Sporadic inclusion body myositis: variability in prevalence and phenotype and influence of the MHC

Sporadic inclusion body myositis (sIBM) is the most common myopathy presenting over the age of 40 years but its prevalence varies considerably in different populations. Genetic factors play a part in the pathogenesis of sIBM and in Caucasians susceptibility has been linked to the HLA-DR3 allele and the 8.1 MHC ancestral haplotype (AH) which is also associated with other autoimmune diseases. The variable prevalence of sIBM in different populations may be related to differences in the population frequency of this haplotype. Our recent observations indicate that the clinical phenotype at presentation is also quite variable and that the influence of the MHC is more complex than previously appreciated with HLA alleles also having modifying effects on the age-at-onset, severity and rate of progression of the disease. Recent recombinant mapping studies of polymorphisms in the Class II/III regions of the MHC by our group have further refined the susceptibility region and have identified a number of candidate genes warranting further investigation. The significance of these findings for the pathogenesis of the disease is discussed.

Fast-twitch sarcomeric and glycolytic enzyme protein loss in inclusion body myositis

Inclusion body myositis (IBM) is an inflammatory disease of skeletal muscle of unknown cause. To further understand the nature of the tissue injury in this disease, we developed methods for large-scale detection and quantitation of proteins in muscle biopsy samples and analyzed proteomic data produced by these methods together with histochemical, immunohistochemical, and microarray data. Twenty muscle biopsy samples from patients with inflammatory myopathies (n = 17) or elderly subjects without neuromuscular disease (n = 3) were profiled by proteomic studies using liquid chromatographic separation of peptides followed by mass spectrometry. Thirteen of the diseased samples additionally underwent microarray studies. Seventy muscle specimens from patients with a range of neuromuscular disorders were examined by ATPase histochemical methods. Smaller numbers of samples underwent immunohistochemical and immunoblot studies. Mass spectrometric studies identified and quantified approximately 300 total distinct proteins in each muscle sample. In IBM and to a lesser extent in polymyositis, proteomic studies confirmed by histochemical, immunohistochemical, and immunoblot studies showed loss of many fast-twitch specific structural proteins and glycolytic enzymes despite relative preservation of transcript levels. Increased abundance of a nuclear membrane protein, immunoglobulins, and two calpain-3 substrates were present. The atrophy present in IBM muscle is accompanied by preferential loss of fast-twitch structural proteins and glycolytic enzymes, particularly glycogen debranching enzyme, with relative preservation of the abundance of their respective transcripts. Although muscle atrophy has long been recognized in IBM, these studies are the first to report specific proteins which are reduced in quantity in IBM muscle.

The hereditary inclusion body myopathy enigma and its future therapy

Hereditary inclusion body myopathy (HIBM) is a genetic muscle disease due to mutations in the gene encoding the enzyme complex UDP-N-acetylglucosamine 2 epimerase-N-acetylmannosamine kinase (GNE), which catalyzes the rate-limiting step in sialic acid production. The review describes some of the disease features that may be relevant for further understanding of the metabolic impairment of HIBM and its future therapy. It also addresses the biochemical basis behind the substrate supplementation therapy designed for this condition.

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase in nuclei and rimmed vacuoles of muscle fibers in DMRV (distal myopathy with rimmed vacuoles)

BACKGROUND

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is a key molecule in the pathogenesis of distal myopathy with rimmed vacuoles (DMRV) and hereditary inclusion body myopathy (HIBM) and almost all such patients have some mutations in GNE. However, subcellular localization of GNE and the mechanism of muscular damage have not been clarified.

METHODS

A rabbit polyclonal antibody for GNE was prepared. Immunohistochemistry was performed using anti-GNE and anti-nuclear protein antibodies. Western blotting with subcellular fractionated proteins was performed to determine subcellular localization of GNE. The sizes of myonuclei were quantified in muscle biopsies from patients with DMRV and amyotrophic lateral sclerosis (ALS).

RESULTS

In DMRV muscles, immunohistochemistry identified GNE in sarcoplasm and specifically in myonuclei and rimmed vacuoles (RV). Nuclear proteins were also found in RVs. Immunohistochemistry showed colocalization of GNE and emerin in C2C12 cells. Western blotting revealed the presence of GNE in nuclear fractions of human embryonic kidney (HEK) 293T cells. The mean size of myonuclei of DMRV was significantly larger than that of ALS.

CONCLUSION

GNE is present in myonuclei near nuclear membrane. Our results suggest that myonuclei are involved in RV formation in DMRV, and that mutant GNE in myonuclei seems to play some role in this process.

An Italian family with inclusion-body myopathy and frontotemporal dementia due to mutation in theVCP gene

Mutations of the valosin-containing protein gene (VCP) are responsible for autosomal-dominant hereditary inclusion-body myopathy associated with frontotemporal dementia and Paget's disease of bone. We identified the p.R155C missense mutation in the VCP gene segregating in an Italian family with three affected siblings, two of whom had a progressive myopathy associated with dementia, whereas one exhibited a progressive myopathy and preclinical signs of Paget's disease of bone. Our study demonstrates that VCP mutations are found in patients of Italian background and may lead to a variable clinical phenotype even within the same kinship.

Brain imaging and neuropsychology in late-onset dementia due to a novel mutation (R93C) of valosin-containing protein

Inclusion body myopathy with Paget disease of bone and frontotemporal dementia (IBMPFD, MIM 167320) is a recently identified autosomal dominant disorder due to mutations in the valosin-containing protein (VCP) that affects muscle, bone and brain. Brain involvement and neuropsychological findings of IBMPFD have not been described in detail. A patient carried a novel heterozygous base pair change, 47832C>T, in the VCP gene that resulted in substitution of an arginine residue by cysteine at position 93 (R93C). He presented first with myopathy while bone involvement remained subclinical. The patient developed behavioral abnormalities in his 60s and showed frank personality change with fluent empty speech at the age of 74 years. This syndrome was best classified as semantic dementia. Magnetic resonance imaging disclosed slight but progressive cerebral atrophy with prominent callosal and frontal white matter loss. Positron emission tomography demonstrated glucose hypometabolism of the frontal and temporal lobes disproportionate to their structural involvement. This first comprehensive clinical and neuroimaging study in IBMPFD may raise the awareness among clinicians as well as basic scientists for this exemplary genetic model of dementia.

Inclusion body myopathy and frontotemporal dementia caused by a novel VCP mutation

Hereditary inclusion body myopathy (IBM) with Paget's disease of the bone (PDB) and frontotemporal dementia (FTD) is a rare autosomal dominant disease caused by mutations in the valosin-containing protein (VCP) gene. We report a novel heterozygous VCP gene mutation (R159C) in a 69-year-old Italian patient presenting with slowly progressive muscle weakness of the distal upper and proximal lower limbs since the age of 50 years, 18 years later FTD supervened. No dementia or myopathies were revealed in the family history covering two generations. Degenerative changes and rimmed vacuoles together with VCP- and ubiquitin-positive cytoplasmic and nuclear aggregates were observed at the muscle biopsy. Several elements support the pathogenic role of the R159C VCP gene mutation: the occurrence at the same codon of a different, previously identified pathogenic mutation within a VCP gene mutational hot-spot, the histopathological and biochemical evidence of muscle VCP accumulation and the combined clinical presentation of IBM and FTD. These findings suggest VCP gene investigation even in apparently sporadic cases.

Inclusion body myositis: current pathogenetic concepts and diagnostic and therapeutic approaches

Inclusion body myositis is the most common acquired muscle disease in older individuals, and its prevalence varies among countries and ethnic groups. The aetiology and pathogenesis of sporadic inclusion body myositis are still poorly understood; however genetic factors, ageing, and environmental triggers might all have a role. Unlike other inflammatory myopathies, sporadic inclusion body myositis causes slowly progressing muscular weakness and atrophy, it has a distinctive pattern of muscle involvement, and is unresponsive to conventional forms of immunotherapy. This review covers the clinical presentation, diagnosis, treatment, and the latest information on genetic susceptibility and pathogenesis of sporadic inclusion body myositis.

GNE protein expression and subcellular distribution are unaltered in HIBM

Mutations in GNE encoding UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) cause hereditary inclusion body myopathy (HIBM). To define the role of GNE mutations in HIBM pathogenesis, GNE protein expression was analyzed. GNE protein is expressed at equal levels in HIBM patients and normal control subjects. Immunofluorescence detection of GNE did not reveal any mislocalization of GNE in skeletal muscle. We conclude that impaired GNE function, not lack of expression, may be the key pathogenic factor in HIBM. For diagnostic purposes, direct genetic analysis of the GNE gene in patients with IBM will remain the mainstay and is not aided by immunohistochemistry or immunoblotting using antibodies against the GNE protein.

Characterization of hereditary inclusion body myopathy myoblasts: possible primary impairment of apoptotic events

Hereditary inclusion body myopathy (HIBM) is a unique muscular disorder caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene. GNE encodes a bi-functional enzyme acting in the biosynthetic pathway of sialic acid. Since the underlying myopathological mechanism leading to the disease phenotype is poorly understood, we have established human myoblasts cultures, derived from HIBM satellite cells carrying the homozygous M712T mutation, and identified cellular and molecular characteristics of these cells. HIBM and control myoblasts showed similar heterogeneous patterns of proliferation and differentiation. Upon apoptosis induction, phosphatidylserine externalization was similar in HIBM and controls. In contrast, the active forms of caspase-3 and -9 were strongly enhanced in most HIBM cultures compared to controls, while pAkt, downregulated in controls, remained high in HIBM cells. These results could indicate impaired apoptotic signaling in HIBM cells. Since satellite cells enable partial regeneration of the post-mitotic muscle tissue, these altered processes could contribute to the muscle mass loss seen in patients. The identification of survival defects in HIBM affected muscle cells could disclose new functions for GNE in muscle cells.

Valosin-containing protein and the pathogenesis of frontotemporal dementia associated with inclusion body myopathy

Frontotemporal dementia with inclusion body myopathy and Paget's disease of bone (IBMPFD) is a rare, autosomal dominant disorder caused by mutations in the gene valosin-containing protein (VCP). The CNS pathology is characterized by a novel pattern of ubiquitin pathology distinct from sporadic and familial frontotemporal lobar degeneration with ubiquitin-positive inclusions without VCP mutations. Yet, the ubiquitin-positive inclusions in IBMPFD also stain for TAR DNA binding protein, a feature that links this rare disease with the pathology associated with the majority of sporadic FTD as well as disease resulting from different genetic alterations. VCP, a member of the AAA-ATPase gene family, associates with a plethora of protein adaptors to perform a variety of cellular processes including Golgi assembly/disassembly and regulation of the ubiquitin-proteasome system. However, the mechanism whereby mutations in VCP lead to CNS, muscle, and bone disease is largely unknown. In this report, we review current literature on IBMPFD, focusing on the pathology of the disease and the biology of VCP with respect to IBMPFD.

Genetics of inclusion-body myositis

Sporadic inclusion-body myositis (sIBM) is the most common acquired muscle disease in Caucasians over the age of 50 years. Pathologically it is marked by inflammatory, degenerative, and mitochondrial changes that interact in a yet-unknown way to cause progressive muscle degeneration and weakness. The cause of the disease is unknown, but it is thought to involve a complex interplay between environmental factors, genetic susceptibility, and aging. The strongest evidence for genetic susceptibility comes from studies of the major histocompatibility complex (MHC), where different combinations of alleles have been associated with sIBM in different ethnic groups. The rare occurrence of familial cases of inclusion-body myositis (fIBM) adds additional evidence for genetic susceptibility. Other candidate genes such as those encoding some of the proteins accumulating in muscle fibers have been investigated, with negative results. The increased understanding of related disorders, the hereditary inclusion-body myopathies (hIBM), may also provide clues to the underlying pathogenesis of sIBM, but to date there is no indication that the genes responsible for these conditions are involved in sIBM. This review summarizes current understanding of the contribution of genetic susceptibility factors to the development of sIBM.

TDP-43 in the ubiquitin pathology of frontotemporal dementia with VCP gene mutations

Frontotemporal dementia with inclusion body myopathy and Paget disease of bone is a rare, autosomal-dominant disorder caused by mutations in the gene valosin-containing protein (VCP). The CNS pathology is characterized by a novel pattern of ubiquitin pathology distinct from sporadic and familial frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U) without VCP mutations. TAR DNA binding protein 43 (TDP-43) was recently identified as a major disease protein in the ubiquitin-positive inclusions of sporadic and familial FTLD-U. To determine whether the ubiquitin pathology associated with mutations in VCP is characterized by the accumulation of TDP-43, we analyzed TDP-43 in the CNS pathology of five patients with VCP gene mutations. Accumulations of TDP-43 colocalized with ubiquitin pathology in inclusion body myopathy and Paget disease of bone, including both intranuclear inclusions and dystrophic neurites. Similar to FTLD-U, phosphorylated TDP-43 was detected only in insoluble brain extracts from affected brain regions. Identification of TDP-43, but not VCP, within ubiquitin-positive inclusions supports the hypothesis that VCP gene mutations lead to a dominant negative loss or alteration of VCP function culminating in impaired degradation of TDP-43. TDP-43 is a common pathologic substrate linking a variety of distinct patterns of FTLD-U pathology caused by different genetic alterations.

Transgenic expression of inclusion body myopathy associated mutant p97/VCP causes weakness and ubiquitinated protein inclusions in mice

Mutations in p97/VCP cause the autosomal-dominant, inherited syndrome inclusion body myopathy (IBM) associated with Paget's disease of the bone and frontotemporal dementia (IBMPFD) (Watts, G.D., Wymer, J., Kovach, M.J., Mehta, S.G., Mumm, S., Darvish, D., Pestronk, A., Whyte, M.P. and Kimonis, V.E. (2004) Inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia is caused by mutant valosin-containing protein. p97/VCP is a multi-functional protein with a role in the ubiquitin-proteasome system (UPS) (Wang, Q., Song, C. and Li, C.C. (2004) Molecular perspectives on p97-VCP: progress in understanding its structure and diverse biological functions. To understand how mutations in this protein lead to a myopathy, we generated several lines of transgenic mice expressing p97/VCP-WT (TgVCP-WT) or the most common IBMPFD mutant, p97/VCP R155H (TgVCP-RH), under a muscle-specific promoter. TgVCP-RH animals, but not controls, became progressively weaker in a dose-dependent manner starting at 6 months of age. Abnormal muscle pathology, which included coarse internal architecture, vacuolation and disorganized membrane morphology with reduced caveolin-3 expression at the sarcolemma developed coincident with the onset of weakness. These changes were not associated with alterations in sarcolemmal integrity as measured by muscle fiber uptake of Evan's blue dye. Even before animals displayed measurable weakness, there was an increase in ubiquitin-containing protein inclusions and high-molecular-weight ubiquitinated proteins, markers of UPS dysfunction. We suggest that this early and persistent increase in ubiquitinated proteins induced by IBMPFD mutations in p97/VCP may ultimately lead to animal weakness and the observed muscle pathology. TgVCP-RH animals will be a valuable tool for understanding the pathogenesis of IBM and the role of the UPS in skeletal muscle.

Distribution of glucocorticoid receptor alpha and beta subtypes in the idiopathic inflammatory myopathies

In contrast with dermatomyositis and polymyositis, inclusion body myositis is unresponsive to glucocorticoid treatment. Glucocorticoid action is mediated through an active glucocorticoid receptor-alpha and negatively regulated by another glucocorticoid receptor isoform. In several autoimmune diseases glucocorticoid receptor-beta up-regulation is involved in glucocorticoid resistance. We studied glucocorticoid receptor distribution in normal and inflammatory myopathy muscle and investigated whether differences in glucocorticoid receptor-alpha and glucocorticoid receptor-beta protein expression are involved in the differential glucocorticoid sensitivity in inclusion body myositis versus polymyositis. Multistep immunofluorescence and Western blotting on fractionated cytoplasmic or nuclear muscle samples were used. Glucocorticoid receptor-alpha was the predominant receptor subtype in muscle and occurred abundantly in myonuclei of control and diseased muscle alike. Glucocorticoid receptor-beta was constitutively expressed on a subset of endothelial cells. No differences between dermatomyositis and the other idiopathic inflammatory myopathies were observed. Increased nuclear glucocorticoid receptor that has dissociated from heat shock protein 90 was found in glucocorticoid treated subjects. Glucocorticoid receptor-alpha and -beta isoform levels were unaltered in muscle tissues from control subjects that had received glucocorticoid treatment prior to biopsy. No differences in relative glucocorticoid receptor-alpha and glucocorticoid receptor-beta protein expression were seen in inclusion body myositis versus polymyositis specimens. Our study indicates that the different glucocorticoid sensitivity in the idiopathic inflammatory myopathies is not related to up- or down-regulation of a given glucocorticoid receptor isoform at the protein level.

Pathogenic point mutations in a transmembrane domain of the epsilon subunit increase the Ca2+ permeability of the human endplate ACh receptor

The epsilon subunit of the human endplate ACh receptor (AChR) is a key determinant of the large fraction of the ACh-evoked current carried by Ca2+ ions (P(f)). Consequently, missense mutations in the epsilon subunit are potential targets for altering the P(f) of human AChR. In this paper we investigate the effects of two pathogenic point mutations in the M2 transmembrane segment AChR epsilon subunit, epsilonT264P and epsilonV259F, that cause slow-channel syndromes (SCS). When expressed in GH4C1 cells, the mutant receptors subunits raise Ca2+ permeability of the receptors approximately 1.5 and approximately 2-fold above that of wild-type, to attain P(f) values of 11.8% (epsilonT264P) and 15.4% (epsilonV259F). The latter value exceeds most P(f) values reported to date for ligand-gated ion channels. Consistent with these findings, the biionic Ca2+ permeability ratio (P(Ca)/P(Cs)) of the mutant AChRs is also increased. Upon repetitive stimulation with ACh, the mutant receptors show an enhanced current run-down compared with wild-type, leading to a strong reduction of their function. We propose that the enhanced Ca2+ permeability of the mutant receptors overrides the protective effect of desensitization and, together with the prolonged opening events of the AChR channel, is an important determinant of the excitotoxic endplate damage in the SCS.

Spätmanifestation einer Polyglykosankörpermyopathie

We report two patients with polyglycosan body disease manifesting in adulthood. Clinical, electrophysiological, and histopathological characteristics of their disorders are summarized, and diagnostic classification is discussed.

Familial inclusion body myositis in a mother and son with different ancestral MHC haplotypes

An Ashkenazi Jewish family in which the mother and a son both have inclusion body myositis (IBM) is reported. The condition developed at an earlier age and was more rapidly progressive and less responsive to treatment in the son than in the mother or other IBM patients in our clinic. Genetic analysis showed that the mother carried alleles of the 8.1 MHC ancestral haplotype (AH; HLA-B8, DRB1*0301), which is found in 85% of IBM patients in Western Australia. The son did not inherit this haplotype, but carried alleles characteristic of the 52.1AH (HLA-B5, DRB1*1502) of paternal origin. The findings indicate that in this family either the 8.1AH or 52.1AH may carry susceptibility for IBM and that the 52.1AH is associated with a more severe and treatment-resistant form of the disease.

Pathological consequences of VCP mutations on human striated muscle

Mutations in the valosin-containing protein (VCP, p97) gene on chromosome 9p13-p12 cause a late-onset form of autosomal dominant inclusion body myopathy associated with Paget disease of the bone and frontotemporal dementia (IBMPFD). We report on the pathological consequences of three heterozygous VCP (R93C, R155H, R155C) mutations on human striated muscle. IBMPFD skeletal muscle pathology is characterized by degenerative changes and filamentous VCP- and ubiquitin-positive cytoplasmic and nuclear protein aggregates. Furthermore, this is the first report demonstrating that mutant VCP leads to a novel form of dilatative cardiomyopathy with inclusion bodies. In contrast to post-mitotic striated muscle cells and neurons of IBMPFD patients, evidence of protein aggregate pathology was not detected in primary IBMPFD myoblasts or in transient and stable transfected cells using wild-type-VCP and R93C-, R155H-, R155C-VCP mutants. Glutathione S-transferase pull-down experiments showed that all three VCP mutations do not affect the binding to Ufd1, Npl4 and ataxin-3. Structural analysis demonstrated that R93 and R155 are both surface-accessible residues located in the centre of cavities that may enable ligand-binding. Mutations at R93 and R155 are predicted to induce changes in the tertiary structure of the VCP protein. The search for putative ligands to the R93 and R155 cavities resulted in the identification of cyclic sugar compounds with high binding scores. The latter findings provide a novel link to VCP carbohydrate interactions in the complex pathology of IBMPFD.

Novel ubiquitin neuropathology in frontotemporal dementia with valosin-containing protein gene mutations

Frontotemporal dementia (FTD) with inclusion body myopathy and Paget disease of bone (IBMPFD) is a rare, autosomal-dominant disorder caused by mutations in the valosin-containing protein (VCP) gene, a member of the AAA-ATPase gene superfamily. The neuropathology associated with sporadic FTD is heterogeneous and includes tauopathies and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). However, there is limited information on the neuropathology in IBMPFD. We performed a detailed, systematic analysis of the neuropathologic changes in 8 patients with VCP mutations. A novel pattern of ubiquitin pathology was identified in IBMPFD that was distinct from sporadic and familial FTLD-U without VCP gene mutations. This was characterized by ubiquitin-positive neuronal intranuclear inclusions and dystrophic neurites. In contrast to FTLD-U, only rare intracytoplasmic inclusions were identified. The ubiquitin pathology was abundant in the neocortex, less robust in limbic and subcortical nuclei, and absent in the dentate gyrus. Only rare inclusions were detected with antibodies to VCP and there was no biochemical alteration in the VCP protein. VCP is associated with a variety of cellular activities, including regulation of the ubiquitin-proteasome system. Our findings are consistent with the hypothesis that the pathology associated with VCP gene mutations is the result of impairment of ubiquitin-based degradation pathways.

Rimmed vacuoles with beta-amyloid and tau protein deposits in the muscle of children with hereditary myopathy

We investigated whether beta-amyloid and tau protein are involved in the formation of inclusion body myositis (IBM)-like inclusions found in children with rimmed vacuoles and congenitally affected muscles. We immunostained muscle biopsy specimens from four children and one 18-year-old boy with congenital myopathy containing rimmed vacuoles and IBM-like inclusions with antibodies against beta-amyloid, tau protein and ubiquitin. Focal accumulations of both beta-amyloid and phosphorylated tau coexisted with tubulofilamentous structures in all cases. Our studies demonstrate for the first time that the full morphological phenotype of IBM including beta-amyloid and tau protein deposits may also develop in children, and that congenital, probably genetic, muscle defects may lead to abnormal protein aggregation in IBM-like inclusions.

Valosin-containing protein gene mutations: clinical and neuropathologic features

BACKGROUND

Hereditary inclusion body myopathy (IBMPFD) with Paget disease of bone (PDB) and frontotemporal dementia (FTD) is a rare multisystem disorder with autosomal dominant inheritance. Recently, missense mutations in the gene encoding valosin-containing protein (VCP) have been found in individuals with IBMPFD. VCP/P97, which exerts a variety of cellular functions, plays a key role in the ubiquitin-proteasome dependent degradation of cytosolic proteins and in the retrotranslocation of misfolded proteins from the endoplasmic reticulum into the cytoplasm.

METHODS

The authors describe the clinical features of two kindreds in which VCP R93C and R155C missense mutations segregate and perform a histopathologic examination of brain, muscle, bone, and liver of three subjects harboring the R155C mutation.

RESULTS

Frontotemporal dementia was present in 100% of affected subjects in Family F1 and 70% in Family F2, as compared with an average of 30% in previously described IBMPFD families. In contrast, PDB was a more inconstant clinical feature. Biochemical and histopathologic data are consistent with the hypothesis that VCP R155C mutation disrupts normal VCP function, leading to diffuse accumulation of ubiquitinated proteins within the cells.

CONCLUSIONS

VCP mutations are present in two families in which FTD is the most prominent symptom. The histopathologic study performed in patients harboring the R155C mutation supports the hypothesis that this mutation disrupts normal VCP function, leading to diffuse accumulation of ubiquitinated proteins within the cells. IBMPFD belongs to a class of genetic diseases associated with an alteration of the ubiquitin-proteasome system.

Different early pathogenesis in myotilinopathy compared to primary desminopathy

Mutations in the human myotilin gene may cause limb-girdle muscular dystrophy 1A and myofibrillar myopathy. Here, we describe a German patient with the clinically distinct disease phenotype of late adult onset distal anterior leg myopathy caused by a heterozygous S55F myotilin mutation. In addition to a thorough morphological and clinical analysis, we performed for the first time a protein chemical analysis and transient transfections. Morphological analysis revealed an inclusion body myopathy with myotilin- and desmin-positive aggregates. The clinical and pathological phenotype considerably overlaps with late onset distal anterior leg myopathy of the Markesbery-Griggs type. Interestingly, all three analyzed myotilin missense mutations (S55F, S60F and S60C) do not lead to gross changes in the total amount of myotilin or to aberrant posttranslational modifications in diseased muscle, as observed in a number of muscular dystrophies. Transiently transfected wild-type and S55F mutant myotilin similarly colocalised with actin-containing stress fibers in BHK-21 cells. Like the wild-type protein, mutated myotilin did not disrupt the endogenous desmin cytoskeleton or lead to pathological protein aggregation in these cells. This lack of an obvious dominant negative effect sharply contrasts to transfections with, for instance, the disease-causing A357P desmin mutant. In conclusion our data indicate that the disorganization of the extrasarcomeric cytoskeleton and the presence of desmin-positive aggregates are in fact late secondary events in the pathogenesis of primary myotilinopathies, rather than directly related. These findings suggest that unrelated molecular pathways may result in seemingly similar disease phenotypes at late disease stages.

[Mechanisms leading to muscle degeneration: molecular mechanisms and therapeutical forecasts]

In inclusion body myositis (IBM), there is muscular amyloidogenesis and inflammation. A related disorder is due to alterations in the ubiquitin pathway involving the valsolin-containing protein leading to IBM, dementia and Paget's disease. Alteration in the dystrophin glycoprotein complex leads to several muscular dystrophies (MD), and the pathogenesis of dystrophin related MD as well as certain limb girdle MD are discussed. Therapeutic strategies involving inhibition of proteolytic cascades as well as inhibition of a negative regulator of muscle growth (myostatin) are briefly introduced. Finally, molecular aspects of the most common form of adult myopathy, myotonic MD, are discussed. This disease is caused by an aberrant splicing mechanism and interference thereof may be useful in designing therapeutic strategies.

Sporadic inclusion body myositis in Japanese is associated with the MHC ancestral haplotype 52.1

In Caucasians, sporadic inclusion body myositis has been associated with the MHC ancestral haplotypes; HLA-A1, B8, DR3 (8.1AH) and HLA-B35, DR1 (35.2AH). It is not known whether these haplotypes carry susceptibility for the disease in other ethnic groups. We report here the results of HLA-B and -DRB1 typing using a high-resolution sequence-based technique in a cohort of 31 Japanese patients with definite sIBM. Patient allele frequencies were 40.3% for HLA-B*5201 (10.7% in controls: p<0.001) and 37.1% for HLA-DRB1*1502 (10% in controls: p<0.001). Both alleles were found together as part of a conserved haplotype (52.1AH) at a frequency of 37.1% in patients (8.4% in controls: p<0.001). This is the first description of a haplotypic MHC association with sporadic inclusion body myositis in Japanese patients. These findings indicate that different MHC ancestral haplotypes are associated with sIBM in different ethnic groups and further emphasize the importance of genetic factors in this condition.

Mechanisms of Disease: genetics of Paget's disease of bone and related disorders

Paget's disease of bone (PDB) is a common disorder in which focal abnormalities of increased bone turnover lead to complications such as bone pain, deformity, pathological fractures, and deafness. PDB has a strong genetic component and several susceptibility loci for the disease have been identified by genome-wide scans. Mutations that predispose individuals to PDB and related disorders have been identified in four genes. The rare PDB-like syndromes of familial expansile osteolysis, early-onset familial PDB, and expansile skeletal hyperphosphatasia are caused by insertion mutations in TNFRSF11A, which encodes receptor activator of nuclear factor (NF)kappaB (RANK)-a critical regulator of osteoclast function. Inactivating mutations in TNFRSF11B, which encodes osteoprotegerin (a decoy receptor for RANK ligand) cause idiopathic hyperphosphatasia, and polymorphisms in this gene seem to increase the risk for classical PDB. Mutations of the sequestosome 1 gene (SQSTM1), which encodes an important scaffold protein in the NFkappaB pathway, are a common cause of classical PDB. The rare syndrome of hereditary inclusion body myopathy, PDB, and fronto-temporal dementia is caused by mutations in the valosin-containing protein (VCP) gene. This gene encodes VCP, which has a role in targeting the inhibitor of NFkappaB for degradation by the proteasome. Several additional genes for PDB remain to be discovered, and it seems likely that they will also involve the RANK-NFkappaB signaling pathway or components of the proteasomal processing of this pathway, underscoring the critical importance of this signaling pathway in bone metabolism and bone disease.

NCAM is hyposialylated in hereditary inclusion body myopathy due to GNE mutations

The authors found that the neural cell adhesion molecule (NCAM) is hyposialylated in hereditary inclusion body myopathy (HIBM) muscle, as suggested by its decreased molecular weight by Western blot. This abnormality represented the only pathologic feature differentiating HIBM due to GNE mutations from other myopathies with similar clinical and pathologic characteristics. If further confirmed in larger series of patients, this may be a useful diagnostic marker of GNE-related HIBM.

Autosomal dominant inclusion body myopathy, Paget disease of bone, and frontotemporal dementia

Autosomal dominant proximal limb girdle or inclusion body myopathy, associated with Paget disease of bone and frontotemporal dementia (IBMPFD) is a recently described disorder that maps to chromosome 9p21.1-p12. We refined the critical locus and identified the gene as the Valosin Containing Protein (VCP) gene, a member of the AAA-ATPase superfamily using a candidate gene approach. Six missense mutations were found to co-segregate with affected individuals only, two of these representing mutation hot spots. We report the clinical and molecular findings in 99 individuals in 13 families. VCP is associated with a variety of cellular activities, including the control of cell cycle, membrane fusion, and the ubiquitin-proteasome degradation pathway. Previous studies have associated VCP mutants in cell lines with vacuole formation and aggregate formation. Identification of VCP as the gene causing IBMPFD has important implications for understanding the pathogenesis of neurodegenerative disorders.

A perspective on sporadic inclusion-body myositis: the role of aging and inflammatory processes

Sporadic inclusion-body myositis (sIBM) is an age-related condition manifested after midlife. This review points out salient features of sIBM that are shared with normal aging in muscle and with inflammatory changes in vascular atheromas and senile plaques of Alzheimer disease (AD). The amyloid precursor protein (APP) and derived Abeta peptides are found in both AD and sIBM. Because transgenic expression of human APP induces sIBM like changes, it is of potential interest that an inducer of APP, IL-1, increases during aging in mouse muscle. Because various subsets of the usual aging changes in aging brain, muscle, and vessels can be attenuated in rodents by caloric intake and possibly in humans by drugs with anti-inflammatory and anticoagulant activities, this study suggests that diet and inflammation may be useful experimental variations in exploring the pathogenesis of sIBM.

Clinical and muscle magnetic resonance imaging study of an Italian family with autosomal dominant inclusion body myopathy not linked to known genetic loci

The objective was to report a clinical, pathological and muscle magnetic resonance (MR) study of an Italian family with an autosomal dominant inclusion body myopathy (AD-IBM). Eight subjects (age range 20-56 years; 5 females and 3 males) belonging to four generations were studied. Onset of disturbances (distal weakness at lower limbs) ranged from 20 to 28 years. CK levels were increased to five times. Only in an early stage oedema of involved muscles has been demonstrated by muscle MR. Quadriceps femoris was characteristically spared; in the last phases a mild involvement of the vasti became evident with persistent sparing of the rectus femori. Rimmed vacuoles and hyperphosphorylated tau filaments were evident at muscle biopsy. Linkage analysis excluded the association of the disease to chromosome loci 14q11, 17p13.1, 2p13, 19p13. The study suggests that quadriceps sparing is a characteristic feature also of AD-IBM. This finding could represent a muscle-image hallmark helpful in diagnosis of autosomal dominant muscular disorders.

Messenger RNA degradation may be inhibited in sporadic inclusion body myositis

OBJECTIVE

To integrate an immune-mediated mechanism and the disturbed protein expression in sporadic inclusion body myositis (IBM).

BACKGROUND

In IBM, abnormal fibers harbor inclusions of some proteins found in the brains of patients with Alzheimer disease (AD). Poly(A)-binding protein 1 (PABP1) is the RNA binding protein that attaches to the poly(A) tail of mRNA and is involved in translation and mRNA degradation. Under stresses, mRNA combined with PABP1 forms cytoplasmic granules called stress granules.

METHODS

Using 12 muscle biopsies with sporadic IBM and 46 controls, the authors localized PABP1 by immunohistochemistry, and poly(A)-containing RNA (poly(A)+ RNA) using the in situ hybridization method. They also immuno-localized HuR, one of the components of stress granules.

RESULTS

In IBM, a proportion of fibers, including those vacuolated, showed an abnormal accumulation of PABP1 immuno-positive deposits. An immunofluorescence study indicated that large PABP1 positive deposits formed conglomerates with poly(A)+ RNA and PABP1 colocalized with HuR. Although PABP1-positive cytoplasmic inclusions were found in disease controls, their aggregates combined with poly(A)+ RNA were only detected in IBM.

CONCLUSIONS

The localization of PABP1 positive deposits in inclusion body myositis (IBM) and other diseases may correspond to the stress granules that are formed under exposure to cellular stresses and the sites of mRNA turnover. The concomitant aggregation of poly(A)+ RNA that is specifically found in IBM may be due to the inhibition of mRNA degradation, which may affect translation. The authors speculate that an autoantibody against mRNA degradation machinery could play a role in this inhibition.

Inclusion body myopathy-associated mutations in p97/VCP impair endoplasmic reticulum-associated degradation

Mutations in the AAA+ protein (ATPase associated with a variety of cellular activities) p97/VCP (valosin-containing protein) cause a dominantly inherited syndrome of inclusion body myopathy with Paget's disease of the bone and fronto-temporal dementia (IBMPFD). p97/VCP is a ubiquitously expressed protein that participates in a number of cellular processes including endoplasmic reticulum-associated degradation (ERAD). p97/VCP aids in the extraction of ubiquitinated proteins from the endoplasmic reticulum (ER) and facilitates their delivery to the proteasome. This study focuses on the effects of disease-associated p97/VCP mutations on this pathway. We show that p97/VCP containing the most prevalent IBMPFD-associated mutation, R155H, has normal ATPase activity and hexameric structure. However, when expressed in cultured cells, both this and a second IBMPFD-associated p97/VCP mutant increase the overall level of ubiquitin-conjugated proteins and specifically impair degradation of mutant DeltaF508-CFTR handled by the ERAD pathway. These effects are similar to those previously described for an ATPase deficient p97/VCP mutant and suggest that IBMPFD mutations impair p97/VCP cellular function. In a subset of cells, IBMPFD mutations also promote formation of aggregates that contain p97/VCP, ubiquitin conjugates and ER-resident proteins. Undegraded mutant DeltaF508-CFTR also accumulates in these aggregates. We conclude that IBMPFD mutations in p97/VCP disrupt ERAD and that this may contribute to the pathogenesis of IBMPFD.

Inclusion body myopathy and Paget disease is linked to a novel mutation in the VCP gene

Mutations in the valosin-containing protein (VCP) on chromosome 9p13-p12 were recently found to be associated with hereditary inclusion body myopathy, Paget disease of the bone, and frontotemporal dementia (IBMPFD). We identified a novel missense mutation in the VCP gene (R159H; 688G>A) segregating with this disease in an Austrian family of four affected siblings, who exhibited progressive proximal myopathy and Paget disease of the bone but without clinical signs of dementia.

IBM-type inclusions in a patient with slow-channel syndrome caused by a mutation in the AChR epsilon subunit

We report a patient with a slow-channel congenital myasthenic syndrome who carries a novel slow-channel mutation in the epsilon subunit of the acetylcholine receptor and has tubulofilamentous inclusion bodies, in skeletal muscle of the type observed in hereditary and sporadic inclusion body myositis. Ultrastructural analysis of a muscle specimen obtained at the age of 9 years showed an endplate myopathy typical of the slow-channel syndrome. Twenty years later, a second muscle specimen again showed the endplate myopathy as well numerous nuclear and cytoplasmic tubulofilamentous inclusion bodies. Molecular genetic studies revealed a novel valine to phenylalanine mutation (epsilonV259F) in the M2 domain of the acetylcholine receptor. Coexistence of the slow-channel syndrome with a feature of IBM has not been observed before.

Discovering statistically significant pathways in expression profiling studies

Accurate and rapid identification of perturbed pathways through the analysis of genome-wide expression profiles facilitates the generation of biological hypotheses. We propose a statistical framework for determining whether a specified group of genes for a pathway has a coordinated association with a phenotype of interest. Several issues on proper hypothesis-testing procedures are clarified. In particular, it is shown that the differences in the correlation structure of each set of genes can lead to a biased comparison among gene sets unless a normalization procedure is applied. We propose statistical tests for two important but different aspects of association for each group of genes. This approach has more statistical power than currently available methods and can result in the discovery of statistically significant pathways that are not detected by other methods. This method is applied to data sets involving diabetes, inflammatory myopathies, and Alzheimer's disease, using gene sets we compiled from various public databases. In the case of inflammatory myopathies, we have correctly identified the known cytotoxic T lymphocyte-mediated autoimmunity in inclusion body myositis. Furthermore, we predicted the presence of dendritic cells in inclusion body myositis and of an IFN-alpha/beta response in dermatomyositis, neither of which was previously described. These predictions have been subsequently corroborated by immunohistochemistry.

Single nucleotide polymorphisms in the dystroglycan gene do not correlate with disease severity in hereditary inclusion body myopathy

Aberrant glycosylation of dystroglycan occurs in certain muscular dystrophies, including hereditary inclusion body myopathy (HIBM). HIBM harbors a widely varying clinical severity and age of onset, which raised the suspicion of the presence of disease modifier genes. We considered the highly polymorphic dystroglycan gene (DAG1) as a feasible candidate modifier gene. DAG1 genomic DNA was sequenced for 32 HIBM patients, mainly of Persian-Jewish descent. Five novel DAG1 single nucleotide polymorphisms (SNPs) were identified, bringing the total number of SNPs to 19. However, no direct correlation between DAG1 SNPs and clinical severity of HIBM could be detected. Several identified SNPs substitute an amino acid and might modulate dystroglycan function or glycosylation status, and deserve further research. These data are valuable for future studies on the role of DAG1 in HIBM and other muscular dystrophies, especially those dystrophies that involve abnormal glycosylation of dystroglycan.

Associations with autoimmune disorders and HLA class I and II antigens in inclusion body myositis

Whether autoimmune mechanisms play a role in the pathogenesis of inclusion body myositis (IBM) is unknown. Human leukocyte antigen (HLA) analysis in 52 patients, including 17 with autoimmune disorders (AIDs), showed that patients were more likely to have antigens from the autoimmune-prone HLA-B8-DR3 ancestral haplotype than healthy control subjects, irrespective of the presence of AIDs. Patients lacked the apparently protective HLA-DR53 antigen. The results provide further support for an autoimmune basis in IBM.

Use of a cell-free system to determine UDP-N-acetylglucosamine 2-epimerase and N-acetylmannosamine kinase activities in human hereditary inclusion body myopathy

Hereditary inclusion body myopathy (HIBM) is an autosomal recessive neuromuscular disorder associated with mutations in uridine diphosphate (UDP)-N-acetylglucosamine (GlcNAc) 2-epimerase (GNE)/N-acetylmannosamine (ManNAc) kinase (MNK), the bifunctional and rate-limiting enzyme of sialic acid biosynthesis. We developed individual GNE and MNK enzymatic assays and determined reduced activities in cultured fibroblasts of patients, with HIBM harboring missense mutations in either or both the GNE and MNK enzymatic domains. To assess the effects of individual mutations on enzyme activity, normal and mutated GNE/MNK enzymatic domains were synthesized in a cell-free in vitro transcription-translation system and subjected to the GNE and MNK enzymatic assays. This cell-free system was validated for both GNE and MNK activities, and it revealed that mutations in one enzymatic domain (in GNE, G135V, V216A, and R246W; in MNK, A631V, M712T) affected not only that domain's enzyme activity, but also the activity of the other domain. Moreover, studies of the residual enzyme activity associated with specific mutations revealed a discrepancy between the fibroblasts and the cell-free systems. Fibroblasts exhibited higher residual activities of both GNE and MNK than the cell-free system. These findings add complexity to the tightly regulated system of sialic acid biosynthesis. This cell-free approach can be applied to other glycosylation pathway enzymes that are difficult to evaluate in whole cells because their substrate specificities overlap with those of ancillary enzymes.

Allelic heterogeneity of GNE gene mutation in two Tunisian families with autosomal recessive inclusion body myopathy

Autosomal recessive hereditary inclusion body myopathy (AR-HIBM), with sparing of the quadriceps, is characterized by adult-onset, with weakness and atrophy of distal lower limb muscles, and typical histopathological findings in muscle biopsy. AR hIBM is associated with mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene on chromosome 9p12-13 . We report two unrelated Tunisian families with clinical and pathological features of AR HIBM. One distinct homozygous GNE missense mutation, M712T, previously reported in Middle Eastern Jewish patients, and a newly identified one, L379H, were found in one patient from each family. We conclude that AR HIBM in Tunisia shows an allelic genetic heterogeneity.

Pathogenic accumulation of APP in fast twitch muscle of IBM patients and a transgenic model

Inclusion body myositis (IBM) is the most common age-related degenerative skeletal muscle disorder. The aberrant intracellular accumulation of the beta-amyloid (Abeta) peptide within skeletal muscle is a pathological hallmark of IBM. Skeletal muscle is comprised of both slow and fast twitch fibers, which are present in different proportions in various muscles. It remains unclear if fast and/or slow twitch fibers are differentially involved in IBM pathogenesis. To better understand the molecular pathogenesis of IBM, we analyzed human IBM muscle biopsies and muscle from a transgenic mouse model of IBM (MCK-betaAPP). Here we report that the majority of histopathologically-affected fibers in human IBM biopsies were type II fast fibers. Skeletal muscle from MCK-betaAPP mice exhibited higher transgene expression and steady-state levels of human betaAPP in fast type IIB fibers compared to slow type I fibers. These findings indicate that fast twitch fibers may selectively accumulate and be more vulnerable to betaAPP- and Abeta-mediated damage in IBM. These findings also highlight parallels between the MCK-betaAPP mice and the human IBM condition.

Mutant ubiquitin UBB+1 is accumulated in sporadic inclusion-body myositis muscle fibers

Mutant ubiquitin (UBB+1), a product of "molecular misreading," is toxic to cells because its ubiquitinated form inhibits the proteasome, contributing to accumulation of misfolded proteins and their ensuing toxicity. The authors demonstrate in 10 sporadic inclusion body myositis (s-IBM) muscle biopsies that UBB+1 is accumulated in aggregates containing amyloid-beta and phosphorylated-tau. In s-IBM, UBB+1 may be pathogenic by inhibiting proteasome, thereby promoting accumulation of cytotoxic misfolded amyloid-beta and phosphorylated-tau.

Gene expression profile in the muscles of patients with inflammatory myopathies: effect of therapy with IVIg and biological validation of clinically relevant genes

To explore the biological significance of gene expression in the pathogenesis of inflammatory myopathies, we performed microarray experiments followed by real-time PCR and immunohistochemistry on muscle biopsies obtained before and after therapy from patients with dermatomyositis (DM) who improved and patients with inclusion body myositis (sIBM) who did not improve after controlled trials with three monthly intravenous immunoglobulin (IVIg) infusions. The pretreatment biopsies showed high expression of immunoglobulin, adhesion molecules, chemokines and cytokine genes in both sIBM and DM (sIBM > DM). In the repeated biopsies of DM patients who clinically improved, 2206 genes were downregulated more than 1.5-fold; in contrast, 1700 of the same genes remained unchanged in sIBM patients who did not improve. Genes markedly downregulated in DM, but not sIBM, were interleukin 22, Kallmann syndrome 1 (KAL-1), an adhesion molecule shown for the first time in muscle, ICAM-1, complement C1q, and several structural protein genes. Because mRNA for KAL-1 was selectively upregulated in vitro by transforming growth factor (TGF) beta1, a fibrogenic cytokine immunolocalized in the endomysial connective tissue of pretreatment DM muscles, the downregulation of both TGF-beta and KAL-1 after IVIg only in DM suggests that these molecules have a functional role in connective tissue proliferation and fibrosis. The improved muscles of DM, but not sIBM, showed upregulation of chemokines CXCL9 (Mig) and CXCL11, and several immunoglobulin-related genes, suggesting an effect on muscle remodelling and regeneration. The results suggest that IVIg modulates several immunoregulatory or structural muscle genes, but only a subset of them associated with inflammatory mediators, fibrosis and muscle remodelling are connected with the clinical response. Gene arrays, when combined with clinical assessments, may provide important information in the pathogenesis of inflammatory myopathies.

Familial inflammatory inclusion body myositis

OBJECTIVE

To compare familial inflammatory inclusion body myositis (IBM) with hereditary inclusion body myopathies and sporadic IBM.

PATIENTS AND METHODS

Clinical, biological, MRI, and histological data were analysed in two siblings with inflammatory IBM and compared with those of patients with sporadic and hereditary IBM.

RESULTS

Both patients had a clinical phenotype of sporadic IBM, which differs from hereditary myopathies because of late age of onset--respectively 65 and 66 years, and different pattern of muscular involvement--asymmetric, mainly distal but also involving quadriceps. MRI showed selective fatty infiltration and oedema in the extensor compartment of thigh muscles. The diagnosis of IBM was confirmed by muscle biopsy, showing muscle fibres containing numerous rimmed vacuoles, a characteristic shared by all types of IBM. In contrast with hereditary IBM, histological analysis also showed inflammatory mononuclear infiltrate invading non-necrotic fibres, ragged red and oxidase c negative fibres, and positive Congo red staining. Moreover, HLA class II typing disclosed DR beta 1 0301 haplotype, which is significantly related to sporadic but not to hereditary IBM. With steroid treatment and monthly intravenous immunoglobulins, the disease was stabilised in both patients at protracted follow up.

CONCLUSION

Sporadic and familial inflammatory IBM share the same clinical, biological, MRI, and histological features.

Two major histocompatibility complex haplotypes influence susceptibility to sporadic inclusion body myositis: critical evaluation of an association with HLA-DR3

Previous studies of sporadic inclusion body myositis (sIBM) have shown a strong association with HLA-DR3 and other components of the 8.1 ancestral haplotype (AH) (HLA-A1, B8, DR3), where the susceptibility locus has been mapped to the central major histocompatibility complex (MHC) region between HLA-DR and C4. Here, the association with HLA-DR3 and other genes in the central MHC and class II region was further investigated in a group of 42 sIBM patients and in an ethnically similar control group (n = 214), using single-nucleotide polymorphisms and microsatellite screening. HLA-DR3 (marking DRB1*0301 in Caucasians) was associated with sIBM (Fisher's test). However, among HLA-DR3-positive patients and controls, carriage of HLA-DR3 without microsatellite and single-nucleotide polymorphism alleles of the 8.1AH (HLA-A1, B8, DRB3*0101, DRB1*0301, DQB1*0201) was marginally less common in patients. Patients showed no increase in carriage of the 18.2AH (HLA-A30, B18, DRB3*0202, DRB1*0301, DQB1*0201) or HLA-DR3 without the central MHC of the 8.1AH, further arguing against HLA-DRB1 as the direct cause of susceptibility. Genes between HLA-DRB1 and HOX12 require further investigation. BTL-II lies in this region and is expressed in muscle. Carriage of allele 2 (exon 6) was more common in patients. BTL-II(E6)*2 is characteristic of the 35.2AH (HLA-A3, B35, DRB1*01) in Caucasians and HLA-DR1, BTL-II(E6)*2, HOX12*2, RAGE*2 was carried by several patients. The 8.1AH and 35.2AH may confer susceptibility to sIBM independently or share a critical allele.

No overall hyposialylation in hereditary inclusion body myopathy myoblasts carrying the homozygous M712T GNE mutation

Hereditary inclusion body myopathy (HIBM) is a unique group of neuromuscular disorders characterized by adult-onset, slowly progressive distal and proximal muscle weakness, which is caused by mutations in UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme in the biosynthetic pathway of sialic acid. In order to investigate the consequences of the mutated GNE enzyme in muscle cells, we have established cell cultures from muscle biopsies carrying either kinase or epimerase mutations. While all myoblasts carrying a mutated GNE gene show a reduction in their epimerase activity, only the cells derived from the patient carrying a homozygous epimerase mutation present also a significant reduction in the overall membrane bound sialic acid. These results indicate that although mutations in each of the two GNE domains result in an impaired enzymatic activity and the same HIBM phenotype, they do not equally affect the overall sialylation of muscle cells. This lack of correlation suggests that the pathological mechanism of the disease may not be linked solely to the well-characterized sialic acid pathway.

α-Dystroglycan does not play a major pathogenic role in autosomal recessive hereditary inclusion-body myopathy

Mutations of the GNE gene are responsible for autosomal recessive hereditary inclusion-body myopathy (HIBM). In this study we searched for the presence of any significant abnormality of alpha-dystroglycan (alpha-DG), a highly glycosylated component of the dystrophin-glycoprotein complex, in 5 HIBM patients which were previously clinically and genetically characterized. Immunocytochemical and immunoblot analysis showed that alpha-DG extracted from muscle biopsies was normally expressed and displayed its typical molecular mass. Immunoblot analysis on the wheat germ lectin-enriched glycoprotein fraction of muscles and primary myotubes showed a reduced amount of alpha-DG in 4 out of 5 HIBM patients, compared to normal and other diseased muscles. However, such altered lectin-binding behaviour, possibly reflecting a partial hyposialylation of alpha-DG, did not affect the laminin binding properties of alpha-DG. Therefore, the subtle changes within the alpha-DG glycosylation pattern, detected in HIBM muscles, likely do not play a key pathogenic role in this disorder.