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Cell membrane transporters facilitate the passage of nucleobases and nucleosides for nucleotide synthesis and metabolism, and are important for the delivery of nucleoside analogues used in anticancer drug therapy. Here, we investigated if cell membrane transporters are involved in the cellular uptake of the nucleoside analogue DNA damage mediator 6-thio-2′-deoxyguanosine (6-thio-dG). A large panel of non–small cell lung cancer (NSCLC) cell lines (73 of 77) were sensitive to 6-thio-dG; only four NSCLC lines were resistant to 6-thio-dG. When analyzed by microarray and RNA sequencing, the resistant NSCLC cell lines clustered together, providing a molecular signature for patients that may not respond to 6-thio-dG. Significant downregulation of solute carrier family 43 A3 (SLC43A3), an equilibrative nucleobase transporter, was identified as a candidate in this molecular resistance signature. High levels of SLC43A3 mRNA predicted sensitivity to 6-thio-dG and therefore SLC43A3 could serve as a promising biomarker for 6-thio-dG sensitivity in patients with NSCLC. Significance: These findings identify a biomarker of resistance to the telomeric DNA damage mediator 6-thio-2′-deoxyguanosine.

Summary Telomerase is an attractive target for anti-tumor therapy as it is almost universally expressed in cancer cells. Here, we show that treatment with a telomere-targeting drug, 6-thio-2′-deoxyguanosine (6-thio-dG), leads to tumor regression through innate and adaptive immune-dependent responses in syngeneic and humanized mouse models of telomerase-expressing cancers. 6-thio-dG treatment causes telomere-associated DNA damages that are sensed by dendritic cells (DCs) and activates the host cytosolic DNA sensing STING/interferon I pathway, resulting in enhanced cross-priming capacity of DCs and tumor-specific CD8+ T cell activation. Moreover, 6-thio-dG overcomes resistance to checkpoint blockade in advanced cancer models. Our results unveil how telomere stress increases innate sensing and adaptive anti-tumor immunity and provide strong rationales for combining telomere-targeting therapy with immunotherapy.