Circulating clonal cells in multiple myeloma do not express CD34 mRNA, as measured by single‐cell and real‐time RT‐PCR assays

The peripheral blood (PB) mononuclear cells in patients with multiple myeloma (MM) have been reported to include CD34-expressing cells that are clonally related to the myeloma cells. To determine whether there were elevated levels of CD34 mRNA or whether CD34+ cells in the PB include myeloma-related cells, we developed a quantitative real-time and a competitive CD34 RT-PCR assay working on single flow-sorted cells. Myeloma-specific cells were detected with allele-specific oligonucleotides (ASO) IgH PCR. PBSC products and mononuclear cell fractions in blood from normal donors, untreated and treated myeloma patients were analysed. When measured by flow cytometry, the numbers of CD34+/CD19+ cells were consistently < 0.1% of the mononuclear cells. In addition, no significant difference was found in the levels of CD34 mRNA between normal subjects and untreated MM patients (P = 0.935). In the treated group of MM patients the CD34 mRNA levels were significantly reduced (P = 0.052) because of the stem cell toxicity of melphalan. Further, no cells clonally related to the MM clone were found within the CD34 compartment, defined by a sort-gate that included all cells expressing CD34 mRNA, as no cells outside the used CD34 sort-gate had a detectable level of CD34 mRNA. We conclude that in myeloma patients, the myeloma clone is not found within the CD34 compartment.