Inhibition of Replication Origins and ATR Synergistically Activates the Innate Immune System in Cancer Cells

Sufficient numbers of activated replication origins are essential for successful DNA replication. While normal cells load replication origins in abundance to the genome, cancer cells often load fewer origins of replication due to genomic alterations such as CCNE1 amplifications. Here we exploit this feature to sensitize cancer cells to ATR inhibition. We show by using siRNA and small molecules that the inhibition of origin activation as well as the reduction of replication origins itself sensitizes ovarian cancer cell lines against ATR inhibition by inducing genomic instability. We further show that in ovarian cancer cells this combinatorial approach leads to an increased genomic instability which manifests in an increase of micronuclei that consequently activate the innate immune system through the cGAS-STING pathway. Notably, no activation of the innate immune system was observed in immortalized fallopian tube cells. However, overexpression of Cyclin E1 in this model leads to a marked increase in genomic instability and innate immune activation. Here we provide preclinical evidence to increase the therapeutic efficacy of ATR inhibitors. Additionally, this approach could help to activate innate immunity and increase T-cell immune infiltration in tumors, providing a rationale for a combination with immune checkpoint inhibitors.