INVESTIGATING ROLES OF THE METABOLIC ENZYME FUMARASE AND THE METABOLITE FUMARATE IN DNA DAMAGE RESPONSE

In eukaryotic cells, DNA is packaged into a structure named chromatin which contains DNA and proteins. Nucleosomes are building blocks of chromatin and contain DNA wrapped around a histone octamer. Chromatin modifications (histone post-translational modifications and histone variants) play central roles in various cellular processes including gene expression and DNA damage response. Chromatin modifying enzymes use metabolites as co-substrates and co-factors, and changes in metabolic pathways and metabolite availability affects chromatin modifications and chromatin-associated functions. Moreover, recent studies have uncovered direct roles of metabolic enzymes in chromatin-associated functions. Fumarase, a TCA cycle enzyme that catalyzes the reversible conversion of fumarate to malate in mitochondria (a hydration reaction), is an example of an enzyme with dual functions in metabolism and genome integrity. Cytoplasmic fraction of yeast fumarase, Fum1p, localizes to the nucleus and promotes growth upon DNA damage. Fum1p promotes homologous recombination by enhancing DNA end resection. Human fumarase is involved in DNA repair by non-homologous end joining. Here, we provide evidence that yeast Fum1p and the histone variant Htz1p are also involved in DNA replication stress response and DNA repair by non-homologous end joining (NHEJ). Using mutants lacking the histone variant HTZ1, we show that high cellular levels of fumarate, by deletion of FUM1 or addition of exogenous fumarate, suppressed the sensitivity to DNA replication stress by modulation of activity of Jhd2p. This suppression required sensors and mediators of the intra-S phase checkpoint, but not factors involved in the processing of replication intermediates. These results imply that high cellular levels of fumarate can confer resistance to DNA replication stress by bypassing or complementing the defects caused by loss of HTZ1 and replication fork processing factors. We also show that upon induction of DSBs, exogenous fumarate conferred resistance to mutants with defects in NHEJ, early steps of homologous recombination (DNA end resection pathway) or late steps of homologous recombination (strand invasion and exchange). Taken together, these results link the metabolic enzyme fumarase and the metabolite fumarate to DNA damage response and show that modulation of DNA damage response by regulating activity of chromatin modifying enzymes is a plausible pathway linking metabolism and nutrient availability to chromatin-associated functions like genome integrity.