Mutations resulting in the inactivation of the tumor suppressor p53 are the most common lesions present in human malignancies, and thereby contribute significantly to the pathogenesis of human cancer. Importantly, the reactivation of wild-type p53 leads to efficient tumor regression indicating the clinical implications for understanding p53 function. In normal cells, the primary function of p53 is to respond to stress by activating multiple target genes that then execute specific pathways that include cell cycle arrest and programmed cell death. The activation kinetics of the various p53 target genes varies significantly, ranging from a rapid induction of cell cycle control genes to a delayed induction of pro-apoptotic genes. Establishing the proper activation kinetics and balance between the p53-response pathways is essential for preventing the propagation of cells that harbor DNA mutations and can lead to tumor development. A fundamental question that remains is how p53 determines which of the diverse transcription programs to activate in order to facilitate the appropriate stress response.
Changes in gene expression that arise in the absence of alterations in the DNA sequence are referred to as “epigenetic.” A primary epigenetic mechanism that contributes to gene regulation includes the posttranslational modifications of histone proteins. Histone modifications function by facilitating direct effects on chromatin structure and through the recruitment of chromatin-associated factors, or “effector proteins” that translate histone modifications into specific biological outcomes. A wide array of histone modifications and effector proteins have been identified, and as a result, we are faced with the challenge of understanding the mechanisms by which histone modifications, both independently and in conjunction with each other, function through effector interactions to regulate gene expression. A role for histone modifications in the regulation of p53 target gene expression is demonstrated by several studies showing an accumulation of histone modifications at p53 target gene promoters in response to stress. Yet, the precise functions of histone modifications in regulating the p53 gene network remain to be elucidated.
The primary goal of our research is to understand the molecular basis for the epigenetic regulation of the gene-selective functions of p53. Toward this goal, we are employing a powerful combination of biochemical, molecular, and genomic approaches. By implementing this multi-faceted strategy, we recently revealed a mechanism by which the prominent histone modification, histone H3 trimethylated at lysine 4 (H3K4me3), through direct interactions with the general initiation factor TAF3/TFIID, facilitates selective p53-dependent activation of cell cycle control genes (Figure 1). By exploiting cell free transcription systems reconstituted with purified factors and recombinant chromatin templates, we identified direct (causal) effects of this histone modification and a functional interplay between this histone mark and core promoter DNA sequences. Thus, our in vitro assays provide a powerful and unique means to identify regulatory mechanisms that function both independently and cooperatively at both the genetic and epigenetic level to mediate coordinated transcriptional control. We are also developing single cell methods to quantitatively measure the temporal dynamics of the recruitment/stabilization of epigenetic readers during a p53 dependent response and to measure whether changes in effector binding correlate with the kinetics and distinct patterns of p53 target gene expression.
Together our ongoing studies will aid in the understanding of how histone modifications precisely contribute to the dynamics of p53 target gene regulation, and will ultimately advance the potential for target based screening approaches that will allow for the development of therapeutic targeting strategies, an issue of great significance in light of the recent successes in targeting histone modification effector proteins/readers for developing cancer therapies.
Dr. Shannon Lauberth received her Ph.D. in Biochemistry and Molecular Biology from Saint Louis University. She performed her postdoctoral training at the Rockefeller University where she was a recipient of a Ruth L. Kirschstein National Research Service Award (NRSA) and an active member of the Anderson Cancer Center.