Aptamer Binding Assay for the E Antigen of Hepatitis B Using Modified Aptamers with G-Quadruplex Structures.

The e antigen of hepatitis B (HBeAg) is positively associated with an increased risk of developing liver cancer and cirrhosis in chronic hepatitis B (CHB) patients. Clinical monitoring of HBeAg provides guidance to the treatment of CHB and the assessment of disease progression. We describe here an affinity binding assay for HBeAg, which takes advantage of G-quadruplex aptamers for enhanced binding and stability. We demonstrate a strategy to improve the binding affinity of aptamers by modifying their sequences upon their G-quadruplex and secondary structures. On the basis of predicting a stable G-quadruplex and a secondary structure, we truncated 19 nucleotides (nt) from the primer regions of an 80-nt aptamer, and the resulting 61-nt aptamer enhanced binding affinity by 19 times (Kd = 1.2 nM). We mutated a second aptamer (40 nt) in one loop region and incorporated pyrrolo-deoxycytidine to replace deoxycytidine in another loop. The modified 40-nt aptamer, with a stable G-quadruplex and two modified loops, exhibited a 100 times higher binding affinity for HBeAg (Kd = 0.4 nM) than the unmodified original aptamer. Using the two newly modified aptamers, one serving as the capture and the other as the reporter, we have developed an improved sandwich binding assay for HBeAg. Analyses of HBeAg in serum samples (concentration ranging from 0.1 to 60 ng/mL) of 10 hepatitis B patients, showing consistent results with clinical tests, demonstrate a successful application of the aptamer modification strategy and the associated aptamer binding assay.