To understand the biological basis of resistance to endocrine
therapy is of utmost importance in patients with
steroid hormone receptor-positive
breast cancer. Not only will this allow us prediction of
therapy success, it may also lead to novel
therapies for patients resistant to current endocrine
therapy. DNA methylation in the promoter regions of genes is a prominent epigenetic gene silencing mechanism that contributes to
breast cancer biology. In the current study, we investigated whether promoter DNA methylation could be associated with resistance to endocrine
therapy in patients with recurrent
breast cancer. Using a microarray-based technology, the promoter DNA methylation status of 117 candidate genes was studied in a cohort of 200
steroid hormone receptor-positive
tumors of patients who received the
antiestrogen tamoxifen as first-line treatment for recurrent
breast cancer. Of the genes analyzed, the promoter DNA methylation status of 10 genes was significantly associated with clinical outcome of
tamoxifen therapy. The association of the promoter hypermethylation of the strongest marker,
phosphoserine aminotransferase (PSAT1) with favorable clinical outcome was confirmed by an independent quantitative DNA methylation detection method. Furthermore, the extent of DNA methylation of PSAT1 was inversely associated with its expression at the
mRNA level. Finally, also at the
mRNA level, PSAT1 was a predictor of
tamoxifen therapy response. Concluding, our work indicates that promoter hypermethylation and
mRNA expression of PSAT1 are indicators of response to
tamoxifen-based endocrine
therapy in
steroid hormone receptor-positive patients with recurrent
breast cancer.