Single-molecule RNA in situ detection in clinical FFPE tissue sections by vsmCISH

Although RNA plays a vital role in gene expression, it is less used as an in situ biomarker for clinical diagnostics than DNA and protein. This is mainly due to technical challenges caused by the low expression level and easy degradation of RNA molecules. To tackle this issue, methods that are sensitive and specific are needed. Here, we present an RNA single-molecule chromogenic in situ hybridization assay based on DNA probe proximity ligation and rolling circle amplification. When the DNA probes hybridize into close proximity to the RNA molecules, they form a V-shape structure and mediate the circularization of circle probes. Thus, our method was termed vsmCISH. We successfully applied our method to assess HER2 mRNA expression status in invasive breast cancer tissue and investigated the utility of albumin mRNA ISH for differentiating primary from metastatic liver cancer. The promising results on clinical samples indicate that our method has great potential for application in diagnosing diseases using RNA biomarkers.