CDC7 kinase promotes replication DNA double-strand breaks upon replication fork stalling and its inhibition causes an inflammatory response and senescence
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CDC7 is an essential serine-threonine kinase that has an important role in the initiation of DNA replication. CDC7 phosphorylates several sites of the MCM helicase complex allowing recruitment of CDC45, among other factors, and stimulating origin firing. In recent years, various inhibitors of CDC7 have been developed as anti-cancer therapeutics. Increased understanding of the cellular response to CDC7 inhibitors is crucial in order to identify the additional roles of CDC7. Although the effect of short-term CDC7 inhibition has been broadly studied, the cellular response to prolonged CDC7 inhibition has been hardly investigated. Novel evidence has demonstrated the role of CDC7 in the cellular response to replication stress. Replication stress can be cause by endogenous or exogenous insults, which cause the slowing or stalling of replication forks. Using an inhibitor of CDC7, this research suggests that CDC7 activity promotes replication fork collapse upon prolonged fork stalling. Further, this research discloses that CDC7 inhibition does not affect EXO1 stability, as previously observed , and do not stimulates RAD51 filaments formation on the reversed forks. Our work in EMBO Reports also showed that CDC7 inhibition protect MRE11 dependent fork degradation in BRCA2 deficient cells, and suggest that CDC7 activity is required for the regulation of MRE11 activity . The results shown in this study, together with the results shown in Rainey et al 2020 , suggest that CDC7 is a key regulator of fork processing and integrity in response to replication stress . This study also investigated the phenotype induced by long-term treatment with two potent and specific CDC7 inhibitors: XL413 and TAK-931. Treatment was performed for 8 days in the breast epithelial cell line, MCF10A, and a block in proliferation and a reduction of S-phase cells were observed upon CDC7 inhibition. RNA-sequencing data from cells treated for 8 days with CDC7 inhibitors, suggest that long-term CDC7 inhibition induces a major transcriptional change in MCF10A. However, only a portion of the differentially expressed gene (DEGs) observed is common between the two CDC7 inhibitors, suggesting that XL413 and TAK-931 do not elicit the same transcriptional response. Nevertheless, an upregulation of various interleukins (ILs), chemokines and cytokines, in both XL413 and TAK-931 treated samples, was observed, suggesting that prolonged CDC7 inhibition can lead to an inflammatory phenotype. Further results suggest that prolonged CDC7 inhibition induces an accumulation of cells with micronuclei and the localization of the Cyclic GMP-AMP Synthase (cGAS) protein, a signaling enzyme that controls immune-sensing of cytosolic DNA, at the micronuclei. This suggests a possible activation of the cGAS-STING pathway upon CDC7 kinase inhibition. In addition, long-term treatment with CDC7 inhibitors causes the accumulation of β-Galactosidase positive cells, a typical marker of senescence cells. However, cells were able to recover after the removal of the inhibitors, suggesting that prolonged CDC7 inhibition induces a senescence-like phenotype in MCF10A. The senescence phenotype is also observed in both MCF7 and MDA-MB231, suggesting that the senescence-like phenotype is a general response to CDC7 inhibition. Lastly, the preliminary results suggest that p53 plays a role in the phenotype observed. Loss of p53 prevents the senescence-like phenotype observed in MCF10A. The results of this studies have important potential implications in the use of CDC7 inhibitors as chemotherapeutics. The induction of senescence causes changes in the cellular physiology of cancer cells, which can be exploited to specifically target and eradicate senescence cancer cells, also called senolysis.