Toxicological housekeeping genes: Do they really keep the House?

Toxicogenomics has provided us with the opportunity of looking at dynamic systems that are growing and changing. The analysis of gene expression patterns using quantitative (real-time) reverse-transcriptase polymerase chain reaction (qRT-PCR) has become an essential requirement in determining the amount of gene expression in toxicological and biological samples. In this regard, the use of housekeeping gene has become a critical demand in normalization of real-time PCR and other toxicogenomic assay. Housekeeping gene is defined as a gene involved in basic functions needed for the sustenance of the cell. Thus, housekeeping genes are defined by specific gene promoter elements and are expressed constitutively in every cell and used in a normal cell to maintain basic cellular functions. But, if does not necessarily that the expressions of housekeeping genes are not regulated. The aim of this study was to provide toxicology researchers who have little experience in molecular biology but find themselves interested or involved with gene expression analysis, with a summary of informations necessary for re-evaluating their procedures. Thus, evaluating the expression patterns of five most commonly used (18S RNA, glucose-6-phosphate, b-actin, b-tubulin and elongation factor-1) housekeeping genes in toxicology, after waterborne exposure of juvenile Atlantic salmon to thyroxine (T4) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE), singly and also in combination, using quantitative (real-time) PCR method. Our data showed that the gene expression patterns for all five studied were modulated according to exposure condition. Particularly, 18S RNA was found to be an unrealistic housekeeping gene since maximum amplification was reached after only 5 qPCR cycles. The implication of using these so-called housekeeping genes in toxicology will be discussed and some practical solutions to these problems will be suggested. The practical question asked in this study is whether housekeeping genes really keep the house of toxicology?