LELAND STANFORD JUNIOR UNIVERSITY, THE
Cancer is caused by the activation of oncogenes and the inactivation of tumor suppressor genes. The targeted inactivation and repair of these gene products may be a specific and effective therapy for cancer. Previously, we have shown that the inactivation of the MYC oncogene is sufficient to induce sustained regression of hematopoietic tumors . Subsequently, we reported that even brief inactivation of MYC is sufficient to induce sustained tumor regression of osteosarcoma [3, 4]. Our results support the general hypothesis that tumors exhibit the phenomena of oncogene addiction [5, 6]. To explain our findings, we reasoned that MYC inactivation induces a permanent change in the ability of cells to induce a cancer-associated gene expressionprogram. Upon MYC inactivation, there are specific and sustained changes in gene expression [Wu et al, PLoS Genetics ; and see Appendix, Shachaf et al, Cancer Research, 2008]. These changes in gene expression are frequently accompanied by permanent changes in the ability of MYC binding to promoter loci, as shown by ChIP. We performed a preliminary ChIP-on-chip analysis for MYC and interrogated changes in binding of other transcription factors in a genome-wide scale. Moreover, we also found that changes in gene expression are associated with specific alterations in chromatin modifications. Importantly, MYC appears to regulate gene expression not just through interactions with the canonical DNA binding sequence (E-Box), but additionally, its binding specificity may be regulated by DNA methylation. We provide new results that illustrate that the TGF-b signaling pathway may play an important role in the mechanism by which MYC inactivation induces changes in gene expression and cellular senescence. Moreover, we provide evidence that MYC regulates DNA methylation. Hence, we hypothesize that MYC inactivation restores auto-regulatory programs, including the induction of CDKIs, through effects on DNA methylation, resulting in the induction of a cellular senescence program. Hence, both cell extrinsic receptor based mechanisms, including TGF-b signaling, as well as cell intrinsic mechanisms, including regulation of DNA methylation, may be critical to oncogene addiction.