Comparison of genomic DNA and cDNA for detection of residual disease after treatment of chronic myeloid leukemia with allogeneic bone marrow transplantation

To test whether patients in remission after allogeneic bone marrow transplantation (BMT) possess a pool of chronic myeloid leukemia (CML) cells that do not express BCR-ABL mRNA, we have compared the results and sensitivity of amplification of BCR-ABL from genomic DNA with conventional reverse transcription-polymerase chain reaction (RT-PCR). Bubble PCR was used to amplify the genomic BCR-ABL translocation breakpoints from chronic-phase DNA of 10 patients with CML who subsequently underwent BMT. After cloning and sequencing of the amplification products, patient-specific ABL primers were synthesized and tested for both specificity and sensitivity in nested or heminested combinations with a variety of primers derived from the major breakpoint cluster region of the BCR gene. In all cases, combinations of primers were selected that enabled the detection of chronic-phase DNA from a specific patient at up to a 10(5)x dilution into DNA from a normal individual. Patterns of residual disease obtained by serial RT- PCR and DNA-PCR analyses of blood and bone marrow samples obtained after BMT were similar for most patients, including one treated for relapse by infusion of donor leukocytes. Of the 24 samples for direct comparison of RT-PCR and DNA-PCR, results were concordant in 19 (79%) cases. Five results were discordant. In two instances, RT-PCR was positive, while PCR from genomic DNA was negative; this discrepancy might have arisen due to the slightly greater sensitivity of RT-PCR compared with DNA-PCR. In three samples from three patients, two of whom had been transplanted in the accelerated phase, PCR from genomic DNA was positive while RT-PCR was negative; this could mean that some CML cells in these samples had a reduced or absent capacity to express BCR-ABL mRNA post-transplant. Of these three patients, one subsequently relapsed; and two are in remission at 21 and 24 months after the discordant result. Thus, the finding of a single DNA-PCR- positive, RT- PCR-negative results does not necessarily predict relapse. Because the great majority of samples (79%) gave concordant results with the two assays, we believe that patients in remission do not generally harbor a substantial pool of CML cells that do not express BCR-ABL mRNA.