Selective transcriptional regulation by Myc in cellular growth control and lymphomagenesis

Global transcriptional and epigenomic analyses in diverse cell types reveal that the primary action of Myc is to up- and downregulate transcription of distinct groups of genes, rather than to amplify transcription of all active genes; general RNA amplification, when observed, is better explained as an indirect consequence of Myc’s action on cellular physiology. Selective gene regulation by Myc The mammalian Myc oncoprotein is a transcription factor that binds to thousands of promoters. Two current models for Myc function propose that it is either a gene-specific regulator of transcription, or a global amplifier of all active genes. Two groups reporting in this issue of Nature present evidence in support of the idea that Myc regulates specific genes. Arianna Sabò et al . analyse Myc genomic distribution and RNA expression profiles during B-cell lymphomagenesis in mice and Susanne Walz et al . compare normal cells and Myc-transformed tumour cells. Although both groups find that Myc overexpression can result in a general increase in gene expression, the effect is an indirect one. Modulated by various other transcription factors, Myc seems to act primarily by regulating specific groups of genes. The c- myc proto-oncogene product, Myc, is a transcription factor that binds thousands of genomic loci 1 . Recent work suggested that rather than up- and downregulating selected groups of genes 1 , 2 , 3 , Myc targets all active promoters and enhancers in the genome (a phenomenon termed ‘invasion’) and acts as a general amplifier of transcription 4 , 5 . However, the available data did not readily discriminate between direct and indirect effects of Myc on RNA biogenesis. We addressed this issue with genome-wide chromatin immunoprecipitation and RNA expression profiles during B-cell lymphomagenesis in mice, in cultured B cells and fibroblasts. Consistent with long-standing observations 6 , we detected general increases in total RNA or messenger RNA copies per cell (hereby termed ‘amplification’) 4 , 5 when comparing actively proliferating cells with control quiescent cells: this was true whether cells were stimulated by mitogens (requiring endogenous Myc for a proliferative response) 7 , 8 or by deregulated, oncogenic Myc activity. RNA amplification and promoter/enhancer invasion by Myc were separable phenomena that could occur without one another. Moreover, whether or not associated with RNA amplification, Myc drove the differential expression of distinct subsets of ta