Current Research

E2F1 and the DNA damage response

E2F1 is a key transcriptional regulator of genes important for cell proliferation and apoptosis. Using transgenic and knockout mouse models, the Johnson lab showed that E2F1 behaves as either an oncogene or a tumor suppressor, depending on the biological context. We are now exploring a novel, non-transcriptional function for E2F1 in DNA repair. We have discovered that E2F1 localizes to sites of both DNA double-strand breaks and UV-induced DNA damage. The accumulation of E2F1 at sites of DNA damage requires its phosphorylation by the ATM or ATR kinases. Moreover, our studies demonstrate that E2F1 in turn recruits histone acetyltransferases and other chromatin-modifying factors to sites of damage. Our findings suggest that E2F1-dependent alterations in chromatin structure stimulates DNA repair by facilitating access to the DNA repair machinery. These findings uncover a new function for E2F1 as an accessory factor for DNA repair in the context of chromatin. We are also developing unique knock-in mouse models to examine the role of E2F1 phosphorylation and other post-translational modifications in regulating E2F1 activities in response to DNA damage. These studies demonstrate that E2F1 is important for maintaining genome integrity and suppressing carcinogenesis.


Our lab has shown that transcription factors such as E2F1 and Rb form nuclear foci upon ionizing radiation and that these foci colocalize with γH2AX, a marker for DNA double-strand breaks (DSBs). We are currently elucidating the role that these factors play in the repair of DSBs.




The p53 R72P polymorphism in cancer development and response to therapy 

The human p53 gene contains a common single nucleotide polymorphism (SNP) that results in either an arginine (R) or proline (P) at position 72 of the p53 protein (R72P). Epidemiological studies demonstrate that this SNP can influence risk for developing some cancers, response to cancer therapies, and survival of cancer patients. In vitro studies suggest that the R72P polymorphism alters the transcriptional and pro-apoptotic activities of p53, although the mechanistic basis for these differences is unclear. The Johnson lab has developed knock-in mouse models in which a portion of the mouse p53 gene is replaced with human p53 sequences encoding either R or P at codon 72. The humanized p53 variants in these mouse models are functional and display the same differences in apoptotic capacity in mouse tissues as have been observed in human cells. We have used these models to examine how this p53 polymorphism modulates the response to chemical carcinogens, an oncogenic virus (HPV) and UV light. These gene-environment studies reveal that the R72P polymorphism has context- and tissue-specific effects on apoptosis and tumor suppression. These models are now being used to understand how the R72P polymorphism modulates breast cancer progression and response to chemotherapies.  


Non-invasive imaging techniques allow us to follow differences in tumor growth and response to therapy in live R72P variant mice.