This project will address a key feature of an organism's response to DNA damage that permits it to survive and divide. Cells as diverse as yeast and humans package their DNA with proteins called histones. One particular chemical modification of histone allows cells to recognize, respond to and repair damage to DNA. However, this histone modification is present in some parts of the DNA but absent in others, and the regions lacking this special modification have a reduced response to damaged DNA. This project will determine how this particular modification is excluded from some histones, and what role this exclusion has in normal cell growth. The outcomes of this work will greatly improve our understanding of how humans and other organisms respond to environmental insults that damage DNA, and provide a vehicle for training graduate students in the thinking and practice of science. This project will also provide undergraduate students with their first experience in scientific research beyond the classroom, and with real-world applications of their science, technology and math skills before entering the work force.
Telomeres are the regions at the ends of chromosomes, and the manner in which the stable telomere ends are distinguished from the unstable ends generated by double-strand breaks elsewhere in chromatin is a fundamental question in the fields of genome integrity and chromosome biology. While double-strand breaks cause growth arrest and promote genomic instability by DNA recombination, telomeres have functions that allow continuous cell growth and the complete replication of the chromosome end. Telomeres are composed of DNA repeat tracts bound by specific proteins, and previous studies have focused on the role of telomere proteins in suppressing recombination and cell cycle arrest. However, recent work in the fission yeast Schizosaccharomyces pombe has implicated an epigenetic modification in this process. Specifically, a key feature that prevents damaged telomeres with bound DNA damage response proteins from causing cell cycle arrest is the lack of dimethylation of histone H4 lysine 20 (H4K20me2) in nucleosomes near the telomere DNA. The H4K20me2 modification is conserved from S. pombe to mammals, and its absence near telomeres is thought to allow them to incur damage and still allow cell growth, making telomeres a DNA damage checkpoint-exempt region of the genome. How telomeres exclude H4K20me2 is unknown. This project will advance our understanding of H4K20me2 exclusion using a rapidly inducible telomere formation system in S. pombe. This system will be used to determine how the H4K20me2-free state is established. Defining how much of the chromosomal end excludes H4K20me2 has important implications for the portions of the genome exempt from DNA damage-induced arrest, and also for our understanding of how cells regulate genome stability. Using a combination of chromatin immunoprecipitation and genetic approaches, this project will determine if the H4K20me2-free state is restricted to telomeres, or also spreads into the adjacent chromatin.
