An oncogene-induced DNA replication stress model for human cancer development
Omar Zgheib1, Ayguel Dereli1, Gwennaelle Versini1, Leandros Vassiliou2, Vassilis G Gorgoulis2, Thanos D Halazonetis1
1University of Geneva, Switzerland; 2University of Athens, Greece
DNA damage checkpoint genes, such as p53, are frequently mutated in human cancer, but the selective pressure for their inactivation remains elusive. Further, most human cancers have chromosomal instability, but the genes whose mutation leads to this type of genomic instability have also remained elusive. We hypothesized that the presence of DNA double-strand breaks (DSBs) in cancer cells could explain both the presence of p53 mutations and the genomic instability. To test this hypothesis, we analysed a panel of human precancerous and cancerous lesions for the presence of DNA damage response markers using immunohistochemistry and for the presence of genomic instability by loss-of-heterozygosity analysis. Similarly, we also examined human cells in tissue culture in which oncogene expression was induced. In a panel of lung hyperplasias, all of which retained wild-type p53 genes, we found signs of a DNA damage response, including histone H2AX and Chk2 phosphorylation, p53 accumulation, focal staining of 53BP1 and apoptosis or senescence. Progression to carcinoma was associated with p53, 53BP1 or Chk2 inactivation and suppression of apoptosis and senescence. A DNA damage response was also observed in dysplastic nevi and in human skin xenografts, in which hyperplasia was induced by overexpression of growth factors. Both lung and experimentally-induced skin hyperplasias showed allelic imbalance at loci (common fragile sites) that are prone to formation of DNA DSBs when DNA replication is compromised. Further, in various model systems in tissue culture, oncogene overexpression led to stalling and collapse of DNA replication forks and generation of DNA DSBs. We conclude that from its earliest stages, cancer development is associated with DNA replication stress, which leads to DNA double-strand breaks, genomic instability and selective pressure for p53 mutations.
