Heterochromatin comprises tightly compacted repetitive regions of eukaryotic chromosomes. It is inherited through mitosis and has roles in transcriptional silencing, centromere specification and genome integrity, which profoundly impact epigenetic mechanisms in health and disease. We have found that the epigenetic inheritance of heterochromatin in the fission yeast S.pombe requires RNA interference (RNAi) to guide histone modification, which occurs during the DNA replication phase of the cell cycle. S.pombe centromeric repeats have an alternating arrangement of small RNA clusters and origins of replication that makes collision of the transcription and replication machineries all but inevitable. We found that RNAi promotes release of RNA polymerase (Pol II) during S phase, allowing completion of DNA replication by the leading strand DNA polymerase, which recruits the histone-modifying Rik1 complex to spread heterochromatin along with DNA replication. In the absence of RNAi, stalled forks are repaired by homologous recombination (HR) without histone modification, so that HR is essential in the absence of RNAi, and reduces the copy number of rDNA repeats. This model may explain the participation of non-coding RNA and DNA repair in many examples of epigenetic silencing, such as imprinting and X-inactivation. Recently, we have found that RNAi is essential for viability in quiescent cells (G0), which are predominant in yeast and play critical roles in cancer, stem cells and neuronal disease. Genetic screens have revealed that the Rik1 complex and Pol II are both involved in this novel function for RNAi, implicating both heterochromatic silencing and DNA repair. S.pombe is an outstanding model system for cell cycle research, heterochromatic silencing, and RNAi. We will examine the roles of DNA replication, RNA Polymerase release, DNA recombination and repair in heterochromatic histone modification mediated by the Rik1 complex and RNAi.
Health relevance/ project merit: Epigenetic mechanisms alter gene function independent of DNA sequence, and have profound effects on health and disease. RNA interference impacts these mechanisms by guiding the modification of histones associated with the DNA, ensuring specificity and avoiding inappropriate gene silencing. We have found that replication of the chromosome during cell division occurs at the same time as RNA interference, and that these mechanisms interact to cause silencing. In a major innovation, we have found that RNAi is also required for entry, maintenance and exit from cellular quiescence, a prevalent cellular condition throughout the human body. The key molecules involved are conserved from yeast to humans, and are implicated in childhood disease, mental retardation, aging and cancer. Our findings suggest that therapies that target these molecules may also impact gene expression and chromosome organization. For example, a more comprehensive understanding of Dicer's function in the face of replication stress may inform cancer treatment, since it suppresses genome instability, a driving force behind tumorigenesis and metastasis. DICER levels in cancer are epigenetically regulated by replication stress and by hypoxia, and DICER mutations are frequent in tumors. Importantly, DICER deficient stem cells cannot differentiate, for example into neurons, reflecting perhaps the inability of RNAi mutants in fission yeast to differentiate into quiescent cells, and relevant to neurological disease as well as cancer.
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| Roche, Benjamin; Arcangioli, Benoît; Martienssen, Rob (2017) New roles for Dicer in the nucleolus and its relevance to cancer. Cell Cycle 16:1643-1653 |
| Roche, Benjamin; Arcangioli, Benoit; Martienssen, Robert (2017) Transcriptional reprogramming in cellular quiescence. RNA Biol 14:843-853 |
| Schorn, Andrea J; Gutbrod, Michael J; LeBlanc, Chantal et al. (2017) LTR-Retrotransposon Control by tRNA-Derived Small RNAs. Cell 170:61-71.e11 |
| He, Haijin; Li, Yang; Dong, Qianhua et al. (2017) Coordinated regulation of heterochromatin inheritance by Dpb3-Dpb4 complex. Proc Natl Acad Sci U S A 114:12524-12529 |
| Roche, B; Arcangioli, B; Martienssen, R A (2016) RNA interference is essential for cellular quiescence. Science 354: |
| Pichugina, T; Sugawara, T; Kaykov, A et al. (2016) A diffusion model for the coordination of DNA replication in Schizosaccharomyces pombe. Sci Rep 6:18757 |
| Grand, Ralph S; Pichugina, Tatyana; Gehlen, Lutz R et al. (2014) Chromosome conformation maps in fission yeast reveal cell cycle dependent sub nuclear structure. Nucleic Acids Res 42:12585-99 |
| Castel, Stephane E; Ren, Jie; Bhattacharjee, Sonali et al. (2014) Dicer promotes transcription termination at sites of replication stress to maintain genome stability. Cell 159:572-83 |
| Kuscu, Canan; Zaratiegui, Mikel; Kim, Hyun Soo et al. (2014) CRL4-like Clr4 complex in Schizosaccharomyces pombe depends on an exposed surface of Dos1 for heterochromatin silencing. Proc Natl Acad Sci U S A 111:1795-800 |
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