Cell stress molecules in the skeletal muscle of GNE myopathy

Background: Mutations of the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine-kinase (GNE)-gene are causally related to GNE myopathy. Yet, underlying pathomechanisms of muscle fibre damage have remained elusive. In sporadic inclusion body myositis (sIBM), the pro-inflammatory cell-stress mediators alpha B-crystallin and inducible nitric oxide synthase (iNOS) are crucial markers of the disease pathology. Methods: 10 muscle biopsies from GNE myopathy patients were analyzed for mRNA-expression of markers of cell-stress, inflammation and beta-amyloid and compared to non-myopathic controls. Using double-labeling immunohistochemistry, serial sections of skeletal muscle biopsies were stained for amyloid precursor protein (APP), major histocompatibility complex (MHC)-I, alpha B-crystallin, neural cell adhesion molecule (NCAM), interleukin (IL)-1 beta, beta-amyloid, iNOS, and phosphorylated neurofilament (P-neurofilament) as well as hematoxylin/eosin histochemistry. Corresponding areas of all biopsies with a total of 2,817 muscle fibres were quantitatively assessed for all markers. Results: mRNA-expression of APP, NCAM, iNOS, TNF-alpha and TGF-beta was higher in GNE myopathy compared to controls, yet this was not statistically significant. The mRNA-expression of APP and alpha B-crystallin significantly correlated with the expression of several pro-inflammatory and cell-stress-associated markers as NCAM, IL-1 beta, TGF-beta, CCL-3, and CCL4. By immunohistochemistry, alpha B-crystallin and iNOS were co-upregulated and the number of fibres positive for alpha B-crystallin, NCAM, MHC-I and iNOS significantly correlated with each other. A large fraction of fibres positive for alpha B-crystallin were double positive for iNOS and vice-versa. Moreover, several fibres with structural abnormalities were positive for alpha B-crystallin and iNOS. Notably, particularly normal appearing fibres displayed an overexpression of these molecules. Conclusions: The cell-stress molecules alpha B-crystallin and iNOS are overexpressed in GNE myopathy muscle and may identify early disease mechanisms. The data help to better understand the pathology of GNE myopathy.