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Morphologic assessment of formalin-fixed, paraffin-embedded (FFPE) tissue samples has been the gold standard for cancer diagnostics for decades due to its excellent preservation of morphology. Personalized medicine increasingly provides individually adapted and targeted therapies for characterized individual diseases enabled by combined morphological and molecular analytical technologies and diagnostics. Performance of morphologic and molecular assays from the same FFPE specimen is challenging because of the negative impact of formalin due to chemical modification and cross-linking of nucleic acids and proteins. A non-cross-linking, formalin-free tissue fixative has been recently developed to fulfil both requirements, i.e., to preserve morphology like FFPE and biomolecules like cryo-preservation. Since FISH is often required in combination with histopathology and molecular diagnostics, we tested the applicability of FISH protocols on tissues treated with this new fixative. We found that formalin post-fixation of histological sections of non-cross-linking, formalin-free and paraffin-embedded (NCFPE) breast cancer tissue generated equivalent results to those with FFPE tissue in human epidermal growth factor receptor 2 (HER2) FISH analysis. This protocol describes how a FISH assay originally developed and validated for FFPE tissue can be used for NCFPE tissues by a simple post-fixation step of histological sections.

Molecular diagnostics in personalized medicine increasingly relies on the combination of a variety of analytical technologies to characterize individual diseases and to select patients for targeted therapies. The gold standard for tissue-based diagnostics is fixation in formalin and embedding in paraffin, which results in excellent preservation of morphology but negatively impacts on a variety of molecular assays. The formalin-free, non-cross-linking PAXgene tissue system preserves morphology in a similar way to formalin, but also preserves biomolecules essentially in a similar way to cryopreservation, which markedly widens the spectrum, sensitivity and accuracy of molecular analytics. In this study, we have developed and tested a protocol for PAXgene-fixed and paraffin-embedded tissues for fluorescent in situ hybridization (FISH). The implementation of a 24-h formalin postfixation step of slides from PAXgene-fixed and paraffin-embedded tissues allowed us to use the assays approved for formalin-fixed and paraffin-embedded tissues. The equivalence of the methodologies was demonstrated by FISH analysis of HER2 amplification in breast cancer cases. The 24-h postfixation step of the slides used for FISH can be well integrated in the routine diagnostic workflow and allows the remaining PAXgene-fixed and paraffin-embedded tissue to be used for further molecular testing., (© 2016 The Authors. International Journal of Experimental Pathology published by John Wiley & Sons Ltd on behalf of Company of the International Journal of Experimental Pathology (CIJEP).)