Department of Molecular Biology, University of Geneva, 1205 Geneva, Switzerland.
Nature. 2018 Mar 1;555(7694):112-116. doi: 10.1038/nature25507. Epub 2018 Feb 21.
Oncogene-induced DNA replication stress contributes critically to the genomic instability that is present in cancer. However, elucidating how oncogenes deregulate DNA replication has been impeded by difficulty in mapping replication initiation sites on the human genome. Here, using a sensitive assay to monitor nascent DNA synthesis in early S phase, we identified thousands of replication initiation sites in cells before and after induction of the oncogenes CCNE1 and MYC. Remarkably, both oncogenes induced firing of a novel set of DNA replication origins that mapped within highly transcribed genes. These ectopic origins were normally suppressed by transcription during G1, but precocious entry into S phase, before all genic regions had been transcribed, allowed firing of origins within genes in cells with activated oncogenes. Forks from oncogene-induced origins were prone to collapse, as a result of conflicts between replication and transcription, and were associated with DNA double-stranded break formation and chromosomal rearrangement breakpoints both in our experimental system and in a large cohort of human cancers. Thus, firing of intragenic origins caused by premature S phase entry represents a mechanism of oncogene-induced DNA replication stress that is relevant for genomic instability in human cancer.
癌基因诱导的 DNA 复制应激对基因组不稳定性至关重要，而基因组不稳定性存在于癌症中。然而，由于难以在人类基因组上定位复制起始点，阐明癌基因如何使 DNA 复制失调一直受到阻碍。在这里，我们使用一种灵敏的检测方法来监测早期 S 期的新生 DNA 合成，在诱导癌基因 CCNE1 和 MYC 前后，我们在细胞中鉴定了数千个复制起始点。值得注意的是，这两种癌基因都诱导了一组新的 DNA 复制起始点的激活，这些起始点位于高度转录的基因内。这些异位起始点通常在 G1 期被转录抑制，但在所有基因区域被转录之前，细胞提前进入 S 期，允许激活的癌基因中的基因内起始点被激活。由于复制和转录之间的冲突，癌基因诱导的起始点的叉容易崩溃，并与我们的实验系统和大量人类癌症中的 DNA 双链断裂形成和染色体重排断点相关。因此，由过早的 S 期进入引起的基因内起始点的激活代表了一种与人类癌症中基因组不稳定性相关的癌基因诱导的 DNA 复制应激机制。
