To an effective therapeutic option. In fact,

To date, only two DnmtIs 5-Azacytidine (AZA) and Decitabine (DAC) are approved for clinical usage, and for the treatment of Myeloid Dysplastic Syndrome (MDS), Acute Myeloid Leukaemia (AML) and Chronic Myelomonocytic Leukaemia (CMML) by the USA Food and Drug Administration. AZA and DAC are classed as cytidine analogues due to the substitution of the carbon atom in the 5th position with a nitrogen, bound to a ribose or deoxyribose molecule respectively. Cellular uptake of both drugs subsequently leads to phosphorylation and reduction processes to generate tri-phosphorylated activated compounds that are integrated into the DNA. Chemically, due to the nitrogen molecule in the 5th position of the DnmtIs, the ?-elimination mechanism cannot proceed and thus leads to sequestration of Dnmts through suicide inhibition and inducing degradation of the enzyme (figure 6). Furthermore, 80-90% of AZA is incorporated into RNA while the rest is incorporated into DNA, whereas DAC is fully incorporated into DNA, which possibly explains DACs increased efficacy and fewer side effects. For example, in the context of Huntington’s disease (HD), therapeutic usage of DAC reduced the amounts of huntingtin (Htt) protein aggregates in striatal neurons, a key hallmark of HD, and rescued transcriptional activity of genes known to be pathologically hypermethylated as well, highlighting a potential therapeutic use of DAC in HD besides the treatment of leukaemias. In the treatment of MDS and AML, both AZA and DAC are utilised for their hypomethylating effect via Dnmt1 inhibition, and thus restoring the transcriptional activity of commonly hypermethylated genes including MDR1 and syndecan 4. Additionally, hypermethylation of Frizzled9 (FZD9), an isoform of the Wnt receptor was also found to be directly correlated with both MDS and AML, and that demethylation via DAC treatment was an effective therapeutic option. In fact, a comprehensive analysis comparing AML patients undergoing AZA therapy or conventional care regimens displayed a significant increase in overall survival (OS) rates towards the AZA treatment group. As mentioned previously, the incidence of Dnmt3A-mutation related leukaemias are on the rise and a study comprised of 92 patients with MDS and similar disorders concluded that patients bearing either or both Dnmt3A and Tet2 mutations increasingly benefited from hypomethylating agents. Conversely, an in-vitro stromal coculture assay exposing primary AML cells to DAC did not display any significant difference due to DAC treatment between WT and Dnmt3A mutant samples, indicating that the precise mode of action of DAC remains to be explored. An intriguing question would be since Dnmt mutations are expected to induce a reduction in global methylation levels, how do hypomethylating drugs offer their beneficial effects? Further research exploring the correlation between gene expression and post-DnmtI treatment in-vivo are imperative to expand our understanding in their clinical applications.