Congenital myasthenic syndrome with episodic apnea in patients homozygous for a CHAT missense mutation

BACKGROUND

The syndrome of congenital myasthenia with episodic apnea (CMS-EA) was previously found to be due to mutations in the choline acetyltransferase gene (CHAT).

OBJECTIVE

To identify the mutations underlying CMS-EA in a Turkish multiplex family.

DESIGN

Direct sequencing of the CHAT gene.

PATIENTS

A consanguineous Turkish family with 2 siblings affected by muscular weakness and episodic respiratory distress.

RESULTS

The sequencing of CHAT coding exons identified a previously unknown missense mutation that affected a highly conserved amino acid residue (I336T). The mutation was absent in 164 control chromosomes.

CONCLUSIONS

The high degree of conservation in different species strongly suggests that I336T is a functionally important amino acid residue. The absence of I336T from a large control sample further supports the pathogenic role of I336T in CMS-EA. This is the second report of CHAT mutations causing presynaptic CMS.

Neonatal convulsions and epileptic encephalopathy in an Italian family with a missense mutation in the fifth transmembrane region of KCNQ2

Mutations in the voltage gated K(+)-channel gene KCNQ2 are known to cause benign familial neonatal convulsions (BFNC), which are characterized by a benign course, spontaneous remission and normal psychomotor development. Most KCNQ2 mutations can be predicted to truncate the protein. Only a few amino acid exchanges have been found, and their localization was restricted to either the pore region or the fourth or sixth transmembrane region (TM). We have now identified the first KCNQ2 mutation located within TM5, affecting a highly conserved serine in amino acid position 247 of the predicted protein. The clinical history of the two affected family members is not compatible with typical BFNC. The poor outcome in the index patient raises the question if at least some KCNQ2 mutations might increase the risk to develop therapy-resistant epilepsy. Additional studies are needed to evaluate the possibility of a causal relationship between KCNQ2 mutations and severe early infantile epilepsy.

Congenital myasthenia in Brahman calves caused by homozygosity for a CHRNE truncating mutation

To elucidate the genetic defect in four previously reported related Brahman calves with severe myasthenic weakness, we determined the genomic structure of the gene encoding the bovine epsilon-subunit (bovCHRNE) of the acetylcholine receptor (AChR). Amplification of DNA isolated from paraplast-embedded tissue samples from one of the myasthenic calves and subsequent sequencing of all bovCHRNE exons revealed a homozygous 20-bp deletion within exon 5 (470del20). The deletion causes a frame shift followed by a premature stop codon in the predicted bovCHRNE protein. Thus, the 470del20 mutation reported here leads to a non-functional allele, explaining the impairment of neuromuscular transmission observed in the affected Brahman calves. With a survival time limited to only several months, the effect on neuromuscular transmission was more pronounced in the calves than that observed in humans homozygous for truncating CHRNE mutations. This may be due to a different capacity to express the fetal-type AChR after birth.

LGI1 is mutated in familial temporal lobe epilepsy characterized by aphasic seizures

Autosomal dominant lateral temporal lobe epilepsy previously has been linked to chromosome 10q22-q24, and recently mutations in the LGI1 gene (Leucine-rich gene, Glioma Inactivated) have been found in some autosomal dominant lateral temporal lobe epilepsy families. We have now identified a missense mutation affecting a conserved cysteine residue in the extracellular region of the LGI1 protein. The C46R mutation is associated with autosomal dominant lateral temporal lobe epilepsy in a large Norwegian family showing unusual clinical features like short-lasting sensory aphasia and auditory symptoms.

Nicotinic acetylcholine receptors and epilepsy

The neuronal nicotinic acetylcholine receptors (nAChRs) have multiple roles in the brain: they are involved in signal transduction by fast synaptic transmission, axo-axonic transmission, and in the modulation of presynaptic transmitter release. Presynaptic nAChRs can increase the release of excitatory as well as of inhibitory transmitters, and can thereby control neuronal excitability. Furthermore, nAChRs which are expressed in fetal brain might also be involved in brain morphogenesis. Thus, the genes coding for the different nAChR subunits are likely candidates for several neurological disorders. The CHRNA4- or CHRNB2 subunits of the nAChR are associated with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), a rare monogenic type of idiopathic epilepsy. Electrophysiological studies demonstrated that ADNFLE mutations are causing both a loss-of-function and a gain-of-function in alpha4/beta2-heteropentameric nAChRs.

A splice-site mutation in GABRG2 associated with childhood absence epilepsy and febrile convulsions

CONTEXT

Missense mutations in the GABRG2 gene, which encodes the gamma 2 subunit of central nervous gamma-aminobutyric acid (GABA)(A) receptors, have recently been described in 2 families with idiopathic epilepsy. In one of these families, the affected individuals predominantly exhibited childhood absence epilepsy and febrile convulsions.

OBJECTIVE

To assess the role of GABRG2 in the genetic predisposition to idiopathic absence epilepsies.

DESIGN

The GABRG2 gene was screened by single-strand conformation analysis for mutations. Furthermore, a population-based association study assessing a common exon 5 polymorphism (C588T) was carried out.

PATIENTS

The sample was composed of 135 patients with idiopathic absence epilepsy and 154 unrelated and ethnically matched controls.

RESULTS

A point mutation (IVS6 + 2T-->G) leading to a splice-donor site mutation in intron 6 was found. The mutation, which is predicted to lead to a nonfunctional protein, cosegregates with the disease status in a family with childhood absence epilepsy and febrile convulsions. The association study did not find any significant differences in the allele and genotype frequencies of the common exon 5 polymorphism (C588T) between patients with idiopathic absence epilepsy and controls (P>.35).

CONCLUSIONS

Our study identified a splice-donor-site mutation that was probably causing a nonfunctional GABRG2 subunit. This mutation occurred in heterozygosity in the affected members of a single nuclear family, exhibiting a phenotypic spectrum of childhood absence epilepsy and febrile convulsions. The GABRG2 gene seems to confer a rare rather than a frequent major susceptibility effect to common idiopathic absence epilepsy syndromes.

Congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) constitute a heterogenous group of inherited disorders in which neuromuscular transmission is compromised by one or more specific mechanisms. Clinical evidence for the diagnosis of a CMS includes a history of increased fatigable weakness since infancy or early childhood, a decremental EMG response, and the absence of acetylcholine receptor (AChR) antibodies. There has been rapid progress in understanding of the molecular basis of CMS. Mutation analysis of the AChR subunits has revealed numerous disease-associated mutations. These mutations alter the response to acetylcholine. It is decreased in the fast-channel syndromes and in primary AChR deficiency; and it is increased in the slow-channel syndrome due to prolonged open-time of the AChR. Acetylcholinesterase deficiency is associated with mutations in the gene encoding the collagenic tail subunit of the enzyme. Mutations in the gene encoding for choline acetyltransferase causes the CMS associated with episodic apnea.

The LGI1 gene involved in lateral temporal lobe epilepsy belongs to a new subfamily of leucine-rich repeat proteins

Recently mutations in the LGI1 (leucine-rich, glioma-inactivated 1) gene have been found in human temporal lobe epilepsy. We have now identified three formerly unknown LGI-like genes. Hydropathy plots and pattern analysis showed that LGI genes encode proteins with large extra- and intracellular domains connected by a single transmembrane region. Sequence analysis demonstrated that LGI1, LGI2, LGI3, and LGI4 form a distinct subfamily when compared to other leucine-rich repeat-containing proteins. In silico mapping and radiation hybrid experiments assigned LGI2, LGI3, and LGI4 to different chromosomal regions (4p15.2, 8p21.3, 19q13.11), some of which have been implicated in epileptogenesis and/or tumorigenesis.

Channelopathies can cause epilepsy in man

Idiopathic epilepsies, which account for up to 40% of all epilepsies, are mainly caused by genetic factors. Most idiopathic epilepsies are due to oligogenic or multifactorial rather than monogenetic inheritance. Nevertheless, most of what is known today about the molecular genetics of idiopathic epilepsies has been found by analysing large families with rare monogenetic forms of the disease. For the first time, gene defects can be linked to certain epilepsies. Mutations in the CHRNA4 or CHRNB subunits of the neuronal nicotinic acetylcholine receptor lead to familial nocturnal frontal lobe epilepsy, while defects in the voltage-gated potassium channels KCNQ2 and KCNQ3 have recently been found to cause benign familial neonatal convulsions. The voltage-gated sodium channel subunits SCN1B, SCN1A and SCN2A as well as the GABRG2 subunit of the GABA(A) receptor are involved in the pathology of the newly described syndrome generalized epilepsy with febrile seizures plus. These rare monogenetic epilepsies can serve as models for further genetic analysis of the common forms of idiopathic epilepsies.

Expression pattern and functional characteristics of two novel splice variants of the two-pore-domain potassium channel TREK-2

Two novel alternatively spliced isoforms of the human two-pore-domain potassium channel TREK-2 were isolated from cDNA libraries of human kidney and fetal brain. The cDNAs of 2438 base pairs (bp) (TREK-2b) and 2559 bp (TREK-2c) encode proteins of 508 amino acids each. RT-PCR showed that TREK-2b is strongly expressed in kidney (primarily in the proximal tubule) and pancreas, whereas TREK-2c is abundantly expressed in brain. In situ hybridization revealed a very distinct expression pattern of TREK-2c in rat brain which partially overlapped with that of TREK-1. Expression of TREK-2b and TREK-2c in human embryonic kidney (HEK) 293 cells showed that their single-channel characteristics were similar. The slope conductance at negative potentials was 163 +/- 5 pS for TREK-2b and 179 +/- 17 pS for TREK-2c. The mean open and closed times of TREK-2b at -84 mV were 133 +/- 16 and 109 +/- 11 micros, respectively. Application of forskolin decreased the whole-cell current carried by TREK-2b and TREK-2c. The sensitivity to forskolin was abolished by mutating a protein kinase A phosphorylation site at position 364 of TREK-2c (construct S364A). Activation of protein kinase C (PKC) by application of phorbol-12-myristate-13-acetate (PMA) also reduced whole-cell current. However, removal of the putative TREK-2b-specific PKC phosphorylation site (construct T7A) did not affect inhibition by PMA. Our results suggest that alternative splicing of TREK-2 contributes to the diversity of two-pore-domain K+ channels.

Tandem pore domain K(+)-channel TASK-3 (KCNK9) and idiopathic absence epilepsies

Recently, the gene coding for the tandem pore domain K(+)-channel TASK-3 (KCNK9) has been localized to the chromosomal region 8q24. Because mutations in ion channel genes have been recognized as an important factor in the etiology of abnormal neuronal excitability, TASK-3 is an interesting candidate gene for epilepsies linked to 8q24. We therefore performed a mutation analysis of the TASK-3 gene in 65 patients with childhood and juvenile absence epilepsy. Only one silent nucleotide exchange (636C/T) was detected in exon 2 of the TASK-3 coding region. No evidence for an allelic association was found between the exon 2 polymorphism and absence epilepsy. Accordingly, genetic variation of the TASK-3 coding region does not play a major role in the etiology of idiopathic absence epilepsies.

Familial temporal lobe epilepsy with aphasic seizures and linkage to chromosome 10q22-q24

PURPOSE

To describe the phenotypic expression of a new family with familial lateral temporal lobe epilepsy with aphasic seizures, and to compare the findings with the clinical features of previously reported families linked to chromosome 10q22-q24.

METHODS

Medical records were collected from 12 living affected members. The patients underwent a personal interview and a clinical neurologic examination. Results from interictal scalp EEGs and neuroimaging examinations were obtained.

RESULTS

The cardinal ictal symptom was a brief sensory aphasia in eight of the patients. In four, this was accompanied by auditory symptoms, usually in the form of monotonous unformed sounds. Simple partial seizures with psychic or somatosensory seizures also were present. Visual ictal symptoms and complex partial seizures were absent. All patients had generalized tonic-clonic seizures. Magnetic resonance imaging (MRI) or computed tomography (CT) did not reveal morphologic correlates. Improvement with age seemed to occur in many patients. Significant linkage to chromosome 10q22-q24 was established by testing 17 polymorphic microsatellite markers.

CONCLUSIONS

The epilepsy of this family appears to represent a variety of autosomal dominant lateral temporal lobe epilepsy. Aphasic seizures and a peculiar seizure-precipitating effect of the activation of speech (initiation or perception) may serve as markers for identifying further families with this phenotype.

Genes and mutations in idiopathic epilepsy

Partial or generalized idiopathic epilepsies, which account for up to 40% of all epilepsies, are characterized by a mostly benign course and no apparent etiology other than a genetic predisposition. So far, the genetic defects underlying three different idiopathic epilepsy syndromes have been identified: mutations in the CHRNA4- or CHRNB subunits of the neuronal nicotinic acetylcholine receptor are found in familial nocturnal frontal lobe epilepsy, while defects in the voltage-gated potassium channels KCNQ2 and KCNQ3 have recently been identified in benign familial neonatal convulsions. The syndrome of "generalized epilepsy with febrile seizures plus" can be caused by mutations affecting the voltage-gated sodium channel subunits SCN1B and SCN1A or the gamma 2-subunit of the GABA(A) receptor. The results of recent molecular studies contributed largely to our understanding of the etiology and pathophysiology of idiopathic epilepsies.

Cloning and mutation analysis of the human potassium channel KCNQ2 gene promoter

Benign familial neonatal convulsions (BFNC) have been previously found to be associated with mutations within the coding region of KCNQ2. We have now cloned and analyzed the promoter region of the human KCNQ2 gene. 5'-RACE identified a transcription start site (TSS) located 200 bp upstream of the ATG start codon. The TSS is located close to a repetitive region containing seven copies of a degenerate 42-mer repeat. Several different luciferase (LUC) reporter plas- mids containing fragments from the KCNQ2 5'-flanking region were constructed and expressed in NT2N and SH-SY5Y cell lines. A core promoter region was found to be located between bp 20 and bp 74 upstream of the TSS. Neither the promoter region nor the repetitive region showed any mutations in 13 index patients from unrelated BFNC families.

Myokymia and neonatal epilepsy caused by a mutation in the voltage sensor of the KCNQ2 K+ channel

KCNQ2 and KCNQ3 are two homologous K(+) channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia/BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC.

Classical Alzheimer features and cholinergic dysfunction: towards a unifying hypothesis?

OBJECTIVE

Our autopsy studies show possible links between classical Alzheimer pathology and decreased expression of nicotinic acetylcholine receptors. For further elucidation we are now using in vitro models. We report preliminary evidence for the impact of beta-amyloid on nicotinic receptor expression in hippocampal dissociation culture.

METHODS

Cultures (E18 rats) were grown in a serum-free medium and incubated at 8 days in vitro for 3 days with 1 microM Abeta1-42. Expression of alpha4, alpha7, and beta2 nicotinic receptor subunit protein was assessed immunohistochemically and rated semiquantitatively.

RESULTS

Abeta1-42 incubation resulted in a massive reduction of alpha4 protein-expressing neurons, this effect was less pronounced for the alpha7 and beta2 subunit protein.

CONCLUSION

These findings provide first evidence for a direct impact of classical Alzheimer pathology features on nicotinic receptor expression in vitro. Our model will be useful for testing the potential of drugs to stop or reverse these effects.

Mutation analysis of the potassium chloride cotransporter KCC3 (SLC12A6) in rolandic and idiopathic generalized epilepsy

Genetic predisposition plays a major role in the etiology of idiopathic epilepsies. The common epilepsy syndromes display a complex pattern of inheritance, with an unknown number of genes contributing to seizure susceptibility. During the last decade linkage studies have narrowed down several candidate regions for susceptibility loci of idiopathic epilepsies. Several lines of evidence point to the existence of an epilepsy susceptibility gene on chromosome 15q14. Evidence for linkage to this region has thus been reported for juvenile myoclonic epilepsy, common subtypes of idiopathic generalized epilepsy (IGE), in addition to the EEG trait 'centrotemporal spikes' in families with rolandic epilepsy. The chromosomal region 15q14 harbours several candidate genes that are involved in the regulation of neuronal excitability. One of the most promising candidate genes is the brain-expressed potassium chloride cotransporter KCC3, given that this class of ion transporter has been implicated in the regulation of neuronal chloride activity. We therefore performed a mutation analysis of KCC3 in the index patients of 23 IGE-families as well as of 16 families with rolandic epilepsy which where selected by positive evidence for linkage to D15S165. Four novel single nucleotide exchanges (SNPs) were identified, none of which change the coding sequence. These results do not support a major role for KCC3 in the etiology of rolandic epilepsy or common subtypes of IGE.

Severe congenital myasthenic syndrome due to homozygosity of the 1293insG epsilon-acetylcholine receptor subunit mutation

Recently, a congenital myasthenic syndrome (CMS) with end-plate acetylcholine receptor (AChR) deficiency due to missense mutations in the genes for the AChR subunit was described. The first observed patient with this CMS was heteroallelic for the two epsilon-AChR subunit mutations epsilon1101insT and epsilon1293insG. This patient had only a moderate phenotype with mild muscle weakness and abnormal fatigue. We have now found homozygosity for the epsilon1293insG mutation in a severely affected CMS patient, who lost the ability to walk in midchildhood and shows profound weakness and muscle wasting. Our observation allows a genotype-phenotype correlation illustrating how differences in the AChR mutation haplotype can profoundly influence disease severity.

Expression of nicotinic acetylcholine receptors in Alzheimer's disease: postmortem investigations and experimental approaches

Nicotinic ligand binding studies have shown rather early that the cholinoceptive system is affected in Alzheimer's disease (AD). Today, molecular histochemistry enables one to study the nicotinic acetylcholine receptor (nAChR) subunit expression on the cellular level in human autopsy brains, in animal models and in in vitro approaches, thus deciphering the distribution of nAChRs and their role as potential therapeutic targets. The studies on the nAChR expression in the frontal and temporal cortex of AD patients and age-matched controls could demonstrate that both, the numbers of alpha4- and alpha7-immunoreactive neurons and the quantitative amount, in particular of the alpha4 protein, were markedly decreased in AD. Because the number of the corresponding mRNA expressing neurons was unchanged these findings point to a translational/posttranslational rather than a transcriptional event as an underlying cause. This assumption is supported by direct mutation screening of the CHRNA4 gene which showed no functionally important mutations. To get more insight into the underlying mechanisms, two model systems organotypic culture and primary hippocampal culture - have been established, both allowing to mimic nAChR expression in vitro. In ongoing studies the possible impact of beta-amyloid (Abeta) on nAChR expression is tested. Preliminary results obtained from primary cultures point to an impaired nAChR expression following Abeta exposure.

Immature end-plates and utrophin deficiency in congenital myasthenic syndrome caused by epsilon-AChR subunit truncating mutations

Congenital myasthenic syndromes (CMS) are inborn disorders due to presynaptic, synaptic, or postsynaptic defects of neuromuscular transmission. Some previously described kinships with typical signs of CMS showed a marked deficiency of acetylcholine receptors (AChR) and utrophin at the neuromuscular junctions. Additionally, the end-plate ultrastructure was immature, with reduced enfolding of the postsynaptic membrane. In two such families, we found truncating mutations of the epsilon-AChR subunit. In family 1, both affected siblings were heteroallelic for a epsilon911delT and a epsilonIVS4+1G-->A mutation within the AChR epsilon-subunit gene (CHRNE). In the affected member of family 2, a epsilon1030delC mutation and a previously described epsilonR64X mutation were found. These deleterious epsilonAChR mutations not only result in AChR deficiency, but also affect end-plate maturation, including the formation of secondary synaptic clefts during ontogenesis.

[Benign familial neonatal convulsions: molecular pathology and diagnosis]

Benign familial neonatal convulsions are a rare monogenic form of idiopathic epilepsy characterized by the onset of frequent brief seizures after the second day of life. The seizures disappear spontaneously within a few weeks, but recurrent seizures later in life are common. Linkage studies located genes to chromosome 20q13.3 and 8q24, and the voltage-gated potassium channels KCNQ2 and KCNQ3 were recently identified. Since then, several mutations have been found leading to haplosufficiency of the ion channel. Functional studies showed that KCNQ2 and KCNQ3 are able to contribute to a heteromeric channel exhibiting kinetic and pharmacological properties similar to those of the native M current, the latter playing an important role in the regulation of neuronal excitability. This overview presents a summary of the molecular, genetic, and electrophysiological findings and discusses them with respect to their clinical relevance.

The new voltage gated potassium channel KCNQ5 and neonatal convulsions

In 1998, mutations in the voltage gated potassium channel gene KCNQ2 were found to be the main cause underlying the autosomal dominant inherited syndrome of benign familial neonatal convulsions (BFNC). In one BFNC family a mutation was found in an homologous gene, KCNQ3. We have now identified another brain-expressed member of this ion channel subfamily, KCNQ5, which maps to chromosome 6q14. On the genomic level KCNQ5 is composed of 14 exons, which are coding for 897 amino acid residues. Mutation analysis made KCNQ5 unlikely as a candidate gene for benign neonatal convulsions in patients with a positive family history for neonatal or early infantile seizures, but without mutations in the KCNQ2 or KCNQ3 genes.