Rodriguez-Muñoz Marina, Serrat Martina, Soler David, Genescà Anna, Anglada Teresa
Department of Cell Biology, Physiology and Immunology, Universitat Autònoma de Barcelona, Bellaterra, Spain.
Front Cell Dev Biol. 2021 Sep 28;9:745195. doi: 10.3389/fcell.2021.745195. eCollection 2021.
Chromosomal instability, the most frequent form of plasticity in cancer cells, often proceeds through the formation of chromosome bridges. Despite the importance of these bridges in tumor initiation and progression, debate remains over how and when they are resolved. In this study, we investigated the behavior and properties of chromosome bridges to gain insight into the potential mechanisms underlying bridge-induced genome instability. We report that bridges may break during mitosis or may remain unbroken until the next interphase. During mitosis, we frequently observed discontinuities in the bridging chromatin, and our results strongly suggest that a substantial fraction of chromosome bridges are broken during this stage of the cell cycle. This notion is supported by the observation that the chromatin flanking mitotic bridge discontinuities is often decorated with the phosphorylated form of the histone H2AX, a marker of DNA breaks, and by MDC1, an early mediator of the cell response to DNA breaks. Also, free 3'OH DNA ends were detected in more than half of the bridges during the final stages of cell division. However, even if detected, the DNA ends of broken bridges are not repaired in mitosis. To investigate whether mitotic bridge breakage depends on mechanical stress, we used experimental models in which chromosome bridges with defined geometry are formed. Although there was no association between spindle pole separation or the distance among non-bridge kinetochores and bridge breakage, we found a direct correlation between the distance between bridge kinetochores and bridge breakage. Altogether, we conclude that the discontinuities observed in bridges during mitosis frequently reflect a real breakage of the chromatin and that the mechanisms responsible for chromosome bridge breakage during mitosis may depend on the separation between the bridge kinetochores. Considering that previous studies identified mechanical stress or biochemical digestion as possible causes of bridge breakage in interphase cells, a multifactorial model emerges for the breakage of chromosome bridges that, according to our results, can occur at different stages of the cell cycle and can obey different mechanisms.
染色体不稳定是癌细胞中最常见的可塑性形式，通常通过形成染色体桥来进行。尽管这些桥在肿瘤起始和进展中很重要，但关于它们如何以及何时被解决仍存在争议。在本研究中，我们研究了染色体桥的行为和特性，以深入了解桥诱导的基因组不稳定的潜在机制。我们报告说，桥可能在有丝分裂期间断裂，或者可能一直不断裂直到下一个间期。在有丝分裂期间，我们经常观察到桥接染色质的不连续性，我们的结果强烈表明，相当一部分染色体桥在细胞周期的这个阶段断裂。这一观点得到以下观察结果的支持：有丝分裂桥不连续两侧的染色质通常被组蛋白H2AX的磷酸化形式修饰，H2AX是DNA断裂的标志物，还有MDC1，它是细胞对DNA断裂反应的早期介质。此外，在细胞分裂的最后阶段，超过一半的桥中检测到游离的3'OH DNA末端。然而，即使检测到，断裂桥的DNA末端在有丝分裂中也不会被修复。为了研究有丝分裂桥断裂是否取决于机械应力，我们使用了形成具有确定几何形状的染色体桥的实验模型。尽管纺锤极分离或非桥动粒之间的距离与桥断裂之间没有关联，但我们发现桥动粒之间的距离与桥断裂之间存在直接相关性。总之，我们得出结论，在有丝分裂期间在桥中观察到的不连续性经常反映染色质的真正断裂，并且在有丝分裂期间负责染色体桥断裂的机制可能取决于桥动粒之间的分离。考虑到先前的研究确定机械应力或生化消化是间期细胞中桥断裂的可能原因，根据我们的结果，出现了一个多因素模型来解释染色体桥的断裂，该断裂可以发生在细胞周期的不同阶段并且可以遵循不同的机制。
