Genetic differences in DNA sequence between two individuals can be manifested as differences in the individual's physical characteristics and functions. The term epigenetics refers to the fact that two organisms, or two cells within the same organism, can have identical genetic information (the same DNA sequence) but have very different characteristics and functions. Epigenetic controls are essential in establishing the hundreds of distinct cell types (e.g., skin, muscle, liver) in complex organisms such as humans. Cells within an organism have the same DNA, and therefore identical genes, but can assume unique morphologies and functions by establishing unique patterns of gene expression, expressing some genes while turning others off. The DNA in cells is organized into structures known as chromosomes. A key mechanism for controlling whether genes are on or off is by altering the structure of the chromosome. Once established, these alterations can become a stable, heritable part of the chromosome. The nature of these structures and the manner in which they are inherited is not clear. To understand how epigenetic mechanisms work this project utilizes a simple organism, budding yeast. Yeast uses an epigenetic gene repression mechanism, known as silencing to control the genes responsible for determining cell type. The fundamental mechanisms of gene regulation and cell growth are remarkably similar in yeast and human cells; however, as it is far easier to perform many types of experiments using yeast, progress is more rapid using the yeast system. This project is focused on determining if the cell machinery responsible for duplicating and partitioning chromosomes as cells divide also has a role in establishing and maintaining silenced chromosomal structures. These studies will also examine the function of the Sir2 silencing protein in yeast. Interestingly, Sir2 proteins in many species affect lifespan, and there are several Sir2-like proteins in humans.
Broader Impacts:
The research pursuits described in this project will be closely integrated into the educational mission of Wesleyan University, an undergraduate-oriented liberal arts university. Projects in the principal investigator's laboratory on epigenetics are integrated into an advanced lab course required of undergraduate majors. This course is designed to familiarize undergraduates with the methods and approaches of the field in the context of pursuing novel research questions. Undergraduates (typically ~3/semester) also join the project leader's research lab, where they join a group of approximately three graduate students. The project leader is also collaborating in developing lessons in genetics with a local high school biology teacher; advanced students from this high school also visit the research lab to shadow graduate students.
Our research addresses basic questions about the influence that the structure of chromosomes has on gene expression and chromosome segregation. An organism's DNA is organized into genes, which constitute the blueprint for how the organism develops. While each cell has the same set of genes, to develop into distinct types of cells (muscle, skin, nerve, etc.) specific genes must be selectively activated or repressed. The term "epigenetics" refers to the fact that two cells can have identical genetic information (the same DNA sequence) but have very different physical characteristics and functions. To understand how epigenetic mechanisms work we are determining how a simple organism, yeast, maintains cell identity. The fundamental mechanisms of gene regulation and cell growth are the same in yeast cells and human cells; however, it is far easier to perform many types of experiments using yeast, and therefore we can progress more rapidly using the yeast system. Yeast uses a gene repression mechanism, known as "silencing", to control the expression of key developmental regulatory genes. Genes are organized on chromosomes, very long DNA molecules. Proteins associated with chromosomes, particularly histones, organize and protect the DNA. Silencing is achieved by altering the structure of the chromosome, which involves modifying the histones. Sir2 is an important modifier of histone genes, and its activity is associated with gene silencing. Sir2 activity has also been linked to increased lifespan in a variety of organisms. A key finding from our research is that GAPDH, an enzyme that maintains basic metabolism in cells, has a novel function in promoting Sir2 activity. This result highlights an interesting signaling mechanism that may tie cell metabolism and nutrition to control of gene expression. A key feature of epigenetics is the ability of gene expression patterns to be inherited as cells divide. We have identified key aspects of chromosome structure that are maintained on genes as DNA is duplicated in the process of making new cells. We have also identified links between the mechanisms that underlie chromosome duplication and segregation and those that mediate gene silencing. In particular, we have reported that a variant histone protein known as H2A.Z controls the timing of gene silencing, and is also crucial for chromosome pairing, a necessary prelude to chromosome distribution to new cells. We reported our results at several scientific meetings, and also published five articles in the scientific literature. Results obtained in this funding period will appear in future publications as well. This research was conducted by a team of undergraduate and graduate students at Wesleyan University. Wesleyan combines the mission of a highly selective undergraduate liberal arts college with the resources and graduate program of a university. Our lab's position at Wesleyan leads to excellent opportunities to train and influence undergraduate and graduate students interested in the life sciences. Our lab engages significant numbers of undergraduate students in our NSF-funded research; this research grant supported the research of approximately ten undergraduate students; five undergraduates were authors on papers published from the lab. Four graduate students received their PhD and continued on to positions in the biomedical sciences. Many of the undergraduate researchers went on to pursue their PhD's (e.g., at Harvard University and Johns Hopkins University). We have new undergraduate and graduate students who continue to avidly pursue these questions.
