Quantitative polymerase chain reaction analysis of mdr1 mRNA in multiple myeloma cell lines and clinical specimens.

We have designed a new polymerase chain reaction (PCR) protocol for the quantitation of mdr1 mRNA in cell lines and clinical specimens. This protocol uses an in vitro-generated RNA molecule as an internal standard. This synthetic RNA contains the same mdr1 primer sequences as the cellular mRNA, but yields a different-sized PCR product after amplification. Since a single primer set is used in quantitation, differences in primer efficiency are not a concern. We have used this assay to measure mdr1 expression in a multiple myeloma cell line, 8226/S, its drug resistant variants 8226/dox6 and 8226/dox40, and tumor samples from 10 patients with B-cell malignancies (9 multiple myeloma, 1 chronic lymphocytic leukemia). 8226/S does not express mdr1 mRNA. 8226/dox6 is 10-fold resistant to doxorubicin, and expresses 32 mdr1 mRNA/10 pg cellular RNA. 8226/dox40 is 140-fold resistant to doxorubicin, and expresses 890 mdr1 mRNA/10 pg cellular RNA. Seven of the 10 patients had levels of mdr1 mRNA expression below that seen in the multidrug-resistant, human multiple myeloma cell line, 8226/dox6. Three patients had levels of mdr1 expression comparable to those seen in 8226/dox6. No patient had levels of mdr1 expression close to that seen in 8226/dox40. Sample RNA integrity is assured by PCR analysis of a different, ubiquitous, cell cycle independent, histone variant, H3.3. This assay will be useful for studying low level mdr1 expression in cell lines and clinical specimens.