Endonuclease-Assisted PAM-free Recombinase Polymerase Amplification Coupling with CRISPR/Cas12a (E-PfRPA/Cas) for Sensitive Detection of DNA Methylation.

DNA methylation is considered as a potential cancer biomarker. The evaluation of DNA methylation level will contribute to the prognosis and diagnosis of cancer. Herein, we propose a novel assay based on endonuclease-assisted protospacer adjacent motif (PAM)-free recombinase polymerase amplification coupling with CRISPR/Cas12a (E-PfRPA/Cas) for sensitive detection of DNA methylation. The methylation-sensitive restriction enzyme (MSRE) is first used to selectively digest unmethylated DNA, while the methylated target remains structurally intact. Therefore, the methylated target can initiate the RPA reaction to generate a large amount of double-stranded DNA (dsDNA). To avoid the dependence of PAM site of CRISPR/Cas12a, one of the RPA primers is designed with 5'-phosphate terminuses. After treating with Lambda, the sequence with 5'-phosphate modification will be degraded, leaving the single-stranded DNA (ssDNA). The CRISPR/Cas12a can accurately locate ssDNA without PAM, then initiating its trans-cleavage activity for further signal amplification. Meanwhile, non-specific amplification can be also avoided under Lambda, effectively filtering the detection background. Benefiting from the specificity of MSRE, the high amplification efficiency of Lambda-assisted RPA, and the self-amplification effect of CRISPR/Cas, the E-PfRPA/Cas assay shows outstanding sensitivity and selectivity, and as low as 0.05% of methylated DNA can be distinguished. Moreover, the lateral flow assay is also introduced to exploit the point-of-care diagnostic platform. Most importantly, the proposed method shows high sensitivity for determination of genomic DNA methylation from cancer cells, indicating its great potential for tumor-specific gene analysis.