Laminopathies of striated muscle are defined as a group of muscular dystrophies associated to dilated cardiomyopathy and conduction and/or rhythm defects (DCM-CD). They are caused by mutations in the LMNA gene encoding two nuclear envelop proteins: lamins A/C (Broers et al. 2006; Worman et Bonne, 2007). In 1999 we identified the first laminopathy: autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD) (Bonne et al. 1999). Since mutations in the EMD gene encoding emerin, another nuclear envelop component, were already known to cause in X-linked EDMD (Bione et al. 1994), it became clear that the nuclear envelop played an important role in the aetiology of neuromuscular disorders. Over the last years, LMNA was implicated in more than 10 disorders affecting either single tissue groups (muscular dystrophies/cardiomyopathies (our work), lipodystrophies, axonal neuropathies) or a combination of tissues (premature aging syndromes) (Worman et Bonne, 2007).
We have established collaborative networks: i) French network on EDMD and other laminopathies (coord. G Bonne/F Leturcq, >50 members, Inserm/AFM rare disease network funding); ii) EUROMEN European consortium (2000-2003, FP5, coord. G Bonne/K Schwartz), followed by the Euro-Laminopathies network (coord. R Foisner, FP6, coord. genetic teams: G Bonne). Through these networks and thanks to a technology transfer in 2000 to the Functional Unit of Cardio & Myogenetic (P Richard, Service de Biochimie Métabolique, GH Pitié-Salpêtrière), we started and are still pursuing the characterization of the clinical and genetic spectrum of these disorders. We identified 155 LMNA mutations in 458 individuals displaying a very variable clinical severity, suggesting a continuum between all laminopathies, from isolated cases of dilated cardiomyopathy to fatal foetal akinesia, including muscular dystrophies with variable degrees of severity  (Worman et Bonne, 2007 ; Bonne et al, 2007 ; Ben Yaou et Bonne, 2008 ; Ben Yaou et Bonne, 2008  ; Bonne et Lampe, 2008). In view of this important diversity, we set up a UMD-LMNA database (www.umd.be:2000) which regroups the clinical and genetic data on all patients carrying a LMNA mutation, reported in the literature and/or by our group. An UMD-EMD mutation database with similar characteristics has also been setup; it compiles all mutations reported in the EMD coding emerin (www.umd.be:2010). To date, the UMD-LMNA database contains 317 LMNA mutations found in 1416 individuals and 60% of these presented a striated muscle laminopathy. The use of this tool allowed to unravel phenotype/genotype correlations (Benedetti et al. 2007), and in particular we identified a group of mutations which lead to severe cases of congenital muscular dystrophies (Quijano-Roy et al. 2007 and in rev). We also characterized several cases of digenism associating LMNA and EMD or DES (desmin) gene mutations (Muntoni et al. 2006, Ben Yaou et al. 2007). Nevertheless, digenism alone does not explain the huge clinical variability of EDMD. In addition, about 40% of patients with EDMD are orphan of a molecular diagnostic. Thus, we set out to identify new genes, major and/or modulators of the EDMD phenotype ((Gueneau et al. 2006) Ph.D. project of L Gueneau, in progress).

 

Lamins A/C, along with lamin B, delineate the nuclear lamina on the internal side of the nuclear envelop. They interact with numerous proteins of the nuclear envelop, DNA and transcription factors. The exact role of lamins A/C remains elusive: they are thought to participate in replication, transcription, chromatin organization and nuclear envelop structure (Worman et Bonne, 2007). The mechanisms by which mutations of the LMNA gene, encoding 2 ubiquitous proteins, cause disorders specifically affecting striated muscle remain to be elucidated. At present, two hypotheses not mutually exclusive are proposed to account for the physiopathology of laminopathies affecting striated muscles. 1) The structural hypothesis supports that lamin mutations affect the structure and mechanical resistance of cells. As striated muscles are subjected to high and prolonged mechanical constraints, it may therefore be a tissue particularly sensible to lamin mutations affecting nucleus and cell resistance. 2) The gene expression hypothesis is supported by the multiple interactions od lamins A/C with chromatin as well as transcription factors. The LMNA mutations potentially would modify these interactions which in turn would modulate the gene expression and also cell profiferation/differentiation process (Worman et Bonne, 2007).
In patient’s tissues, we demonstrated that LMNA mutations not only alter nuclear envelop integrity, but also perturb genome organization (Meaburn et al. 2007) or activate the proteasome pathway in one case with a nonsense mutation (Muchir et al. 2006). The pattern of muscle fibres degeneration of observed in laminopathies seems different from that observed in muscular dystrophy due to plasma membrane defects (Mittelbronn et al, 2006).
To carry our pathophysiological studies further, we developed 2 mouse models, KI-LmnaH222P and KI-LmnadelK32, which reproduce 2 mutations that were identified in EDMD patients with a classical or a severe phenotype, respectively. Homozygous KI-LmnaH222P/H222P mice develop a muscular and cardiac dystrophy similar to that seen in EDMD (Arimura et al. 2005). Transcriptome analysis showed that their cardiomyopathy correlates with the activation of the MAPK pathway, preceding the onset of clinical signs (Muchir et al. 2007 ; Muchir et al. 2007). This pathway is also triggered by emerin deficiency in a KO-Emd model, which is a genetic model for X-linked EDMD (Muchir et al. 2007). Homozygous KI-LmnadelK32 mice die at 23 weeks of age and display delayed muscle maturation. At the molecular level, the deletion of amino-acid K32 leads to reduced levels of mutant lamins A/C proteins but not of the transcripts. Because lysine 32 is localized within the dimerization domain of lamins A/C, it seems likely that its deletion might decrease the protein stability (Bertrand et al. 2007).

 

Progressing in the comprehension of pathophysiological and genetic mechanisms enable us to envision therapeutic approaches for laminopathies of striated muscle. Indeed much emphasis is put on the cardiac involvement, because of its severity (van Berlo et al. 2005). In patients with conserved left ventricular function, implanted defibrillator was proven to be useful (Meune et al. 2006). With regards to diagnostic tools, a lot of efforts concentrate on DTI and NMR imaging which allow early detection of ventricular dysfunction (Smith et al. 2006). The KI-LmnaH222P mice constitute an excellent model for testing therapeutic approaches. We investigated N-acetylcysteine (NAC), a glutathione precursor, on 6-month old females. A treatment of one month restored glutathione levels, decreased TNF-alpha levels, ameliorated cardiac function and slowed fibrosis progression (Decostre et al. 2007). This study is being pursued as an ANR-GIS-Rare Disease Project.