Abstract
This chapter focuses on RNA in situ hybridization (RNA ISH) technology and applications of RNA ISH in anatomic pathology. Over the last several years, novel RNA ISH methods that utilize signal amplification, such as RNAscope, have dramatically improved the performance of the approach and led to the development of RNA ISH assays for a variety of applications across anatomic pathology, with a focus on infectious disease detection and tumor characterization. One of the most important applications is the detection of high-risk types of HPV in head and neck squamous cell carcinomas, an application with both high diagnostic sensitivity and specificity when compared to other detection techniques, such as DNA ISH and p16 immunohistochemistry (IHC). Identification of both high-risk and low-risk types of HPV in cervical biopsy specimens to improve diagnostic accuracy for low-grade squamous intraepithelial lesions (LGSILs) is another application with demonstrated clinical utility. Other compelling infectious disease applications include detection of cytomegalovirus (CMV) and Epstein-Barr virus (EBV), with assays for important pathogens like SARS-CoV-2 and Mycobacterium tuberculosis showing great promise. Applications of RNAscope in tumor characterization have become increasingly important, including detection of albumin expression in hepatocellular carcinoma and intrahepatic cholangiocarcinoma, and detection of KIM-1 expression in renal cell carcinoma. Furthermore, RNAscope has been shown to detect immunoglobulin kappa and lambda light chains in the assessment of clonality in lymphoid proliferations with similar diagnostic sensitivity and specificity to flow cytometry. Finally, several recent reports indicate that RNAscope can be used to detect gene fusions and amplifications, such as those involving the ALK and MDM2 genes, potentially providing an alternative approach to fluorescent in situ hybridization (FISH) and other technologies.