p53 controls a powerful stress response and is a quintessential tumor suppressor. The discovery of p63 and p73, two p53 family members, provoked much speculation about their individual and collective functions. However, while a strong consensus exists that both genes play a major role in cancer, current data is conflicting whether their nature is tumor suppressive, oncogenic or in some context both. Although their function in development was immediately apparent from gene ablation studies, delineating their exact role in cancer remains elusive due to a lack of clear genetic data in humans and mice. Due to a second internal promoter and alternative splicing, p63 and p73 are complex bipolar genes giving rise to multiple isoforms that can simplistically be viewed as "Two Opposing-Genes-in-One". Moreover, in sharp contrast to the ubiquitous mutational alteration of p53, p63 and p73 are characterized by a virtual absence of inactivating mutations, and instead exhibit aberrant expression of specific isoforms in tumors. This grant aims at elucidating the in vivo role of both genes in specific oncogenic contexts and is based on two premises. First, normal tissues exhibit a striking specificity for tissue types and isoforms in their p63 and p73 expression, an important fact that up to now received little attention, yet likely holds the key to a clearer understanding of these two genes in cancer. Second, global p63 and p73 KO mice (missing all isoforms) can be very informative, since by canceling all isoforms, the biologically dominant function among the two opposing classes of proteins within a specific tissue and a given oncogenic stress will sort itself out and be revealed. Mindful of tissue and isoform specificity, we will use mouse genetics and cell biological in vitro studies to determine the role of p63 and p73 in specific human cancers. Aim I Based on our preliminary data, p73 plays an autonomous role - distinct from p53 - as a genomic stability factor in primary cells, which is linked to DNA damage signaling and DNA repair. We will identify its mechanisms and targets. Aim II The role of p73 in cancer in vivo. By generating p73 nullizygosity in existing oncogenic mouse models, we will determine whether p73 loss influences B-lymphomagenesis and carcinomas. Aim III Based on the recent finding that TAp63 is the unique member of the p53 family that mediates DNA damage-induced apoptosis in the female germ line, we will test the hypothesis that TAp63 also protects the male germ line and is a tumor suppressor in human testicular cancers (seminomas and non-seminomas). Aim IV We will determine whether specific classes of human B-lymphomas sustain p63 loss-of-function mutations. Also, by generating p63 heterozygosity in existing oncogenic mouse models, we will determine whether p63 loss influences B- lymphomagenesis and breast cancer. PUBLIC HEALTH RELEVANCE: The p53 gene controls a powerful stress response and is a quintessential tumor suppressor in human cancers. There is ample evidence to suggest that two related genes, called p63 and p73, also play crucial roles in human cancer. Using well characterized, physiologically relevant mouse models, we will define the precise role of these genes in the development and progression of several cancers, including lymphoma, breast and testicular cancer. These data will provide prognostic markers and possibly targets for therapeutic intervention.