An improved method for absolute quantification of mRNA using multiplex polymerase chain reaction: determination of renin and angiotensinogen mRNA levels in various tissues.

We have developed a multiplex, competitive, reverse-transcriptase polymerase chain reaction (RT-PCR) method which measures absolute levels of renin, angiotensinogen, and the housekeeping transcript elongation factor-1 alpha (EF-1 alpha) mRNA. Sample RNA was simultaneously titrated with serial dilutions of renin, angiotensinogen, and EF-1 alpha competitor RNAs which flanked the endogenous concentrations of target transcripts. The samples were coreverse transcribed in the presence of random primers and resulting first-strand cDNA was coamplified for 10-15 cycles with [32P]-dCTP and primers for renin angiotensinogen, after which EF-1 alpha primers were added. Amplified DNA was separated by electrophoresis on polyacrylamide gel and radioactivity in the bands was quantified by direct radioanalytical scanning. Three conditions were necessary to obtain absolute quantification of renin and angiotensinogen mRNA levels: (a) exogenous competitor RNA was used to control for tube-to-tube variability in the efficiencies of reverse transcription and amplification; (b) Sample RNA was titrated with flanking concentrations of competitor RNA to correct for intraassay differences in the efficiency of amplification due to concentration differences between competitor and target templates; and (c) a housekeeping transcript EF-1 alpha was used to control for tube-to-tube differences in RNA loading and/or degradation. We show that the multiplex RT-PCR method is precise and accurate over approximately three logs of transcript concentration and sensitive to less than 5 and 0.5 fg for renin and angiotensinogen mRNA, respectively. This method will be useful for absolute quantification of target mRNAs, especially when the amount of sample RNA is limited or unknown and/or the gene expression is low.