Microarray analysis of p53 target gene expression patterns in the spleen and thymus in response to ionizing radiation.

The ability of p53 to induce apoptosis through transactivation of its target genes is critical for its function as a tumor suppressor. To identify the critical p53 target genes that mediate apoptosis through these pathways, we examined p53-dependent apoptosis in vivo where these apoptotic pathways are intact. p53 wild-type and null animals were irradiated and the global p53 gene expression patterns in response to ionizing radiation were examined in two tissues (spleen and thymus) which undergo p53-dependent apoptosis in response to ionizing radiation. We found that the vast majority of genes that increased or decreased in a p53-dependent manner did so in a tissue specific manner. Although the spleen and thymus had similar levels of p53-dependent apoptosis after ionizing radiation, there was very little overlap in the induced or repressed targets. This suggested that distinct targets may mediate apoptosis in these tissues. In addition, these microarray analyses also led to the identification of several novel p53 targets. Several of these targets appear to be involved in known p53 functions such as the induction of apoptosis (bim) or the initiation of DNA repair (DinB). However, some p53 targets (both confirmed and unconfirmed) link p53 to the anti-tumor immune response (MIP 1-alpha). Further characterization of the tissue specific mediators of p53-dependent responses will greatly increase our understanding of p53 function.