Effects of DZNep and 5-Aza-CdR on gene expression in MCF7 cells

DNA methylation, histone modifications, and nucleosomal occupancy collaborate to cause silencing of tumor related genes in cancer. The development of drugs that target these processes is therefore important for cancer therapy. Inhibitors of DNA methylation and histone deacetylation have already been approved by the FDA for the treatment of hematologic malignancies. However, drugs that target the other mechanisms still need to be developed. Recently, 3-deazaneplanocin A (DZNep) was reported to selectively inhibit the trimethylation of lysine 27 on histone H3 (H3K27me3) and lysine 20 on histone H4 (H4K20me3) as well as re-activate silenced genes in cancer cells. This finding opens the door to pharmacological inhibition of histone methylation and we therefore wanted to further study the mechanism of action of 3-deazaneplanocin A in cancer cells. Western blot analysis showed that two other drugs, sinefungin and adenosine-dialdehyde (Adox), have similar effects on the trimethylation H3K27 as 3-deazaneplanocin A and that DZNep is not selective, but globally inhibits histone methylation. Intriguingly, chromatin immunoprecipitation of various histone modifications and microarray analysis show DZNep acts via a different pathway to 5-aza-2´-deoxycytidine (5-azaCdR), a DNA methyltransferase inhibitor and gives us an interesting insight into how chromatin structure effects gene expression. We also determine the kinetics of gene activation in order to understand if the induced changes were somatically heritable. We have found that upon removal of DZNep, gene expression is reduced to its original state suggesting that there is a homeostatic mechanism which returns the histone modifications to their “ground state” after DZNep treatment. Not only do these studies show the strong need for further development of histone methylation inhibitors but also allow us to better understand how chromatin structure affects gene expression.