Mice engineered with a range of mutations in the tumor suppressor protein p53 have allowed scientists to separate as distinct functions the responses of the protein to DNA damage and cancerous changes. The multidisciplinary team found that separate domains within the p53 protein are responsible for activating distinct subsets of genes that ultimately force the cell cycle arrest or apoptosis of cells with damaged DNA, or the death of cancerous cells.
In fact, the investigators report, p53 can still act as a tumor suppressor even when the part of the protein that is responsible for shutting down cells in response to DNA damage has been inactivated. The researchers hope their findings could enable the development of drugs that switch off the part of p53 responsible for killing off noncancerous tissue subjected to radiotherapy or chemotherapy without compromising p53’s tumor suppressor function.
The research group comprised scientists from Stanford University School of Medicine, MIT, and the University of California’s Helen Diller Family Comprehensive Cancer Center. Their findings appear in Cell in a paper titled “Distinct p53 Transcriptional Programs Dictate Acute DNA-Damage Responses.”
The extent to which p53’s ability to respond to DNA damage is linked to its effects as tumor suppressor has been controversial. The situation is complicated by the fact that p53 contains two distinct transcriptional activation domains, TAD1 and TAD2, whose discrete functions and relative contributions to p53 function are not fully understood.
To try and further understand the transcriptional mechanisms of p53 tumor suppression, the researchers used knockin mice expressing a series of transactivation mutants with two or more amino acid alterations in either the first, second, or both TADs.
Studies with these mice showed that the protein’s responses to either acute genotoxic stress or oncogenic stimuli hinge on distinct p53 transcriptional activation requirements associated with different target gene-expression programs. The findings indicated that the activity of TAD1, but not TAD2, was essential for triggering cell cycle arrest and apoptosis in response to DNA damage.
Intriguingly, although the TAD1 knockout couldn’t activate the vast majority of p53’s pool of target genes, the largely disabled protein was still able to function as a tumor suppressor. In fact, p53 was also still able to respond to oncogenic changes if TAD2 was disabled instead of TAD1. Both TADs had to be inactivated before the tumor suppressor activity of the protein was completely deactivated.
The functional redundancy of the first and second p53 TADs in terms of tumor suppressor activities could explain the lack of p53 TAD mutations in human cancer, the authors note. “The majority of reported p53 mutations in cancer target the p53 DNA-binding domain, where one amino acid alteration can ablate both p53 DNA binding and transactivation activity.
"Although mutating both TADs can also severely cripple p53 tumor suppression function, it is less probable that the four amino acid substitutions required for loss of function would occur during carcinogenesis.”
Knocking out the activity of TAD1 allowed the researchers to look more closely at the small subset of genes activated by functional TAD2 in response to the cell becoming cancerous. This pool included not only predicted genes involved in cell signaling, cytoskeletal functioning, and DNA repair but also a number of completely new genes.
“Our studies demonstrate that the domains defined in vitro are indeed essential for transcriptional activation by p53 in vivo and further expand our understanding by revealing that TADs display selectivity according to context,” the authors conclude. “It may be that activation of genes important for DNA-damage responses relies on specific co-factor contacts with the first TAD of p53, whereas activation of genes involved in tumor suppression requires an alternate co-factor(s) interacting with both TADs.
“The idea that the DNA-damage pathway downstream of chronic genotoxic stress may be mechanistically different from that downstream of acute genotoxic injury provides a potential resolution to the controversy regarding the role of DNA-damage signaling in activating p53 in nascent tumors and warrants further investigation.”
As well as the potential development of drugs that could help prevent radio- and chemotherapy-related damage to normal tissues, the authors suggest their findings hint at more direct therapeutic strategies for cancer that would restore p53 function in tumors by reactivating the p53 targets specifically involved in tumor suppression.