A Role for E2F Activities in Determining the Fate of Myc- Induced Lymphomagenesis Rachel E. Rempel1, Seiichi Mori1,2, Maura Gasparetto3, Michele A. Glozak4, Eran R. Andrechek1, Steven B. Adler1, Nina M. Laakso1, Anand S. Lagoo 5 , Robert Storms 6 , Clay Smith3, Joseph R. Nevins1* 1 Duke Institute for Genome Sciences and Policy, Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina, United States of America, 2 Oncology Research Institute, National University of Singapore, Singapore, Singapore, 3 Terry Fox Laboratory/BC Cancer Research Center, Vancouver, British Columbia, Canada, 4 Department of Interdisciplinary Oncology, Moffitt Cancer Center, Tampa, Florida, United States of America, 5 Department of Pathology, Duke University Medical Center, Durham, North Carolina, United States of America, 6 Department of Medicine, Duke University Medical Center, Durham, North Carolina, United States of America Abstract The phenotypic heterogeneity that characterizes human cancers reflects the enormous genetic complexity of the oncogenic process. This complexity can also be seen in mouse models where it is frequently observed that in addition to the initiating genetic alteration, the resulting tumor harbors additional, somatically acquired mutations that affect the tumor phenotype. To investigate the role of genetic interactions in the development of tumors, we have made use of the Em-myc model of pre- B and B cell lymphoma. Since various studies point to a functional interaction between Myc and the Rb/E2F pathway, we have investigated the role of E2F activities in the process of Myc-induced lymphomagenesis. Whereas the absence of E2F1 and E2F3 function has no impact on Myc-mediated tumor development, the absence of E2F2 substantially accelerates the time of tumor onset. Conversely, tumor development is delayed by the absence of E2F4. The enhanced early onset of tumors seen in the absence of E2F2 coincides with an expansion of immature B lineage cells that are likely to be the target for Myc oncogenesis. In contrast, the absence of E2F4 mutes the response of the lineage to Myc and there is no expansion of immature B lineage cells. We also find that distinct types of tumors emerge from the Em-myc mice, distinguished by different patterns of gene expression, and that the relative proportions of these tumor types are affected by the absence of either E2F2 or E2F4. From these results, we conclude that there are several populations of tumors that arise from the Em-myc model, reflecting distinct populations of cells that are susceptible to Myc-mediated oncogenesis and that the proportion of these cell populations is affected by the presence or absence of E2F activities. Citation: Rempel RE, Mori S, Gasparetto M, Glozak MA, Andrechek ER, et al. (2009) A Role for E2F Activities in Determining the Fate of Myc-Induced Lymphomagenesis. PLoS Genet 5(9): e1000640. doi:10.1371/journal.pgen.1000640 Editor: Jason D. Lieb, The University of North Carolina at Chapel Hill, United States of America Received February 9, 2009; Accepted August 10, 2009; Published September 11, 2009 Copyright: 2009 Rempel et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The source of funding for this work is the National Institutes of Health, grant 5-RO1-CA-106520-04. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: j.nevins@duke.edu Introduction A hallmark of human cancer is genetic complexity, reflecting the acquisition of multiple mutations and gene rearrangements that give rise to the tumor phenotype. Indeed, recent large-scale DNA sequencing efforts have provided direct evidence for this complexity, revealing large numbers of alterations that character- ize various tumor types [1–4]. Undoubtedly, this genetic complexity of cancer underlies much of the challenge in developing effective therapeutic strategies. Not only is it likely that combinations of drugs will be necessary to match the complexity and effectively treat these tumors but equally important is the ability to identify subgroups of cancers that represent more homogeneous mechanisms of disease. An ability to model the complexity that gives rise to the tumor heterogeneity seen in human cancers would clearly enhance the understanding of the oncogenic process but also would enable the development and testing of combination therapeutics that might match this complexity. Mouse models of cancer have generally employed the use of an activated oncogene or the disruption of a tumor suppressor gene to initiate the oncogenic process. Although this represents a defined genetic alteration, it is also true that in most instances this single event is not sufficient to allow for tumor development. This can be seen in the often protracted latency of tumor development as well as the identification of specific additional genetic alterations that appear in these tumors. An example of a well-studied genetic model for the analysis of pre-B and B cell lymphoma is the Em-myc transgenic mouse. In the Em-myc transgenic mouse c-myc is constitutively expressed in the B lineage [5,6]. The resulting polyclonal expansion of pre-B cells is initially limited by increased apoptosis [7]. Additional mutations, many of which inactivate the p53 tumor suppressor pathway [8], then arise. This leads to the emergence of a clonal pre-B or B cell lymphoma by six months of age in mice of a mixed C57Bl/6 and 129 strain background. Myc has been shown to induce a large number of genes that contribute to cell proliferation. These include the direct transcrip- tional activation of D cyclin genes, the cdk4 gene encoding the kinase partner for cyclin D, and the Cdc25A gene encoding the phosphatase that removes negative regulatory phosphates from the Cdks. The induction of Cyclin D/cdk4 activity leads directly to the phosphorylation of Rb and thus activation of E2Fs. Numerous PLoS Genetics | www.plosgenetics.org 1 September 2009 | Volume 5 | Issue 9 | e1000640