During mitosis, chromosomes need to be divided equally to two daughter cells in order to maintain genome stability. Any loss of genetic information leads to aneuploidy, genome instability, and genetic diseases including birth defects or cancer. It is therefore crucial to molecularly understand in full detail how chromosome segregation is regulated. The crucial genomic locus for genome stability is the centromere on every chromosome since kinetochore formation takes place at specialized chromatin structure and the spindle microtubule attachment requires the kinetochore to create forces to pull the chromsomes apart during mitosis. Centromeric chromatin structure is highly conserved but the underlying DNA sequence is very variable, and it has long been accepted that centromeres are defined epigenetically by their specialized chromatin composition instead of any specialized DNA sequence. The epigenetic determinant of centromeric chromatin is the high occupancy of the histone H3-variant CENP-A. Centromeric chromatin is embedded in large blocks of so called pericentromeric heterochromatin, highly condense and repetitive DNA sequences that stretch over megabases of the genome and make up about 4% of the human or Drosophila genome. These repetitive regions have been ignored for a long time because of its repetitiveness and its lack of protein coding genes. But in recent years, researchers noticed that heterochromatin plays important roles in genome maintenance. We previously found that repeat regions from pericentromeric chromatin of Drosophila melanogaster are transcribed in mitosis and that these transcripts are important for genome stability. These transcripts do not code for proteins but belong to the class of non-coding RNAs. These RNAs are involved in diverse processes from transcriptional and translational control to immune response and developmental regulation. Important for the projects described in this proposal is that non-coding RNAs have emerged as crucial components of chromatin structures and have implications in epigenetic processes. Identifying the nature and function of transcripts that associate with or derive from (peri-) centromeric repeat regions and their interacting proteins is the primary aim of this ERC consolidator grant. The major objectives of our work on centromeric RNAs is to, firstly, identify all RNAs that associate and therefore potentially influence centromeric chromatin. Since regulatory RNAs usually function in complex with proteins, we also set out to identify and functionally characterize proteins that localize to centromeric chromatin through RNA. Last but not least, it is important to identify if these RNAs or the protein-RNA complexes are misregulated in different human diseases. If we fully understand their regulation under physiological conditions we hope to then also understand how their misregulation, for instance in certain cancer entities, lead to disease progression and will ultimately help us to progress to disease prevention. This ERC grant has given us some important insights into how RNA transcripts are regulated at centromeres and how these RNAs influence the function of centromeres and the inherticance of centromeric chromatin.
