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.

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.

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.

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.

Pharmacokinetics and clinical efficacy of 6'-sialyllactose in patients with GNE myopathy: Randomized pilot trial.

GNE myopathy, caused by biallelic mutations in the GNE gene, is characterized by initial ankle dorsiflexor weakness and rimmed vacuoles in the muscle histopathology, resulting in reduced sialic acid production. Sialyllactose is a source of sialic acid. We performed a pilot clinical trial to analyze the pharmacokinetic properties of 6'-sialyllactose (6SL) and evaluated the safety, and efficacy of oral 6SL in patients with GNE myopathy. Ten participants were in the pharmacokinetic study, and 20 in the subsequent clinical trial. For the pharmacokinetic study, participants were administered either 3 g (low-dose) or 6 g (high-dose) of 6SL in a single dose. Plasma concentrations of 6SL, sialic acid, and sialic acid levels on the surface of red blood cells were periodically assessed in blood samples. Patients were randomly allocated to test (low- and high-dose groups) or placebo groups for the trial. Motor function, ambulation, plasma 6SL and sialic acid concentrations, GNE myopathy-functional activity scale scores, and MRI findings were assessed. 6SL was well tolerated, except for self-limited gastrointestinal discomfort. Free sialic acid in both low- and high-dose groups significantly increased at 6 and 12 weeks, but not in the placebo group. In the high-dose group, proximal limb powers improved with daily 6SL. Considering the fat fraction on muscle MRI, results in the high-dose group were superior to those in the low-dose group. 6SL may be a good candidate for GNE myopathy therapeutics as it induces an increase or reduces the decrease in limb muscle power, attenuates muscle degeneration, and improves the biochemical properties of sialic acid.

[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.

GNE myopathy: can homozygous asymptomatic subjects give a clue for the identification of protective factors?

GNE myopathy is caused by bi allelic recessive mutations in the GNE gene. The largest identified cohort of GNE myopathy patients carries a homozygous mutation- M743T (the "Middle Eastern" mutation). More than 160 such patients in 67 families have been identified by us. Mean onset in this cohort is 30 years (range 17-48) with variable disease severity. However, we have identified two asymptomatic females, homozygous for M743T in two different families, both with affected siblings. The first showed no myopathy when examined at age 76 years. The second has no sign of disease at age 60 years. Since both agreed only for testing of blood, we performed exome and RNA sequencing of their blood and that of their affected siblings. Various filtering layers resulted in 2723 variant loci between symptomatic and asymptomatic individuals, representing 1364 genes. Among those, 39 genes are known to be involved in neuromuscular diseases, and only in two of them the variant is located in the proper exon coding region, resulting in a missense change. Surprisingly, only 27 genes were significantly differentially expressed between the asymptomatic and the GNE myopathy affected individuals, with three overexpressed genes overlapping between exome and RNA sequencing. Although unable to unravel robust candidate genes, mostly because of the very low number of asymptomatic individuals analyzed, and because of the tissue analyzed (blood and not muscle), this study resulted in relatively restricted potential candidate protective genes, emphasizing the power of using polarized phenotypes (completely asymptomatic vs clearly affected individuals) with the same genotype to unmask those genes which could be used as targets for disease course modifiers.

Dysferlinopathy in Tunisia: clinical spectrum, genetic background and prognostic profile.

Dysferlinopathy is a rare group of hereditary muscular dystrophy with an autosomal recessive mode of inheritance caused by a mutation in the DYSF gene. It encodes for the dysferlin protein, which has a crucial role in multiple cellular processes, including muscle fiber membrane repair. This deficit has heterogeneous clinical presentations. In this study, we collected 20 Tunisian patients with a sex ratio of 1 and a median age of 50.5 years old (Interquartile range (IQR) = [36,5-54,75]). They were followed for periods ranging from 5 to 48 years. The median age at onset was 17 years old (IQR = [16,8-28,4]). Five major phenotypes were identified: Limb-girdle muscular dystrophy (LGMDR2) (35%), a proximodistal phenotype (35%), Miyoshi myopathy (10%),  Distal myopathy with anterior tibial onset (DMAT) (10%), and asymptomatic HyperCKemia (10%). At the last evaluation, more than half of patients (55%) were on wheelchair. Loss of ambulation occurred generally during the fourth decade. After 20 years of disease progression, two patients with a proximodistal phenotype (10%) developed dilated cardiomyopathy and mitral valve regurgitation. Restrictive respiratory syndrome was observed in three patients (DMAT: 1 patient, proximodistal phenotype: 1 patient, LGMDR2: 1 patient). Genetic study disclosed five mutations. We observed clinical heterogeneity between families and even within the same family. Disease progression was mainly slow to intermediate regardless of the phenotype.

Efficacy confirmation study of aceneuramic acid administration for GNE myopathy in Japan.

BACKGROUND: A rare muscle disease, GNE myopathy is caused by mutations in the GNE gene involved in sialic acid biosynthesis. Our recent phase II/III study has indicated that oral administration of aceneuramic acid to patients slows disease progression. METHODS: We conducted a phase III, randomized, placebo-controlled, double-blind, parallel-group, multicenter study. Participants were assigned to receive an extended-release formulation of aceneuramic acid (SA-ER) or placebo. Changes in muscle strength and function over 48 weeks were compared between treatment groups using change in the upper extremity composite (UEC) score from baseline to Week 48 as the primary endpoint and the investigator-assessed efficacy rate as the key secondary endpoint. For safety, adverse events, vital signs, body weight, electrocardiogram, and clinical laboratory results were monitored. RESULTS: A total of 14 patients were enrolled and given SA-ER (n = 10) or placebo (n = 4) tablets orally. Decrease in least square mean (LSM) change in UEC score at Week 48 with SA-ER (- 0.115 kg) was numerically smaller as compared with placebo (- 2.625 kg), with LSM difference (95% confidence interval) of 2.510 (- 1.720 to 6.740) kg. In addition, efficacy was higher with SA-ER as compared with placebo. No clinically significant adverse events or other safety concerns were observed. CONCLUSIONS: The present study reproducibly showed a trend towards slowing of loss of muscle strength and function with orally administered SA-ER, indicating supplementation with sialic acid might be a promising replacement therapy for GNE myopathy. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov (NCT04671472).

Distal myopathy.

Distal myopathies are a group of genetic, primary muscle diseases. Patients develop progressive weakness and atrophy of the muscles of forearm, hands, lower leg, or feet. Currently, over 20 different forms, presenting a variable age of onset, clinical presentation, disease progression, muscle involvement, and histological findings, are known. Some of them are dominant and some recessive. Different variants in the same gene are often associated with either dominant or recessive forms, although there is a lack of a comprehensive understanding of the genotype-phenotype correlations. This chapter provides a description of the clinicopathologic and genetic aspects of distal myopathies emphasizing known etiologic and pathophysiologic mechanisms.

Hypertrophic Cardiomyopathy Complicated by Post-COVID-19 Myopericarditis in Patient with ANO5-Related Distal Myopathy.

A 60-year-old male with hypertrophic cardiomyopathy, conduction disorders, post-COVID-19 myopericarditis and heart failure was admitted to the hospital's cardiology department. Blood tests revealed an increase in CPK activity, troponin T elevation and high titers of anticardiac antibodies. Whole exome sequencing showed the presence of the pathogenic variant NM_213599:c.2272C>T of the ANO5 gene. Results of the skeletal muscle biopsy excluded the diagnosis of systemic amyloidosis. Microscopy of the muscle fragment demonstrated sclerosis of the perimysium, moderate lymphoid infiltration, sclerosis of the microvessels, dystrophic changes and a lack of cross striations in the muscle fibers. Hypertrophy of the LV with a low contractile ability, atrial fibrillation, weakness of the distal skeletal muscles and increased plasma CPK activity and the results of the skeletal muscle biopsy suggested a diagnosis of a late form of distal myopathy (Miyoshi-like distal myopathy, MMD3). Post-COVID-19 myopericarditis, for which genetically modified myocardium could serve as a favorable background, caused heart failure decompensation.

Phase II/III Study of Aceneuramic Acid Administration for GNE Myopathy in Japan.

BACKGROUND: GNE myopathy is an ultra-rare muscle disease characterized by a reduction in the synthesis of sialic acid derived from pathogenic variants in the GNE gene. No treatment has been established so far. OBJECTIVE: We evaluated the safety and efficacy of oral supplementation of aceneuramic acid in patients with GNE myopathy. METHODS: This multicenter, placebo-controlled, double-blind study comprised genetically confirmed GNE myopathy patients in Japan who were randomly assigned into treatment groups of sialic acid-extended release (SA-ER) tablets (6 g/day for 48 weeks) or placebo groups (4:1). The primary endpoint of effectiveness was set as the change in total upper limb muscle strength (upper extremity composite [UEC] score) from the start of administration to the final evaluation time point. RESULTS: Among the 20 enrolled patients (SA-ER group, 16; placebo group, 4), 19 completed this 48-week study. The mean value of change in UEC score (95% confidence interval [CI]) at 48 weeks was -0.1 kg (-2.1 to 2.0) in the SA-ER group and -5.1 kg (-10.4 to 0.3) in the placebo group. The least squares mean difference (95% CI) between the groups in the covariance analysis was 4.8 kg (-0.3 to 9.9; P = 0.0635). The change in UEC score at 48 weeks was significantly higher in the SA-ER group compared with the placebo group (P = 0.0013) in the generalized estimating equation test repeated measurement analysis. In one patient in the SA-ER group, who was found to be pregnant 2 weeks after drug administration fetal death with tangled umbilical cord occurred at 13 weeks after the discontinuation of treatment. No other serious adverse effects were observed. CONCLUSIONS: The present study indicates that oral administration of SA-ER tablets is effective and safe in patients with GNE myopathy in Japan.

Long-term favorable prognosis in late onset dominant distal titinopathy: Tibial muscular dystrophy.

BACKGROUND AND PURPOSE: Tibial muscular dystrophy (TMD) is a dominant late onset distal titinopathy. It was first described in Finnish patients 3 decades ago. TMD patients with several other TTN mutations occur in many European populations. In this retrospective study, we were able to obtain longitudinal follow-up data of the disease progression over 15 years in 137 TMD patients. METHODS: We retrieved clinical data retrospectively from three examinations spanning a period of 15 years. The data were analyzed in R. Frequencies, percentages, and median values were used to describe data. Probability values were determined with the chi-squared test. RESULTS: In the cohort, the first symptoms were walking difficulties (97.8%) and weakness in distal lower limbs (98.5%). The progression of the weakness in distal lower limbs was moderate, and in the proximal lower limbs and proximal upper limbs it was mild. The distal upper limbs were not affected. Magnetic resonance imaging results indicated fatty degeneration preferentially in lower leg anterior muscles, gluteus minimus, and hamstring muscles. Serum creatine kinase values in the cohort were mostly normal (40.7%) or mildly elevated (53.7%). The data suggest that 50% of patients need walking aids by the age of 88 years. CONCLUSIONS: Despite individual variability of severity, the overall disability due to walking difficulties and upper limb weakness remained moderate even at very advanced ages, and cardiomyopathy did not develop due to the titin defect alone. The acquired results promote the correct identification of TMD, and the obtained trajectories of disease evolution can be used as natural history data for any therapeutic intervention.

Morpholino-Mediated Exons 28-29 Skipping of Dysferlin and Characterization of Multiexon-skipped Dysferlin using RT-PCR, Immunoblotting, and Membrane Wounding Assay.

Dysferlinopathies are a group of disabling muscular dystrophies  that includes limb girdle muscular dystrophy type 2B (LGMD2B), Miyoshi myopathy, and distal myopathy with anterior tibial onset (DMAT) as the main phenotypes. They are associated with molecular defects in DYSF, which encodes dysferlin, a key player in sarcolemmal homeostasis. Previous investigations have suggested that exon skipping may be a promising therapy for many patients with dysferlinopathies. It was reported that exons 28-29 of DYSF are dispensable for dysferlin functions. Here, we present a method for multiexon skipping of DYSF exons 28-29 using a cocktail of two phosphorodiamidate morpholino oligomers (PMOs) on cells derived from a dystrophinopathy patient. Also, we describe assays to characterize the multiexon skipped dysferlin at several levels by using one-step RT-PCR, immunoblotting, and a membrane wounding assay.

Multidimensional analyses of the pathomechanism caused by the non-catalytic GNE variant, c.620A>T, in patients with GNE myopathy.

GNE myopathy is a distal myopathy caused by biallelic variants in GNE, which encodes a protein involved in sialic acid biosynthesis. Compound heterozygosity of the second most frequent variant among Japanese GNE myopathy patients, GNE c.620A>T encoding p.D207V, occurs in the expected number of patients; however, homozygotes for this variant are rare; three patients identified while 238 homozygotes are estimated to exist in Japan. The aim of this study was to elucidate the pathomechanism caused by c.620A>T. Identity-by-descent mapping indicated two distinct c.620A>T haplotypes, which were not correlated with age onset or development of myopathy. Patients homozygous for c.620A>T had mildly decreased sialylation, and no additional pathogenic variants in GNE or abnormalities in transcript structure or expression of other genes related to sialic acid biosynthesis in skeletal muscle. Structural modeling of full-length GNE dimers revealed that the variant amino acid localized close to the monomer interface, but far from catalytic sites, suggesting functions in enzymatic product transfer between the epimerase and kinase domains on GNE oligomerization. In conclusion, homozygotes for c.620A>T rarely develop myopathy, while symptoms occur in compound heterozygotes, probably because of mildly decreased sialylation, due to partial defects in oligomerization and product trafficking by the mutated GNE protein.