Mismatch repair (MMR)-deficient cancers are characterized by microsatellite instability (MSI) and hypermutation. However, it remains unclear how MSI and hypermutation arise and contribute to cancer development. Here, we show that MSI and hypermutation are triggered by replication stress in an MMR-deficient background, enabling clonal expansion of cells harboring ARF/p53-module mutations and cells that are resistant to the anti-cancer drug camptothecin. While replication stress-associated DNA double-strand breaks (DSBs) caused chromosomal instability (CIN) in an MMR-proficient background, they induced MSI with concomitant suppression of CIN via a PARP-mediated repair pathway in an MMR-deficient background. This was associated with the induction of mutations, including cancer-driver mutations in the ARF/p53 module, via chromosomal deletions and base substitutions. Immortalization of MMR-deficient mouse embryonic fibroblasts (MEFs) in association with ARF/p53-module mutations was ~60-fold more efficient than that of wild-type MEFs. Thus, replication stress-triggered MSI and hypermutation efficiently lead to clonal expansion of cells with abrogated defense systems.

Replication-stressassociated DSBs accumulate after exposure to ionizing radiation Such DSBs risk genomic destabilization and associated mutagenesis The resulting genomic rearrangements and mutations lead to clonal evolutionThe radiation-associated risks arise at wide ranges of radiation doses and dose rates

Genomic destabilisation is associated with the induction of mutations, including those in cancer-driver genes, and subsequent clonal evolution of cells with abrogated defence systems. Such mutations are not induced when genome stability is maintained; however, the mechanisms involved in genome stability maintenance remain elusive. Here, resveratrol (and related polyphenols) is shown to enhance genome stability in mouse embryonic fibroblasts, ultimately protecting the cells against the induction of mutations in the ARF/p53 pathway. Replication stress-associated DNA double-strand breaks (DSBs) that accumulated with genomic destabilisation were effectively reduced by resveratrol treatment. In addition, resveratrol transiently stabilised the expression of histone H2AX, which is involved in DSB repair. Similar effects on the maintenance of genome stability were observed for related polyphenols. Accordingly, we propose that polyphenol consumption can contribute to the suppression of cancers that develop with genomic instability, as well as lifespan extension. Most cancers are associated with genomic instability, which can be categorised as chromosomal instability or microsatellite instability (MSI) 1. Genomic destabilisation is a major cause of mutations, including those in cancer-driver genes, and can lead to clonal evolution of cells with abrogated defence systems, such as those containing mutations in the ARF/p53 pathway 2. Genome stability maintenance would likely prevent the formation of mutations and suppress cancer development; however, it is still unclear if genome stability can be maintained in vivo and whether this process can indeed suppress the occurrence of cancer. Genomic instability is caused by the erroneous repair of DNA double-strand breaks (DSBs); paradoxically, the DNA repair systems of most cancers that develop with genomic instability are genetically normal 3. The mechanisms by which normal cells accumulate DSBs remain unclear, but DSBs widely accumulate in pre-cancerous cells and are accompanied by genomic instability 2,4-6. In vitro, replication stress-associated DSBs and the associated genomic instability are observed in cells subjected to aberrant growth stimulation 2 or overexpression of oncogenes such as c-Myc and E2F1 6-8. Possibly reflecting the correlation between cancer development and age, DSBs accumulate with age in vivo and with cultivating passages in vitro 9 , suggesting that ageing cells are defective in DSB repair. DSB repair deficiency is probably related at least in part to a reduction in the level of H2AX. This histone mediates DSB repair and is required for genome stability maintenance, and H2AX expression levels are attenuated when the growth rate of normal cells slows down 10,11. In fact, such cells are defective in repairing replication stress-associated DSBs 10 ,

In the developing central nervous system (CNS), oligodendrocyte precursor cells (OPCs) migrate along blood vessels and are widely distributed in the CNS. Meanwhile, OPCs require survival factors from the extracellular microenvironment. In other tissues, laminins, heterotrimetric (αβγ) extracellular matrix proteins, promote cell migration and survival. However, the expression pattern and functions of laminins in OPC development remain poorly understood. In the present study, we first investigated the expression of laminin α chains, which bind to cell surface receptors such as integrins, in the postnatal murine brain. We found that laminin α1, α2, α4, and α5 chains were expressed around blood vessels and OPCs attached the laminin α chain-positive vessels. We then evaluated the effect of these laminins on OPCs activity using recombinant laminin E8s (LME8s) that are minimally active fragments of the laminin isoforms. OPCs attached on LM211E8, LM411E8, and LM511E8, containing laminin α2, α4, and α5 chains, respectively, through integrin β1. Further, these three LME8s promoted migration of OPCs, and OPC survival was prolonged on either LM411E8 or LM511E8 via the activation of focal adhesion kinase. Together, our findings suggest that laminins expressed surrounding blood vessels positively regulate migration and survival of OPCs through the integrin β1-FAK pathway.