The Existence of MTH1-independent 8-oxodGTPase Activity in Cancer Cells as a Compensatory Mechanism against On-target Effects of MTH1 Inhibitors

Abstract

Investigations into the human 8-oxodGTPase, MutT Homolog 1 (MTH1), have risen sharply since the first-in-class MTH1 inhibitors were reported to be highly tumoricidal. However, MTH1 as a cancer therapeutic target is currently controversial because subsequently developed inhibitors did not exhibit similar cytotoxic effects. Here, we provide the first direct evidence for MTH1-independent 8-oxodGTPase function in human cancer cells and human tumors, using a novel ATP-releasing guanine-oxidized (ARGO) chemical probe. Our studies show that this functionally redundant 8-oxodGTPase activity is not decreased by five different published MTH1-targeting small molecules or by MTH1 depletion. Significantly, while only the two first-in-class inhibitors, TH588 and TH287, reduced cancer cell viability, all five inhibitors evaluated in our studies decreased 8-oxodGTPase activity to a similar extent. Thus, the reported efficacy of the first-in-class MTH1 inhibitors does not arise from their inhibition of MTH1-specific 8-oxodGTPase activity. Comparison of DNA strand breaks, genomic 8-oxoguanine incorporation, or alterations in cellular oxidative state by TH287 versus the noncytotoxic inhibitor, IACS-4759, contradict that the cytotoxicity of the former results solely from increased levels of oxidatively damaged genomic DNA. Thus, our findings indicate that mechanisms unrelated to oxidative stress or DNA damage likely underlie the reported efficacy of the first-inclass inhibitors. Our study suggests that MTH1 functional redundancy, existing to different extents in all cancer lines and human tumors evaluated in our study, is a thus far undefined factor which is likely to be critical in understanding the importance of MTH1 and its clinical targeting in cancer.

OGG1 co-inhibition antagonizes the tumor-inhibitory effects of targeting MTH1

“…We have previously shown that MTH1 expression and 8-oxo-dGTPase activity are elevated in human lung adenocarcinomas relative to counterpart normal tissues [ 4 , 19 , 37 , 38 ]. To assess whether OGG1/MTH1 co-inhibition is likely to enhance anti-tumor outcomes, we evaluated whether OGG1 levels were elevated globally in human lung adenocarcinoma specimens or in tumor subsets without enhanced MTH1 expression.…”

“…To further investigate the molecular effects of MTH1/OGG1 co-inhibition, we treated A549 cells with a novel small molecule OGG1/MTH1 co-inhibitor SU0383 31 as well as with SU0268, IACS-4759 (a second-generation MTH1 inhibitor with minimal off-target effects [ 15 , 19 ]) or with a combination of SU0268/IACS-4759, the two molecules which make up the SU0383 scaffold ( Fig. 5 A).…”

“…5 B). As we have previously published, IACS-4759 alone also did not produce any significant anti-tumor cytotoxicity, presumably due to its inability to target non-MTH1 8-oxo-dGTPase activity [ 19 ]. In general, across the majority of treatment conditions, we did not observe any results to suggest an enhanced anti-tumor advantage to co-inhibiting MTH1/OGG1 over the single inhibitor treatments ( Fig.…”

“…However, a body of in vitro and in vivo work has shown that genomic 8-oxo-dG, in and of itself, is not a genotoxic lesion but instead a mildly mutagenic one [ 13 ]. In contrast to the TH inhibitors, other independently developed MTH1 inhibitors have been unable to produce robust anti-tumor effects, despite comparable or better target engagement [ [14] , [15] , [16] , [17] , [18] ] and similar inhibition of MTH1 8-oxo-dGTPase activity and genomic 8-oxo-dG incorporation [ 19 ]. We and others have surmised that the cytotoxicity of TH588 and TH287 is not due to their inhibition of MTH1 8-oxo-dGTPase activity but rather due to their direct production of reactive oxygen species (ROS) or other off-target activity [ [19] , [20] , [21] ].…”

Biomarkers of nucleic acid oxidation – A summary state-of-the-art

“…Indeed, this process has focused interest on using inhibition of 8-oxodGTPases as a therapeutic target in cancer [ 62 ] as, due to their faster rate of proliferation, and increased intracellular ROS, cancer cells are thought to have greater reliance on enzymes, such as the 8-oxodGTPases [ [63] , [64] , [65] ]. However, the results from the various studies exploring this area have been mixed between positive [ [66] , [67] , [68] ] and negative [ 69 , 70 ], although this may arise from differences in the inhibitors, the experimental systems being used [ 71 ], and redundancy in the pathways being inhibited [ 72 ].…”