Chromosomal Instability Mediated by Telomere Insufficiency

所有真核生物都以线性DNA的形式维持核遗传物质。DNA的末端称为端粒，对染色体的稳定是必不可少的。当端粒功能丧失时，染色体会发生端到端的融合，产生染色体异常和细胞死亡。我们之前已经报道过，在分裂酵母中，端粒功能障碍可以通过染色体的自我循环来绕过。在本研究中，我们分离了新的裂解酵母端粒成分spRap1和spRif1。我们进一步证明，在没有端粒重复的情况下，部分端粒功能可以在表观遗传上保持不变。因此，当端粒DNA短暂丢失时，端粒功能可能会得到很好的维持。当端粒DNA严重缩短时，正常的哺乳动物细胞会出现一种称为细胞衰老的状态，在这种状态下，细胞将不可逆转地停止生长。我们发现，应激诱导的MAPK p38在这一过程中起着重要作用。当p38被激活时，细胞以Rb依赖的方式诱导细胞周期停滞，并以Rb非依赖的方式表现出形态变化，这表明p38下游通路分叉。使用SV40DNA体外复制系统，我们发现端粒DNA和染色质不是DNA复制的良好底物。因此，人们期望有一种促进体内端粒复制的机制。我们发现非洲爪哇卵提取液中的端粒染色质是动态变化的。综上所述，所有这些观察表明，端粒是一个特别容易受到染色质压力影响的基因座，也是癌细胞染色体不稳定的原因之一。

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