Ultra-sensitive detection of melanoma NRAS mutant ctDNA based on programmable endonucleases

Melanoma is a complex and often fatal disease, with NRAS being one of the most frequently mutated genes in this type of cancer. Liquid biopsies, specifically tests for circulating tumor DNA (ctDNA), represent a promising and less invasive approach to diagnosis. This study aims to develop an ultra-sensitive assay for detecting melanoma NRAS mutant ctDNA.

To detect rare NRAS mutant ctDNA, we developed the NRAS PASEA assay by screening CRISPR-Cas proteins that recognize the PAM sequence 5'-TTN-3'. This method employs CRISPR-Cas proteins to continuously shear wild-type alleles during isothermal amplification, resulting in exponential amplification of mutant alleles to a detectable level by Sanger sequencing.

The developed NRAS Q61R/L/K mutation detection method can detect simulated ctDNA samples with mutant allele fractions (MAF) as low as 0.01 % with 30 mins of PASEA treatment. Notably, the NRAS Q61 K mutation was accurately identified by FnCas12a-based NRAS PASEA, even with the nucleotide at the "N" position in the PAM site "TTN." The method successfully detected ctDNA in patients with malignant melanoma. All patients (5/5) from 15 melanoma blood samples with NRAS Q61R (4/4) and NRAS Q61 K (1/1) mutations were accurately identified, with no false positives among patients with wildtype NRAS Q61.

Detecting ctDNA from peripheral blood samples is highly significant for melanomas in areas where imaging evaluation is challenging. Our assay demonstrated 100 % consistency with tumor tissue NGS, providing a new analytical strategy for companion diagnosis and dynamic assessment of therapeutic efficacy and disease progression in melanoma.