Calcium level and autophagy defect in GNE mutants of rare neuromuscular disorder.

Rare genetic disorders are low in prevalence and hence there is little or no attention paid to them in the mainstream medical industry. One of the ultra-rare neuromuscular disorders, GNE myopathy is caused due to biallelic mutations in the bifunctional enzyme, GNE (UDP N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase). It catalyses the rate-limiting step in sialic acid biosynthesis. There are no effective treatments for GNE myopathy as the pathomechanism is poorly understood. Pathologically, the disease is characterized by the formation of rimmed vacuoles that contain aggregates of β-amyloid, tau, presenilin etc proteins in muscle biopsy samples. Accumulation of aggregated proteins in the cells may occur due to the failure of the regulated autophagy phenomenon. In the present study, we aim to understand the effect of GNE mutations on autophagy. The cytosolic calcium levels in GNE mutant cells were found to be altered in a GNE mutation-specific manner. The chaperone levels, such as HSP70 and PDI, as well as autophagic markers (LC3II/I ratios) were altered in the GNE mutant cells. Treatment with BAPTA-AM, calcium chelator, significantly restored cytosolic calcium levels in some GNE mutant cells as well as autophagic marker levels and autophagic punctae formation. The effect on the calcium signalling cascade involving CaMKKβ/AMPK/mTOR was studied in the GNE mutant cells. Our study provides insights into the role of calcium in autophagic vacuole formation in the cells with GNE mutations that will have significance towards understanding the pathomechanism of GNE Myopathy and drug target identification for the rare disease.