Antigen Retrieval for Immunostaining of Modified Cytosine Species

Immunostaining (also called as immunofluorescence) is a fluorescence labeling method to stain one or more epitopes of interest on DNA and/or protein using specific antibodies. Cytosine modifications can be detected quantitatively by immunostaining. The protocol commonly includes sequential steps. These include fixation, permeabilization, antigen retrieval, blocking, incubation with primary and secondary antibodies, and visualization under the microscope followed by image-based intensity analysis of staining. Each step is important, but antigen retrieval is especially necessary for DNA epitopes such as cytosine modifications as antibodies can access cytosines in DNA only once the DNA double-strand is denatured and DNA-packaging proteins have been removed. Hydrochloric acid is commonly used for this purpose. However, there are additional treatments with enzymes to enhance antigen retrieval and improve the detection by increasing staining intensity. This chapter describes current methodology for improving antigen retrieval for the staining of the cytosine modifications 5'-methylcytosine (5meC), 5'-hydroxymethylcytosine (5hmC), 5'-formylcytosine (5fC), and 5'-carboxycytosine (5caC).