Topoisomerase I poison-triggered immune gene activation is markedly reduced in human small-cell lung cancers by impairment of the cGAS/STING pathway

Background: Current immunotherapy strategies have contrasting clinical results in human lung cancer patients as small-cell lung cancers (SCLC) often show features of immunological cold tumours. Topoisomerase 1 (TOP1) poisons are effective antitumor drugs with good efficacy against lung cancers. Methods: We used molecular, genetic and bioinformatic approaches to determine the mechanism of micronuclei formation induced by two TOP1 poisons in different human cancer cells, including SCLC cell lines. Results: TOP1 poisons stimulate similar levels of micronuclei in all tested cell lines but downstream effects can vary markedly. TOP1 poisons increase micronuclei levels with a mechanism involving R-loops as overexpression of RNaseH1 markedly reduces or abolishes both H2AX phosphorylation and micronuclei formation. TOP1 poison-induced micronuclei activate the cGAS/STING pathway leading to increased expression of immune genes in HeLa cells, but not in human SCLC cell lines, mainly due to lack of STING and/or cGAS expression. Moreover, the expression of STING and antigen-presenting machinery genes is generally downregulated in patient tumours of human lung cancer datasets. Conclusions: Altogether, our data reveal an immune signalling mechanism activated by TOP1 poisons, which is often impaired in human SCLC tumours.