Gene encoding a new RING-B-box-Coiled-coil protein is mutated in mulibrey nanism.

Mulibrey nanism (for muscle-liver-brain-eye nanism, MUL; MIM 253250) is an autosomal recessive disorder that involves several tissues of mesodermal origin, implying a defect in a highly pleiotropic gene. Characteristic features include severe growth failure of prenatal onset and constrictive pericardium with consequent hepatomegaly. In addition, muscle hypotonia, J-shaped sella turcica, yellowish dots in the ocular fundi, typical dysmorphic features and hypoplasia of various endocrine glands causing hormonal deficiency are common. About 4% of MUL patients develop Wilms' tumour. MUL is enriched in the Finnish population, but is rare elsewhere. We previously assigned MUL to chromosome 17q22-q23 and constructed a physical contig over the critical MUL region. The region has now been further refined by haplotype analysis and new positional candidate genes have been localized. We identified a gene with four independent MUL-associated mutations that all cause a frameshift and predict a truncated protein. MUL is ubiquitously expressed and encodes a new member of the RING-B-box-Coiled-coil (RBCC) family of zinc-finger proteins, whose members are involved in diverse cellular functions such as developmental patterning and oncogenesis.

The neuronal ceroid lipofuscinoses in human EPMR and mnd mutant mice are associated with mutations in CLN8.

The neuronal ceroid lipofuscinoses (NCLs) are a genetically heterogeneous group of progressive neurodegenerative disorders characterized by the accumulation of autofluorescent lipopigment in various tissues. Progressive epilepsy with mental retardation (EPMR, MIM 600143) was recently recognized as a new NCL subtype (CLN8). It is an autosomal recessive disorder characterized by onset of generalized seizures between 5 and 10 years, and subsequent progressive mental retardation. Here we report the positional cloning of a novel gene, CLN8, which is mutated in EPMR. It encodes a putative transmembrane protein. EPMR patients were homozygous for a missense mutation (70C-->G, R24G) that was not found in homozygosity in 433 controls. We also cloned the mouse Cln8 sequence. It displays 82% nucleotide identity with CLN8, conservation of the codon harbouring the human mutation and is localized to the same region as the motor neuron degeneration mouse, mnd, a naturally occurring mouse NCL (ref. 4). In mnd/mnd mice, we identified a homozygous 1-bp insertion (267-268insC, codon 90) predicting a frameshift and a truncated protein. Our data demonstrate that mutations in these orthologous genes underlie NCL phenotypes in human and mouse, and represent the first description of the molecular basis of a naturally occurring animal model for NCL.

Unstable minisatellite expansion causing recessively inherited myoclonus epilepsy, EPM1.

Progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1; MIM 254800) is an autosomal recessive disorder that occurs with a low frequency in many populations but is more common in Finland and the Mediterranean region. It is characterized by stimulus-sensitive myoclonus and tonic-clonic seizures with onset at age 6-15 years, typical electroencephalographic abnormalities and a variable rate of progression between and within families. Following the initial mapping of the EPM1 gene to chromosome 21 (ref. 6) and the refinement of the critical region to a small interval, positional cloning identified the gene encoding cystatin B (CST6), a cysteine protease inhibitor, as the gene underlying EPM1 (ref. 10). Levels of messenger RNA encoded by CST6 were dramatically decreased in patients. A 3' splice site and a stop codon mutation were identified in three families, leaving most mutations uncharacterized. In this study, we report a novel type of disease-causing mutation, an unstable 15- to 18-mer minisatellite repeat expansion in the putative promoter region of the CST6 gene. The mutation accounts for the majority of EPM1 patients worldwide. Haplotype data are compatible with a single ancestral founder mutation. The length of the repeat array differs between chromosomes and families, but changes in repeat number seem to be comparatively rare events.

Expression and lysosomal targeting of CLN7, a major facilitator superfamily transporter associated with variant late-infantile neuronal ceroid lipofuscinosis.

Neuronal ceroid lipofuscinoses (NCLs) constitute a group of progressive neurodegenerative disorders resulting from mutations in at least eight different genes. Mutations in the most recently identified NCL gene, MFSD8/CLN7, underlie a variant of late-infantile NCL (vLINCL). The MFSD8/CLN7 gene encodes a polytopic protein with unknown function, which shares homology with ion-coupled membrane transporters. In this study, we confirmed the lysosomal localization of the native CLN7 protein. This localization of CLN7 is not impaired by the presence of pathogenic missense mutations or after genetic ablation of the N-glycans. Expression of chimeric and full-length constructs showed that lysosomal targeting of CLN7 is mainly determined by an N-terminal dileucine motif, which specifically binds to the heterotetrameric adaptor AP-1 in vitro. We also show that CLN7 mRNA is more abundant in neurons than astrocytes and microglia, and that it is expressed throughout rat brain, with increased levels in the granular layer of cerebellum and hippocampal pyramidal cells. Interestingly, this cellular and regional distribution is in good agreement with the autofluorescent lysosomal storage and cell loss patterns found in brains from CLN7-defective patients. Overall, these data highlight lysosomes as the primary site of action for CLN7, and suggest that the pathophysiology underpinning CLN7-associated vLINCL is a cell-autonomous process.

Galactolipid deficiency in the early pathogenesis of neuronal ceroid lipofuscinosis model Cln8mnd : implications to delayed myelination and oligodendrocyte maturation.

AIMS: CLN8 deficiency underlies one of a group of devastating childhood neurodegenerative disorders, the neuronal ceroid lipofuscinoses. The function of the CLN8 protein is currently unknown, but a role in lipid metabolism has been proposed. In human CLN8 diseased brains, alterations in lipid composition have been detected. To further investigate the connection of CLN8 to lipid metabolism, we characterized the lipid composition of early symptomatic Cln8-deficient mouse (Cln8(mnd)) brains. METHODS: For lipid profiling, Cln8(mnd) cerebral cortical tissue was analysed by liquid chromatography/mass spectrometry. Galactolipid synthesis was measured through enzyme activity and real-time mRNA expression analyses. Based on the findings, myelination and white matter integrity were studied by immunohistochemistry, stereological methods, electron microscopy and magnetic resonance imaging. The development of myelin-forming oligodendrocytes was also studied in vitro. RESULTS: Sphingolipid profiling showed a selective reduction in myelin-enriched galactolipids. The mRNA expression and activity of UDP-galactose:ceramide galactosyltransferase (CGT), the key enzyme in the galactolipid synthesis, was reduced in the Cln8(mnd) brain. Expression of oligodendrocyte markers suggests a maturation defect. The amount of myelin was reduced in 1-month-old Cln8(mnd) mice, but reached normal levels by 5 months of age. The level of Cln8 gene expression followed the developmental pattern of myelin formation and was high in primary oligodendrocytes. CONCLUSIONS: Taken together, these observations suggest that galactolipid deficiency and delayed myelin maturation characterize the early CLN8 disease pathogenesis through a maturation defect of oligodendrocytes.

SCARB2 mutations in progressive myoclonus epilepsy (PME) without renal failure.

OBJECTIVE: Mutations in SCARB2 were recently described as causing action myoclonus renal failure syndrome (AMRF). We hypothesized that mutations in SCARB2 might account for unsolved cases of progressive myoclonus epilepsy (PME) without renal impairment, especially those resembling Unverricht-Lundborg disease (ULD). Additionally, we searched for mutations in the PRICKLE1 gene, newly recognized as a cause of PME mimicking ULD. METHODS: We reviewed cases of PME referred for diagnosis over two decades in which a molecular diagnosis had not been reached. Patients were classified according to age of onset, clinical pattern, and associated neurological signs into "ULD-like" and "not ULD-like." After exclusion of mutations in cystatin B (CSTB), DNA was examined for sequence variation in SCARB2 and PRICKLE1. RESULTS: Of 71 cases evaluated, 41 were "ULD-like" and five had SCARB2 mutations. None of 30 "not ULD-like" cases were positive. The five patients with SCARB2 mutations had onset between 14 and 26 years of age, with no evidence of renal failure during 5.5 to 15 years of follow-up; four were followed until death. One living patient had slight proteinuria. A subset of 25 cases were sequenced for PRICKLE1 and no mutations were found. INTERPRETATION: Mutations in SCARB2 are an important cause of hitherto unsolved cases of PME resembling ULD at onset. SCARB2 should be evaluated even in the absence of renal involvement. Onset is in teenage or young adult life. Molecular diagnosis is important for counseling the patient and family, particularly as the prognosis is worse than classical ULD.

Deletions in the VPS13B (COH1) gene as a cause of Cohen syndrome.

Cohen syndrome is an autosomal recessive disorder that is characterized by mental retardation, facial dysmorphism, microcephaly, retinal dystrophy, truncal obesity, joint laxity and intermittent neutropenia. Mutations in the VPS13B (COH1) gene underlie Cohen syndrome. In approximately 70% of the patients mutations in the gene are identified on both alleles, while in about 30% only a mutation in a single allele or no mutant allele is detected. The VPS13B locus was recently added to the growing list of benign copy number variants. We hypothesized that patients with unexplained Cohen syndrome would harbour deletions affecting the VPS13B locus. We screened 35 patients from 26 families with targeted array CGH and identified 7 copy number alterations: 2 homozygous and 5 heterozygous deletions. Our results show that deletions are an important cause of Cohen syndrome and screening for copy number alterations of VPS13B should be an integral part of the diagnostic work-up of these patients. These findings have important consequences for the diagnosis of patients with genetic disorders in general since, as we highlight, rare benign copy number variants can underly autosomal recessive disorders and lead to disease in homozygous state or in compound heterozygosity with another mutation.

Mutations in the gene encoding cystatin B in progressive myoclonus epilepsy (EPM1)

Progressive myoclonus epilepsy of the Unverricht-Lundborg type (EPM1) is an autosomal recessive inherited form of epilepsy, previously linked to human chromosome 21q22.3. The gene encoding cystatin B was shown to be localized to this region, and levels of messenger RNA encoded by this gene were found to be decreased in cells from affected individuals. Two mutations, a 3' splice site mutation and a stop codon mutation, were identified in the gene encoding cystatin B in EPM1 patients but were not present in unaffected individuals. These results provide evidence that mutations in the gene encoding cystatin B are responsible for the primary defect in patients with EPM1.

Variation of CNV distribution in five different ethnic populations.

Recent studies have revealed a new type of variation in the human genome encompassing relatively large genomic segments ( approximately 100 kb-2.5 Mb), commonly referred to as copy number variation (CNV). The full nature and extent of CNV and its frequency in different ethnic populations is still largely unknown. In this study we surveyed a set of 12 CNVs previously detected by array-CGH. More than 300 individuals from five different ethnic populations, including three distinct European, one Asian and one African population, were tested for the occurrence of CNV using multiplex ligation-dependent probe amplification (MLPA). Seven of these loci indeed showed CNV, i.e., showed copy numbers that deviated from the population median. More precise estimations of the actual genomic copy numbers for (part of) the NSF gene locus, revealed copy numbers ranging from two to at least seven. Additionally, significant inter-population differences in the distribution of these copy numbers were observed. These data suggest that insight into absolute DNA copy numbers for loci exhibiting CNV is required to determine their potential contribution to normal phenotypic variation and, in addition, disease susceptibility.

Mutations in the sodium channel gene SCN2A cause neonatal epilepsy with late-onset episodic ataxia.

Mutations in SCN2A cause epilepsy syndromes of variable severity including neonatal-infantile seizures. In one case, we previously described additional childhood-onset episodic ataxia. Here, we corroborate and detail the latter phenotype in three further cases. We describe the clinical characteristics, identify the causative SCN2A mutations and determine their functional consequences using whole-cell patch-clamping in mammalian cells. In total, four probands presented with neonatal-onset seizures remitting after five to 13 months. In early childhood, they started to experience repeated episodes of ataxia, accompanied in part by headache or back pain lasting minutes to several hours. In two of the new cases, we detected the novel mutation p.Arg1882Gly. While this mutation occurred de novo in both patients, one of them carries an additional known variant on the same SCN2A allele, inherited from the unaffected father (p.Gly1522Ala). Whereas p.Arg1882Gly alone shifted the activation curve by -4 mV, the combination of both variants did not affect activation, but caused a depolarizing shift of voltage-dependent inactivation, and a significant increase in Na(+) current density and protein production. p.Gly1522Ala alone did not change channel gating. The third new proband carries the same de novo SCN2A gain-of-function mutation as our first published case (p.Ala263Val). Our findings broaden the clinical spectrum observed with SCN2A gain-of-function mutations, showing that fairly different biophysical mechanisms can cause a convergent clinical phenotype of neonatal seizures and later onset episodic ataxia.

Expression of MUL, a gene encoding a novel RBCC family ring-finger protein, in human and mouse embryogenesis.

We studied the expression of MUL, a gene encoding a novel member of the RING-B-Box-Coiled Coil family of zinc finger proteins that underlies the human inherited disorder, Mulibrey nanism. In early human and mouse embryogenesis MUL is expressed in dorsal root and trigeminal ganglia, liver and in epithelia of multiple tissues.

Two translocations of chromosome 15q associated with dyslexia.

Developmental dyslexia is characterised by difficulties in learning to read. As reading is a complex cognitive process, multiple genes are expected to contribute to the pathogenesis of dyslexia. The genetics of dyslexia has been a target of molecular studies during recent years, but so far no genes have been identified. However, a locus for dyslexia on chromosome 15q21 (DYX1) has been established in previous linkage studies. We have identified two families with balanced translocations involving the 15q21-q22 region. In one family, the translocation segregates with specific dyslexia in three family members. In the other family, the translocation is associated with dyslexia in one family member. We have performed fluorescence in situ hybridisation (FISH) studies to refine the position of the putative dyslexia locus further. Our results indicate that both translocation breakpoints on 15q map within an interval of approximately 6-8 Mb between markers D15S143 and D15S1029, further supporting the presence of a locus for specific dyslexia on 15q21.

Diagnostic criteria, clinical characteristics, and natural history of Cohen syndrome.

Cohen syndrome is a rare, recessively inherited condition associated with facial dysmorphism, developmental delay, and visual disability. A delay in making the diagnosis commonly occurs, contributed to by the lack of a definitive molecular test and the clinical variability of published case reports. A specific clinical phenotype has been delineated in a homogeneous cohort of Finnish Cohen syndrome patients, but the applicability of their diagnostic criteria to non-Finnish patients has been debated. Detailed delineation of Cohen syndrome in patients from outside Finland is therefore warranted. We report on the clinical features of 33 non-Finnish Cohen syndrome patients. Variability within the clinical spectrum is identified and the natural history of Cohen syndrome described. Diagnostic guidelines for facilitating accurate and early diagnosis are discussed. Results from molecular genetic analysis using markers located within the previously mapped COH1 critical region support allelic but not genetic heterogeneity in this UK cohort.

Refined mapping of the Cohen syndrome gene by linkage disequilibrium.

The Cohen syndrome is a rare autosomal recessively inherited disorder. Contrary to many case reports published elsewhere, the phenotype is uniform in Finland including nonprogressive mental and motor retardation, typical dysmorphic features, granulocytopenia and marked ophthalmological changes. By linkage analysis in five Finnish multiplex nuclear families, the COH1 locus for the Cohen syndrome was recently assigned to a 10-cM region between loci D8S270 and D8S521 on the long arm of chromosome 8. Here we present results of linkage disequilibrium and haplotype analysis in an extended panel of 16 Finnish COH1 families using new markers localized in the COH1 region. By inferring historical recombinations in conserved haplotypes the COH1 gene was assigned in the region of marker loci D8S1808, D8S1762 and D8S546. Calculations of genetic distances based on linkage disequilibrium suggest that the most likely localization of COH1 is in the immediate vicinity of marker locus D8S1762. Haplotype analysis suggests the occurrence of one main COH1 mutation and possibly one or two rare ones in Finland. This information will be useful in the positional cloning of the gene.

Positional cloning and characterisation of the human DLGAP2 gene and its exclusion in progressive epilepsy with mental retardation.

In search of the gene for progressive epilepsy with mental retardation (EPMR) we identified DLGAP2, the human homolog of the gene encoding the rat PSD-95/SAP90-associated protein-2 (Dlgap2). We extended the transcript in both the 5' and 3' directions and characterised the genomic structure of the approximately 10 kb gene. Sequence comparisons of human DLGAP2 cDNA sequences obtained from human testis and brain cDNA libraries with homologous rat genes suggest alternative splicing in the 5' end of the gene. The 5' coding sequence of the testis cDNA is complete, whereas based on homology with the rat gene 103 bp of coding sequence may still be missing in the 5' end of the DLGAP2 brain transcript. DLGAP2 was excluded as the gene responsible for EPMR.

Characterization of the cystatin B gene promoter harboring the dodecamer repeat expanded in progressive myoclonus epilepsy, EPM1.

Mutations in the gene encoding cystatin B (CSTB) are responsible for the primary defect in progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1). A novel and unique type of disease-causing mutation, an unstable dodecamer repeat expansion, accounts for the majority of EPM1 patients world-wide. This minisatellite repeat expansion, located in the putative promoter of CSTB 175 bp upstream from the translation initiation codon, appears to downregulate CSTB gene expression in vivo. We report here the characterization of the CSTB promoter using different promoter-luciferase gene constructs. Transient transfections of cultured mammalian cells suggest that the region from -670 to -1 bp from the translation initiation codon functions as the CSTB promoter. Active binding to five Sp1 and four AP1 sites as well as weak binding to an androgen response element (ARE) half site was demonstrated by electrophoretic mobility shift assays. The effect of the minisatellite expansion on the promoter activity was evaluated by comparing the activity of constructs containing wild-type and expanded alleles. An increase in the number of dodecamer units from three to 19 repeats lowered transcription in vitro by 10-fold. Northern analysis of lymphoblastoid RNA from individuals with 'premutation' length dodecamer repeat (12-17 copies) expansions showed decreased levels of CSTB mRNA expression. These data indicate that expansion of the dodecamer repeat located in the proximal promoter of CSTB severely disrupts the function of the promoter and thereby reduces transcription of CSTB.

A patient with 2 different repeat expansion mutations.

BACKGROUND: Many inherited progressive encephalopathies have a poor outcome, and some are caused by repeat expansion mutations. How would the presence of 2 different expansion mutations affect the phenotype? OBJECTIVE: To describe a patient who has 2 distinct, rare genetic disorders: myotonic dystrophy (DM, OMIM 160900) and progressive myoclonus epilepsy of the Unverricht-Lundborg type (EPM1, OMIM 254800). Both conditions are caused by repeat expansion mutations. They affect the central nervous system causing mental retardation, but also produce a wide spectrum of disabilities in daily living. SETTING: Referral center. METHODS: Clinical description with accompanying photographs, electroencephalography and magnetic resonance imaging; DNA analysis of both of the mutations and chromosomal analysis with prometaphase spreads. RESULTS: The patient had clinical characteristics and findings of both myotonic dystrophy and progressive myoclonus epilepsy of the Unverricht-Lundborg type. Electroencephalographic recordings over a 3-year period showed typical findings for myoclonus epilepsy. The patient had no gross anomalies in brain magnetic resonance imaging. She had a normal karyotype, and both of the diagnoses were confirmed at the molecular level with the direct detection of the mutations. CONCLUSIONS: Despite having 2 different progressive inherited disorders affecting the central nervous system, the patient, at age 28 years, showed only mild mental retardation with very slow progression. However, clear deterioration in activities of daily living has taken place during last 3 years. Arch Neurol. 2000;57:1199-1203

Progressive myoclonus epilepsy of Unverricht-Lundborg type: a clinical and molecular genetic study of a family from the United States with four affected sibs.

We describe clinical and molecular genetic data on a family from the United States in which four of five sibs are affected with progressive myoclonus epilepsy of Unverricht-Lundborg type. The gene for this disorder (EPM1) has previously been mapped to the distal region of chromosome 21. Molecular genetic results suggest that the disease gene in this family is linked to the same region of chromosome 21. Crossover events in the family help refine the gene localization by placing EPM1 between loci CBS and D21S112.

Brainstem involvement in Unverricht-Lundborg disease (EPM1): An MRI and (1)H MRS study.

MRI of the brain and proton MRS ((1)H MRS) of the pons and dentate were obtained in 10 patients with genetically confirmed Unverricht-Lundborg disease (EPM1) and 20 control subjects. Patients with EPM1 showed (p < or = 0.01) loss of bulk of the basis pontis, medulla, and cerebellar hemispheres. Cerebral atrophy was present in six patients. The N-acetylaspartate/creatine and choline/creatine ratios were reduced in the pons but not in the dentate (p < or = 0.005). Brainstem involvement could play a role in pathophysiology of EPM1.

Linkage studies in progressive myoclonus epilepsy: Unverricht-Lundborg and Lafora's diseases.

The progressive myoclonus epilepsies (PME) are a heterogeneous group of rare genetic disorders. Unverricht-Lundborg disease and Lafora's disease are two major classic forms of PME. We recently assigned the gene for Unverricht-Lundborg disease (EPM1) to human chromosome 21 band q22.3. We have now refined the localization of EPM1 by linkage analysis between the disease phenotype and nine DNA markers in 13 Finnish families. Loci MX1 and CD18 flank the EPM1 interval, which spans a distance of about 3.5 megabases. In this 20-centimorgan interval, no recombinations were detected between EPM1 and marker loci BCEI, D21S19, D21S42, D21S113, D21S154, and PFKL. Within this interval a maximum multipoint lod score of 11.04 was reached at loci D21S154-PFKL. In two Swedish families with Unverricht-Lundborg disease no recombinations were detected. In three Italian families with Lafora's disease the linkage results suggested that EPM1 is not the locus for Lafora's disease.