Measurement of minimal residual disease (MRD) in B-cell acute lymphoblastic leukemia (B-ALL) is integrated in treatment protocols due to it prognostic significance. In Ph+ B-ALL BCR-ABL1 transcripts are the preferential molecular targets for MRD assessment. However it is known that BCR-ABL1 may persist not only in leukemic cells but in myeloid and other cells. In this case MRD determined by other methods may clarify the real status of the disease.
To compare results of MRD study in Ph+ B-ALL patients performed by reverse transcription PCR (RT-PCR, BCR-ABL1) and multicolor flow cytometry (MFC).
The main principle of Ph + RALL-2012 is non-intensive treatment but non-interruptive with simultaneous administration of tyrosine kinase inhibitors. Study of MRD included 12 patients (m = 5, f = 7, median age = 39). MRD was determined after 36, 70, 133 and 190 days of protocol. BCR-ABL1 transcripts were evaluated by RT-PCR with sensitivity of at least 0.01%. MRD by MFC was analyzed by 6-color flow cytometry with minimal sensitivity of 0.01%.
On +36 day MRD analysis was performed for 8 patients. All MFC-studies were positive and MRD varied from 0.0014% to 17.5% (median = 0.22%). 7 patients had detectable BCR-ABL1 transcript (0.84% to 72.34%) and 1 patient with MFC-MRD 0.0014% was RT-PCR negative.
On +70 day MRD was determined in 10 pts. Only 1 pts was MRD- negative by both methods. In 9 pts PCR-MRD was found (0.007-29.63%), but only in 5 of them MFC-MRD was positive too (0.005- 5.88%).
On +133 day MRD was measured in 10 patients. Two pts were PCR and MFC -negative and 8 pts had BCR-ABL1 varied from 0.008 to 0.44% but 6 of them were MFC-negative.
On +190 day MRD was detected in 8 pts. MRD by PCR was found in 7 of them and MFC-MRD was positive in 2 samples. Results of two methods mismatched in 5 patients.
The parallel study of MRD by MFC and RT-PCR was performed in 37 samples. The concordance of the results was in 20 cases (16 double-positive and 4 double-negative, 54%), and 16 (43%) cases were PCR+MFC− and 1(2.7%) case was PCR-MFC+. The R2 was 0.99, and the proportion of tumor cells determined by RT-PCR was 4 times greater than in MFC data (fig. 1).
On 36 day, the greatest concordance of MRD results was observed, probably due to low clearance of tumor cells. However, on 70 and 133 days, negative results were often observed by MFC while BCR-ABL1 transcripts persisted. This could happen due to persistence of BCL-ABL1 transcript in non-leukemic cells or due to higher sensitivity of RT-PCR. This conclusion may improve the predictive ability of the MRD tests in the prospective studies.