Clinical and genetic heterogeneity in chromosome 9p associated hereditary inclusion body myopathy: exclusion of GNE and three other candidate genes

Glycation Interferes with the Activity of the Bi-Functional UDP-N-Acetylglucosamine 2-Epimerase/N-Acetyl-mannosamine Kinase (GNE)

Mutations in the gene coding for the bi-functional UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme of the sialic acid biosynthesis, are responsible for autosomal-recessive GNE myopathy (GNEM). GNEM is an adult-onset disease with a yet unknown exact pathophysiology. Since the protein appears to work adequately for a certain period of time even though the mutation is already present, other effects appear to influence the onset and progression of the disease. In this study, we want to investigate whether the late onset of GNEM is based on an age-related effect, e.g., the accumulation of post-translational modifications (PTMs). Furthermore, we also want to investigate what effect on the enzyme activity such an accumulation would have. We will particularly focus on glycation, which is a PTM through non-enzymatic reactions between the carbonyl groups (e.g., of methylglyoxal (MGO) or glyoxal (GO)) with amino groups of proteins or other biomolecules. It is already known that the levels of both MGO and GO increase with age. For our investigations, we express each domain of the GNE separately, treat them with one of the glycation agents, and determine their activity. We demonstrate that the enzymatic activity of the N-acetylmannosamine kinase (GNE-kinase domain) decreases dramatically after glycation with MGO or GO-with a remaining activity of 13% ± 5% (5 mM MGO) and 22% ± 4% (5 mM GO). Whereas the activity of the UDP-N-acetylglucosamine 2-epimerase (GNE-epimerase domain) is only slightly reduced after glycation-with a remaining activity of 60% ± 8% (5 mM MGO) and 63% ± 5% (5 mM GO).

GNE myopathy: History, etiology, and treatment trials

GNE myopathy is an ultrarare muscle disease characterized by slowly progressive muscle weakness. Symptoms typically start in early adulthood, with weakness and atrophy in the tibialis anterior muscles and with slow progression over time, which largely spares the quadriceps muscles. Muscle biopsy shows atrophic fibers and rimmed vacuoles without inflammation. Inherited in an autosomal recessive manner, patients with GNE myopathy carry mutations in the GNE gene which affect the sialic acid synthesis pathway. Here, we look at the history and clinical aspects of GNE myopathy, as well as focus on prior treatment trials and challenges and unmet needs related to this disorder.

Tissue-specific isoform expression of GNE gene in human tissues

Mutations in the sialic acid biosynthesis enzyme GNE lead to a late-onset, debilitating neuromuscular disorder, GNE myopathy, characterized by progressive skeletal muscle weakness. The mechanisms responsible for skeletal muscle specificity, late-onset, and disease progression are unknown. Our main aim is to understand the reason for skeletal muscle-specific phenotype. To answer this question, we have analyzed the expression profile of the GNE gene and its multiple mRNA variants in different human tissues. A combinatorial approach encompassing bioinformatics tools and molecular biology techniques was used. NCBI, Ensembl, and GTEx were used for data mining. The expression analysis of GNE and its variants was performed with cDNA tissue panel using PCR and targeted RNA-seq. Among nine different GNE isoforms reported in this study, transcript variants 1, X1, and X2 were not tissue specific. Transcript variants 1, 6, X1, and X2, were found in skeletal muscles suggesting their possible role in GNE myopathy. In the current study, we present new data about GNE expression patterns in human tissues. Our results suggest that there may be a link between tissue-specific pathology and isoform pattern in skeletal muscles, which could provide clues for the development of new treatment strategies for GNE myopathy.

Sialic Acids and Their Influence on Human NK Cell Function

Sialic acids are sugars with a nine-carbon backbone, present on the surface of all cells in humans, including immune cells and their target cells, with various functions. Natural Killer (NK) cells are cells of the innate immune system, capable of killing virus-infected and tumor cells. Sialic acids can influence the interaction of NK cells with potential targets in several ways. Different NK cell receptors can bind sialic acids, leading to NK cell inhibition or activation. Moreover, NK cells have sialic acids on their surface, which can regulate receptor abundance and activity. This review is focused on how sialic acids on NK cells and their target cells are involved in NK cell function.

A cross-sectional analysis of clinical evaluation in 35 individuals with mutations of the valosin-containing protein gene

Inclusion body myopathy (IBM) associated with Paget disease of the bone and frontotemporal dementia or IBMPFD is an autosomal dominant degenerative disorder caused by mutations in the valosin-containing protein (VCP) gene. We aim to establish a detailed clinical phenotype of VCP disease amongst 35 (28 affected individuals, 7 presymptomatic gene carriers) individuals versus 14 unaffected first-degree relatives in 14 families to establish useful biomarkers for IBMPFD and identify the most meaningful tests for monitoring disease progression in future clinical trials. Comprehensive studies included the Inclusion Body Myositis Functional Rating Scale (IBMFRS) and fatigue severity scale questionairres, strength measurements using the Manual Muscle Test with Medical Research Council (MRC) scales, hand-held dynamometry using the microFET and Biodex dynamometers, 6 minute walk test (6MWT), and pulmonary function studies. Strong correlation was observed between the IBMFRS and measurements of muscle strength with dynamometry and the other functional tests, indicating that it may be utilized in long-term follow-up assessments due to its relative simplicity. This cross-section study represents the most comprehensive evaluation of individuals with VCP disease to date and provides a useful guide for evaluating and possible monitoring of muscle weakness and pulmonary function progression in this unique cohort of individuals.

Myogenic differentiation of VCP disease-induced pluripotent stem cells: A novel platform for drug discovery

Valosin Containing Protein (VCP) disease is an autosomal dominant multisystem proteinopathy caused by mutations in the VCP gene, and is primarily associated with progressive muscle weakness, including atrophy of the pelvic and shoulder girdle muscles. Currently, no treatments are available and cardiac and respiratory failures can lead to mortality at an early age. VCP is an AAA ATPase multifunction complex protein and mutations in the VCP gene resulting in disrupted autophagic clearance. Due to the rarity of the disease, the myopathic nature of the disorder, ethical and practical considerations, VCP disease muscle biopsies are difficult to obtain. Thus, disease-specific human induced pluripotent stem cells (hiPSCs) now provide a valuable resource for the research owing to their renewable and pluripotent nature. In the present study, we report the differentiation and characterization of a VCP disease-specific hiPSCs into precursors expressing myogenic markers including desmin, myogenic factor 5 (MYF5), myosin and heavy chain 2 (MYH2). VCP disease phenotype is characterized by high expression of TAR DNA Binding Protein-43 (TDP-43), ubiquitin (Ub), Light Chain 3-I/II protein (LC3-I/II), and p62/SQSTM1 (p62) protein indicating disruption of the autophagy cascade. Treatment of hiPSC precursors with autophagy stimulators Rapamycin, Perifosine, or AT101 showed reduction in VCP pathology markers TDP-43, LC3-I/II and p62/SQSTM1. Conversely, autophagy inhibitors chloroquine had no beneficial effect, and Spautin-1 or MHY1485 had modest effects. Our results illustrate that hiPSC technology provide a useful platform for a rapid drug discovery and hence constitutes a bridge between clinical and bench research in VCP and related diseases.

Missing genetic variations in GNE myopathy: rearrangement hotspots encompassing 5'UTR and founder allele

GNE myopathy is an autosomal recessive distal myopathy caused by loss-of-function mutations in the GNE gene, which encodes UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE), a key enzyme in sialic-acid biosynthesis. By comprehensive screening of manifesting patients using a fine-mapped targeted next-generation sequencing (NGS), we identified copy number variations (CNVs) in 13 patients from 11 unrelated families. The nine unique CNVs largely vary in size from 0.3 to 72 kb. Over half of the cases carry different deletions spanning merely exon 2, which contains the 5' untranslated region (5'UTR) of the muscle major transcript hGNE1. Of most unique CNVs, either the telomeric or the centromeric breakpoint locates within intron 2, indicating rearrangement hotspots. Haplotype analysis suggested the existence of a founder allele with exon 2 deletion. The breakpoints for all CNVs were determined by long-range PCR and sequencing. All of the breakpoints of gross deletion/duplications reside within directly oriented pairs of Alu repeats. The results of this study firstly widen the spectra of mutations to CNVs encompassing 5'UTR, underscoring the pivotal role of the hGNE1 transcript. Alu-mediated non-recurrent CNVs may have been overlooked in a wide variety of recessive phenotypes, especially in those associated with genomic Alu-rich genes such as GNE.

IBMPFD Disease-Causing Mutant VCP/p97 Proteins Are Targets of Autophagic-Lysosomal Degradation

The ubiquitin-proteasome system (UPS) degrades soluble proteins and small aggregates, whereas macroautophagy (autophagy herein) eliminates larger protein aggregates, tangles and even whole organelles in a lysosome-dependent manner. VCP/p97 was implicated in both pathways. VCP/p97 mutations cause a rare multisystem disease called IBMPFD (Inclusion Body Myopathy with Paget's Disease and Frontotemporal Dementia). Here, we studied the role IBMPFD-related mutants of VCP/p97 in autophagy. In contrast with the wild-type VCP/p97 protein or R155C or R191Q mutants, the P137L mutant was aggregate-prone. We showed that, unlike commonly studied R155C or R191Q mutants, the P137L mutant protein stimulated both autophagosome and autolysosome formation. Moreover, P137L mutant protein itself was a substrate of autophagy. Starvation- and mTOR inhibition-induced autophagy led to the degradation of the P137L mutant protein, while preserving the wild-type and functional VCP/p97. Strikingly, similar to the P137L mutant, other IBMPFD-related VCP/p97 mutants, namely R93C and G157R mutants induced autophagosome and autolysosome formation; and G157R mutant formed aggregates that could be cleared by autophagy. Therefore, cellular phenotypes caused by P137L mutant expression were not isolated observations, and some other IBMPFD disease-related VCP/p97 mutations could lead to similar outcomes. Our results indicate that cellular mechanisms leading to IBMPFD disease may be various, and underline the importance of studying different disease-associated mutations in order to better understand human pathologies and tailor mutation-specific treatment strategies.

Targeted excision of VCP R155H mutation by Cre-LoxP technology as a promising therapeutic strategy for valosin-containing protein disease

Inclusion body myopathy associated with Paget's disease of the bone and frontotemporal dementia is attributed to mutations in the valosin-containing protein (VCP) gene, mapped to chromosomal region 9p13.3-12. Affected individuals exhibit scapular winging and die from progressive muscle weakness and cardiac and respiratory failure in their 40s to 50s. Mutations in the VCP gene have also been associated with amyotrophic lateral sclerosis in 10-15% of individuals with hereditary inclusion body myopathy and 2-3% of isolated familial amyotrophic lateral sclerosis. Currently, there are no effective treatments for VCP-related myopathy or dementia. To determine the effects of targeted excision of the most common R155H mutation in VCP disease, we generated the Cre-ER™-VCPR155H/+ tamoxifen-inducible model. We administered tamoxifen (0.12 mg/g body weight) or corn oil (vehicle) to the pregnant dams by oral gavage and monitored survival and muscle strength measurements of the pups until 18 months of age. We confirmed efficient removal of exons 4 and 5 and recombination of the mutant/floxed VCP copies by Q-PCR analyses. The activity and specificity of Cre recombinase was confirmed by immunostaining. Herein, we report that Cre-ER™-VCPR155H/+ mice demonstrated improved muscle strength and quadriceps fibers architecture, autophagy signaling pathway, reduced brain neuropathology, decreased apoptosis, and less severe Paget-like bone changes. The Cre-ER™-VCPR155H/+ mouse model provides proof of principle by demonstrating that removal of the mutated exons could be beneficial to patients with VCP-related neurodegenerative diseases, and serves as an excellent platform in understanding the underlying pathophysiological mechanism(s) in the hopes of a promising therapeutic approach.

Psychological Impact of Predictive Genetic Testing in VCP Inclusion Body Myopathy, Paget Disease of Bone and Frontotemporal Dementia

Inclusion Body Myopathy associated with Paget's disease of bone and Fronto-temporal Dementia, also known as multisystem proteinopathy is an autosomal dominant, late onset neurodegenerative disorder caused by mutations in Valosin containing protein (VCP) gene. This study aimed to assess uptake and decision making for predictive genetic testing and the impact on psychological well-being. Individuals who had participated in the gene discovery study with a 50 % a priori risk of inheriting VCP disease were sent a letter of invitation offering genetic counseling and testing and were also invited to participate in this psychosocial study. A total of 102 individuals received an invitation and 33 individuals participated in genetic counseling and testing (32.3 %) with 29 completing baseline questionnaires. Twenty completed the follow-up post-test Hospital Anxiety and Depression Scale questionnaire including 13 of the 18 who had tested positive. Mean risk perception at baseline was 50.1 %. Reasons for testing included planning for the future, relieving uncertainty, informing children and satisfying curiosity. At baseline, one quarter of the participants had high levels of anxiety. However, scores were normal one year following testing. In this small cohort, one third of individuals at 50 % risk chose pre-symptomatic testing. Although one quarter of those choosing testing had high anxiety at baseline, this was not evident at follow-up.

A Fine Balance of Dietary Lipids Improves Pathology of a Murine Model of VCP-Associated Multisystem Proteinopathy

The discovery of effective therapies and of disease mechanisms underlying valosin containing protein (VCP)-associated myopathies and neurodegenerative disorders remains elusive. VCP disease, caused by mutations in the VCP gene, are a clinically and genetically heterogeneous group of disorders with manifestations varying from hereditary inclusion body myopathy, Paget’s disease of bone, frontotemporal dementia (IBMPFD), and amyotrophic lateral sclerosis (ALS). In the present study, we examined the effects of higher dietary lipid percentages on VCP^R155H/R155H, VCP^R155H/+ and Wild Type (WT) mice from birth until 15 months of age by immunohistochemical and biochemical assays. Findings illustrated improvement in the muscle strength, histology, and autophagy signaling pathway in the heterozygote mice when fed 9% lipid-enriched diets (LED). However, increasing the LED by 12%, 30%, and 48% showed no improvement in homozygote and heterozygote survival, muscle pathology, lipid accumulation or the autophagy cascade. These findings suggest that a balanced lipid supplementation may have a therapeutic strategy for patients with VCP-associated multisystem proteinopathies.

Rapamycin and chloroquine: the in vitro and in vivo effects of autophagy-modifying drugs show promising results in valosin containing protein multisystem proteinopathy

Mutations in the valosin containing protein (VCP) gene cause hereditary Inclusion body myopathy (hIBM) associated with Paget disease of bone (PDB), frontotemporal dementia (FTD), more recently termed multisystem proteinopathy (MSP). Affected individuals exhibit scapular winging and die from progressive muscle weakness, and cardiac and respiratory failure, typically in their 40s to 50s. Histologically, patients show the presence of rimmed vacuoles and TAR DNA-binding protein 43 (TDP-43)-positive large ubiquitinated inclusion bodies in the muscles. We have generated a VCP^R155H/+ mouse model which recapitulates the disease phenotype and impaired autophagy typically observed in patients with VCP disease. Autophagy-modifying agents, such as rapamycin and chloroquine, at pharmacological doses have previously shown to alter the autophagic flux. Herein, we report results of administration of rapamycin, a specific inhibitor of the mechanistic target of rapamycin (mTOR) signaling pathway, and chloroquine, a lysosomal inhibitor which reverses autophagy by accumulating in lysosomes, responsible for blocking autophagy in 20-month old VCP^R155H/+ mice. Rapamycin-treated mice demonstrated significant improvement in muscle performance, quadriceps histological analysis, and rescue of ubiquitin, and TDP-43 pathology and defective autophagy as indicated by decreased protein expression levels of LC3-I/II, p62/SQSTM1, optineurin and inhibiting the mTORC1 substrates. Conversely, chloroquine-treated VCP^R155H/+ mice revealed progressive muscle weakness, cytoplasmic accumulation of TDP-43, ubiquitin-positive inclusion bodies and increased LC3-I/II, p62/SQSTM1, and optineurin expression levels. Our in vitro patient myoblasts studies treated with rapamycin demonstrated an overall improvement in the autophagy markers. Targeting the mTOR pathway ameliorates an increasing list of disorders, and these findings suggest that VCP disease and related neurodegenerative multisystem proteinopathies can now be included as disorders that can potentially be ameliorated by rapalogs.

In vitro studies in VCP-associated multisystem proteinopathy suggest altered mitochondrial bioenergetics

Mitochondrial dysfunction has recently been implicated as an underlying factor to several common neurodegenerative diseases, including Parkinson's disease, Alzheimer's and amyotrophic lateral sclerosis (ALS). Valosin containing protein (VCP)-associated multisystem proteinopathy is a new hereditary disorder associated with inclusion body myopathy, Paget disease of bone (PDB), frontotemporal dementia (FTD) and ALS. VCP has been implicated in several transduction pathways including autophagy, apoptosis and the PINK1/Parkin cascade of mitophagy. In this report, we characterized VCP patient and mouse fibroblasts/myoblasts to examine their mitochondrial dynamics and bioenergetics. Using the Seahorse XF-24 technology, we discovered decreased spare respiratory capacity (measurement of extra ATP that can be produced by oxidative phosphorylation in stressful conditions) and increased ECAR levels (measurement of glycolysis), and proton leak in VCP human fibroblasts compared with age- and sex-matched unaffected first degree relatives. We found decreased levels of ATP and membrane potential, but higher mitochondrial enzyme complexes II+III and complex IV activities in the patient VCP myoblasts when compared to the values of the control cell lines. These results suggest that mutations in VCP affect the mitochondria's ability to produce ATP, thereby resulting in a compensatory increase in the cells' mitochondrial complex activity levels. Thus, this novel in vitro model may be useful in understanding the pathophysiology and discovering new drug targets of mitochondrial dynamics and physiology to modify the clinical phenotype in VCP and related multisystem proteinopathies (MSP).

Global gene expression profiling in R155H knock-in murine model of VCP disease

Dominant mutations in the valosin-containing protein (VCP) gene cause inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia, which is characterized by progressive muscle weakness, dysfunction in bone remodeling, and frontotemporal dementia. More recently, VCP has been linked to 2% of familial amyotrophic lateral sclerosis cases. VCP plays a significant role in a plethora of cellular functions including membrane fusion, transcription activation, nuclear envelope reconstruction, postmitotic organelle reassembly, and cell cycle control. To elucidate the pathological mechanisms underlying the VCP disease progression, we have previously generated a VCP(R155H/+) mouse model with the R155H mutation. Histological analyses of mutant muscle showed vacuolization of myofibrils, centrally located nuclei, and disorganized muscle fibers. Global expression profiling of VCP(R155H/+) mice using gene annotations by DAVID identified key dysregulated signaling pathways including genes involved in the physiological system development and function, diseases and disorders, and molecular and cellular functions. There were a total of 212 significantly dysregulated genes, several of which are involved in the regulation of proteasomal function and NF-κB signaling cascade. Findings of the gene expression study were validated by using quantitative reverse transcriptase polymerase chain reaction analyses to test genes involved in various signaling cascades. This investigation reveals the importance of the VCP(R155H/+) mouse model in the understanding of cellular and molecular mechanisms causing VCP-associated neurodegenerative diseases and in the discovery of novel therapeutic advancements and strategies for patients suffering with these debilitating disorders.

A case report comparing clinical, imaging and neuropsychological assessment findings in twins discordant for the VCP p.R155C mutation

Inclusion body myopathy, Paget disease of bone and/or frontotemporal dementia is an autosomal dominant disease caused by mutations in the Valosin Containing Protein (VCP) gene. We compared clinical findings including MRI images and neuropsychological assessment data in affected and unaffected twin brothers aged 56 years from a family with the p.R155C VCP gene mutation. The affected twin presented with a 10 year history of progressive proximal muscle weakness, difficulty swallowing, gastroesophageal reflux, fecal incontinence, and peripheral neuropathy. Comprehensive neuropsychological testing revealed rapid cognitive decline in the absence of any behavioral changes in a span of 1 year. This case illustrates that frontotemporal dementia related cognitive impairment may precede behavioral changes in VCP disease as compared with predominance of behavioral impairment reported in previous studies. Our findings suggest that there is a need to establish VCP disease specific tools and normative rates of decline to detect pre-clinical cognitive impairment among affected individuals.

Lipid-enriched diet rescues lethality and slows down progression in a murine model of VCP-associated disease

Valosin-containing protein (VCP)-associated disease caused by mutations in the VCP gene includes combinations of a phenotypically heterogeneous group of disorders such as hereditary inclusion body myopathy, Paget's disease of bone, frontotemporal dementia and amyotrophic lateral sclerosis. Currently, there are no effective treatments for VCP myopathy or dementia. VCP mouse models carrying the common R155H mutation include several of the features typical of the human disease. In our previous investigation, VCP(R155H/R155H) homozygous mice exhibited progressive weakness and accelerated pathology prior to their early demise. Herein, we report that feeding pregnant VCP(R155H/+) heterozygous dams with a lipid-enriched diet (LED) results in the reversal of the lethal phenotype in VCP(R155H/R155H) homozygous offspring. We examined the effects of this diet on homozygous and wild-type mice from birth until 9 months of age. The LED regimen improved survival, motor activity, muscle pathology and the autophagy cascade. A targeted lipidomic analysis of skeletal muscle and liver revealed elevations in tissue levels of non-esterified palmitic acid and ceramide (d18:1/16:0), two lipotoxic substances, in the homozygous mice. The ability to reverse lethality, increase survival, and ameliorate myopathy and lipids deficits in the VCP(R155H/R155H) homozygous animals suggests that lipid supplementation may be a promising therapeutic strategy for patients with VCP-associated neurodegenerative diseases.

Murine isoforms of UDP-GlcNAc 2-epimerase/ManNAc kinase: Secondary structures, expression profiles, and response to ManNAc therapy

The bifunctional enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) catalyzes the first two committed steps in sialic acid synthesis. Non-allosteric GNE gene mutations cause the muscular disorder GNE myopathy (also known as hereditary inclusion body myopathy), whose exact pathology remains unknown. Increased knowledge of GNE regulation, including isoform regulation, may help elucidate the pathology of GNE myopathy. While eight mRNA transcripts encoding human GNE isoforms are described, we only identified two mouse Gne mRNA transcripts, encoding mGne1 and mGne2, homologous to human hGNE1 and hGNE2. Orthologs of the other human isoforms were not identified in mice. mGne1 appeared as the ubiquitously expressed, major mouse isoform. The mGne2 encoding transcript is differentially expressed and may act as a tissue-specific regulator of sialylation. mGne2 expression appeared significantly increased the first 2 days of life, possibly reflecting the high sialic acid demand during this period. Tissues of the knock-in Gne p.M712T mouse model had similar mGne transcript expression levels among genotypes, indicating no effect of the mutation on mRNA expression. However, upon treatment of these mice with N-acetylmannosamine (ManNAc, a Gne substrate, sialic acid precursor, and proposed therapy for GNE myopathy), Gne transcript expression, in particular mGne2, increased significantly, likely resulting in increased Gne enzymatic activities. This dual effect of ManNAc supplementation (increased flux through the sialic acid pathway and increased Gne activity) needs to be considered when treating GNE myopathy patients with ManNAc. In addition, the existence and expression of GNE isoforms needs consideration when designing other therapeutic strategies for GNE myopathy.

Genotype-phenotype studies of VCP-associated inclusion body myopathy with Paget disease of bone and/or frontotemporal dementia

Valosin containing protein (VCP) disease associated with inclusion body myopathy, Paget disease of the bone and frontotemporal dementia is a progressive autosomal dominant disorder caused by mutations in Valosin containing protein gene. To establish genotype-phenotype correlations we analyzed clinical and biochemical markers from a database of 190 members in 27 families harboring 10 missense mutations. Individuals were grouped into three categories: symptomatic, presymptomatic carriers and noncarriers. The symptomatic families were further divided into ten groups based on their VCP mutations. There was marked intra and inter-familial variation; and significant genotype-phenotype correlations were difficult to establish because of small numbers. Nevertheless when comparing the two most common mutations, R155C mutation was found to be more severe, with an earlier onset of myopathy and Paget (p = 0.03). Survival analysis of all subjects revealed an average life span after diagnosis of myopathy and Paget of 18 and 19 years respectively, and after dementia only 6 years. R155C had a reduced survival compared to the R155H mutation (p = 0.03).We identified amyotrophic lateral sclerosis (ALS) was diagnosed in 13 individuals (8.9%) and Parkinson's disease in five individuals (3%); however, there was no genotypic correlation. This study represents the largest dataset of patients with VCP disease and expands our understanding of the natural history and provides genotype-phenotype correlations in this unique disease.

UDP-GlcNAc 2-Epimerase/ManNAc Kinase (GNE): A Master Regulator of Sialic Acid Synthesis

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is the key enzyme of sialic acid biosynthesis in vertebrates. It catalyzes the first two steps of the cytosolic formation of CMP-N-acetylneuraminic acid from UDP-N-acetylglucosamine. In this review we give an overview of structure, biochemistry, and genetics of the bifunctional enzyme and its complex regulation. Furthermore, we will focus on diseases related to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase.

A progressive translational mouse model of human valosin-containing protein disease: the VCP(R155H/+) mouse

INTRODUCTION

Mutations in the valosin-containing protein (VCP) gene cause hereditary inclusion body myopathy (IBM) associated with Paget disease of bone (PDB), and frontotemporal dementia (FTD). More recently, these mutations have been linked to 2% of familial amyotrophic lateral sclerosis (ALS) cases. A knock-in mouse model offers the opportunity to study VCP-associated pathogenesis.

METHODS

The VCP(R155H/+) knock-in mouse model was assessed for muscle strength and immunohistochemical, Western blot, apoptosis, autophagy, and microPET/CT imaging analyses.

RESULTS

VCP(R155H/+) mice developed significant progressive muscle weakness, and the quadriceps and brain developed progressive cytoplasmic accumulation of TDP-43, ubiquitin-positive inclusion bodies, and increased LC3-II staining. MicroCT analyses revealed Paget-like lesions at the ends of long bones. Spinal cord demonstrated neurodegenerative changes, ubiquitin, and TDP-43 pathology of motor neurons.

CONCLUSIONS

VCP(R155H/+) knock-in mice represent an excellent preclinical model for understanding VCP-associated disease mechanisms and future treatments.

The homozygote VCP(R¹⁵⁵H/R¹⁵⁵H) mouse model exhibits accelerated human VCP-associated disease pathology

Valosin containing protein (VCP) mutations are the cause of hereditary inclusion body myopathy, Paget's disease of bone, frontotemporal dementia (IBMPFD). VCP gene mutations have also been linked to 2% of isolated familial amyotrophic lateral sclerosis (ALS). VCP is at the intersection of disrupted ubiquitin proteasome and autophagy pathways, mechanisms responsible for the intracellular protein degradation and abnormal pathology seen in muscle, brain and spinal cord. We have developed the homozygous knock-in VCP mouse (VCP(R155H/R155H)) model carrying the common R155H mutations, which develops many clinical features typical of the VCP-associated human diseases. Homozygote VCP(R155H/R155H) mice typically survive less than 21 days, exhibit weakness and myopathic changes on EMG. MicroCT imaging of the bones reveal non-symmetrical radiolucencies of the proximal tibiae and bone, highly suggestive of PDB. The VCP(R155H/R155H) mice manifest prominent muscle, heart, brain and spinal cord pathology, including striking mitochondrial abnormalities, in addition to disrupted autophagy and ubiquitin pathologies. The VCP(R155H/R155H) homozygous mouse thus represents an accelerated model of VCP disease and can be utilized to elucidate the intricate molecular mechanisms involved in the pathogenesis of VCP-associated neurodegenerative diseases and for the development of novel therapeutic strategies.

Radiological features of Paget disease of bone associated with VCP myopathy

OBJECTIVE

Mutations in the Valosin-containing protein (VCP) gene cause a unique disorder characterized by classic Paget disease of bone (PDB), inclusion body myopathy, and frontotemporal dementia (IBMPFD). Our objective was to analyze the radiographic features of PDB associated with VCP mutations since there is a dearth of literature on the PDB component of VCP disease.

MATERIALS AND METHODS

Radiographic bone surveys were examined in 23 individuals with VCP mutation and compared with their unaffected relatives. Laboratory testing relevant for VCP disease was performed in all individuals.

RESULTS

Of the 17 affected individuals with clinical manifestations of VCP disease, 16 of whom had myopathy, radiographic analysis revealed classic PDB in 11 individuals (65%). The mean age of diagnosis for myopathy was 43.8 years and for PDB was 38.1 years of age. Radiological evidence of PDB was seen in one individual (16%) amongst six clinically asymptomatic VCP mutation carriers. Alkaline phosphatase was a useful marker for diagnosing PDB in VCP disease.

CONCLUSIONS

Radiographic findings of classic PDB are seen in 52% of individuals carrying VCP mutations at a significantly younger age than conventional PDB. Screening for PDB is warranted in at-risk individuals because of the benefit of early treatment with the new powerful bisphosphonates that hold the potential for prevention of disease.

Identification, tissue distribution, and molecular modeling of novel human isoforms of the key enzyme in sialic acid synthesis, UDP-GlcNAc 2-epimerase/ManNAc kinase

UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) catalyzes the first two committed steps in sialic acid synthesis. In addition to the three previously described human GNE isoforms (hGNE1-hGNE3), our database and polymerase chain reaction analysis yielded five additional human isoforms (hGNE4-hGNE8). hGNE1 is the ubiquitously expressed major isoform, while the hGNE2-hGNE8 isoforms are differentially expressed and may act as tissue-specific regulators of sialylation. hGNE2 and hGNE7 display a 31-residue N-terminal extension compared to hGNE1. On the basis of similarities to kinases and helicases, this extension does not seem to hinder the epimerase enzymatic active site. hGNE3 and hGNE8 contain a 55-residue N-terminal deletion and a 50-residue N-terminal extension compared to hGNE1. The size and secondary structures of these fragments are similar, and modeling predicted that these modifications do not affect the overall fold compared to that of hGNE1. However, the epimerase enzymatic activity of GNE3 and GNE8 is likely absent, because the deleted fragment contains important substrate binding residues in homologous bacterial epimerases. hGNE5-hGNE8 have a 53-residue deletion, which was assigned a role in substrate (UDP-GlcNAc) binding. Deletion of this fragment likely eliminates epimerase enzymatic activity. Our findings imply that GNE is subject to evolutionary mechanisms to improve cellular functions, without increasing the number of genes. Our expression and modeling data contribute to elucidation of the complex functional and regulatory mechanisms of human GNE and may contribute to further elucidating the pathology and treatment strategies of the human GNE-opathies sialuria and hereditary inclusion body myopathy.

Biochemical characterization of the M712T-mutation of the UDP-N-acetylglucosamine 2-epimerase/N-acetyl-mannosaminekinase in hereditary inclusion body myopathy

Hereditary inclusion body myopathy is a neuromuscular disorder characterized by muscle weakness with a late onset and slow progression. It is caused by mutations of the gene encoding UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE). One of the most frequent mutations is an exchange of methionine to threonine at position 712 (M712T). Here we analyzed wildtype (wt) and M712T-mutated (M712T) GNE. We identified threonine 712 as an additional possible phosphorylation site and found by two-dimensional gel-electrophoresis a lower isoelectric point compared to wt-GNE. This lower isoelectric point could be partially reversed back to the wildtype isoelectric point after treatment with protein phosphatase. Furthermore, in contrast to wt-GNE, a significant fraction of M712T-GNE was in the insoluble fraction. Finally, by using bimolecular fluorescence complementation we demonstrate that the M712T mutation does not disrupt the formation of GNE-oligomers.

VCP associated inclusion body myopathy and paget disease of bone knock-in mouse model exhibits tissue pathology typical of human disease

Dominant mutations in the valosin containing protein (VCP) gene cause inclusion body myopathy associated with Paget's disease of bone and frontotemporal dementia (IBMPFD). We have generated a knock-in mouse model with the common R155H mutation. Mice demonstrate progressive muscle weakness starting approximately at the age of 6 months. Histology of mutant muscle showed progressive vacuolization of myofibrils and centrally located nuclei, and immunostaining shows progressive cytoplasmic accumulation of TDP-43 and ubiquitin-positive inclusion bodies in quadriceps myofibrils and brain. Increased LC3-II staining of muscle sections representing increased number of autophagosomes suggested impaired autophagy. Increased apoptosis was demonstrated by elevated caspase-3 activity and increased TUNEL-positive nuclei. X-ray microtomography (uCT) images show radiolucency of distal femurs and proximal tibiae in knock-in mice and uCT morphometrics shows decreased trabecular pattern and increased cortical wall thickness. Bone histology and bone marrow derived macrophage cultures in these mice revealed increased osteoclastogenesis observed by TRAP staining suggestive of Paget bone disease. The VCP(R155H/+) knock-in mice replicate the muscle, bone and brain pathology of inclusion body myopathy, thus representing a useful model for preclinical studies.

Regulation and pathophysiological implications of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) as the key enzyme of sialic acid biosynthesis

The key enzyme for the biosynthesis ofN-acetylneuraminic acid, from which all other sialic acids are formed, is the bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). GNE is a highly conserved protein found throughout the animal kingdom. Its highest expression is seen in the liver and placenta. GNE is regulated by a variety of biochemical means, including tetramerization promoted by the substrate UDP-GlcNAc, phosphorylation by protein kinase C and feedback inhibition by CMP-Neu5Ac, which is defect in the human disease sialuria. GNE knock-out in mice leads to embryonic lethality, emphasizing the crucial role of this key enzyme for sialic acid biosynthesis. The metabolic capacity to synthesize sialic acid and CMP-sialic acid upon ManNAc loads is amazingly high. An additional characteristic of GNE is its interaction with proteins involved in the regulation of development, which might play a crucial role in the hereditary inclusion body myopathy. Due to the importance of increased concentrations of tumor-surface sialic acid, first attempts to find inhibitors of GNE have been successful.

Biochemical characterization of human and murine isoforms of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE)

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is the key enzyme for the biosynthesis of sialic acids, terminal components of glycoconjugates associated with a variety of physiological and pathological processes. Different protein isoforms of human and mouse GNE, deriving from splice variants, were predicted recently: GNE1 represents the GNE protein described in several studies before, GNE2 and GNE3 are proteins with extended and deleted N-termini, respectively. hGNE2, recombinantly expressed in insect and mamalian cells, displayed selective reduction of UDP-GlcNAc 2-epimerase activity by the loss of its tetrameric state, which is essential for full enzyme activity. hGNE3, which had to be expressed in Escherichia coli, only possessed kinase activity, whereas mGNE1 and mGNE2 showed no significant differences. Our data therefore suggest a role of GNE1 in basic supply of cells with sialic acids, whereas GNE2 and GNE3 may have a function in fine-tuning of the sialic acid pathway.

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.

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.

Prediction of three different isoforms of the human UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is the key enzyme of the biosynthesis of sialic acids, terminal components of glycoconjugates associated with a variety of cellular processes. Two novel isoforms of human GNE, namely GNE2 and GNE3, which possess extended and deleted N-termini, respectively, were characterized. GNE2 was also found in other species like apes, rodents, chicken or fish, whereas GNE3 seems to be restricted to primates. Both, GNE2 and GNE3, displayed tissue specific expression patterns, therefore may contribute to the complex regulation of sialic acid metabolism.

HDDD2 is a familial frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions caused by a missense mutation in the signal peptide of progranulin

OBJECTIVE

Familial autosomal dominant frontotemporal dementia with ubiquitin-positive, tau-negative inclusions in the brain linked to 17q21-22 recently has been reported to carry null mutations in the progranulin gene (PGRN). Hereditary dysphasic disinhibition dementia (HDDD) is a frontotemporal dementia with prominent changes in behavior and language deficits. A previous study found significant linkage to chromosome 17 in a HDDD family (HDDD2), but no mutation in the MAPT gene. Longitudinal follow-up has enabled us to identify new cases and to further characterize the dementia in this family. The goals of this study were to develop research criteria to classify the different clinical expressions of dementia observed in this large kindred, to identify the causal mutation in affected individuals and correlate this with phenotypic characteristics in this pedigree, and to assess the neuropathological characteristics using immunohistochemical techniques.

METHODS

In this study we describe a detailed clinical, pathological and mutation analysis of the HDDD2 kindred.

RESULTS

Neuropathologically, HDDD2 represents a familial frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions (FTLD-U). We developed research classification criteria and identified three distinct diagnostic thresholds, which helped localize the disease locus. The chromosomal region with the strongest evidence of linkage lies within the minimum critical region for FTLD-U. Sequencing of each exon of the PGRN gene led to the identification of a novel missense mutation, Ala-9 Asp, within the signal peptide.

INTERPRETATION

HDDD2 is an FTLD-U caused by a missense mutation in the PGRN gene that cosegregates with the disease and with the disease haplotype in at-risk individuals. This mutation is the first reported pathogenic missense mutation in the signal peptide of the PGRN gene causing FTLD-U. In light of the previous reports of null mutations and its position in the gene, two possible pathological mechanisms are proposed: (1) the protein may accumulate within the endoplasmic reticulum due to inefficient secretion; and (2) mutant RNA may have a lower expression because of degradation via nonsense-mediated decay.

Roles for UDP-GlcNAc 2-epimerase/ManNAc 6-kinase outside of sialic acid biosynthesis: modulation of sialyltransferase and BiP expression, GM3 and GD3 biosynthesis, proliferation, and apoptosis, and ERK1/2 phosphorylation

Roles for UDP-GlcNAc 2-epimerase/ManNAc 6-kinase (GNE) beyond controlling flux into the sialic acid biosynthetic pathway by converting UDP-GlcNAc to N-acetylmannosamine are described in this report. Overexpression of recombinant GNE in human embryonic kidney (HEK AD293) cells led to an increase in mRNA levels for ST3Gal5 (GM3 synthase) and ST8Sia1 (GD3 synthase) as well as the biosynthetic products of these sialyltransferases, the GM3 and GD3 gangliosides. Conversely, down-regulation of GNE by RNA interference methods had the opposite, but consistent, effect of lowering ST3Gal5 and ST8Sia1 mRNAs and reducing GM3 and GD3 levels. Control experiments ensured that GNE-mediated changes in sialyltransferase expression and ganglioside biosynthesis were not the result of altered flux through the sialic acid pathway. Interestingly, exogenous GM3 and GD3 also changed the expression of GNE and led to reduced ST3Gal5 and ST8Sia1 mRNA levels, demonstrating a reciprocating feedback mechanism where gangliosides regulate upstream biosynthetic enzymes. Cellular responses to the GNE-mediated changes in ST3Gal5 and ST8Sia1 expression and GM3 and GD3 levels were investigated next. Conditions that led to reduced ganglioside production (e.g. short hairpin RNA exposure) stimulated proliferation, whereas conditions that resulted in increased ganglioside levels (e.g. recombinant GNE and exogenous gangliosides) led to reduced proliferation with a concomitant increase in apoptosis. Finally, changes to BiP expression and ERK1/2 phosphorylation consistent with apoptosis and proliferation, respectively, were observed. These results provide examples of specific biochemical pathways, other than sialic acid metabolism, that are influenced by GNE.

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.

Localization of UDP-GlcNAc 2-epimerase/ManAc kinase (GNE) in the Golgi complex and the nucleus of mammalian cells

The bifunctional enzyme UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) is essential for early embryonic development and catalyzes the rate limiting step in sialic acid biosynthesis. Although epimerase and kinase activities have been attributed to GNE, little is known about the regulation, differential expression, and subcellular localization of GNE in vivo. Mutations in GNE cause a rare inherited muscle disorder in humans called hereditary inclusion body myopathy (HIBM). However, the role of GNE in HIBM pathogenesis has not been defined yet. Here, we show that the GNE protein is expressed in various mammalian cells and tissues with highest levels found in cancer cells and liver. In human skeletal muscle, GNE protein is developmentally regulated: high levels are found in immature myoblasts but low levels in mature skeletal muscle. The GNE protein colocalizes with resident proteins of the Golgi compartment in a variety of human cells including muscle. Drug-induced disruption of the Golgi and subsequent recovery reveals co-distribution of GNE along with Golgi-targeted proteins. This subcellular localization of GNE is in good agreement with its established role as the key enzyme of sialic acid biosynthesis, since the sialylation of glycoconjugates takes place in the Golgi complex. Surprisingly, GNE is also detected in the nucleus. Upon nocodazole treatment, GNE redistributes to the cytoplasm suggesting that GNE may act as a nucleocytoplasmic shuttling protein. A regulatory role for GNE shifting between the nuclear and the Golgi compartment is proposed. Further insight into GNE regulation may promote the understanding of HIBM pathogenesis.

Myositis: an update on pathogenesis

PURPOSE OF REVIEW

The etiology and much about the pathogenesis of the inflammatory myopathies remain a mystery. In this review, we investigate recent research efforts to understand the pathogenesis of the diverse entities of polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM), diseases that result from interactions between environmental risk factors and genetic susceptibility.

RECENT FINDINGS

Over the past year, there has been considerable progress toward better understanding of IBM, with relatively few developments toward understanding PM and DM. Although these diseases may share some common clinical phenotypic and serologic components, they differ on a molecular and cellular level.

SUMMARY

The need for definitive, safer therapies in these diseases makes vital the search for defining detailed pathogenesis of inflammation and muscle fiber damage at the molecular level.

Inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia is caused by mutant valosin-containing protein

Inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia (IBMPFD) is a dominant progressive disorder that maps to chromosome 9p21.1-p12. We investigated 13 families with IBMPFD linked to chromosome 9 using a candidate-gene approach. We found six missense mutations in the gene encoding valosin-containing protein (VCP, a member of the AAA-ATPase superfamily) exclusively in all 61 affected individuals. Haplotype analysis indicated that descent from two founders in two separate North American kindreds accounted for IBMPFD in approximately 50% of affected families. VCP is associated with a variety of cellular activities, including cell cycle control, membrane fusion and the ubiquitin-proteasome degradation pathway. Identification of VCP as causing IBMPFD has important implications for other inclusion-body diseases, including myopathies, dementias and Paget disease of bone (PDB), as it may define a new common pathological ubiquitin-based pathway.