Safety, pharmacokinetics and sialic acid production after oral administration of N-acetylmannosamine (ManNAc) to subjects with GNE myopathy

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 β1 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 (SKM‑GNEHz). Treatment with BGP-15 improved GNE epimerase activity by 40 % and reduced ER stress by 45 % for SKM‑GNEHz. 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 SKM‑GNEHz. Treatment with CGA recovered epimerase activity by 49 %, F-actin assembly by 132 % and cell migration upto 41 % (24 h) in SKM‑GNEHz. Our study shows that treatment with these small effector molecules reduces the detrimental phenotype observed in SKM‑GNEHz, thereby, providing insights into potential therapeutic targets for GNEM.

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.

Novel GNE missense variants impair de novo sialylation and cause defective angiogenesis in the developing brain in mice

ABSTRACT

Glucosamine (UDP-N-acetyl)-2-epimerase and N-acetylmannosamine (ManNAc) kinase (GNE) is a cytosolic enzyme in de novo sialic acid biosynthesis. Congenital deficiency of GNE causes an autosomal recessive genetic disorder associated with hereditary inclusion body myopathy and macrothrombocytopenia. Here, we report a pediatric patient with severe macrothrombocytopenia carrying 2 novel GNE missense variants, c.1781G>A (p.Cys594Tyr, hereafter, C594Y) and c.2204C>G (p.Pro735Arg, hereafter, P735R). To investigate the biological significance of these variants in vivo, we generated a mouse model carrying the P735R mutation. Mice with homozygous P735R mutations exhibited cerebral hemorrhages as early as embryonic day 11 (E11), which subsequently progressed to large hemorrhages in the brain and spinal cord, and died between E11.5 and E12.5. Defective angiogenesis such as distended vascular sprouts were found in neural tissues and embryonic megakaryocytes were abnormally accumulated in the perineural vascular plexus in mutant mouse embryos. Furthermore, our in vitro experiments indicated that both C594Y and P735R are loss-of-function mutations with respect to de novo sialic acid biosynthesis. Overall, this study reveals a novel role for GNE-mediated de novo sialic acid biosynthesis in mouse embryonic angiogenesis.

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.

Oral sialic acid supplementation in NANS-CDG: Results of a single center, open-label, observational pilot study

NANS-CDG is a congenital disorder of glycosylation (CDG) caused by biallelic variants in NANS, encoding an essential enzyme in de novo sialic acid synthesis. It presents with intellectual developmental disorder (IDD), skeletal dysplasia, neurologic impairment, and gastrointestinal dysfunction. Some patients suffer progressive intellectual neurologic deterioration (PIND), emphasizing the need for a therapy. In a previous study, sialic acid supplementation in knockout nansa zebrafish partially rescued skeletal abnormalities. Here, we performed the first in-human pre- and postnatal sialic-acid study in NANS-CDG. In this open-label observational study, 5 patients with NANS-CDG (range 0-28 years) were treated with oral sialic acid for 15 months. The primary outcome was safety. Secondary outcomes were psychomotor/cognitive testing, height and weight, seizure control, bone health, gastrointestinal symptoms, and biochemical and hematological parameters. Sialic acid was well tolerated. In postnatally treated patients, there was no significant improvement. For the prenatally treated patient, psychomotor and neurologic development was better than two other genotypically identical patients (one treated postnatally, one untreated). The effect of sialic acid treatment may depend on the timing, with prenatal treatment potentially benefiting neurodevelopmental outcomes. Evidence is limited, however, and longer-term follow-up in a larger number of prenatally treated patients is required.

N-acetylneuraminate pyruvate lyase controls sialylation of muscle glycoproteins essential for muscle regeneration and function

Deleterious variants in N-acetylneuraminate pyruvate lyase (NPL) cause skeletal myopathy and cardiac edema in humans and zebrafish, but its physiological role remains unknown. We report generation of mouse models of the disease: NplR63C, carrying the human p.Arg63Cys variant, and Npldel116 with a 116-bp exonic deletion. In both strains, NPL deficiency causes drastic increase in free sialic acid levels, reduction of skeletal muscle force and endurance, slower healing and smaller size of newly formed myofibers after cardiotoxin-induced muscle injury, increased glycolysis, partially impaired mitochondrial function, and aberrant sialylation of dystroglycan and mitochondrial LRP130 protein. NPL-catalyzed degradation of sialic acid in the muscle increases after fasting and injury and in human patient and mouse models with genetic muscle dystrophy, demonstrating that NPL is essential for muscle function and regeneration and serves as a general marker of muscle damage. Oral administration of N-acetylmannosamine rescues skeletal myopathy, as well as mitochondrial and structural abnormalities in NplR63C mice, suggesting a potential treatment for human patients.

Development of Assays to Measure GNE Gene Potency and Gene Replacement in Skeletal Muscle

BACKGROUND

GNE myopathy (GNEM) is a severe muscle disease caused by mutations in the UDP-GlcNAc-2-epimerase/ManNAc-6-kinase (GNE) gene, which encodes a bifunctional enzyme required for sialic acid (Sia) biosynthesis.

OBJECTIVE

To develop assays to demonstrate the potency of AAV gene therapy vectors in making Sia and to define the dose required for replacement of endogenous mouse Gne gene expression with human GNE in skeletal muscles.

METHODS

A MyoD-inducible Gne-deficient cell line, Lec3MyoDI, and a GNE-deficient human muscle cell line, were made and tested to define the potency of various AAV vectors to increase binding of Sia-specific lectins, including MAA and SNA. qPCR and qRT-PCR methods were used to quantify AAV biodistribution and GNE gene expression after intravenous delivery of AAV vectors designed with different promoters in wild-type mice.

RESULTS

Lec3 cells showed a strong deficit in MAA binding, while GNE-/-MB135 cells did not. Overexpressing GNE in Lec3 and Lec3MyoDI cells by AAV infection stimulated MAA binding in a dose-dependent manner. Use of a constitutive promoter, CMV, showed higher induction of MAA binding than use of muscle-specific promoters (MCK, MHCK7). rAAVrh74.CMV.GNE stimulated human GNE expression in muscles at levels equivalent to endogenous mouse Gne at a dose of 1×1013vg/kg, while AAVs with muscle-specific promoters required higher doses. AAV biodistribution in skeletal muscles trended higher when CMV was used as the promoter, and this correlated with increased sialylation of its viral capsid.

CONCLUSIONS

Lec3 and Lec3MyoDI cells work well to assay the potency of AAV vectors in making Sia. Systemic delivery of rAAVrh74.CMV.GNE can deliver GNE gene replacement to skeletal muscles at doses that do not overwhelm non-muscle tissues, suggesting that AAV vectors that drive constitutive organ expression could be used to treat GNEM.

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.

Diet-Related and Gut-Derived Metabolites and Health Outcomes: A Scoping Review

We conducted a scoping review to map available evidence about the health impact of gut microbiota-derived metabolites. We searched PubMed and Embase for studies that assessed the health impact of ten metabolites on any health condition: deoxycholate or deoxycholic acid (DCA), lithocholate or lithocholic acid (LCA), glycolithocholate or glycolithocholic acid, glycodeoxycholate or glycodeoxycholic acid, tryptamine, putrescine, d-alanine, urolithins, N-acetylmannosamine, and phenylacetylglutamine. We identified 352 eligible studies with 168,072 participants. Most (326, 92.6%) were case-control studies, followed by cohort studies (14, 4.0%), clinical trials (8, 2.3%), and cross-sectional studies (6, 1.7%). Most studies assessed the following associations: DCA on hepatobiliary disorders (64 studies, 7976 participants), colorectal cancer (19 studies, 7461 participants), and other digestive disorders (27 studies, 2463 participants); LCA on hepatobiliary disorders (34 studies, 4297 participants), colorectal cancers (14 studies, 4955 participants), and other digestive disorders (26 studies, 2117 participants); putrescine on colorectal cancers (16 studies, 94,399 participants) and cancers excluding colorectal and hepatobiliary cancers (42 studies, 4250 participants). There is a need to conduct more prospective studies, including clinical trials. Moreover, we identified metabolites and conditions for which systemic reviews are warranted to characterize the direction and magnitude of metabolite-disease associations.

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.

Recent advances in establishing a cure for GNE myopathy

PURPOSE OF REVIEW

GNE myopathy is a rare autosomal recessive disease caused by biallelic variants in the GNE gene, which encodes an enzyme involved in sialic acid biosynthesis. No drugs are approved for the treatment of GNE myopathy. Following proof-of-concept of sialic acid supplementation efficacy in mouse models, multiple clinical trials have been conducted. Here, we review clinical trials of sialic acid supplementation therapies and provide new insights into the additional clinical features of GNE myopathy.

RECENT FINDINGS

Clinical trials of sialic acid supplementation have been conducted in Europe, the USA, Japan, and South Korea. Some clinical trials of NeuAc-extended release tablets demonstrated amelioration of decline in upper extremity muscle strength; however, no significant improvement was observed in phase 3 trials in Europe and USA. A phase 2 trial of ManNAc showed slowed decline of both upper and lower extremity strength. GNE myopathy patient registries have been established in Europe and Japan, and have provided information on extramuscular manifestations such as thrombocytopenia, respiratory dysfunction, and sleep apnea syndrome. Sensitive and reliable biomarkers, and a disease-specific functional activity scale, have also been investigated.

SUMMARY

We discuss recent advances in establishing a GNE myopathy cure, and discuss other prospective therapeutic options, including gene therapy.

The role of amyloid β in the pathological mechanism of GNE myopathy

GNE myopathy is a hereditary muscle disorder characterized by muscle atrophy and weakness initially involving the lower distal extremities. The treatment of GNE myopathy mainly focuses on a sialic acid deficiency caused by a mutation in the GNE gene, but it has not achieved the expected effect. The main pathological features of GNE myopathy are myofiber atrophy and rimmed vacuoles, including accumulation of amyloid β, which is mainly found in atrophic muscle fibers. Although the role of amyloid β and other misfolded proteins on the nervous system has been widely recognized, the cause and process of the formation of amyloid β in the pathological process of GNE myopathy are unclear. In addition, amyloid β has been reported to be linked to quality control mechanisms of proteins, such as molecular chaperones, the ubiquitin–proteasome system, and the autophagy-lysosome system. Herein, we summarize the possible reasons for amyloid β deposition and illustrate amyloid β-mediated events in the cells and their role in muscle atrophy in GNE myopathy. This review represents an overview of amyloid β and GNE myopathy that could help identify a potential mechanism and thereby a plausible therapeutic for the disease.

Novel compound heterozygous mutations in a GNE myopathy with congenital thrombocytopenia: A case report and literature review

We reported a GNE myopathy with congenital thrombocytopenia on a young male patient. He presented with a 3‐year history of lower distal extremity weakness initially affecting his legs. The weakness slowly progressed to lower proximal legs and upper arms last 6 months. Whole‐exome sequencing revealed that the patient harbored two heterozygous gene mutations, including a novel insertion mutation c.*1037_*1038CACACACACACACACACACACA and c.C478T in exome 12 and 3 of the GNE gene (NM_001128227), respectively. The levels of serum sialic acid in this patient were considerably decreased. Muscle MRI imaging showed the anterior and medial parts of his quadriceps were heavily affected by this disease. Hematoxylin and eosin staining showed prominent rimmed vacuoles with a lack of inflammatory response in the atrophied muscle. We also undertook a review of the current literature, searching for reports in which the GNE gene mutation caused the thrombocytopenia with or without muscle weakness. This new gene mutation finding broadens the GNE disease genotype spectrum, and further investigation of the relationship between GNE gene mutations and the heterogeneity of its clinical manifestations is needed.

Human apolipoprotein E isoforms are differentially sialylated and the sialic acid moiety in ApoE2 attenuates ApoE2-Aβ interaction and Aβ fibrillation

The APOE genotype is the most prominent genetic risk factor for the development of late-onset Alzheimer''s disease (LOAD); however, the underlying mechanisms remain unclear. In the present study, we found that the sialylation profiles of ApoE protein in the human brain are significantly different among the three isoforms, with ApoE2 exhibiting the most abundant sialic acid modification whereas ApoE4 had the least. We further observed that the sialic acid moiety in ApoE2 significantly affected the interaction between ApoE2 and Aβ peptides. The removal of sialic acid in ApoE2 increased the ApoE2 binding affinity for the Aβ17-24 region of Aβ and promoted Aβ fibrillation. These findings provide a plausible explanation for the well-documented differential roles of ApoE isoforms in Aβ pathogenesis. Specifically, compared to the other two isotypes, the higher expression of sialic acid in ApoE2 may contribute to the less potent interaction between ApoE2 and Aβ and ultimately the slower rate of brain Aβ deposition, a mechanism thought to underlie ApoE2-mediated decreased risk for AD. Future studies are warranted to determine whether the differential sialylation in ApoE isoforms may also contribute to some of their other distinct properties, such as their divergent preferences in associations with lipids and lipoproteins, as well as their potential impact on neuroinflammation through modulation of microglial Siglec activity. Overall, our findings lead to the insight that the sialic acid structure is an important posttranslational modification (PTM) that alters ApoE protein functions with relevance for AD.

Visualizing Muscle Sialic Acid Expression in the GNED207VTgGne-/- Cmah-/- Model of GNE Myopathy: A Comparison of Dietary and Gene Therapy Approaches

BACKGROUND

GNE myopathy (GNEM) is a rare, adult-onset, inclusion body myopathy that results from partial loss of function mutations in the GNE gene. GNE encodes UDP-GlcNAc epimerase/Mannose-6 kinase, a protein with two enzymatic activities that comprise the committed step in biosynthesis of sialic acid (SA), an essential glycan that appears on the terminal positions of many extracellular oligosaccharide chains. These GNE mutations can cause a reduction of SA in many tissues, although pathology is restricted to skeletal muscles through a poorly understood mechanism.

OBJECTIVE

Despite recent advances in the field, it remains unclear which therapeutic avenue is most promising for the restoration of SA level in skeletal muscle affected by GNEM. Our objective was to assess dietary and gene therapy strategies for GNEM in Cmah-deficient GNED207VTgGne-/- mice, a model that allows for the visualization of orally delivered N-glycolylneuraminic acid (Neu5Gc), one of the two predominant SA forms in muscle.

METHODS

Methods included in situ physiology studies of the tibialis anterior muscle, studies of ambulation and limb grip strength, and muscle staining using MAA, SNA, and anti-Neu5Gc antibody, along with qPCR, qRT-PCR, western blot, and HPLC studies to assess virally introduced DNA, GNE gene expression, GNE protein expression, and SA expression.

RESULTS

We found that a diet enriched in Neu5Gc-containing glycoproteins had no impact on Neu5Gc immunostaining in muscles of GNEM model mice. Delivery of a single high dose oral Neu5Gc therapy, however, did increase Neu5Gc immunostaining, though to levels below those found in wild type mice. Delivery of a single dose of GNE gene therapy using a recombinant Adeno Associated Virus (rAAV) vector with a liver-specific or a muscle-specific promoter both caused increased muscle Neu5Gc immunostaining that exceeded that seen with single dose monosaccharide therapy.

CONCLUSIONS

Our findings indicate that dietary loading of Neu5Gc-containing glycoproteins is not effective in increasing muscle Neu5Gc expression, while single dose oral Neu5Gc monosaccharide or GNE gene therapy are. Neu5Gc immunostaining, however, showed greater changes than did lectin staining or HPLC analysis. Taken together, these results suggest that Neu5Gc immunostaining may be more sensitive technique to follow SA expression than other more commonly used methods and that liver expression of GNE may contribute overall muscle SA content.

The fate of orally administered sialic acid: First insights from patients with N-acetylneuraminic acid synthase deficiency and control subjects

BACKGROUND

In NANS deficiency, biallelic mutations in the N-acetylneuraminic acid synthase (NANS) gene impair the endogenous synthesis of sialic acid (N-acetylneuraminic acid) leading to accumulation of the precursor, N-acetyl mannosamine (ManNAc), and to a multisystemic disorder with intellectual disability. The aim of this study was to determine whether sialic acid supplementation might be a therapeutic avenue for NANS-deficient patients.

METHODS

Four adults and two children with NANS deficiency and four adult controls received oral NeuNAc acid (150 mg/kg/d) over three days. Total NeuNAc, free NeuNAc and ManNAc were analyzed in plasma and urine at different time points.

RESULTS

Upon NeuNAc administration, plasma free NeuNAc increased within hours (P < 0.001) in control and in NANS-deficient individuals. Total and free NeuNAc concentrations also increased in the urine as soon as 6 h after beginning of oral administration in both groups. NeuNAc did not affect plasma and urinary ManNAc, that remained higher in NANS deficient subjects than in controls (day 1-3; all P < 0.01). Oral NeuNAc was well tolerated with no significant side effects.

DISCUSSION

Orally administered free NeuNAc was rapidly absorbed but also rapidly excreted in the urine. It did not change ManNAc levels in either patients or controls, indicating that it may not achieve enough feedback inhibition to reduce ManNAc accumulation in NANS-deficient subjects. Within the limitations of this study these results do not support a potential for oral free NeuNAc in the treatment of NANS deficiency but they provide a basis for further therapeutic approaches in this condition.

Safety and efficacy of N-acetylmannosamine (ManNAc) in patients with GNE myopathy: an open-label phase 2 study

Purpose

To evaluate the safety and efficacy of N-acetylmannosamine (ManNAc) in GNE myopathy, a genetic muscle disease caused by deficiency of the rate-limiting enzyme in N-acetylneuraminic acid (Neu5Ac) biosynthesis.

Methods

We conducted an open-label, phase 2, single-center (NIH, USA) study to evaluate oral ManNAc in 12 patients with GNE myopathy (ClinicalTrials.gov NCT02346461). Primary endpoints were safety and biochemical efficacy as determined by change in plasma Neu5Ac and sarcolemmal sialylation. Clinical efficacy was evaluated using secondary outcome measures as part of study extensions, and a disease progression model (GNE-DPM) was tested as an efficacy analysis method.

Results

Most drug-related adverse events were gastrointestinal, and there were no serious adverse events. Increased plasma Neu5Ac (+2,159 nmol/L, p < 0.0001) and sarcolemmal sialylation (p = 0.0090) were observed at day 90 compared to baseline. A slower rate of decline was observed for upper extremity strength (p = 0.0139), lower extremity strength (p = 0.0006), and the Adult Myopathy Assessment Tool (p = 0.0453), compared to natural history. Decreased disease progression was estimated at 12 (γ = 0.61 [95% CI: 0.09, 1.27]) and 18 months (γ = 0.55 [95% CI: 0.12, 1.02]) using the GNE-DPM.

Conclusion

ManNAc showed long-term safety, biochemical efficacy consistent with the intended mechanism of action, and preliminary evidence clinical efficacy in patients with GNE myopathy.

Population Pharmacokinetic Model of N-acetylmannosamine (ManNAc) and N-acetylneuraminic acid (Neu5Ac) in Subjects with GNE Myopathy

BACKGROUND

GNE myopathy is a rare genetic muscle disease resulting from deficiency in an enzyme critical for the biosynthesis of N-acetylneuraminic acid (Neu5Ac, sialic acid). The uncharged Neu5Ac precursor, N-acetylmannosamine (ManNAc), is under development as an orphan drug for treating GNE myopathy.

METHODS

A semi-mechanistic population pharmacokinetic model was developed to simultaneously characterize plasma ManNAc and its metabolite Neu5Ac following oral administration of ManNAc to subjects with GNE myopathy. Plasma ManNAc and Neu5Ac pharmacokinetic data were obtained from two clinical studies (ClinicalTrials.gov identifiers NCT01634750, NCT02346461) and were simultaneously modeled using NONMEM.

RESULTS

ManNAc and Neu5Ac plasma concentrations were obtained from 34 subjects with GNE myopathy (16 male, 18 female, median age 39.5 years). The model parameter estimates included oral absorption rate (k_a) = 0.256 h^-1, relative bioavailability relationship with dose (F-Dose) slope = -0.405 (where F = 1 for 6-g dose), apparent clearance (CL_M/F) = 631 L/h, volume of distribution (V_M/F) = 506 L, Neu5Ac elimination rate constant (k_out) = 0.283 h^-1, initial ManNAc to Neu5Ac conversion (SLP₀) = 0.000619 (ng/mL)^-1 and at steady-state (SLP_SS) = 0.00334 (ng/mL)^-1, with a rate-constant of increase (k_inc) = 0.0287 h^-1. Goodness-of-fit plots demonstrated an acceptable and unbiased fit to the plasma ManNAc and Neu5Ac concentration data. Visual predictive checks demonstrated reasonable agreement between the 5th, 50th, and 95th percentiles of the observed and simulated data.

CONCLUSIONS

This population pharmacokinetic model can be used to evaluate ManNAc dosing regimens and to calculate Neu5Ac production and exposure following oral administration of ManNAc in subjects with GNE myopathy.

Pregnancy in GNE myopathy patients: a nationwide repository survey in Japan

BACKGROUND

GNE myopathy is an autosomal recessive adult-onset distal myopathy. While a few case reports have described the progression of GNE myopathy during pregnancy, to our knowledge, none have examined disease progression after delivery or obstetric complications.

OBJECTIVE

This study aimed to reveal maternal complications, newborn complications, and the impact of pregnancy on disease progression in GNE myopathy patients.

METHODS

We conducted a questionnaire survey on pregnancy, delivery, and newborns involving female GNE myopathy patients who are currently registered in a national registry in Japan.

RESULTS

The response rate for the questionnaire survey was 60.0% (72/120). Of the 72 respondents, 44 (61.1%) had pregnancy experience (average, 1.8 pregnancies; 53 pregnancies before onset and 28 after onset). The incidence of threatened abortion was 26.9% among post-onset pregnancies, which was higher compared to those of the general Japanese population (p = 0.03). No other maternal or infant complications were commonly observed. Over 80% were unaware of changes in disease progression during pregnancy (mean age, 32.8 ± 3.5 years) or after delivery (32.9 ± 3.8 years), while 19.0% experienced disease exacerbation within a year after delivery (30.0 ± 1.0 years). Six patients developed myopathy within a year after delivery (29.7 ± 4.6 years), while none developed myopathy during pregnancy.

CONCLUSIONS

There were no serious maternal or newborn complications, and subjective progression did not differ during or after delivery in the majority of GNE myopathy patients. However, our findings suggest the importance of considering the possibility of threatened abortion and disease progression after delivery.

Quantitation of cytidine-5'-monophospho-N-acetylneuraminic acid in human leukocytes using LC-MS/MS: method development and validation

The biosynthesis of sialic acid (Neu5Ac) leads to the intracellular production of cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-Neu5Ac), the active sialic acid donor to nascent glycans (glycoproteins and glycolipids) in the Golgi. UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase myopathy is a rare autosomal recessive muscular disease characterized by progressive muscle weakness and atrophy. To quantify the intracellular levels of CMP-Neu5Ac as well as N-acetylmannosamine (ManNAc) and Neu5Ac in human leukocytes, we developed and validated robust liquid chromatography-tandem mass spectrometry methods. A fit-for-purpose approach was implemented for method validation. Hydrophilic interaction chromatography was used to retain three hydrophilic analytes. The human leukocyte pellets were lysed and extracted in a methanol-water mixture and the leukocyte extract was used for LC-MS/MS analysis. The lower limits of quantitation for ManNAc, Neu5Ac and CMP-Neu5Ac were 25.0, 25.0 and 10.0 ng/ml, respectively. These validated methods were applied to a clinical study.

Rationale and Design for a Phase 1 Study of N-Acetylmannosamine for Primary Glomerular Diseases

Introduction

Sialic acids are important contributors to the polyanionic component of the glomerular filtration barrier, which regulates permeability selectivity. Pathologic glomerular hyposialylation, associated with podocyte effacement, has been implicated in human and mouse glomerulopathies. Oral treatment with N-acetylmannosamine (ManNAc), the uncharged precursor of sialic acid, ameliorates glomerular pathology in different models of glomerular disease.

Methods

Here we explore the sialylation status of kidney biopsies obtained from 27 subjects with various glomerular diseases using lectin histochemistry.

Results

We identified severe glomerular hyposialylation in 26% of the biopsies. These preliminary findings suggest that this condition may occur relatively frequently and may be a novel target for therapy. We describe the background, rationale, and design of a phase 1 study to test safety, tolerability, and pharmacokinetics of ManNAc in subjects with primary podocyte diseases.

Conclusion

We recently demonstrated that ManNAc was safe and well tolerated in a first-in-human phase 1 study in subjects with UDP-N-acetylglucosamine (GlcNAc) 2-epimerase/ManNAc kinase (GNE) myopathy, a disorder of impaired sialic acid synthesis. Using previous preclinical and clinical data, we propose to test ManNAc therapy for subjects with primary glomerular diseases. Even though the exact mechanisms, affected cell types, and pathologic consequences of glomerular hyposialylation need further study, treatment with this physiological monosaccharide could potentially replace or supplement existing glomerular diseases therapies.

Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS

BACKGROUND

The etiology of steroid-resistant nephrotic syndrome, which manifests as FSGS, is not completely understood. Aberrant glycosylation is an often underestimated factor for pathologic processes, and structural changes in the glomerular endothelial glycocalyx have been correlated with models of nephrotic syndrome. Glycans are frequently capped by sialic acid (Sia), and sialylation's crucial role for kidney function is well known. Human podocytes are highly sialylated; however, sialylation's role in podocyte homeostasis remains unclear.

METHODS

We generated a podocyte-specific sialylation-deficient mouse model (PCmas^-/- ) by targeting CMP-Sia synthetase, and used histologic and ultrastructural analysis to decipher the phenotype. We applied CRISPR/Cas9 technology to generate immortalized sialylation-deficient podocytes (asialo-podocytes) for functional studies.

RESULTS

Progressive loss of sialylation in PCmas^-/- mice resulted in onset of proteinuria around postnatal day 28, accompanied by foot process effacement and loss of slit diaphragms. Podocyte injury led to severe glomerular defects, including expanded capillary lumen, mesangial hypercellularity, synechiae formation, and podocyte loss. In vivo, loss of sialylation resulted in mislocalization of slit diaphragm components, whereas podocalyxin localization was preserved. In vitro, asialo-podocytes were viable, able to proliferate and differentiate, but showed impaired adhesion to collagen IV.

CONCLUSIONS

Loss of cell-surface sialylation in mice resulted in disturbance of podocyte homeostasis and FSGS development. Impaired podocyte adhesion to the glomerular basement membrane most likely contributed to disease development. Our data support the notion that loss of sialylation might be part of the complex process causing FSGS. Sialylation, such as through a Sia supplementation therapy, might provide a new therapeutic strategy to cure or delay FSGS and potentially other glomerulopathies.

A phase 3 randomized study evaluating sialic acid extended-release for GNE myopathy

Objective

To investigate the efficacy and safety of aceneuramic acid extended-release (Ace-ER), a treatment intended to replace deficient sialic acid, in patients with GNE myopathy.

Methods

UX001-CL301 was a phase 3, double-blind, placebo-controlled, randomized, international study evaluating the efficacy and safety of Ace-ER in patients with GNE myopathy. Participants who could walk ≥200 meters in a 6-minute walk test at screening were randomized 1:1, and stratified by sex, to receive Ace-ER 6 g/d or placebo for 48 weeks and assessed every 8 weeks. The primary endpoint was change in muscle strength over 48 weeks measured by upper extremity composite (UEC) score. Key secondary endpoints included change in lower extremity composite (LEC) score, knee extensor strength, and GNE myopathy–Functional Activity Scale (GNEM-FAS) mobility domain score. Safety assessments included adverse events (AEs), vital signs, and clinical laboratory results.

Results

Eighty-nine patients were randomized (Ace-ER n = 45; placebo n = 44). Change from baseline to week 48 for UEC score between treatments did not differ (least square mean [LSM] Ace-ER −2.25 kg vs placebo −2.99 kg; LSM difference confidence interval [CI] 0.74 [−1.61 to 3.09]; p = 0.5387). At week 48, there was no significant difference between treatments for the change in key secondary endpoints: LEC LSM difference (CI) −1.49 (−5.83 to 2.86); knee extension strength −0.40 (−2.38 to 1.58); and GNEM-FAS mobility domain score −0.72 (−2.01 to 0.57). Gastrointestinal events were the most common AEs.

Conclusions

Ace-ER was not superior to placebo in improving muscle strength and function in patients with GNE myopathy.

Classification of evidence

This study provides Class I evidence that for patients with GNE myopathy, Ace-ER does not improve muscle strength compared to placebo.

Progression of GNE Myopathy Based on the Patient-Reported Outcome

Background and Purpose

GNE myopathy is a rare progressive myopathy caused by biallelic mutations in the GNE gene, and frequently accompanied by rimmed vacuoles in muscle pathology. The initial symptom of foot drop or hip-girdle weakness eventually spreads to all limbs over a period of decades. Recent advances in pathophysiologic research have facilitated therapeutic trials aimed at resolving the core biochemical defect. However, there remains unsettled heterogeneity in its natural course, which confounds the analysis of therapeutic outcomes. We performed the first large-scale study of Korean patients with GNE myopathy.

Methods

We gathered the genetic and clinical profiles of 44 Korean patients with genetically confirmed GNE myopathy. The clinical progression was estimated retrospectively based on a patient-reported questionnaire on the status of the functional joint sets and daily activities.

Results

The wrist and neck were the last joints to lose antigravity functionality irrespective of whether the weakness started from the ankle or hip. Two-thirds of the patients could walk either independently or with an aid. The order of losing daily activities could be sorted from standing to eating. Patients with limb-girdle phenotype showed an earlier age at onset than those with foot-drop onset. Patients with biallelic kinase domain mutations tended to progress more rapidly than those with epimerase and kinase domain mutations.

Conclusions

The reported data can guide the clinical management of GNE myopathy, as well as provide perspective to help the development of clinical trials.

The unfolding spectrum of inherited distal myopathies

Distal myopathies are a group of rare muscle diseases characterized by distal weakness at onset. Although acquired myopathies can occasionally present with distal weakness, the majority of distal myopathies have a genetic etiology. Their age of onset varies from early-childhood to late-adulthood while the predominant muscle weakness can affect calf, ankle dorsiflexor, or distal upper limb muscles. A spectrum of muscle pathological changes, varying from nonspecific myopathic changes to rimmed vacuoles to myofibrillar pathology to nuclei centralization, have been noted. Likewise, the underlying molecular defect is heterogeneous. In addition, there is emerging evidence that distal myopathies can result from defective proteins encoded by genes causative of neurogenic disorders, be manifestation of multisystem proteinopathies or the result of the altered interplay between different genes. In this review, we provide an overview on the clinical, electrophysiological, pathological, and molecular aspects of distal myopathies, focusing on the most recent developments in the field. Muscle Nerve 59:283-294, 2019.

GNE Myopathy: Etiology, Diagnosis, and Therapeutic Challenges

GNE myopathy, previously known as hereditary inclusion body myopathy (HIBM), or Nonaka myopathy, is a rare autosomal recessive muscle disease characterized by progressive skeletal muscle atrophy. It has an estimated prevalence of 1 to 9:1,000,000. GNE myopathy is caused by mutations in the GNE gene which encodes the rate-limiting enzyme of sialic acid biosynthesis. The pathophysiology of the disease is not entirely understood, but hyposialylation of muscle glycans is thought to play an essential role. The typical presentation is bilateral foot drop caused by weakness of the anterior tibialis muscles with onset in early adulthood. The disease slowly progresses over the next decades to involve skeletal muscles throughout the body, with relative sparing of the quadriceps until late stages of the disease. The diagnosis of GNE myopathy should be considered in young adults presenting with bilateral foot drop. Histopathologic findings on muscle biopsies include fiber size variation, atrophic fibers, lack of inflammation, and the characteristic "rimmed" vacuoles on modified Gomori trichome staining. The diagnosis is confirmed by the presence of pathogenic (mostly missense) mutations in both alleles of the GNE gene. Although there is no approved therapy for this disease, preclinical and clinical studies of several potential therapies are underway, including substrate replacement and gene therapy-based strategies. However, developing therapies for GNE myopathy is complicated by several factors, including the rare incidence of disease, limited preclinical models, lack of reliable biomarkers, and slow disease progression.

Role of IGF-1R in ameliorating apoptosis of GNE deficient cells

Sialic acids (SAs) are nine carbon acidic amino sugars, found at the outermost termini of glycoconjugates performing various physiological and pathological functions. SA synthesis is regulated by UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) that catalyzes rate limiting steps. Mutations in GNE result in rare genetic disorders, GNE myopathy and Sialuria. Recent studies indicate an alternate role of GNE in cell apoptosis and adhesion, besides SA biosynthesis. In the present study, using a HEK cell-based model for GNE myopathy, the role of Insulin-like Growth Factor Receptor (IGF-1R) as cell survival receptor protein was studied to counter the apoptotic effect of non-functional GNE. In the absence of functional GNE, IGF-1R was hyposialylated and transduced a downstream signal upon IGF-1 (IGF-1R ligand) treatment. IGF-1 induced activation of IGF-1R led to AKT (Protein Kinase B) phosphorylation that may phosphorylate BAD (BCL2 Associated Death Promoter) and its dissociation from BCL2 to prevent apoptosis. However, reduced ERK (Extracellular signal-regulated kinases) phosphorylation in GNE deficient cells after IGF-1 treatment suggests downregulation of the ERK pathway. A balance between the ERK and AKT pathways may determine the cell fate towards survival or apoptosis. Our study suggests that IGF-1R activation may rescue apoptotic cell death of GNE deficient cell lines and has potential as therapeutic target.

CDG Therapies: From Bench to Bedside

Congenital disorders of glycosylation (CDG) are a group of genetic disorders that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. More than 100 different disorders have been reported and the number is rapidly increasing. Since glycosylation is an essential post-translational process, patients present a large range of symptoms and variable phenotypes, from very mild to extremely severe. Only for few CDG, potentially curative therapies are being used, including dietary supplementation (e.g., galactose for PGM1-CDG, fucose for SLC35C1-CDG, Mn²⁺ for TMEM165-CDG or mannose for MPI-CDG) and organ transplantation (e.g., liver for MPI-CDG and heart for DOLK-CDG). However, for the majority of patients, only symptomatic and preventive treatments are in use. This constitutes a burden for patients, care-givers and ultimately the healthcare system. Innovative diagnostic approaches, in vitro and in vivo models and novel biomarkers have been developed that can lead to novel therapeutic avenues aiming to ameliorate the patients’ symptoms and lives. This review summarizes the advances in therapeutic approaches for CDG.

Quantification of lectin fluorescence in GNE myopathy muscle biopsies

INTRODUCTION

GNE myopathy is an adult-onset muscle disorder characterized by impaired sialylation of (muscle) glycans, detectable by lectin histochemistry. We describe a standardized method to quantify (lectin-) fluorescence in muscle sections, applicable for diagnosis and response to therapy for GNE myopathy.

METHODS

Muscle sections were fluorescently labeled with the sialic acid-binding Sambucus nigra agglutinin (SNA) lectin and antibodies to sarcolemma residence protein caveolin-3 (CAV-3). Entire tissue sections were imaged in tiles and fluorescence was quantified.

RESULTS

SNA fluorescence co-localizing with CAV-3 was ∼50% decreased in GNE myopathy biopsies compared with muscle-matched controls, confirming previous qualitative results.

DISCUSSION

This quantitative fluorescence method can accurately determine sialylation status of GNE myopathy muscle biopsies. This method is adaptable for expression of other membrane-associated muscle proteins, and may be of benefit for disorders in which therapeutic changes in expression are subtle and difficult to assess by other methods. Muscle Nerve 58: 286-292, 2018.