A novel autosomal dominant limb-girdle muscular dystrophy (LGMD 1F) maps to 7q32.1-32.2

LGMD D2 TNPO3-Related: From Clinical Spectrum to Pathogenetic Mechanism

Limb-girdle muscular dystrophies (LGMDs) are clinically and genetically heterogeneous diseases presenting with a wide clinical spectrum. Autosomal dominant LGMDs represent about 10–15% of LGMDs and include disorders due to defects of DNAJB6, transportin-3 (TNPO3), HNRNPDL, Calpain-3 (CAPN3), and Bethlem myopathy. This review article aims to describe the clinical spectrum of LGMD D2 TNPO3-related, a rare disease due to heterozygous mutation in the TNPO3 gene. TNPO3 encodes for transportin-3, which belongs to the importin beta family and transports into the nucleus serine/arginine-rich (SR) proteins, such as splicing factors, and HIV-1 proteins, thus contributing to viral infection. The purpose of this review is to present and compare the clinical features and the genetic and histopathological findings described in LGMD D2, performing a comparative analytical description of all the families and sporadic cases identified. Even if the causative gene and mutations of this disease have been identified, the pathogenic mechanisms are still an open issue; therefore, we will present an overview of the hypotheses that explain the pathology of LGMD D2 TNPO3-related.

Inhibition of autophagy rescues muscle atrophy in a LGMDD2 Drosophila model

Limb-girdle muscular dystrophy D2 (LGMDD2) is an ultrarare autosomal dominant myopathy caused by mutation of the normal stop codon of the TNPO3 nuclear importin. The mutant protein carries a 15 amino acid C-terminal extension associated with pathogenicity. Here we report the first animal model of the disease by expressing the human mutant TNPO3 gene in Drosophila musculature or motor neurons and concomitantly silencing the endogenous expression of the fly protein ortholog. A similar genotype expressing wildtype TNPO3 served as a control. Phenotypes characterization revealed that mutant TNPO3 expression targeted at muscles or motor neurons caused LGMDD2-like phenotypes such as muscle degeneration and atrophy, and reduced locomotor ability. Notably, LGMDD2 mutation increase TNPO3 at the transcript and protein level in the Drosophila model Upregulated muscle autophagy observed in LGMDD2 patients was also confirmed in the fly model, in which the anti-autophagic drug chloroquine was able to rescue histologic and functional phenotypes. Overall, we provide a proof of concept of autophagy as a target to treat disease phenotypes and propose a neurogenic component to explain mutant TNPO3 pathogenicity in diseased muscles.

LGMD. Identification, description and classification

The term ‘limb girdle muscular dystrophy’ (LGMD) was first used in the seminal paper by Walton and Nattrass in 1954, were they identified LGMD as a separate clinical entity In LGMD description it is pointed out that the category of LGMD most likely comprises a heterogeneous group of disorders. After that the clinical entity was discussed but the LMGD nosography reached a permanent classification during two ENMC workshops held in 1995 and 2017, in the last one an operating definition of LGMD was agreed. This last classification included dystrophies with proximal or distal-proximal presentation with evidence at biopsy of fibre degeneration and splitting, high CK, MRI imaging consistent with degenerative changes, fibro-fatty infiltration present in individuals that reached independent walking ability. To be considered in this group at least two unrelated families should be identified.

A review is done of the first genetic characterisation of a number of LGMDs during the late twentieth century and a historical summary is given regarding how these conditions were clinically described and identified, the progresses done from identification of genetic loci, to protein and gene discoveries are reported. The LGMD described on which such historical progresses were done are the recessive calpainopathy (LGMD 2A/R1), dysferlinopathy (LGMD 2B/R2), sarcoglycanopathy (LGMD 2C-2F/R3-R6) types and the dominant type due to TPNO3 variants named transportinopathy (LGMD 1F/D2). Because of new diagnostic techniques such as exome and genome sequencing, it is likely that many other subtypes of LGMD might be identified in the future, however the lesson from the past discoveries can be useful for scientists and clinicians.

Manipulative evidence and medical interventions: some qualifications

The notion of causal evidence in medicine has been the subject of wide philosophical debate in recent years. The notion of evidence has been discussed mostly in connection with Evidence Based Medicine and, more in general, with the assessment of causal nexus in medical, and especially research contexts. "Manipulative evidence" is one of the notions of causal evidence that has stimulated much debate. It has been defined in slightly different ways, attributed different relevance, and recently placed at the core of Gillies' "action-related theory of causality", a view specifically meant to address causation in medicine. While in general sympathetic to Gillies' account, and totally convinced of the relevance of manipulative evidence and different sorts of interventions in the biomedical sciences, we believe that some further qualifications are needed to allow the notion of manipulative evidence to better express features of medical practice. In particular, we provide some qualification of the role of "interventional evidence" proposed by Gillies, suggesting a distinction between "interventional evidence" and "evidence for interventions". A case study from research on rare diseases is analyzed in depth and a multifaceted notion of manipulative evidence put forward that allows better understanding of what manipulations in medical contexts amount to and what their targets are.

A new family with transportinopathy: increased clinical heterogeneity

We describe a family with a novel TNPO3 mutation of limb–girdle muscular dystrophy D2 (or LGMD 1F), a rare muscle disorder with autosomal dominant inheritance, first identified in an Italo-Spanish family where the causative defect has been found to be due to TNPO3 gene mutation, encoding transportin-3 protein (TNPO3). We present the clinical, histopathological and muscle magnetic resonance imaging (MRI) features in two patients, mother and son Hungarian origin, affected by LGMD D2 and correlate their clinical, MRI and histopathological data found in this condition. The affected son presented early pelvic girdle muscle weakness and thin muscles similar to a congenital myopathy; the mother was less compromised and had an LGMD phenotype. Muscle MRI showed a very pronounced lower limb muscle atrophy in both patients. The most relevant change obtained in the child muscle biopsy was a generalized type 1 fibre atrophy. The two patients presented the same mutation, but a different phenotype has been observed in mother and son.

A novel pathogenic variant in TNPO3 in a Hungarian family with limb-girdle muscular dystrophy 1F

Limb-girdle muscular dystrophies (LGMDs) are a group of genetically heterogeneous muscular diseases that predominantly affect the proximal muscles. Pathogenic variants in TNPO3 have been associated with a rare, autosomal dominant limb-girdle muscular dystrophy 1F (LGMD1F) in a large Italian-Spanish family and an isolated LGMD1F case. Here we present two individuals from a Hungarian family with an early-onset, slowly progressive muscular dystrophy. Both the female proband and her affected son had delayed early motor milestones including first walking at 14 months and 18 months, respectively. Both present with progressive weakness of facial, bulbar, axial, and distal muscles especially of the lower extremities. Electromyography indicated myogenic damage and muscle biopsy from the proband showed myopathic alterations with sarcoplasmic masses and signs of mitochondrial dysfunction. Exome sequencing of the female proband identified a novel c.2767delC p.(Arg923AspfsTer17) variant in TNPO3. Sanger sequencing confirmed the presence of the TNPO3 variant in the affected son; the unaffected son did not have the variant. The identification of the c.2767delC variant further supports the clinical significance of TNPO3 and expands the clinical spectrum of TNPO3-associated LGMD1F.

Limb-girdle muscular dystrophies: where next after six decades from the first proposal (Review)

Limb-girdle muscular dystrophies (LGMD) are a heterogeneous group of disorders, which has led to certain investigators disputing its rationality. The mutual feature of LGMD is limb-girdle affection. Magnetic resonance imaging (MRI), perioral skin biopsies, blood-based assays, reverse-protein arrays, proteomic analyses, gene chips and next generation sequencing are the leading diagnostic techniques for LGMD and gene, cell and pharmaceutical treatments are the mainstay therapies for these genetic disorders. Recently, more highlights have been shed on disease biomarkers to follow up disease progression and to monitor therapeutic responsiveness in future trials. In this study, we review LGMD from a variety of aspects, paying specific attention to newly evolving research, with the purpose of bringing this information into the clinical setting to aid the development of novel therapeutic strategies for this hereditary disease. In conclusion, substantial progress in our ability to diagnose and treat LGMD has been made in recent decades, however enhancing our understanding of the detailed pathophysiology of LGMD may enhance our ability to improve disease outcome in subsequent years.

Next-generation sequencing identifies transportin 3 as the causative gene for LGMD1F

Limb-girdle muscular dystrophies (LGMD) are genetically and clinically heterogeneous conditions. We investigated a large family with autosomal dominant transmission pattern, previously classified as LGMD1F and mapped to chromosome 7q32. Affected members are characterized by muscle weakness affecting earlier the pelvic girdle and the ileopsoas muscles. We sequenced the whole exome of four family members and identified a shared heterozygous frame-shift variant in the Transportin 3 (TNPO3) gene, encoding a member of the importin-β super-family. The TNPO3 gene is mapped within the LGMD1F critical interval and its 923-amino acid human gene product is also expressed in skeletal muscle. In addition, we identified an isolated case of LGMD with a new missense mutation in the same gene. We localized the mutant TNPO3 around the nucleus, but not inside. The involvement of gene related to the nuclear transport suggests a novel disease mechanism leading to muscular dystrophy.

Limb-girdle muscular dystrophy 1F is caused by a microdeletion in the transportin 3 gene

In 2001, we reported linkage of an autosomal dominant form of limb-girdle muscular dystrophy, limb-girdle muscular dystrophy 1F, to chromosome 7q32.1-32.2, but the identity of the mutant gene was elusive. Here, using a whole genome sequencing strategy, we identified the causative mutation of limb-girdle muscular dystrophy 1F, a heterozygous single nucleotide deletion (c.2771del) in the termination codon of transportin 3 (TNPO3). This gene is situated within the chromosomal region linked to the disease and encodes a nuclear membrane protein belonging to the importin beta family. TNPO3 transports serine/arginine-rich proteins into the nucleus, and has been identified as a key factor in the HIV-import process into the nucleus. The mutation is predicted to generate a 15-amino acid extension of the C-terminus of the protein, segregates with the clinical phenotype, and is absent in genomic sequence databases and a set of >200 control alleles. In skeletal muscle of affected individuals, expression of the mutant messenger RNA and histological abnormalities of nuclei and TNPO3 indicate altered TNPO3 function. Our results demonstrate that the TNPO3 mutation is the cause of limb-girdle muscular dystrophy 1F, expand our knowledge of the molecular basis of muscular dystrophies and bolster the importance of defects of nuclear envelope proteins as causes of inherited myopathies.

A practical approach to molecular diagnostic testing in neuromuscular diseases

Molecular diagnosis is an important aspect in the care of patients with neuromuscular disorders. Because of the rapidly evolving nature of the field, the approach to obtaining a molecular diagnosis may be challenging. This article provides a general approach to molecular diagnostic testing while reviewing the principles of genetics and genetic disorders and the indications and limitations of testing methods in common hereditary neuromuscular disorders.

A human skeletal muscle interactome centered on proteins involved in muscular dystrophies: LGMD interactome

Background

The complexity of the skeletal muscle and the identification of numerous human disease-causing mutations in its constitutive proteins make it an interesting tissue for proteomic studies aimed at understanding functional relationships of interacting proteins in both health and diseases.

Method

We undertook a large-scale study using two-hybrid screens and a human skeletal-muscle cDNA library to establish a proteome-scale map of protein-protein interactions centered on proteins involved in limb-girdle muscular dystrophies (LGMD). LGMD is a group of more than 20 different neuromuscular disorders that principally affect the proximal pelvic and shoulder girdle muscles.

Results and conclusion

The interaction network we unraveled incorporates 1018 proteins connected by 1492 direct binary interactions and includes 1420 novel protein-protein interactions. Computational, experimental and literature-based analyses were performed to assess the overall quality of this network. Interestingly, LGMD proteins were shown to be highly interconnected, in particular indirectly through sarcomeric proteins. In-depth mining of the LGMD-centered interactome identified new candidate genes for orphan LGMDs and other neuromuscular disorders. The data also suggest the existence of functional links between LGMD2B/dysferlin and gene regulation, between LGMD2C/γ-sarcoglycan and energy control and between LGMD2G/telethonin and maintenance of genome integrity. This dataset represents a valuable resource for future functional investigations.

Exome sequencing reveals DNAJB6 mutations in dominantly-inherited myopathy

OBJECTIVE

To identify the causative gene in an autosomal dominant limb-girdle muscular dystrophy (LGMD) with skeletal muscle vacuoles.

METHODS

Exome sequencing was used to identify candidate mutations in the studied pedigree. Genome-wide linkage was then used to narrow the list of candidates to a single disease-associated mutation. Additional pedigrees with dominant or sporadic myopathy were screened for mutations in the same gene (DNAJB6) using exome sequencing. Skeletal muscle from affected patients was evaluated with histochemistry and immunohistochemical stains for dystrophy-related proteins, SMI-31, TDP43, and DNAJB6.

RESULTS

Exome analysis in 3 affected individuals from a family with dominant LGMD and vacuolar pathology identified novel candidate mutations in 22 genes. Linkage analysis excluded all variants except a Phe93Leu mutation in the G/F domain of the DNAJB6 gene, which resides within the LGMD locus at 7q36. Analysis of exome sequencing data from other pedigrees with dominant myopathy identified a second G/F domain mutation (Pro96Arg) in DNAJB6. Affected muscle showed mild dystrophic changes, vacuoles, and abnormal aggregation of proteins, including TDP-43 and DNAJB6 itself.

INTERPRETATION

Mutations within the G/F domain of DNAJB6 are a novel cause of dominantly-inherited myopathy. DNAJB6 is a member of the HSP40/DNAJ family of molecular co-chaperones tasked with protecting client proteins from irreversible aggregation during protein synthesis or during times of cellular stress. The abnormal accumulation of several proteins in patient muscle, including DNAJB6 itself, suggest that DNAJB6 function is compromised by the identified G/F domain mutations.

Childhood onset of limb-girdle muscular dystrophy

Limb-girdle muscular dystrophies comprise a rare heterogeneous group of genetic muscular dystrophies, involving 15 autosomal recessive subtypes and seven autosomal dominant subtypes. Autosomal recessive dystrophy is far more common than autosomal dominant dystrophy. Typical clinical features include progressive limb muscle weakness and atrophy (proximal greater than distal), varying from very mild to severe. Significant overlap of clinical phenotypes, with genetic and clinical heterogeneity, constitutes the rule for this group of diseases. Muscle biopsies are useful for histopathologic and immunolabeling studies, and DNA analysis is the gold standard to establish the specific form of muscular dystrophy. A definitive diagnosis among various subtypes is challenging, and the data presented here provide neuromuscular clinicians with additional information to help attain that goal.

A new locus on 3p23-p25 for an autosomal-dominant limb-girdle muscular dystrophy, LGMD1H

Limb-girdle muscular dystrophies (LGMDs) are a genetically heterogeneous group of neuromuscular disorders with a selective or predominant involvement of shoulder and pelvic girdles. We clinically examined 19 members in a four-generation Italian family with autosomal-dominant LGMD. A total of 11 subjects were affected. Clinical findings showed variable expressivity in terms of age at onset and disease severity. Five subjects presented with a slowly progressive proximal muscle weakness, in both upper and lower limbs, with onset during the fourth-fifth decade of life, which fulfilled the consensus diagnostic criteria for LGMD. Earlier onset of the disease was observed in a group of patients presenting with muscle weakness and/or calf hypertrophy, and/or occasionally high CK and lactate serum levels. Two muscle biopsies showed morphological findings compatible with MD associated with subsarcolemmal accumulation of mitochondria and the presence of multiple mitochondrial DNA deletions. A genome-wide scan performed using microsatellite markers mapped the disease on chromosome 3p23-p25.1 locus in a 25-cM region between markers D3S1263 and D3S3685. The highest two-point LOD score was 3.26 (theta=0) at marker D3S1286 and D3S3613, whereas non-parametric analysis reached a P-value=0.0004. Four candidate genes within the refined region were analysed but did not reveal any mutations. Our findings further expand the clinical and genetic heterogeneity of LGMDs.

Ins and outs of therapy in limb girdle muscular dystrophies

Muscular dystrophies are hereditary degenerative muscle diseases that cause life-long disability in patients. They comprise the well-known Duchenne Muscular Dystrophy (DMD) but also the group of Limb Girdle Muscular Dystrophies (LGMD) which account for a third to a fourth of DMD cases. From the clinical point of view, LGMD are characterised by predominant effects on the proximal limb muscles. The LGMD group is still growing today and consists of 19 autosomal dominant and recessive forms (LGMD1A to LGMD1G and LGMD2A to LGMD2M). The proteins involved are very diverse and include sarcomeric, sarcolemmal and enzymatic proteins. With respect to this variability and in line with the intense search for a potent therapeutic approach for DMD, many different strategies have been tested in rodent models. These include replacing the lost function by gene transfer or stem cell transplantation, using a related protein for functional substitution, increasing muscle mass, or blocking the molecular pathological mechanisms by pharmacological means to alleviate the symptoms. The purpose of this review is to summarize current data arising from these preclinical studies and to examine the potential of the tested strategies to lead to clinical applications.

Extensive scanning of the calpain-3 gene broadens the spectrum of LGMD2A phenotypes

BACKGROUND

The limb girdle muscular dystrophies (LGMD) are a heterogeneous group of Mendelian disorders highlighted by weakness of the pelvic and shoulder girdle muscles. Seventeen autosomal loci have been so far identified and genetic tests are mandatory to distinguish among the forms. Mutations at the calpain 3 locus (CAPN3) cause LGMD type 2A.

OBJECTIVE

To obtain unbiased information on the consequences of CAPN3 mutations.

PATIENTS

530 subjects with different grades of symptoms and 300 controls.

METHODS

High throughput denaturing HPLC analysis of DNA pools.

RESULTS

141 LGMD2A cases were identified, carrying 82 different CAPN3 mutations (45 novel), along with 18 novel polymorphisms/variants. Females had a more favourable course than males. In 94% of the more severely affected patient group, the defect was also discovered in the second allele. This proves the sensitivity of the approach. CAPN3 mutations were found in 35.1% of classical LGMD phenotypes. Mutations were also found in 18.4% of atypical patients and in 12.6% of subjects with high serum creatine kinase levels.

CONCLUSIONS

A non-invasive and cost-effective strategy, based on the high throughput denaturing HPLC analysis of DNA pools, was used to obtain unbiased information on the consequences of CAPN3 mutations in the largest genetic study ever undertaken. This broadens the spectrum of LGMD2A phenotypes and sets the carrier frequency at 1:103.

The childhood limb-girdle muscular dystrophies

The heterogeneous childhood limb-girdle muscular dystrophies have originally been defined as a group of autosomal recessive and dominant diseases with progressive weakness and wasting of shoulder and pelvic-girdle muscles. Over the last 12 years, the underlying genetic defects for many of the diseases have been identified and insight into pathomechanisms of disease has been gained. At the same time, improved diagnostic techniques have allowed to extend the phenotypic spectrum for many of these devastating conditions, which showed that clinical symptoms and pathological findings are not restricted to skeletal muscles. Childhood limb-girdle muscular dystrophies are systemic diseases that often affect the musculoskeletal, respiratory, and cardiovascular system and that can go along with central nervous system involvement and gastrointestinal symptoms. The systemic nature of the diseases requires adequate management strategies that improve symptoms, longevity, and quality of life of the patients. As we are entering an era of translational research the need for precise molecular diagnoses, a thorough understanding of the natural history of the diseases and guidelines for standardized assessments of the patients become even more relevant. In this review, the best characterized childhood limb-girdle muscular dystrophies are discussed and their management aspects highlighted.

Hutterite brothers both affected with two forms of limb girdle muscular dystrophy: LGMD2H and LGMD2I

Limb girdle muscular dystrophy (LGMD) is very common in the Hutterite population of the North American Prairies. We have recently reported the homozygous c.1459G>A mutation in TRIM32 associated with LGMD2H. We have also identified Hutterite patients with LGMD2I, homozygous for the common c.826C>A mutation in FKRP. To date, all Hutterites with LGMD have been shown to be homozygous for either the TRIM32 or FKRP mutation. We now report a Hutterite family in which both parents and five sons were all found to be homozygous for the TRIM32 mutation. The father had slowly progressive proximal muscle weakness, whereas three sons and their mother, all currently asymptomatic, had normal physical examinations. The remaining two sons (7 and 10 years old), presented with mild decrease in stamina, had normal neuromuscular examinations and were found to be homozygous for the FKRP mutation in addition to the TRIM32 mutation. These two boys do not differ in age at or mode of presentation, physical findings, or serum CK levels compared to age-matched individuals affected with LGMD2I alone. This suggests that the effects of these two mutations are not acting synergistically at this time. It remains to be seen whether there will be signs of interaction between these two mutations as the patients get older.

Molecular etiopathogenesis of limb girdle muscular and congenital muscular dystrophies: Boundaries and contiguities

The muscular dystrophies are a heterogeneous group of inherited disorders characterized by progressive muscle wasting and weakness. These disorders present a large clinical variability regarding age of onset, patterns of skeletal muscle involvement, heart damage, rate of progression and mode of inheritance. Difficulties in classification are often caused by the relatively common sporadic occurrence of autosomal recessive forms as well as by intrafamilial clinical variability. Furthermore recent discoveries, particularly regarding the proteins linking the sarcolemma to components of the extracellular matrix, have restricted the gap existing between limb girdle (LGMD) and congenital muscular dystrophies (CMD). Therefore a renewed definition of boundaries between these two groups is required. Molecular genetic studies have demonstrated different causative mutations in the genes encoding a disparate collection of proteins involved in all aspects of muscle cell biology. These novel skeletal muscle genes encode highly diverse proteins with different localization within or at the surface of the skeletal muscle fibre, such as the sarcolemmal muscle membrane (dystrophin, sarcoglycans, dysferlin, caveolin-3), the extracellular matrix (alpha2 laminin, collagen VI), the sarcomere (telethonin, myotilin, titin, nebulin and ZASP), the muscle cytosol (calpain-3, TRIM32), the nucleus (emerin, lamin A/C) and the glycosilation pathway enzymes (fukutin and fukutin related proteins). The accumulating knowledge about the role of these different proteins in muscle pathology has led to a profound change in the original phenotype-based classification and shed new light on the molecular pathogenesis of these disorders.

A mutation in the dimerization domain of filamin c causes a novel type of autosomal dominant myofibrillar myopathy

Myofibrillar myopathy (MFM) is a human disease that is characterized by focal myofibrillar destruction and pathological cytoplasmic protein aggregations. In an extended German pedigree with a novel form of MFM characterized by clinical features of a limb-girdle myopathy and morphological features of MFM, we identified a co-segregating, heterozygous nonsense mutation (8130G-->A; W2710X) in the filamin c gene (FLNC) on chromosome 7q32.1. The mutation is the first found in FLNC and is localized in the dimerization domain of filamin c. Functional studies showed that, in the truncated mutant protein, this domain has a disturbed secondary structure that leads to the inability to dimerize properly. As a consequence of this malfunction, the muscle fibers of our patients display massive cytoplasmic aggregates containing filamin c and several Z-disk-associated and sarcolemmal proteins.

Limb-girdle muscular dystrophy in childhood

LGMD refers to a class of muscular dystrophies with onset in the proximal muscles. They are genetically heterogeneous, with both autosomal recessive and dominant forms. The autosomal recessive forms are more common and in general follow a more severe course compared to the dominant forms. It is important to reach a specific genetic diagnosis beyond making a group diagnosis of LGMD to provide adequate genetic counseling, to predict risks for the patient such as the development of cardiomyopathy, and to be able to take advantage of specific treatments when they become available. Establishing a specific diagnosis requires knowledge about the individual clinical features, expert analysis of the muscule biopsy, and the guided initiation of appropriate genetic testing.

Molecular diagnosis of inheritable neuromuscular disorders. Part II: Application of genetic testing in neuromuscular disease

Molecular genetic advances have led to refinements in the classification of inherited neuromuscular disease, and to methods of molecular testing useful for diagnosis and management of selected patients. Testing should be performed as targeted studies, sometimes sequentially, but not as wasteful panels of multiple genetic tests performed simultaneously. Accurate diagnosis through molecular testing is available for the vast majority of patients with inherited neuropathies, resulting from mutations in three genes (PMP22, MPZ, and GJB1); the most common types of muscular dystrophies (Duchenne and Becker, facioscapulohumeral, and myotonic dystrophies); the inherited motor neuron disorders (spinal muscular atrophy, Kennedy's disease, and SOD1 related amyotrophic lateral sclerosis); and many other neuromuscular disorders. The role of potential multiple genetic influences on the development of acquired neuromuscular diseases is an increasingly active area of research.

Klinik und Genetik der Gliederg�rteldystrophien

Limb girdle muscular dystrophies (LGMDs) are a genetically heterogeneous group of primary myopathies involving progressive weakness and wasting of the muscles in the hip and shoulder girdles, with distal spread to the bulbar or respiratory musculature in rare cases. Depending on the mode of genetic transmission, six autosomal dominant forms (LGMD1A-F, 10-25%) and ten autosomal recessive forms (LGMD2A-J, 75-90%) are currently known. The prevalence of LGMDs is 0.8/100,000. These conditions are caused by mutations in genes encoding for myotilin (5q31, LGMD1A), lamin A/C (1q11-q21.2, LGMD1B), caveolin-3 (3p25, LGMD1C), unknown proteins (7q, LGMD1D, 6q23, LGMD1E, 7q32.1-32.2., LGMD1F), calpain-3 (15q15.1-21.1, LGMD2A), dysferlin (2p13.3-13.1, LGMD2B), gamma-sarcoglycan (13q12, LGMD2C), alpha-sarcoglycan, also known as adhalin (17q12-q21.3, LGMD2D), beta-sarcoglycan (4q12, LGMD2E), delta-sarcoglycan (5q33-q34, LGMD2F), telethonin (17q11-q12, LGMD2G), E3-ubiquitin ligase (9q31-q34.1, LGMD2H), fukutin-related protein (19q13.3, LGMD2I), and titin (2q31, LGMD2J). Cardiac involvement has been described for LGMD1B-E, LGMD2C-G, and LGMD2I. The time of onset varies between early childhood and middle age. There is no male or female preponderance. Disease progression and life expectancy vary widely, even among different members of the same family. The diagnosis is based primarily on DNA analysis. The history, clinical neurological examinations, blood chemistry investigations, electromyography, and muscle biopsy also provide information that is helpful for the diagnosis. No causal therapy is currently available.

A new form of autosomal dominant limb-girdle muscular dystrophy (LGMD1G) with progressive fingers and toes flexion limitation maps to chromosome 4p21

Limb-girdle muscular dystrophy (LGMD) is a genetic disorder characterized by progressive weakness of pelvic and scapular girdles and great clinical variability. It is a highly heterogeneous disease with 16 identified loci: six of them autosomal dominant (AD) (LGMD1) and 10 autosomal recessive (AR) (LGMD2). The responsible genes are known for three of the AD-LGMD and for all 10 AR-LGMD. Linkage analysis excluded these 16 loci in a Brazilian-Caucasian family with 12 patients affected by AD late-onset LGMD associated with progressive fingers and toes flexion limitation. Biceps muscle biopsy from one of the patients showed a predominantly myopathic histopathological pattern, associated with rimmed vacuoles. A genomewide scan was performed which mapped a new locus for this disorder at 4p21 with a maximum two-point lod score of 6.62 for marker D4S2964. Flanking markers place this locus between D4S2947 and D4S2409, within an interval of 9 cM. We propose to classify this AD form of LGMD as LGMD1G.

Limb-girdle muscular dystrophies - from genetics to molecular pathology

The limb-girdle muscular dystrophies are a diverse group of muscle-wasting disorders characteristically affecting the large muscles of the pelvic and shoulder girdles. Molecular genetic analyses have demonstrated causative mutations in the genes encoding a disparate collection of proteins involved in all aspects of muscle cell biology. Muscular dystrophy includes a spectrum of disorders caused by loss of the linkage between the extracellular matrix and the actin cytoskeleton. Within this are the forms of limb-girdle muscular dystrophy caused by deficiencies of the sarcoglycan complex and by aberrant glycosylation of alpha-dystroglycan caused by mutations in the fukutin-related protein gene. However, other forms of this disease have distinct pathophysiological mechanisms. For example, deficiency of dysferlin disrupts sarcolemmal membrane repair, whilst loss of calpain-3 may exert its pathological influence either by perturbation of the IkappaBalpha/NF-kappaB pathway, or through calpain-dependent cytoskeletal remodelling. Caveolin-3 is implicated in numerous cell-signalling pathways and involved in the biogenesis of the T-tubule system. Alterations in the nuclear lamina caused by mutations in laminA/C, sarcomeric changes in titin, telethonin or myotilin at the Z-disc, and subtle changes in the extracellular matrix proteins laminin-alpha2 or collagen VI can all lead to a limb-girdle muscular dystrophy phenotype, although the specific pathological mechanisms remain obscure. Differential diagnosis of these disorders requires the careful application of a broad range of disciplines: clinical assessment, immunohistochemistry and immunoblotting using a panel of antibodies and extensive molecular genetic analyses.