Molecular basis of Mendelian idiopathic epilepsies

Very large G protein-coupled receptor 1 regulates myelin-associated glycoprotein via Gαs/Gαq-mediated protein kinases A/C

VLGR1 (very large G protein-coupled receptor 1), also known as MASS1 (monogenic audiogenic seizure susceptible 1), is an orphan G protein-coupled receptor that contains a large extracellular N terminus with 35 calcium-binding domains. A truncating mutation in the Mass1 gene causes autosomal recessive, sound-induced seizures in the Frings mouse. However, the function of MASS1 and the mechanism underlying Frings mouse epilepsy are not known. Here, we found that MASS1 protein is enriched in the myelinated regions of the superior and inferior colliculi, critical areas for the initiation and propagation of audiogenic seizures. Using a panel of myelin antibodies, we discovered that myelin-associated glycoprotein (MAG) expression is dramatically decreased in Frings mice. MASS1 inhibits the ubiquitylation of MAG, thus enhancing the stability of this protein, and the calcium-binding domains of MASS1 are essential for this regulation. Furthermore, MASS1 interacts with Gαs/Gαq and activates PKA and PKC in response to extracellular calcium. Suppression of signaling by MASS1 RNAi or a specific inhibitor abrogates MAG up-regulation. We postulate that MASS1 senses extracellular calcium and activates cytosolic PKA/PKC pathways to regulate myelination by means of MAG protein stability in myelin-forming cells of the auditory pathway. Further work is required to determine whether MAG dysregulation is a cause or consequence of audiogenic epilepsy and whether there are other pathways regulated by MASS1.

Familial Risks for Main Neurological Diseases in Siblings Based on Hospitalizations in Sweden

Recent successes in identifying the underlying genetic mechanisms for neurological diseases, particularly for their Mendelian forms, have had profound implications for their diagnostics, treatment and classification. However, there has never been an attempt to compare familial risks in a systematic way among and between the main neurological diseases. Familial risks were here defined for siblings who were hospitalized because of a neurological disease in Sweden. A nationwide database for neurological diseases was constructed by linking the Multigeneration Register of 0- to 69-year-old siblings to the Hospital Discharge Register for the years 1987 to 2001. Standardized risk ratios were calculated for affected sibling pairs by comparing them to those whose siblings had no neurological disease. There were three main results. First, it was shown that all disease groups had a familial risk, with the exception of transient ischemic attacks, and the risks could be ranked from the highest (3451) for Huntington's disease to the lowest (2.1) for inflammatory diseases. Second, increased familial risks were shown for disease subtypes for which susceptibility genes or familial clustering have not been demonstrated previously, including multiple sclerosis, sleep apnea, nerve, nerve root and plexus disorders, and cerebral palsy. Third, based on the available sample size there was no convincing evidence for familial comorbidity between the disease groups, suggesting that the factors causing familial aggregation, probably usually heritable genes, are distinct for each subtype. The high familial risks for neurological disease imply heritable etiology and opportunities for identification of further susceptibility genes.

High incidence of pediatric idiopathic epilepsy is associated with familial and autosomal dominant disease in Eastern Newfoundland

PURPOSE

To describe the incidence and epidemiology of pediatric idiopathic epilepsy (IE) in Newfoundland and Labrador.

METHODS

All children in Newfoundland and Labrador aged 0-15 years with IE were ascertained through the provincial neurology clinic at the Janeway Child Health Centre. Family history, medical history and blood samples were obtained from probands and relatives. Two genes, SCN1A and KCNQ2, were screened for mutations by direct sequencing.

RESULTS

The mean annual incidence of IE for the population of children living in the Avalon region of Newfoundland from 2000 to 2004 was 107 per 100,000. This rate is approximately three-fold greater than rates reported in other developed countries. Of 117 families with IE eligible for study, 86 (74%) provided detailed pedigree data. Multiple different epilepsy phenotypes were identified. Fifty-five families (64%) had a positive family history. Eight of these had family histories compatible with autosomal dominant (AD) inheritance and these families lived in five different geographic isolates. DNA was obtained from 21 families (79 individuals). The two previously identified mutations in Newfoundland families with epilepsy were sequenced and excluded as pathogenic sites in all but one family which had a mutation in SCN1A.

CONCLUSION

The incidence of IE is high in the Avalon Peninsula of Newfoundland and the rate of familial disease is high throughout the province of Newfoundland and Labrador. The distribution of familial and AD IE in different geographic isolates, together with the clinical heterogeneity of disease suggests substantial genetic heterogeneity. It is likely that other novel mutations will be identified in this population.

A novel microdeletion syndrome involving 5q14.3-q15: clinical and molecular cytogenetic characterization of three patients

Molecular karyotyping is being increasingly applied to delineate novel disease causing microaberrations and related syndromes in patients with mental retardation of unknown aetiology. We report on three unrelated patients with overlapping de novo interstitial microdeletions involving 5q14.3-q15. All three patients presented with severe psychomotor retardation, epilepsy or febrile seizures, muscular hypotonia and variable brain and minor anomalies. Molecular karyotyping revealed three overlapping microdeletions measuring 5.7, 3.9 and 3.6 Mb, respectively. The microdeletions were identified using single nucleotide polymorphism (SNP) arrays (Affymetrix 100K and Illumina 550K) and array comparative genomic hybridization (1 Mb Sanger array-CGH). Confirmation and segregation studies were performed using fluorescence in situ hybridization (FISH) and quantitative PCR. All three aberrations were confirmed and proven to have occurred de novo. The boundaries and sizes of the deletions in the three patients were different, but an overlapping region of around 1.6 Mb in 5q14.3 was defined. It included five genes: CETN3, AC093510.2, POLR3G, LYSMD3 and the proximal part of GPR98/MASS1, a known epilepsy gene. Haploinsufficiency of GPR98/MASS1 is probably responsible for the seizure phenotype in our patients. At least one other gene contained in the commonly deleted region, LYSMD3, shows a high level of central nervous expression during embryogenesis and is also, therefore, a good candidate gene for other central nervous system (CNS) symptoms, such as psychomotor retardation, brain anomalies and muscular hypotonia of the 5q14.3 microdeletion syndrome.

Familial risks for common diseases: etiologic clues and guidance to gene identification

Familial clustering of a disease is a direct indicator of a possible heritable cause, provided that environmental sharing can be excluded. If the familial clustering is lacking, the likelihood of a heritable influence is also small. In the era of genome scans, the consideration of data on heritability should be important in the assessment of the likely success of the genome scan. The availability of a Multigeneration Register in Sweden provides a reliable access to families throughout the last century. This Register has been extensively used to study a number of different diseases through linkage to the Hospital Discharge Register. In the present article we review the obtained and some unpublished results for nine main disease classes. For each of these, familial risks are given for four disease subtypes. As measures of familial clustering we use risks between siblings, twins and spouses. Disease correlation between spouses suggests environmental sharing and a higher correlation between siblings and particularly twins shows heritable effects. We will also comment on the established susceptibility genes and the risks conferred by them. The data suggest high heritabilities for chronic obstructive pulmonary disease, asthma, noninfective enteritis and colitis, cerebral palsy and endocrine and metabolic diseases. Among the performed first-generation genome scans on various diseases, the success appears to be related to the a priori heritability estimates. To our knowledge this is a first attempt to summarize familial risks for a large number of diseases using data from a single population on which reasonable uniform diagnostic criteria have been applied.

Neurodevelopment in Seizure-prone and Seizure-resistant Rat Strains: Recognizing Conflicts in Management

Cytoarchitectural alterations during central nervous system (CNS) development are believed to underlie aberrations in brain morphology that lead to epilepsy. We have recently reported marked reductions in hippocampal and white matter volumes along with relative ventriculomegaly in a rat strain bred to be seizure-prone (FAST) compared to a strain bred to be seizure-resistant (SLOW) (Gilby et al., 2002, American Epilepsy Society 56th Annual Meeting). This study was designed to investigate deviations in gene expression during late-phase embryogenesis within the brains of FAST and SLOW rats. In this way, we hoped to identify molecular mechanisms operating differentially during neurodevelopment that might ultimately create the observed differences in brain morphology and/or seizure susceptibility. Using Superarray technology, we compared the expression level of 112 genes, known to play a role in neurodevelopment, within whole brains of embryonic day 21 (E21) FAST and SLOW rats. Results revealed that while most genes investigated showed near equivalent expression levels, both Apolipoprotein E (APOE) and the beta2 subunit of the voltage-gated sodium channel (SCN2beta) were significantly underexpressed in brains of the seizure-prone embryos. Currently, these transcripts have no known interactions during embryogenesis; however, they have both been independently linked to seizure disposition and/or neurodevelopmental aberrations leading to epilepsy. Thus, alterations in the timing and/or degree of expression for APOE and SCN2beta may be important to developmental cascades that ultimately give rise to the differing brain morphologies, behaviors, and/or seizure vulnerabilities that characterize these strains.

Impact of GPCRs in clinical medicine: monogenic diseases, genetic variants and drug targets

By virtue of their large number, widespread distribution and important roles in cell physiology and biochemistry, G-protein-coupled receptors (GPCR) play multiple important roles in clinical medicine. Here, we focus on 3 areas that subsume much of the recent work in this aspect of GPCR biology: (1) monogenic diseases of GPCR; (2) genetic variants of GPCR; and (3) clinically useful pharmacological agonists and antagonists of GPCR. Diseases involving mutations of GPCR are rare, occurring in <1/1000 people, but disorders in which antibodies are directed against GPCR are more common. Genetic variants, especially single nucleotide polymorphisms (SNPs), show substantial heterogeneity in frequency among different GPCRs but have not been evaluated for some GPCR. Many therapeutic agonists and antagonists target GPCR and show inter-subject variability in terms of efficacy and toxicity. For most of those agents, it remains an open question whether genetic variation in primary sequence of the GPCR is an important contributor to such inter-subject variability, although this is an active area of investigation.

Genetic interactions among cortical malformation genes that influence susceptibility to convulsions in C. elegans

Epilepsy is estimated to affect 1-2% of the world population, yet remains poorly understood at a molecular level. We have previously established the roundworm Caenorhabditis elegans as a model for investigating genetic susceptibilities to seizure-like convulsions in vivo. Here we investigate the behavioral consequences of decreasing the activity of nematode gene homologs within the LIS1 pathway that are associated with a human cortical malformation termed lissencephaly. Bioinformatic analysis revealed the nud-2 gene, encoding the worm homolog of mammalian effectors of LIS1, termed NDE1 and NDEL1. Phenotypic analysis of animals targeted by RNA interference (RNAi) was performed using a pentylenetetrazole (PTZ) exposure paradigm to induce convulsions. Worms depleted for LIS1 pathway components (NUD-1, NUD-2, DHC-1, CDK-5, and CDKA-1) exhibited significant convulsions following PTZ and RNAi treatment. Strains harboring fluorescent markers for GABAergic neuronal architecture and synaptic vesicle trafficking were employed to discern putative mechanisms accounting for observed convulsion behaviors. We found that depletion of LIS1 pathway components resulted in defective GABA synaptic vesicle trafficking. We also utilized combinations of specific genetic backgrounds to create a sensitized state for convulsion susceptibility and discovered that convulsion effects were significantly enhanced when LIS-1 and other pathway components were compromised within the same animals. Thus, interactions among gene products with LIS-1 may mediate intrinsic thresholds of neuronal synchrony.

Familial Risks for Epilepsy among Siblings Based on Hospitalizations in Sweden

PURPOSE

Epilepsy is a common disabling condition, with high heritability according to twin studies. Characterization of familial risks for common subtypes of epilepsy will advance the search for the heritable causes of these conditions and their underlying mechanisms. We aim at defining familial risks for siblings to be hospitalized because of epilepsy.

METHODS

A nationwide ad hoc epilepsy database was constructed by linking the Multigeneration Register on 0- to 69-year-old siblings to the Hospital Discharge Register for data on epilepsies covering the years 1987-2001. Standardized risk ratios (SIRs) were calculated for affected sibling pairs by comparing them to those whose siblings had no epilepsy.

RESULTS

Among a total of 26,799 hospitalized cases, 598 affected siblings were identified with a familial SIR of 2.35; the SIR was highest at ages 0-4 years (6.82). Infantile spasms showed the highest risk for any subtype (10.45), when a co-sibling was diagnosed with any epilepsy. When both siblings were diagnosed with a concordant (same) subtype of epilepsy, the SIRs were high, i.e. 8.43 for generalized idiopathic epilepsy, 2.56 for partial epilepsy, 24.72 for status epilepticus and 24.39 for other epilepsies. Generalized idiopathic epilepsy was also associated with grand mal (4.06) and other epilepsies (7.61). The numbers of cases were small but concordant diagnoses always showing higher SIRs compared with discordant diagnoses.

CONCLUSIONS

Within the limits of the present sample size, our results suggest high familial aggregation for certain subtypes of epilepsy for which distinct genetic mechanisms may underlie.

Recessive symptomatic focal epilepsy and mutant contactin-associated protein-like 2

Contactin-associated protein-like 2 (CASPR2) is encoded by CNTNAP2 and clusters voltage-gated potassium channels (K(v)1.1) at the nodes of Ranvier. We report a homozygous mutation of CNTNAP2 in Old Order Amish children with cortical dysplasia, focal epilepsy, relative macrocephaly, and diminished deep-tendon reflexes. Intractable focal seizures began in early childhood, after which language regression, hyperactivity, impulsive and aggressive behavior, and mental retardation developed in all children. Resective surgery did not prevent the recurrence of seizures. Temporal-lobe specimens showed evidence of abnormalities of neuronal migration and structure, widespread astrogliosis, and reduced expression of CASPR2.

A locus for generalized tonic-clonic seizure susceptibility maps to chromosome 10q25-q26

Inheritance patterns in twins and multiplex families led us to hypothesize that two loci were segregating in subjects with juvenile myoclonic epilepsy (JME), one predisposing to generalized tonic-clonic seizures (GTCS) and a second to myoclonic seizures. We tested this hypothesis by performing genome-wide scan of a large family (Family 01) and used the results to guide analyses of additional families. A locus was identified in Family 01 that was linked to GTCS (10q25-q26). Model-based multipoint analysis of the 10q25-q26 locus showed a logarithm of odds (LOD) score of 2.85; similar results were obtained with model-free analyses (maximum nonparametric linkage [NPL] of 2.71; p = 0.0019). Analyses of the 10q25-q26 locus in 10 additional families assuming heterogeneity revealed evidence for linkage in four families; model-based and model-free analyses showed a heterogeneity LOD (HLOD) of 2.01 (alpha = 0.41) and maximum NPL of 2.56 (p = 0.0027), respectively, when all subjects with GTCS were designated to be affected. Combined analyses of all 11 families showed an HLOD of 4.04 (alpha = 0.51) and maximum NPL score of 4.20 (p = 0.000065). Fine mapping of the locus defined an interval of 4.45Mb. These findings identify a novel locus for GTCS on 10q25-q26 and support the idea that distinct loci underlie distinct seizure types within an epilepsy syndrome such as JME.

Erythromelalgia: A hereditary pain syndrome enters the molecular era

In contrast with acquired pain syndromes, molecular substrates for hereditary pain disorders have been poorly understood. Familial erythromelalgia (Weir Mitchell's disease), also known as primary erythermalgia, is an autosomal dominant disorder characterized by burning pain in the extremities in response to warm stimuli or moderate exercise. The cause of this disorder has been enigmatic, and treatment has been empirical and not very effective. Recent studies, however, have shown that familial erythromelalgia is a channelopathy caused by mutations in the gene encoding the Na(v)1.7 sodium channel which lead to altered channel function. Selective expression of Na(v)1.7 within dorsal root ganglion neurons including nociceptors (in which this channel is targeted to sensory terminals, close to impulse trigger zones) and within sympathetic ganglion neurons explains why patients experience pain but do not suffer from seizures or other manifestations of altered excitability within central nervous system neurons. Erythromelalgia is the first human disorder in which it has been possible to associate an ion channel mutation with chronic neuropathic pain. Identification of mutations within a peripheral neuron-specific sodium channel suggests the possibility of rational therapies that target the affected channel. Moreover, because some other pain syndromes, including acquired disorders, involve altered sodium channel function, erythromelalgia may emerge as a model disease that holds more general lessons about the molecular neurobiology of chronic pain.