Evaluation of methods for preserving PTEN antigenicity in stored paraffin sections

Reduced phosphatase and tensin homolog (PTEN) expression in breast cancer confers susceptibility to phosphatidylinositol 3 kinase/Akt pathway inhibitors. As inhibitors of this pathway are in development for treatment of breast and other cancers, it is of interest to minimize the frequency at which PTEN signal is reduced because of poor antigen preservation, rather than to decreased protein expression. Methods for preserving antigenicity include coating sections in paraffin, refrigerated storage, desiccation, and storage in nitrogen. This study examined various combinations of the first 2 methods with respect to PTEN immunoreactivity. This was assessed semiquantitatively in freshly cut sections of paraffin blocks from 26 breast cancer specimens. The histoscore was compared with that observed in cut sections stored for 2 to 6 months with or without paraffin coating at room temperature (RT; 20 to 25°C) or at 2 to 8°C. At 2 months, the histoscore was strongest in freshly cut sections. Among the other treatments, refrigerated paraffin-coated sections had the highest histoscore followed by refrigerated uncoated sections. It was lower in RT paraffin-coated sections. Uncoated sections stored at RT showed the greatest signal loss. The greatest magnitude of changes between the various storage conditions and freshly cut slides ranged from 15% for refrigerated paraffin-coated slides to 75% for RT uncoated slides at 4 months. Although freshly cut paraffin-embedded sections were optimal, sections can be stored with adequate antigen preservation for 2 months using refrigeration with or without paraffin coating, and for 4 months using refrigeration with paraffin coating.