O 6-methylguanine-DNA methyltransferase (MGMT) is an enzyme that removes alkyl groups at the O 6-position of guanine in DNA. MGMT expression is reduced or absent in many tumor types derived from a diverse range of tissues, most notably in glioma. Low MGMT expression confers significant sensitivity to DNA alkylating agents such as temozolomide (TMZ), providing a natural therapeutic index over normal tissue. In this study, we sought to identify novel approaches which could maximally exploit the therapeutic index between tumor cells and normal tissues based on MGMT expression, as a means to enhance selective tumor cell killing. TMZ, unlike other alkylators, activated the Ataxia Telangiectasia and Rad3-related (ATR)-Checkpoint Kinase 1 (Chk1) axis in a manner that was highly dependent on MGMT status. TMZ induced growth delay, DNA double-strand breaks, and G2/M cell cycle arrest, which led to ATR-dependent phosphorylation of Chk1; this effect was dependent on reduced MGMT expression. Treatment of MGMT-deficient cells with TMZ increased sensitivity to ATR inhibitors both in vitro and in vivo across numerous tumor cell types. Taken together, this study reveals a novel approach for selectively targeting MGMT-deficient cells with ATR inhibitors and TMZ. As ATR inhibitors are currently being tested in clinical trials, and TMZ is a commonly used chemotherapeutic, this approach is clinically actionable. Furthermore, this interaction potently exploits a DNA-repair defect found in many cancers.

The methylation status of the O6-methylguanine methyltransferase (MGMT) gene promoter has been widely accepted as a prognostic biomarker for treatment with the alkylator, temozolomide (TMZ). In the absence of promoter methylation, the MGMT enzyme removes O6-methylguanine (O6-meG) lesions. In the setting of MGMT-promoter methylation (MGMT-), the O6-meG lesion activates the mismatch repair (MMR) pathway which functions to remove the damage. Our group reported that loss of MGMT expression via MGMT promoter silencing modulates activation of ataxia telangiectasia and RAD3 related protein (ATR) in response to TMZ treatment, which is associated with synergistic tumor-cell killing. Whether or not MMR proteins are involved in ATR activation in MGMT-cells upon alkylation damage remains poorly understood. To investigate the function of MMR in ATR activation, we created isogenic cell lines with knockdowns of the individual human MMR proteins MutS homolog 2 (MSH2), MutS homolog 6 (MSH6), MutS homolog 3 (MSH3), MutL homolog 1 (MLH1), and PMS1 homolog 2 (PMS2). Here, we demonstrate that MSH2, MSH6, MLH1 and PMS2, specifically, are involved in the activation of the ATR axis after TMZ exposure, whereas MSH3 is likely not. This study elucidates a potential mechanistic understanding of how the MMR system is involved in ATR activation by TMZ in glioblastoma cells, which is important for targeting MMR-mutated cancers.

Mutations in the isocitrate dehydrogenase-1 and -2 (IDH1/2) genes were first identified in glioma and acute myeloid leukemia (AML), and subsequently found in multiple other tumor types. These neomorphic mutations convert the normal product of enzyme, α-ketoglutarate (αKG), to the oncometabolite 2-hydroxyglutarate (2HG). Our group recently demonstrated that 2HG suppresses the high-fidelity homologous recombination (HR) DNA repair pathway, resulting in a state referred to as ‘BRCAness’, which confers exquisite sensitivity to poly(ADP-ribose) polymerase (PARP) inhibitors. In this study, we sought to elucidate sensitivity of IDH1/2-mutant cells to DNA damage response (DDR) inhibitors and, whether combination therapies could enhance described synthetic lethal interactions. Here, we report that ATR (ataxia telangiectasia and Rad3-related protein kinase) inhibitors are active against IDH1/2-mutant cells, and that this activity is further potentiated in combination with PARP inhibitors. We demonstrate this interaction across multiple cell line models with engineered and endogenous IDH1/2 mutations, with robust anti-tumor activity in vitro and in vivo. Mechanistically, we found ATR and PARP inhibitor treatment induces premature mitotic entry, which is significantly elevated in the setting of IDH1/2-mutations. These data highlight the potential efficacy of targeting HR defects in IDH1/2-mutant cancers and support the development of this combination in future clinical trials.

<div>Abstract<p>O⁶-methylguanine-DNA methyltransferase (MGMT) is an enzyme that removes alkyl groups at the O⁶-position of guanine in DNA. MGMT expression is reduced or absent in many tumor types derived from a diverse range of tissues, most notably in glioma. Low MGMT expression confers significant sensitivity to DNA alkylating agents such as temozolomide, providing a natural therapeutic index over normal tissue. In this study, we sought to identify novel approaches that could maximally exploit the therapeutic index between tumor cells and normal tissues based on MGMT expression, as a means to enhance selective tumor cell killing. Temozolomide, unlike other alkylators, activated the ataxia telangiectasia and Rad3–related (ATR)–checkpoint kinase 1 (Chk1) axis in a manner that was highly dependent on MGMT status. Temozolomide induced growth delay, DNA double-strand breaks, and G₂–M cell-cycle arrest, which led to ATR-dependent phosphorylation of Chk1; this effect was dependent on reduced MGMT expression. Treatment of MGMT-deficient cells with temozolomide increased sensitivity to ATR inhibitors both <i>in vitro</i> and <i>in vivo</i> across numerous tumor cell types. Taken together, this study reveals a novel approach for selectively targeting MGMT-deficient cells with ATR inhibitors and temozolomide. As ATR inhibitors are currently being tested in clinical trials, and temozolomide is a commonly used chemotherapeutic, this approach is clinically actionable. Furthermore, this interaction potently exploits a DNA-repair defect found in many cancers.</p>Significance:<p>Monofunctional alkylating agents sensitize MGMT-deficient tumor cells to ATR inhibitors.</p></div>

The methylation status of the O6-methyl guanine methyltransferase (MGMT) gene promoter is a prognostic biomarker for treatment with the alkylator, temozolomide (TMZ) in many solid tumors including gliomas and colorectal cancers. It is well established that patients with a methylated MGMT promoter (MGMT-) who are treated with the TMZ have a better overall survival than patients with an unmethylated MGMT promoter (MGMT+). The enzyme produced by the MGMT gene is responsible for removing cytotoxic O6-methylguanine (O6-meG) lesions formed by TMZ. In the MGMT- setting, the O6-meG lesion activates the mismatch repair (MMR) pathway which functions to remove the damage. Published work from our group reported differential activation of the ataxia telangiectasia and RAD3 related protein (ATR) in MGMT- and MGMT+ glioblastoma multiforme (GBM) cells in response to TMZ treatment, as demonstrated through the phosphorylation of CHK1. Though it is known that MMR proteins are involved in ATR activation, the specific MMR proteins required for ATR activation by TMZ-induced alkyl lesions remain unknown in the MGMT- setting. Here, we demonstrate that the MMR protein MSH2 plays a role in ATR activation in the presence of O6-meG lesions. We show that there is potent synergy with ATRi and TMZ in the MGMT- MSH2-proficient GBM cell line, which is abrogated in an shMSH2 MGMT- GBM cell line. Additionally, we observe decreased levels of pCHK1 in the shMSH2 MGMT- setting compared to the MGMT- MSH2-proficient cells, suggesting that MSH2 is integral in the activation of ATR upon TMZ treatment. This study elucidates a potential role for MSH2 in ATR activation. Mechanistic understanding of how the MMR system is involved in ATR activation by TMZ can ultimately be exploited for therapeutic gain. Citation Format: Sachita Ganesa, Amrita D. Sule, Christopher B. Jackson, Ranjini K. Sundaram, Ranjit S. Bindra. MSH2 is necessary for temozolomide-induced ATR activation in MGMT-methylated cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1179.