Inactivation of the Myc−Induced DNA Damage Response Promotes Lymphomagenesis and Treatment Resistance In Vivo.

Activation of the DNA damage response (DDR) was recently claimed to serve as a critical anti−cancer barrier in early human tumorigenesis (Bartkova−J et al., and Gorgoulis−VG et al., Nature, 2005). As a consequence, deregulation of the DDR machinery may be required for tumor manifestation.

Myc activation is an oncogenic event frequently detected in malignant lymphomas. It is known that constitutive Myc expression provokes apoptosis via the ARF/p53 pathway as a cellular failsafe mechanism to counteract malignant transformation, but the potential role of Myc signaling to the DDR machinery, mediated via the Atm (ataxia telangiectasia mutated) kinase, as another putative tumor suppressive restraint remains still elusive.

Utilizing a tamoxifen−regulatable Myc system as well as RNA interference in vitro, we found in cell−based models that acute induction of Myc, associated with an increased production of reactive oxygen species (ROS) and marks of DNA strand breaks, leads to p53 activation which is mainly mediated by Atm and its upstream regulator PP5.

To further examine the role of Atm during Myc−driven tumorigenesis in vivo, we intercrossed Atm knockout mice to Eμ−myc transgenic mice that develop B−cell lymphomas. Lymphomas formed significantly faster in the absence of Atm, mostly due to a significant apoptotic defect in Atm−/− lymphomas compared to Atm+/+ derived (referred to as ‘control’) lymphomas.

In accordance with our in vitro findings, we detected significantly elevated levels of ROS−induced oxidative DNA damage and γ−H2AX foci in Myc−driven lymphomas. Importantly, constitutive expression of Myc selected against components of the Atm−governed DDR in a subset of control lymphomas. Like Atm deficiency, these lesions predetermined a poor response in subsequent cytotoxicity assays in vitro and anticancer therapy in vivo, since DNA damaging treatments also produce apoptosis via PP5/Atm.

The role for ROS as inducers of DNA damage during the execution of Myc−induced apoptosis was further substantiated by the finding that Eμ−myc transgenic mice continuously exposed to the ROS scavenger N−acetylcysteine (starting prenatally) developed lymphomas with significantly reduced levels of spontaneous apoptosis and with less marks of DNA damage (i.e. γ−H2AX foci), but a retained ability to respond to anticancer therapy in vitro and in vivo.

Hence, Atm eliminates pre−neoplastic lesions by converting oncogenic signaling into apoptosis, while selection for an attenuated Atm response permits lymphoma formation, and, in turn, has profound implications for reduced efficacy of DNA damaging therapies.