Dynamic changes of nuclear RNA foci in proliferating DM1 cells

Nuclear RNA foci are molecular hallmarks of myotonic dystrophy type 1 (DM1). However, no designated study has investigated their formation and changes in proliferating cells. Proliferating cells, as stem cells, consist of an important cellular pool in the human body. The revelation of foci changes in these cells might shed light on the effects of the mutation on these specific cells and tissues. In this study, we used human DM1 iPS-cell-derived neural stem cells (NSCs) as cellular models to investigate the formation and dynamic changes of RNA foci in proliferating cells. Human DM1 NSCs derived from human DM1 iPS cells were cultured under proliferation conditions and nonproliferation conditions following mitomycin C treatment. The dynamic changes of foci during the cell cycle were investigated by fluorescence in situ hybridization. We found RNA foci formed and dissociated during the cell cycle. Nuclear RNA foci were most prominent in number and size just prior to entering mitosis (early prophase). During mitosis, most foci disappeared. After entering interphase, RNA foci accumulated again in the nuclei. After stopping cell dividing by treatment of mitomycin C, the number of nuclear RNA foci increased significantly. In summary, DM1 NSC nuclear RNA foci undergo dynamic changes during cell cycle, and mitosis is a mechanism to decrease foci load in the nuclei, which may explain why dividing cells are less affected by the mutation. The dynamic changes need to be considered when using foci as a marker to monitor the effects of therapeutic drugs.