Aptamer-Assisted Detection of the Altered Expression of Estrogen Receptor Alpha in Human Breast Cancer

An increase in the expression of estrogen receptors (ER) and the expanded population of ER-positive cells are two common phenotypes of breast cancer. Detection of the aberrantly expressed ER alpha in breast cancer is carried out using ER alpha-antibodies and radiolabelled ligands to make decisions about cancer treatment and targeted therapy. Capitalizing on the beneficial advantages of aptamer over the conventional antibody or radiolabelled ligand, we have identified a DNA aptamer that selectively binds and facilitates the detection of ERa in human breast cancer tissue sections. The aptamer is identified using the high throughput sequencing assisted SELEX screening. Biophysical characterization confirms the binding and formation of a thermodynamically stable complex between the identified DNA aptamer (ERaptD4) and ER alpha (Ka = 1.55 +/- 0.298x10(8) M-1; Delta H = 4.32x10(4)+/- 801.1 cal/mol; Delta S = -108 cal/mol/deg). Interestingly, the specificity measurements suggest that the ERaptD4 internalizes into ER alpha-positive breast cancer cells in a target-selective manner and localizes specifically in the nuclear region. To harness these characteristics of ERaptD4 for detection of ER alpha expression in breast cancer samples, we performed the aptamer-assisted histochemical analysis of ER alpha in tissue samples from breast cancer patients. The results were validated by performing the immunohistochemistry on same samples with an ER alpha-antibody. We found that the two methods agree strongly in assay output (kappa value = 0.930, pvalue <0.05 for strong ER alpha positive and the ER alpha negative samples; kappa value = 0.823, pvalue <0.05 for the weak/moderate ER+ve samples, n = 20). Further, the aptamer stain the ER alpha-positive cells in breast tissues without cross-reacting to ER alpha-deficient fibroblasts, adipocytes, or the inflammatory cells. Our results demonstrate a significant consistency in the aptamer-assisted detection of ER alpha in strong ER alpha positive, moderate ER alpha positive and ER alpha negative breast cancer tissues. We anticipate that the ERaptD4 aptamer targeting ER alpha may potentially be used for an efficient grading of ER alpha expression in cancer tissues.