Recently, post-translational modifications on histone tails have been shown to be responsible for epigenetic regulation of gene expression. One of these modifications is methylation of lysine residues. The first enzyme discovered to accomplish demethylation of these modified histones was histone lysine demethylase (LSD1). LSD1 specifically removes methyl groups from di- and mono-methylated lysines at position 4 of histone 3. This enzyme contained tightly bound FAD and the products of the reaction were the demethylated lysine residue and formaldehyde. The mechanism of LSD1 demethylation is analogous to the mechanism previously postulated for DMGDH, i.e. oxidation of the N-methyl bond to the methylene imine followed by hydrolysis to generate formaldehyde. This suggested that THF might also be involved in the LSD1 reaction to scavenge the formaldehyde produced.
Our hypotheses are that THF is bound to native LSD1 by analogy to DMGDH and SDH and that the bound THF serves to protect the FAD class of histone demethylases from the destructive effects of formaldehyde generation by formation of 5,10-methylene-THF. We present pilot data showing that decreased folate in livers as a result of dietary folate deficiency is associated with increased levels of methylated lysine 4 of histone 3. This can be a result of decreased LSD1 activity resulting from the decreased folate available to scavenge the formaldehyde produced at the active site caused by the folate deficiency. Because LSD1 can regulate gene expression this suggests that folate may play a more important role than simply serving as a carrier of one-carbon units and be a factor in other diseases associated with low folate.