Genetic linkage of Welander distal myopathy to chromosome 2p13

Dynamics of T-Cell Intracellular Antigen 1-Dependent Stress Granules in Proteostasis and Welander Distal Myopathy under Oxidative Stress

T-cell intracellular antigen 1 (TIA1) is an RNA-binding protein that is primarily involved in the post-transcriptional regulation of cellular RNAs. Furthermore, it is a key component of stress granules (SGs), RNA, and protein aggregates that are formed in response to stressful stimuli to reduce cellular activity as a survival mechanism. TIA1 p.E384K mutation is the genetic cause of Welander distal myopathy (WDM), a late-onset muscular dystrophy whose pathogenesis has been related to modifying SG dynamics. In this study, we present the results obtained by analyzing two specific aspects: (i) SGs properties and dynamics depending on the amino acid at position 384 of TIA1; and (ii) the formation/disassembly time-course of TIA1WT/WDM-dependent SGs under oxidative stress. The generation of TIA1 variants—in which the amino acid mutated in WDM and the adjacent ones were replaced by lysines, glutamic acids, or alanines—allowed us to verify that the inclusion of a single lysine is necessary and sufficient to alter SGs dynamics. Moreover, time-lapse microscopy analysis allowed us to establish in vivo the dynamics of TIA1WT/WDM-dependent SG formation and disassembly, after the elimination of the oxidizing agent, for 1 and 3 h, respectively. Our observations show distinct dynamics between the formation and disassembly of TIA1WT/WDM-dependent SGs. Taken together, this study has allowed us to expand the existing knowledge on the role of TIA1 and the WDM mutation in SG formation.

A Heterologous Cell Model for Studying the Role of T-Cell Intracellular Antigen 1 in Welander Distal Myopathy

Welander distal myopathy (WDM) is a muscle dystrophy characterized by adult-onset distal muscle weakness, prevalently impacting the distal long extensors of the hands and feet. WDM is an autosomal dominant disorder caused by a missense mutation (c.1362G>A; p.E384K) in the TIA1 (T-cell intracellular antigen 1) gene, which encodes an RNA-binding protein basically required for the posttranscriptional regulation of RNAs. We have developed a heterologous cell model of WDM to study the molecular and cellular events associated with mutated TIA1 expression. Specifically, we analyzed how this mutation affects three regulatory functions mediated by TIA1: (i) control of alternative SMN2 (survival motor neuron 2) splicing; (ii) formation, assembly, and disassembly of stress granules; and (iii) mitochondrial dynamics and its consequences for mitophagy, autophagy, and apoptosis. Our results show that whereas WDM-associated TIA1 expression had only a mild effect on SMN2 splicing, it led to suboptimal adaptation to environmental stress, with exacerbated stress granule formation that was accompanied by mitochondrial dysfunction and autophagy. Overall, our observations indicate that some aspects of the cell phenotype seen in muscle of patients with WDM can be recapitulated by ectopic expression of WDM-TIA1 in embryonic kidney cells, highlighting the potential of this model to investigate the pathogenesis of this degenerative disease and possible therapeutics.

GNE Myopathy With Novel Mutations and Pronounced Paraspinal Muscle Atrophy

GNE myopathy is characterized by distal muscle weakness, and caused by recessive mutations in GNE. Its onset is characteristically in young adulthood, although a broad spectrum of onset age is known to exist. A large number of mutations in GNE are pathogenic and this clinical phenotype can be difficult to differentiate clinically from other late-onset myopathies. We describe two families with novel mutations in GNE, and describe their clinical and MRI features. We also describe the presence of striking paraspinal muscle involvement on MRI of the lumbar spine, which is an under-recognized feature of GNE myopathy.

T-cell intracellular antigens in health and disease

T-cell intracellular antigen 1 (TIA1) and TIA1-related/like protein (TIAR/TIAL1) are 2 proteins discovered in 1991 as components of cytotoxic T lymphocyte granules. They act in the nucleus as regulators of transcription and pre-mRNA splicing. In the cytoplasm, TIA1 and TIAR regulate and/or modulate the location, stability and/or translation of mRNAs. As knowledge of the different genes regulated by these proteins and the cellular/biological programs in which they are involved increases, it is evident that these antigens are key players in human physiology and pathology. This review will discuss the latest developments in the field, with physiopathological relevance, that point to novel roles for these regulators in the molecular and cell biology of higher eukaryotes.

CanvasDB: a local database infrastructure for analysis of targeted- and whole genome re-sequencing projects

CanvasDB is an infrastructure for management and analysis of genetic variants from massively parallel sequencing (MPS) projects. The system stores SNP and indel calls in a local database, designed to handle very large datasets, to allow for rapid analysis using simple commands in R. Functional annotations are included in the system, making it suitable for direct identification of disease-causing mutations in human exome- (WES) or whole-genome sequencing (WGS) projects. The system has a built-in filtering function implemented to simultaneously take into account variant calls from all individual samples. This enables advanced comparative analysis of variant distribution between groups of samples, including detection of candidate causative mutations within family structures and genome-wide association by sequencing. In most cases, these analyses are executed within just a matter of seconds, even when there are several hundreds of samples and millions of variants in the database. We demonstrate the scalability of canvasDB by importing the individual variant calls from all 1092 individuals present in the 1000 Genomes Project into the system, over 4.4 billion SNPs and indels in total. Our results show that canvasDB makes it possible to perform advanced analyses of large-scale WGS projects on a local server. Database URL: https://github.com/UppsalaGenomeCenter/CanvasDB.

Distal myopathies

In this article, distal myopathy syndromes are discussed. A discussion of the more traditional distal myopathies is followed by discussion of the myofibrillar myopathies. Other clinically and genetically distinctive distal myopathy syndromes usually based on single or smaller family cohorts are reviewed. Other neuromuscular disorders that are important to recognize are also considered, because they show prominent distal limb weakness.

Distal myopathies

The distal myopathies are a heterogeneous group of genetic disorders defined by a predominant distal weakness at onset or throughout the evolution of the disease and by pathological data supporting a myopathic process. The number of genes associated with distal myopathies continues to increase. Fourteen distinct distal myopathies are currently defined by their gene and causative mutations, compared to just five entities delineated on clinical grounds two decades ago. The known proteins affected in the distal myopathies are of many types and include a significant number of sarcomeric proteins. The useful indicators for clinicians to direct towards a correct molecular diagnosis are the mode of inheritance, the age at onset, the pattern of muscle involvement, the serum creatine kinase level and the muscle pathology findings. This review gives an overview of the clinical and genetic characteristics of the currently identified distal myopathies with emphasis on some recent findings.

Welander distal myopathy is caused by a mutation in the RNA-binding protein TIA1

OBJECTIVE

A study was undertaken to identify the molecular cause of Welander distal myopathy (WDM), a classic autosomal dominant distal myopathy.

METHODS

The genetic linkage was confirmed and defined by microsatellite and single nucleotide polymorphism haplotyping. The whole linked genomic region was sequenced with targeted high-throughput and Sanger sequencing, and coding transcripts were sequenced on the cDNA level. WDM muscle biopsies were studied by Western blotting and immunofluorescence microscopy. Splicing of TIA1 and its target genes in muscle and myoblast cultures was analyzed by reverse transcriptase polymerase chain reaction. Mutant TIA1 was characterized by cell biological studies on HeLa cells, including quantification of stress granules by high content analysis and fluorescence recovery after photobleaching (FRAP) experiments.

RESULTS

The linked haplotype at 2p13 was narrowed down to <806 kb. Sequencing by multiple methods revealed only 1 segregating coding mutation, c.1362 G>A (p.E384K) in the RNA-binding protein TIA1, a key component of stress granules. Immunofluorescence microscopy of WDM biopsies showed a focal increase of TIA1 in atrophic and vacuolated fibers. In HeLa cells, mutant TIA1 constructs caused a mild increase in stress granule abundance compared to wild type, and showed slower average fluorescence recovery in FRAP.

INTERPRETATION

WDM is caused by mutated TIA1 through a dominant pathomechanism probably involving altered stress granule dynamics.

Distal muscular dystrophies

Distal muscular dystrophies are a group of inherited primary muscle disorders showing progressive weakness and atrophy preferentially in the hands, forearm, lower legs, or feet. Extensive progress in understanding the molecular genetic background has changed the classification and extended the list of confirmed entities to almost 20 different disorders, making the differential diagnostic procedure both easier and more difficult. Distal phenotypes first have to be differentiated from neurogenic disorders. The axonal form of Charcot-Marie-Tooth disease with late-onset distal weakness and distal forms of chronic spinal muscular atrophy may mimic those of the distal dystrophies. Increasing numbers of reports suggest increasing awareness of distal phenotypes in muscular dystrophy. Some disorders regularly progress eventually to involve proximal muscle, whereas others, such as tibial muscular dystrophy titinopathy (Udd), Welander distal myopathy, and distal myosinopathy (Laing), remain distal throughout the patient's lifetime. Pathologically there is a gradual degeneration and loss of muscle fibers with replacement by fibrous and fatty connective tissue, similar to the proximal forms of muscular dystrophy, frequently, but not always with rimmed vacuolar degenerative change. Strikingly, many of the genes involved in distal dystrophies code for sarcomeric proteins. However, the genetic programs leading to preferential involvement of distal muscles have remained unknown.

Kelch-like homologue 9 mutation is associated with an early onset autosomal dominant distal myopathy

Distal myopathies are a heterogeneous group of disorders characterized by progressive weakness and muscular atrophy, beginning in distal limb muscles and affecting proximal limb muscles at a later stage. We studied a large German kindred with 10 affected members. Weakness and atrophy of the anterior tibial muscles started between the ages of 8 and 16 years, followed by atrophy of intrinsic hand muscles. Progression was slow, and patients retained the ability to walk until the seventh decade. Serum creatinine kinase levels were increased in the range of 150–1400 U/l. Muscle biopsies showed myopathic changes, whereas immunohistochemistry showed normal expression of marker proteins for muscular dystrophies. Patients had reduced sensation with stocking-glove distribution in the distal limbs in later life. Nerve conduction studies revealed no evidence of neuropathy. Genome-wide linkage analysis in this family revealed a new locus for distal myopathy at 9p21.2-p22.3 (multipoint logarithm of the odds ratio = 4.21). By positional cloning we found a heterozygous mutation L95F in the Kelch-like homologue 9 gene, encoding a bric-a-brac Kelch protein. Molecular modelling indicated that the mutation may interfere with the interaction of the bric-a-brac domain with Cullin 3. Coimmunoprecipitation experiments confirmed that the mutation reduces association with Cullin 3 in the Kelch-like homologue 9-Cullin 3–E3 ubiquitin ligase complex, which is involved in ubiquitin-dependent protein degradation. We identified a unique form of early onset autosomal dominant distal myopathy which is associated with a Kelch-like homologue 9 mutation and interferes with normal skeletal muscle through a novel pathogenetic mechanism.

Genetics and pathogenesis of distal muscular dystrophies

Distal myopathies are distal muscular dystrophies because they are genetic disorders with progressive loss of muscle tissue. The true distal dystrophies not only show a distal onset; they also remain more distal than proximal throughout the course of the disease. Currently almost 20 different entities of distal muscular dystrophies have been genetically determined, compared to just five entities delineated on clinical grounds in the 1980s. Half of the genes underlying these disorders have been associated with distal phenotypes only, whereas the other genes can manifest also with other than distal phenotypes such as proximal, scapuloperoneal or generalized phenotypes. Interestingly, most of the genes causing distal muscular dystrophies code for protein components of the sarcomere, in contrast to the proximal dystrophies in which most of the genes cause defects in sarcolemmal proteins. The reason for why some gene defects predominantly affect distal muscles is not well understood. The fact that the majority of these defects are due to structural and functional components of the sarcomere is intriguing but so far it does not provide further clues for understanding or for therapeutic approaches. The highly selective involvement of muscles in many of the distal dystrophies is even less well understood.

Molecular biology of distal muscular dystrophies--sarcomeric proteins on top

During the last 10 years several muscular dystrophies within the group of distal myopathies have been clarified as to the molecular genetic cause of the disease. Currently, the next steps are carried out to identify the molecular pathogenesis downstream of the gene defects. Some early ideas on what is going on in the muscle cells based on the defect proteins are emerging. However, in no single distal muscular dystrophy these efforts have yet reached the point where direct trials for therapy would have been launched, and in many distal dystrophies the causative gene is still lacking. When comparing the gene defects in the distal dystrophies with the more common proximal muscular dystrophies such as dystrophinopathies or limb-girdle muscular dystrophies, there is a striking difference: the genes for distal dystrophies encode sarcomere proteins whereas the genes for proximal dystrophies more often encode sarcolemmal proteins.

Distal myopathies

PURPOSE OF REVIEW

The distal myopathies are a heterogeneous group of disorders that pose a challenge to both the clinician and geneticist. This article summarizes the findings of recent clinical, genetic and molecular studies and the current diagnostic approach to this group of patients.

RECENT FINDINGS

Publications over the past 5 years describe a number of new clinical phenotypes and genetic loci and further emphasize the overlap in clinical phenotype between a number of these disorders and between the distal and limb girdle myopathies and hereditary inclusion body myopathies. Recent studies have led to the identification of the genes and mutations responsible for early onset (Laing) myopathy and tibial (Udd) myopathy, and for distal myopathy with rimmed vacuoles (Nonaka), which has been shown to be allelic with quadriceps sparing hereditary inclusion body myopathy (IBM2), and have elucidated the underlying pathogenetic mechanisms in these conditions. New diagnostic approaches using magnetic resonance imaging, and a blood-based assay for dysferlin deficiency, have also been reported.

SUMMARY

These findings have important implications for future genetic linkage and gene expression studies and for the diagnostic approach to patients with a distal myopathy phenotype. They also hold promise for the eventual development of therapies for this group of disorders.

Mutations in the slow skeletal muscle fiber myosin heavy chain gene (MYH7) cause laing early-onset distal myopathy (MPD1)

We previously linked Laing-type early-onset autosomal dominant distal myopathy (MPD1) to a 22-cM region of chromosome 14. One candidate gene in the region, MYH7, which is mutated in cardiomyopathy and myosin storage myopathy, codes for the myosin heavy chain of type I skeletal muscle fibers and cardiac ventricles. We have identified five novel heterozygous mutations--Arg1500Pro, Lys1617del, Ala1663Pro, Leu1706Pro, and Lys1729del in exons 32, 34, 35, and 36 of MYH7--in six families with early-onset distal myopathy. All five mutations are predicted, by in silico analysis, to locally disrupt the ability of the myosin tail to form the coiled coil, which is its normal structure. These findings demonstrate that heterozygous mutations toward the 3' end of MYH7 cause Laing-type early-onset distal myopathy. MYH7 is the fourth distal-myopathy gene to have been identified.

Abnormal lysosomal and ubiquitin-proteasome pathways in 19p13.3 distal myopathy

We describe a second large Italian kindred with autosomal dominant vacuolar myopathy characterized by variable severity, adult-onset weakness of distal limb muscles, and no cardiac involvement. At least 19 individuals over four generations are affected. Histopathological and immunochemical features of the vacuoles, present in many fibers, indicate protein degradation abnormalities with dysregulation of the lysosomal pathway and activation of the ubiquitin-proteasomal pathway. Linkage analysis localized the defect to the 19p13.3 locus in a region with no known genes. We speculate that the primary defect may be an abnormality in the lysosomal degradation pathway or related components.

Muscle magnetic resonance imaging shows distinct diagnostic patterns in Welander and tibial muscular dystrophy

OBJECTIVES

This is a report on a retrospective muscle magnetic resonance imaging (MRI) study on 11 patients affected by Welander distal myopathy (WDM) and 22 patients with tibial muscular dystrophy (TMD) carried out in order to define the pattern and characteristics of muscle involvement.

RESULTS

WDM patients showed involvement of gastrocnemius, soleus, tibial anterior (TA) and extensor digitorum longus (EDL), as well as hamstrings and hip adductor muscles. TMD patients showed involvement of the TA and EDL muscles, and in some patients also hamstring and posterior compartment muscles of the legs. Some patients showed asymmetry of muscle involvement.

CONCLUSION

We conclude that muscle MRI examination proved to be very useful in the determination of the exact pattern of muscle involvement in WDM and TMD. Clinical testing using the Medical Research Council scale is not sensitive enough to establish the pattern of muscle involvement in focal muscle diseases.

Linkage to two separate loci in a family with a novel distal myopathy phenotype (MPD3)

We recently described a new type of adult onset distal myopathy (MPD3) with autosomal dominant inheritance. The onset of symptoms is around the age of 30 and the characteristic first symptoms include clumsiness of the hands and stumbling. The thenar and hypothenar muscles are involved at the onset. The disease progressed to the intrinsic muscles of the hands, both anterior and posterior muscle compartments of the lower legs, the forearm muscles, and later to the proximal muscles. Dystrophic changes with rimmed vacuoles were observed in the muscle biopsy. We have performed a genome wide scan here in order to identify the MPD3 locus. Unexpectedly, markers on two distinct chromosomal regions 8p22-q11 and 12q13-q22, provided significant evidence for linkage in this family. Multipoint linkage analyses produced equal maximum multipoint LOD score of 3.01 for both chromosomal regions and haplotype analysis showed a specific haplotype segregating with the disease for both loci. It is thus impossible to distinguish between two loci without additional family material. Two obvious regional candidate genes, encoding muscular proteins became subjects for sequence analyses, the gene for myosin light chain 1 slow-twitch muscle A on 12q13 and the muscle specific exons of ankyrin 1 on 8p11. No mutations were identified in the coding sequence.

Myopathies distales

Distal myopathies are a heterogeneous group of genetic disorders characterized clinically by progressive muscular weakness and atrophy beginning in the hands or feet, and pathologically by myopathic changes in skeletal muscles. Five distinct distal myopathies are identified, among them four have been recently defined by their gene and causative mutations. They are classified according to age at onset, mode of inheritance, and muscle groups initially involved into the following: Laing myopathy (infancy onset, autosomal dominant inheritance, onset in anterior compartment of legs) caused by mutations in a myosin gene (MYH7) on chromosome 14q; Nonaka myopathy (early adult onset, autosomal recessive inheritance, onset in anterior compartment of legs), identical to quadriceps-sparing familial inclusion myopathy, caused by mutations in the GNE gene on chromosome 9p-q; Miyoshi myopathy (early adult onset, autosomal recessive inheritance, onset in posterior compartment of legs) caused by mutations in the dysferlin gene on chromosome 2p; Welander myopathy (late adult onset, autosomal dominant inheritance, onset in hands) linked to chromosome 2p; Udd/Markesbery-Griggs myopathy (late adult onset, autosomal dominant inheritance, onset in anterior compartment of legs) caused by mutations in the titin gene on chromosome 2q. Except for Miyoshi myopathy, which has a striking elevated serum creatine kinase level and the typical findings of muscular dystrophy, most of the distal myopathies have normal or midly elevated creatine kinase levels and share the common pathologic feature of rimmed vacuoles.

Muscular dystrophies: influence of physical conditioning on the disease evolution

PURPOSE OF REVIEW

To summarize the current knowledge of the effects of physical activity on muscular dystrophies.

RECENT FINDINGS

Although the usefulness of exercise training in muscular dystrophy patients has been debated for many years, only a limited number of articles addressing this issue have been published to date. Existing studies on the effects of strength training in patients with muscular dystrophies have shown promising results, but interpretations are hampered by several methodological shortcomings.

SUMMARY

The scientific basis for solid recommendations of different exercise regimens in muscular dystrophies is poor, but existing data suggest beneficial effects of adopting an active lifestyle. Low- to moderate-intensity resistance and aerobic training may be recommended in slowly progressive myopathic disorders. To date, there is no evidence to support the recommendation of high-resistance exercise regimens over low-moderate intensity exercise. In rapidly progressive myopathies, which are due to aberrant structural proteins such as Duchenne muscular dystrophy, the use of high-resistance and eccentric training should be avoided. There is still, however, no evidence that physical training can influence the evolution of muscular dystrophies in the long term.

Welander distal myopathy outside the Swedish population: phenotype and genotype

Welander distal myopathy is a late onset disorder that is mainly seen in Sweden. It is linked to chromosome 2p13 and all Swedish patients show a common shared haplotype, indicating a founder mutation. Here we report the clinical manifestations, magnetic resonance imaging, pathophysiology and haplotype analysis of Welander patients in the Finnish population. The clinical examination of patients from 12 different families showed a distal myopathy with onset in the long extensor muscles of the hands and fingers, also seen in Swedish Welander patients. Muscle biopsies showed characteristic myopathic changes. Haplotype analysis with the five polymorphic markers that make up the common core haplotype, seen in the Swedish patients, revealed that this haplotype is also co-segregating in the Finnish patients and a common ancestry is therefore further supported for patients with Welander distal myopathy.

Genetic diseases of muscle

In the last twenty years, the genetic basis for most of the inherited myopathies and muscular dystrophies has been unveiled. Diseases have been found to result from loss of function of structural components of the muscle basal lamina (e.g., MCD1A), sarcolemma (e.g., the sarcoglycanopathies), nucleus (e.g., EDMD) and sarcomere (e.g., the nemaline myopathies). A few have been associated with abnormalities in the genes for muscle enzymes (e.g., calpain and fukutin). Alternate mechanisms of pathogenesis have also recently been suggested by mutations lying outside of coding regions, such as the "field effect" of chromosomal mutations in DM2. In the future, we will likely identify the genes responsible for the remaining disorders, including many of the distal myopathies. In addition, we may also find skeletal muscle diseases associated with some of the presently non-implicated muscle proteins: syntropin, dystrobrevin, epsilon-sarcoglycan and sarcospan. The next steps may be to identify and understand the relationship of modifier genes producing the phenotypic heterogeneity of many of these diseases and to characterize those and other targets for therapeutic intervention, whether by gene therapy or by pharmacological treatment.

Genomic organization of the DCTN1-SLC4A5 locus encoding both NBC4 and p150(Glued)

In eukaryotes, it is rare for a single gene to encode two functionally unrelated proteins. p150(Glued) is a component of the dynactin heteromultimeric complex of proteins which is required for dynein-mediated vesicle and organelle transport by microtubules. NBC4 is an electrogenic sodium bicarbonate cotransporter, which regulates intracellular pH. Here we report that NBC4 and p150(Glued) are encoded by the same locus, DCTN1-SLC4A5. We have characterized the genomic organization of the human DCTN1-SLC4A5 locus which spans approximately 230 kilobases on chromosome 2p13 and contains 66 exons. This information should allow the study of potential genomic alterations of DCTN1-SLC4A5 in patients with diseases mapping to this genomic region.

Early onset chromosome 14-linked distal myopathy (Laing)

A dominantly inherited form of distal myopathy with onset in early childhood was first reported in a 4-generation Australian family in 1995. In the present report we provide further information on the clinical phenotype and natural history of this myopathy, and on the electromyogram and magnetic resonance imaging findings in affected individuals. The pattern of muscle involvement was similar to that in the 'tibial' forms of distal myopathy such as the Finnish (Udd) and Markesbery-Griggs types, with additional involvement of the finger extensors and of some more proximal limb and neck muscles. However, the age of onset was earlier than in these other myopathies and rimmed vacuoles were not found in biopsies from two affected individuals. Evidence of possible anticipation was found in one branch of the family. The gene locus for this myopathy had been mapped to 14q11.2-q13. The linkage region has been refined to a 24 cM region between D14S283 and D14S49 and mutations have been excluded in the PABP2 gene for oculopharyngeal muscular dystrophy which lies within this region.

Sporadic inclusion body myositis and hereditary inclusion body myopathy

Sporadic inclusion body myositis (s-IBM) is a common but under-recognized myopathy in individuals over 50 years of age. An awareness of the clinical phenotype and of the electrodiagnostic and histopathologic features should lead to improved recognition, and should minimize confusion with polymyositis, motor neuron disease, and other neuromuscular disorders. Treatment efficacy has been difficult to judge because of the insidious progression of the disease over many years, but immunomodulating therapy is generally less effective than in polymyositis and dermatomyositis, and may not be effective at all in many patients. The hereditary inclusion body myopathies (h-IBM) are a heterogeneous group of recessively and dominantly inherited vacuolar myopathies that share some histologic features with s-IBM. Oxidative stress may play a role in the pathogenesis of both s-IBM and h-IBM.

Inclusion body myositis: genetic factors, aberrant protein expression, and autoimmunity

Sporadic inclusion body myositis (s-IBM) is an inflammatory myopathy mainly affecting elderly individuals. It has a chronic progressive course leading to severe disability. Immunosuppressive treatment is in most instances ineffective. S-IBM is morphologically characterized by mononuclear cell infiltrates and vacuolated muscle fibers with pathologic accumulation of a large number of different proteins. Recent research has focused on the expression of various factors that may contribute to the inflammatory reaction and the typical inclusions. This review summarizes the new information on genetic factors, abnormal protein expression and inflammation, which provides a basis for linking the different typical morphologic features of s-IBM to a cascade of pathogenic events.

Clinical and genetic aspects of distal myopathies

Although most muscle disorders produce proximal weakness, some myopathies may manifest predominantly or exclusively distal weakness. Although several congenital, inflammatory, or metabolic myopathies may produce mainly distal weakness, there are several distinct entities, typically referred to as distal myopathies. Most of these are inherited conditions. The distal myopathies are rare, but characteristic clinical and histological features aid in their identification. Advances in molecular genetics have led to the identification of the gene lesions responsible for several of these entities and have also expanded our understanding of the genetic relationships of distal myopathies to other inherited disorders of muscle. This review summarizes current knowledge of the clinical and molecular aspects of the distal myopathies.

Distal myopathies

Distal myopathies are frequently encountered in the Nordic countries, and are now being increasingly recognized elsewhere. Three new descriptions of distal myopathy phenotypes have been published in the past year. At the same time there has been considerable progress in molecular genetics and in understanding the molecular pathophysiology underlying distal myopathies. Membrane-associated dysferlin, which was the first gene in which mutations were identified, is shown to cause a distal phenotype. The ability to make a molecular diagnosis has increased awareness of dysferlinopathy - Miyoshi myopathy. Since most entities have been linked to specific chromosomal loci, it is likely that other distal myopathies will soon be better recognized by their molecular genetic definitions.

Characterization of the murine Lbx2 promoter, identification of the human homologue, and evaluation as a candidate for Alström syndrome

The murine Lbx2 gene is a member of the ladybird family of homeobox genes, which is expressed in the developing urogenital system, eye, and brain. Using transgenic mice, we demonstrate that 9 kb of the 5' flanking region of mouse Lbx2 is able to direct expression of a reporter gene in a tissue-specific manner recapitulating the endogenous expression pattern. This regulatory region provides a novel reagent allowing for transgenic expression in the developing urogenital ridge. In addition, we describe the identification of the human homologue, LBX2. Comparison of the human LBX2 and mouse Lbx2 sequences upstream of the coding regions reveals sequence conservation suggesting conserved regulatory regions. Both the human LBX2 and the mouse Lbx2 genes have similar genomic structures and are composed of two exons separated by an intron. We mapped the mouse Lbx2 gene to 35 cM on chromosome 6 and the human LBX2 gene to a homologous region of chromosome 2p13. This is a candidate region for several inherited disorders, including Alström syndrome, a disorder that includes ocular, urogenital, and renal abnormalities. Given the expression pattern of Lbx2, the chromosomal location in humans, and the potential function of mammalian ladybird genes, we have begun to analyze patients with ocular disorders and those with Alström syndrome for mutations in LBX2. Although polymorphisms were identified, our results indicate that mutations in the coding region of LBX2 do not account for Alström syndrome in the six kindreds analyzed.

Muscle training in muscular dystrophies

There has been a debate for many years on whether muscular training is beneficial or harmful for patients with myopathic disorders and the role of exercise training in the management of these patients is still controversial. Much of this confusion is because of the lack of well-designed controlled training studies on this heterogenic group of disorders. Because effective therapies are still lacking, the patients have to rely on symptomatic treatment in which continuous physiotherapy plays an important role. There is thus still a need for studies evaluating the short- and long-term effects of muscular training in different types of myopathic disorders. We need to elucidate whether muscular training can increase strength and resistance to fatigue, but most importantly, we need to clarify whether training can improve specific functional abilities of the patient with myopathy. Future studies should give us specific information on what type of training, endurance or strength training, is to be preferred for different myopathies. The effect of strength training in one type of muscle disorder is not directly applicable to another, but is largely dependent on the underlying biological defect. From the studies published so far, high-resistance strength training at submaximal and possibly also at near-maximal levels seem beneficial, at least in the short perspective for slowly progressive myopathic disorders. However, the long-term effects of such training have not been systematically studied. In rapidly progressive myopathies, which are caused by deficient structural proteins such as in Duchenne's muscular dystrophy, the use of high-resistance training is far more controversial and questionable. If exercise regimens are to be used, they should preferably commence in the early stages of the disease, at which time there is still a substantial amount of trainable muscle fibres.

Autosomal dominant distal myopathy: further evidence of a chromosome 14 locus

In 1995 Laing et al. (Am J Hum Genet 56(1995)422) described a single family with nine members affected by an autosomal dominant infantile onset distal myopathy. This family generated a LOD score of 2.6 for a locus on chromosome 14. We describe two families with an infantile onset distal myopathy: a new family with four affected members and the family previously described by Scoppetta et al. (Acta Neurol Scand 92(1955)122) in both of which haplotype segregation was compatible with linkage to the same chromosome 14 locus, generating LOD scores of 0.9 at a penetrance of 100% for the markers D14S283 and D14S64 (theta=0) in both families. The loci for autosomal recessive hereditary inclusion body myopathy and Nonaka myopathy on chromosome 9 and for autosomal dominant distal myopathy of Markesberry-Griggs and Udd on chromosome 2q31-33 were excluded by linkage analysis. The disease followed a uniform course with selective wasting of the anterior tibial muscles, starting in infancy and recognizable by a characteristic clinical sign of the 'hanging big toe'. This was followed by slow progression, with involvement of the finger and wrist extensor muscles in the third decade and proximal limb muscles in the fourth decade. Interestingly, we also found evidence of an accompanying mild peripheral neuropathy in the oldest individual with hypomyelination of numerous large myelinated fibres. In addition, this patient's muscle biopsy also showed autophagic vacuoles and numerous intranuclear tubulo-filamentous inclusions of 15-20 nm diameter. Given that all three families with infantile onset distal myopathy are compatible with linkage to the same locus on chromosome 14, this study supports evidence for, and enlarges the clinical and neuropathological spectrum of the distal myopathy on chromosome 14.

Tissue-specific splicing of Omi stress-regulated endoprotease leads to an inactive protease with a modified PDZ motif

Omi is a human serine protease whose catalytic domain is homologous to a bacterial heat shock endoprotease (HtrA), a protein indispensable to the survival of bacteria at elevated temperatures. Omi is expressed ubiquitously, and its protein product is predominantly localized in the endoplasmic reticulum of mammalian cells. Here we present the genomic structure of Omi, consisting of eight exons located on human chromosome 2p12-p13. Furthermore, we describe an alternatively splice form of Omi (D-Omi) that is expressed predominantly in the kidney, colon, and thyroid. D-Omi lacks peptide sequence encoded by two exons (exons III and VII). The absence of exon VII leads to a protein with a modified PDZ domain unable to interact with a known partner, the Mxi2 protein. The absence of exon III affects the catalytic domain and leads to a protein with no detectable protease activity. Our studies suggest that D-Omi may have a unique role in the normal function of kidney, colon, and thyroid.