Impairment of ceramide synthesis causes a novel progressive myoclonus epilepsy.

OBJECTIVE: Alterations of sphingolipid metabolism are implicated in the pathogenesis of many neurodegenerative disorders. METHODS: We identified a homozygous nonsynonymous mutation in CERS1, the gene encoding ceramide synthase 1, in 4 siblings affected by a progressive disorder with myoclonic epilepsy and dementia. CerS1, a transmembrane protein of the endoplasmic reticulum (ER), catalyzes the biosynthesis of C18-ceramides. RESULTS: We demonstrated that the mutation decreases C18-ceramide levels. In addition, we showed that downregulation of CerS1 in a neuroblastoma cell line triggers ER stress response and induces proapoptotic pathways. INTERPRETATION: This study demonstrates that impairment of ceramide biosynthesis underlies neurodegeneration in humans.

Novel compound heterozygous mutations in TBC1D24 cause familial malignant migrating partial seizures of infancy.

Early-onset epileptic encephalopathies (EOEEs) are a group of rare devastating epileptic syndromes of infancy characterized by severe drug-resistant seizures and electroencephalographic abnormalities. The current study aims to determine the genetic etiology of a familial form of EOEE fulfilling the diagnosis criteria for malignant migrating partial seizures of infancy (MMPSI). We identified two inherited novel mutations in TBC1D24 in two affected siblings. Mutations severely impaired TBC1D24 expression and function, which is critical for maturation of neuronal circuits. The screening of TBC1D24 in an additional set of eight MMPSI patients was negative. TBC1D24 loss of function has been associated to idiopathic infantile myoclonic epilepsy, as well as to drug-resistant early-onset epilepsy with intellectual disability. Here, we describe a familial form of MMPSI due to mutation in TBC1D24, revealing a devastating epileptic phenotype associated with TBC1D24 dysfunction.

TBC1D24, an ARF6-interacting protein, is mutated in familial infantile myoclonic epilepsy.

Idiopathic epilepsies (IEs) are a group of disorders characterized by recurrent seizures in the absence of detectable brain lesions or metabolic abnormalities. IEs include common disorders with a complex mode of inheritance and rare Mendelian traits suggesting the occurrence of several alleles with variable penetrance. We previously described a large family with a recessive form of idiopathic epilepsy, named familial infantile myoclonic epilepsy (FIME), and mapped the disease locus on chromosome 16p13.3 by linkage analysis. In the present study, we found that two compound heterozygous missense mutations (D147H and A509V) in TBC1D24, a gene of unknown function, are responsible for FIME. In situ hybridization analysis revealed that Tbc1d24 is mainly expressed at the level of the cerebral cortex and the hippocampus. By coimmunoprecipitation assay we found that TBC1D24 binds ARF6, a Ras-related family of small GTPases regulating exo-endocytosis dynamics. The main recognized function of ARF6 in the nervous system is the regulation of dendritic branching, spine formation, and axonal extension. TBC1D24 overexpression resulted in a significant increase in neurite length and arborization and the FIME mutations significantly reverted this phenotype. In this study we identified a gene mutation involved in autosomal-recessive idiopathic epilepsy, unveiled the involvement of ARF6-dependent molecular pathway in brain hyperexcitability and seizures, and confirmed the emerging role of subtle cytoarchitectural alterations in the etiology of this group of common epileptic disorders.

A novel pedigree with familial cortical myoclonic tremor and epilepsy (FCMTE): clinical characterization, refinement of the FCMTE2 locus, and confirmation of a founder haplotype.

PURPOSE: We describe the clinical, neurophysiologic, and genetic features of a new, large family with familial cortical myoclonic tremor and epilepsy (FCMTE). METHODS: Reliable clinical information was obtained on the 127 members. Thirty-one collaborative individuals were assessed by a detailed clinical interview and a complete neurologic examination. A polygraphic study was conducted in 15 patients, back-averaging analysis and somatosensory evoked potentials with C-reflex study in four. The genetic study investigated 30 subjects with microsatellite markers at three loci on chromosomes 8q (FCMTE1), 2p (FCMTE2), and 5p (FCMTE3). KEY FINDINGS: The pedigree included 25 affected members (M/F: 9/16). We studied 16 of the 19 living affected members (M/F: 5/11; mean age 47.8 years). Cortical myoclonic tremor (CMT) was associated with generalized seizures in 10 patients (62.5%). The mean age at onset of CMT and seizures was 28.1 and 33.8 years, respectively. Four patients (25%) reported a slow progression of CMT, with severe gait impairment in one. Psychiatric disorders of variable severity recurred in 37.5% of cases. Rhythmic bursts at 7-15 Hz were recorded in all 11 affected members tested. Additional neurophysiologic investigations disclosed a cortical origin of myoclonus in all patients tested. Generalized epileptiform discharges were recorded in 25% of cases, and a photoparoxysmal response in 31%. Genetic analysis established linkage to the FCMTE2 locus on chromosome 2p11.1-2q12.2 (OMIM 607876) and narrowed the critical interval to a 10.4 Mb segment. Haplotype analysis in the present family identified a founder haplotype identical to that previously observed in families from the same geographic area. SIGNIFICANCE: This study confirms evidence of a founder effect in Italian families and reduces the number of positional candidate genes in the FCMTE2 locus to 59, thereby contributing to future gene identification by Next Generation Sequencing approaches.

Genetic testing in benign familial epilepsies of the first year of life: clinical and diagnostic significance.

PURPOSE: To dissect the genetics of benign familial epilepsies of the first year of life and to assess the extent of the genetic overlap between benign familial neonatal seizures (BFNS), benign familial neonatal-infantile seizures (BFNIS), and benign familial infantile seizures (BFIS). METHODS: Families with at least two first-degree relatives affected by focal seizures starting within the first year of life and normal development before seizure onset were included. Families were classified as BFNS when all family members experienced neonatal seizures, BFNIS when the onset of seizures in family members was between 1 and 4 months of age or showed both neonatal and infantile seizures, and BFIS when the onset of seizures was after 4 months of age in all family members. SCN2A, KCNQ2, KCNQ3, PPRT2 point mutations were analyzed by direct sequencing of amplified genomic DNA. Genomic deletions involving KCNQ2 and KCNQ3 were analyzed by multiple-dependent probe amplification method. KEY FINDINGS: A total of 46 families including 165 affected members were collected. Eight families were classified as BFNS, 9 as BFNIS, and 29 as BFIS. Genetic analysis led to the identification of 41 mutations, 14 affecting KCNQ2, 1 affecting KCNQ3, 5 affecting SCN2A, and 21 affecting PRRT2. The detection rate of mutations in the entire cohort was 89%. In BFNS, mutations specifically involve KCNQ2. In BFNIS two genes are involved (KCNQ2, six families; SCN2A, two families). BFIS families are the most genetically heterogeneous, with all four genes involved, although about 70% of them carry a PRRT2 mutation. SIGNIFICANCE: Our data highlight the important role of KCNQ2 in the entire spectrum of disorders, although progressively decreasing as the age of onset advances. The occurrence of afebrile seizures during follow-up is associated with KCNQ2 mutations and may represent a predictive factor. In addition, we showed that KCNQ3 mutations might be also involved in families with infantile seizures. Taken together our data indicate an important role of K-channel genes beyond the typical neonatal epilepsies. The identification of a novel SCN2A mutation in a family with infantile seizures with onset between 6 and 8 months provides further confirmation that this gene is not specifically associated with BFNIS and is also involved in families with a delayed age of onset. Our data indicate that PRRT2 mutations are clustered in families with BFIS. Paroxysmal kinesigenic dyskinesia emerges as a distinctive feature of PRRT2 families, although uncommon in our series. We showed that the age of onset of seizures is significantly correlated with underlying genetics, as about 90% of the typical BFNS families are linked to KCNQ2 compared to only 3% of the BFIS families, for which PRRT2 represents the major gene.

Autosomal recessive progressive myoclonus epilepsy due to impaired ceramide synthesis.

Autosomal recessive progressive myoclonus epilepsy due to impaired ceramide synthesis is an extremely rare condition, so far reported in a single family of Algerian origin presenting an unusual, severe form of progressive myoclonus epilepsy characterized by myoclonus, generalized tonic-clonic seizures and moderate to severe cognitive impairment, with probable autosomal recessive inheritance. Disease onset was between 6 and 16 years of age. Genetic study allowed to identify a homozygous nonsynonymous mutation in CERS1, the gene encoding ceramide synthase 1, a transmembrane protein of the endoplasmic reticulum (ER), catalyzes the biosynthesis of C18-ceramides. The mutation decreased C18-ceramide levels. In addition, downregulation of CerS1 in neuroblastoma cell line showed activation of ER stress response and induction of proapoptotic pathways. This observation demonstrates that impairment of ceramide biosynthesis underlies neurodegeneration in humans.