Generation of epitope-specific hCG aptamers through a novel targeted selection approach

Human chorionic gonadotropin (hCG) is a glycoprotein hormone used as a biomarker for several medical conditions, including pregnancy, trophoblastic and nontrophoblastic cancers. Most commercial hCG tests rely on a combination of antibodies, one of which is usually specific to the C-terminal peptide of the beta-subunit. However, cleavage of this region in many hCG degradation variants prevents rapid diagnostic tests from quantifying all hCG variants in serum and urine samples. An epitope contained within the core fragment, beta 1, represents an under-researched opportunity for developing immunoassays specific to most variants of hCG. In the study described here, we report on a SELEX procedure tailored towards the identification of two pools of aptamers, one specific to the beta-subunit of hCG and another to the beta 1 epitope within it. The described SELEX procedure utilized antibody-blocked targets, which is an underutilized strategy to exert negative selection pressure and in turn direct aptamer enrichment to a specific epitope. We report on the first aptamers, designated as R4_64 and R6_5, each capable of recognising two distinct sites of the hCG molecule-the beta-subunit and the (presumably) beta 1-epitope, respectively. This study therefore presents a new SELEX approach and the generation of novel aptamer sequences that display potential hCG-specific biorecognition.