Testing the role of p53 in the expression of genetic instability and apoptosis in ataxia-telangiectasia

[Direct role of p53 on homologous recombination]

The tumor suppressor gene p53, which is the most frequently mutated gene in human tumors, controls cell cycle checkpoint and apoptosis via the transactivation of the transcription of a collection of genes. These activities avoid proliferation of cell bearing alteration of genetic material. However, like a two-edged sword, p53 can also directly participate to genome stability maintenance by repressing homologous recombination (HR), independently of the transactivation activity. This parallel activity allows to limit the deleterious consequences on an excess of HR. Beside genetic interactions, p53 protein physically interacts with both HR proteins and HR intermediates (heteroduplex and Holliday junctions). The core domain of p53 is required for interaction with Rad51 at an early step and the carboxy-terminal domain of p53 is involved in the interaction with Rad54 and HR intermediates, at a late step. We discuss here the putative consequences of this parallel activity of p53 on genome stability, speciation and tumor protection.

p53 and recombination intermediates: role of tetramerization at DNA junctions in complex formation and exonucleolytic degradation

Heteroduplex joints represent intermediates of Rad51-dependent recombination processes, which are recognized by p53 with extremely high affinities, in a manner independent of the DNA sequence content. To determine the structural elements required for complex formation, we monitored DNA-binding by protection against restriction endonuclease cleavage. We show that wild-type (wt) p53 interacts with heteroduplex joints in the proximity of the flexible junction. Association of p53 within this junction region was also observed with preformed Rad51-heteroduplex complexes, whereas SSB counteracted p53 binding. At a distance of 31 bp from the junction p53 established very few contacts with the heteroduplex, despite the presence of an A-G mismatch. Consistently, p53-dependent exonucleolytic degradation decreased when we raised the distance between the junction and the heteroduplex terminus by 27 bp. Different from the cancer-related mutant p53(273H), which did not recognize the junction, tetramerization defective p53-1262 was protection competent but displayed reduced complex stability in gel shifts. Moreover, p53-1262 performed exonucleolytic activities towards ssDNA like wtp53, but reduced degradation of heteroduplex joints. These results suggest that during recombination wild-type p53, as a tetramer, stably binds to strand transfer regions, enabling the protein to exonucleolytically correct heteroduplex intermediates early after strand invasion.

Dissociation of p53-mediated suppression of homologous recombination from G1/S cell cycle checkpoint control

The tumor suppressor p53 is considered as the guardian of the genome which is activated following genotoxic stress. In many cell types, p53 mediates G1 cell cycle arrest as the predominant cellular response. Inactivation of wild-type p53 leads to loss of G1/S checkpoint control and to genomic instability, including increased spontaneous homologous recombination (HR). To determine whether regulation of the G1/S checkpoint is required for suppression of HR, we assessed recombination events using a plasmid substrate that stably integrated into the genome of p53-null mouse fibroblasts. Exogenous expression of a temperature-sensitive p53 protein (Ala135 to Val), which had lost trans-activation function and could not regulate G1/S transition when in mutant conformation, reduced HR rates to the same extent as wild-type p53. Furthermore, a p53 construct with an alternatively-spliced carboxy terminus also retained this ability in the absence of both activities, G1/S control and non-sequence specific DNA binding as mediated by the carboxy terminus. Our data dissociate regulation of HR by p53 from its role as a cell cycle checkpoint protein. The results support a model which extends p53's role as a guardian of the genome to include transactivation-independent regulatory functions in DNA repair, replication and recombination.