Now, in a new study, researchers at The Wistar Institute have identified a carefully orchestrated series of molecular modifications to p53 that must occur for it to perform its normal function, which is to initiate the transcription of genes involved in growth control. The findings give a clearer picture of the system in which p53 is a central player and may suggest new ways to combat an array of cancers in which p53 is dysfunctional. More generally, the study begins to show how many other proteins that act directly on DNA, as p53 does, might also be tightly managed by similar sets of closely interacting molecules. A report on the research appears in the December issue of Molecular Cell, published today.
"Our findings show that p53's ability to suppress tumors depends on a cascade of molecular changes that occur after the molecule binds to the DNA," says Shelley L. Berger, Ph.D., an associate professor in molecular genetics and senior author on the report. "The data also outline a general mechanism by which many DNA-binding proteins, including other transcription factors like p53, might be regulated. So, in terms of understanding gene control, the implications could be quite broad."
The normal function of p53 is to monitor the replication of DNA during the cycle of cell division. If DNA damage is detected, p53 is responsible for either arresting the cycle until repairs can be made or sending the cell into apoptosis, or regulated cell death. When p53 is unable to perform this function, the frequent result is cancerous growth of the cell.
Earlier work by Berger and others, including Wistar associate professor Thanos Halazonetis, D.D.S., Ph.D., a coauthor on the current study, had shown that p53 uses a family of molecules called HATs, or histone acetyltransferases, to activate the genes that it controls. HATs, which act on small proteins called histones, are themselves at heart of larger understanding of gene control that is still developing.
In this emerging scheme, long strands of DNA are seen to coil themselves around histones to create sub-chromosomal structures called nucleosomes. Genes along the tightly wrapped DNA cannot be physically accessed by the cellular machinery of transcription, and so their expression is repressed. The coils of DNA around the histones must first be loosened to permit gene expression. HATs are enzymes that add an acetyl molecule to the histone, which has the effect of loosening the DNA coils.
To further explore HAT activity in relation to p53, Berger and her colleagues created mutations at the four specific sites on p53 where HATs are known to acetylate the molecule, thus disabling the process.
"What we found was that the mutations at p53's acetylation sites significantly reduced its efficiency as a transcriptional activator and almost entirely abrogated its ability to arrest the cell cycle," says Wistar staff scientist Nickolai A. Barlev, Ph.D., lead author on the Molecular Cell study. "This was the first direct evidence that acetylation is critical for p53's function."
"While we saw no effect on p53's ability to bind to DNA," says Berger, "the molecule was severely limited in its ability to activate transcription without the acetylation events that would ordinarily follow binding to DNA. We also demonstrated the acetylation process to be multi-step: acetylation of p53 triggers the histone acetylation required to promote gene transcription."
In addition to senior author Berger, lead author Barlev, and collaborating coauthor Halazonetis, the remaining coauthors on the Molecular Cell study are Lin Liu, Nabil H. Chehab, Kyle Mansfield, and Kimberly G. Harris, all at The Wistar Institute. The research was supported by a National Cancer Institute grant.
The Wistar Institute is an independent nonprofit research institution dedicated to discovering the causes and cures for major diseases, including cancer and AIDS. The Institute is a National Cancer Institute-designated Cancer Center - one of the nation's first, funded continuously since 1968, and one of only 10 focused on basic research. Founded in 1892, Wistar was the first independent institution devoted to medical research and training in the nation. Since the Institute's inception, Wistar scientists have helped to improve world health through the development of vaccines against rabies, rubella, rotavirus, cytomegalovirus, and other viruses and the identification of genes associated with breast, lung, prostate and other cancers.
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Journal
Molecular Cell