Rna-Protein Phase Separation In Cancer: Investigating Human Satellite Ii Rna Structure And Function

Human Satellite II, a tandemly repeated stretch of DNA found near the centromeres of most human chromosomes, is not transcribed into RNA in normal human cells, but is expressed in many human cancer cell lines and tissues. HSATII transcripts accumulate in the nucleus adjacent to their sites of transcription and recruit nuclear regulatory proteins within these large nuclear foci. Recent evidence suggests that RNA and proteins within the nuclear environment can phase separate into liquid-like droplets, creating abberant structures, some of which are known to contribute to pathological effects. Here, we present evidence that HSATII RNA folds on itself to form defined secondary structures, which could enable HSATII RNA to participate in a variety of RNA-RNA or RNA-protein interactions. Indeed, we show that HSATII RNA phase separates into liquid-like droplets, a process that is dependent on the size and sequence of the individual transcripts. A dependence on length suggests that HSATII RNA phase separation requires either a certain complexity of secondary structure or minimum number of contacts between single molecules. We hypothesize that HSATII RNA droplets differentially solubilize specific proteins via RNA-protein interactions, which could explain the observed sequestration of genomic regulatory proteins to nuclear HSATII RNA accumulations. Phase separation offers a powerful lens through which to begin to understand how misregulation of transcription, via the creation of liquid-like droplets, can disrupt regulatory processes, whose slight alterations can tip the balance in favor of further misregulation.