Modulating DNA Damage Responses for Improved Breast Cancer Therapy
Ilcheva, Mariya Ilcheva
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The main focus of this work is to improve the efficacy of breast cancer therapy, either by utilizing novel agents that induce specific types of DNA damage resulting in metabolic changes, or by modulating factors that are involved in the repair of toxic DNA double-strand breaks (DSBs). The detoxifying enzyme, NQO1 (NAD(P)H:quinone oxidoreductase-1), is a promising therapeutic target due to its over-expression in many solid cancers, and very low presence in normal cells. Agents, such as β-lapachone and deoxynyboquinone (DNQ), which target NQO1 enzyme to induce programmed necrosis in solid tumors, have shown great promise. However, they have not been able to reveal the full potential of an NQO1-activated anticancer agent due to their low solubility, and more potent tumor-selective compounds are needed. Based on its structure and mode of action, isobutyl-DNQ (IB-DNQ) was recently added to the spectrum of NQO1 substrates. IB-DNQ increases NQO1 processing, enhancing both the potency and the selectivity of its anticancer properties, making it a highly efficient NQO1 substrate, and thus an outstanding anticancer agent. IB-DNQ is a promising antitumor agent whose mechanism of action has not been elucidated yet. We found that IB-DNQ killed breast cancer cells in an NQO1-dependent manner with greater potency than β-lapachone or DNQ. IB-DNQ treatment caused extensive DNA lesions, PARP1 hyperactivation, and severe NAD+ /ATP depletion leading to µ-calpain-mediated cell death (NAD+-Keresis). Next, we tested for synergy between IB-DNQ and the base excision repair (BER) inhibitor, methoxyamine (MeOX). Methoxyamine potentiated IB-DNQ cytotoxicity and allowed the use of very low doses of IB-DNQ, thereby potentially reducing any side-effects. Future studies in vivo will be geared toward proving the equivalent antitumor efficacy of IB-DNQ to β-lapachone and DNQ, but with much greater potency at lower doses. In addition, this study examined factors that modulate the response of BRCA1/2-deficient breast cancers to PARP1 inhibitors. Interestingly, a significant number of BRCA1-deficient breast cancers exhibit aberrantly reduced expression of the Mre11 protein, an important player in DNA damage detection and repair. We assessed the role of Mre11 in the response of BRCA1-deficient breast cancers to PARP1 inhibitors (PARP1i) and found that Mre11 depletion resulted in a significant increase in radial chromosomal structures after IR or PARP1i treatments. These aberrations were indicative of a diminished capacity to repair DSBs by homologous recombination (HR) and subsequent repair of lesions by error-prone pathways, such as NHEJ. Loss of Mre11 abrogated HR repair pathway in BRCA1-deficient cancers, as seen with reduced Rad51 foci, and increased the sensitivity of these tumors to PARP1 inhibitors, thus Mre11 status may be an important prognostic factor in the treatment of BRCA1-deficient breast cancers with PARP inhibitors. Another factor whose inhibition might hyper-sensitize breast cancers to PARP1 inhibitors is CDK1/2 due to the regulatory role that these kinases play in HR. Therefore, we explored the inhibition of CDK1/2 activity as a way to sensitize BRCA1-proficient cancers to PAPR1 inhibition. We found that CDK1/2 inhibition further abrogated DSB repair in BRCA1-deficient cancers, leading to heightened sensitivity to PARP1 inhibitors. These results indicate that inhibition of CDK1/2 could create a state of "BRCAness", which could expand the efficacy of PARP1 inhibitors not only for BRCA1/2-deficient cancers, but also to BRCA1-proficient ones.