Genome destabilization-associated phenotypes arising as a consequence of therapeutic treatment are suppressed by Olaparib.

Malignancy is often associated with therapeutic resistance and metastasis, usually arising after therapeutic treatment. These include radio- and chemo-therapies, which cause cancer cell death by inducing DNA double strand breaks (DSBs). However, it is still unclear how resistance to these DSBs is induced and whether it can be suppressed. Here, we show that DSBs induced by camptothecin (CPT) and radiation jeopardize genome stability in surviving cancer cells, ultimately leading to the development of resistance. Further, we show that cytosolic DNA, accumulating as a consequence of genomic destabilization, leads to increased cGAS/STING-pathway activation and, ultimately, increased cell migration, a precursor of metastasis. Interestingly, these genomic destabilization-associated phenotypes were suppressed by the PARP inhibitor Olaparib. Recognition of DSBs by Rad51 and genomic destabilization were largely reduced by Olaparib, while the DNA damage response and cancer cell death were effectively increased. Thus, Olaparib decreases the risk of therapeutic resistance and cell migration of cells that survive radio- and CPT-treatments.

Exploiting Transient Radical Cations as Brønsted Acids for Allylic C-H Heteroarylation of Enol Silyl Ethers.

Intermediary radical cations, generated through single-electron oxidation of enol silyl ethers by excited Ir-based photocatalysts, can be exploited as Brønsted acids for the activation of heteroarylcyanides. This strategy enables the direct allylic C-H heteroarylation of enol silyl ethers under visible-light irradiation.

Resveratrol and its Related Polyphenols Contribute to the Maintenance of Genome Stability.

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.

Genomic-Destabilization-Associated Mutagenesis and Clonal Evolution of Cells with Mutations in Tumor-Suppressor Genes.

The development of cancer is driven by genomic instability and mutations. In general, cancer develops via multiple steps. Each step involves the clonal evolution of cells with abrogated defense systems, such as cells with mutations in cancer-suppressor genes. However, it remains unclear how cellular defense systems are abrogated and the associated clonal evolution is triggered and propagated. In this manuscript, we review current knowledge regarding mutagenesis associated with genomic destabilization and its relationship with the clonal evolution of cells over the course of cancer development, focusing especially on mechanistic aspects.

Replication stress triggers microsatellite destabilization and hypermutation leading to clonal expansion in vitro.

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.

Mismatch repair dependence of replication stress-associated DSB recognition and repair.

Most cancers develop with one of two types of genomic instability, namely, chromosomal instability (CIN) or microsatellite instability (MSI). Both are induced by replication stress-associated DNA double-strand breaks (DSBs). The type of genomic instability that arises is dependent on the choice of DNA repair pathway. Specifically, MSI is induced via a PolQ-dependent repair pathway called microhomology-mediated end joining (MMEJ) in a mismatch repair (MMR)-deficient background. However, it is unclear how the MMR status determines the choice of DSB repair pathway. Here, we show that replication stress-associated DSBs initially targeted by the homologous recombination (HR) system were subsequently hijacked by PolQ-dependent MMEJ in MMR-deficient cells, but persisted as HR intermediates in MMR-proficient cells. PolQ interacting with MMR factors was effectively loaded onto damaged chromatin in an MMR-deficient background, in which merged MRE11/γH2AX foci also effectively formed. Thus, the choice of DNA repair pathway according to the MMR status determines whether CIN or MSI is induced.

Onset of deaminase APOBEC3B induction in response to DNA double-strand breaks.

Deamination of 5-methyl cytosine is a major cause of cancer-driver mutations in inflammation-associated cancers. The deaminase APOBEC3B is expressed in these cancers and causes mutations under replication stress; however, the mechanisms by which APOBEC3B mediates deamination and its association with genomic disorders are still unclear. Here, we show that APOBEC3B is stabilized to induce deamination reaction in response to DNA double-strand breaks (DSBs), resulting in the formation of long-lasting DSBs. Uracil, the major deamination product, is subsequently targeted by base excision repair (BER) through uracil-DNA glycosylase 2 (UNG2); hence late-onset DSBs arise as by-products of BER. The frequency of these delayed DSBs was increased by treatment of cells with a PARP inhibitor, and was suppressed following knock-down of UNG2. The late-onset DSBs were induced in an ATR-dependent manner. Those secondary DSBs were persistent, unlike DSBs directly caused by γ-ray irradiation. Overall, these results suggest that the deaminase APOBEC3B is induced in response to DSBs, leading to long-lasting DSB formation in addition to mutagenic 5me-C>T transition induction.

Sensitization of Cancer Cells to Radiation and Topoisomerase I Inhibitor Camptothecin Using Inhibitors of PARP and Other Signaling Molecules.

Radiation and certain anticancer drugs damage DNA, resulting in apoptosis induction in cancer cells. Currently, the major limitations on the efficacy of such therapies are development of resistance and adverse side effects. Sensitization is an important strategy for increasing therapeutic efficacy while minimizing adverse effects. In this manuscript, we review possible sensitization strategies for radiation and anticancer drugs that cause DNA damage, focusing especially on modulation of damage repair pathways and the associated reactions.