Lucas J N, Sachs R K
Lawrence Livermore National Laboratory, University of California, Livermore 94551.
Proc Natl Acad Sci U S A. 1993 Feb 15;90(4):1484-7. doi: 10.1073/pnas.90.4.1484.
Ionizing radiation induces DNA double-strand breaks (DSB), which interact pairwise to produce chromosome aberrations. There have long been two main competing theories of such pairwise DSB-DSB interactions. The "classical" theory asserts that an unrepaired DSB makes two ends that separate within the cell nucleus, with each end subsequently able to join any similar (nontelomeric) end. The "exchange" theory asserts that at a DSB the chromatin does not separate completely; rather the DSB ends remain associated until repair, or an illegitimate recombination involving another DSB, occurs. The DSB-DSB interaction mechanism was tested by using three-color fluorescence in situ hybridization to paint chromosomes and observe "three-color triplets": three broken and misrejoined chromosomes having cyclically permuted colors. We observed 18 "three-color triplets" in 2000 cells after 2.25 Gy of gamma-irradiation. On the exchange model in its standard form such three-color triplets cannot occur, so this model is inconsistent with the observations. On the classical model, formalized as a discrete time Markov chain embedded at the transitions of a continuous time Markov chain, the frequency of occurrence of three-color triplets can be computed by Monte Carlo simulations. The number of three-color triplets predicted mathematically by the classical model was found to be slightly larger than the observed number. Thus our data, together with our computer simulations, exclude the standard form of the exchange model but are compatible with the classical model. The results are also compatible with other, more complicated models.
电离辐射会导致DNA双链断裂（DSB），这些双链断裂会两两相互作用，从而产生染色体畸变。长期以来，关于这种双链断裂与双链断裂的两两相互作用，存在两种主要的相互竞争的理论。“经典”理论认为，一个未修复的双链断裂会产生两个末端，它们在细胞核内分离，随后每个末端都能够与任何类似的（非端粒）末端连接。“交换”理论认为，在双链断裂处，染色质不会完全分离；相反，双链断裂的末端在修复或涉及另一个双链断裂的非法重组发生之前，会一直保持关联。通过使用三色荧光原位杂交技术对染色体进行标记，并观察“三色三联体”（即三条断裂并重新错接的染色体，其颜色呈循环排列），对双链断裂与双链断裂的相互作用机制进行了测试。在2.25 Gy的γ射线照射后，我们在2000个细胞中观察到了18个“三色三联体”。在标准形式的交换模型中，这种三色三联体不可能出现，因此该模型与观察结果不一致。在经典模型中，它被形式化为嵌入在连续时间马尔可夫链转换中的离散时间马尔可夫链，通过蒙特卡罗模拟可以计算出三色三联体出现的频率。经典模型通过数学预测的三色三联体数量略高于观察到的数量。因此，我们的数据以及计算机模拟结果排除了交换模型的标准形式，但与经典模型相符。这些结果也与其他更复杂的模型相符。
