BCR-ABL-positive progenitors in chronic myeloid leukaemia patients in complete cytogenetic remission after treatment with interferon-a A NDREAS R EITER, 1,2 S TEPHEN B. MARLEY, 1 A NDREAS H OCHHAUS, 2 J ASTINDER S OHAL, 1 P IA R AANANI, 1 RU ¨ DIGER H EHLMANN, 2 MYRTLE Y. G ORDON, 1 J OHN M. G OLDMAN 1 AND N ICHOLAS C. P. C ROSS 1 1 Department of Haematology, Imperial College School of Medicine, Hammersmith Hospital, London, U.K., and 2 Medizinische Klinik, Klinikum Mannheim der Universita ¨t Heidelberg, Mannheim, Germany Received 24 April 1998; accepted for publication 18 June 1998 Summary. To determine the source of residual disease detected in patients with chronic myeloid leukaemia (CML) in complete cytogenetic remission (n ¼ 8) after treatment with interferon-a (IFN-a), we have tested CFU-GM colonies grown from bone marrow mononuclear cells or from plastic- adherent (PD) cells for BCR-ABL mRNA using a nested multiplex RT-PCR. We compared our results with those obtained by analysis of colonies from newly diagnosed patients (n ¼ 4) and patients achieving no cytogenetic response (n ¼ 1) or incomplete cytogenetic response to treatment with IFN-a (n ¼ 5). A total of 1239 informative colonies were analysed. A small proportion of BCR-ABL- positive colonies was detected in all eight patients in complete cytogenetic remission, suggesting the persistence of leukaemia that could potentially lead to relapse. The overall proportion of BCR-ABL-positive colonies in patients achieving a cytogenetic response to IFN-a correlated with the levels of BCR-ABL transcripts detected in the peripheral blood by competitive RT-PCR (P ¼ 0·004). We conclude that residual disease detected in the peripheral blood of complete cytogenetic responders to IFN-a is at least partly derived from clonogenic myeloid cells. It is probable that the leukaemia clone in CML is only very rarely or never entirely eradicated by treatment with IFN-a. Keywords: CML, BCR-ABL, RT-PCR, IFN. Chronic myeloid leukaemia (CML) is characterized by the reciprocal translocation t(9;22)(q34;q11), which results in the formation of the Philadelphia (Ph) chromosome and the chimaeric BCR-ABL fusion gene. BCR-ABL transcripts can be detected by reverse transcription polymerase chain reaction (RT-PCR), and this highly sensitive method can be used for the detection of minimal residual disease (MRD) in CML patients after treatment. Interferon-a (IFN-a) is now the standard initial treatment for CML patients not eligible for allogeneic bone marrow transplantation (BMT). The achievement of partial or complete cytogenetic remission (<35% Ph-positive meta- phases) on IFN-a therapy is associated with improved survival (The Italian Cooperative Study Group on CML, 1994; Allan et al, 1995; Kantarjian et al, 1995; Guilhot et al, 1997). However, in contrast to patients after BMT, residual BCR-ABL-positive cells remain detectable by RT-PCR in the great majority of IFN-a-treated patients even after achieve- ment of complete cytogenetic remission (Hochhaus et al, 1996). It is currently unclear which cellular compartment contributes to the detectable residual disease in these patients. The aim of our study was to determine if the residual disease detected in unfractioned peripheral blood leucocytes in complete cytogenetic responders to IFN-a is attributable to BCR-ABL-positive clonogenic myeloid precursors rather than, for example, BCR-ABL-positive lymphoid cells or more mature myeloid cells. The presence or absence of BCR-ABL mRNA was determined in granulocyte-macro- phage colony-forming cells (CFU-GM) grown directly from bone marrow mononuclear cells (MNC) from patients treated with IFN-a. To minimize the possibility of false negative results due to the amplification of small numbers of target molecules, we utilized a new nested multiplex RT-PCR that simultaneously amplifies BCR-ABL and normal BCR transcripts as a control for adequate RNA/cDNA quality. To British Journal of Haematology , 1998, 102, 1271–1278 1271  1998 Blackwell Science Ltd Correspondence: Dr N. C. P. Cross, Department of Haematology, Imperial College School of Medicine, Hammersmith Hospital, DuCane Road, London W12 0NN.