Two new p73 splice variants, γ and δ, with different transcriptional activity

p73 has been recently identified as a new structural and functional homologue of the transcription factor p53. It is expressed in either a full-length form, alpha, or a shorter beta mRNA variant, with exon 13 spliced out. Here we report the identification and functional characterization of two new p73 splicing variants, gamma (splicing out exon 11) and delta (splicing out exons 11, 12, and 13). Both gamma and delta p73 variants are expressed in human peripheral blood lymphocytes, primary keratinocytes, and different tumor cell lines, including neuroblastoma, glioblastoma, melanoma, hepatoma, and leukemia. The expression pattern of the four p73 splicing variants differs in both primary cells of different lineage and established cell lines even within the same type of tumor. A two-hybrid assay was used to characterize the homodimeric and heterodimeric interactions between the p73 variants, and showed that neither p73 gamma nor p73 delta interact with p53, whereas p73 gamma showed strong interactions with all p73 isoforms, and p73 delta binds efficiently p73 alpha and p73 gamma but only weakly p73 beta. At the functional level, p73 gamma is significantly less efficient in activating transcription of the p21(Waf1/Cip1) promoter than p53 or p73 beta, whereas the effect of p73 delta is intermediate and comparable to that of p73 alpha. The ability of the different p73 variants to affect cell growth in p53 null osteosarcoma SAOS-2 cells correlates with their transcriptional activity on the p21(Waf1/Cip1) promoter: p73 beta is the most efficient in inhibiting colony formation, whereas p73 gamma is almost ineffective. Our results suggest that p73 isoforms may be differentially regulated, with four different isoforms capable of interacting among themselves and with p53. The relative expression level of each splice variant may modulate p73 transcriptional and growth suppression activities by affecting heterodimer formation.