Environmental Factor, February 2011, National Institute of Environmental Health Sciences
Assigning a function to histone modifying enzymes
By Sophie Bolick
Dent's lab uses yeast and mouse models to understand the function of histone modifying enzymes. (Photo courtesy of Sharon Dent)
Lecture host Trevor Archer is also principal investigator and head of the NIEHS Chromatin and Gene Expression Group. (Photo courtesy of Steve McCaw)
Sharon Dent, Ph.D., a leading scientist in the field of chromatin biology, may have had a sense of déjà vu on her recent visit to give a presentation on "The Secret Lives of Histone Modifying Enzymes" at NIEHS Jan. 13. Rising through the faculty ranks to her current position as chair of the Department of Carcinogenesis at the University of Texas M. D. Anderson Cancer Center in Smithville, Texas, Dent says she now fondly remembers her days as a postdoctoral fellow at NIH.
The Dent(http://www.mdanderson.org/education-and-research/departments-programs-and-labs/labs/dent-laboratory/index.html) lab's research program focuses on understanding the function and regulation of histone modifying enzymes and their role in cancer and other diseases, an area which has seen explosive growth since the first histone acetyltransferases were discovered in 1996. She began by describing what she thinks are the three major breakthroughs in the field over the last ten years.
The first important advance was defining the enzymes governing acetylation, methylation, and phosphorylation events at histones. Second was the finding that these modifications can regulate one another. And, third is discovering histone modifications not only alter the interactions they have with DNA, but also act as ligands for binding of other proteins.
"All of these things together have led to the idea that there's information in these marks," stated Dent.
Cross-talk on non-histone proteins
Accurate chromosome segregation during mitosis relies on centromeres of sister chromatids to attach to microtubules from opposite spindle poles, a process mediated by kinetochores. Regulation occurs, in part, through a series of phosphorylation and dephosphorylation events. In yeast, the Aurora kinase, Ipl1, plays a key role in mediating chromosome segregation events, while Dam1 connects microtubules to the kinetochore.
One-well established function for the Set1 methyltransferase is to methylate the lysine 4 residue of histone H3. Dent's group identified a novel function for Set1, separate from histone methylation, by performing a series of elegant experiments. Set1 methylates Lys233 in Dam1, which subsequently prevents phosphorylation of neighboring serines around this lysine residue from phosphorylation by Ipl1. The phosphatase Glc7 also plays an important role in maintaining just the right amount of phosphorylation for proper chromosome segregation to occur. Dent terms this the "Goldilocks model," because too little or too much phosphorylation also negatively impacts proper chromosome segregation. This finding was one of the first reports demonstrating cross-regulation on non-histone proteins.
Gcn5 and developmental processes in mice
Dent's group also uses mouse models to study the function of histone modifying enzymes. Her work on the histone acetyltransferase Gcn5 demonstrated its involvement in mammalian development. Mice lacking Gcn5 (Gcn5 -/-) die early in embryogenesis, while mice with point mutations in the catalytic domain of Gcn5 (Gcn5hat/hat) die later in embryogenesis and have neural tube defects. These phenotypic observations led Dent to conclude, "Gcn5 functions early in development, but independent of its histone acetyltransferase activity."
In examining the chromosomes from the Gcn5 -/- mice, it appeared they were fused together. "These turned out to be telomeric fusions that were occurring," Dent explained. Upon further analysis, there were decreased levels of the telomere-associated proteins TRF1 and POT1a in the Gcn5 -/- cells.
The SAGA complex contains numerous modules, including Gcn5 and ubiquitin-specific protease (USP22). A series of experiments showed that USP22 interacted with TRF1. Loss of Gcn5 from cells leads to a loss of USP22 from the SAGA complex, increasing TRF1 protein turnover, and thereby explaining the initial results. This was the first experimental evidence linking the SAGA complex to its role in telomere maintenance. Elucidating the function of Gcn5 is important for understanding the molecular basis of neural tube defects and cancer.
Hosted by a former postdoctoral colleague at NIH, Chief of the Laboratory of Molecular Carcinogenesis (LMC) Trevor Archer, Ph.D.(http://www.niehs.nih.gov/research/atniehs/labs/lmc/cge/index.cfm), Dent presented her groundbreaking research as part of the weekly LMC seminar series.
(Sophie Bolick, Ph.D., is a postdoctoral fellow with the Molecular and Genetic Epidemiology Group in the Laboratory of Molecular Carcinogenesis.)