p53 must be competent for transcriptional regulation to suppress tumor formation.

In vitro studies suggest that effective tumor suppression by p53 requires multiple domains to execute transcription-dependent and transcription-independent functions. We generated a mutant p53 allele in mice, p53(W25QL26S) (p53(QS)), containing an inactive transactivation domain to evaluate the importance of transactivation for p53-mediated tumor suppression. Recently, we discovered that the allele also contains a valine substitution for alanine at codon 135, which borders the DNA-binding domain. We found that p53(QSval135) bound to chromatin albeit less well than p53(QSala135), but both were equally deficient in transcriptional regulation, apoptosis induction in mouse embryo fibroblasts (MEFs), and suppression of tumor formation by E1A, Ha-Ras transformed MEFs. p53(QSval135) mice and p53-null mice exhibited identical tumor development kinetics and spectra in spontaneous and oncogene-initiated tumorigenicity assays, when tested in a homo- and heterozygous configuration. The p53(QSval135) allele did not have dominant negative functions and behaved as a null allele. Taken together, these data indicate that effective tumor suppression requires the transcriptional regulation function of p53, and they suggest that transactivation independent functions of p53 are unlikely to contribute significantly to tumor suppression in vivo.

c-Myc can induce DNA damage, increase reactive oxygen species, and mitigate p53 function: a mechanism for oncogene-induced genetic instability.

Oncogene overexpression activates p53 by a mechanism posited to involve uncharacterized hyperproliferative signals. We determined whether such signals produce metabolic perturbations that generate DNA damage, a known p53 inducer. Biochemical, cytological, cell cycle, and global gene expression analyses revealed that brief c-Myc activation can induce DNA damage prior to S phase in normal human fibroblasts. Damage correlated with induction of reactive oxygen species (ROS) without induction of apoptosis. Deregulated c-Myc partially disabled the p53-mediated DNA damage response, enabling cells with damaged genomes to enter the cycle, resulting in poor clonogenic survival. An antioxidant reduced ROS, decreased DNA damage and p53 activation, and improved survival. We propose that oncogene activation can induce DNA damage and override damage controls, thereby accelerating tumor progression via genetic instability.