Functional characterization of GNE mutations prevalent in Asian subjects with GNE myopathy, an ultra-rare neuromuscular disorder

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.

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

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

Role of HSP70 chaperone in protein aggregate phenomenon of GNE mutant cells: Therapeutic lead for GNE Myopathy

Limited treatment options and research in understanding the pathomechanisms of rare diseases has raised concerns about their therapeutic development. One such poorly understood ultra-rare neuromuscular disorder is GNE Myopathy (GNEM) which is caused due to mutation in key sialic acid biosynthetic enzyme, GNE. Treatment with sialic acid or its derivatives/precursors slows the disease progression, but curative strategies need to be explored further. Pathologically, muscle biopsy samples of GNEM patients reveal rimmed vacuole formation due to aggregation of β-amyloid, Tau, presenilin proteins with unknown mechanism. The present study aims to understand the mechanism of protein aggregate formation in GNE mutant cells to decipher role of chaperones in disease phenotype. The pathologically relevant GNE mutations expressed as recombinant proteins in HEK cells was used as a model system for GNEM to estimate extent of protein aggregation. We identified HSP70, a chaperone, as binding partner of GNE. Downregulation of HSP70 with altered BAG3, JNK, BAX expression levels was observed in GNE mutant cells. The cell apoptosis was observed in GNE mutation specific manner. An activator of HSP70 chaperone, BGP-15, rescued the phenotypic defects due to GNE mutation, thereby, reducing protein aggregation significantly. The results were further validated in rat skeletal muscle cell lines carrying single Gne allele. Our study suggests that HSP70 activators can be a promising therapeutic target in the treatment of ultra-rare GNE Myopathy disease.