The Lafora disease gene product laforin interacts with HIRIP5, a phylogenetically conserved protein containing a NifU-like domain

Lafora disease is an autosomal recessive type of progressive myoclonus epilepsy caused by mutations in the EPM2A gene. The EPM2A gene-encoded protein laforin is a dual-specificity phosphatase that associates with polyribosomes. Because the cellular functions of laforin are largely unknown, we used the yeast-two hybrid system to screen for protein(s) that interact with laforin. We found that laforin interacts with a phylogenetically conserved protein HIRIP5 that harbors a NifU-like domain. Both in vitro and in vivo assay have shown that the interaction is specific and that laforin probably uses its N-terminal CBD-4 domain to interact with the C-terminal NifU-like domain of the HIRIP5 protein. HIRIP5 encodes a cytosolic protein and is expressed ubiquitously, perhaps reflecting a house-keeping function. The presence of a NifU-like domain in the HIRIP5 protein raises an interesting possibility that it may be involved in iron homeostasis. Although the significance of the interaction between HIRIP5 and laforin proteins is not yet fully known, because laforin dephosphorylated HIRIP5 in vitro, HIRIP5 promises to be an interesting laforin-binding partner and would contribute to the understanding of the molecular pathology of Lafora disease.

Genetic mapping of a new Lafora progressive myoclonus epilepsy locus (EPM2B) on 6p22

BACKGROUND

Lafora disease is a progressive myoclonus epilepsy with polyglucosan accumulations and a peculiar neurodegeneration with generalised organellar disintegration. It causes severe seizures, leading to dementia and eventually death in early adulthood.

METHODS

One Lafora disease gene, EPM2A, has been identified on chromosome 6q24. Locus heterogeneity led us to search for a second gene using a genome wide linkage scan in French-Canadian families.

RESULTS

We mapped a second Lafora disease locus, EPM2B, to a 2.2 Mb region at 6p22, a region known to code for several proteins, including kinesins. Kinesins are microtubule dependent motor proteins that are involved in transporting cellular components. In neurones, they play a major role in axonal and dendritic transport.

CONCLUSION

Analysis of the present locus in other non-EPM2A families will reveal whether there is further locus heterogeneity. Identification of the disease gene will be of major importance towards our understanding of the pathogenesis of Lafora disease.

Juvenile myoclonic epilepsy: linkage to chromosome 6p12 in Mexico families

Juvenile myoclonic epilepsy is a common subtype of idiopathic epilepsy accounting for 4-11% of all epilepsies. We reported previously significant evidence of linkage between chromosome 6p12-11 microsatellites and the clinical epilepsy and EEG traits of JME families from Belize and Los Angeles. To narrow the JME region, we ascertained and genotyped 31 new JME families from Mexico using a later generation of Généthon microsatellites. Two point linkage analyses obtained significant Z(max) values of 3.70 for D6S1573 and 2.65 for D6S1714 at theta(m = f) = 0.10, and 3.49 for D6S465, 2.11 for D6S1960 at theta(m = f) = 0.05 assuming autosomal dominant inheritance with 70% age-dependent penetrance. Multipoint LOD score curve peaked at 4.21 for D6S1573. Haplotype and recombination analysis reduced the JME region to 3.5 cM flanked by D6S272 and D6S1573. These results provide confirmatory evidence that a major susceptibility gene for JME exists in chromosome 6p12 in Spanish-Amerinds of Mexico.

Identification and mutational analysis of candidate genes for juvenile myoclonic epilepsy on 6p11-p12: LRRC1, GCLC, KIAA0057 and CLIC5

Juvenile myoclonic epilepsy (JME) is one of the most frequent hereditary epilepsies characterized by myoclonic and tonic-clonic convulsions beginning at 8-20 years of age. Genetic studies have revealed four major chromosomal loci on 6p21.3, 6p11-12, 6q24, and 15q14 as candidate regions harboring genes responsible for JME. Previously we reported the region on 6p11-p12 (EJM1), and here we report the identification and mutational analysis of candidate genes for EJM1. One of those is a leucine-rich repeat-containing 1 (LRRC1) gene that is composed of 14 exons and codes for 524 amino acid residues. In Northern analysis, 7 kb transcripts of LRRC1 gene were detected in multiple tissues, most strongly, in heart, lung, and kidney. Mutation analysis of LRRC1 gene in 20 JME patients from ten families revealed one nucleotide substitution that lead to amino acid exchange (c.577 A>G; Ile193Val). This variation, however, did not co-segregate with the disease phenotype. We further performed mutational analyses of CLIC5, KIAA0057 and GCLC genes in or flank to the EJM1 region. These analyses did not provide any evidences that these genes are responsible for the JME phenotype, and suggested that these may not be the EJM1 gene.

[Juvenile myoclonic epilepsy in chromosome 6p12: clinical and genetic advances]

Amongst idiopathic generalized epilepsies, juvenile myoclonic epilepsy (JME) is the most common, accounting for 12% to 30% of all epilepsies in the Western world. Classic JME consists of awakening myoclonias, grand mal convulsions and EEG 4 to 6 Hz polyspike waves that appear in adolescence. Probands and affected family members do not have pyknoleptic 3Hz spike and wave absences. However, in 10 to 30% of patients, rare or spanioleptic polyspike wave absences appear. In 1988,1995,1996,we mapped classic JME to a 7 cM locus in chromosome 6p12 11, called EJM1, using families from Los Angeles and Belize. In 2001,we studied one large family from Belize and 21 new families from Los Angeles and Mexico Cities, aided by a BAC/PAC based physical map and 6 new dinucleotide repeats, to narrow EJM1 to an interval between D6S272 and D6S1573. In 2002, we found myoclonin, the putative gene for typical JME in 6p12. At the congress, we will reveal the identity of the myoclonin gene, its putative function and discuss the significance of this discovery in the JME population at large.

The Use of 2-Deoxy-2-[18F]Fluoro- D-Glucose (FDG-PET) Positron Emission Tomography in the Routine Diagnosis of Epilepsy

PURPOSE

Positron emission tomography with 2-deoxy fluoroglucose positron emission tomography (18-FDG-PET) is widely used in the pre-surgical evaluation of subjects with epilepsy, but little is known of its usefulness in a non-surgical population.

PROCEDURES

We analyzed the sensitivity of PET as a diagnostic tool in a large unselected population of epilepsy subjects. Pre-surgical and non-surgical portions of this population were individually assessed as well. The relationship of PET abnormalities to other neurodiagnostic tests was examined. Statistical assessment relied primarily on contingency tables (chi-square tests), with ANOVA or non-parametric assessment used as necessary.

RESULTS

While PET was more likely to identify areas of decreased metabolism in the surgical population than in the non-surgical populations, it nevertheless found a significant number of abnormalities in the total population and in the non-surgical group alone. Even in groups in which the clinical diagnosis was unknown, abnormalities were found 40% of the time. PET was useful as an exclusionary diagnostic tool for non-epileptic seizures (NES) and primary generalized epilepsies (PGE) with sensitivity, specificity, and accuracy > 90%. The PET was somewhat more sensitive than magnetic resonance imaging (MRI) in finding abnormalities in the total population, but was less sensitive than electroencephalography (EEG).

CONCLUSION

PET may be a useful diagnostic tool in the general epilepsy population even when a definitive clinical diagnosis is not suggested by other modalities.

Genotype-phenotype correlations for EPM2A mutations in Lafora's progressive myoclonus epilepsy: exon 1 mutations associate with an early-onset cognitive deficit subphenotype

Mutations in the EPM2A gene encoding a dual-specificity phosphatase (laforin) cause an autosomal recessive fatal disorder called Lafora's disease (LD) classically described as an adolescent-onset stimulus-sensitive myoclonus, epilepsy and neurologic deterioration. Here we related mutations in EPM2A with phenotypes of 22 patients (14 families) and identified two subsyndromes: (i) classical LD with adolescent-onset stimulus-sensitive grand mal, absence and myoclonic seizures followed by dementia and neurologic deterioration, and associated mainly with mutations in exon 4 (P = 0.0007); (ii) atypical LD with childhood-onset dyslexia and learning disorder followed by epilepsy and neurologic deterioration, and associated mainly with mutations in exon 1 (P = 0.0015). To understand the two subsyndromes better, we investigated the effect of five missense mutations in the carbohydrate-binding domain (CBD-4; coded by exon 1) and three missense mutations in the dual phosphatase domain (DSPD; coded by exons 3 and 4) on laforin's intracellular localization in HeLa cells. Expression of three mutant proteins (T194I, G279S and Y294N) in DSPD formed ubiquitin-positive cytoplasmic aggregates, suggesting that they were folding mutants set for degradation. In contrast, none of the three CBD-4 mutants showed cytoplasmic clumping. However, CBD-4 mutants W32G and R108C targeted both cytoplasm and nucleus, suggesting that laforin had diminished its usual affinity for polysomes. Our data, thus, represent the first report of a novel childhood syndrome for LD. Our results also provide clues for distinct roles for the CBD-4 and DSP domains of laforin in the etiology of two subsyndromes of LD.

Targeted disruption of the Epm2a gene causes formation of Lafora inclusion bodies, neurodegeneration, ataxia, myoclonus epilepsy and impaired behavioral response in mice

Mutations in the EPM2A gene encoding a dual-specificity phosphatase (laforin) cause Lafora disease (LD), a progressive and invariably fatal epilepsy with periodic acid-Schiff-positive (PAS+) cytoplasmic inclusions (Lafora bodies) in the central nervous system. To study the pathology of LD and the functions of laforin, we disrupted the Epm2a gene in mice. At two months of age, homozygous null mutants developed widespread degeneration of neurons, most of which occurred in the absence of Lafora bodies. Dying neurons characteristically exhibit swelling in the endoplasmic reticulum, Golgi networks and mitochondria in the absence of apoptotic bodies or fragmentation of DNA. As Lafora bodies become more prominent at 4-12 months, organelles and nuclei are disrupted. The Lafora bodies, present both in neuronal and non-neural tissues, are positive for ubiquitin and advanced glycation end-products only in neurons, suggesting different pathological consequence for Lafora inclusions in neuronal tissues. Neuronal degeneration and Lafora inclusion bodies predate the onset of impaired behavioral responses, ataxia, spontaneous myoclonic seizures and EEG epileptiform activity. Our results suggest that LD is a primary neurodegenerative disorder that may utilize a non-apoptotic mechanism of cell death.

Advances in the genetics of progressive myoclonus epilepsy

The genetic progressive myoclonus epilepsies (PMEs) are clinically characterized by the triad of stimulus sensitive myoclonus (segmental lightning like muscular jerks), epilepsy (grand mal and absences) and progressive neurologic deterioration (dementia, ataxia, and various neurologic signs depending on the cause). Etiologically heterogenous, PMEs are rare and mostly autosomal recessive disorders, with the exception of autosomal dominant dentatorubral-pallidoluysian atrophy and mitochondrial encephalomyopathy with ragged red fibers (MERRF). In the last five years, specific mutations have been defined in Lafora disease (gene for laforin or dual specificity phosphatase in 6q24), Unverricht-Lundborg disease (cystatin B in 21q22.3), Jansky-Bielschowsky ceroid lipofuscinoses (CLN2 gene for tripeptidyl peptidase 1 in 11q15), Finnish variant of late infantile ceroid lipofuscinoses (CLN5 gene in 13q21-32 encodes 407 amino acids with two transmembrane helices of unknown function), juvenile ceroid lipofuscinoses or Batten disease (CLN3 gene in 16p encodes 438 amino acid protein of unknown function), a subtype of Batten disease and infantile ceroid lipofuscinoses of the Haltia-Santavuori type (both are caused by mutations in palmitoyl-protein thiosterase gene at 1p32), dentadorubropallidoluysian atrophy (CAG repeats in a gene in 12p13.31) and the mitochondrial syndrome MERRF (tRNA Lys mutation in mitochondrial DNA). In this review, we cover mainly these rapid advances.

A novel gene in the chromosomal region for juvenile myoclonic epilepsy on 6p12 encodes a brain-specific lysosomal membrane protein

Juvenile myoclonic epilepsy (JME) is the most frequent and, hence, most important form of hereditary grand mal epilepsy. Genetic linkage, haplotype, and recombination analyses have indicated that 6p11-12 (EJM1) is one of the candidate regions harboring a gene responsible for JME. In efforts to identify a gene responsible for JME, we identified several expressed sequences in the EJM1 critical region. Here we report the identification and characterization of a gene, named C6orf33, in the EJM1 region. Northern blot analysis showed that C6orf33 is predominantly expressed in brain but in mice, testis shows additional transcripts. C6orf33 is predicted to encode a novel approximately 40-kDa membrane protein, LMPB1, that defines a novel protein family by having highly conserved orthologs in eukaryotes and three putative paralogs in human. Biochemical and immunocytochemical studies revealed that LMPB1 is indeed an integral membrane protein that targets to lysosomal structures. LMPB1 may be involved in specialized lysosomal functions that are unique to brain and testis, and the C6orf33 gene is of interest as a candidate for EJM1.

T-STAR gene: fine mapping in the candidate region for childhood absence epilepsy on 8q24 and mutational analysis in patients

Childhood absence epilepsy (CAE) is one of the most common epilepsies in children. At least four phenotypic subcategories of CAE have been proposed. Among them, a subtype persisting with tonic-clonic seizures has been mapped to 8q24 (ECA1 MIM 600131). By constructing a physical map for the 8q24 region, we recently narrowed the ECA1 locus to a 1.5-Mb region. In the present communication, we show that T-STAR gene is located within the ECA1 region. T-STAR is a novel member of STAR (for signal transduction and activation of RNA) family, and is predicted to encode a spermatogenesis related RNA-binding protein. T-STAR is located within the markers D8S2049 and D8S1753 and its complete coding region spans nine exons. In addition to its known expression in testis, moderate level of transcripts for T-STAR gene was detected in brain, heart and is highly abundant in skeletal muscle. Mutational analysis for the T-SATR gene in CAE families did not show any sequence variation in the coding region, and this suggests that the T-STAR gene is not involved in the pathogenesis of persisting CAE. However, genomic organization of T-STAR gene characterized in the present report might help in understanding the biological functions of T-STAR as well as its suspected involvement in other disorders mapped on this region.

Regional and developmental expression of Epm2a gene and its evolutionary conservation

Lafora's disease, an autosomal recessive progressive myoclonus epilepsy, is caused by mutations in the EPM2A gene encoding a dual-specificity phosphatase (DSP) named laforin. Here, we analyzed the developmental and regional expression of murine Epm2a and discussed its evolutionary conservation. A phylogenetic analysis indicated that laforin is evolutionarily distant from other DSPs. Southern zoo blot analysis suggested that conservation of Epm2a gene is limited to mammals. Laforin orthologs (human, mouse, and rat) display more than 94% similarity. All missense mutations known in Lafora disease patients affect conserved residues, suggesting that they may be essential for laforin's function. Epm2a is expressed widely in various organs but not homogeneously in brain. The levels of Epm2a transcripts in mice brains increase postnatally, attaining its highest level in adults. The most intense signal was detected in the cerebellum, hippocampus, cerebral cortex, and the olfactory bulb. Our results suggest that Epm2a is functionally conserved in mammals and is involved in growth and maturation of neural networks.

Laforin, defective in the progressive myoclonus epilepsy of Lafora type, is a dual-specificity phosphatase associated with polyribosomes

The progressive myoclonus epilepsy of Lafora type is an autosomal recessive disorder caused by mutations in the EPM2A gene. EPM2A is predicted to encode a putative tyrosine phosphatase protein, named laforin, whose full sequence has not yet been reported. In order to understand the function of the EPM2A gene, we isolated a full-length cDNA, raised an antibody and characterized its protein product. The full-length clone predicts a 38 kDa laforin that was very close to the size detected in transfected cells. Recombinant laforin was able to hydrolyze phosphotyrosine as well as phosphoserine/threonine substrates, demonstrating that laforin is an active dual-specificity phosphatase. Biochemical, immunofluorescence and electron microscopic studies on the full-length laforin expressed in HeLa cells revealed that laforin is a cytoplasmic protein associated with polyribosomes, possibly through a conformation-dependent protein-protein interaction. We analyzed the intracellular targeting of two laforin mutants with missense mutations. Expression of both mutants resulted in ubiquitin-positive perinuclear aggregates suggesting that they were misfolded proteins targeted for degradation. Our results suggest that laforin is involved in translational regulation and that protein misfolding may be one of the molecular bases of the Lafora disease phenotype caused by missense mutations in the EPM2A gene.

Childhood absence epilepsy in 8q24: refinement of candidate region and construction of physical map

Childhood absence epilepsy (CAE), one of the common idiopathic generalized epilepsies, accounts for 8 to 15% of all childhood epilepsies. Inherited as an autosomal dominant trait, frequent absence attacks start in early or midchildhood and disappear by 30 years of age or may persist through life. Recently, we mapped the locus for CAE persisting with tonic-clonic seizures to chromosome 8q24 (ECA1) by genetic linkage analysis. As a further step in the identification of the ECA1 gene, we constructed a bacterial artificial chromosome- and yeast artificial chromosome-based physical map for the 8q24 region, spanning about 3 Mb between D8S1710 and D8S523. Accurately ordered STS markers within the physical map aided in the analysis of haplotypes and recombinations and reduced the ECA1 region to 1.5 Mb flanked by D8S554 and D8S502. Pairwise analysis in six families confirmed linkage with a pooled lod score of 4.10 (θ = 0) at D8S534. The sequence-ready physical map as well as the narrowed candidate region described here should contribute to the identification of the ECA1 gene.

Mutation spectrum and predicted function of laforin in Lafora's progressive myoclonus epilepsy

BACKGROUND

Lafora's disease is a progressive myoclonus epilepsy with pathognomonic inclusions (polyglucosan bodies) caused by mutations in the EPM2A gene. EPM2A codes for laforin, a protein with unknown function. Mutations have been reported in the last three of the gene's exons. To date, the first exon has not been determined conclusively. It has been predicted based on genomic DNA sequence analysis including comparison with the mouse homologue.

OBJECTIVES

1) To detect new mutations in exon 1 and establish the role of this exon in Lafora's disease. 2) To generate hypotheses about the biological function of laforin based on bioinformatic analyses.

METHODS

1) PCR conditions and components were refined to allow amplification and sequencing of the first exon of EPM2A. 2) Extensive bioinformatic analyses of the primary structure of laforin were completed.

RESULTS

1) Seven new mutations were identified in the putative exon 1. 2) Laforin is predicted not to localize to the cell membrane or any of the organelles. It contains all components of the catalytic active site of the family of dual-specificity phosphatases. It contains a sequence predicted to encode a carbohydrate binding domain (coded by exon 1) and two putative glucohydrolase catalytic sites.

CONCLUSIONS

The identification of mutations in exon 1 of EPM2A establishes its role in the pathogenesis of Lafora's disease. The presence of potential carbohydrate binding and cleaving domains suggest a role for laforin in the prevention of accumulation of polyglucosans in healthy neurons.

Identification of new and common mutations in the EPM2A gene in Lafora disease

Lafora disease is a teenage onset progressive myoclonus epilepsy caused by mutations in the EPM2A gene. In this report, we describe new mutations within EPM2A, review the known mutations to date to identify the most common, and describe three simple tests for prenatal and carrier screening.

Exclusion of the JRK/JH8 gene as a candidate for human childhood absence epilepsy mapped on 8q24

Childhood absence epilepsy (CAE), one of the most common epilepsies in children, is genetically and phenotypically heterogeneous. One of the genes responsible for human CAE associated with tonic-clonic seizures has been mapped to chromosome band 8q24 by genetic linkage analysis and is termed ECA1. Recently, we isolated and mapped the JRK/JH8 gene, a human homologue of the mouse epilepsy gene, jerky, on 8q24. The epilepsy phenotype of the mice with inactivated jerky gene as well as its chromosomal localization proposed JRK/JH8 as a prominent candidate for the CAE gene. To confirm whether the JRK/JH8 gene is responsible for ECA1, we performed mutational analyses in the coding region of JRK/JH8 in two CAE families mapped on 8q24, using heteroduplex and direct sequencing methods. We identified seven nucleotide changes, two of which lead to amino acid substitutions. However, these changes did not co-segregate with the disease phenotype. In addition, we redefined the location of JRK/JH8 to be more than 4 Mb distant from D8S502 and ECA1. Thus, negative results of mutation analyses and detailed physical mapping exclude JRK/JH8 as the ECA1 gene.