388. Hypersensitivity of MYH16 Fibers (M Fibers) to Myodegeneration in Canine Muscular Dystrophy

Golden Retriever Muscular Dystrophy (GRMD) dogs frequently develop severe restriction to mouth opening, a condition known as trismus, as one of their earliest physical signs of the disease. Trismus is generally due to fibrosis and/or tonic contraction of the muscles of mastication, the temporalis and the masseter muscle. These muscles are primarily composed of unique myofibers exhibiting a masticatory myosin heavy chain encoded by the MYH 16 gene (M fibers). M fibers have been shown to produce the highest specific force among mammalian myofibers, based on their atypical crossbridge kinetics. Here, we show for a first time a morphological analysis of M fibers for the purposes of their classification, distribution and proportion in the masticatory muscles in normal and GRMD dogs based on immunofluorescence staining by a highly specific anti-MYH16 antibody. Our data demonstrate that in both temporalis and masseter muscles, the predominant M fibers are larger in size when compared to any other striated myofiber. A comparative histological analysis of these and other muscles in GRMD dogs demonstrates a significant relative increase in the proportion of injured, hypercontracted M fibers, a highly significant relative abundance of regenerating fibers expressing the embryonic myosin isoform encoded by the MYH3 gene, and prominent fibrosis when compared to the other affected striated muscles. These results suggest that the higher specific force developed by M fibers, due to the MYH16 encoded MyHC isoform, accelerates the fibers’ rate of degeneration in response to myonecrotizing sarcolemmal injury in the absence of dystrophin. Trismus is not seen in Duchenne MD, suggesting that the deletional frameshift in the human MYH16 gene (Nature 428: 373-4, 415-8) dramatically reduces the relative rate of sarcolemmal injury. These observations may offer novel insights into the physiological role of dystrophin in striated muscles, and provide an especially sensitive test for use in experimental therapeutics in the canine models for DMD.