Functional consequences of an LMNA mutation associated with a new cardiac and non-cardiac phenotype

Heritable dilated cardiomyopathy is a genetically highly heterogeneous disease. To date 17 different chromosomal loci have been described for autosomal dominant forms of dilated cardiomyopathy with or without additional clinical manifestations. Among the 10 mutated genes associated with dilated cardiomyopathy, the lamin A/C (LMNA) gene has been reported in forms associated with conduction-system disease with or without skeletal muscle myopathy. For the first time, we report here a French family affected with a new phenotype composed of an autosomal dominant severe dilated cardiomyopathy with conduction defects or atrial/ventricular arrhythmias, and a specific quadriceps muscle myopathy. In all previously reported cases with both cardiac and neuromuscular involvement, neuromuscular disorders preceded cardiac abnormalities. The screening of the coding sequence of the LMNA gene on all family members was performed and we identified a missense mutation (R377H) in the lamin A/C gene that cosegregated with the disease in the family. Cell transfection experiments showed that the R377H mutation leads to mislocalization of both lamin and emerin. These results were obtained in both muscular (C2C12) and non-muscular cells (COS-7). This new phenotype points out the wide spectrum of neuromuscular and cardiac manifestations associated with lamin A/C mutations, with the functional consequence of this mutation seemingly associated with a disorganization of the lamina.     

Mutations in the LMNA gene encoding lamin A/C

Very recently, mutations within the LMNA gene on chromosome 1q21.2 were shown to result in forms of muscular dystrophy, conduction-system disease, cardiomyopathy, and partial lipodystrophy. The LMNA gene encodes for the nucleophilic A-type lamins, lamin A and lamin C. These isoforms are generated by different splicing within exon 10 of LMNA. Thus lamin A/C is, besides emerin, the first known nucleophilic protein which plays a role in human disease. To date, 41 different mutations, predominantly missense, in the LMNA gene are known causing variable phenotypes. Twenty-three different mutations of LMNA have so far been shown to cause autosomal-dominant Emery-Dreifuss muscular dystrophy (EDMD2), three mutations were reported to cause limb-girdle muscular dystrophy (LGMD1B), eight mutations are known to result in dilated cardiomyopathy (CMD1A), and seven mutations were reported to cause familial partial lipodystrophy (FPL). The reports of lamin mutations including the corresponding phenotype are of great interest in order to gain insights into the function of lamin A/C. Here we summarize the mutations published to date in LMNA encoding lamin A/C.     

Novel and recurrent mutations in lamin A/C in patients with Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by slowly progressive muscle wasting and weakness; early contractures of the elbows, Achilles tendons, and spine; and cardiomyopathy associated with cardiac conduction defects. Clinically indistinguishable X-linked and autosomal forms of EDMD have been described. Mutations in the STA gene, encoding the nuclear envelope protein emerin, are responsible for X-linked EDMD, while mutations in the LMNA gene encoding lamins A and C by alternative splicing have been found in patients with autosomal dominant, autosomal recessive, and sporadic forms of EDMD. We report mutations in LMNA found in four familial and seven sporadic cases of EDMD, including seven novel mutations. Nine missense mutations and two small in-frame deletions were detected distributed throughout the gene. Most mutations (7/11) were detected within the LMNA exons encoding the central rod domain common to both lamins A/C. All of these missense mutations alter residues in the lamin A/C proteins conserved throughout evolution, implying an essential structural and/or functional role of these residues. One severely affected patient possesed two mutations, one specific to lamin A that may modify the phenotype of this patient. Mutations in LMNA were frequently identified among patients with sporadic and familial forms of EDMD. Further studies are needed to identify the factors modifying disease phenotype among patients harboring mutations within lamin A/C and to determine the effect of various mutations on lamin A/C structure and function.     

Cardiac involvement in Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a common form of muscular dystrophy frequently involving cardiac muscle, thus leading to dilated cardiomyopathy. Clinical outcome and prognosis is frequently determined by the involvement of the cardiac conduction system causing symptomatic bradyarrhythmias, as well as tachyarrhythmias and, if untreated, frequent sudden cardiac death. Typical features of the cardiac involvement of EDMD are presented, caused by a novel missense mutation in the splice receptor sequence of intron 6 of the LMNA gene on chromosome 1, encoding for the lamin A/C gene, consistent with the autosomal dominant form of EDMD.     

Novel and recurrent mutations in lamin A/C in patients with Emery‐Dreifuss muscular dystrophy

Emery‐Dreifuss muscular dystrophy (EDMD) is characterized by slowly progressive muscle wasting and weakness; early contractures of the elbows, Achilles tendons, and spine; and cardiomyopathy associated with cardiac conduction defects. Clinically indistinguishable X‐linked and autosomal forms of EDMD have been described. Mutations in the STA gene, encoding the nuclear envelope protein emerin, are responsible for X‐linked EDMD, while mutations in the LMNA gene encoding lamins A and C by alternative splicing have been found in patients with autosomal dominant, autosomal recessive, and sporadic forms of EDMD. We report mutations in LMNA found in four familial and seven sporadic cases of EDMD, including seven novel mutations. Nine missense mutations and two small in‐frame deletions were detected distributed throughout the gene. Most mutations (7/11) were detected within the LMNA exons encoding the central rod domain common to both lamins A/C. All of these missense mutations alter residues in the lamin A/C proteins conserved throughout evolution, implying an essential structural and/or functional role of these residues. One severely affected patient possesed two mutations, one specific to lamin A that may modify the phenotype of this patient. Mutations in LMNA were frequently identified among patients with sporadic and familial forms of EDMD. Further studies are needed to identify the factors modifying disease phenotype among patients harboring mutations within lamin A/C and to determine the effect of various mutations on lamin A/C structure and function. © 2001 Wiley‐Liss, Inc.     

Lamin A/C truncation in dilated cardiomyopathy with conduction disease

Background  

Mutations in the gene encoding the nuclear membrane protein lamin A/C have been associated with at least 7 distinct diseases including autosomal dominant dilated cardiomyopathy with conduction system disease, autosomal dominant and recessive Emery Dreifuss Muscular Dystrophy, limb girdle muscular dystrophy type 1B, autosomal recessive type 2 Charcot Marie Tooth, mandibuloacral dysplasia, familial partial lipodystrophy and Hutchinson-Gilford progeria.     

Cardiac dysrhythmias,cardiomyopathy and muscular dystrophy in patients with Emery-Dreifuss muscular dystrophy and limb-girdle muscular dystrophy type 1B

Emery-Dreifuss muscular dystrophy (EDMD) and limb-girdle muscular dystrophy type 1B (LGMD1B) are characterized by cardiac dysrhythmias, late-onset cardiomyopathy, slowly progressive skeletal myopathy and contractures of the neck, elbows and ankles. The causative mutation is either in the emerin gene (X-linked recessive EDMD) or lamin A/C gene (autosomal dominant EDMD2 or LGMD1B). We report three cases of EDMD, EDMD2 and LGMD1B. A 14-yr-old boy showed limitation of cervical flexion and contractures of both elbows and ankles. Sinus arrest with junctional escape beats was noted. He was diagnosed as X-linked recessive EDMD (MIM 310300). A 28-yr-old female showed severe wasting and weakness of humeroperoneal muscles. Marked limitation of cervical flexion and contractures of both elbows and ankles were noted. Varying degrees of AV block were noted. She was diagnosed as autosomal dominant EDMD2 (MIM 181350). A 41-yr-old female had contractures of both ankles and limb-girdle type muscular dystrophy. ECG revealed atrial tachycardia with high grade AV block. She was diagnosed as autosomal dominant LGMD1B (MIM 159001). Cardiac dysrhythmias in EDMD and LGMD1B include AV block, bradycardia, atrial tachycardia, atrial fibrillation, and atrial standstill, causing sudden death necessitating pacemaker implantation. Cardiologists should know about these unusual genetic diseases with conduction defects, especially in young adults.     

Identification of mutations in the gene encoding lamins A/C in autosomal dominant limb girdle muscular dystrophy with atrioventricular conduction disturbances (LGMD1B)

LGMD1B is an autosomal dominantly inherited, slowly progressive limb girdle muscular dystrophy, with age-related atrioventricular cardiac conduction disturbances and the absence of early contractures. The disease has been linked to chromosome 1q11-q21. Within this locus another muscular dystrophy, the autosomal dominant form of Emery-Dreifuss muscular dystrophy (AD-EDMD) has recently been mapped and the corresponding gene identified. AD-ADMD is characterized by early contractures of elbows and Achilles tendons and a humero-peroneal distribution of weakness combined with a cardiomyopathy with conduction defects. The disease gene of AD-EDMD is LMNA which encodes lamins A/C, two proteins of the nuclear envelope. In order to identify whether or not LGMD1B and AD-EDMD are allelic disorders, we carried out a search for mutations in the LMNA gene in patients with LGMD1B. For this, PCR/SSCP/sequencing screening was carried out for the 12 exons of LMNA on DNA samples of individuals from three LGMD1B families that were linked to chromo-some 1q11-q21. Mutations were identified in all three LGMD1B families: a missense mutation, a deletion of a codon and a splice donor site mutation, respectively. The three mutations were identified in all affected members of the corresponding families and were absent in 100 unrelated control subjects. The present identification of mutations in the LMNA gene in LGMD1B demonstrates that LGMD1B and AD-EDMD are allelic disorders. Further analysis of phenotype-genotype relationship will help to clarify the variability of the phenotype observed in these two muscular dystrophies.     

The R482Q lamin A/C mutation that causes lipodystrophy does not prevent nuclear targeting of lamin A in adipocytes or its interaction with emerin

Most pathogenic missense mutations in the lamin A/C gene identified so far cause autosomal-dominant dilated cardiomyopathy and/or Emery-Dreifuss muscular dystrophy. A few specific mutations, however, cause a disease with remarkably different clinical features: FPLD, or familial partial lipodystrophy (Dunnigan-type), which mainly affects adipose tissue. We have prepared lamin A with a known FPLD mutation (R482Q) by in vitro mutagenesis. Nuclear targeting of lamin A in transfected COS cells, human skeletal muscle cells or mouse adipocyte cell cultures (pre- and post-differentiation) was not detectably affected by the mutation. Quantitative in vitro measurements of lamin A interaction with emerin using a biosensor also showed no effect of the mutation. The results show that the loss of function of R482 in lamin A/C in FPLD does not involve loss of ability to form a nuclear lamina or to interact with the nuclear membrane protein, emerin.     

Two related Dutch families with a clinically variable presentation of cardioskeletal myopathy caused by a novel S13F mutation in the desmin gene

Desmin-related myopathy is characterised by skeletal muscle weakness often combined with cardiac involvement. Mutations in the desmin gene have been described as a cause of desmin-related myopathy (OMIM 601419). We report here on two distantly related Dutch families with autosomal dominant inheritance of desmin-related myopathy affecting 15 family members. A highly heterogeneous clinical picture is apparent, varying from isolated dilated cardiomyopathy to a more generalised skeletal myopathy and mild respiratory problems. Morphological analysis of muscle biopsies revealed intracytoplasmic desmin aggregates (desmin and p62 staining). In both families we identified an identical novel pathogenic heterozygous missense mutation, S13F, in the 'head' domain of the desmin gene which cosegregates with the disease phenotype. This is the 5th reported missense mutation located at the 'head' domain of the desmin gene and the first reported Dutch family with desmin-related myopathy. This article illustrates the importance of analysing the desmin gene in patients with (familial) cardiac conduction disease, dilated cardiomyopathy and/or a progressive skeletal myopathy resembling limb-girdle muscular dystrophy.     

Emery-Dreifuss肌营养不良症的研究进展

Emery-Dreifuss肌营养不良症是一种相对良性的肌营养不良类型。其遗传方式为X-连锁隐性、常染色体显性和隐性遗传。EMD基因和LMNA基因是引起X-连锁EDMD和常染色体遗传EDMD的致病基因，编码产物分别为emerin蛋白和核纤层蛋白（lamin）A/C。该病确切的发病机制目前尚不清楚，临床特点表现为早期出现关节挛缩，受累肌肉呈肱－腓分布并伴有心脏受累。致病基因的研究使基因治疗该病成为可能。     

Meta-analysis of clinical characteristics of 299 carriers of LMNA gene mutations: do lamin A/C mutations portend a high risk of sudden death?

This study evaluated common clinical characteristics of patients with lamin A/C gene mutations that cause either isolated dilated cardiomyopathy or dilated cardiomyopathy in association with skeletal muscular dystrophy. We pooled clinical data of all published carriers of lamin A/C gene mutations as cause of skeletal and/or cardiac muscle disease and reviewed ECG findings. Cardiac dysrhythmias were reported in 92% of patients after the age of 30 years; heart failure was reported in 64% after the age of 50. Sudden death was the most frequently reported mode of death (46%) in both the cardiac and the neuromuscular phenotype. Carriers of lamin A/C gene mutations often received a pacemaker (28%). However, this intervention did not alter the rate of sudden death. Review of the ECG findings typically showed a low amplitude P wave and prolongation of the PR interval with a narrow QRS complex. This meta-analysis suggests that cardiomyopathy due to lamin A/C gene mutations portends a high risk of sudden death, and that this risk does not differ between subjects with predominantly cardiac or neuromuscular disease. This implies then that all carriers of a lamin A/C gene mutation need to be carefully screened with particular emphasis also on tachyarrhythmias. Prospective studies are needed to evaluate risk stratification and proper treatment strategies.     

Overlapping syndrome with familial partial lipodystrophy,Dunnigan variety and cardiomyopathy due to amino‐terminal heterozygous missense lamin A/C mutations

Subramanyam L, Simha V, Garg A. Overlapping syndrome with familial partial lipodystrophy, Dunnigan variety and cardiomyopathy due to amino‐terminal heterozygous missense lamin A/C mutations. Familial partial lipodystrophy, Dunnigan variety (FPLD) is a well‐recognized autosomal dominant disorder due to heterozygous missense mutations in lamin A/C (LMNA) gene. Most of the FPLD patients harbor mutations in the C‐terminal of the lamin A/C and do not develop cardiomyopathy. On the other hand, affected subjects from three FPLD pedigrees with heterozygous R28W, R60G and R62G LMNA mutations in the amino‐terminal had associated cardiomyopathy presenting as premature onset of congestive heart failure, dilated cardiomyopathy and conduction system disturbances. We report three new FPLD pedigrees presenting with cardiomyopathy associated with heterozygous LMNA mutations in the amino‐terminal region. Two of them had previously reported R60G and R62G mutations and one has a novel D192V mutation. Affected subjects belonging to the pedigree with heterozygous R62G mutation had atrial fibrillation and required pacemaker implantation. The affected subjects from the other pedigrees with R60G and D192V mutations developed severe cardiomyopathy requiring defibrillator implantation and cardiac transplantation before 30 years of age in some and premature death in the fourth decade in others. Thus, our report provides further evidence of association of a multisystem dystrophy syndrome in FPLD patients harboring amino‐terminal mutations in LMNA. Increased understanding of the genotype–phenotype association might help devise clinical strategies aimed at preventing devastating manifestations of cardiomyopathy including heart failure, arrhythmias and sudden death. Furthermore, the underlying molecular mechanisms by which these amino‐terminal mutations cause lipodystrophy as well as cardiomyopathy remain to be understood.     

一个常染色体显性遗传Emery-Dreifuss型肌营养不良家系的基因突变分析

目的 探讨一个常染色体显性遗传Emery-Dreifuss型肌营养不良(Emery-Dreifuss muscular dystrophy,EDMD)家系的临床、病理及遗传学特点.方法 收集家系中2例患者(先证者及女儿)的临床资料及骨骼肌标本,行组织化学染色病理分析;收集先证者及家系成员(3代7人)血液DNA标本,采用聚合酶链反应和DNA直接测序方法 对LMNA基因进行突变检测;明确基因变异位点后对家系行单倍型分析.结果 先证者具有典型的EDMD临床表现:关节挛缩、进行性加重的肌无力和肌萎缩、心脏传导异常;骨骼肌活检病理示肌源性合并轻度神经源性改变;2例患者LMNA基因第9外显子发现杂合错义突变1583(C→G)(T528R),表型正常的其他家系成员未发现该突变;单倍型分析显示先证者及女儿具有相同的致病单倍型.结论 报道了中国人常染色体显性遗传EDMD患者的表现型及基因型.     

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is characterised by early contractures, slowly progressive muscle wasting and weakness with a distinctive humero-peroneal distribution and cardiac conduction defects leading to dilated cardiomyopathy. The genes known to be responsible for EDMD encode proteins associated with the nuclear envelope: the emerin and the lamins A and C.     

Activation of MAPK in hearts of EMD null mice: similarities between mouse models of X-linked and autosomal dominant Emery Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is an inherited disorder characterized by slowly progressive skeletal muscle weakness in a humero-peroneal distribution, early contractures and prominent cardiomyopathy with conduction block. Mutations in EMD, encoding emerin, and LMNA, encoding A-type lamins, respectively, cause X-linked and autosomal dominant EDMD. Emerin and A-type lamins are proteins of the inner membrane of the nuclear envelope. Whereas the genetic cause of EDMD has been described and the proteins well characterized, little is known on how abnormalities in nuclear envelope proteins cause striated muscle disease. In this study, we analyzed genome-wide expression profiles in hearts from Emd knockout mice, a model of X-linked EDMD, using Affymetrix GeneChips. This analysis showed a molecular signature similar to that we previously described in hearts from Lmna H222P knock-in mice, a model of autosomal dominant EDMD. There was a common activation of the ERK1/2 branch of the mitogen-activated protein kinase (MAPK) pathway in both murine models, as well as activation of downstream targets implicated in the pathogenesis of cardiomyopathy. Activation of MAPK signaling appears to be a cornerstone in the development of heart disease in both X-linked and autosomal dominant EDMD.     

Expanding the phenotype of LMNA mutations in dilated cardiomyopathy and functional consequences of these mutations

Aims: Mutations in the lamin A/C gene (LMNA) have been reported to be involved in dilated cardiomyopathy (DCM) associated with conduction system disease and/or skeletal myopathy. The aim of this study was to perform a mutational analysis of LMNA in a large white population of patients affected by dilated cardiomyopathy with or without associated symptoms.

Methods: We performed screening of the coding sequence of LMNA on DNA samples from 66 index cases, and carried out cell transfection experiments to examine the functional consequences of the mutations identified.

Results: A new missense (E161K) mutation was identified in a family with early atrial fibrillation and a previously described (R377H) mutation in another family with a quadriceps myopathy associated with DCM. A new mutation (28insA) leading to a premature stop codon was identified in a family affected by DCM with conduction defects. No mutation in LMNA was found in cases with isolated dilated cardiomyopathy. Functional analyses have identified potential physiopathological mechanisms involving identified mutations, such as haploinsufficiency (28insA) or intermediate filament disorganisation (E161K, R377H).

Conclusion: For the first time, a specific phenotype characterised by early atrial fibrillation is associated with LMNA mutation. Conversely, mutations in LMNA appear as a rare cause of isolated dilated cardiomyopathy. The variable phenotypes observed in LMNA-DCM might be explained by the variability of functional consequences of LMNA mutations.

     

Identification of lamin A/C ( LMNA) gene mutations in Korean patients with autosomal dominant Emery-Dreifuss muscular dystrophy and limb-girdle muscular dystrophy 1B

Mutations in the LMNA gene encoding lamins A and C by alternative splicing have been found to cause at least four different kinds of genetic disorders: autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD2; MIM 181350); limb-girdle muscular dystrophy type 1B (LGMD1B; MIM 159001); dilated cardiomyopathy type 1A (CMD1A; MIM 115200); and familial partial lipodystrophy (FPLD; MIM 151660). Recently, we have studied two Korean patients with atrioventricular conduction defects. They had variable extents of muscular dystrophy; one patient was diagnosed with EDMD2 and the other with LGMD1B. We performed a mutation analysis of the LMNA gene by direct sequencing and found two different missense mutations: R249Q and R377L, in the EDMD2 and LGMD1B patient, respectively. The R249Q mutation is located within the central rod domain of the LMNA gene, and has been described in at least five unrelated sporadic EDMD2 patients. On the other hand, the R377L mutation, also located within the rod domain, is a novel mutation, although a histidine substitution instead of leucine (R377H) has been reported previously in an LGMD1B patient. To our knowledge, this is the first report of LMNA gene mutations in Korean patients with EDMD2 and LGMD1B. Received: November 19, 2001 / Accepted: February 8, 2002     

Nuclear sequestration of delta-sarcoglycan disrupts the nuclear localization of lamin A/C and emerin in cardiomyocytes

Sarcoglycan is a membrane-associated protein complex found at the plasma membrane of cardiomyocytes and skeletal myofibers. Recessive mutations of delta-sarcoglycan that eliminate expression, and therefore function, lead to cardiomyopathy and muscular dystrophy by producing instability of the plasma membrane. A dominant missense mutation in the gene encoding delta-sarcoglycan was previously shown to associate with dilated cardiomyopathy in humans. To investigate the mechanism of dominantly inherited cardiomyopathy, we generated transgenic mice that express the S151A delta-sarcoglycan mutation in the heart using the alpha-myosin heavy-chain gene promoter. Similar to the human delta-sarcoglycan gene mutation, S151A delta-sarcoglycan transgenic mice developed dilated cardiomyopathy at a young age with enhanced lethality. Instead of placement at the plasma membrane, delta-sarcoglycan was found in the nucleus of S151A delta-sarcoglycan cardiomyocytes. Retention of delta-sarcoglycan in the nucleus was accompanied by partial nuclear sequestration of beta- and gamma-sarcoglycan. Additionally, the nuclear-membrane-associated proteins, lamin A/C and emerin, were mislocalized throughout the nucleoplasm. Therefore, the S151A delta-sarcoglycan gene mutation acts in a dominant negative manner to produce trafficking defects that disrupt nuclear localization of lamin A/C and emerin, thus linking together two common mechanisms of inherited cardiomyopathy.