Molecular Genetic Analysis of Fetal Tissues from a Family Affected by Myotonic Dystrophy

Development of the pathological phenotype in patients with myotonic dystrophy (MD1) largely depends on toxic effects of expanded DMPK gene repeats (CUG(exp)) transcription and their interaction with binding proteins. The relationship between DMPK CUG(exp), MBNL1 and MBNL2 has so far been studied on tissue cultures (myoblast cell lines) and transgenic animals. In this report, the first in situ molecular genetic analysis of fetal tissue from a family affected by MD1 is presented. Intrauterine development, maturation and differentiation of skeletal muscles of the fetus (360 g, 22nd week of pregnancy) were only slightly delayed. In the fetus, paternally inherited expansion of the CTG repeat in the DMPK gene (350 CTG) was confirmed by molecular analysis. Subsequent histopathological examination revealed signs of myotonic dystrophy. Fetal tissue obtained at autopsy (skeletal muscles, esophagus, stomach and intestines), were studied by histopathological, immunofluorescence (expression of MBNL1/MBNL2 proteins) and in situ hybridization (DMPK CUG(exp)) methods. Intranuclear CUG(exp)-containing foci were present in skeletal muscle fibers, muscularis externa of the esophagus, stomach and intestines, vascular smooth muscle, neurons and Schwann cells of intrinsic ganglionic plexuses, and epithelial cells. MBNL1 protein was largely co-localized with the CUG(exp) foci in all tissues examined. Contrarily, MBNL2 protein was also detected in the tissue cytoplasm. The presence of DMPK transcript with expanded CUG repeat and MBNL1 protein in the intranuclear foci of MD1 fetal tissues studied may theoretically result in sequestration of the protein and thus contribute to generation of the MD1 phenotype.