Identifying novel components of human tumour suppressor networks

textabstractThe p53 pathway plays a central role in protecting cells from becoming tumorigenic. Reactivation of the pathway is under extensive study in the development of anti-cancer drugs. Although a lot is known about the pathway, the way it is inactivated in human tumours harbouring the wild-type p53 gene is in many cases unknown. This raises the question if we indeed know all the components of this important tumour suppressor pathway. In this thesis, we aimed to identify novel components of the p53 tumour suppressor network. Besides focussing on protein-coding genes, we also set out to identify non-coding gene components. To reach these goals, we made use of genetic screens and genome-wide p53-binding analysis. First, we describe the identification of the bromodomain-containing protein BRD7 as a transcriptional cofactor of p53 and show that BRD7 is required for proper p53 activation in response to certain cellular stresses. We further demonstrate that BRD7 expression is low and the gene encoding BRD7 is frequently deleted in human breast tumours harbouring wild-type p53, implying that loss of BRD7 expression is a way to neutralize the p53 pathway. Second, genome-wide p53-binding profiling reveals specific interactions of p53 with promoter regions of nearby genes. Interestingly, we found p53 binding at nongenic regions that possess evolutionary highly conserved p53-binding sites and all known hallmarks of enhancer regions. We demonstrate that these p53-bound domains indeed have p53-dependent enhancer activity and produce non-coding transcripts, termed enhancer RNAs (eRNAs). Importantly, the domains interact with multiple distantly located genes and we show that eRNA production is required for transcriptional induction of some of these genes. Third, we discuss the involvement of an oncogenic microRNA cluster, miR-17-92, in the neoplastic transformation of human primary cells. We propose that the microRNAs in this cluster cooperate to dampen the retinoblastoma tumour suppressor pathway