Detection of bcr-abl mRNA in single progenitor colonies from patients with chronic myeloid leukemia by PCR: comparison with cytogenetics and PCR from uncultured cells.

Bone marrow and/or peripheral blood of patients with chronic myeloid leukemia (CML) was investigated by the following three parameters: Ph' chromosome, bcr-abl expression in fresh blood and/or bone marrow, and bcr-abl expression in single hematopoietic progenitor colonies generated from blood and/or bone marrow. Expression of bcr-abl was proven by a reverse "nested primer" polymerase chain reaction (PCR) that is able to detect 1 pg of hybrid mRNA. We performed 108 investigations on 68 patients containing all three parameters: 12 on untreated patients, seven after interferon-alpha (IFN-alpha), seven after low-dose cytosine arabinoside (Ara-C), 22 after cyclic high-dose hydroxyurea (HU), 49 after allogeneic BMT, five before and three after stem cell mobilization, and three after autologous stem cell transplantation (ASCT). In 53 cases (49%), cytogenetics and PCR gave identical results. In 40 cases (37%), PCR from single colonies gave additional information compared to cytogenetics (e.g., mosaic in colonies when all metaphases were positive or negative). Most interesting were the results of one patient after IFN, one patient after ASCT, and 10 patients after BMT (14 investigations = 13%), showing only Ph'-negative mitoses accompanied by a negative nested primer PCR from fresh blood/bone marrow but single bcr-abl-positive progenitor colonies. False-positive results could be widely excluded by repeated insertion of negative controls into the experiments. One explanation for these results could be that CML, progenitors survive in the patient's body by being inactive and not proliferating. These cells express no or very little RNA and bcr-abl is not detectable by reverse PCR. When stimulated ex vivo in a colony assay by external growth factors, cells proliferate and produce detectable amounts of hybrid mRNA. The value of these observations is not clear. A follow-up of the patients will show if such sleeping progenitors can be activated in vivo. Concluding our observations, we can say that in special cases (therapy follow-up, detection of minimal residual disease) it could be useful to perform a PCR analysis of single progenitors in parallel with the routine investigations.