Folate deficiency and DNA-methyltransferase inhibition modulate G-quadruplex frequency

G-quadruplexes (G4) are highly stable tetra-stranded DNA secondary structures known to mediate gene regulation and to trigger genomic instability events during replication. G4 structural stability can be affected by DNA methylation and oxidation modifications; thus nutrients such as folate that have the ability to alter these processes could potentially modify the genomic occurrence of G4 elements. Hela cells were cultured in a range of folate concentrations or in the presence or absence of 5-aza-2'-deoxycytidine, a DNA-methyltransferase inhibitor. G4 structures were then quantified by immunofluorescence using an automated quantitative imaging system. G4 frequency in Hela cells and nuclei area mean were increased in 20nM folate medium compared with 2000nM folate, as well as in the presence of 5-aza-2'-deoxycytidine when compared to cells non-exposed to 5-aza-2'-deoxycytidine. These changes were exacerbated when pyridostatin, a G4 stabilising ligand, was added to the culture medium. G4 intensity in Hela cells cultured in deficient folate condition with pyridostatin was highly correlated with DNA damage as measured by γH2AX immunofluorescence (r = 0.71). This study showed for the first time that cellular G4 balance is modifiable by low folate concentrations and that these changes may occur as a consequence of DNA hypomethylation. Although the exact mechanism by which these changes occur is unclear, these findings establish the possibility that nutrients could be utilised as a tool for sustaining genome integrity by modifying G4 frequency at a cellular level.