Problems related to the interpretation of autoradiographic data on gene expression using common constitutive transcripts as controls.

The 28S rRNA, a ribosomal RNA, and the ACTB and GAPD mRNAs, coding respectively for beta-actin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), are frequently presented as controls of modulated gene expression. These transcripts were quantified by replicate slot-blot autoradiography and image analysis in mammary epithelial cells and fibroblasts from breast tissues. Each cell-type group comprised strains with different pathological backgrounds, growth rates, antigenic phenotypes and culture histories. The effects of a differentiating agent (cholera toxin) and/or a tumor promoter (12-O-tetradecanoyl-phorbol-13-acetate) were also examined. Despite the impression that visual examination of autoradiographs might create, image analysis suggests that 28S rRNA, ACTB and GAPD are substantially and independently influenced by the above biological factors and by the drugs. Therefore, these transcripts represent specifically regulated cellular activities and may not be taken as alternative indicators of the overall transcription rate or of the amount of material being examined. Instead, such nonspecific variation may be accurately measured and removed from quantitative data using a principal component function. A methodology that allows comparison of expression (or amplification) patterns between genes, between experiments or, even, between laboratories is presented with an example of quantification of transcripts related to cell-growth, differentiation, signaling and cancer.