Quantitative RT-PCR luminometric hybridization assay with an RNA internal standard for cytokeratin-19 mRNA in peripheral blood of patients with breast cancer

Objectives: To develop a highly sensitive quantitative RT-PCR hybridization assay for the determination of CK-19 mRNA in peripheral blood of patients with breast cancer. Patients and methods: Quantification of CK-19 mRNA was based on the coamplification of CK-19 mRNA with a recombinant CK-19 RNA internal standard (CK-19 RNA-IS) through RT-PCR. The biotinylated amplification products were immobilized on steptavidin coated wells, hybridized with digoxigenin labeled probes and determined through an antidigoxigenin antibody conjugated to alkaline phosphatase by luminometric detection. The developed luminometric hybridization assay was validated with samples containing total RNA of known amounts from CK-19 expressing cells (MCF-7) in the presence of 1 μg total RNA isolated from peripheral blood mononuclear cells (PBMC) of healthy controls and a constant amount of CK-19 RNA-IS. The method was applied for the quantitative determination of CK-19 mRNA in the peripheral blood of 26 healthy volunteers, 14 patients with stage IV breast cancer and 37 patients with stage I/II breast cancer before chemotherapy. Results: Luminescence ratios for CK-19 mRNA and CK-19 RNA-IS were linearly related to the number of MCF-7 cells within the range of 1 to 2000 cells. The overall reproducibility of the assay (between-run) varied between 8.9% and 13.4%. The method can clearly detect CK-19 mRNA from 1 MCF-7 cell in the presence of 106 normal PBMC and is highly specific as none of the 26 healthy controls tested had detectable CK-19 mRNA levels, while 10 out of 14 (71.4%) and 9 out of 37 (24.3%) patients with stage IV and stage I/II breast cancer, respectively, were tested positive. Conclusion: The developed quantitative RT-PCR hybridization assay for CK-19 is reproducible, highly sensitive and specific, and can be used for a large-scale prospective evaluation of clinical samples.