141st ENMC International Workshop Inaugural Meeting of the EURO-Laminopathies Project Nuclear Envelope-linked Rare Human Diseases: From Molecular Pathophysiology towards Clinical Applications 10–12 March 2006, Naarden, The Netherlands

Abstract

1. IntroductionEURO-Laminopathies is a European Commission-funded research project, including 13 scientific group leaders in the fields of human genetics, clinical research, structural biology, molecular cell biology, and pharmaceutical industry, and one administrative manager from eight countries (Austria, France, Germany, Italy Israel, Spain, Switzerland and the United Kingdom). The consortium held its kick-off meeting as an International ENMC Workshop in Naarden on the weekend of March 10–12, 2006. The EURO-Laminopathies project aims at understanding the molecular mechanisms of laminopathies, which are rare human diseases linked to mutations in genes encoding nuclear envelope proteins, such as A-type lamins (LMNA), proteins involved in the post-translational processing of A-type lamins (ZMPSTE24), and lamin-binding proteins (EMD, LBR, LAP2). Laminopathies are clinically manifested after birth, can affect different tissues and progressively develop during childhood or adolescence, often leading to early death. Efficient therapies have been hampered by the lack of understanding the molecular disease mechanisms. The ultimate goal of the project thus is to identify reliable diagnostic markers and drug targets in order to rationally develop new therapeutic interventions and to improve existing therapies for laminopathy patients.During the workshop, new insights into the clinical and genetic spectrum of laminopathies and into clinical trials on the treatment of lipodystrophy-type laminopathy patients were given and future prospects on novel therapeutic approaches and theranostic tests for the validation of therapies were presented. Furthermore, the consortium discussed clinical and basic research approaches in order to analyze the effects of disease-causing mutations in A-type lamins and in one of their prominent binding partners, Lamina-associated polypeptide 2-alpha (LAP2α) on the atomic structure, interactions, and assembly properties of the proteins and on their potential roles in chromatin organization, gene expression, and differentiation of adult muscle and adipose stem cells.2. BackgroundLamins are major architectural proteins in the nuclei of eukaryotic cells [1Burke B. Stewart C.L. The laminopathies: the functional architecture of the nucleus and its contribution to disease.Annu Rev Genomics Hum Genet. 2006; 7: 369-405Crossref PubMed Scopus (128) Google Scholar, 2Gruenbaum Y. Margalit A. Goldman R.D. Shumaker D.K. Wilson K.L. The nuclear lamina comes of age.Nat Rev Mol Cell Biol. 2005; 6: 21-31Crossref PubMed Scopus (686) Google Scholar]. B-type lamins are expressed in all cells and are essential for cell viability, while A-type lamins are expressed primarily in differentiated cells and are involved in tissue homeostasis and function. Mutations in A-type lamins and their binding partners cause a variety of disease phenotypes, collectively called laminopathies [3Broers J.L. Ramaekers F.C. Bonne G. Yaou R.B. Hutchison C.J. Nuclear lamins: laminopathies and their role in premature ageing.Physiol Rev. 2006; 86: 967-1008Crossref PubMed Scopus (445) Google Scholar, 4Capell B.C. Collins F.S. Human laminopathies: nuclei gone genetically awry.Nat Rev Genet. 2006; 7: 940-952Crossref PubMed Scopus (405) Google Scholar]. These diseases can affect muscle, adipose, nerve, bone, and skin tissues or cause premature ageing. Based on known and proposed functions of lamins, various disease hypotheses have been proposed to explain the molecular basis of laminopathies, but it remains unclear how much a particular disease mechanism can contribute to a given clinical phenotype [2Gruenbaum Y. Margalit A. Goldman R.D. Shumaker D.K. Wilson K.L. The nuclear lamina comes of age.Nat Rev Mol Cell Biol. 2005; 6: 21-31Crossref PubMed Scopus (686) Google Scholar, 5Gotzmann J. Foisner R. A-type lamin complexes and regenerative potential: a step towards understanding laminopathic diseases?.Histochem Cell Biol. 2006; 125: 33-41Crossref PubMed Scopus (90) Google Scholar, 6Hutchison C.J. Worman H.J. A-type lamins: guardians of the soma?.Nat Cell Biol. 2004; 6: 1062-1067Crossref PubMed Scopus (174) Google Scholar]. The mechanical hypothesis predicts that mutations in lamins and lamin-binding proteins alter their structure and weaken their stability, either by interfering with proper folding of the proteins or by affecting the assembly of lamina protein complexes, thereby predisposing cells and tissues to physical damage. This model seems reasonable particularly for muscle tissue, as lamins provide structural stability to the nucleus and muscle is exposed to physical stress. In line with this hypothesis, laminopathy patient often have structural abnormalities of their cell nuclei. However, the structural disease model cannot explain all clinical pathologies, as unaffected tissues also show deformed nuclei. The gene expression hypothesis proposes that mutations in lamins disrupt interactions of lamins with transcriptional regulators, such as the adipocyte-specific transcription factor sterol response element binding protein, and affect tissue-specific patterns of gene expression. Furthermore, lamins are also involved in epigenetic pathways regulating heterochromatin formation through numerous interactions of lamin complexes with DNA and chromatin proteins [[7]Vlcek S, Foisner R. A-type lamin networks in light of laminopathic diseases. Biochim Biophys Acta 2006, published online.Google Scholar]. Mutations in A-type lamins may cause gross changes in higher order chromatin structure and gene expression. In line with this hypothesis nuclei from patient cells often lack the peripheral heterochromatin. The cell proliferation/differentiation hypothesis is mainly based on the in vivo interaction between lamins A/C and LAP2α and the tumor suppressor retinoblastoma protein [8Dorner D. Vlcek S. Foeger N. Gajewski A. Makolm C. Gotzmann J. Hutchison C.J. Foisner R. Lamina-associated polypeptide 2{alpha} regulates cell cycle progression and differentiation via the retinoblastoma-E2F pathway.J Cell Biol. 2006; 173: 83-93Crossref PubMed Scopus (127) Google Scholar, 9Pekovic V. Harborth J. Broers J.L. Ramaekers F.C. van Engelen B. Lammens M. von Zglinicki T. Foisner R. Hutchison C. Markiewicz E. Nucleoplasmic LAP2{alpha}-lamin A complexes are required to maintain a proliferative state in human fibroblasts.J Cell Biol. 2007; 176: 163-172Crossref PubMed Scopus (100) Google Scholar], which is required for muscle and adipocyte differentiation. It has been suggested that stem cells in laminopathy patients have a defective differentiation potential and cannot effectively regenerate tissues.3. Genetic and clinical features of laminopathiesDisease-causing mutations are currently reported for 11 genes encoding nuclear envelope components (LMNA, LMNB1, LMNB2, EMD, LAP2, LBR, LEMD3, ZMPSTE24, SYNE-1, NUP62, DYT1). Among them, the major group of diseases is caused either by mutations in the lamin A/C gene (i.e. primary laminopathies) or by mutations in the ZMPSTE24 gene affecting the correct post-translational processing of prelamin A and thus considered as secondary laminopathies. Primary laminopathies can be classified into 5 types affecting either specific tissue in isolated fashion, i.e. (1) the striated muscles, (2) the peripheral nerves, and (3) the adipose tissue; or in a systemic way several tissues with (4) the premature ageing syndromes and their related disorders, named also “systemic laminopathies”. Finally, numerous heterogeneous clinical situations have been reported and form the fifth group of disorders that comprise overlapping phenotypes characterized by the coexistence of two or more tissue involvements, suggesting a real continuum within the different types of laminopathies [[3]Broers J.L. Ramaekers F.C. Bonne G. Yaou R.B. Hutchison C.J. Nuclear lamins: laminopathies and their role in premature ageing.Physiol Rev. 2006; 86: 967-1008Crossref PubMed Scopus (445) Google Scholar]. Gisèle Bonne, Jacqueline Capeau, Nicolas Lévy, and Manfred Wehnert reported that up to now, more than 211 mutations of the LMNA gene in more than 1037 individuals have been identified, of whom about 60% presented laminopathies affecting the striated muscles (Emery Dreifuss Muscular Dystrophy; Limb Girdle Muscular Dystrophy; Dilated Cardiomyopathy with Conduction Defects), 25% laminopathies affecting the adipose issue (Dunningan type familial partial lipodystrophy), 6% presented with premature aging syndromes (Hutchinson–Gilford Progeria; Atypical Werner Syndrome) and 3% with axonal neuropathies. Faced with this very wide diversity, a Universal mutation database (UMD)-LMNA database has been established which brings together all the clinical and genetic data concerning the mutations described by our networks as well as those reported in the literature (http://www.umd.be:2000). Similar UMD mutation databases were also created for the EMD (http://www.umd.be:2010) and ZMPSTE24 gene (soon available on the UMD web site). These mutation databases are useful tools for analyzing phenotype/genotype relation in this complex group of disorders. It was also discussed that in addition to mutations in the lamin A genes, an increasing number of lamin A-interacting proteins, such as emerin, LAP2α, and MAN1 have been linked to similar diseases showing clinically overlapping phenotypes with the lamin-linked laminopathies [3Broers J.L. Ramaekers F.C. Bonne G. Yaou R.B. Hutchison C.J. Nuclear lamins: laminopathies and their role in premature ageing.Physiol Rev. 2006; 86: 967-1008Crossref PubMed Scopus (445) Google Scholar, 7Vlcek S, Foisner R. A-type lamin networks in light of laminopathic diseases. Biochim Biophys Acta 2006, published online.Google Scholar]. The number of laminopathy-linked genes is likely to increase, since a growing number of patients with laminopathy-type pathologies do not have mutations in any of the known disease genes. In this context, the human geneticist and clinician partners of the EURO-Laminopathies consortium will mainly focus on: (1) the further description of LMNA, ZMPSTE24, EMD and LAP2 gene mutations and of the clinical spectrum of associated diseases, (2) the search for new genes responsible for closely related disorders in both a gene and a syndrome candidate approach, (3) the understanding of the clinical variability of these disorders through the analyses of possible phenotype/genotype relations as well as the identification of modifier genes and/or polymorphisms.4. Molecular disease mechanisms4.1 Lamin structure and assemblyLamins represent the principal molecular building blocks of the nuclear lamina, a filamentous meshwork closely adhering to the inner nuclear membrane. Based on their primary sequence lamins are members of the superfamily of intermediate filament proteins with a conserved tripartite domain organization, consisting of a central α-helical rod domain flanked by a short N-terminal head domain and a large globular C-terminal tail domain [[10]Herrmann H. Aebi U. Intermediate filaments: molecular structure, assembly mechanism, and integration into functionally distinct intracellular Scaffolds.Annu Rev Biochem. 2004; 73: 749-789Crossref PubMed Scopus (546) Google Scholar]. Harald Herrmann discussed studies on the in vitro assembly of lamins into filamentous and fibrous structures reminiscent of intermediate filament-like filaments and paracrystalline fibers [[11]Foeger N. Wiesel N. Lotsch D. Mucke N. Kreplak L. Aebi U. Gruenbaum Y. Herrmann H. Solubility properties and specific assembly pathways of the B-type lamin from Caenorhabditis elegans.J Struct Biol. 2006; 155: 340-350Crossref PubMed Scopus (50) Google Scholar]. However, whereas vertebrate cytoplasmic intermediate filament dimers first associate laterally into so-called unit-length filaments before these longitudinally anneal and radially compact to yield mature 10-nm filaments [[12]Herrmann H. Aebi U. Intermediate filament assembly: fibrillogenesis is driven by decisive dimer–dimer interactions.Curr Opin Struct Biol. 1998; 8: 177-185Crossref PubMed Scopus (141) Google Scholar], lamin dimers first polymerize head-to-tail into long protofilaments before these associate laterally into filamentous and paracrystalline arrays [[13]Stuurman N. Heins S. Aebi U. Nuclear lamins: their structure, assembly, and interactions.J Struct Biol. 1998; 122: 42-66Crossref PubMed Scopus (594) Google Scholar]. The molecular basis for the different assembly pathways of cytoplasmic intermediate filaments and lamins is poorly understood but recent studies on the elucidation of the atomic structure of N- and C-terminal domains of the central rod domain, which are involved in the head to tail association of lamin dimers during assembly, brought first insights into potential interaction mechanisms. Ueli Aebi reported that the crystal structure of the coil 2B dimer of human lamin A revealed an overall structure similar to the homologous cytoplasmic intermediate filament segment in human vimentin [14Strelkov S.V. Herrmann H. Geisler N. Wedig T. Zimbelmann R. Aebi U. Burkhard P. Conserved segments 1A and 2B of the intermediate filament dimer: their atomic structures and role in filament assembly.Embo J. 2002; 21: 1255-1266Crossref PubMed Scopus (225) Google Scholar, 15Strelkov S.V. Schumacher J. Burkhard P. Aebi U. Herrmann H. Crystal structure of the human lamin A coil 2B dimer: implications for the head-to-tail association of nuclear lamins.J Mol Biol. 2004; 343: 1067-1080Crossref PubMed Scopus (169) Google Scholar]. However, the distribution of charged residues and the patterns of intra- and interhelical salt bridges were different.Because of the described structural roles of lamins, it is widely believed that the lamina acts as a cellular equivalent of a tensegrity device, i.e. a load bearing structure that provides resilience and an ability to resist deformation forces [[16]Hutchison C.J. Lamins: building blocks or regulators of gene expression?.Nat Rev Mol Cell Biol. 2002; 3: 848-858Crossref PubMed Scopus (247) Google Scholar]. Shear stress, for example, has been shown to induce structural remodeling of the nucleus [[17]Deguchi S. Maeda K. Ohashi T. Sato M. Flow-induced hardening of endothelial nucleus as an intracellular stress-bearing organelle.J Biomech. 2005; 38: 1751-1759Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar]. Such physical stimuli may critically involve the nuclear lamina, not only as a nuclear architectural moiety but also as a ‘mechanical signal transducer’ coupling the cytoskeleton to the nuclear interior. Thus, an altered behavior or loss of resistance to mechanical stress may be a common phenomenon in laminopathies [[18]Lammerding J. Hsiao J. Schulze P.C. Kozlov S. Stewart C.L. Lee R.T. Abnormal nuclear shape and impaired mechanotransduction in emerin-deficient cells.J Cell Biol. 2005; 170: 781-791Crossref PubMed Scopus (276) Google Scholar]. Furthermore, the lamina plays important roles in organizing peripheral chromatin, positioning the nuclear pore complexes within the nuclear envelope, and coupling the cytoskeleton to the nuclear envelope and the nuclear skeleton.The more than 200 different mutations in the LMNA gene, which give rise to the multiple disease phenotypes, have been reported to be spread across the entire gene sequence. As yet, the hypothesis that a particular lamin A mutation position determines the laminopathy phenotype has not been proven. So far, only one report documented that mutations within lamin A’s C-terminal Ig-like domain (residues 430–545), which destabilize its three-dimensional fold are causing muscular dystrophy. In contrast, charged residues residing on the surface of this domain that may be involved in the interaction of lamin A with other proteins are causing lipodystrophy when mutated [[19]Krimm I. Ostlund C. Gilquin B. Couprie J. Hossenlopp P. Mornon J.P. Bonne G. Courvalin J.C. Worman H.J. Zinn-Justin S. The Ig-like structure of the C-terminal domain of lamin A/C, mutated in muscular dystrophies, cardiomyopathy, and partial lipodystrophy.Structure. 2002; 10: 811-823Abstract Full Text Full Text PDF PubMed Scopus (236) Google Scholar]. Besides the Ig-fold and the above mentioned coil 2B, high resolution structures of other lamin domains are not available except for atomic models of coil 1A [[15]Strelkov S.V. Schumacher J. Burkhard P. Aebi U. Herrmann H. Crystal structure of the human lamin A coil 2B dimer: implications for the head-to-tail association of nuclear lamins.J Mol Biol. 2004; 343: 1067-1080Crossref PubMed Scopus (169) Google Scholar]. In order to reveal potential molecular consequences at the atomic level of disease-causing mutations in the LMNA and the LAP2α genes, the EURO-Laminopathies structural biology team proposed to elucidate the crystal structure of wild-type and disease variants of lamins and LAP2α. They will apply a ‘divide-and-conquer’ crystallographic approach based on the analysis of specifically designed protein fragments and complexes of fragments. Furthermore they presented plans to develop novel assays to test lamin in vitro assembly regimes and to investigate the molecular effects of disease-causing mutations on the assembly and mechanical stability of lamin complexes.4.2 Nuclear and chromatin organization in laminopathiesCytological studies show that the position of chromatin in nuclei is not random. Each chromosome maintains a discrete domain within the nucleus [20Cremer T. Cremer C. Chromosome territories, nuclear architecture and gene regulation in mammalian cells.Nat Rev Genet. 2001; 2: 292-301Crossref PubMed Scopus (1671) Google Scholar, 21Misteli T. Beyond the sequence: cellular organization of genome function.Cell. 2007; 128: 787-800Abstract Full Text Full Text PDF PubMed Scopus (882) Google Scholar] and large proportions of condensed chromatin are localized at the nuclear periphery [[22]Boyle S. Gilchrist S. Bridger J.M. Mahy N.L. Ellis J.A. Bickmore W.A. The spatial organization of human chromosomes within the nuclei of normal and emerin-mutant cells.Hum Mol Genet. 2001; 10: 211-219Crossref PubMed Scopus (512) Google Scholar]. Furthermore, in mammalian cells, the position of chromosomes that are more transcriptionally active (chromosome 19, active X-chromosome) is more towards the interior of nuclei, whereas the gene-poor chromosomes (chromosome 18, inactive X-chromosome) are positioned towards the nuclear periphery [22Boyle S. Gilchrist S. Bridger J.M. Mahy N.L. Ellis J.A. Bickmore W.A. The spatial organization of human chromosomes within the nuclei of normal and emerin-mutant cells.Hum Mol Genet. 2001; 10: 211-219Crossref PubMed Scopus (512) Google Scholar, 23Shumaker D.K. Dechat T. Kohlmaier A. Adam S.A. Bozovsky M.R. Erdos M.R. Eriksson M. Goldman A.E. Khuon S. Collins F.S. Jenuwein T. Goldman R.D. Mutant nuclear lamin A leads to progressive alterations of epigenetic control in premature aging.Proc Natl Acad Sci USA. 2006; 103: 8703-8708Crossref PubMed Scopus (542) Google Scholar]. Joanna Bridger presented evidence that chromosome position may also be altered in cells containing specific LMNA mutations.It is now well established that lamins A/C are involved in chromatin organization at the nuclear periphery [[2]Gruenbaum Y. Margalit A. Goldman R.D. Shumaker D.K. Wilson K.L. The nuclear lamina comes of age.Nat Rev Mol Cell Biol. 2005; 6: 21-31Crossref PubMed Scopus (686) Google Scholar]. Nuclei of progeria fibroblasts expressing the mutant lamin A (LAΔ0) lose their peripheral heterochromatin and a histone methylation mark of pericentric heterochromatin (lysine 9 of histone H3) is reduced, as well as the association of this mark with heterochromatin protein 1α [[23]Shumaker D.K. Dechat T. Kohlmaier A. Adam S.A. Bozovsky M.R. Erdos M.R. Eriksson M. Goldman A.E. Khuon S. Collins F.S. Jenuwein T. Goldman R.D. Mutant nuclear lamin A leads to progressive alterations of epigenetic control in premature aging.Proc Natl Acad Sci USA. 2006; 103: 8703-8708Crossref PubMed Scopus (542) Google Scholar]. Interestingly, downregulation of the mutant lamin A (LAΔ0) restores the heterochromatin pattern at the nuclear periphery [[24]Scaffidi P. Misteli T. Reversal of the cellular phenotype in the premature aging disease Hutchinson–Gilford progeria syndrome.Nat Med. 2005; 11: 440-445Crossref PubMed Scopus (448) Google Scholar], demonstrating that expression of the mutated protein is directly linked to chromatin reorganization. Nadir Maraldi and Giovanna Lattanzi presented evidence that LNMA mutations in other laminopathies may also alter chromatin organization. For example, nuclei derived from familial partial lipodystrophy patient cells contain farnesylated lamin A (typically found in progeria [[25]Goldman R.D. Shumaker D.K. Erdos M.R. Eriksson M. Goldman A.E. Gordon L.B. Gruenbaum Y. Khuon S. Mendez M. Varga R. Collins F.S. Accumulation of mutant lamin A causes progressive changes in nuclear architecture in Hutchinson–Gilford progeria syndrome.Proc Natl Acad Sci USA. 2004; 101: 8963-8968Crossref PubMed Scopus (800) Google Scholar]) and show chromatin abnormalities [[26]Capanni C. Mattioli E. Columbaro M. Lucarelli E. Parnaik V.K. Novelli G. Wehnert M. Cenni V. Maraldi N.M. Squarzoni S. Lattanzi G. Altered prelamin A processing is a common mechanism leading to lipodystrophy.Hum Mol Genet. 2005; 14: 1489-1502Crossref PubMed Scopus (180) Google Scholar]. This also suggests a direct correlation between accumulation of farnesylated lamin A and chromatin defects in laminopathic disorders.Both pre-B-type lamins and prelamin A have a C-terminal CaaX motif that is subjected to several post-translational modifications [[27]Mattout A. Dechat T. Adam S.A. Goldman R.D. Gruenbaum Y. Nuclear lamins, diseases and aging.Curr Opin Cell Biol. 2006; 18: 335-341Crossref PubMed Scopus (140) Google Scholar]. First, the cysteine is farnesylated, then the last three residues are cleaved off and the cysteine undergoes methyl esterification. Prelamin A undergoes an additional cleavage that removes the 15 C-terminal amino acids, including the farnesyl group. Both cleavage steps in the maturation of lamin A are probably catalyzed by the zinc-metalloproteinase ZMPSTE24 and are dependent on the sequence of processing steps. In the majority of cases progeria-linked mutated lamin A cannot undergo the final proteolytic cleavage step due to a loss of the proteolytic cleavage site, thus staying permanently farnesylated. Giovanni Lattanzi and Nadir Maraldi are testing the effect of inhibiting each of these steps in lamin A maturation on chromatin organization, as a means of potential therapeutic interventions in patients. They showed that treatment of progeria cells with farnesyl-transferase inhibitors can rescue heterochromatin organization [[28]Columbaro M. Capanni C. Mattioli E. Novelli G. Parnaik V.K. Squarzoni S. Maraldi N.M. Lattanzi G. Rescue of heterochromatin organization in Hutchinson–Gilford progeria by drug treatment.Cell Mol Life Sci. 2005; 62: 2669-2678Crossref PubMed Scopus (133) Google Scholar].Do changes in cell shape, nuclear lamina composition and chromatin organization as seen in progeria fibroblasts also occur in normal cells that are getting old? Yosef Gruenbaum tested this hypothesis in Caenorhabditis elegans. They found that nuclear architecture in most non-neuronal cell types undergoes progressive and stochastic alteration as the animal ages and that the rate of this alteration is affected by mutations in the insulin/insulin-like growth factor like signaling pathway [[29]Haithcock E. Dayani Y. Neufeld E. Zahand A.J. Feinstein N. Mattout A. Gruenbaum Y. Liu J. Age-related changes of nuclear architecture in Caenorhabditis elegans.Proc Natl Acad Sci USA. 2005; 102: 16690-16695Crossref PubMed Scopus (196) Google Scholar]. These changes are accompanied by changes in the distribution of heterochromatin markers. He also showed that reducing the levels of lamin and lamin-associated LEM domain proteins can lead to shortening of the life span. These data correlate with recent results showing that the LAΔ0 splicing isoform that is detected in progeria cells is also present in low amounts in normal cells [[30]Scaffidi P. Misteli T. Lamin A-dependent nuclear defects in human aging.Science. 2006; 312: 1059-1063Crossref PubMed Scopus (858) Google Scholar].4.3 Lamins in the control of cell proliferation and differentiationSeveral transcriptional regulator proteins and signaling components can form lamin-dependent complexes, which regulate gene expression and signal transduction [2Gruenbaum Y. Margalit A. Goldman R.D. Shumaker D.K. Wilson K.L. The nuclear lamina comes of age.Nat Rev Mol Cell Biol. 2005; 6: 21-31Crossref PubMed Scopus (686) Google Scholar, 7Vlcek S, Foisner R. A-type lamin networks in light of laminopathic diseases. Biochim Biophys Acta 2006, published online.Google Scholar]. A novel concept was presented by Roland Foisner and Chris Hutchison, postulating that in addition to the peripheral lamin complexes, lamins in the nucleoplasm also interact and control transcriptional regulators. Dorner et al. have recently reported that nucleoplasmic complexes of lamin A and LAP2α bind to the cell cycle regulator protein retinoblastoma (pRb) and affect its function in the E2F-pRb pathway that controls cell cycle progression and differentiation [[8]Dorner D. Vlcek S. Foeger N. Gajewski A. Makolm C. Gotzmann J. Hutchison C.J. Foisner R. Lamina-associated polypeptide 2{alpha} regulates cell cycle progression and differentiation via the retinoblastoma-E2F pathway.J Cell Biol. 2006; 173: 83-93Crossref PubMed Scopus (127) Google Scholar]. These findings are in line with other reports on lamin A-deficient mouse fibroblasts, providing evidence for the involvement of lamins in cell cycle arrest, by either stabilizing pRb protein [[31]Nitta R.T. Jameson S.A. Kudlow B.A. Conlan L.A. Kennedy B.K. Stabilization of the retinoblastoma protein by A-type nuclear lamins is required for INK4A-mediated cell cycle arrest.Mol Cell Biol. 2006; 26: 5360-5372Crossref PubMed Scopus (70) Google Scholar] or by controlling its phosphorylation/dephosphorylation [[32]Van Berlo J.H. Voncken J.W. Kubben N. Broers J.L. Duisters R. van Leeuwen R.E. Crijns H.J. Ramaekers F.C. Hutchison C.J. Pinto Y.M. A-type lamins are essential for TGF-beta1 induced PP2A to dephosphorylate transcription factors.Hum Mol Genet. 2005; 14: 2839-2849Crossref PubMed Scopus (118) Google Scholar]. This has lead to the formulation of a novel intriguing disease model, proposing that mutations in lamin A or in lamin-binding proteins (emerin and LAP2α) can interfere with their functions in cell cycle control [[5]Gotzmann J. Foisner R. A-type lamin complexes and regenerative potential: a step towards understanding laminopathic diseases?.Histochem Cell Biol. 2006; 125: 33-41Crossref PubMed Scopus (90) Google Scholar]. This would lead to an imbalance of cell proliferation versus differentiation in adult stem cells consequently impairing tissue homeostasis and regeneration. Indeed, expression of disease-causing lamin variants in myoblast cell cultures [[33]Favreau C. Higuet D. Courvalin J.C. Buendia B. Expression of a mutant lamin A that causes Emery-Dreifuss muscular dystrophy inhibits in vitro differentiation of C2C12 myoblasts.Mol Cell Biol. 2004; 24: 1481-1492Crossref PubMed Scopus (118) Google Scholar] or loss of lamin A expression [[34]Frock R.L. Kudlow B.A. Evans A.M. Jameson S.A. Hauschka S.D. Kennedy B.K. Lamin A/C and emerin are critical for skeletal muscle satellite cell differentiation.Genes Dev. 2006; 20: 486-500Crossref PubMed Scopus (201) Google Scholar] have been shown to impair muscle differentiation in vitro. Also, expression profiling of muscle tissue derived from Emery Dreifuss Muscular Dystrophy patients indicated a deregulation of the pRb/MyoD pathway which controls muscle differentiation [[35]Bakay M. Wang Z. Melcon G. Schiltz L. Xuan J. Zhao P. Sartorelli V. Seo J. Pegoraro E. Angelini C. Shneiderman B. Escolar D. Chen Y.W. Winokur S.T. Pachman L.M. Fan C. Mandler R. Nevo Y. Gordon E. Zhu Y. Dong Y. Wang Y. Hoffman E.P. Nuclear envelope dystrophies show a transcriptional fingerprint suggesting disruption of Rb-MyoD pathways in muscle regeneration.Brain. 2006; 129: 996-1013Crossref PubMed Scopus (234) Google Scholar]. Several members in the EURO-Laminopathies consortium will test this hypothesis by investigating in vitro and in vivo muscle and adipocyte differentiation in patient cells and in cells derived from available and newly established laminopathy mouse models (e.g. the knock-in mouse models expressing Emery Dreifuss Muscular Dystrophy-linked H222P lamin A variant [[36]Arimura T. Helbling-Leclerc A. Massart C. Varnous S. Niel F. Lacene E. Fromes Y. Toussaint M. Mura A.M. Keller D.I. Amthor H. Isnard R. Malissen M. Schwartz K. Bonne G. Mouse model carrying H222P-Lmna mutation develops muscular dystrophy and dilated cardiomyopathy similar to human striated muscle laminopathies.Hum Mol Genet. 2005; 14: 155-169Crossref PubMed Scopus (243) Google Scholar]). In human fibroblasts Pekovic et al. recently found that loss of LAP2α and/or loss of nucleoplasmic lamin complexes in human fibroblasts caused an initial acceleration in cell cycle progression, but subsequently initiated cell cycle arrest and cellular senescence [[9]Pekovic V. Harborth J. Broers J.L. Ramaekers F.C. van Engelen B. Lammens M. von Zglinicki T. Foisner R. Hutchison C. Markiewicz E. Nucleoplasmic LAP2{alpha}-lamin A complexes are required to maintain a proliferative state in human fibroblasts.J Cell Biol. 2007; 176: 163-172Crossref PubMed Scopus (100) Google Scholar]. It will be extremely interesting to test whether this cellular phenotype is linked to pathologies detected in laminopathic patients.5. Drug target search and therapeutic approachesAlthough laminopathies have become a subject of intense research in the past few year