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

Although RNA plays a vital role in the process of gene expression, it is less used as an in situ biomarker for clinical diagnostics compared to DNA and protein. This is mainly due to technical challenges caused by the low expression level and easy degradation of RNA molecules themselves. 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 on the RNA molecules, they form V shape structure and mediate the circularization of circle probes. Thus, our method was termed vsmCISH. We not only successfully applied our method to assess HER2 RNA mRNA expression status in invasive breast cancer tissue, but also to investigate the utility of albumin mRNA ISH for differentiating primary from metastatic liver cancer. The promising results on clinical samples indicates the great potential of our method to be applied in the diagnosis of disease using RNA biomarkers.