Mutations in NHLRC1 cause progressive myoclonus epilepsy

Lafora progressive myoclonus epilepsy is characterized by pathognomonic endoplasmic reticulum (ER)-associated polyglucosan accumulations. We previously discovered that mutations in EPM2A cause Lafora disease. Here, we identify a second gene associated with this disease, NHLRC1 (also called EPM2B), which encodes malin, a putative E3 ubiquitin ligase with a RING finger domain and six NHL motifs. Laforin and malin colocalize to the ER, suggesting they operate in a related pathway protecting against polyglucosan accumulation and epilepsy.

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.

Laforin, the most common protein mutated in Lafora disease, regulates autophagy

Lafora disease (LD) is an autosomal recessive, progressive myoclonus epilepsy, which is characterized by the accumulation of polyglucosan inclusion bodies, called Lafora bodies, in the cytoplasm of cells in the central nervous system and in many other organs. However, it is unclear at the moment whether Lafora bodies are the cause of the disease, or whether they are secondary consequences of a primary metabolic alteration. Here we describe that the major genetic lesion that causes LD, loss-of-function of the protein laforin, impairs autophagy. This phenomenon is confirmed in cell lines from human patients, mouse embryonic fibroblasts from laforin knockout mice and in tissues from such mice. Conversely, laforin expression stimulates autophagy. Laforin regulates autophagy via the mammalian target of rapamycin kinase-dependent pathway. The changes in autophagy mediated by laforin regulate the accumulation of diverse autophagy substrates and would be predicted to impact on the Lafora body accumulation and the cell stress seen in this disease that may eventually contribute to cell death.

Abnormal metabolism of glycogen phosphate as a cause for Lafora disease

Lafora disease is a progressive myoclonus epilepsy with onset in the teenage years followed by neurodegeneration and death within 10 years. A characteristic is the widespread formation of poorly branched, insoluble glycogen-like polymers (polyglucosan) known as Lafora bodies, which accumulate in neurons, muscle, liver, and other tissues. Approximately half of the cases of Lafora disease result from mutations in the EPM2A gene, which encodes laforin, a member of the dual specificity protein phosphatase family that is able to release the small amount of covalent phosphate normally present in glycogen. In studies of Epm2a(-/-) mice that lack laforin, we observed a progressive change in the properties and structure of glycogen that paralleled the formation of Lafora bodies. At three months, glycogen metabolism remained essentially normal, even though the phosphorylation of glycogen has increased 4-fold and causes altered physical properties of the polysaccharide. By 9 months, the glycogen has overaccumulated by 3-fold, has become somewhat more phosphorylated, but, more notably, is now poorly branched, is insoluble in water, and has acquired an abnormal morphology visible by electron microscopy. These glycogen molecules have a tendency to aggregate and can be recovered in the pellet after low speed centrifugation of tissue extracts. The aggregation requires the phosphorylation of glycogen. The aggregrated glycogen sequesters glycogen synthase but not other glycogen metabolizing enzymes. We propose that laforin functions to suppress excessive glycogen phosphorylation and is an essential component of the metabolism of normally structured glycogen.

Progressive myoclonic epilepsies: a review of genetic and therapeutic aspects

The progressive myoclonic epilepsies (PMEs) are a group of symptomatic generalised epilepsies caused by rare disorders, most of which have a genetic component, a debilitating course, and a poor outcome. Challenges with PME arise from difficulty with diagnosis, especially in the early stages of the illness, and further problems of management and drug treatment. Recent advances in molecular genetics have helped achieve better understanding of the different disorders that cause PME. We review the PMEs with emphasis on updated genetics, diagnosis, and therapeutic options.

The phosphatase laforin crosses evolutionary boundaries and links carbohydrate metabolism to neuronal disease

Lafora disease (LD) is a progressive myoclonic epilepsy resulting in severe neurodegeneration followed by death. A hallmark of LD is the accumulation of insoluble polyglucosans called Lafora bodies (LBs). LD is caused by mutations in the gene encoding the phosphatase laforin, which reportedly exists solely in vertebrates. We utilized a bioinformatics screen to identify laforin orthologues in five protists. These protists evolved from a progenitor red alga and synthesize an insoluble carbohydrate whose composition closely resembles LBs. Furthermore, we show that the kingdom Plantae, which lacks laforin, possesses a protein with laforin-like properties called starch excess 4 (SEX4). Mutations in the Arabidopsis thaliana SEX4 gene results in a starch excess phenotype reminiscent of LD. We demonstrate that Homo sapiens laforin complements the sex4 phenotype and propose that laforin and SEX4 are functional equivalents. Finally, we show that laforins and SEX4 dephosphorylate a complex carbohydrate and form the only family of phosphatases with this activity. These results provide a molecular explanation for the etiology of LD.

Lafora's disease: towards a clinical, pathologic, and molecular synthesis

Lafora's disease is one of five inherited progressive myoclonus epilepsy syndromes. It is an autosomal-recessive disorder with onset in late childhood or adolescence. Characteristic seizures include myoclonic and occipital lobe seizures with visual hallucinations, scotomata, and photoconvulsions. The course of the disease consists of worsening seizures and an inexorable decline in mental and other neurologic functions that result in dementia and death within 10 years of onset. Pathology reveals pathognomonic polyglucosan inclusions that are not seen in any other progressive myoclonus epilepsy. Lafora's disease is one of several neurologic conditions associated with brain polyglucosan bodies. Why Lafora's polyglucosan bodies alone are associated with epilepsy is unknown and is discussed in this article. Up to 80% of patients with Lafora's disease have mutations in the EPM2A gene. Although common mutations are rare, simple genetic tests to identify most mutations have been established. At least one other still-unknown gene causes Lafora's disease. The EPM2A gene codes for the protein laforin, which localizes at the plasma membrane and the rough endoplasmic reticulum and functions as a dual-specificity phosphatase. Work toward establishing the connection between laforin and Lafora's disease polyglucosans is underway, as are attempts to replace it into the central nervous system of patients with Lafora's disease.

Expanded Repeat in Canine Epilepsy

Epilepsy afflicts 1% of humans and 5% of dogs. We report a canine epilepsy mutation and evidence for the existence of repeat-expansion disease outside humans. A canid-specific unstable dodecamer repeat in the Epm2b (Nhlrc1) gene recurrently expands, causing a fatal epilepsy and contributing to the high incidence of canine epilepsy. Tracing the repeat origins revealed two successive events, starting 50 million years ago, unique to canid evolution. A genetic test, presented here, will allow carrier and presymptomatic diagnosis and disease eradication. Clinicopathologic characterization establishes affected animals as a model for Lafora disease, the most severe teenage-onset human epilepsy.

Recent advances in the molecular basis of Lafora's progressive myoclonus epilepsy

Lafora's disease (LD) is an autosomal recessive and fatal form of progressive myoclonus epilepsy with onset in late childhood or adolescence. LD is characterised by the presence of intracellular polyglucosan inclusions, called Lafora bodies, in tissues including the brain, liver and skin. Patients have progressive neurologic deterioration, leading to death within 10 years of onset. No preventive or curative treatment is available for LD. At least three genes underlie LD, of which two have been isolated and mutations characterised: EPM2A and NHLRC1. The EPM2A gene product laforin is a protein phosphatase while the NHLRC1 gene product malin is an E3 ubiquitin ligase that ubiquitinates and promotes the degradation of laforin. Analyses of the structure and function of these gene products suggest defects in post-translational modification of proteins as the common mechanism that leads to the formation of Lafora inclusion bodies, neurodegeneration and the epileptic phenotype of LD. In this review, we summarise the available information on the genetic basis of LD, and correlate these advances with the rapidly expanding information about the mechanisms of LD gained from studies on both cell biological and animal models. Finally, we also discuss a possible mechanism to explain the locus heterogeneity observed in LD.

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.

Laforin preferentially binds the neurotoxic starch-like polyglucosans, which form in its absence in progressive myoclonus epilepsy

Lafora disease (LD) is a fatal and the most common form of adolescent-onset progressive epilepsy. Fulminant endoplasmic reticulum (ER)-associated depositions of starch-like long-stranded, poorly branched glycogen molecules [known as polyglucosans, which accumulate to form Lafora bodies (LBs)] are seen in neuronal perikarya and dendrites, liver, skeletal muscle and heart. The disease is caused by loss of function of the laforin dual-specificity phosphatase or the malin E3 ubiquitin ligase. Towards understanding the pathogenesis of polyglucosans in LD, we generated a transgenic mouse overexpressing inactivated laforin to trap normal laforin's unknown substrate. The trap was successful and LBs formed in liver, muscle, neuronal perikarya and dendrites. Using immunogold electron microscopy, we show that laforin is found in close proximity to the ER surrounding the polyglucosan accumulations. In neurons, it compartmentalizes to perikaryon and dendrites and not to axons. Importantly, it binds polyglucosans, establishing for the first time a direct association between the disease-defining storage product and disease protein. It preferentially binds polyglucosans over glycogen in vivo and starch over glycogen in vitro, suggesting that laforin's role begins after the appearance of polyglucosans and that the laforin pathway is involved in monitoring for and then preventing the formation of polyglucosans. In addition, we show that the laforin interacting protein, EPM2AIP1, also localizes on the polyglucosan masses, and we confirm laforin's intense binding to LBs in human LD biopsy material.

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.

Lafora disease due to EPM2B mutations: a clinical and genetic study

OBJECTIVE

To study EPM2B gene mutations and genotype-phenotype correlations in patients with Lafora disease.

METHODS

The authors performed a clinical and mutational analysis of 25 patients, from 23 families, diagnosed with Lafora disease who had not shown mutations in the EPM2A gene.

RESULTS

The authors identified 18 mutations in EPM2B, including 12 novel mutations: 4 nonsense mutations (R265X, C26X, W219X, and E67X), a 6-base pair (bp) microdeletion resulting in a two amino acid deletion (V294_K295del), a 4-bp insertion resulting in a frameshift mutation (S339fs12), and 6 missense mutations (D308A, I198N, C68Y, E67Q, P264H, and D233A). In our data set of 77 families with Lafora disease, 54 (70.1%) tested probands have mutations in EPM2A, 21 (27.3%) in EPM2B, and 2 (2.6%) have no mutations in either gene. The course of the disease was longer in patients with EPM2B mutations vs patients with EPM2A mutations.

CONCLUSIONS

Genetic allelic heterogeneity is present in Lafora disease associated with mutations in EPM2B. Patients with mutations in EPM2A and EPM2B express similar clinical manifestation, although patients with EPM2B-associated Lafora disease seem to have a slightly milder clinical course. The lack of mutations in EPM2A and EPM2B in two families could be because of the presence of mutations in noncoding, nontested regions or the existence of an additional gene associated with Lafora disease.

Progressive myoclonus epilepsy with polyglucosans (Lafora disease): evidence for a third locus

Lafora disease (LD) is the most common teenage-onset progressive myoclonus epilepsy. It is caused by recessive mutations in the EPM2A or EPM2B genes. The authors describe a family with three affected members with no mutations in either gene. Linkage and haplotype analyses exclude both loci from causative involvement in this family. Therefore, a third LD locus is predicted. Its identification will be a crucial element in the understanding of the biochemical pathway underlying the generation of Lafora bodies and LD.

Advances in lafora progressive myoclonus epilepsy

Abstract Lafora progressive myoclonus epilepsy is an autosomal recessive, fatal, generalized polyglucosan storage disorder that occurs in childhood or adolescence with stimulus sensitive epilepsy (resting and action myoclonias, grand mal, and absence), dementia, ataxia and rapid neurologic deterioration. Mutations in EPM2A/laforin cause 58% of cases and mutations in EPM2B/malin cause 35% of cases. Accumulating evidence points to Lafora disease as primarily a disorder of cell death with impaired clearance of misfolded proteins, as shown by ubiquitin-positive aggresomes in HeLa cells transfected with mutated laforin, ubiquitin-positive polyglucosan inclusion bodies, and malin/E3 ubiquitin ligase polyubiquitination of laforin. How polyglucosan inclusion bodies accumulate is still a mystery. Polyglucosan accumulates hypothetically because of an overactive polyglucosan biosynthetic pathway or a breakdown in polyglucosan degradation. Five separate laboratories are looking for the biochemical pathways that connect laforin and malin to polyglucosan synthesis or degradation. A curative therapy for human Lafora disease with laforin replacement therapy using neutral pegylated immunoliposomes is being investigated.

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.

Increased endoplasmic reticulum stress and decreased proteasomal function in lafora disease models lacking the phosphatase laforin

Background

Lafora progressive myoclonus epilepsy (Lafora disease; LD) is a fatal autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in either the EPM2A gene, encoding the dual specificity phosphatase laforin, or the EPM2B gene, encoding the E3-ubiquitin ligase malin. Previously, we and others have shown that both proteins form a functional complex that regulates glycogen synthesis by a novel mechanism involving ubiquitination and proteasomal degradation of at least two proteins, glycogen synthase and R5/PTG. Since laforin and malin localized at the endoplasmic reticulum (ER) and their regulatory role likely extend to other proteins unrelated to glycogen metabolism, we postulated that their absence may also affect the ER-unfolded protein response pathway.

Methodology/Principal Findings

Here, we demonstrate that siRNA silencing of laforin in Hek293 and SH-SY5Y cells increases their sensitivity to agents triggering ER-stress, which correlates with impairment of the ubiquitin-proteasomal pathway and increased apoptosis. Consistent with these findings, analysis of tissue samples from a LD patient lacking laforin, and from a laforin knockout (Epm2a-/-) mouse model of LD, demonstrates constitutive high expression levels of ER-stress markers BIP/Grp78, CHOP and PDI, among others.

Conclusions/Significance

We demonstrate that, in addition to regulating glycogen synthesis, laforin and malin play a role protecting cells from ER-stress, likely contributing to the elimination of unfolded proteins. These data suggest that proteasomal dysfunction and ER-stress play an important role in the pathogenesis of LD, which may offer novel therapeutic approaches for this fatal neurodegenerative disorder.

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.

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.

Clinical and Genetic Findings in 26 Italian Patients with Lafora Disease

PURPOSE

EPM2B mutations have been found in a variable proportion of patients with Lafora disease (LD). Genotype-phenotype correlations suggested that EPM2B patients show a slower course of the disease, with delayed age at death, compared with EPM2A patients. We herein report clinical and genetic findings of 26 Italian LD patients.

METHODS

Disease progression was evaluated by means of a disability scale based on residual motor and cognitive functions and daily living and social abilities, at 4 years from the onset. Mutational analysis was performed by sequencing the coding regions of the EPM2A and EPM2B genes.

RESULTS

Age at onset ranged from 8.5 to 18.5 years (mean, 13.7+/-2.6). The mean duration of follow-up was 7.1+/-3.9 years. Daily living activities and social interactions were preserved in five of 24 patients. The remaining patients showed moderate to extremely severe limitations of daily living and social abilities. Sixteen (72%) of 22 families showed mutations in the EPM2B gene, and five (22%), in the EPM2A gene. One family showed no mutations. A novel EPM2B mutation also was identified.

CONCLUSIONS

In our series, EPM2B mutations occurred in 72% of families, thus indicating that EPM2B is the major gene for LD in the Italian population. Moreover, we found that six of 17 EPM2B patients preserved daily living activities and social interactions at 4 years from onset, suggesting a slow disease progression. Additional clinical and functional studies will clarify whether specific mutations may influence the course of the disease in LD patients.

Late-onset and Slow-progressing Lafora Disease in Four Siblings with EPM2B Mutation

We report a family with four brothers affected by Lafora disease (LD). Mean age at onset was 19.5 years (range, 17-21). In all cases, the initial obvious symptoms were diffuse myoclonus and occasional generalized tonic-clonic seizures (GTCSs), followed by cognitive difficulties. Severity of myoclonus, seizure diaries, and neurologic and neuropsychological status were finally evaluated in March 2005. The duration of follow-up was >10 years for three subjects. Daily living activities and social interaction were preserved in all cases and, overall, the progression of the disease was slow. Genetic study revealed the homozygous mutation D146N in the EPM2B gene. We suggest that this mutation may be associated with a less severe LD phenotype.

The carbohydrate-binding domain of Lafora disease protein targets Lafora polyglucosan bodies

Lafora's disease (LD) is an autosomal recessive and fatal form of epilepsy with onset in late childhood or adolescence. One of the characteristic features of LD pathology is the presence of periodic acid-Schiff (PAS) positive Lafora inclusion bodies. Lafora bodies are present primarily in neurons, but they have also been found in other organs. Histochemical and biochemical studies have indicated that Lafora bodies are composed mainly of polysaccharides. The LD gene, EPM2A, encodes a 331 amino acid long protein named laforin that contains an N-terminal carbohydrate-binding domain (CBD) and a C-terminal dual-specificity phosphatase domain (DSPD). Here we demonstrate that the CBD of laforin targets the protein to Lafora inclusion bodies and this property could be evolutionarily conserved. We also tested in vitro the effects of five LD missense mutations on laforin's affinity to Lafora body. While the missense mutant W32G failed to bind to purified Lafora body, four other mutants (S25P, E28L, F88L, and R108C) did not show any effect on the binding affinity. Based on these observations we propose the existence of a laforin-mediated glycogen metabolic pathway regulating the disposal of pathogenic polyglucosan inclusions. This is the first report demonstrating a direct association between the LD gene product and the disease-defining storage product, the Lafora bodies.

Hyperphosphorylation and aggregation of Tau in laforin-deficient mice, an animal model for Lafora disease

Lafora progressive myoclonous epilepsy (Lafora disease; LD) is caused by mutations in the EPM2A gene encoding a dual specificity protein phosphatase named laforin. Our analyses on the Epm2a gene knock-out mice, which developed most of the symptoms of LD, reveal the presence of hyperphosphorylated Tau protein (Ser(396) and Ser(202)) as neurofibrillary tangles (NFTs) in the brain. Intriguingly, NFTs were also observed in the skeletal muscle tissues of the knock-out mice. The hyperphosphorylation of Tau was associated with increased levels of the active form of GSK3 beta. The observations on Tau protein were replicated in cell lines using laforin overexpression and knockdown approaches. We also show here that laforin and Tau proteins physically interact and that the interaction was limited to the phosphatase domain of laforin. Finally, our in vitro and in vivo assays demonstrate that laforin dephosphorylates Tau, and therefore laforin is a novel Tau phosphatase. Taken together, our study suggests that laforin is one of the critical regulators of Tau protein, that the NFTs could underlie some of the symptoms seen in LD, and that laforin can contribute to the NFT formation in Alzheimer disease and other tauopathies.

Novel NHLRC1 mutations and genotype-phenotype correlations in patients with Lafora's progressive myoclonic epilepsy

BACKGROUND

Lafora's progressive myoclonic epilepsy (Lafora's disease) is an autosomal recessive neurodegenerative disorder characterised by the presence of polyglucosan intracellular inclusions called Lafora bodies. Mutations in two genes, EPM2A and NHLRC1, have been shown to cause the disease. A previous study showed mutations in the EPM2A gene in 14 Lafora's disease families and excluded the involvement of this gene in five other families who were biopsy proven to have the disease.

OBJECTIVE

To relate the genetic findings to the clinical course of the disease.

METHODS

As part of an ongoing mutational study of the Lafora's disease genes, five new families with the disease were recruited and the genetic analysis was extended to screen the entire coding region of the NHLRC1 gene. Genotype-phenotype correlations were carried out.

RESULTS

Seven NHLRC1 mutations were identified, including five novel mutations (E91K, D195N, P218S, F216_D233del, and V359fs32), in eight families with Lafora's disease. On relating the genetic findings to the clinical course of the disease it was shown that patients with NHLRC1 mutations had a slower rate of disease progression (p<0.0001) and thus appeared to live longer than those with EPM2A mutations. A simple DNA based test is described to detect the missense mutation C26S (c.76T-->A) in the NHLRC1 gene, which is prevalent among French Canadians.

CONCLUSIONS

Patients with NHLRC1 mutations have a slower rate of disease progression than those with EPM2A mutations.