Ultrasensitive Electrochemical Detection of miRNA-21 Using a Zinc Finger Protein Specific to DNA–RNA Hybrids

Both high sensitivity and high specificity are crucial for detection of miRNAs that have emerged as important clinical biomarkers. Just Another Zinc finger proteins (JAZ, ZNF346) bind preferably (but nonsequence-specifically) to DNA-RNA hybrids over single-stranded RNAs, single-stranded DNAs, and double-stranded DNAs. We present an ultrasensitive and highly specific electrochemical method for miRNA-21 detection based on the selective binding of JAZ to the DNA-RNA hybrid formed between a DNA capture probe and a target miRNA-21. This enables us to use chemically stable DNA as a capture probe instead of RNA as well as to apply a standard sandwich-type assay format to miRNA detection. High signal amplification is obtained by (i) enzymatic amplification by alkaline phosphatase (ALP) coupled with (ii) electrochemical-chemical-chemical (ECC) redox cycling involving an ALP product (hydroquinone). Low nonspecific adsorption of ALP-conjugated JAZ is obtained using a polymeric self-assembled-monolayer-modified and casein-treated indium-tin oxide electrode. The detection method can discriminate between target miRNA-21 and nontarget nucleic acids (DNA-DNA hybrid, single-stranded DNA, miRNA-125b, miRNA-155, single-base mismatched miRNA, and three-base mismatched miRNA). The detection limits for miRNA-21 in buffer and 10-fold diluted serum are approximately 2 and 30 fM, respectively, indicating that the detection method is ultrasensitive. This detection method can be readily extended to multiplex detection of miRNAs with only one ALP-conjugated JAZ probe due to its nonsequence-specific binding character. We also believe that the method could offer a promising solution for point-of-care testing of miRNAs in body fluids.