Clinical, Histopathologic, and Genetic Features of Patients With Myofibrillary and Distal Myopathies: Experience From the Italian Network.

BACKGROUND AND OBJECTIVES: The diagnostic process for myofibrillar myopathies (MFM) and distal myopathies (DM) is particularly complex because of the large number of causative genes, the existence of still molecularly undefined disease entities, and the overlapping features between the 2 categories. This study aimed to characterize a large cohort of patients affected by MFM and DM and identify the most important diagnostic and prognostic aspects of these diseases. METHODS: Patients with either a myopathological diagnosis of MFM or a clinical diagnosis of DM were included in this retrospective multicentric national study. Demographic, genetic, clinical, and histopathologic data of anonymized patients were collected from the neuromuscular centers of the Italian Association of Myology network. RESULTS: Data regarding 132 patients with MFM (mean age 57.0 ± 15.8 years, 49% female) and 298 patients with DM (mean age 50.7 ± 15.9 years, 40% female) were gathered from 20 neuromuscular centers. 69 patients fulfilled the criteria for both groups (distal myopathies with myofibrillar pathology, DM-MP). Molecular confirmation was achieved in 63% of the patients. Fifty-two percent of the patients with MFM carried pathogenic variants in either DES (n = 30), MYOT (n = 20), or DNAJB6 (n = 18), which were also the most frequent disease-causing genes in DM-MP, while GNE (n = 44) and MYH7 (n = 23) were the genes most commonly carrying pathogenic variants in DM. The mean age at onset varied from <25 years in patients with causative variants in MYH7 and DYSF to 59 years in patients with myotilinopathies. Cardiac involvement was reported in 29% of patients with MFM and 16% of patients with DM, with DES and MYH7 variants significantly associated with the development of cardiomyopathy. Respiratory impairment was more prevalent in patients with TTN and DES variants and rare in other disorders such as GNE myopathy and dysferlinopathies, which were instead associated, together with DNAJB6-related and PLIN4-related myopathies, with the risk of losing ambulation during the disease course. DISCUSSION: The Italian cohort of patients with MFM and DM recapitulates the phenotypic heterogeneity and the partial overlap between the 2 groups. However, in relative contrast to the encountered phenotypic variability, only 5 genes accounted for most of the molecular diagnoses. Specific genetic entities are associated with significantly increased risk of developing cardiorespiratory complications or loss of ambulation, which has relevant prognostic implications.

A novel mutation alters GNE bifunctional enzyme activity and leads to familial inherited GNE diseases.

Distal myopathies are a group of rare heterogeneous diseases that are mostly caused by genetic factors. At least 20 genes have been associated with distal myopathies. We performed whole-exome sequencing to identify the genetic cause of disease in a family with distal myopathy. Following the American College of Medical Genetics and Genomics (ACMG) guidelines, we analyzed the sequencing results and screened suspicious mutations based on mutation frequency, functional impact, and disease inheritance pattern. The harmfulness of the mutations was predicted using bioinformatics methods, and the pathogenic mutations were determined. We identified a novel amino acid mutation (NP_005467.1:p.S663L) on the GNE gene that may cause familial distal myopathy. This mutation is the result of the simultaneous mutation of two adjacent nucleotides (c.1988C > T, c.1989C > A) in the codon. First, we measured the mRNA and protein expression of the GNE gene in the lymphoblastoid cell lines (LCLs) of the probands and their family members. Second, GNE vectors carrying the novel mutation, two other known pathogenic mutations, and the wild-type gene were constructed and transfected into HEK293T cells. The enzymatic activity of these GNE variants was investigated and showed that the p.S663L mutation significantly reduced the activity of the bifunctional GNE enzyme without altering the expression level of the GNE protein. Furthermore, the mutation may also alter the immunogenicity of the 3' end of the GNE protein, potentially affecting its oligomer formation. In this study, a novel GNE gene mutation that may cause distal myopathy was identified, expanding the spectrum of genetic mutations associated with this disease.

Altered autophagic flux in GNE mutant cells of Indian origin: Potential drug target for GNE myopathy.

Autophagy phenomenon in the cell maintains proteostasis balance by eliminating damaged organelles and protein aggregates. Imbalance in autophagic flux may cause accumulation of protein aggregates in various neurodegenerative disorders. Regulation of autophagy by either calcium or chaperone play a key role in the removal of protein aggregates from the cell. The neuromuscular rare genetic disorder, GNE Myopathy, is characterized by accumulation of rimmed vacuoles having protein aggregates of ß-amyloid and tau that may result from altered autophagic flux. In the present study, the autophagic flux was deciphered in HEK cell-based model for GNE Myopathy harbouring GNE mutations of Indian origin. The refolding activity of HSP70 chaperone was found to be reduced in GNE mutant cells compared to wild type controls. The autophagic markers LC3II/I ratio was altered with increased number of autophagosome formation in GNE mutant cells compared to wild type cells. The cytosolic calcium levels were also increased in GNE mutant cells of Indian origin. Interestingly, treatment of GNE mutant cells with HSP70 activator, BGP-15, restored the expression and refolding activity of HSP70 along with autophagosome formation. Treatment with calcium chelator, BAPTA-AM restored the cytoplasmic calcium levels and autophagosome formation but not LC3II/I ratio significantly. Our study provides insights towards GNE mutation specific response for autophagy regulation and opens up a therapeutic advancement area in calcium signalling and HSP70 function for GNE related Myopathy.

A therapeutic leap: how myosin inhibitors moved from cardiac interventions to skeletal muscle myopathy solutions.

The myosin inhibitor mavacamten has transformed the management of obstructive hypertrophic cardiomyopathy (HCM) by targeting myosin ATPase activity to mitigate cardiac hypercontractility. This therapeutic mechanism has proven effective for patients with HCM independent of having a primary gene mutation in myosin. In this issue of the JCI, Buvoli et al. report that muscle hypercontractility is a mechanism of pathogenesis underlying muscle dysfunction in Laing distal myopathy, a disorder characterized by mutations altering the rod domain of ß myosin heavy chain. The authors performed detailed physiological, molecular, and biomechanical analyses and demonstrated that myosin ATPase inhibition can correct a large extent of muscle abnormalities. The findings offer a therapeutic avenue for Laing distal myopathy and potentially other myopathies. This Commentary underscores the importance of reevaluating myosin activity's role across myopathies in general for the potential development of targeted myosin inhibitors to treat skeletal muscle disorders.

Large phenotypic diversity by genotype in patients with GNE myopathy: 10 years after the establishment of a national registry in Japan.

BACKGROUND: GNE myopathy is an ultra-rare autosomal recessive distal myopathy caused by pathogenic variants of the GNE gene, which encodes a key enzyme in sialic acid biosynthesis. The present study aimed to examine the long-term progression of GNE myopathy, genotype-phenotype correlations, and complications to provide useful information for predicting patient progression and designing clinical trials using a large collection of registry data over a 10-year period. METHODS: We analyzed 220 Japanese patients with GNE myopathy from a national registry in Japan. Diagnoses were confirmed by genetic curators based on genetic analysis reports. We analyzed registration sheets and annually updated items completed by attending physicians. RESULTS: In total, 197 of 220 participants (89.5%) carried p.D207V or p.V603L in at least one allele. The median disease duration to loss of ambulation was estimated to be 10 years in p.V603L homozygotes (n = 48), whereas more than 90% of p.D207V/p.V603L compound heterozygotes were estimated to be ambulatory even 20 years after disease onset according to Kaplan-Meier analysis (p < 0.001). Moreover, participants with a younger age of onset lost ambulation earlier regardless of genotype. A decline in respiratory function was observed as the disease progressed, particularly in p.V603L homozygotes, whereas none of the p.D207V/p.V603L compound heterozygotes showed a decline. CONCLUSIONS: The present study demonstrated large differences in disease progression and respiratory function between genotypes. Moreover, age of onset was found to be an indicator of disease severity regardless of genotype in GNE myopathy patients. These results may help stratify patients in clinical trials and predict disease progression.

A Laing distal myopathy-associated proline substitution in the ß-myosin rod perturbs myosin cross-bridging activity.

Proline substitutions within the coiled-coil rod region of the ß-myosin gene (MYH7) are the predominant mutations causing Laing distal myopathy (MPD1), an autosomal dominant disorder characterized by progressive weakness of distal/proximal muscles. We report that the MDP1 mutation R1500P, studied in what we believe to be the first mouse model for the disease, adversely affected myosin motor activity despite being in the structural rod domain that directs thick filament assembly. Contractility experiments carried out on isolated mutant muscles, myofibrils, and myofibers identified muscle fatigue and weakness phenotypes, an increased rate of actin-myosin detachment, and a conformational shift of the myosin heads toward the more reactive disordered relaxed (DRX) state, causing hypercontractility and greater ATP consumption. Similarly, molecular analysis of muscle biopsies from patients with MPD1 revealed a significant increase in sarcomeric DRX content, as observed in a subset of myosin motor domain mutations causing hypertrophic cardiomyopathy. Finally, oral administration of MYK-581, a small molecule that decreases the population of heads in the DRX configuration, significantly improved the limited running capacity of the R1500P-transgenic mice and corrected the increased DRX state of the myofibrils from patients. These studies provide evidence of the molecular pathogenesis of proline rod mutations and lay the groundwork for the therapeutic advancement of myosin modulators.

Potential small effector molecules restoring cellular defects due to sialic acid biosynthetic enzyme deficiency: Pathological relevance to GNE myopathy.

GNEM (GNE Myopathy) is a rare neuromuscular disease caused due to biallelic mutations in sialic acid biosynthetic GNE enzyme (UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine Kinase). Recently direct or indirect role of GNE in other cellular functions have been elucidated. Hyposialylation of IGF-1R leads to apoptosis due to mitochondrial dysfunction while hyposialylation of ß1 integrin receptor leads to altered F-actin assembly, disrupted cytoskeletal organization and slow cell migration. Other cellular defects in presence of GNE mutation include altered ER redox state and chaperone expression such as HSP70 or PrdxIV. Currently, there is no cure to treat GNEM. Possible therapeutic trials focus on supplementation with sialic acid, ManNAc, sialyllactose and gene therapy that slows the disease progression. In the present study, we analyzed the effect of small molecules like BGP-15 (HSP70 modulator), IGF-1 (IGF-1R ligand) and CGA (cofilin activator) on cellular phenotypes of GNE heterozygous knock out L6 rat skeletal muscle cell line (SKM­GNEHz). Treatment with BGP-15 improved GNE epimerase activity by 40 % and reduced ER stress by 45 % for SKM­GNEHz. Treatment with IGF-1 improved epimerase activity by 37.5 %, F-actin assembly by 100 %, cell migration upto 36 % (36 h) and atrophy by 0.44-fold for SKM­GNEHz. Treatment with CGA recovered epimerase activity by 49 %, F-actin assembly by 132 % and cell migration upto 41 % (24 h) in SKM­GNEHz. Our study shows that treatment with these small effector molecules reduces the detrimental phenotype observed in SKM­GNEHz, thereby, providing insights into potential therapeutic targets for GNEM.

Pseudoexon activation by deep intronic variation in GNE myopathy with thrombocytopenia.

INTRODUCTION/AIMS: GNE myopathy is a rare autosomal recessive disorder caused by pathogenic variants in the GNE gene, which is essential for the sialic acid biosynthesis pathway. Although over 300 GNE variants have been reported, some patients remain undiagnosed with monoallelic pathogenic variants. This study aims to analyze the entire GNE genomic region to identify novel pathogenic variants. METHODS: Patients with clinically compatible GNE myopathy and monoallelic pathogenic variants in the GNE gene were enrolled. The other GNE pathogenic variant was verified using comprehensive methods including exon 2 quantitative polymerase chain reaction and nanopore long-read single-molecule sequencing (LRS). RESULTS: A deep intronic GNE variant, c.862+870C>T, was identified in nine patients from eight unrelated families. This variant generates a cryptic splice site, resulting in the activation of a novel pseudoexon between exons 5 and 6. It results in the insertion of an extra 146 nucleotides into the messengerRNA (mRNA), which is predicted to result in a truncated humanGNE1(hGNE1) protein. Peanut agglutinin（PNA） lectin staining of muscle tissues showed reduced sialylation of mucin O-glycans on sarcolemmal glycoproteins. Notably, a third of patients with the c.862+870C>T variant exhibited thrombocytopenia. A common core haplotype harboring the deep intronic GNE variant was found in all these patients. DISCUSSION: The transcript with pseudoexon activation potentially affects sialic acid biosynthesis via nonsense-mediated mRNA decay, or resulting in a truncated hGNE1 protein, which interferes with normal enzyme function. LRS is expected to be more frequently incorporated in genetic analysis given its efficacy in detecting hard-to-find pathogenic variants.

Recessive GNE Mutations in Korean Nonaka Distal Myopathy Patients with or without Peripheral Neuropathy.

Autosomal recessive Nonaka distal myopathy is a rare autosomal recessive genetic disease characterized by progressive degeneration of the distal muscles, causing muscle weakness and decreased grip strength. It is primarily associated with mutations in the GNE gene, which encodes a key enzyme of sialic acid biosynthesis (UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase). This study was performed to find GNE mutations in six independent distal myopathy patients with or without peripheral neuropathy using whole-exome sequencing (WES). In silico pathogenic prediction and simulation of 3D structural changes were performed for the mutant GNE proteins. As a result, we identified five pathogenic or likely pathogenic missense variants: c.86T>C (p.Met29Thr), c.527A>T (p.Asp176Val), c.782T>C (p.Met261Thr), c.1714G>C (p.Val572Leu), and c.1771G>A (p.Ala591Thr). Five affected individuals showed compound heterozygous mutations, while only one patient revealed a homozygous mutation. Two patients revealed unreported combinations of combined heterozygous mutations. We observed some specific clinical features, such as complex phenotypes of distal myopathy with distal hereditary peripheral neuropathy, an earlier onset of weakness in legs than that of hands, and clinical heterogeneity between two patients with the same set of compound heterozygous mutations. Our findings on these genetic causes expand the clinical spectrum associated with the GNE mutations and can help prepare therapeutic strategies.

Resilience in patients and family caregivers living with congenital disorders of glycosylation (CDG): a quantitative study using the brief resilience coping scale (BRCS).

BACKGROUND: Patients and family caregivers living with Congenital Disorders of Glycosylation (CDG) experience a heavy burden, which can impact their resiliency and quality of life. The study's purpose was to measure the resilience levels of patients and family caregivers living with CDG using the brief resilience coping scale. METHODS: We conducted an observational, cross-sectional study with 23 patients and 151 family caregivers living with CDG. Descriptive analyses were performed to characterize patients with CDG and family caregivers' samples. Additionally, we assessed correlations between resilience and specific variables (e.g., age, academic degree, time until diagnosis) and examined resilience differences between groups (e.g., sex, marital status, occupation, professional and social support). RESULTS: GNE myopathy was the most prevalent CDG among patients, while in family caregivers was PMM2-CDG. Both samples showed medium levels of resilience coping scores. Individuals with GNE myopathy had significantly higher scores of resilience compared to patients with other CDG. Resilience was positively correlated with educational degree in patients with CDG. Family caregivers had marginally significant higher scores of resilience coping if they received any kind of professional support or had contact with other families or people with the same or similar disease, compared with unsupported individuals. CONCLUSIONS: Despite the inherited difficulties of living with a life-threatening disease like CDG, patients and family caregivers showed medium resilient coping levels. Resilience scores changed significantly considering the CDG genotype, individual's academic degree and professional and social support. These exploratory findings can empower the healthcare system and private institutions by promoting the development of targeted interventions to enhance individuals` coping skills and improve the overall well-being and mental health of the CDG community.

Miyoshi myopathy associated with spine rigidity and multiple contractures: a case report.

BACKGROUND: Dysferlinopathy is a phenotypically heterogeneous group of hereditary diseases caused by mutations in the DYSF gene. Early contractures are considered rare, and rigid spine syndrome in dysferlinopathy has been previously reported only once. CASE PRESENTATION: We describe a 23-year-old patient with Miyoshi myopathy with a rigid spine and multiple contractures, a rare phenotypic variant. The disease first manifested when the patient was 13 years old, with fatigue of the gastrocnemius muscles and the development of pronounced contractures of the Achilles tendons, flexors of the fingers, and extensors of the toes, followed by the involvement of large joints and the spine. Magnetic resonance imaging revealed signs of connective tissue and fatty replacement of the posterior muscles of the thighs and lower legs. Edema was noted in the anterior and medial muscle groups of the thighs, lower legs, and the multifidus muscle of the back. Whole genome sequencing revealed previously described mutations in the DYSF gene in exon 39 (c.4282 C > T) and intron 51 (c.5785-824 C > T). An immunohistochemical analysis and Western blot showed the complete absence of dysferlin protein expression in the muscle fibers. CONCLUSIONS: This case expands the range of clinical and phenotypic correlations of dysferlinopathy and complements the diagnostic search for spine rigidity.

Phenotypic and genotypic analysis of a patient with Miyoshi myopathy caused by truncated protein.

Dysferlin protein deficiency can cause neuromuscular dysfunction, resulting in autosomal recessive dysferlinopathy, which is caused by DYSF gene mutation. Dysferlin proteins belongs to the Ferlin1-like protein family and are associated with muscle membrane repair and regeneration. In China, pathogenic mutations of the protein often result in two clinical phenotypes of Miyoshi muscular or limb band muscular dystrophy type 2B. It is clinically characterized by progressive muscle weakness and elevated serum creatine kinase. The data of the child were collected, blood samples of the child and his family members were collected, and whole exome sequencing (WES) was performed. The recombinant expression vector was constructed, the function of the mutation was verified by minigene, and the pathogenicity of the mutation was further analyzed by combining with biological information analysis. The patient initially presented with asymptomatic elevation of serum creatine kinase（CK）. Then progressive lower limb weakness, mainly distal limb weakness. Large amounts of scattered necrosis, myogenic lesions, and complete deletion of dysferlin protein were observed under muscle biopsy, which further improved genetic detection. Whole exome sequencing showed compound mutations (c.1397 + 1_1397 + 3del and c.1375dup p.M459Nfs*15) in DYSF gene. c.1375dup p.M459Nfs*15 have been reported. The other mutation is the deletion of c.1397 + 1_1397 + 3 in Intron15, which is an intron mutation that may affect splicing and the pathogenesis is still unknown. Minigene splicing assay verified that c.1397 + 1_1397 + 3del resulted in exon15 skipping and produced a premature termination codon. We report a novel pathogenic mutation in DYSF gene with Miyoshi myopathy and demonstrate this variant causes skipping of exon15 by minigene splicing assay. We point out the need of conducting functional analysis to verify the pathogenicity of intronic mutation. The finding enriches the mutation spectrum of DYSF gene and laid a foundation for future studies on the correlation between genotype and phenotype.

Understanding pathophysiology of GNE myopathy and current progress towards drug development.

GNE myopathy is a rare genetic neuromuscular disease that is caused due to mutations in the GNE gene responsible for sialic acid biosynthesis. Foot drop is the most common initial symptom observed in GNE myopathy patients. There is slow progressive muscle weakness in the lower and upper extremities while the quadriceps muscles are usually spared. The exact pathophysiology of the disease is unknown. Besides sialic acid biosynthesis, recent studies suggest either direct or indirect involvement of GNE in other cellular functions such as protein aggregation, apoptosis, ER stress, cell migration, HSP70 chaperone activity, autophagy, muscle atrophy, and myogenesis. Both animal and in vitro cell-based model systems are generated to elucidate the mechanism of GNE myopathy and evaluate the efficacy of therapies. The many therapeutic avenues explored include supplementation with sialic acid derivatives or precursors and gene therapy. Recent studies suggest other therapeutic options such as modulators of HSP70 chaperone (BGP-15), cofilin activator (CGA), and ligands like IGF-1 that may help to rescue cellular defects due to GNE dysfunction. This review provides an overview of the pathophysiology associated with GNE function in the cell and promising therapeutic leads to be explored for future drug development.

Growth deficiency and enhanced basal immunity in Arabidopsis thaliana mutants of EDM2, EDM3 and IBM2 are genetically interlinked.

Mutants of the Arabidopsis thaliana genes, EDM2 (Enhanced Downy Mildew 2), EDM3 (Enhanced Downy Mildew 3) and IBM2 (Increase in Bonsai Methylation 2) are known to show defects in a diverse set of defense and developmental processes. For example, they jointly exhibit enhanced levels of basal defense and stunted growth. Here we show that these two phenotypes are functionally connected by their dependency on the salicylic acid biosynthesis gene SID2 and the basal defense regulatory gene PAD4. Stunted growth of edm2, edm3 and ibm2 plants is a consequence of up-regulated basal defense. Constitutively enhanced activity of reactive oxygen species-generating peroxidases, we observed in these mutants, appears also to contribute to both, their enhanced basal defense and their growth retardation phenotypes. Furthermore, we found the histone H3 demethylase gene IBM1, a direct regulatory target of EDM2, EDM3 and IBM2, to be at least partially required for the basal defense and growth-related effects observed in these mutants. We recently reported that EDM2, EDM3 and IBM2 coordinate basal immunity with the timing of the floral transition by gradually reducing the extent of this defense mechanism prior to flowering. Together with these observations, data presented here show that at least some of the diverse phenotypic effects in edm2, edm3 and ibm2 mutants are genetically interlinked and functionally connected. Our new results show that repression of basal immunity by EDM2, EDM3 and IBM2 limits negative impact on growth and development.

GNE deficiency impairs Myogenesis in C2C12 cells and cannot be rescued by ManNAc supplementation.

GNE myopathy (GNEM) is a late-onset muscle atrophy, caused by mutations in the gene for the key enzyme of sialic acid biosynthesis, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). With an incidence of one to nine cases per million it is an ultra-rare, so far untreatable, autosomal recessive disease. Several attempts have been made to treat GNEM patients by oral supplementation with sialic acid precursors (e.g. N-acetylmannosamine, ManNAc) to restore sarcolemmal sialylation and muscle strength. In most studies, however, no significant improvement was observed. The lack of a suitable mouse model makes it difficult to understand the exact pathomechanism of GNEM and many years of research have failed to identify the role of GNE in skeletal muscle due to the lack of appropriate tools. We established a CRISPR/Cas9-mediated Gne-knockout cell line using murine C2C12 cells to gain insight into the actual role of the GNE enzyme and sialylation in a muscular context. The main aspect of this study was to evaluate the therapeutic potential of ManNAc and N-acetylneuraminic acid (Neu5Ac). Treatment of Gne-deficient C2C12 cells with Neu5Ac, but not with ManNAc, showed a restoration of the sialylation level back to wild type levels-albeit only with long-term treatment, which could explain the rather low therapeutic potential. We furthermore highlight the importance of sialic acids on myogenesis, for C2C12 Gne-knockout myoblasts lack the ability to differentiate into mature myotubes.

Titin copy number variations associated with dominant inherited phenotypes.

BACKGROUND: Titinopathies are caused by mutations in the titin gene (TTN). Titin is the largest known human protein; its gene has the longest coding phase with 364 exons. Titinopathies are very complex neuromuscular pathologies due to the variable age of onset of symptoms, the great diversity of pathological and muscular impairment patterns (cardiac, skeletal muscle or mixed) and both autosomal dominant and recessive modes of transmission. Until now, only few CNVs in TTN have been reported without clear genotype-phenotype associations. METHODS: Our study includes eight families with dominant titinopathies. We performed next-generation sequencing or comparative genomic hybridisation array analyses and found CNVs in the TTN gene. We characterised these CNVs by RNA sequencing (RNAseq) analyses in six patients' muscles and performed genotype-phenotype inheritance association study by combining the clinical and biological data of these eight families. RESULTS: Seven deletion-type CNVs in the TTN gene were identified among these families. Genotype and RNAseq results showed that five deletions do not alter the reading frame and one is out-of-reading frame. The main phenotype identified was distal myopathy associated with contractures. The analysis of morphological, clinical and genetic data and imaging let us draw new genotype-phenotype associations of titinopathies. CONCLUSION: Identifying TTN CNVs will further increase diagnostic sensitivity in these complex neuromuscular pathologies. Our cohort of patients enabled us to identify new deletion-type CNVs in the TTN gene, with unexpected autosomal dominant transmission. This is valuable in establishing new genotype-phenotype associations of titinopathies, mainly distal myopathy in most of the patients.

Evaluation of N-Acetylmannosamine Administration to Restore Sialylation in GNE-Deficient Human Embryonal Kidney Cells.

BACKGROUND: A key mechanism in the neuromuscular disease GNE myopathy (GNEM) is believed to be that point mutations in the GNE gene impair sialic acid synthesis - maybe due to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) activity restrictions - and resulting in muscle tissue loss. N-acetylmannosamine (ManNAc) is the first product of the bifunctional GNE enzyme and can therefore be regarded as a precursor of sialic acids. This study investigates whether this is also a suitable substance for restoring the sialic acid content in GNE-deficient cells. METHODS: A HEK-293 GNE-knockout cell line was generated using CRISPR-Cas9 and analyzed for its ability to synthesize sialic acids. The cells were then supplemented with ManNAc to compensate for possible GNE inactivity and thereby restore sialic acid synthesis. Sialic acid levels were monitored by immunoblot and high performance liquid chromatography (HPLC). RESULTS: The HEK-293 GNE-knockout cells showed almost no polysialylation signal (immunoblot) and a reduced overall (-71%) N-acetylneuraminic acid (Neu5Ac) level (HPLC) relative to total protein and normalized to wild type level. Supplementation of GNE-deficient HEK-293 cells with 2 mM ManNAc can restore polysialylation and free intracellular sialic acid levels to wild type levels. The addition of 1 mM ManNAc is sufficient to restore the membrane-bound sialic acid level. CONCLUSIONS: Although the mechanism behind this needs further investigation and although it remains unclear why adding ManNAc to GNE-deficient cells is sufficient to elevate polysialylation back to wild type levels - since this substance is also converted by the GNE, all of this might yet prove helpful in the development of an appropriate therapy for GNEM.

Distal myopathy due to digenic inheritance of TIA1 and SQSTM1 variants in two unrelated Spanish patients.

Welander distal myopathy typically manifests in late adulthood and is caused by the founder TIA1 c.1150G>A (p.Glu384Lys) variant in families of Swedish and Finnish descent. Recently, a similar phenotype has been attributed to the digenic inheritance of TIA1 c.1070A>G (p.Asn357Ser) and SQSTM1 c.1175C>T (p.Pro392Leu) variants. We describe two unrelated Spanish patients presenting with slowly progressive gait disturbance, distal-predominant weakness, and mildly elevated creatine kinase (CK) levels since their 6th decade. Electromyography revealed abnormal spontaneous activity and a myopathic pattern. Muscle magnetic resonance imaging (MRI) showed marked fatty replacement in distal leg muscles. A muscle biopsy, performed on one patient, revealed myopathic changes with rimmed vacuoles. Both patients carried the TIA1 p.Asn357Ser and SQSTM1 p.Pro392Leu variants. Digenic inheritance is supported by evidence from unrelated pedigrees and a plausible biological interaction between both proteins in protein quality control processes. Recent functional studies and additional case descriptions further support this. Clinical suspicion is necessary to seek both variants.

[Efficacy of Aceneuramic Acid for Distal Myopathy with Rimmed Vacuoles].

Distal myopathy with rimmed vacuoles (DMRV), also known as GNE myopathy, is a rare disease affecting the distal muscles, such as the tibialis anterior muscle. The GNE gene, which codes for a key enzyme in the sialic acid biosynthesis pathway, is mutated in a homozygous or compound heterozygous manner, and the lack of sialic acid in skeletal muscle is the critical underlying mechanism in DMRV pathogenesis. DMRV mouse models were established, and supplementation with sialic acid improved the phenotypes of the models. A phase 1 clinical trial using aceneuramic acid was conducted at Tohoku University Hospital, Japan, followed by trials using a slow-release product. A phase II/III study, subsequent extended trial, and confirmatory trial were also conducted. Regulatory approval is currently under review.