Effect of scrapie on the stability of housekeeping genes.

Scrapie is the archetype of prion diseases, fatal neurodegenerative disorders that affect humans and animals. Gene expression analysis of normal and infected sheep may provide clues to clarify the molecular mechanisms involved in the neuropathology of these diseases. Real time quantitative PCR has become a powerful and accurate technique for examination of transcription patterns in different biological conditions. One of the critical steps in the comparison of transcription profiles is the selection of stable genes for normalization of expression data. In this work, we have investigated the effect of scrapie on the stability of eight commonly used housekeeping genes in the central nervous system of sheep. We found that their stability decreased in scrapie-infected tissues, with the effect of the disease most evident in the medulla oblongata, a highly affected area of the brain stem. The risk of choosing inappropriate housekeeping genes for expression analysis was evaluated. Although the stability of each reference gene was suitable, a wide variation in expression of target genes (BAX and BCL2) was observed when only one or two housekeeping genes were used to normalize. However, reliable results were obtained with a normalization factor based on three reference genes, regardless of their position in a stability ranking.