A.P.4: Cytoplasmic bodies in the muscle of HMERF patients with TTN A150/FN3 119 and kinase domain mutations – An immunofluorescent analysis

Abstract

Hereditary myopathy with early respiratory failure (HMERF) is an early adult-onset muscular dystrophy, with weakness predominantly in proximal and respiratory muscles. A mutation in the titin kinase domain was originally reported as one cause of the disease. However, the genetic background of HMERF has recently turned out to be more complex, with titin A150/FN3 119 domain mutations playing a central role. The HMERF muscle histopathology consists of myopathic-dystrophic changes with protein aggregation pathology, including autophagic rimmed vacuoles and myofibrillar pathology. A distinctive feature is the presence of cytoplasmic bodies, which are dense, often spherical bodies located either in the fiber periphery or in central regions. Muscle biopsies from HMERF patients with different mutation combinations, including the recessive p.P30091L, the dominant p.C30071R and the kinase domain recessive p.R32450W, were analyzed using immunofluorescence. Various kinds of protein accumulation were observed. The cytoplasmic bodies were positive F-actin (phalloidin), myotilin, ubiquitin, MURF1, DNAJB6 and NBR1, but negative for alphaB-crystallin, desmin, p62 and several titin antibodies, including A-band TTN. Outside the bodies, p62 and NBR1 formed distinct dense aggregates, whereas alphaB-crystallin, desmin and TTN accumulated in more diffuse manner, both in the periphery and central fiber areas. The function or binding partners of the titin FN3 119 domain are not yet known. However, mutations in it trigger a molecular cascade leading to accumulation of ubiquitinated proteins, which exhausts the degradation capacity of the muscle fibers. This finding emphasizes the role of titin as a central signaling protein, regulating protein turnover, proteasomal and autophagic degradation. Both the FN3 119 domain and the kinase domain seem to be key players in this regulation.