Members of the Runx and MYC families have been implicated as collaborating oncogenes. The mechanism of this potent collaboration is elucidated in this study of Runx2/MYC mice. As shown previously, ectopic expression of Runx2 in the thymus leads to a preneoplastic state defined by an accumulation of cells with an immature phenotype and a low proliferative rate. We now show that c-MYC overexpression is sufficient to rescue proliferation and to release the differentiation block imposed by Runx2. Analysis of Runx2-expressing lymphomas reveals a consistently low rate of apoptosis, in contrast to lymphomas of MYC mice which are often highly apoptotic. The low apoptosis phenotype is dominant in Runx2/ MYC tumors, indicating that Runx2 confers a potent survival advantage to MYC-expressing tumor cells. The role of the p53 pathway in Runx2/MYC tumors was explored on a p53 heterozygote background. Surprisingly, functional p53 was retained in vivo, even after transplantation, whereas explanted tumor cells displayed rapid allele loss in vitro. Our results show that Runx2 and MYC overcome distinct ''fail-safe'' responses and that their selection as collaborating genes is due to their ability to neutralize each other's negative growth effect. Furthermore, the Runx2/MYC combination overcomes the requirement for genetic inactivation of the p53 pathway in vivo. (Cancer Res 2006; 66(4): 2195-201)

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In this study, we have exploited the power of insertional mutagenesis to elucidate tumor progression pathways in mice carrying two oncogenes (MYC/Runx2) that collaborate to drive early lymphoma development. Neonatal infection of these mice with Moloney murine leukemia virus resulted in accelerated tumor onset with associated increases in clonal complexity and lymphoid dissemination. Large-scale analysis of retroviral integration sites in these tumors revealed a profound bias towards a narrow range of target genes, including Jdp2 (Jundm2), D cyclin, and Pim family genes. Remarkably, direct PCR analysis of integration hotspots revealed that every progressing tumor consisted of multiple clones harboring hits at these loci, giving access to large numbers of independent insertion events and uncovering the contrasting mutagenic mechanisms operating at each target gene. Direct PCR analysis showed that high-frequency targeting occurs only in the tumor environment in vivo and is specific for the progression gene set. These results indicate that early lymphomas in MYC/Runx2 mice remain dependent on exogenous growth signals, and that progression can be achieved by constitutive activation of pathways converging on a cell cycle checkpoint that acts as the major rate-limiting step for lymphoma outgrowth. [Cancer Res 2007;67(11):5126-33]

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Retroviral insertional mutagenesis (RIM) is a powerful tool for cancer genomics that was combined in this study with deep sequencing (RIM/DS) to facilitate a comprehensive analysis of lymphoma progression. Transgenic mice expressing two potent collaborating oncogenes in the germ line (CD2-MYC, -Runx2) develop rapid onset tumours that can be accelerated and rendered polyclonal by neonatal Moloney murine leukaemia virus (MoMLV) infection. RIM/DS analysis of 28 polyclonal lymphomas identified 771 common insertion sites (CISs) defining a ‘progression network’ that encompassed a remarkably large fraction of known MoMLV target genes, with further strong indications of oncogenic selection above the background of MoMLV integration preference. Progression driven by RIM was characterised as a Darwinian process of clonal competition engaging proliferation control networks downstream of cytokine and T-cell receptor signalling. Enhancer mode activation accounted for the most efficiently selected CIS target genes, including Ccr7 as the most prominent of a set of chemokine receptors driving paracrine growth stimulation and lymphoma dissemination. Another large target gene subset including candidate tumour suppressors was disrupted by intragenic insertions. A second RIM/DS screen comparing lymphomas of wild-type and parental transgenics showed that CD2-MYC tumours are virtually dependent on activation of Runx family genes in strong preference to other potent Myc collaborating genes (Gfi1, Notch1). Ikzf1 was identified as a novel collaborating gene for Runx2 and illustrated the interface between integration preference and oncogenic selection. Lymphoma target genes for MoMLV can be classified into (a) a small set of master regulators that confer self-renewal; overcoming p53 and other failsafe pathways and (b) a large group of progression genes that control autonomous proliferation in transformed cells. These findings provide insights into retroviral biology, human cancer genetics and the safety of vector-mediated gene therapy.