A novel genetic locus for juvenile myoclonic epilepsy at chromosome 5q12-q14

KIF2A regulates the development of dentate granule cells and postnatal hippocampal wiring

Kinesin super family protein 2A (KIF2A), an ATP-dependent microtubule (MT) destabilizer, regulates cell migration, axon elongation, and pruning in the developing nervous system. KIF2A mutations have recently been identified in patients with malformed cortical development. However, postnatal KIF2A is continuously expressed in the hippocampus, in which new neurons are generated throughout an individual's life in established neuronal circuits. In this study, we investigated KIF2A function in the postnatal hippocampus by using tamoxifen-inducible Kif2a conditional knockout (Kif2a-cKO) mice. Despite exhibiting no significant defects in neuronal proliferation or migration, Kif2a-cKO mice showed signs of an epileptic hippocampus. In addition to mossy fiber sprouting, the Kif2a-cKO dentate granule cells (DGCs) showed dendro-axonal conversion, leading to the growth of many aberrant overextended dendrites that eventually developed axonal properties. These results suggested that postnatal KIF2A is a key length regulator of DGC developing neurites and is involved in the establishment of precise postnatal hippocampal wiring.

ADGRV1 is implicated in myoclonic epilepsy

OBJECTIVE

To investigate the significance of variation in ADGRV1 (also known as GPR98, MASS1, and VLGR1), MEF2C, and other genes at the 5q14.3 chromosomal locus in myoclonic epilepsy.

METHODS

We studied the epilepsy phenotypes of 4 individuals with 5q14.3 deletion and found that all had myoclonic seizures. We then screened 6 contiguous genes at 5q14.3, MEF2C, CETN3, MBLAC2, POLR3G, LYSMD3, and ADGRV1, in a 95-patient cohort with epilepsy and myoclonic seizures. Of these genes, point mutations in MEF2C cause a phenotype involving seizures and intellectual disability. A role for ADGRV1 in epilepsy has been proposed previously, based on a recessive mutation in the Frings mouse model of audiogenic seizures, as well as a shared homologous region with another epilepsy gene, LGI1.

RESULTS

Six patients from the myoclonic epilepsy cohort had likely pathogenic ultra-rare ADGRV1 variants, and statistical analysis showed that ultra-rare variants were significantly overrepresented when compared to healthy population data from the Genome Aggregation Database. Of the remaining genes, no definite pathogenic variants were identified.

SIGNIFICANCE

Our data suggest that the ADGRV1 variation contributes to epilepsy with myoclonic seizures, although the inheritance pattern may be complex in many cases. In patients with 5q14.3 deletion and epilepsy, ADGRV1 haploinsufficiency likely contributes to seizure development. The latter is a shift from current thinking, as MEF2C haploinsufficiency has been considered the main cause of epilepsy in 5q14.3 deletion syndrome. In cases of 5q14.3 deletion and epilepsy, seizures likely occur due to haploinsufficiency of one or both of ADGRV1 and MEF2C.

Genetic generalized epilepsies

In the International League Against Epilepsy's most recent revision of classification and terminology, the term idiopathic epilepsy, previously used to describe those epilepsies whose cause was unknown, but presumed genetic, has been removed. It has been replaced by the term genetic epilepsy, only to be used to describe epilepsy in which the etiology has a known or presumed genetic defect in which seizures are the core symptom of the disorder. The purpose of this article was to review the electroclinical spectrum of those epilepsies that would fall under this new designation of genetic epilepsies in the context of specific generalized epilepsy syndromes providing an update in the clinical, electroencephalographic, and genetic findings in these syndromes.

Interstitial deletion 5q14.3q21.3 with MEF2C haploinsufficiency and mild phenotype: when more is less

An 18-year-old female with mild mental disability (global IQ 69), febrile seizures with subsequent myoclonic/grand mal epilepsy, and subtle morphologic changes is described with del 5(q14.3q21.3) by karyotype and minimal DNA deletion of 21.08 Mb by array comparative genomic hybridization microarray analysis (arr chr5:83,592,798-104,671,993 X1) that encompasses at least 50 genes. Included in the deletion interval is the MEF2C gene that usually causes severe mental disability when haploinsufficient, illustrating the complexity of clinic-cytogenetic correlation even with defined segmental aneuploidy. Interaction of MEF2C with the deleted febrile seizure (FEB4) and juveline myoclonic epilepsy (EJM4) loci plus the G-protein receptor (GPR98/MASS1/Usher syndrome) gene may moderate the phenotype, perhaps through common regulation by calcium.

A locus for juvenile myoclonic epilepsy maps to 2q33-q36

We performed a whole genome linkage analysis in a three-generation south Indian family with multiple members affected with juvenile myoclonic epilepsy (JME). The maximum two-point LOD score obtained was 3.32 at recombination fraction (theta) = 0 for D2S2248. The highest multipoint score of 3.59 was observed for the genomic interval between D2S2322 and D2S2228 at the chromosomal region 2q33-q36. Proximal and distal boundaries of the critical genetic interval were defined by D2S116 and D2S2390, respectively. A 24-Mb haplotype was found to co-segregate with JME in the family. While any potentially causative variant in the functional candidate genes, SLC4A3, SLC23A3, SLC11A1 and KCNE4, was not detected, we propose to examine brain-expressed NRP2, MAP2, PAX3, GPR1, TNS1 and DNPEP, and other such positional candidate genes to identify the disease-causing gene for the disorder.

Genetic linkage study of an autosomal recessive form of juvenile myoclonic epilepsy in a consanguineous Tunisian family

Juvenile myoclonic epilepsy (JME) is the most common idiopathic generalized epilepsies (IGEs), affecting 12-30% of all epilepsies in medical centers. To date genetic linkage studies have revealed putative loci on different chromosomes, but these findings are still inconclusive about which gene precisely is responsible for the disease. Here, we report the genetic and clinical analysis of a (JME) consanguineous Tunisian family with four affected children out of eight. A genome-wide search was carried out by using the Affymetrix GeneChip Mapping 500K NspI chip. Pairewise logarithm of the odds (LOD) scores were calculated with MERLIN (1.1) assuming an autosomal recessive model, and a complementary homozygous mapping analysis was performed with AutoSNPa software. The genome-wide parametric linkage analysis showed suggestive linkage to chromosome 2q. Interactive visual analysis of SNP data using AutoSNPa revealed two large regions of shared homozygosity by descent on 2q23.3 and on 2q24.1. We decided to sequence the exons of the two genes coding for such proteins located in 2q23.3, CACNB4 and 2q24.1, KCNJ3. No nucleotide variation--comprising the previously reported mutations--was detected.

Exploring Candidate Genes for Epilepsy by Computational Disease-Gene Identification Strategy

Epilepsy is a complex disease with a strong genetic component. So far, studies have focused on experimental validation or genome-wide linkage scans for epilepsy susceptibility genes in multiple populations. We have used four bioinformatic tools (SNPs3D, PROSPECTR and SUSPECTS, GenWanderer, PosMed) to analyze 16 susceptibility loci selected from a literature search. Pathways and regulatory network analyses were performed using the Ingenuity Pathways Analysis (IPA) software. We identified a subset of 48 candidate epilepsy susceptibility genes. Five significant canonical pathways, in four typical networks, were identified: GABA receptor signaling, interleukin-6 (IL-6) signaling, G-protein coupled receptor signaling, type 2 diabetes mellitus signaling and airway inflammation in asthma. We concluded that online analytical tools provide a powerful way to reveal candidate genes which can greatly reduce experimental time. Our study contributes to further experimental tests for epilepsy susceptibility genes.