The toxic lesion emanating from chemotherapy that targets the DNA was initially debated, but eventually the DNA double strand break (DSB) ultimately prevailed. The reasoning was in part based on the perception that repairing a fractured chromosome necessitated intricate processing or condemned the cell to death. Genetic evidence for the DSB model was also provided by the extreme sensitivity of cells that were deficient in DSB repair. In particular, sensitivity characterized cells harboring mutations in the hereditary breast/ovarian cancer genes, BRCA1 or BRCA2, that function in the repair of DSBs by homologous recombination (HR). Along with functions in HR, BRCA proteins were found to prevent DSBs by protecting stalled replication forks from nuclease degradation. Coming full-circle, BRCA mutant cancer cells that gained resistance to genotoxic chemotherapy often displayed restored DNA repair by HR and/or restored fork protection (FP) implicating that the therapy was tolerated when DSB repair was intact or DSBs were prevented. Despite this well-supported paradigm that has been the impetus for targeted cancer therapy, here we argue that the toxic DNA lesion conferring response is instead single stranded DNA (ssDNA) gaps. We discuss the evidence that persistent ssDNA gaps formed in the wake of DNA replication rather than DSBs are responsible for cell killing following treatment with genotoxic chemotherapeutic agents. We also highlight that proteins, such as BRCA1, BRCA2, and RAD51 known for canonical DSB repair also have critical roles in normal replication as well as replication gap suppression (RGS) and repair. We review the literature that supports the idea that widespread gap induction proximal to treatment triggers apoptosis in a process that does not need or stem from DSB induction. Lastly, we discuss the clinical evidence for gaps and how to exploit them to enhance genotoxic chemotherapy response.

最初对靶向 DNA 的化学疗法产生的毒性损伤进行了辩论，但最终 DNA 双链断裂 (DSB) 最终占了上风。推理部分基于这样一种观念，即修复断裂的染色体需要复杂的处理或使细胞死亡。DSB 模型的遗传证据也由缺乏 DSB 修复的细胞的极端敏感性提供。特别是，敏感性表征了在遗传性乳腺癌/卵巢癌基因BRCA1或BRCA2中具有突变的细胞，这些突变在通过同源重组 (HR) 修复 DSB 中起作用。除了在 HR 中的功能外，还发现 BRCA 蛋白通过保护停滞的复制叉免受核酸酶降解来防止 DSB。轮回，BRCA获得对基因毒性化疗耐药的突变癌细胞通常通过 HR 和/或恢复的叉保护 (FP) 显示恢复的 DNA 修复，这表明当 DSB 修复完整或 DSB 被阻止时治疗是耐受的。尽管这种得到很好支持的范式一直是靶向癌症治疗的推动力，但我们在此认为，赋予反应的毒性 DNA 病变是单链 DNA (ssDNA) 缺口。我们讨论了在 DNA 复制后形成的持久性 ssDNA 间隙而不是 DSB 导致用基因毒性化学治疗剂治疗后细胞杀伤的证据。我们还强调了以经典 DSB 修复而闻名的 BRCA1、BRCA2 和 RAD51 等蛋白质在正常复制以及复制间隙抑制 (RGS) 和修复中也具有关键作用。我们回顾了支持这一观点的文献，即治疗附近的广泛间隙诱导在不需要或源于 DSB 诱导的过程中触发细胞凋亡。最后，我们讨论了差距的临床证据以及如何利用它们来增强基因毒性化疗反应。