Elevated expression of c-Myc occurs frequently in human cancers and is associated with tumor aggression and a remarkable range of cellular phenotypes, but the effect of high levels of c-Myc on global gene regulation in tumor cells is poorly understood. A widely held view is that high levels of c-Myc lead to newly activated or repressed “Myc target genes”. The results described here and by Nie, et al., (2012)
in this issue, support a different model: c-Myc accumulates in the promoter regions of active genes across the cancer cell genome and causes transcriptional amplification, producing increased levels of transcripts within the cell's gene expression program.
Transcription factors such as the myogenic regulator MyoD or the pluripotency regulators Oct4 and Sox2, when newly expressed at high levels in cells, will bind and establish new enhancers that modify the expression program and alter cell state (Graf and Enver, 2009
). In contrast, expression of high levels of c-Myc in tumor cells produces a very different result. In the tumor cell systems described here, increased levels of c-Myc result in increased binding to active genes and little binding to genes that were inactive at low c-Myc levels, and there is little change in the set of actively transcribed genes. As the levels of c-Myc increase, high affinity E-box sites at active core promoters become saturated and lower affinity E-box variant sequences at promoters and enhancers become occupied. This behavior of c-Myc is consistent with evidence that the open chromatin environment at active promoters is important for binding by this factor (Guccione et al., 2006
). Furthermore, enhancers loop into proximity of core promoters at active genes (Bulger and Groudine, 2011
; Gondor and Ohlsson, 2009
; Kagey et al., 2010
; Ong and Corces, 2011
) and this proximity may facilitate binding of c-Myc to nearby enhancer elements once binding to sites in core promoters is saturated. The functional consequence of elevated c-Myc binding at active genes is increased recruitment of the pause release factor P-TEFb, increased transcriptional elongation and a global increase in transcript levels.
The model that oncogenic c-Myc acts primarily in transcriptional amplification of the tumor cell's gene expression program provides an explanation for the diverse effects of oncogenic c-Myc on gene expression in different tumor cells. Numerous gene expression studies have identified sets of genes whose expression levels are altered by changes in c-Myc levels (Dang et al., 2006
; Ji et al., 2011
; Kim et al., 2006
; Schlosser et al., 2005
; Schuhmacher et al., 2001
; Zeller et al., 2003
). These c-Myc signatures vary greatly across cell types (Chandriani et al., 2009
), making it difficult to ascribe the broad range of cellular effects produced by oncogenic c-Myc to a key set of target genes. This variation in c-Myc signatures would be expected with transcriptional amplification of the different gene expression programs inherent in different cancer cells. Some variation in c-Myc signatures may also be due to a limitation of microarray-based analysis, which typically involves a normalization step that assumes similar levels of total RNA and thus does not detect amplification of the entire gene expression program. Although our evidence indicates that c-Myc functions primarily in amplification, there are a small number of genes whose expression is reduced in cells with high levels of c-Myc. These genes may be directly repressed by c-Myc or their repression may be due to indirect effects; for example, elevated levels of a negative regulator might overwhelm c-Myc-induced amplification of some genes.
Transcriptional amplification may explain why c-Myc plays a critical role in tumorigenesis in a wide variety of human tissues. While numerous cellular pathways can be mutated to provide an initial signal for increased cell growth and proliferation, transcriptional amplification could provide the sweeping changes in cellular physiology necessary for aggressive cellular growth and proliferation. Translational capacity and aerobic energy metabolism are among the cellular functions that are limiting for the growth of tumor cells and an increase in the levels of transcripts for this machinery would increase the levels of its rate limiting components (Dai and Lu, 2008
; Ruggero, 2009
). It is also possible that the increase in essentially all components of the gene expression program provides cells with an advantage when adapting to the multiple mutated pathways that characterize most tumor cells.
Elevated expression of c-Myc occurs frequently in cancer. The model that c-Myc can promote tumorigenesis through transcriptional amplification suggests that therapies focused on the molecular mechanisms involved in RNA Pol II pause control and elongation may be valuable for clinical treatment of many different tumors.