Knowledge Bank

The cell cycle, which comprises the series of events that lead to a cell's division, is regulated by a family of proteins called the Cyclins. These have fluctuating levels of expression across different stages of the cell cycle, and are themselves regulated by the E2F family of transcription factors; these can bind to a specific consensus sequence and can activate and repress gene expression. One of their targets is Cyclin A2 (Ccna2/CCNA2), a cyclin that plays a major role in DNA synthesis and entry into the mitotic or meiotic phase. The in vivo relevance of E2F regulation, particularly of specific targets like Ccna2 is not yet fully understood, and its study is important to understanding the conserved nature of the protein throughout evolution. To evaluate the importance of E2F-mediated regulation of Ccna2 in vivo, we generated a mouse line with a knock-in mutation at the putative E2F binding site of the Ccna2 promoter. Although mice are viable and appear healthy, we observed infertility and early testicular atrophy in males homozygous for the promoter mutation. Using histological analysis, real-time PCR (qPCR), and antibody-mediated visualization of CCNA2 and stem cell marker localization in testes, we identified a defect in spermatogenesis initiation and sustenance, an aberrant Ccna2 expression profile, and a late but sustained upregulation of stem cell activity throughout development. Collectively, these data suggest that the precise E2F-mediated regulation of Ccna2 transcription is required for proper stem cell differentiation during spermatogenesis. The genomic acquisition of E2F binding sites can, therefore, be said to serve as an evolutionary mechanism that can bestow conserved proteins novel functions in different tissues. Further inquiry into how these transcription factors regulate their individual targets and how this regulation is important in vivo is likely to continue revealing novel functions and phenotypes, increasing our understanding of the cell cycle and the many ways it is and could be involved in human disease.