Chromogenic in situ hybridization - A practical alternative for fluorescence in situ hybridization to detect HER-2/neu oncogene amplification in archival breast cancer samples

Determination of HER-2/neu oncogene amplification has become necessary for selection of breast cancer patients for trastuzumab (Herceptin) therapy. Fl uorescence in situ hybridization (FISH) is currently regarded as a gold sta ndard method for detecting HER2/neu amplification, but it is not very pract ical for routine histopathological laboratories. We evaluated a new modific ation of in situ hybridization, the chromogenic ill situ hybridization (CIS H), which enables detection of HER-2/neu gene copies with conventional pero xidase reaction. Archival formalin-fixed paraffin-embedded tumor tissue sec tions were pretreated (by heating In a microwave oven and using enzyme dige stion) and hybridized with a digoxigenin-labeled DNA probe. The probe was d etected with antidigoxigenin fluorescein, anti-fluorescein peroxidase, and diaminobenzidine. Gene copies visualized by CISH could be easily distinguis hed with a x40 objective in hematoxylin-stained tissue sections. HER-2/neu amplification typically appeared as large peroxidase-positive intranuclear gene copy clusters. CISH and FISH (according to Vysis, made from frozen pul verized tumor samples) correlated well in a series of 157 breast cancers (k appa coefficient, 0.81). The few different classifications were mostly beca use of low-level amplifications by FISH that were negative by CISH and immu nohistochemistry with monoclonal antibody CB-11, We conclude that CISH, usi ng conventional bright-field microscopy in evaluation, is a useful alternat ive for determination of HER-2/neu amplification in paraffin-embedded tumor samples, especially for confirming the immunohistochemical staining result s.