Physical and transcriptional map of the hereditary inclusion body myopathy locus on chromosome 9p12-p13

Substantial deficiency of free sialic acid in muscles of patients with GNE myopathy and in a mouse model

GNE myopathy (GNEM), also known as hereditary inclusion body myopathy (HIBM), is a late- onset, progressive myopathy caused by mutations in the GNE gene encoding the enzyme responsible for the first regulated step in the biosynthesis of sialic acid (SA). The disease is characterized by distal muscle weakness in both the lower and upper extremities, with the quadriceps muscle relatively spared until the late stages of disease. To explore the role of SA synthesis in the disease, we conducted a comprehensive and systematic analysis of both free and total SA levels in a large cohort of GNEM patients and a mouse model. A sensitive LC/MS/MS assay was developed to quantify SA in serum and muscle homogenates. Mean serum free SA level was 0.166 μg/mL in patients and 18% lower (p<0.001) than that of age-matched control samples (0.203 μg/mL). In biopsies obtained from patients, mean free SA levels of different muscles ranged from 0.046–0.075 μg/μmol Cr and were markedly lower by 72–85% (p<0.001) than free SA from normal controls. Free SA was shown to constitute a small fraction (3–7%) of the total SA pool in muscle tissue. Differences in mean total SA levels in muscle from patients compared with normal controls were less distinct and more variable between different muscles, suggesting a small subset of sialylation targets could be responsible for the pathogenesis of GNEM. Normal quadriceps had significantly lower levels of free SA (reduced by 39%) and total SA (reduced by 53%) compared to normal gastrocnemius. A lower SA requirement for quadriceps may be linked to the reported quadriceps sparing in GNEM. Analysis of SA levels in Gne^M743T/M743T mutant mice corroborated the human study results. These results show that serum and muscle free SA is severely reduced in GNEM, which is consistent with the biochemical defect in SA synthesis associated with GNE mutations. These results therefore support the approach of reversing SA depletion as a potential treatment for GNEM patients.

A novel autosomal dominant inclusion body myopathy linked to 7q22.1-31.1

We describe a novel autosomal dominant hereditary inclusion body myopathy (HIBM) that clinically mimics limb girdle muscular dystrophy in a Chinese family. We performed a detailed clinical assessment of 36 individuals spanning four generations. The age of onset ranged from the 30s to the 50s. Hip girdle, neck flexion and axial muscle weakness were involved at an early stage. This disease progressed slowly, and a shoulder girdle weakness appeared later in the disease course. Muscle biopsies showed necrotic, regenerating, and rimmed vacuolated fibers as well as congophilic inclusions in some of the fibers. Electron micrograph revealed cytoplasmic inclusions of 15–21 nm filaments. A genomewide scan and haplotype analyses were performed using an Illumina Linkage-12 DNA Analysis Kit (average spacing 0.58 cM), which traced the disease to a new locus on chromosome 7q22.1–31.1 with a maximum multi-point LOD score of 3.65. The critical locus for this unique disorder, which is currently referred to as hereditary inclusion body myopathy 4 (HIBM4), spans 8.78 Mb and contains 65 genes. This localization raises the possibility that one of the genes clustered within this region may be involved in this disorder.

Hereditary inclusion body myopathy: a decade of progress

Hereditary Inclusion Body Myopathy (HIBM) is an autosomal recessive, quadriceps sparing type commonly referred to as HIBM but also termed h-IBM or Inclusion Body Myopathy 2 (IBM2). The clinical manifestations begin with muscle weakness progressing over the next 10-20 years uniquely sparing the quadriceps until the most advanced stage of the disease. Histopathology of an HIBM muscle biopsy shows rimmed vacuoles on Gomori's trichrome stain, small fibers in groups and tubulofilaments without evidence of inflammation. In affected individuals distinct mutations have been identified in the GNE gene, which encodes the bifunctional enzyme uridine diphospho-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase/N-acetyl-mannosamine (ManNAc) kinase (GNE/MNK). GNE/MNK catalyzes the first two committed steps in the biosynthesis of acetylneuraminic acid (Neu5Ac), an abundant and functionally important sugar. The generation of HIBM animal models has led to novel insights into both the disease and the role of GNE/MNK in pathophysiology. Recent advances in therapeutic approaches for HIBM, including administration of N-acetyl-mannosamine (ManNAc), a precursor of Neu5Ac will be discussed.

Distal myopathy with rimmed vacuoles: impaired O-glycan formation in muscular glycoproteins

Distal myopathy with rimmed vacuoles (DMRV), is an autosomal recessive disorder with early adult onset, displays distal dominant muscular involvement and is characterized by the presence of numerous rimmed vacuoles in the affected muscle fibers. The pathophysiology of DMRV has not been clarified yet, although the responsible gene was identified as that encoding UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase involved in the biosynthesis of sialic acids. To identify defective carbohydrate moieties of muscular glycoproteins from DMRV patients, frozen skeletal muscle sections from seven patients with DMRV, as well as normal and pathological controls, were treated with or without sialidase or N-glycosidase F followed by lectin staining and lectin blotting analysis. The sialic acid contents of the O-glycans in the skeletal muscle specimens from the DMRV patients were also measured. We found that Arachis hypogaea agglutinin (PNA) lectin reacted strongly with sarcolemmal glycoproteins in the DMRV patients but not with those in control subjects. alpha-Dystroglycan from the DMRV patients strongly associated with PNA lectin, although that from controls did not. The sialic acid level of the O-glycans in the DMRV muscular glycoproteins with molecular weights of 30 to 200 kd was reduced to 60 to 80% of the control level. The results show that impaired sialyl O-glycan formation in muscular glycoproteins, including alpha-dystroglycan, occurs in DMRV.

A novel homozygous missense mutation in the GNE gene of a patient with quadriceps-sparing hereditary inclusion body myopathy associated with muscle inflammation

An adult-onset hereditary inclusion body myopathy with sparing of the quadriceps muscle was originally described in Iranian Jews and assigned to a locus on chromosome 9p12-p13. Recently, mutations of the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) gene were reported to cause hereditary inclusion body myopathy and one type of distal myopathy in a world-wide distribution. Importantly, the lack of muscle inflammation was used to distinguish hereditary inclusion body myopathy from the sporadic form of inclusion body myopathy. We report a case of a quadriceps-sparing myopathy in a non-Jewish, Iranian patient with a high degree of muscle inflammation. A novel homozygous G-to-A mutation (128933G-->A) in exon 7 changing a valine to isoleucine (V367I) in the epimerase domain of the GNE gene was found. We conclude that muscle inflammation is not sufficient to exclude the diagnosis of hereditary inclusion body myopathy.

BACE1 and BACE2 in pathologic and normal human muscle

BACE1 and BACE2 are recently discovered enzymes participating in processing of amyloid beta precursor protein (AbetaPP). Their discovery is contributing importantly to understanding the mechanism of amyloid-beta generation, and hence the pathogenesis of Alzheimer's disease (AD). Sporadic inclusion-body myositis (s-IBM) and hereditary inclusion-body myopathy (h-IBM) are progressive muscle diseases in which overproduction of AbetaPP and accumulation of its presumably toxic proteolytic product amyloid-beta (Abeta) in abnormal muscle fibers appear to play an important upstream role in the pathogenic cascade. In normal human muscle AbetaPP was also shown to be present and presumably playing a role (a) at neuromuscular junctions and (b) during muscle development. To investigate whether BACE1 and BACE2 play a role in normal and diseased human muscle, we have now studied them by immunocytochemistry and immunoblotting in 35 human muscle biopsies, including: 5 s-IBM; 5 chromosome-9p1-linked quadriceps-sparing h-IBM; and 25 control muscle biopsies. In addition, expression of BACE1 and BACE2 was studied in normal cultured human muscle. Our studies demonstrate that BACE1 and BACE2 (a) are expressed in normal adult muscle at the postsynaptic domain of neuromuscular junctions, and in cultured human muscle; (b) are accumulated in the form of plaque-like inclusions in both s-IBM and h-IBM vacuolated muscle fibers; and (c) are immunoreactive in necrotizing muscle fibers. Accordingly, BACE1 and BACE2 participate in normal and abnormal processes of human muscle, suggesting that their functions are broader than previously thought.

Genetic and environmental risk factors for idiopathic inflammatory myopathies

Although the studies discussed are beginning to reveal a number of genetic and possible environmental risk factors for myositis, further investigations are needed to fully understand and classify these syndromes. The difficulties in this process include small numbers of subjects with varying disease phenotypes available for study, polygenic risk factors for which it remains unclear which are primary and which are secondary or linked genes, and the lack of validated environmental exposure assessment tools. New technologies and international collaborative approaches, however, may overcome some of these difficulties and allow us to identify genetic and environmental risk factors, as well as the critical gene-environment interactions in the IIM and its subgroups. Nonetheless, our understanding of these diseases is still in the early stages. Although we have learned a great deal about these disorders through detailed investigations over the last several decades, we have even further to go to understand the environmental triggers and genetic susceptibilities for the myositis syndromes.

A novel mutation in the GNE gene and a linkage disequilibrium in Japanese pedigrees

Distal myopathy with rimmed vacuoles (DMRV) is an autosomal recessive muscular disorder characterized by weakness of the anterior compartment of the lower limbs with onset in early adulthood and sparing of the quadricep muscles. The UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) gene was recently identified as the causative gene for hereditary inclusion body myopathy (HIBM). To investigate whether DMRV and HIBM are allelic diseases, we conducted mutational analysis of the GNE gene of six Japanese DMRV pedigrees and found that all the pedigrees share a homozygous mutation (V572L) associated with a strong linkage disequilibrium, suggesting a strong founder effect in Japanese DMRV pedigrees.

Cloning and characterization of a novel human gene RNF38 encoding a conserved putative protein with a RING finger domain

RING finger (C3HC4-type zinc finger) is a variant zinc finger motif present in a large family of functionally distinct proteins. We describe the cloning and characterization of a novel human transcript RNF38 encoding a new member of the RING finger protein family. The complete mRNA consists of about 6.8 kb widely expressed in human tissues as a single transcript, most abundantly in testis. The predicted proline-rich protein consists of 432 amino acid residues with a coiled-coil motif and a RING-H2 motif (C3H2C2) at its carboxy-terminus. High degree homology was found between the human protein and hypothetical peptides from several other species including Rattus norvegicus, Mus musculus, and Drosophila melanogaster, indicating a significant conservation throughout evolution. The RNF38 genomic structure was determined and comprises at least 13 exons extending over more than 65 kb in the genome, 78 kb centromeric to the GNE gene on human chromosome 9p12-p13. The involvement of this chromosomal segment in a large number of human diseases and in particular in various types of malignancies urges the assessment of the potential functional role of RNF38 in these disorders.

Cloning and characterization of a human novel gene C9orf19 encoding a conserved putative protein with an SCP-like extracellular protein domain

A novel human transcript, C9orf19, mapped to the genomic region involved in hereditary inclusion body myopathy (IBM2) at chromosome 9p12-p13, has been cloned and characterized. A single cDNA clone consisting of the full-length 1.9 kb transcript has been isolated from a human placenta cDNA library and further analyzed. Genomic characterization of the C9orf19 gene identified five exons extending over 27.2 kb of genomic DNA, located 12 kb centromeric to the tumor suppressor RECK gene. C9orf19 mRNA is expressed in a wide range of adult tissues as a single transcript, most abundantly in lung and peripheral blood leukocytes. The predicted protein contains the SCP-like extracellular protein signature classified to IPR001283, a family of evolutionary related proteins with extracellular domains, which includes the human glioma pathogenesis-related protein (GliPR), the human testis specific glycoprotein (TPX-1), and several other extracellular proteins from rodents (SCP), insects venom allergens (Ag5, Ag3), plants pathogenesis proteins (PR-1) and yeast hypothetical proteins. Homology searches with the deduced 154 amino acid protein sequence of C9orf19 revealed highly similar proteins in mouse, drosophila, nematode and yeast. Mutation analysis of C9orf19 in IBM2 patients excluded it as the disease causing gene and revealed four single nucleotide polymorphisms within and in the vicinity of the gene, which will certainly be useful tools to study its potential role in several human diseases mapped to chromosome 9p12-p13. Parallel to this study, the gene termed GNE, approximately 50 kb centromeric to C9orf19, was shown to be the disease causing gene in IBM2.

Clinical delineation and localization to chromosome 9p13.3-p12 of a unique dominant disorder in four families: hereditary inclusion body myopathy, Paget disease of bone, and frontotemporal dementia

Autosomal dominant myopathy, Paget disease of bone, and dementia constitute a unique disorder (MIM 605382). Here we describe the clinical, biochemical, radiological, and pathological characteristics of 49 affected (23 male, 26 female) individuals from four unrelated United States families. Among these affected individuals 90% have myopathy, 43% have Paget disease of bone, and 37% have premature frontotemporal dementia. EMG shows myopathic changes and muscle biopsy reveals nonspecific myopathic changes or blue-rimmed vacuoles. After candidate loci were excluded, a genome-wide screen in the large Illinois family showed linkage to chromosome 9 (maximum LOD score 3.64 with marker D9S301). Linkage analysis with a high density of chromosome 9 markers generated a maximum two-point LOD score of 9.29 for D9S1791, with a maximum multipoint LOD score of 12.24 between D9S304 and D9S1788. Subsequent evaluation of three additional families demonstrating similar clinical characteristics confirmed this locus, refined the critical region, and further delineated clinical features of this unique disorder. Hence, autosomal dominant inclusion body myopathy (HIBM), Paget disease of bone (PDB), and frontotemporal dementia (FTD) localizes to a 1.08-6.46 cM critical interval on 9p13.3-12 in the region of autosomal recessive IBM2.

The UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase gene is mutated in recessive hereditary inclusion body myopathy

Hereditary inclusion body myopathy (HIBM; OMIM 600737) is a unique group of neuromuscular disorders characterized by adult onset, slowly progressive distal and proximal weakness and a typical muscle pathology including rimmed vacuoles and filamentous inclusions. The autosomal recessive form described in Jews of Persian descent is the HIBM prototype. This myopathy affects mainly leg muscles, but with an unusual distribution that spares the quadriceps. This particular pattern of weakness distribution, termed quadriceps-sparing myopathy (QSM), was later found in Jews originating from other Middle Eastern countries as well as in non-Jews. We previously localized the gene causing HIBM in Middle Eastern Jews on chromosome 9p12-13 (ref. 5) within a genomic interval of about 700 kb (ref. 6). Haplotype analysis around the HIBM gene region of 104 affected people from 47 Middle Eastern families indicates one unique ancestral founder chromosome in this community. By contrast, single non-Jewish families from India, Georgia (USA) and the Bahamas, with QSM and linkage to the same 9p12-13 region, show three distinct haplotypes. After excluding other potential candidate genes, we eventually identified mutations in the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) gene in the HIBM families: all patients from Middle Eastern descent shared a single homozygous missense mutation, whereas distinct compound heterozygotes were identified in affected individuals of families of other ethnic origins. Our findings indicate that GNE is the gene responsible for recessive HIBM.