Nesprin-2 is a novel scaffold protein for telethonin and FHL-2 in the cardiomyocyte sarcomere

Nesprins comprise a family of multi-isomeric scaffolding proteins, forming the linker of nucleoskeleton-and-cytoskeleton complex with lamin A/C, emerin and SUN1/2 at the nuclear envelope. Mutations in nesprin-1/-2 are associated with Emery-Dreifuss muscular dystrophy (EDMD) with conduction defects and dilated cardiomyopathy (DCM). We have previously observed sarcomeric staining of nesprin-1/-2 in cardiac and skeletal muscle, but nesprin function in this compartment remains unknown. In this study, we show that specific nesprin-2 isoforms are highly expressed in cardiac muscle and localize to the Z-disc and I band of the sarcomere. Expression of GFP-tagged nesprin-2 giant spectrin repeats 52 to 53, localized to the sarcomere of neonatal rat cardiomyocytes. Yeast two-hybrid screening of a cardiac muscle cDNA library identified telethonin and four-and-half LIM domain (FHL)-2 as potential nesprin-2 binding partners. GST pull-down and immunoprecipitation confirmed the individual interactions between nesprin-2/telethonin and nesprin-2/FHL-2, and showed that nesprin-2 and telethonin binding was dependent on telethonin phosphorylation status. Importantly, the interactions between these binding partners were impaired by mutations in nesprin-2, telethonin, and FHL-2 identified in EDMD with DCM and hypertrophic cardiomyopathy patients. These data suggest that nesprin-2 is a novel sarcomeric scaffold protein that may potentially participate in the maintenance and/or regulation of sarcomeric organization and function.

A multistage sequencing strategy pinpoints novel candidate alleles for Emery-Dreifuss muscular dystrophy and supports gene misregulation as its pathomechanism

Background

As genome-wide approaches prove difficult with genetically heterogeneous orphan diseases, we developed a new approach to identify candidate genes. We applied this to Emery-Dreifuss muscular dystrophy (EDMD), characterised by early onset contractures, slowly progressive muscular wasting, and life-threatening heart conduction disturbances with wide intra- and inter-familial clinical variability. Roughly half of EDMD patients are linked to six genes encoding nuclear envelope proteins, but the disease mechanism remains unclear because the affected proteins function in both cell mechanics and genome regulation.

Methods

A primer library was generated to test for mutations in 301 genes from four categories: (I) all known EDMD-linked genes; (II) genes mutated in related muscular dystrophies; (III) candidates generated by exome sequencing in five families; (IV) functional candidates — other muscle nuclear envelope proteins functioning in mechanical/genome processes affected in EDMD. This was used to sequence 56 unlinked patients with EDMD-like phenotype.

Findings

Twenty-one patients could be clearly assigned: 18 with mutations in genes of similar muscular dystrophies; 3 with previously missed mutations in EDMD-linked genes. The other categories yielded novel candidate genes, most encoding nuclear envelope proteins with functions in gene regulation.

Interpretation

Our multi-pronged approach identified new disease alleles and many new candidate EDMD genes. Their known functions strongly argue the EDMD pathomechanism is from altered gene regulation and mechanotransduction due to connectivity of candidates from the nuclear envelope to the plasma membrane. This approach highlights the value of testing for related diseases using primer libraries and may be applied for other genetically heterogeneous orphan diseases.

Funding

The Wellcome Trust, Muscular Dystrophy UK, Medical Research Council, European Community's Seventh Framework Programme “Integrated European –omics research project for diagnosis and therapy in rare neuromuscular and neurodegenerative diseases (NEUROMICS)”.

Immunohistochemistry on a panel of Emery-Dreifuss muscular dystrophy samples reveals nuclear envelope proteins as inconsistent markers for pathology

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Altered distribution of EDMD-linked proteins is not a general characteristic of EDMD.

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Tissue-specific proteins exhibit altered distributions in some EDMD patients.

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Variation in redistributed proteins in EDMD may underlie its clinical variability.

Reports of aberrant distribution for some nuclear envelope proteins in cells expressing a few Emery–Dreifuss muscular dystrophy mutations raised the possibility that such protein redistribution could underlie pathology and/or be diagnostic. However, this disorder is linked to 8 different genes encoding nuclear envelope proteins, raising the question of whether a particular protein is most relevant. Therefore, myoblast/fibroblast cultures from biopsy and tissue sections from a panel of nine Emery–Dreifuss muscular dystrophy patients (4 male, 5 female) including those carrying emerin and FHL1 (X-linked) and several lamin A (autosomal dominant) mutations were stained for the proteins linked to the disorder. As tissue-specific nuclear envelope proteins have been postulated to mediate the tissue-specific pathologies of different nuclear envelopathies, patient samples were also stained for several muscle-specific nuclear membrane proteins. Although linked proteins nesprin 1 and SUN2 and muscle-specific proteins NET5/Samp1 and Tmem214 yielded aberrant distributions in individual patient cells, none exhibited defects through the larger patient panel. Muscle-specific Tmem38A normally appeared in both the nuclear envelope and sarcoplasmic reticulum, but most patient samples exhibited a moderate redistribution favouring the sarcoplasmic reticulum. The absence of striking uniform defects in nuclear envelope protein distribution indicates that such staining will be unavailing for general diagnostics, though it remains possible that specific mutations exhibiting protein distribution defects might reflect a particular clinical variant. These findings further argue that multiple pathways can lead to the generally similar pathologies of this disorder while at the same time the different cellular phenotypes observed possibly may help explain the considerable clinical variation of EDMD.

Gene-rich chromosomal regions are preferentially localized in the lamin B deficient nuclear blebs of atypical progeria cells

More than 20 mutations in the gene encoding A-type lamins (LMNA) cause progeria, a rare premature aging disorder. The major pathognomonic hallmarks of progeria cells are seen as nuclear deformations or blebs that are related to the redistribution of A- and B-type lamins within the nuclear lamina. However, the functional significance of these progeria-associated blebs remains unknown. We have carried out an analysis of the structural and functional consequences of progeria-associated nuclear blebs in dermal fibroblasts from a progeria patient carrying a rare point mutation p.S143F (C428T) in lamin A/C. These blebs form microdomains that are devoid of major structural components of the nuclear envelope (NE)/lamina including B-type lamins and nuclear pore complexes (NPCs) and are enriched in A-type lamins. Using laser capture microdissection and comparative genomic hybridization (CGH) analyses, we show that, while these domains are devoid of centromeric heterochromatin and gene-poor regions of chromosomes, they are enriched in gene-rich chromosomal regions. The active form of RNA polymerase II is also greatly enriched in blebs as well as nascent RNA but the nuclear co-activator SKIP is significantly reduced in blebs compared to other transcription factors. Our results suggest that the p.S143F progeria mutation has a severe impact not only on the structure of the lamina but also on the organization of interphase chromatin domains and transcription. These structural defects are likely to contribute to gene expression changes reported in progeria and other types of laminopathies.

Rapamycin treatment of Mandibuloacral dysplasia cells rescues localization of chromatin-associated proteins and cell cycle dynamics

Lamin A is a key component of the nuclear lamina produced through post-translational processing of its precursor known as prelamin A. LMNA mutations leading to farnesylated prelamin A accumulation are known to cause lipodystrophy, progeroid and developmental diseases, including Mandibuloacral dysplasia, a mild progeroid syndrome with partial lipodystrophy and altered bone turnover. Thus, degradation of prelamin A is expected to improve the disease phenotype. Here, we show different susceptibilities of prelamin A forms to proteolysis and further demonstrate that treatment with rapamycin efficiently and selectively triggers lysosomal degradation of farnesylated prelamin A, the most toxic processing intermediate. Importantly, rapamycin treatment of Mandibuloacral dysplasia cells, which feature very low levels of the NAD-dependent sirtuin SIRT-1 in the nuclear matrix, restores SIRT-1 localization and distribution of chromatin markers, elicits release of the transcription factor Oct-1 and determines shortening of the prolonged S-phase. These findings indicate the drug as a possible treatment for Mandibuloacral dysplasia.

Abnormal proliferation and spontaneous differentiation of myoblasts from a symptomatic female carrier of X-linked Emery-Dreifuss muscular dystrophy

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X-linked female presenting with EDMD1 not explained by uneven X-inactivation.

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First EDMD blood phenotype with highly lobulated lymphocytes in EDMD1 patient.

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Found high incidence of spontaneous differentiation in cultured patient myoblasts.

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Faster proliferation of emerin-null than emerin-positive EDMD1 patient myoblasts.

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Loss of satellite cells from the above might explain EDMD pathology.

Emery–Dreifuss muscular dystrophy (EDMD) is a neuromuscular disease characterized by early contractures, slowly progressive muscular weakness and life-threatening cardiac arrhythmia that can develop into cardiomyopathy. In X-linked EDMD (EDMD1), female carriers are usually unaffected. Here we present a clinical description and in vitro characterization of a mildly affected EDMD1 female carrying the heterozygous EMD mutation c.174_175delTT; p.Y59* that yields loss of protein. Muscle tissue sections and cultured patient myoblasts exhibited a mixed population of emerin-positive and -negative cells; thus uneven X-inactivation was excluded as causative. Patient blood cells were predominantly emerin-positive, but considerable nuclear lobulation was observed in non-granulocyte cells – a novel phenotype in EDMD. Both emerin-positive and emerin-negative myoblasts exhibited spontaneous differentiation in tissue culture, though emerin-negative myoblasts were more proliferative than emerin-positive cells. The preferential proliferation of emerin-negative myoblasts together with the high rate of spontaneous differentiation in both populations suggests that loss of functional satellite cells might be one underlying mechanism for disease pathology. This could also account for the slowly developing muscle phenotype.

Muscular dystrophy-associated SUN1 and SUN2 variants disrupt nuclear-cytoskeletal connections and myonuclear organization

Proteins of the nuclear envelope (NE) are associated with a range of inherited disorders, most commonly involving muscular dystrophy and cardiomyopathy, as exemplified by Emery-Dreifuss muscular dystrophy (EDMD). EDMD is both genetically and phenotypically variable, and some evidence of modifier genes has been reported. Six genes have so far been linked to EDMD, four encoding proteins associated with the LINC complex that connects the nucleus to the cytoskeleton. However, 50% of patients have no identifiable mutations in these genes. Using a candidate approach, we have identified putative disease-causing variants in the SUN1 and SUN2 genes, also encoding LINC complex components, in patients with EDMD and related myopathies. Our data also suggest that SUN1 and SUN2 can act as disease modifier genes in individuals with co-segregating mutations in other EDMD genes. Five SUN1/SUN2 variants examined impaired rearward nuclear repositioning in fibroblasts, confirming defective LINC complex function in nuclear-cytoskeletal coupling. Furthermore, myotubes from a patient carrying compound heterozygous SUN1 mutations displayed gross defects in myonuclear organization. This was accompanied by loss of recruitment of centrosomal marker, pericentrin, to the NE and impaired microtubule nucleation at the NE, events that are required for correct myonuclear arrangement. These defects were recapitulated in C2C12 myotubes expressing exogenous SUN1 variants, demonstrating a direct link between SUN1 mutation and impairment of nuclear-microtubule coupling and myonuclear positioning. Our findings strongly support an important role for SUN1 and SUN2 in muscle disease pathogenesis and support the hypothesis that defects in the LINC complex contribute to disease pathology through disruption of nuclear-microtubule association, resulting in defective myonuclear positioning.

Emery-Dreifuss muscular dystrophy (EDMD) is an inherited disorder involving muscle wasting and weakness, accompanied by cardiac defects. The disease is variable in its severity and also in its genetic cause. So far, 6 genes have been linked to EDMD, most encoding proteins that form a structural network that supports the nucleus of the cell and connects it to structural elements of the cytoplasm. This network is particularly important in muscle cells, providing resistance to mechanical strain. Weakening of this network is thought to contribute to development of muscle disease in these patients. Despite rigorous screening, at least 50% of patients with EDMD have no detectable mutation in the 6 known genes. We therefore undertook screening and identified mutations in two additional genes that encode other components of the nuclear structural network, SUN1 and SUN2. Our findings add to the genetic complexity of this disease since some individuals carry mutations in more than one gene. We also show that the mutations disrupt connections between the nucleus and the structural elements of cytoplasm, leading to mis-positioning and clustering of nuclei in muscle cells. This nuclear mis-positioning is likely to be another factor contributing to pathogenesis of EDMD.

Contribution of SUN1 mutations to the pathomechanism in muscular dystrophies

Mutations in several genes encoding nuclear envelope (NE) associated proteins cause Emery-Dreifuss muscular dystrophy (EDMD). We analyzed fibroblasts from a patient who had a mutation in the EMD gene (p.L84Pfs*6) leading to loss of Emerin and a heterozygous mutation in SUN1 (p.A203V). The second patient harbored a heterozygous mutation in LAP2alpha (p.P426L) and a further mutation in SUN1 (p.A614V). p.A203V is located in the N-terminal domain of SUN1 facing the nucleoplasm and situated in the vicinity of the Nesprin-2 and Emerin binding site. p.A614V precedes the SUN domain, which interacts with the KASH domain of Nesprins in the periplasmic space and forms the center of the LINC complex. At the cellular level, we observed alterations in the amounts for several components of the NE in patient fibroblasts and further phenotypic characteristics generally attributed to laminopathies such as increased sensitivity to heat stress. The defects were more severe than observed in EDMD cells with mutations in a single gene. In particular, in patient fibroblasts carrying the p.A203V mutation in SUN1, the alterations were aggravated. Moreover, SUN1 of both patient fibroblasts exhibited reduced interaction with Lamin A/C and when expressed ectopically in wild-type fibroblasts, the SUN1 mutant proteins exhibited reduced interactions with Emerin as well.

Progeroid laminopathy with restrictive dermopathy-like features caused by an isodisomic LMNA mutation p.R435C

The clinical course of a female patient affected by a progeroid syndrome with Restrictive Dermopathy (RD)-like features was followed up. Besides missing hairiness, stagnating weight and growth, RD-like features including progressive skin swelling and solidification, acrocontractures, osteolysis and muscular hypotension were observed until the patient died at the age of 11 months. A homozygous LMNA mutation c.1303C>T (p.R435C) was found by Sanger sequencing. Haplotyping revealed a partial uniparental disomy of chromosome 1 (1q21.3 to 1q23.1) including the LMNA gene. In contrast to reported RD patients with LMNA mutations, LMNA p.R435C is not located at the cleavage site necessary for processing of prelamin A by ZMPSTE24 and leads to a distinct phenotype combining clinical features of Restrictive Dermopathy, Mandibuloacral Dysplasia and Hutchinson-Gilford Progeria. Functionally, LMNA p.R435C is associated with increasing DNA double strand breaks and decreased recruitment of P53 binding protein 1 (53BP1) to DNA-damage sites indicating delayed DNA repair. The follow-up of the complete clinical course in the patient combined with functional studies showed for the first time that a progressive loss of lamin A rather than abnormal accumulation of prelamin A species could be a pathophysiological mechanism in progeroid laminopathies, which leads to DNA repair deficiency accompanied by advancing tissue degeneration.

Familial partial lipodystrophy, mandibuloacral dysplasia and restrictive dermopathy feature barrier-to-autointegration factor (BAF) nuclear redistribution

Prelamin A processing impairment is a common feature of a restricted group of rare genetic alterations/disorders associated with a wide range of clinical phenotypes. Changes in histone posttranslational modifications, alterations in non-histone chromatin proteins and chromatin disorganization have been specifically linked to impairment of specific, distinct prelamin A processing steps, but the molecular mechanism involved in these processes is not yet understood . In this study, we show that the accumulation of wild-type prelamin A detected in restrictive dermopathy (RD), as well as the accumulation of mutated forms of prelamin A identified in familial partial lipodystrophy (FPLD) and mandibuloacral dysplasia (MADA), affect the nuclear localization of barrier-to-autointegration factor (BAF), a protein able to link lamin A precursor to chromatin remodeling functions. Our findings, in accordance with previously described results, support the hypothesis of a prelamin A involvement in BAF nuclear recruitment and suggest BAF-prelamin A complex as a protein platform usually activated in prelamin A-accumulating diseases. Finally, we demonstrate the involvement of the inner nuclear membrane protein emerin in the proper localization of BAF-prelamin A complex.

LINC complex alterations in DMD and EDMD/CMT fibroblasts

Emery-Dreifuss muscular dystrophy (EDMD) is a late onset-disease characterized by skeletal muscle wasting and heart defects with associated risk of sudden death. The autosomal dominant form of the disease is caused by mutations in the LMNA gene encoding LaminA and C, the X-linked form results from mutations in the gene encoding the inner nuclear membrane protein Emerin (STA). Both Emerin and LaminA/C interact with the nuclear envelope proteins Nesprin-1 and -2 and mutations in genes encoding C-terminal isoforms of Nesprin-1 and -2 have also been implicated in EDMD. Here we analyse primary fibroblasts from patients affected by either Duchenne muscular dystrophy (DMD) or Emery-Dreifuss muscular dystrophy/Charcot-Marie-Tooth syndrome (EDMD/CMT) that in addition to the disease causing mutations harbour mutations in the Nesprin-1 gene and in the SUN1 and SUN2 gene, respectively. SUN proteins together with the Nesprins form the core of the LINC complex which connects the nucleus with the cytoskeleton. The mutations are accompanied by changes in cell adhesion, cell migration, senescence, and stress response, as well as in nuclear shape and nuclear envelope composition which are changes characteristic for laminopathies. Our results point to a potential influence of mutations in components of the LINC complex on the clinical outcome and the molecular pathology in the patients.

Distinct association of the nuclear pore protein Nup153 with A- and B-type lamins

The nuclear envelope (NE) is a double membrane physical barrier, which separates the nucleus from the cytoplasm. Underlying the NE are the nuclear lamins, which in combination with inner nuclear membrane proteins form the lamina. The lamina is crucial for maintaining the structural integrity of the nucleus and for positioning of nuclear pore complexes (NPCs) within the NE. The nucleoporin Nup153 has previously been reported to bind to B-type lamins. However, the specificity of this interaction is not well established. Here we show that Nup153 exhibits multiple binding sites for A- and B-type lamins. Using GST-pull down assays, we found that both the N-terminal domain of Nup153 and its C terminus associate with the Ig-fold domain of A- and B-type lamins. By employing purified Nup153 and lamin proteins in blot overlay assays we revealed that both the N-terminal and the C-terminal domain of Nup153 are directly interacting with the lamins. Moreover, we provide evidence that mutations in the lamin A Ig-fold domain selectively affect Nup153-binding, suggesting that Nup153 may play a role in lamin-associated diseases, known as laminopathies. Together our results indicate a far more intricate interplay between Nup153 and nuclear lamins than previously accepted.

Nesprins, but not sun proteins, switch isoforms at the nuclear envelope during muscle development

Nesprins are a family of nuclear transmembrane proteins anchored via Sun proteins to the nuclear membrane. Analysis of nesprins during human muscle development revealed an increase in nesprin-1-giant during early myogenesis in vitro. During the transition from immature to mature muscle fibres in vivo, nesprin-2 partly replaced nesprin-1 at the nuclear envelope and short nesprin isoforms became dominant. Sun1 and Sun2 proteins remained unchanged during this fibre maturation. In emerin-negative skin fibroblasts, nesprin-2-giant was relocated from the nuclear envelope to the cytoplasm, not to the endoplasmic reticulum, while nesprin-1 remained at the nuclear envelope. In emerin-negative keratinocytes lacking nesprin-1, nesprin-2 remained at the nuclear envelope. HeLa cell nuclear envelopes lacked nesprin-1, which was the dominant form in myoblasts, while a novel 130-kD nesprin-2 isoform dominated Ntera-2 cells. The results suggest the possibility of isoform-specific and tissue-specific roles for nesprins in nuclear positioning.

Mutations of the FHL1 gene cause Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a rare disorder characterized by early joint contractures, muscular dystrophy, and cardiac involvement with conduction defects and arrhythmias. So far, only 35% of EDMD cases are genetically elucidated and associated with EMD or LMNA gene mutations, suggesting the existence of additional major genes. By whole-genome scan, we identified linkage to the Xq26.3 locus containing the FHL1 gene in three informative families belonging to our EMD- and LMNA-negative cohort. Analysis of the FHL1 gene identified seven mutations, in the distal exons of FHL1 in these families, three additional families, and one isolated case, which differently affect the three FHL1 protein isoforms: two missense mutations affecting highly conserved cysteines, one abolishing the termination codon, and four out-of-frame insertions or deletions. The predominant phenotype was characterized by myopathy with scapulo-peroneal and/or axial distribution, as well as joint contractures, and associated with a peculiar cardiac disease characterized by conduction defects, arrhythmias, and hypertrophic cardiomyopathy in all index cases of the seven families. Heterozygous female carriers were either asymptomatic or had cardiac disease and/or mild myopathy. Interestingly, four of the FHL1-mutated male relatives had isolated cardiac disease, and an overt hypertrophic cardiomyopathy was present in two. Expression and functional studies demonstrated that the FHL1 proteins were severely reduced in all tested patients and that this was associated with a severe delay in myotube formation in the two patients for whom myoblasts were available. In conclusion, FHL1 should be considered as a gene associated with the X-linked EDMD phenotype, as well as with hypertrophic cardiomyopathy.

Nuclear protein import is reduced in cells expressing nuclear envelopathy-causing lamin A mutants

Lamins, which form the nuclear lamina, not only constitute an important determinant of nuclear architecture, but additionally play essential roles in many nuclear functions. Mutations in A-type lamins cause a wide range of human genetic disorders (laminopathies). The importance of lamin A (LaA) in the spatial arrangement of nuclear pore complexes (NPCs) prompted us to study the role of LaA mutants in nuclear protein transport. Two mutants, causing prenatal skin disease restrictive dermopathy (RD) and the premature aging disease Hutchinson Gilford progeria syndrome, were used for expression in HeLa cells to investigate their impact on the subcellular localization of NPC-associated proteins and nuclear protein import. Furthermore, dynamics of the LaA mutants within the nuclear lamina were studied. We observed affected localization of NPC-associated proteins, diminished lamina dynamics for both LaA mutants and reduced nuclear import of representative cargo molecules. Intriguingly, both LaA mutants displayed similar effects on nuclear morphology and functions, despite their differences in disease severity. Reduced nuclear protein import was also seen in RD fibroblasts and impaired lamina dynamics for the nucleoporin Nup153. Our data thus represent the first study of a direct link between LaA mutant expression and reduced nuclear protein import.

Restrictive dermopathy--a lethal congenital laminopathy. Case report and review of the literature

Restrictive dermopathy (RD) is a rare, fatal, and genetically heterogeneous laminopathy with a predominant autosomal recessive heredity pattern. The phenotype can be caused by mutations in either LMNA (primary laminopathy) or ZMPSTE24 (secondary laminopathy) genes but mostly by homozygous or compound heterozygous ZMPSTE24 mutations. Clinicopathologic findings are unique, allowing a specific diagnosis in most cases. We describe a premature newborn girl of non-consanguineous parents who presented a rigid, translucent and tightly adherent skin, dysmorphic facies, multiple joint contractures and radiological abnormalities. The overall clinical, radiological, histological, and ultrastructural features were typical of restrictive dermopathy. Molecular genetic analysis revealed a homozygous ZMPSTE24 mutation (c.1085_1086insT). Parents and sister were heterozygous asymptomatic carriers. We conclude that RD is a relatively easy and consistent clinical and pathological diagnosis. Despite recent advances in our understanding of RD, the pathogenetic mechanisms of the disease are not entirely clarified. Recognition of RD and molecular genetic diagnosis are important to define the prognosis of an affected child and for recommending genetic counseling to affected families. However, the outcome for a live born patient in the neonatal period is always fatal.

Impaired nuclear functions lead to increased senescence and inefficient differentiation in human myoblasts with a dominant p.R545C mutation in the LMNA gene

We have studied myoblasts from a patient with a severe autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD) caused by an arginine 545 to cystein point mutation (p.R545C) in the carboxy-terminal domain of the lamin A/C gene. This mutation has pleiotropic cellular effects on these myoblasts as demonstrated by nuclear structural defects, exhibiting lobulations which increase with cell passages in culture. The organization of both lamin A/C and its inner nuclear membrane partner emerin are altered, eventually showing a honeycomb pattern upon immunofluorescence microscopy. In addition, the distribution of histone H3 trimethylated at lysine 27 and of phosphorylated RNA polymerase II, markers of inactive and active chromatin domains, respectively, are altered suggesting an impact on gene expression. Patient myoblasts also presented a high index of senescence in ex vivo culture. Moreover, our data show for the first time in an AD-EDMD context that the 20S core particle of the proteasome was inactivated. With cell passages, the 20S core protein progressively accumulated into discrete nuclear foci that largely colocalized with promyelocytic leukemia (PML) bodies while p21 accumulated throughout the nuclear compartment. Proteasome inactivation has been linked to normal cellular ageing. Our data indicate that it may also contribute to premature senescence in AD-EDMD patient myoblasts. Finally, when transferred to low-serum medium, patient myoblasts were deficient in ex vivo differentiation, as assessed by the absence of myotube formation and myogenin induction. Altogether, these data suggest that the LMNA mutation p.R545C impairs both proliferation and differentiation capacities of myoblasts as part of the pathogenesis of AD-EDMD.

Defects in cell spreading and ERK1/2 activation in fibroblasts with lamin A/C mutations

In-frame mutations in nuclear lamin A/C lead to a multitude of tissue-specific degenerative diseases known as the 'laminopathies'. Previous studies have demonstrated that lamin A/C-null mouse fibroblasts have defects in cell polarisation, suggesting a role for lamin A/C in nucleo-cytoskeletal-cell surface cross-talk. However, this has not been examined in patient fibroblasts expressing modified forms of lamin A/C. Here, we analysed skin fibroblasts from 3 patients with Emery-Dreifuss muscular dystrophy and from 1 with dilated cardiomyopathy. The emerin-lamin A/C interaction was impaired in each mutant cell line. Mutant cells exhibited enhanced cell proliferation, collagen-dependent adhesion, larger numbers of filopodia and smaller cell spread size, compared with control cells. Furthermore, cell migration, speed and polarization were elevated. Mutant cells also showed an enhanced ability to contract collagen gels at early time points, compared with control cells. Phosphotyrosine measurements during cell spreading indicated an initial temporal lag in ERK1/2 activation in our mutant cells, followed by hyper-activation of ERK1/2 at 2 h post cell attachment. Deregulated ERK1/2 activation is linked with cardiomyopathy, cell spreading and proliferation defects. We conclude that a functional emerin-lamin A/C complex is required for cell spreading and proliferation, possibly acting through ERK1/2 signalling.

Lamin A Ser404 is a nuclear target of Akt phosphorylation in C2C12 cells

Akt/PKB is a central activator of multiple signaling pathways coupled with a large number of stimuli. Although both localization and activity of Akt in the nuclear compartment are well-documented, most Akt substrates identified so far are located in the cytoplasm, while nuclear substrates have remained elusive. A proteomic-based search for nuclear substrates of Akt was undertaken, exploiting 2D-electrophoresis/MS in combination with an anti-Akt phosphosubstrate antibody. This analysis indicated lamin A/C as a putative substrate of Akt in C2C12 cells. In vitro phosphorylation of endogenous lamin A/C by recombinant Akt further validated this result. Moreover, by phosphopeptide analysis and point mutation, we established that lamin A/C is phosphorylated by Akt at Ser404, in an evolutionary conserved Akt motif. To delve deeper into this, we raised an antibody against the lamin A Ser404 phosphopeptide which allowed us to determine that phosphorylation of lamin A Ser404 is triggered by the well-known Akt activator insulin, and is therefore to be regarded as a physiological response. Remarkably, expression of S404A lamin A in primary cells from healthy tissue caused the nuclear abnormalities that are a hallmark of Emery-Dreifuss muscular dystrophy (EDMD) cells. Indeed, it is known that mutations at several sites in lamin A/C cause autosomal dominant EDMD. Very importantly, we show here that Akt failed to phosphorylate lamin A/C in primary cells from an EDMD-2 patient with lamin A/C mutated in the Akt consensus motif. Together, our data demonstrate that lamin A/C is a novel signaling target of Akt, and implicate Akt phosphorylation of lamin A/C in the correct function of the nuclear lamina.

Restrictive dermopathy: a rare laminopathy

BACKGROUND

Restrictive dermopathy (RD) belongs to the laminopathies and mostly shows an autosomal recessive heredity pattern. This rare genetic disorder is lethal for the newborn in the neonatal period. Clinical and pathological findings are distinctive and allow for a specific diagnosis in most cases. Furthermore, polyhydramnios, decreased foetal movement, facial dysmorphisms and arthrogryposis are characteristic of RD. Respiratory insufficiency leads to an early neonatal death.

METHODS

We present the case of an affected infant and a review of the previously reported cases in the literature.

RESULTS

The infant showed thin, shiny skin with exfoliating desquamation, a small, round and open mouth, low-set ears, a small pinched nose, joint contractures at all four extremities and distinctive pulmonic atelectasis. It died 3 h and 20 min post-partum. Histologically, the skin showed the typical pattern of an RD with the epidermis covered by an exfoliated, hyperkeratotic horn layer, clearly hypoplastic hair follicles and a considerably reduced dermis thickness, although it had a massive subcutaneous adipose tissue. Electron microscopically, the diagnosis was confirmed.

CONCLUSIONS

It is important to know about this disease and to distinguish it from others like keratinization malfunctions such as ichtyosis, congenital, developmental and akinesia disturbance, etc., to know the prognosis for the affected newborn and to provide sufficient (genetic) counselling to the families. This disorder is caused by dominant mutations of the LMNA (primary laminopathy) or recessive mutations of the ZMPSTE24 (FACE1) (secondary laminopathy) genes.

Nesprin-2 giant safeguards nuclear envelope architecture in LMNA S143F progeria cells

The S143F lamin A/C point mutation causes a phenotype combining features of myopathy and progeria. We demonstrate here that patient dermal fibroblast cells have dysmorphic nuclei containing numerous blebs and lobulations, which progressively accumulate as cells age in culture. The lamin A/C organization is altered, showing intranuclear and nuclear envelope (NE) aggregates and presenting often a honeycomb appearance. Immunofluorescence microscopy showed that nesprin-2 C-terminal isoforms and LAP2alpha were recovered in the cytoplasm, whereas LAP2beta and emerin were unevenly localized along the NE. In addition, the intranuclear organization of acetylated histones, histone H1 and the active form of RNA polymerase II were markedly different in patient cells. A subpopulation of mutant cells, however, expressing the 800 kDa nesprin-2 giant isoform, did not show an overt nuclear phenotype. Ectopic expression of p.S143F lamin A in fibroblasts recapitulates the patient cell phenotype, whereas no effects were observed in p.S143F LMNA keratinocytes, which highly express nesprin-2 giant. Overexpression of the mutant lamin A protein had a more severe impact on the NE of nesprin-2 giant deficient fibroblasts when compared with wild-type. In summary, our results suggest that the p.S143F lamin A mutation affects NE architecture and composition, chromatin organization, gene expression and transcription. Furthermore, our findings implicate a direct involvement of the nesprins in laminopathies and propose nesprin-2 giant as a structural reinforcer at the NE.

Nesprin-1 and -2 are involved in the pathogenesis of Emery Dreifuss muscular dystrophy and are critical for nuclear envelope integrity

Emery-Dreifuss muscular dystrophy (EDMD) is a heterogeneous late-onset disease involving skeletal muscle wasting and heart defects caused, in a minority of cases, by mutations in either of two genes encoding the inner nuclear membrane (INM) proteins, emerin and lamins A/C. Nesprin-1 and -2 are multi-isomeric, spectrin-repeat proteins that bind both emerin and lamins A/C and form a network in muscle linking the nucleoskeleton to the INM, the outer nuclear membrane, membraneous organelles, the sarcomere and the actin cytoskeleton. Thus, disruptions in nesprin/lamin/emerin interactions might play a role in the muscle-specific pathogenesis of EDMD. Screening for DNA variations in the genes encoding nesprin-1 (SYNE1) and nesprin-2 (SYNE2) in 190 probands with EDMD or EDMD-like phenotypes identified four heterozygous missense mutations. Fibroblasts from these patients exhibited nuclear morphology defects and specific patterns of emerin and SUN2 mislocalization. In addition, diminished nuclear envelope localization of nesprins and impaired nesprin/emerin/lamin binding interactions were common features of all EDMD patient fibroblasts. siRNA knockdown of nesprin-1 or -2 in normal fibroblasts reproduced the nuclear morphological changes and mislocalization of emerin and SUN2 observed in patient fibroblasts. Taken together, these data suggest that EDMD may be caused, in part, by uncoupling of the nucleoskeleton and cytoskeleton because of perturbed nesprin/emerin/lamin interactions.

The retinol acid receptor B gene is hypermethylated in patients with familial partial lipodystrophy

Mutations in the LMNA gene cause various phenotypes including partial lipodystrophy, muscular dystrophies, and progeroid syndromes. The specific mutation position within the LMNA sequence can partially predict the phenotype, but the underlying mechanisms for the development of these different phenotypes are still unclear. To investigate whether different DNA methylation patterns contribute to the development of different phenotypes caused by LMNA mutations, we analyzed a panel of ten candidate genes related to fat metabolism, aging, and a tendency to different methylation patterns: CSPG2, ESR1, IGF1R, IGFR2, LMNA, MLH1, RANBP1, RARB, ZMPSTE24, and TGFBR1. We studied two independent families each comprising three individuals affected by familial partial lipodistrophy type 2 (FPLD2). Affected members in each family carried two different mutations of the LMNA gene (R482L and R471G respectively). In addition, we analyzed four progeria patients (2xLMNA/C G608G, 1xLMNA/C S143F, and 1xZMPSTE24 IVS9-Ex10) and seven healthy adults. The gene encoding retinoic acid receptor B (RARB) showed a higher methylation in all six patients with FPLD2 when compared with the progeria patients with other LMNA mutations as well as the healthy controls (P<0.05). All other investigated genes showed no difference in the methylation patterns between the groups. A drug-induced inhibition of the retinol pathway is discussed as the key pathway for developing HAART-associated lipodystrophy and our data support a possible role of the retinol pathway in the development of lipodystrophy phenotypes.

Myofiber degeneration in autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD) (LGMD1B)

Autosomal dominant Emery-Dreifuss muscular dystrophy is caused by mutations in the LMNA gene that code for the nuclear membrane protein lamin A/C. We investigated skeletal muscle fibers from several muscles for cytoplasmic degenerative changes in three patients with genetically confirmed Emery-Dreifuss muscular dystrophy. Methods included quantitative light and electron microscopy and PCR-based mutational analysis.

RESULTS

The degenerative pathway was characterized by the gradual replacement of individual myofibers by connective tissue. Early stages of degeneration typically involved only a segment of the cross-sectional area of a myofiber. Intermediate stages consisted of myofiber shrinkage due to "shedding" of peripheral cytoplasmic portions into the endomysial space, and fragmentation of the myofibers by interposed collagen fibrils. Empty basement membrane sheaths surrounded by abundant deposits of extracellular matrix marked the end stage of the degenerative process. The nuclear number-to-cytoplasmic area in myofibers of one patient increased with increasing cross-sectional area, suggesting that satellite cell fusion with myofibers may have compensated for myofiber shrinkage. The pattern of degeneration described herein differs from muscular dystrophies with plasma membrane defects (dystrophinopathy, dysferlinopathy) and explains the frequently found absence of highly elevated serum creatine kinase levels in autosomal dominant Emery-Dreifuss muscular dystrophy.

A pathogenic mechanism leading to partial lipodistrophy and prospects for pharmacological treatment of insulin resistance syndrome

The understanding of a common complex phenotype such as insulin resistance can be favoured by evaluation of monogenic syndromes. Clinical definition, pathogenesis, and therapeutical strategies for the insulin resistance syndrome can thus be improved by the characterization at the molecular genetic level of monogenic forms of lipodystrophies. Here we report experimental evidence on the pathogenic mechanism underlying insulin resistance in a rare form of laminopathy, due to mutation of the LMNA gene coding for lamin A/C, the Dunnigan-type familial partial lipodystrophy (FPLD). The defect, consisting in the intranuclear accumulation of mutant unprocessed precursors of lamin A, reduces the amount of the DNA-bound adipocyte transcription factor sterol regulatory element binding protein 1 (SREBP1) and lowers the peroxisome proliferator-activated receptor (PPARgamma) expression, causing the impairment of pre-adipocyte differentiation. The treatment with the PPARgamma ligand troglitazone (TDZ) is able to rescue the adipogenic program. Since FPLD recapitulates the essential metabolic abnormalities of the common insulin resistance syndrome, the beneficial effects of TDZ on monogenic lipodystrophies might provide a clue as to the future treatment strategies also for the common syndrome of insulin resistance.

Pre-Lamin A processing is linked to heterochromatin organization

Pre-lamin A undergoes subsequent steps of post-translational modification at its C-terminus, including farnesylation, methylation, and cleavage by ZMPSTE24 metalloprotease. Here, we show that accumulation of different intermediates of pre-lamin A processing in nuclei, induced by expression of mutated pre-lamin A, differentially affected chromatin organization in human fibroblasts. Unprocessed (non-farnesylated) pre-lamin A accumulated in intranuclear foci, caused the redistribution of LAP2alpha and of the heterochromatin markers HP1alpha and trimethyl-K9-histone 3, and triggered heterochromatin localization in the nuclear interior. In contrast, the farnesylated and carboxymethylated lamin A precursor accumulated at the nuclear periphery and caused loss of heterochromatin markers and Lap2alpha in enlarged nuclei. Interestingly, pre-lamin A bound both HP1alpha and LAP2alpha in vivo, but the farnesylated form showed reduced affinity for HP1alpha. Our data show a link between pre-lamin A processing and heterochromatin remodeling and have major implications for understanding molecular mechanisms of human diseases linked to mutations in lamins.

Mutations of the LMNA gene can mimic autosomal dominant proximal spinal muscular atrophy

The molecular basis of autosomal dominant spinal muscular atrophy (AD-SMA) is largely unknown. Because the phenotypic spectrum of diseases caused by LMNA mutations is extremely broad and includes myopathies, neuropathies, and cardiomyopathies designated as class 1 laminopathies, we sequenced the LMNA gene in index patients with the clinical picture of proximal SMA, who had a family history suggestive of autosomal dominant inheritance. Among the 19 families investigated, two showed pathogenic mutations of the LMNA gene, resulting in the diagnosis of a class 1 laminopathy in about 10% of our series. We found one novel truncating mutation (c.1477C > T, Q493X) and one previously described missense mutation (c.1130G > T, R377H) in the LMNA gene of two unrelated patients with adult-onset proximal SMA followed by cardiac involvement 14 and 22 years after the onset of weakness. The pedigrees of both families revealed a high frequency of cardiac abnormalities or sudden deaths. Our findings extend the spectrum of laminopathies and are of relevance for genetic counseling and clinical care of families presenting with adult-onset proximal SMA. Particularly, if neurogenic atrophy is combined with a cardiac disease in a family, this should prompt LMNA mutation analysis.

Lamin A/C assembly defects in Emery-Dreifuss muscular dystrophy can be regulated by culture medium composition

Emery-Dreifuss muscular dystrophy results from mutations in either emerin or lamin A/C and is caused by loss of some unknown function of emerin-lamin A/C complexes. This function must be of special importance in the skeletal and cardiac muscles that are affected by the disease. Some lamin A/C mutant proteins form 'nuclear foci' in the nucleoplasm when overexpressed by transient transfection and similar aggregates have been seen in cultured skin fibroblasts from patients with Emery-Dreifuss muscular dystrophy, suggesting that mis-assembly of the A-type lamina may be involved in the pathogenesis. Whereas an earlier study of cultured skin fibroblasts compared several different missense mutations in lamin A/C, we have chosen to study one particular Emery-Dreifuss mutation (R249Q) in greater detail. We found that the proportion of fibroblast nuclei containing abnormal lamin A/C aggregates can vary from 0.5 to 23.6% depending on the culture conditions. In particular, switching from a 'slow growth' medium to 'rapid growth' media increased both the number and size of nuclear aggregates. Similar results were obtained with fibroblasts from a second unrelated patient with the same mutation. In contrast to these aggregates of endogenous lamin A/C, 'nuclear foci' formed after transfection of mouse embryo fibroblasts by mutant lamin A/C were not affected by culture conditions. Faulty assembly of the nuclear lamina by mutated lamin A/C molecules could be partly responsible for the disease phenotype, though this has not been proven. The present study suggests that inappropriate lamin A/C assembly may be preventable by manipulation of cell growth conditions.

Lamin A/C-dependent localization of Nesprin-2, a giant scaffolder at the nuclear envelope

The vertebrate proteins Nesprin-1 and Nesprin-2 (also referred to as Enaptin and NUANCE) together with ANC-1 of Caenorhabditis elegans and MSP-300 of Drosophila melanogaster belong to a novel family of alpha-actinin type actin-binding proteins residing at the nuclear membrane. Using biochemical techniques, we demonstrate that Nesprin-2 binds directly to emerin and the C-terminal common region of lamin A/C. Selective disruption of the lamin A/C network in COS7 cells, using a dominant negative lamin B mutant, resulted in the redistribution of Nesprin-2. Furthermore, using lamin A/C knockout fibroblasts we show that lamin A/C is necessary for the nuclear envelope localization of Nesprin-2. In normal skin where lamin A/C is differentially expressed, strong Nesprin-2 expression was found in all epidermal layers, including the basal layer where only lamin C is present. This indicates that lamin C is sufficient for proper Nesprin-2 localization at the nuclear envelope. Expression of dominant negative Nesprin-2 constructs and knockdown studies in COS7 cells revealed that the presence of Nesprin-2 at the nuclear envelope is necessary for the proper localization of emerin. Our data imply a scaffolding function of Nesprin-2 at the nuclear membrane and suggest a potential involvement of this multi-isomeric protein in human disease.

Altered pre-lamin A processing is a common mechanism leading to lipodystrophy

Lipodystrophies are a heterogeneous group of human disorders characterized by the anomalous distribution of body fat associated with insulin resistance and altered lipid metabolism. The pathogenetic mechanism of inherited lipodystrophies is not yet clear; at the molecular level they have been linked to mutations of lamin A/C, peroxisome proliferator-activated receptor (PPARgamma) and other seemingly unrelated proteins. In this study, we examined lamin A/C processing in three laminopathies characterized by lipodystrophic phenotypes: Dunnigan type familial partial lipodystrophy, mandibuloacral dysplasia and atypical Werner's syndrome. We found that the lamin A precursor was specifically accumulated in lipodystrophy cells. Pre-lamin A was located at the nuclear envelope and co-localized with the adipocyte transcription factor sterol regulatory element binding protein 1 (SREBP1). Using co-immunoprecipitation experiments, we obtained the first demonstration of an in vivo interaction between SREBP1 and pre-lamin A. Binding of SREBP1 to the lamin A precursor was detected in patient fibroblasts as well as in control fibroblasts forced to accumulate pre-lamin A by farnesylation inhibitors. In contrast, SREBP1 did not interact in vivo with mature lamin A or C in cultured fibroblasts. To gain insights into the effect of pre-lamin A accumulation in adipose tissue, we inhibited lamin A precursor processing in 3T3-L1 pre-adipocytes. Our results show that pre-lamin A sequesters SREBP1 at the nuclear rim, thus decreasing the pool of active SREBP1 that normally activates PPARgamma and causing impairment of pre-adipocyte differentiation. This defect can be rescued by treatment with troglitazone, a known PPARgamma ligand activating the adipogenic program.

p.S143F mutation in lamin A/C: A new phenotype combining myopathy and progeria

We report a young girl with a phenotype combining early-onset myopathy and a progeria. She had myopathy and marked axial weakness during the first year of life; progeroid features, including growth failure, sclerodermatous skin changes, and osteolytic lesions, developed later. We identified the underlying cause to be a hitherto unreported de novo missense mutation in the LMNA gene (S143F) encoding the nuclear envelope proteins lamins A and C. Although LMNA mutations have been known to cause Hutchinson-Gilford progeria syndrome and Emery-Dreifuss muscular dystrophy, this is the first report of a patient combining features of these two phenotypes because of a single mutation in LMNA.

Nuclear envelope alterations in fibroblasts from patients with muscular dystrophy, cardiomyopathy, and partial lipodystrophy carrying lamin A/C gene mutations

Mutations in LMNA, the gene that encodes nuclear lamins A and C, cause up to eight different diseases collectively referred to as "laminopathies." These diseases affect striated muscle, adipose tissue, peripheral nerve, and bone, or cause features of premature aging. We investigated the consequences of LMNA mutations on nuclear architecture in skin fibroblasts from 13 patients with different laminopathies. Western-blotting showed that none of the mutations examined led to a decrease in cellular levels of lamin A or C. Regardless of the disease, we observed honeycomb nuclear structures and nuclear envelope blebs in cells examined by immunofluorescence microscopy. Concentrated foci of lamin A/C in the nucleoplasm were also observed. Only mutations in the head and tail domains of lamins A and C significantly altered the nuclear architecture of patient fibroblasts. These results confirm that mutations in lamins A and C may lead to a weakening of a structural support network in the nuclear envelope in fibroblasts and that nuclear architecture changes depend upon the location of the mutation in different domains of lamin A/C.

Progeria: a new kind of Laminopathy-- report of the First European Symposium on Progeria and creation of EURO-Progeria, a European Consortium on Progeria and related disorders

Progeria is a rare, genetically determined condition characterized by accelerated aging in children. Its name is derived from Greek (Geron) and means "prematurely old". The classic type is the Hutchinson-Gilford Progeria Syndrome (HGPS), which was first described in England in 1886 by Dr. Jonathan Hutchinson (1) and again in 1904 by Dr. Hastings Gilford (2). Since then and up to now, very little advancement toward the understanding of this devastating disorder has been accomplished. In early 2003 a French group succeeded in identifying point mutations in the LMNA gene, encoding A-type lamins, as the main cause of this disorder. These results were concomitantly confirmed by an American group, who identified mutations in LMNA, working on a large cohort of patients (3,4). HGPS is thus the most severe disorder added to the expanding list of "laminopathies", diseases caused by mutations in the LMNA gene encoding A-type lamins. To date, up to ten disorders are associated with mutations in LMNA. These disorders are diverse, both in their symptomatology and pattern of inheritance (see below and table 1). Due to the extremely low prevalence of progeria and the putative functional links between progeria and other premature aging disorders, setting-up a network about these disorders has become an absolute necessity. A reunion of families with a child affected with progeria, gathered within the European Progeria Family Circle, was held from September 25th to 29th 2003 in Magdeburg, Germany. In parallel to this event, a scientific symposium centered on clinical and molecular update of HGPS and related syndromes was organised. Several international experts, including clinical and molecular geneticists, cell biologists involved in the field of laminopathies, as well as paediatricians and other physicians with clinical experience in diagnosis, treatment and research on progeria and progeria-like syndromes presented their experience as well as their research projects and yet unpublished results. The main discussed topics as well as the developing research fields on progeria and related premature ageing disorders will be presented here.

Failure of lamin A/C to functionally assemble in R482L mutated familial partial lipodystrophy fibroblasts: altered intermolecular interaction with emerin and implications for gene transcription

Familial partial lipodystrophy is an autosomal dominant disease caused by mutations of the LMNA gene encoding alternatively spliced lamins A and C. Abnormal distribution of body fat and insulin resistance characterize the clinical phenotype. In this study, we analyzed primary fibroblast cultures from a patient carrying an R482L lamin A/C mutation by a morphological and biochemical approach. Abnormalities were observed consisting of nuclear lamin A/C aggregates mostly localized close to the nuclear lamina. These aggregates were not bound to either DNA-containing structures or RNA splicing intranuclear compartments. In addition, emerin did not colocalize with nuclear lamin A/C aggregates. Interestingly, emerin failed to interact with lamin A in R482L mutated fibroblasts in vivo, while the interaction with lamin C was preserved in vitro, as determined by coimmunoprecipitation experiments. The presence of lamin A/C nuclear aggregates was restricted to actively transcribing cells, and it was increased in insulin-treated fibroblasts. In fibroblasts carrying lamin A/C nuclear aggregates, a reduced incorporation of bromouridine was observed, demonstrating that mutated lamin A/C in FPLD cells interferes with RNA transcription.

Expression of lamin A mutated in the carboxyl-terminal tail generates an aberrant nuclear phenotype similar to that observed in cells from patients with Dunnigan-type partial lipodystrophy and Emery-Dreifuss muscular dystrophy

Autosomal dominantly inherited missense mutations in lamins A and C cause familial partial lipodystrophy of the Dunnigan-type (FPLD), and myopathies including Emery-Dreifuss muscular dystrophy (EDMD). While mutations responsible for FPLD are restricted to the carboxyl-terminal tails, those responsible for EDMD are spread throughout the molecules. We observed here the same structural abnormalities in the nuclear envelope and chromatin of fibroblasts from patients with FPLD and EDMD, harboring missense mutations at codons 482 and 453, respectively. Similar nuclear alterations were generated in fibroblasts, myoblasts, and preadipocytes mouse cell lines overexpressing lamin A harboring either of these two mutations. A large variation in sensitivity to lamin A overexpression was observed among the three cell lines, which was correlated with their variable endogenous content in A-type lamins and emerin. The occurrence of nuclear abnormalities was reduced when lamin B1 was coexpressed with mutant lamin A, emphasizing the functional interaction of the two types of lamins. Transfected cells therefore develop similar phenotypes when expressing lamins mutated in the carboxyl-terminal tail at sites responsible for FPLD or EDMD.

Frequent low penetrance mutations in the Lamin A/C gene, causing Emery Dreifuss muscular dystrophy

Emery Dreifuss muscular dystrophy is a genetically heterogeneous disorder characterized by the clinical triad of early onset contractures, progressive muscular wasting and weakness with humeroperoneal distribution and cardiac conduction defects. Mutations in the Lamin A/C (LMNA) gene are responsible for the autosomal dominant and the autosomal recessive forms. Familiar and sporadic patients carrying mutations in the LMNA gene show high variability in the clinical symptomatology and age of onset. In this report, we describe four families harboring missense mutations in the LMNA gene and we show that the effect of mutations ranges from silent to fully penetrant. We suggest that incomplete penetrance of dominant mutations in the LMNA gene is a common feature and we emphasize the significance of mutational analysis in relatives of sporadic cases of laminopathies, as asymptomatic carriers face high risk of sudden cardiac death.

[Hauptmann-Thannhauser muscular dystrophy and differential diagnosis of myopathies associated with contractures]

Hauptmann-Thannhauser muscular dystrophy is characterized by the clinical triad of early-onset contractures of elbow, Achilles tendons, and cervical spine, slowly progressive humeroperoneal muscle wasting and weakness, and life-threatening cardiac involvement with conduction blocks manifesting in the third decade. Hauptmann-Thannhauser muscular dystrophy is due to mutations in the LMNA gene affecting the nuclear envelope proteins lamin A and C. We present a 16-year-old German boy with typical muscular involvement and contractures and typical course of Hauptmann-Thannhauser muscular dystrophy due to the novel missense mutation R401C. The data of this family suggest a lower penetrance of muscular and especially cardiac symptoms than expected. Autosomal-dominant Hauptmann-Thannhauser muscular dystrophy and X-chromosomal Emery-Dreifuss muscular dystrophy are not clearly distinguishable by phenotypic criteria. Other muscular diseases associated with contractures and congenital or childhood onset are reviewed.

Expression pattern and further characterization of human MAGED2 and identification of rodent orthologues

In a search for genes involved in X-linked mental retardation we have analyzed the expression pattern and genomic structure of human MAGED2. This gene is a member of a new defined MAGE-D cluster in Xp11.2, a hot spot for X-linked mental retardation. Rat and mouse orthologues have been isolated. In contrast to the genes of the MAGE-A, MAGE- B and MAGE-C clusters, MAGED2 is expressed ubiquitously. High expression was detected in specific brain regions and in the interstitium of testes. Five SNPs in the coding region of human MAGED2 were characterized and their allele frequencies determined in a German and Turkish population.

Both emerin and lamin C depend on lamin A for localization at the nuclear envelope

Physical interactions between lamins and emerin were investigated by co-immunoprecipitation of in vitro translated proteins. Emerin interacted with in vitro translated lamins A, B1 and C in co-immunprecipitation reactions. Competition reactions revealed a clear preference for interactions between emerin and lamin C. Structural associations between lamins and emerin were investigated in four human cell lines displaying abnormal expression and/or localisation of lamins A and C. In each cell line absence of lamins A and C from the nuclear envelope (NE) was correlated with mis-localisation of endogenous and exogenous emerin to the ER. In two cell lines that did not express lamin A but did express lamin C, lamin C as well as emerin was mis-localised. When GFP-lamin A was expressed in SW13 cells (which normally express only very low levels of endogenous lamin A and mis-localise endogenous emerin and lamin C), all three proteins became associated with the NE. When GFP-lamin C was expressed in SW13 cells neither the endogenous nor the exogenous lamin C was localised to the NE and emerin remained in the ER. Finally, lamins A and C were selectively eliminated from the NE of HeLa cells using a dominant negative mutant of lamin B1. Elimination of these lamins from the lamina led to the accumulation of emerin as aggregates within the ER. Our data suggest that lamin A is essential for anchorage of emerin to the inner nuclear membrane and of lamin C to the lamina.

Genomic organization of the human complex I 13-kDa subunit gene NDUFA5

We have characterized the human gene NDUFA5 encoding a 13-kDa subunit of the mitochondrial respiratory chain complex I (NADH: ubiquinone oxidoreductase). The gene contains 5 exons and 4 introns, and spans 14 kb of genomic DNA. In the untranscribed region we observed potential transcription factor binding sites. We determined a single nucleotide variant (C/T) at -318, and its frequency in the German population. The functional gene was localised by FISH to 7q31 and by radiation hybrid panel near marker D7S648 in YAC 883_a_2.

Clinical and molecular genetic spectrum of autosomal dominant Emery-Dreifuss muscular dystrophy due to mutations of the lamin A/C gene

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by early contractures of the elbows and Achilles tendons, slowly progressive muscle wasting and weakness, and life-threatening cardiomyopathy with conduction blocks. We recently identified LMNA encoding two nuclear envelope proteins, lamins A and C, to be implicated in the autosomal dominant form of EDMD. Here, we report on the variability of the phenotype and spectrum of LMNA mutations in 53 autosomal dominant EDMD patients (36 members of 6 families and 17 sporadic cases). Twelve of the 53 patients showed cardiac involvement exclusively, although the remaining 41 all showed muscle weakness and contractures. We were able to identify a common phenotype among the patients with skeletal muscle involvement, consisting of humeroperoneal wasting and weakness, scapular winging, rigidity of the spine, and elbow and Achilles tendon contractures. The disease course was generally slow, but we observed either a milder phenotype characterized by late onset and a mild degree of weakness and contractures or a more severe phenotype with early presentation and a rapidly progressive course in a few cases. Mutation analysis identified 18 mutations in LMNA (i.e., 1 nonsense mutation, 2 deletions of a codon, and 15 missense mutations). All the mutations were distributed between exons 1 and 9 in the region of LMNA that is common to lamins A and C. LMNA mutations arose de novo in 76% of the cases; 2 of these de novo mutations were typical hot spots, and 2 others were identified in 2 unrelated cases. There was no clear correlation between the phenotype and type or localization of the mutations within the gene. Moreover, a marked inter- and intra-familial variability in the clinical expression of LMNA mutations exists, ranging from patients expressing the full clinical picture of EDMD to those characterized only by cardiac involvement, which points toward a significant role of possible modifier genes in the course of this disease. In conclusion, the high proportion of de novo mutations together with the large spectrum of both LMNA mutations and the expression of the disease should now prompt screening for LMNA in familial and sporadic cases of both EDMD and dilated cardiomyopathy associated with conduction system disease.

Six novel mutations in the emerin gene causing X-linked Emery-Dreifuss muscular dystrophy

Mutations in the emerin gene, also referred to as the STA- or EMD-gene, have been found to be the cause of X-linked Emery-Dreifuss muscular dystrophy (EMD). For the present study an optimized set of primers was designed to amplify and sequence each of the six emerin gene exons, including the intron/exon boundaries. All emerin gene exons of 30 unrelated EMD patients have been screened by heteroduplex analysis. Aberrant patterns of single exons were found in seven patients. Direct sequencing of the respective exons revealed six novel mutations distributed in the promotor region and exons 3-6 (delta nt -19 to -40; delta AG nt 620-621; ins A nt 895; delta AT nt 908-909; C-->A nt 1420; ins TA nt 1570). By this study, the first mutations in the promotor region and in exon 5 have been identified. Each of the 25 mutations that have been described so far, including those from the present study, abolishes the synthesis of functional emerin. The mutations were submitted to the EMD Mutation database (http://www.path.cam.ac.uk/emd).

Cardiac responses to exercise in competitive child cyclists

PURPOSE

Cardiovascular responses to exercise in highly trained child endurance athletes have not been well-defined. This study compared hemodynamic responses with progressive cycle exercise in seven competitive child cyclists (mean age 11.9 yr) compared with 39 age-matched untrained boys.

METHODS

Doppler echocardiography and gas exchange variables were utilized to assess cardiovascular changes during submaximal and maximal exercise.

RESULTS

Mean VO2max was 60.0 (+/-6.0) and 47.0 (+/-5.8) mL x kg(-1) x min(-1) in the cyclists and nonathletes, respectively. At rest and maximal exercise, the cyclists demonstrated greater stroke index than the untrained subjects (resting mean 59 (+/-6) vs 44 (+/-9) mL x m(-2); maximal mean 76 (+/-6) vs 60 (+/-11) mL x m(-2)), but the ratio of maximal:rest stroke index was similar in both groups (1.31 for cyclists, 1.41 for nonathletes). Both groups showed a plateau in stroke volume beyond low-intensity work levels. No significant difference was observed in maximal arteriovenous oxygen difference.

CONCLUSIONS

These findings indicate that 1) maximal stroke volume is the critical determinant of the high VO2max in child cyclists and 2) factors that influence resting stroke volume are important in defining VO2max differences between child endurance athletes and untrained boys.