Myc-mediated reprogramming of RNA polymerases.
Growth is a fundamental process of life and its de-regulation is implicated in multiple human diseases. Cellular growth is determined by the content and synthesis of ribosomes. Ribosomal biogenesis requires the coordinated activity of all three nuclear RNA polymerases (RNAPI, -II, -III). The oncogene MYC is indispensable for development and maintenance of most human tumors. MYC´s evolutionary conserved physiological function is to promote cell growth. MYC binds to thousands of genes transcribed by all three RNAPs. The mechanisms through which MYC induces cellular growth remains unclear and four aspects will be analyzed in my group:
(I) MYC is a central regulator of cell growth and global regulator (“amplifier”) of transcription in primary cells, yet MYC-driven tumors are characterized by specific gene expression profiles that suggest a much more restricted and gene-specific role in transcription during oncogenesis. In the first set of experiments, I will develop and test a quantitative model of MYC binding to chromatin that aims to resolve this apparent discrepancy.
(II) Two chromatin-binding complexes of MYC are known: MYC/MAX complexes bind to Eboxes and their predominant targets are genes involved in ribosome biogenesis and translation (RiBi genes). Genes transcribed by RNAPIII are bound by MYC/TFIIIB complexes that do not require Max. Neither of these complexes is responsible for binding of MYC to the majority of ribosomal protein (RP) gene promoters and I aim to identify and characterize the missing complex of MYC.
(III) The existence of different chromatin-binding complexes suggests that MYC not only induces a general increase in ribosomal biogenesis, but also co-ordinates the synthesis of different ribosomal components. MYC has been suggested to act at the elongation step of transcription, but this remains to be directly demonstrated and I will use direct measurements of elongation to measure MYC function in a global manner. Our preliminary work has led to a model in which MYC catalyzes the transfer of elongation factors onto RNA polymerase to promote transcriptional elongation. I will test this model by purifying RNA polymerase holo-enzymes and analyze the effect of MYC on holoenzyme composition. Performing these experiments in response to an experimentally induced imbalance in the production of ribosome components (e.g. by selective blocking RNAPI transcription) will test whether MYC indeed co-ordinates rather than simply stimulates ribosome biogenesis.
(IV) Preliminary data obtained in collaboration with Reuven Agami (NKI) show that MYC not only activates the expression of ribosomal protein genes on a transcriptional level, but also enhances their translation. I hypothesize that enhancing translation of selected groups of mRNPs is also a direct function of Myc and propose to unravel the underlying mechanisms and its impact on cellular growth.