The principal methyl-donor S-adenosylmethionine (SAM) is synthesized from methionine and ATP, and participates in more cellular reactions than any other molecule except for ATP. As the co-factor for all chromatin methylation events, SAM is critical for the maintenance and transmission of epigenetic information. It is reasonable to predict a cellular system that monitors intracellular SAM concentrations and prevents initiation of cell division when SAM levels are too low to ensure faithful duplication of chromatin methylation during S-phase. This concept is reminiscent of the cell cycle checkpoint idea and we thus refer to it as the SAM-checkpoint. For my thesis work I propose to generate a molecular understanding for how cells monitor intracellular SAM concentrations and how this information is transmitted to the cell cycle machinery to induce the SAM-checkpoint. I will examine how cells monitor intracellular SAM levels and how DNA replication is blocked when SAM levels fall below a critical threshold concentration. My hypothesis is that SAM sensing is linked to translational control of a small number of "sensory" proteins. I suggest that select mRNAs of these sensory proteins are regulated at the level of mRNA cap methylation. The mechanism of this regulation will be addressed via purification of N7 methyl-capped mRNAs from cells experiencing SAM limitation. Furthermore, models have been developed that will allow me to address fundamental questions about epigenetic stability and the effect of dietary limitations on epigenetics. To assess epigenetic stability I will use yeast reporter strains modified to evade the SAM-checkpoint and monitor histone methylation of yeast cells in SAM limiting conditions. Similar experiments using inducible knock-down of methionine adenosyltransferase, the enzyme responsible for SAM synthesis, will address related questions in the mammalian system. For my thesis work I propose to generate a molecular understanding for how cells monitor intracellular SAM concentrations and how this information is transmitted to the cell cycle machinery to induce the SAM-checkpoint. This research may generate a paradigm for metabolite-regulated cell cycle checkpoints and will approach fundamental questions about epigenetics. My proposed studies will generate a first insight into how metabolic pathways are connected to cell function at the molecular level. My proposal specifically addresses the interaction of metabolic pathways and stability of epigenetic information. Results may have important consequences for our understanding of dietary factors and supplements such as SAMe, folates, or vitamin B12. My thesis proposal will only begin to address nutrient imbalance as a factor of epigenetic changes. However, I will develop tools to allow others to expand research on the interaction between nutrients and epigenetics.
Cell cycle progression relies on availability of key metabolites. When cells experience insufficient levels of metabolites the cell cycle is halted, which can result in cell death. To date, it is mostly unknown how metabolites are sensed and how information assessing metabolite shortages directly transmit to cell cycle machinery and trigger cell cycle arrest. Cancer cells show an altered metabolism and are addicted to certain metabolic pathways to sustain their uncontrolled growth. Thus, a molecular understanding of the integration of cell cycle progression and metabolism are critical for the identification of novel cancer therapeutics.