Inactivation of cell death is a major step in tumor development, and p53, a tumor suppressor frequently mutated in cancer, is a critical mediator of cell death. While a role for p53 in apoptosis is well established, direct links to other pathways controlling cell death are unknown. Here we describe DRAM (damage-regulated autophagy modulator), a p53 target gene encoding a lysosomal protein that induces macroautophagy, as an effector of p53-mediated death. We show that p53 induces autophagy in a DRAM-dependent manner and, while overexpression of DRAM alone causes minimal cell death, DRAM is essential for p53-mediated apoptosis. Moreover, analysis of DRAM in primary tumors revealed frequent decreased expression often accompanied by retention of wild-type p53. Collectively therefore, these studies not only report a stress-induced regulator of autophagy but also highlight the relationship of DRAM and autophagy to p53 function and damage-induced programmed cell death.

Intact p73 function is shown to be an important determinant of cellular sensitivity to anticancer agents. Inhibition of p73 function by dominant-negative proteins or by mutant p53 abrogates apoptosis and cytotoxicity induced by these agents. A polymorphism encoding either arginine (72R) or proline (72P) at codon 72 of p53 influences inhibition of p73 by a range of p53 mutants identified in squamous cancers. Clinical response following cisplatin-based chemo-radiotherapy for advanced head and neck cancer is influenced by this polymorphism, cancers expressing 72R mutants having lower response rates than those expressing 72P mutants. Polymorphism in p53 may influence individual responsiveness to cancer therapy.

The acetyl-transferase Tip60 might influence tumorigenesis in multiple ways. First, Tip60 is a co-regulator of transcription factors that either promote or suppress tumorigenesis, such as Myc and p53. Second, Tip60 modulates DNA-damage response (DDR) signalling, and a DDR triggered by oncogenes can counteract tumour progression. Using E(mu)-myc transgenic mice that are heterozygous for a Tip60 gene (Htatip) knockout allele (hereafter denoted as Tip60+/- mice), we show that Tip60 counteracts Myc-induced lymphomagenesis in a haplo-insufficient manner and in a time window that is restricted to a pre- or early-tumoral stage. Tip60 heterozygosity severely impaired the Myc-induced DDR but caused no general DDR defect in B cells. Myc- and p53-dependent transcription were not affected, and neither were Myc-induced proliferation, activation of the ARF-p53 tumour suppressor pathway or the resulting apoptotic response. We found that the human TIP60 gene (HTATIP) is a frequent target for mono-allelic loss in human lymphomas and head-and-neck and mammary carcinomas, with concomitant reduction in mRNA levels. Immunohistochemical analysis also demonstrated loss of nuclear TIP60 staining in mammary carcinomas. These events correlated with disease grade and frequently concurred with mutation of p53. Thus, in both mouse and human, Tip60 has a haplo-insufficient tumour suppressor activity that is independent from-but not contradictory with-its role within the ARF-p53 pathway. We suggest that this is because critical levels of Tip60 are required for mounting an oncogene-induced DDR in incipient tumour cells, the failure of which might synergize with p53 mutation towards tumour progression.

ASPP = apoptosis stimulating protein of p53; A-T = ataxia-telangectasia; ATM = ataxia-telangectasia mutated; ATR = ataxia-telangectasia and Rad3-related protein; Chk2 = human homologue of Rad53; DCIS = ductal carcinoma in situ; LOH = loss of heterozygosity; maspin = mammary serine protease inhibitor; MDM2 = mouse double minute 2; p14 ARF = alternative product of the INK4 gene; σ = 14-3-3sigma.Available online http://breast-cancer-research.com/content/4/2/070 IntroductionFirst described in 1979, and initially believed to be an oncogene, p53 was the first tumour suppressor gene to be identified. p53 functions to eliminate and inhibit the proliferation of abnormal cells, thereby preventing neoplastic development. Abrogation of the negative growth regulatory functions of p53 occurs in many, perhaps all, human tumours. The p53 signalling pathway is in 'standby' mode under normal cellular conditions. Activation occurs in response to cellular stresses, and several independent pathways of p53 activation have been identified that appear to be dependent on distinct upstream regulatory kinases [1] (Fig. 1). These include an ataxia-telangectasia mutated (ATM)/human homologue of Rad53 (Chk2)-dependent pathway activated by DNA double-strand breaks, a second pathway dependent on the alternative product of the INK4 gene, p14 ARF (which is activated by expression of oncogenes), and a third pathway whose activity is increased by cytotoxic anti-tumour agents and ultraviolet light, but is independent of ATM, Chk2 and p14 ARF . Activation of this pathway may be mediated by other kinases such as the ATM relative ataxiatelangectasia and Rad3-related protein (ATR) [1].Activation results in an increase in the levels of p53 protein due to reduced mouse double minute 2 (MDM2)-dependent proteolytic degradation, and increased affinity of p53 for DNA. Whereas phosphorylation of the N-terminus may affect the stability of p53, lysine acetylation and/or serine phosphorylation in the C-terminus of the protein promotes DNA binding. As a result of activation, the wild-type protein acquires sequence-specific DNA binding activity, and an increasing number of genes are being identified as transcriptional targets of wild-type p53 [2]. These can be placed into a number of classes according to their functions. Abstract p53 mutation remains the most common genetic change identified in human neoplasia. In breast cancer, p53 mutation is associated with more aggressive disease and worse overall survival. The frequency of mutation in p53 is, however, lower in breast cancer than in other solid tumours. Changes, both genetic and epigenetic, have been identified in regulators of p53 activity and in some downstream transcriptional targets of p53 in breast cancers that express wild-type p53. Molecular pathological analysis of the structure and expression of constituents of the p53 pathway is likely to have value in diagnosis, in prognostic assessment and, ultimately, in treatment of breast cancer. Review The p53 pathway in breast cancer

A single-nucleotide polymorphism (SNP) in exon 4 results in expression of either arginine (72R) or proline (72P) at codon 72 of p53. We demonstrate that the in vitro response of cells exposed to anticancer agents is strongly influenced by this SNP in wild-type p53. In inducible systems and in cells expressing the endogenous protein, expression of 72P wild-type p53 results in a predominant G1 arrest, with only a minor apoptosis, at drug concentrations causing extensive apoptosis in cells expressing the 72R wild-type variant. The superior apoptosis-inducing activity of the 72R form correlates with more efficient induction of specific apoptosis-associated genes, and is maximal in the presence of serine 46 (S46). In vivo, the outcome of chemo-radiotherapy of squamous carcinomas is more favourable in cancers retaining a wild-type 72R allele, such cases having higher response rates and longer survival than those with wild-type 72P. Together, these results reveal that this SNP is an important determinant of response to anticancer agents in cells expressing wild-type p53. Analysis of complete p53 genotype (mutation and SNP) merits detailed investigation as a simple means for prediction of treatment response and survival in clinical oncology.