, the molecu- lar bases have not yet been de \201 ned; it is poss ible that different mutations can cause similar phenotypes in different families or in pati ents from different geographical areas (Table 2. 1 ). Genetic research in epilepsy represents an area of great interest for both clinical purpose s and for understanding of the mecha- nisms underlying epilepsy. In the past, genetic studi es on families and twins contributed to the de \201 nition of genetic epilepsy, and especi ally to the evaluation of the risk of familial occurrence. After the \201 rst report in 1 988 of the chromosomal mapping of juvenile myoclonic epilepsy ( 1 ), several additional loc i were demonstrated to be implicated in syndromic epilepsies. In 1995, the \201 rst muta- tion in the gene coding for the nicotinic acetylcholine receptor was identi \201 ed in a family with autosomal dominant nocturnal frontal lobe epilepsy ( 2 ). The subsequent identi \201 cation of new epilepsy genes has greatly improved our understanding of the pathophys io- logical mechanisms underlying epilepsy and has favored research into experimental model s and new therapeutic strategies. A targeted 1 2 o x f o r d t e x t b o o k Table 2.1 Epilepsy syndromes with known genetic basis Epilepsy syndromes Type of seizures Gene Protein Sodium channel, neuronal type 1, subunit Locus OMIM Generalized epilepsy with febrile seizures plus (GEFS + ) Febrile and afebrile, complex partial, generalized tonic \200\223clonic, absences, myocloni c SCN1A 2q24.3 182389 o f Sodium channel, neuronal type 1, subunit Gamma-aminobutyric acid receptor, -2 Sodium channel, neuronal type 1, subunit SCN1B 19q13.1 600235 e p i l e p s y GABRG2 5q31.1 \200\223q33.1 137164 Severe myoclonic epilepsy of infancy (SMEI) Febrile, partial, absences, myoclonic, generalized tonic \200\223clonic SCN1A 2q24.3 182389 a n d Benign familial neonatal seizures (BFNS) Multifocal neonatal convulsions, generalized tonic \200\223clonic KCNQ2 Potassium channel, voltage-gated, KQT-like subfamily, member 2 20q13.3 602235 e p i l e p t i c KCNQ3 Potassium channel, voltage-gated, KQT-like subfamily, member 3 Sodium channel, voltage-gate d, type 2, subunit 8q24 602232 Benign familial neonatal \201\204 infantile seizures (BFNIS) Multifocal neonatal convulsions, generalized tonic \200\223clonic SCN2A 2q23 \200\223q24.3 182390 s e i z u r e s ATPase, Na + /K + Transporting, -2 polypeptide Benign familial infantile seizures (BFIS) with familial hemiplegic migraine Multifocal neonatal convulsions, generalized tonic \200\223clonic, hemiplegic migraine ATP1A2 1q21 \200\223q23 182340 Autosomal dominant nocturnal frontal lobe epilepsy (ADNLFE) Partial nocturnal motor seizures with hyperkinetic or tonic manifestations CHRNA4 Cholinergic receptor, neuronal nicotinic, polypeptide 4 20q13.2 \200\223q13.3 118504 CHRNB2 Cholinergic receptor, neuronal nicotinic, polypeptide 2 1q21 118507 Cholinergic receptor, neuronal nicotinic, polypeptide 2 8p21 118502 CHRNA2 Autosomal dominant temporal lobe epilepsy (ADTLE) Partial seizures with auditory or visual hallucinations Leucine-rich, glioma-inactivated 1 10q24 604619 LGI1 Gamma-aminobutyric acid receptor, -2 Childhood absence epilepsy (CAE) Absences, tonic \200\223clonic 5q31.1 \200\223q33.1 137164 GABRG2 Chloride channel 2 Gamma-aminobutyric acid receptor, -1 3q26 600570 CLCN2 Juvenile myoclonic epilepsy (JME) Myoclonic, tonic \200\223clonic 5q34 \200\223q35 137160 GABRA1 Juvenile myoclonic epilepsy (JME) Myoclonic, tonic \200\223clonic EF-hand domain (C-terminal)-containing protein 1 6p12.2 608815 EFHC1 Infantile spasms, West syndrome Infantile spasms, hypsarrhythmia Gene homeobox aristaless related Xq22.13 300382 ARX Early infantile epileptic encephalopathy Myoclonic, infantile spasms CDKL5 Cyclin-dependent kinase 5 Xp22X 300203 Epilepsy and mental retardation restricted to females Febrile, partial, absences, myoclonic, generalized tonic \200\223clonic PCDH19 Protocadherin 19 Xq22 300460 Epileptic encephalopathy (Ohtahara syndrome) Tonic, infantile spasms Syntaxin binding protein 1 9q34.1 602926 STXBP1 CHAPTER 2 neurogenetics of epilepsy 13 Severe myoclonic epilepsy of infancy or Dravet syndrome Dravet syndrome starts at about 6 m onths of age in previously healthy infants, typically with prolonged generalized or hemiclo nic febrile seizures. Between 1 and 4 years, other types of seizures appear, including myoc lonic, partial, and absence seizures. Hyperthermia, such as fever or a warm bath, often pre cipitates seizures ( 6 ). Development in the \201 rst year of life is normal but subseque ntly slows and may regress. The EEG may be normal until age 2 years when generalized spike wave activity is seen; approximately 10 % of patients are photosensitive. The magnetic reso nance imaging (MRI) scan is either normal or shows non-speci \201 c features ( 7 , 8 ). mutations in human embryonic kidney cells or Xenopus oocytes has revealed both gain- and lo ss-of-function mechanisms. However, loss-of function seems to be the predominant mecha- nis m of action causing FS and GEFS + , which is in agreement with genetic and functional studi es in Dravet syndrome ( 25 ). Two recently published mouse models for SCN1A, in which loss of function mutations were introduced into the endogenous mouse gene, exhibited spontaneous seizures and reduced sodium currents with decreased sodium-channel expression selectively affecting inhibitory interneurons ( 26 , 27 ). These \201 ndings suggest that Dravet synd rome, and maybe the other SCN1A -linked seizure disorders, are caused by a decreased excita bility of GABAergic interneurons owing to SCN1A haploinsuf \201 ciency ( 26 , 27 ). GABA r eceptors were long suspected to be involved in epileptogenesis. Functional expres- sion of some GABRG2 mutations, identified in patients with GEFS + , revealed a pronounced loss-of-f unction by altered gating or defective trafficking and reduced surface expression as a comm on pathogenic mechanism ( 25 ). Hence, these mutations reduce the main mechanism for neuron al inhibition in the brain, which can explain the occurrence of seizures. Dravet syndrome is related to SCN1A mutations in at least 85 % of cases. The majority of pa tients exhibiting mutations of this gene carry de novo mutations (90 % ); about 40 % of the se mutations are truncation and 40 % are missense mutations. 10 % of patients who are negat ive on sequence-based mutational analysis, have copy number variations including exonic del etions or duplications that can involve several exons or the whole gene ( 9 , 10 ). Some ra re patients have a mutation in the GABRG2 gene ( 11 ). Germline mosaicism may result in sib lings with Dravet syndrome born from an unaffected or mildly unaffected parent carrying a l ow level of mosaicism for the mutation ( 12 , 13 ). Benign epilepsies of the \201 rst year of life Benign epilepsies of the \201 rst year o f life represent a group of syn- dromes that are de \201 ned as \200\230benign \200\231 b ecause their clinical manifes- tations, which occur in otherwise asymptomatic babies, regre ss and eventually disappear spontaneously. These forms are quite rare and are transmitted w ith an autosomal dominant pattern of inher- itance. Molecular diagnosis, where possible, is important in order to avoid unnecessary invasive testing and support genetic counsel- ling . The clinical manifestations of these forms of epilepsy are rather similar, while age of o nset is variable. SCN1A mutations are also commonly found in the borderline variant of severe myoclonic epile psy of infancy (SMEB), whose sep- aration from Dravet syndrome may be arbitrary ( 14 ). Mut ations are less commonly found in patients that have been categorized within different subg roups exhibiting various elements of Dravet syndrome ( 15 \200\223 17 ). Generalized epilepsy with febrile seizures plus Generalized epilepsy with febrile seizures plus (GEFS + ) is a famil- ial epilepsy syndrome, diagnosed on the basis of at least two in di- viduals with GEFS + phenotypes in a family. The GEFS + spectrum denotes the phenotypic heterogeneity observed in families includ- ing febrile seizures (FS) and febrile seizures p lus (FS + ). Overlap with classical idiopathic generalized epilepsy (IGE) is also seen. The course and response to antiepileptic drugs may be considera- bly variable within the same family: in some patients FS are rare, or disappear after a few years, while in other indivi duals within the same family, epilepsy is severe and drug resistant. GEFS + was orig- inall y recognized because of remarkable large autosomal dominant pedigrees with 60 \200\22370 % penetrance. It is likely that most cases, however, occur in small families or are sporadic ( 18 ). Benign familial neonatal seizures Benign familial neonatal seizures (BFNS) are characterize d by clusters of seizures that appear from the \201 rst days of life up to the 3rd month, and disappear spontaneously after weeks to months. Seizures have a focal onset, often with hemitonic or hemiclonic symptoms or with apnoeic spells, or can clinically appear as gener - alized. Interictal EEG is usually normal. The rare available ictal EEGs show focal and ge neralized discharges. The risk of seizures recurring later in life is about 15 % ( 28 ). Al though psychomotor development is usually normal, an increasing number of cases with learni ng disability have recently been described ( 29 ). BFNS are autosomal dominantly inherited with a penetrance of 85 % . Most patients have mutations in the gene encoding the K + chann el voltage-dependent, KQT-like subtype member ( KCNQ2 ), and deletions/duplications involvi ng one or more exons of KCNQ2 ( 30 \200\223 32 ). A small proportion of families carry mut ations in the associated gene voltage-dependent K + channel, KQT-like subtype, member 3 ( K CNQ3 ) ( 33 ). KCNQ2 and KCNQ3 form a hetero- meric K + channel, which determines the M-cur rent, in \202 uencing the membrane potential at rest ( 34 ). Co-expression of the hetero- meric wild-type and mutant KCNQ2/3 channels usually revealed a reduction of about 20 \200 \22330 % in the resulting potassium current, which is apparently suf \201 cient to cause B FNS ( 35 ). Although the reduction of the potassium current can cause epileptic seizures by a subthreshold membrane depolarization, which increases neuronal \201 ring, it is not fu lly understood why seizures preferentially occur in neonates ( 36 ). It is possible that th e neonatal brain be more Mutations in genes that encode alpha and beta subunits of the voltage-gated Na + channel ( SCN1A and SCN1B ) have been associ- ated with GEFS + ( 19 , 20 ). Missense mutations in SCN 1A are the commonest identi \201 ed molecular abnormalities and are found in about 10 % of families ( 21 ). In some families, mutations in the gene encoding the gamma 2 subunit of t he GABA A receptor ( GABRG2 ) have been identi \201 ed ( 22 , 23 ). The phenotypic variabi lity observed in GEFS + could be linked to the combined action of mutations in different ge nes. The neuronal sodium (Na + ) channels consist of two subunits: alpha and beta (Fig. 2. 1 ). In vitro functional studies have shown that mutations in the gene that encode the alpha 1 s ubunit deter- mine a persistent depolarization of the Na + current resulting in neuronal hy perexcitability ( 24 ). The expression of many SCN1A oxford textbook of epilepsy and epileptic seizures 14 A D I D II D III D IV + + + S4 + + + + + + S4 + + + + + + S4 + + + + + + S4 + + + S1 S2 S3 S5 S6 S1 S2 S3 S5 S6 S1 S2 S3 S5 S6 S1 S2 S3 S5 S6 NH2 COOH D III D IV B + + + + + + + + + + + + + + + + + + + D I D II Fig. 2.1 A) Scheme of the structure of the linearized voltage-gated sodium channel -subu nit. The -subunit consists of four repeated domains (DI \200\223DIV), each of which is i n turn composed of 6 -helix transmembrane segments (S1 \200\223S6). Segment S4 is the vo ltage sensor. The S5 and S6 segments and the linkers form the pore that acts as ion selecti vity \201 lter. -subunits, not represented here, modulate the kinetic and the voltage- dependence of the channel. B) Three-dimensional con \201 guration of the sodium channel. Autosomal dominant nocturnal frontal lobe epilepsy Autosomal dominant nocturnal frontal lob e epilepsy (ADNFLE) includes frequent brief seizures that occur from childhood, with hyperk inetic or tonic manifestations, typically in clusters at night ( 46 ). Ictal video-EEG stud ies have revealed partial seizures origi- nating from the frontal lobe but also from parts of the insula and temporal lobe suggesting a defect of a broader network ( 47 , 48 ). Penet rance is estimated at approximately 70 \200\22380 % . A mutation was identi \201 ed in the gene CHRNA4 encoding the a4-subunit of a neu- ronal nicotinic acetylcholine receptor as th e \201 rst ion channel muta- tion found in an inherited form of epilepsy ( 2 ). Altogethe r, six mutations in CHRNA4 , three in CHRNB2 , which encodes the beta2-subunit of neuronal nicotinic acetylcholine receptor, and one in CHRNA2 , encoding the neuronal nicotinic acety lcholine receptor alpha2-subunit, have been reported so far (25, 49 \200\223 52 ). These r eceptors are heteropentamers consisting of various combi- nations of subunits. The alpha4-b eta2 combination is the most represented in the thalamus and cerebral cortex (Fig. 2. 2 ). All the identi \201 ed mutations reside in the pore-forming M2 transmem- brane segments. T he exact pathomechanism is not fully under- stood, but an increased acetylcholine sensitivi ty could be the main common gating defect of the mutations. vulnerable to changes, even small, of neuronal excitability. An alternative is that KCNQ2 a nd KCNQ3 channels, when mutated, are replaced by other K + channels that become functional after the \201 rst months of life. Benign familial neonatal-infantile seizures Benign familial neonatal-infantile seizures (BF NIS) are character- ized by seizures similar to those observed in children with BFNS. Howev er, age at onset of seizures ranges from the neonatal period to infancy in different family members, with a mean onset age of 3 months. Remission occurs by 12 months with a very low risk of later seizures ( 37 , 38 ). Mutations in SCN2A have been identi \201 ed in most fa milies with BFNS ( 39 ). Benign familial infantile seizures Benign familial infantile seizures (BFIS) are characteri zed by sei- zures similar to those observed in BNFS with an onset at around 6 months ( 40 ) . Linkage studies have identi \201 ed two loci for this type of epilepsy, one in the peric entromeric region of chromosome 16 and another on chromosome 19 ( 41 , 42 ). However, the d isease gene has not yet been identi \201 ed. Some families also have paroxys- mal dyskines ia beginning in later childhood (infantile convulsions and choreoathetosis) ( 41 ). Long-te rm prognosis for seizure remis- sion is excellent. Several families with mutations in the A TP1A2 gene have benign infantile seizures in conjunction with hemiplegic migraine ( 43 ). R are families with mutations in SCN2A or KCNQ2 have also been described in which only infant ile seizures occur ( 44 , 45 ). Autosomal dominant temporal lobe epilepsy Autosomal dominant temporal lobe epilepsy (ADTLE) is a form of autosomal dominant partial epilepsy associated to auditory symp- toms. The \201 rst clinical manifestations, usually occurring during CHAPTER 2 neurogenetics of epilepsy 15 A childhood or adolescence, are auditory hallucinations, sometimes accompanied by vision or o lfactory manifestations, or dizziness ( 53 , 54 ). Mutations in the LGI1 gene (leucine-rich , glioma-inactivated 1) have been associated with ADTLE in several families ( 55 , 56 ). Th e LGI1 pathogenetic mechanism remains to be clarified. Functional inactivation of one allel e leads to ADTLE, whereas silencing of both alleles has been observed in several high-grade gliomas ( 57 ). The LGI1 protein harbors a domain consisting of a sevenfold repeat of 44 a mino acids, the EAR (epilepsy-associated repeat) domain ( 58 ). This domain is common to MA SS1, a large G-protein coupled receptor, mutated in the Frings murine model of audiogenic e pilepsy ( 59 ). COOH NH2 TM2 TM3 TM4 TM1 Febrile seizures FS are classi \201 ed as a nosological entity distinct from epilepsy. FS affect 2 \200\2234 % of children between the ages of 3 months and 6 years ( 60 ). The diffe rential diagnosis includes seizures with fever in a child with epilepsy, seizures with cent ral nervous system infection, or an acute metabolic disturbance ( 60 ). A positive family h istory is one of the few known risk factors for febrile seizures, with recur- rence risk ra tios of 3 \200\2235 in \201 rst-degree relatives ( 61 ). About 3 \200\2236 % of children with FS will have epilepsy later in life, including idiopathic generalized epilepsy and tem poral lobe epilepsy with hippocampal sclerosis that might be related to prolonged FS ( 62 ) . Segregation analysis of a population-based sample showed that most FS have polygenic inhe ritance, although a small proportion have auto- somal-dominant inheritance ( 63 ). Using la rge, multigeneration families with presumed autosomal dominant inheritance, four loci for F S have been identi \201 ed (FEB1 \200\223FEB4) but no gene defect has yet been identi \201 ed ( 64 ). B ( ) Single gene severe epilepsies and epileptic encephalopathies Some types of severe epilepsie s have a genetic basis. Patients often have severe epilepsy with early onset and infantile spasms, delayed neurological and cognitive development, and behavioral prob- lems. Some exa mples of genes in which, in recent years, mutations have been identi \201 ed in patients w ith particular forms of severe epi- lepsy are discussed in this section. C TM4 2 TM2 TM4 4 TM3 TM1 TM3 TM1 TM2 pore TM4 2 TM4 4 TM3 TM1 TM3 TM1 TM2 TM2 TM4 2 PCDH19 (protocadherin 19) PCDH19 mutations have been associated to epilepsy and mental reta rdation limited to females (EFMR) ( 65 ), an X-linked disorder surprisingly affecting only females and sparing transmitting males, but also often appearing de novo ( 66 ). It is char acterized by a vari- able clinical presentation, including slow development from birth, nor mal early development followed by regression starting at sei- zure onset, and normal develo pment without regression ( 65 , 67 ). The epilepsy spectrum associated with PCDH19 mutation s is in turn variable and includes mild focal epilepsy starting in infancy or epilepsy with recurrent episodes of focal or generalized seizures in series or status epilepticus trigge red by fever ( 66 ). Protocadherin 19 is part of the protocadherin delta-2 subclass of the cadherin superfamily. Protocadherin members are expressed predominantly in the nervous syst em, where they have a role in establishing neu- ronal connections and in signal transductio n at synaptic mem- branes ( 65 ). Males which are hemizygous for PCDH19 mutations have norm al cognitive level and no epilepsy. Cellular interference has been proposed to explain the discrepancy between the clinical TM3 TM1 TM2 Fig. 2.2 A) General structure of a single subunit of the nicotinic receptor. B) Nicotinic r eceptors are formed by \201 ve subunits (pentamers), symmetrically arranged to delimit a pore through which cations \202 ow, with Na + and Ca 2 + incoming, and K + outgoing. Nine different types of -subunits and four different types of -subunits are known. Isofor ms for subunits , , and are not yet known. Numerous receptor subtypes that have spe ci \201 c anatomical locations (e.g. muscle or neuronal) are generated by multiple combina tions of different types of these subunits. In the central nervous system, the pentameric s tructures of the receptor are composed of -subunit homodimers or of -subunits/ -su bunits combinations. C) Top view of the structure of a neuronal nicotinic receptor composed by two 4-subunits (CHRNA4) and three 2-subunits (CHRNB2). Abnormalities in this rece ptor, caused by mutations of its subunits, cause nocturnal frontal lobe epilepsy. oxford textbook of epilepsy and epileptic seizures 16 spasms ( 88 , 89 ). The transition from EIEE to West syndrome occurs in 75 % of individuals with EIEE ( 86 , 87 ). STXBP1 protein plays an essential role in synaptic vesicle release and secretion of neurotransmitters ( 90 ). The Ohtahara syndrome \200\223infantile spasms s pectrum can also be caused by polyalanine expansions of the ARX gene ( 71 , 77 ). Available information suggests a mutation rate of about 6 % for ARX in boys with isolated or X-linke d infantile spasms and no brain lesions ( 71 ) and up to 37 % for STXBP1 in infants with Oh tahara syndrome ( 90 ). However, larger series are needed to fully elucidate the phenotypic spectrum and better understand the causative role of this gene in epilepsy. manifestations of heterozygous females and hemizygous males ( 68 ). This model suggests tha t if an individual has two populations of protocadherin cells (mutated and non-mutated) the n a pathologi- cal phenotype occurs. A normal female or a transmitting male have only one p rotocadherin population of cells, protocadherin- wildtype or protocadherin-mutant cells, re spectively, then they do not present a pathological phenotype. The development of geneti- c ally modi \201 ed animal models will allow this puzzling disease mechanism to be better ex plored. ARX (aristaless-related homeobox) The ARX gene, on chromosome Xp22, is a transcription fact or that belongs to a family of paired class homeobox genes, and plays an important role in embryogenesis, especially in the development of the central nervous system ( 69 ). To date, ARX mutations have been identi \201 ed in about 10 different clinical conditions, with or without brain malformations ( 70 \200\223 72 ). Malformation phenotypes, including X-link ed lissencephaly with abnormal genitalia (XLAG), XLAG with severe hydrocephalus, and Proud syndrome (agenesis of the corpus callosum with abnormal genitalia), are associated with pro tein trun- cation mutations and missense mutations in the homeobox ( 70 , 73 ). Non-malform ation phenotypes are associated with missense muta- tions outside of the homeobox or expans ion of the second polyA tract and include X-linked infantile spasms (ISS)/West syndrome, Pa rtington syndrome (mental retardation with mild distal dystonia), and non-speci \201 c X-l inked mental retardation (XLMR) ( 70 , 73 ). Expansions in the \201 rst polyA tract cause X-linked infantile spasms (ISSX)/West syndrome (tonic spasms with clustering, severe psy- chomotor delay and hypsarrhythmia on the EEG) ( 74 , 75 ), a severe epileptic-dyskinetic en cephalopathy ( 71 ), tonic seizures and dysto- nia without infantile spasms ( 76 ), and Oht ahara syndrome ( 77 ). Progressive myoclonus epilepsies These genetically heterogeneous disorders (Table 2. 2 ) sh are clini- cal features that include action myoclonus, myoclonic jerks that are segmental a nd arrhythmic, appearing both at rest and as stimu- lus sensitive manifestations, epileptic seizures, predominantly gen- eralized tonic \200\223clonic. Progressive mental deteriorati on, cerebellar and extrapyramidal signs are also present in a variable proportion of patien ts ( 91 , 92 ). Progressive myoclonus epilepsies (PMEs) rep- resent less than 1 % of all ep ilepsy cases and have a variable geo- graphic and ethnic distribution. Many gene defects un derlying PMEs have been identi \201 ed (Table 2. 3 ) but in a considerable pro- portion of patients the cause remains unknown. Most PMEs result from the intracellular accumulation o f abnormal deposit material ( 93 ). The age onset, the rapid progression of the symptoms, a nd the prognosis are different and depend on the speci \201 c etiology and on the type of causative mutations. The diagnostic approach varies in relation to the suspected form (Tabl e 2. 4 ). While the Unverricht \200\223Lundborg disease, although disabling, can reach a de gree of stabilization, all other forms progress relentlessly, leading to severe disability or death in matter of years. CDKL5 (cyclin-dependent, kinase-like 5) Mutations in the X-linked gene CDKL5 cause early on set intracta- ble seizures, severe developmental delay and, often, subsequent appearance of Rett syndrome (RTT) \200\223like features ( 78 ). The phe- notypic spectrum associated wit h CDKL5 mutations also includes X-linked infantile spasms syndrome (ISSX) ( 79 ), a form of myo- clonic encephalopathy ( 80 ), and severe encephalopathy with refractory seizures ( 81 ). Mutations in CDKL5 are mainly found in females, suggesting gestational lethality in mal es ( 82 ). Parental germline mosaicism has been reported ( 83 ). In affected females, a see mingly normal early development, followed by onset of intrac- table seizures between the \201 rst days and 4th month of life are early key diagnostic criteria. Severe developmental delay with regression become apparent after seizures onset. Seizures are usually mani- fes ted as infantile spasms, or prolonged tonic seizures followed by spasms and myoclonus, with a peculiar electroclinical pattern ( 84 ) variably associated with, migrating focal seizur es during the course. Overall, 16.3 % of girls with early-onset intractable epilepsy, with or without infantile spasms, exhibit either mutations or genomic deletions involving CDKL5 ( 85 ). Idiopathic generalized epilepsies with complex inheritance Mode of inheritance The idiopath ic generalized epilepsies (IGEs) represent 20 \200\22330 % of all epilepsies and include a group of syndromes characterized by Table 2.2 Clinical classi \201 cation of progressive myoclonic epilepsies PMEs type Age of onset Unverricht \200\223Lundborg disease (ULD) 7 \200\22316 years Lafora disease (LD) 6 \200\22319 years Neuronal ceroid-lipofuscinoses (NCLs): \227\206 Infantile (Haltia \200\223Santavuori) \227\206 Late infantile (Jansky \200\223Bielschowsky) \227\206 Intermediate (Lake or Cavan agh) \227\206 Juvenile (Spielmeyer \200\223Vogt) \227\206 Adult (Kufs) 6 months \200\2232 years 1 \200\2234 years 5 \200\2238 years 4 \200\22314 years 15 \200\223 50 years STXBP1 (syntaxin binding protein 1) Mutations in the STXBP1 gene have been associated with Ohtahara syndrome or early infantile epileptic encephalopathy (EIEE), char- acterized by ea rly onset of tonic spasms, seizure intractability, a characteristic suppression-burst patte rn on the EEG and poor outcome with severe psychomotor retardation ( 86 , 87 ). STXBP1 muta tions have also been found in some children with infantile Myoclonic epilepsy with ragged-red \201 bres (MERRF) 3 \200\22365 years 8 \200\22315 years Sialidosis: \227\206 Type 1 \227\206 Galactosialidosis (type 2) Dentato-rubro-pallido-luysian atrophy (DRPLA) Childhood Gaucher disease (type III) Variable CHAPTER 2 neurogenetics of epilepsy 17 Table 2.3 Molecular genetics of progressive myoclonic epilepsies PMEs type Pattern of inheritance Locus Gene Unverricht \200\223Lundborg disease (ULD) AR 21q22.3 CSTB Lafora disease (LD) AR 6q 24.3 6p22.3 EPM2A NHLRC1 Neuronal ceroid-lipofuscinoses (NCLs): Infantile Late infantile Finnish variant Gipsy varia nt Turkish variant Juvenile Juvenile variant Juvenile variant Adult Congenital AR AR AR AR AR AR AR AR AR AD AR 1p34.2 11p15.4 13q22.3 15q23 4q28.2 16p12.1 8p23.3 \200\223 15q23 20q13.33 11p15.5 PPT1 TPP1 CLN5 CLN6 MFSD8 CLN3 CLN8 CLN9 CLN4 DNAJC5 CTSD tRNA Lys Myoclonic epilepsy with ragged-red \201 bres (MERRF) Mitochondrial Mt DNA Sialidosis: Type I Galactosialidosis AR AR 6p21.33 20q13.12 NEU1 CTSA Gaucher disease, type III AR 1q22 GBA Dentato-rubro-pallido-luysian atrophy (DRPLA) AD 12p13.31 ATN1 AD, autosomal dominant; AR, autosomal recessive; Mt, mitochondrial. Table 2.4 Diagnostic tools for progressive myoclonic epilepsies