Gisèle Bonne

A new mutation of the lamin A/C gene leading to autosomal dominant axonal neuropathy, muscular dystrophy, cardiac disease, and leuconychia

The LMNA gene encodes two nuclear envelope proteins, lamins A and C, derived from alternative splicing. First identified in autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD),1 mutations in this gene are implicated in up to seven diseases including autosomal recessive EDMD (AR-EDMD),2 limb-girdle muscular dystrophy type 1B (LGMD1B),3 dilated cardiomyopathy with conduction defects (DCM-CD),4,5 autosomal dominant partial lipodystrophy of Dunnigan type,6 autosomal recessive axonal Charcot-Marie-Tooth disease (AR-CMT2),7 mandibuloacral dysplasia,8 and Hutchinson-Gilford progeria syndrome.9,10 In addition, some patients appear to have a combination of these different phenotypes11,12 or a clinical variant including skin abnormalities.13 To extend the clinical spectrum of laminopathies, we report a previously undescribed dominant missense mutation, E33D, identified in LMNA and clinically characterised by the combination of axonal neuropathy with myopathic features, cardiac disease including dilated cardiomyopathy, conduction disturbances and arrhythmia, and leuconychia. The LMNA gene is therefore the first gene implicated in both autosomal dominant and recessive forms of CMT2. The pedigree of a white family originating from the south west of France is shown in fig 1. The index case (II-5) and his affected daughter (III-13) were neurologically and cardiologically assessed by one of our team; only partial information was available for other affected members through questioning of patient III-13. The clinical features of all the affected members are shown in table 1. The results of nerve electrophysiological examination of patients II-5 and III-13 are shown in table 2. A muscle CT scan performed for patient II-5 showed wasting and marked fatty infiltration predominating in paraspinal, vasti, hamstring, and gastrocnemius muscles (fig 2). Fig 3 shows the fingernails of patients II-5 and III-13, exhibiting leuconychia. View this table: Table 1 Clinical features of the affected family members View this table: Table 2 Electrophysiological study of patients II-5 and III-13 Figure 1 Pedigree of the family. Arrow …

Multitissular involvement in a family with LMNA and EMD mutations Role of digenic mechanism

Background: Mutations in the EMD and LMNA genes, encoding emerin and lamins A and C, are responsible for the X-linked and autosomal dominant and recessive forms of Emery–Dreifuss muscular dystrophy (EDMD). LMNA mutations can also lead to several other disorders, collectively termed laminopathies, involving heart, fat, nerve, bone, and skin tissues, and some premature ageing syndromes. Methods: Fourteen members of a single family underwent neurologic, electromyographic, and cardiologic assessment. Gene mutation and protein expression analyses were performed for lamins A/C and emerin. Results: Clinical investigations showed various phenotypes, including isolated cardiac disease (seven patients), axonal neuropathy (one patient), and a combination of EDMD with axonal neuropathy (two patients), whereas five subjects remained asymptomatic. Genetic analyses identified the coincidence of a previously described homozygous LMNA mutation (c.892C→T, p. R298C) and a new in-frame EMD deletion (c.110-112delAGA, p. delK37), which segregate independently. Analyses of the contribution of these mutations showed 1) the EMD codon deletion acts in X-linked dominant fashion and was sufficient to induce the cardiac disease, 2) the combination of both the hemizygous EMD and the homozygous LMNA mutations was necessary to induce the EDMD phenotype, 3) emerin was present in reduced amount in EMD-mutated cells, and 4) lamin A/C and emerin expression was most dramatically affected in the doubly mutated fibroblasts. Conclusions: This highlights the crucial role of lamin A/C–emerin interactions, with evidence for synergistic effects of these mutations that lead to Emery–Dreifuss muscular dystrophy as the worsened result of digenic mechanism in this family.

Laminopathies disrupt epigenomic developmental programs and cell fate

Mutations in lamin A/C and emerin change myogenic cell fate by disrupting heterochromatin tethering to the nuclear envelope. Bad LADS alter muscle cell fate The lamin A/C (LMNA) gene encodes a key structural protein (lamin) in the nuclear envelope, a double lipid bilayer membrane that separates the cytoplasm and nucleus. The nuclear lamina—a fibrillar network organized around the nuclear membrane’s inner face—performs both structural and functional roles and interacts with chromatin at a great many genomic regions—called lamina-associated domains (LADs)—which are heterochromatic (transcriptionally repressed) domains. Mutations in LMNA cause Emery-Dreifuss muscular dystrophy (EDMD). Perovanovic and colleagues now show that these mutations interfere with the building of lamin A–associated heterochromatin in an allele-specific manner. The authors used a variety of techniques to map heterochromatin-euchromatin (transcriptionally active) transitions during the differentiation of muscle cells. They found that the EDMD mutations messed up the normal chromatin-structure (epigenomic) transitions during the differentiation of mouse cells and fibroblasts from muscular dystrophy patients who carried LMNA mutations. During muscle formation, normal heterochromatin to euchromatin transitions that take place at myogenic loci drive muscle differentiation. Muscle biopsies from patients carrying mutations that cause EDMD showed a loss of heterochromatin formation at the Sox2 pluripotency locus, which continued to be expressed and inhibited muscle cell differentiation. The new work suggests that so-called nuclear envelopathies, such as EDMD, are caused by aberrant LADS that disrupt developmental epigenetic programming. The nuclear envelope protein lamin A is encoded by the lamin A/C (LMNA) gene, which can contain missense mutations that cause Emery-Dreifuss muscular dystrophy (EDMD) (p.R453W). We fused mutated forms of the lamin A protein to bacterial DNA adenine methyltransferase (Dam) to define euchromatic-heterochromatin (epigenomic) transitions at the nuclear envelope during myogenesis (using DamID-seq). Lamin A missense mutations disrupted appropriate formation of lamin A–associated heterochromatin domains in an allele-specific manner—findings that were confirmed by chromatin immunoprecipitation–DNA sequencing (ChIP-seq) in murine H2K cells and DNA methylation studies in fibroblasts from muscular dystrophy patient who carried a distinct LMNA mutation (p.H222P). Observed perturbations of the epigenomic transitions included exit from pluripotency and cell cycle programs [euchromatin (open, transcribed) to heterochromatin (closed, silent)], as well as induction of myogenic loci (heterochromatin to euchromatin). In muscle biopsies from patients with either a gain- or change-of-function LMNA gene mutation or a loss-of-function mutation in the emerin gene, both of which cause EDMD, we observed inappropriate loss of heterochromatin formation at the Sox2 pluripotency locus, which was associated with persistent mRNA expression of Sox2. Overexpression of Sox2 inhibited myogenic differentiation in human immortalized myoblasts. Our findings suggest that nuclear envelopathies are disorders of developmental epigenetic programming that result from altered formation of lamina-associated domains.

Differentiating Emery-Dreifuss muscular dystrophy and collagen VI-related myopathies using a specific CT scanner pattern

Bethlem myopathy and Ullrich congenital muscular dystrophy are part of the heterogeneous group of collagen VI-related muscle disorders. They are caused by mutations in collagen VI (ColVI) genes (COL6A1, COL6A2, and COL6A3) while LMNA mutations cause autosomal dominant Emery-Dreifuss muscular dystrophy. A muscular dystrophy pattern and contractures are found in all three conditions, making differential diagnosis difficult especially in young patients when cardiomyopathy is absent. We retrospectively assessed upper and lower limb muscle CT scans in 14 Bethlem/Ullrich patients and 13 Emery-Dreifuss patients with identified mutations. CT was able to differentiate Emery-Dreifuss muscular dystrophy from ColVI-related myopathies in selected thigh muscles and to a lesser extent calves muscles: rectus femoris fatty infiltration was selectively present in Bethlem/Ullrich patients while posterior thigh muscles infiltration was more prominently found in Emery-Dreifuss patients. A more severe fatty infiltration particularly in the leg posterior compartment was found in the Emery-Dreifuss group.

A novel genetic variant in the transcription factor Islet-1 exerts gain of function on myocyte enhancer factor 2C promoter activity.

The transcription factor Islet‐1 (ISL1) is a marker of cardiovascular progenitors and is essential for mammalian cardiogenesis. An ISL1 haplotype has recently been associated with congenital heart disease. In this study we evaluated whether ISL1 variants are associated with hypertrophic (HCM), dilated (DCM), arrhythmogenic right ventricular cardiomyopathy (ARVC), or with Emery–Dreifuss muscular dystrophy (EDMD).

Lamin and the heart

Lamins A and C are intermediate filament nuclear envelope proteins encoded by the LMNA gene. Mutations in LMNA cause autosomal dominant severe heart disease, accounting for 10% of dilated cardiomyopathy (DCM). Characterised by progressive conduction system disease, arrhythmia and systolic impairment, lamin A/C heart disease is more malignant than other common DCMs due to high event rates even when the left ventricular impairment is mild. It has several phenotypic mimics, but overall it is likely to be an under-recognised cause of DCM. In certain clinical scenarios, particularly familial DCM with early conduction disease, the pretest probability of finding an LMNA mutation may be quite high. Recognising lamin A/C heart disease is important because implantable cardioverter defibrillators need to be implanted early. Promising oral drug therapies are within reach thanks to research into the mitogen-activated protein kinase (MAPK) and affiliated pathways. Personalised heart failure therapy may soon become feasible for LMNA, alongside personalised risk stratification, as variant-related differences in phenotype severity and clinical course are being steadily elucidated. Genotyping and family screening are clinically important both to confirm and to exclude LMNA mutations, but it is the three-pronged integration of such genetic information with functional data from in vivo cardiomyocyte mechanics, and pathological data from microscopy of the nuclear envelope, that is properly reshaping our LMNA knowledge base, one variant at a time. This review explains the biology of lamin A/C heart disease (genetics, structure and function of lamins), clinical presentation (diagnostic pointers, electrocardiographic and imaging features), aspects of screening and management, including current uncertainties, and future directions.

Truncated prelamin A expression in HGPS-like patients: a transcriptional study

Premature aging syndromes are rare genetic disorders mimicking clinical and molecular features of aging. A recently identified group of premature aging syndromes is linked to mutation of the LMNA gene encoding lamins A and C, and is associated with nuclear deformation and dysfunction. Hutchinson–Gilford progeria syndrome (HGPS) was the first premature aging syndrome linked to LMNA mutation and its molecular bases have been deeply investigated. It is due to a recurrent de novo mutation leading to aberrant splicing and the production of a truncated and toxic nuclear lamin A precursor (prelamin AΔ50), also called progerin. In this work and based on the literature data, we propose to distinguish two main groups of premature aging laminopathies: (1) HGPS and HGP-like syndromes, which share clinical features due to hampered processing and intranuclear toxic accumulation of prelamin A isoforms; and (2) APS (atypical progeria syndromes), due to dominant or recessive missense mutations affecting lamins A and C. Among HGPS-like patients, several deleted prelamin A transcripts (prelamin AΔ50, AΔ35 and AΔ90) have been described. The purpose of this work was to characterize those transcripts in eight patients affected with HGP-like rare syndromes. When fibroblasts were available, the relationships between the presence and ratios of these transcripts and other parameters were studied, aiming to increase our understanding of genotype–phenotype relationships in HGPS-like patients. Altogether our results evidence that progerin accumulation is the major pathogenetic mechanism responsible for HGP-like syndromes due to mutations near the donor splice site of exon 11.

Lamins and nesprin-1 mediate inside-out mechanical coupling in muscle cell precursors through FHOD1

LINC complexes are crucial for the response of muscle cell precursors to the rigidity of their environment, but the mechanisms explaining this behaviour are not known. Here we show that pathogenic mutations in LMNA or SYNE-1 responsible for severe muscle dystrophies reduced the ability of human muscle cell precursors to adapt to substrates of different stiffness. Plated on muscle-like stiffness matrix, mutant cells exhibited contractile stress fibre accumulation, increased focal adhesions, and higher traction force than controls. Inhibition of Rho-associated kinase (ROCK) prevented cytoskeletal defects, while inhibiting myosin light chain kinase or phosphorylation of focal adhesion kinase was ineffective. Depletion or inactivation of a ROCK-dependent regulator of actin remodelling, the formin FHOD1, largely rescued morphology in mutant cells. The functional integrity of lamin and nesprin-1 is thus required to modulate the FHOD1 activity and the inside-out mechanical coupling that tunes the cell internal stiffness to match that of its soft, physiological-like environment.

Pediatric laminopathies: Whole‐body MRI fingerprint and comparison with SEPN1‐myopathy

We sought to define the whole‐body MRI (WB‐MRI) fingerprint of muscle involvement in pediatric LMNA‐related dystrophy (LMNA‐RD) and to compare it with SEPN1‐related myopathy (SEPN1‐RM).

Expression of cytochrome c oxidase subunits encoded by mitochondrial or nuclear DNA in the muscle of patients with zidovudine myopathy.

The present study was carried out to determine whether a selective decrease of mitochondrial (mt) DNA-encoded cytochrome c oxidase (CCO) subunits occurs in zidovudine myopathy, as expected with a compound known to induce selective mtDNA depletion. Fourteen HIV-infected patients with zidovudine myopathy were studied. Thirteen had partial CCO deficiency assessed by histochemistry. Western blot analysis of CCO subunits (II/III, IV, Va, Vb, VIa, VIb, VIc, VIIa, VIIb, and VIIc) was performed on muscle biopsy samples. We evaluated the mtDNA-encoded subunits to nuclear DNA-encoded subunits ratio with the II/III to IV ratio. Patients had either a selective decrease of mtDNA-encoded CCO subunits (3 patients), or an overall decrease affecting both mtDNA-and nuclear DNA-encoded subunits (5 patients), or a normal expression of CCO subunits (6 patients). Positive correlations could not be established between the pattern of expression of CCO subunits and total zidovudine intake, degree of inflammation, and percentages of ragged-red fibers or CCO-deficient fibers. The finding of a decrease of both mtDNA- and nuclear DNA-encoded CCO subunits suggests that a factor additional to zidovudine could be implicated in the pathogenesis of the myopathy, at least in some patients. New insights into the pathogenesis of zidovudine myopathy might come from the use of more sensitive methods, including evaluation of CCO subunits in single fibers.