Transcriptional role of cyclin D1 in development revealed by a genetic–proteomic screen

Abstract

A proteomics approach to screening for cyclin D1-binding proteins in various mouse organs reveals proteins involved in transcription regulation. Cyclin D1 is found to be associated with the promoter regions of many genes, and a role for cyclin D1 in regulating Notch1 gene expression in the developing mouse retina is dissected in detail. These findings suggest that cyclin D1 has a transcription regulatory function during development, in addition to its well-established cell-cycle roles. Although cyclin D1 is frequently overexpressed in human cancers, the full range of its functions in normal development and oncogenesis is unclear. Here, tagged cyclin D1 knock-in mouse strains are developed to allow a search for cyclin D1-binding proteins in different mouse organs using high-throughput mass spectrometry. The results show that, in addition to its established cell cycle roles, cyclin D1 has an in vivo transcriptional function in mouse development. Cyclin D1 belongs to the core cell cycle machinery, and it is frequently overexpressed in human cancers1,2. The full repertoire of cyclin D1 functions in normal development and oncogenesis is unclear at present. Here we developed Flag- and haemagglutinin-tagged cyclin D1 knock-in mouse strains that allowed a high-throughput mass spectrometry approach to search for cyclin D1-binding proteins in different mouse organs. In addition to cell cycle partners, we observed several proteins involved in transcription. Genome-wide location analyses (chromatin immunoprecipitation coupled to DNA microarray; ChIP-chip) showed that during mouse development cyclin D1 occupies promoters of abundantly expressed genes. In particular, we found that in developing mouse retinas—an organ that critically requires cyclin D1 function3,4—cyclin D1 binds the upstream regulatory region of the Notch1 gene, where it serves to recruit CREB binding protein (CBP) histone acetyltransferase. Genetic ablation of cyclin D1 resulted in decreased CBP recruitment, decreased histone acetylation of the Notch1 promoter region, and led to decreased levels of the Notch1 transcript and protein in cyclin D1-null (Ccnd1-/-) retinas. Transduction of an activated allele of Notch1 into Ccnd1-/- retinas increased proliferation of retinal progenitor cells, indicating that upregulation of Notch1 signalling alleviates the phenotype of cyclin D1-deficiency. These studies show that in addition to its well-established cell cycle roles, cyclin D1 has an in vivo transcriptional function in mouse development. Our approach, which we term ‘genetic–proteomic’, can be used to study the in vivo function of essentially any protein.