Chromosomal Instability Mediated by Telomere Insufficiency

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All eukaryotes maintain nuclear genetic materials as linear DNAs. The end of DNAs is called the telomere, and is essential for the stability of chromosomes. When telomere functions are lost, the chromosomes undergo the end-to-end fusion, producing aberrant chromosomes and cell death. We previously have reported that telomere dysfunction can be bypassed by self-circularizing the chromosome in fission yeast. In this study, we isolated novel fission yeast telomere components, spRapl and spRif1. We furthermore demonstrated that part of telomeric functions can be maintained epigenetically in the absence of telomeric repeats. Thus, the telomere function may be maintained robustly when telomere DNA is transiently lost. When the telomeric DNA is critically shortened, normal mammalian cells exhibit a state called cellular senescence, where the cells irreversibly stop growing. We found that the stress-induced MAPK p38 plays an important role in this process. When p38 is activated, the cells induce cell cycle arrest in an Rb-dependent manner, and exhibit morphological changes in an Rb-independent manner, suggesting that the downstream pathway of p38 bifurcates. Using the SV40 DNA replication system in vitro, we found that telomeric DNA and chromatin are not good substrates for DNA replication. Accordingly, it is expected that there is a mechanism facilitating the telomeric replication in vivo. We found that telomeric chromatin is dynamically changed in Xenopus egg extracts. Together, all of these observations suggest that the telomere is one of the loci that are particularly vulnerable to chromatin stress, and contribute to the chromosomal instability in cancer cells