CELL NUCLEAR ALTERATIONS IN MYOTONIC DYSTROPHY

In the cell nucleus, genes are transcribed, and the primary transcripts undergo molecular processing which generates mature RNAs to be exported to the cytoplasm. The events leading to the formation of mature RNAs are chronologically and spatially ordered, and they mostly occur on distinct ribonucleoprotein (RNP)-containing structures. Defects in the RNA maturation pathways have been related to diseases leading to muscle dystrophy: in myotonic dystrophy type 1 (DM1) and type 2 (DM2), the characteristic multisystemic features (e.g., myotonia, muscular dystrophy, dilated cardiomyopathy, cardiac conduction defects, cataracts, insulin-resistance, and disease-specific serological abnormalities) are caused by the expansion of two distinct nucleotide sequences: (CTG)n in the 3’ untranslated region of the DMPK gene on chromosome 19q13 in DM1, and (CCTG)n in the first intron of the ZNF9 gene on chromosome 3q21 in DM2. Combining biomolecular and cytochemical techniques, it has been demonstrated that the basic mechanisms of both DMs reside in the nuclear sequestration of the expanded RNAs: CUG- and CCUG-containing transcripts accumulate in intranuclear foci in DM1 and DM2 cells respectively, and alter the regulation and intranuclear localization of the RNA-binding proteins CUGBP1 and MBLN, which are necessary for the physiological processing of pre-mRNA. Using immunocytochemical techniques at light and electron microscopy, we have demonstrated that MBNL1-containing foci in DM2 cells also sequester snRNPs and hnRNPs, splicing factors involved in the early phases of transcript processing; this strengthens the hypothesis that the multifactorial phenotype of dystrophic patients could be due to a general alteration of the pre-mRNA post-transcriptional pathway. Interestingly, we also demonstrated that, in skeletal muscles of DM1 and DM2 patients, splicing and cleavage factors accumulate in myonuclei, suggesting an impairment of pre-mRNA processing reminiscent of the nuclear alterations typical of sarcopenia (i.e. the loss of muscle mass and function physiologically occurring during ageing). Moreover, in an in vitro study, we observed that satellite-cell-derived DM2 myoblasts show cell senescence alterations and impairment of the pre-mRNA maturation pathways earlier than the myoblasts from healthy patient. These results suggest possible common cellular mechanisms responsible for skeletal muscle wasting in different pathologies.