FOCUS: A Dual-Mismatch crRNA Strategy Unlocks High-Fidelity One-Step SNV Detection with Cas12a

CRISPR/Cas12a has emerged as a powerful tool for nucleic acid detection; however, its clinical utility is severely hampered by intrinsic limitations in single-nucleotide variant (SNV) discrimination, reliance on pre-processed single-stranded DNA (ssDNA) templates, and cumbersome multi-step workflows. Here, we report a novel molecular design principle by engineering crRNAs with site-specific dual mismatches (positions 12 and 14 relative to the PAM), which we systematically demonstrate to drastically enhance the SNV discrimination capability of Cas12a. Leveraging this breakthrough, we developed FOCUS (Fast One-step CRISPR-based Universalizable SNV detection system), an all-in-one CRISPR sensing platform that enables isothermal detection of SNVs from double-stranded DNA (dsDNA) by integrating amplification and detection in a single reaction system. FOCUS achieved attomolar-level sensitivity (13.15 aM) and ultrafast readout (< 20 min) for distinguishing the highly homologous survival motor neuron 1 (SMN1) and SMN2 genes-the gold standard challenge for SNV genotyping in spinal muscular atrophy (SMA) diagnostics. To validate its clinical translatability, FOCUS was successfully adapted to a low-cost, equipment-free assay using lateral flow strips and UV visualization, facilitating point-of-care testing (POCT). In a comprehensive validation across 175 clinical samples, FOCUS exhibited 100% diagnostic concordance with gold-standard methods for SMA (21 samples), high-risk HPV 16/18 (27 samples), Staphylococcus aureus (20 samples), and SARS-CoV-2 (107 samples). Collectively, our study establishes a generalizable engineering strategy for Cas12a crRNAs and presents FOCUS as a robust, versatile, and field-deployable solution for precision SNV genotyping, underscoring the translational medicine value of FOCUS in molecular diagnostics.