Breast cancer (BC) is the most common cancer among women in the world and Brazil, after nonmelanoma skin cancer. Polymorphisms in genes involved in the folate pathway have been associated as possible etiological factors of this disease. Thymidylate synthase (TYMS) codes for the thymidylate synthase, responsible for the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). TYMS has a polymorphic tandem repeat in the 5'-UTR region (TSER), which generally contains a triple (3R) or double (2R) repeat of a 28 bp sequence. It is thought that the TSER variants are functionally relevant and are associated with BC risk. Another polymorphism in TYMS is 1494del6 and consists of the variation of a 6 bp sequence (TTAAAG) at position 1494 of the 3'-UTR region. These allelic variants are closely related to the level of expression of the enzyme. Methylenetetrahydrofolate reductase codes for the 5,10- methylenetetrahydrofolate reductase, which regulates the balance between cell methylation and nucleic acid synthesis, providing methyl groups for the conversion of homocysteine to methionine. Within the polymorphisms of MTHFR, the SNPs C677T and A1298C generate a reduced enzymatic activity, affecting the synthesis of nucleic acids and the availability of methyl groups for biochemical processes, which could increase BC risk. Methionine synthase (MTR) codes for the methionine synthase, which catalyzes the remethylation of homocysteine to methionine, an essential amino acid and precursor of S-adenosylmethionine, which is a universal donor of methyl groups involved in methylation reactions, including DNA methylation. The role of this polymorphism in cancer risk is still controversial. The aim of this study was to determine if the polymorphisms of the TYMS, MTHFR and MTR genes increase the BC risk. We worked with 61 samples of patients and 35 controls, it was carried out DNA extraction and purification, PCR amplification of DNA fragments including polymorphisms and their subsequent analysis directly through gel visualization, by PCR-RFLP and/or by automatic sequencing. Genotypic and allelic frequencies were determined and were related to the clinical characteristics of the patients and the molecular type of tumor. An analysis of statistical significance was carried out to evaluate the associations of all the polymorphisms with the risk of developing BC and the clinical characteristics of the patients. It was found that 3R allele of TSER and T and C alleles of C677T and A1298C could be associated to BC, although without statistical significance, and TSER and 1494del6 polymorphisms of TYMS could be related to the risk of developing more aggressive breast tumors, although the association is not statistically significant.
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