*The Importance of the Fifth Nucleotide in DNA: Uracil DOI: http://dx.doi.org/10.5772/intechopen.110267*

studies have confirmed that DNA-cytosine deaminase or U:G mismatches constitute a major mutational DNA lesion that contributes to various diseases, including cancer development and progression [93]. To counteract uracil in DNA, the cell has evolved the two proteins TS and dUTPase (see Section 4.1), which lowers the dUTP:dTTP ratio and thereby reduces dUTP use by DNA polymerases by having more dTTP available. The first step is by dUTPase, which converts dUTP into dUMP; thus, dUTP levels are decreased and TS's substrate (dUMP) is increased simultaneously. TS then converts dUMP into dTMP by attaching a methyl group to the C5 position of U's aromatic ring, the methyl group being donated by 5,10-methylenetetrahydrofolate (5,10-CH2THF, a folate derivative). Two extra kinase steps are then needed to convert dTMP into dTTP, which can be readily processed for DNA synthesis.

Originally, it was rationalized that targeting the pyrimidine biosynthesis pathway would be an effective method in treating cancer, based on several findings including the observation that U was specifically incorporated into the nucleic acid fractions of rat tumours [94]. This eventually led to the development of the fluoropyrimidine 5-Fluoruracil (5-FU) in 1957 [95], one of the best know chemotherapies for cancer treatment in modern times. 5-FU's mechanism of action as an anti-cancer therapy seem to be complicated but what is known is that metabolites produced from it can are incorporated into DNA and RNA, which induces cellular stress and then subsequent cell death in cancer cells. Additionally, one of 5-FU's metabolites (FdUMP) is able to inhibit and it is through this method a range of fluoropyrimidines and antifolates (which indirectly inhibit TS) have been designed and used for treatment of large range or cancer types, in which their uses still seem to be growing today [96].

Building on 5-FU's clinical success, several additional cancer therapies such Pemetrexed, Capecitabine, Methotrexate and Raltitrexed were developed and are currently used in cancer chemotherapy regimens to inhibit TS and modify the cancer cell's viability as a consequence of the depletion of dTTP pools, called *thymine less-death*. Thymine less-death results from an imbalance in dUTP/TTP levels, where dTTP is depleted and there is an increase in dUTP, misincorporation of uracil into DNA and following attempted repair results in DNA double-strand breaks (DSB) [97, 98]. To a global understanding of involved mechanism it is important to highlight that the dUMP pools needed for TMP biosynthesis depends on dCMP deamination and UDP reduction by deoxycytidylate deaminase (DCD) and RNR, respectively [39].
