ArticlesPrognostic value of minimal residual disease in acute lymphoblastic leukaemia in childhood
Introduction
Current cytotoxic treatment protocols induce complete remission according to cytomorphological criteria in 95–98% of children with acute lymphoblastic leukaemia (ALL).1, 2, 3 However, 25–30% of patients with childhood ALL relapse, implying that not all leukaemic cells are eradicated. Cytomorphological criteria are not sufficient for adequate assessment of the remission status because the threshold of detection is 1–5% of leukaemic cells. More sensitive techniques are required to determine efficacy of treatment and to improve stratification of treatment protocols.
Three types of techniques allow detection of minimal residual disease (MRD) of 10−3–10−6 (one leukaemic cell in 103–106 cells): flow-cytometric immunophenotyping (detecting aberrant protein expression by the ALL cells); PCR analysis of breakpoint fusion regions of chromosome aberrations; and detection of clone-specific immunoglobulin (Ig) and T-cell receptor (TCR) gene rearrangements by PCR amplification.4, 5, 6, 7 The third technique is the most widely used for MRD studies in patients with ALL.
Rearrangements of Ig and TCR genes result in junctional regions that can be regarded as fingerprint-like clone-specific sequences owing to deletion and random insertion of nucleotides.8 PCR-based MRD detection by clone-specific junctional regions generally reaches sensitivities of 10−4–10−5. For this purpose, oligonucleotide primers are designed at opposite sides of the junctional region. To discriminate between the leukaemia-derived PCR products and PCR products of normal cells with comparable rearrangements, the amplification products are generally hybridised to a patient-specific junctional-region probe.4, 5, 6, 7
Although rearrangements of Ig and TCR genes are clone-specific, they are not directly linked to the oncogenic process. Continuing rearrangements and secondary rearrangements might change the junctional regions, which are identified at diagnosis as PCR targets for MRD studies.9, 10, 11, 12 It seems to be important to monitor patients with ALL with two or more independent Ig/TCR targets to prevent false-negative results during follow-up.7, 11
Several retrospective and small prospective studies with short follow-up indicate that the degree of MRD in ALL in childhood has prognostic value, although the results of these studies are not fully concordant. Absence of residual disease after remission induction is associated with good prognosis.13, 14, 15 However, about half of patients with childhood ALL remain MRD positive after remission induction. Consequently, the degree of MRD has predictive value.13, 14 This is in line with cytomorphological studies after 1 week of treatment, which showed that a slow rate of reduction in numbers of blast cells in blood or bone marrow during induction therapy was related to higher relapse rates. However, these cytomorphological techniques can identify only fewer than 30% of patients who are going into relapse.1, 16
A steady disappearance of MRD during follow-up, as determined by PCR, is associated with a favourable prognosis.17 Persistence of MRD generally leads to clinical relapse.18 Low degrees of MRD after treatment might be associated with late relapse, but absence of MRD at the end of treatment is not sufficient to deem the patient cured.19
Despite these preliminary MRD data, it is still not clear whether and how MRD information can be applied in clinical decision making. We did a large prospective multicentre study to assess the clinical relevance of PCR-based MRD detection at multiple time points during follow-up of 240 children with ALL who were treated according to national protocols of the International BFM (Berlin-Frankfurt-Münster) Study Group (I-BFM-SG) in Austria, Germany, Italy, and Netherlands.
Section snippets
Patients and cell samples
Bone-marrow samples were taken, at diagnosis (March, 1991-May, 1995) and at up to nine times during follow-up, from 625 patients with childhood ALL. Recruitment for the study was done in a limited number of I-BFM-SG centres, because of logistics and medical ethical aspects of the study. All children were treated according to protocols of the Austrian BFM group (protocol ALL-BFM 90), the German BFM Group (protocol ALL-BFM 90), the Associazione Italiana di Ematologia ed Oncologia Pediatrica
PCR targets identified at diagnosis
In 148 (62%) of 240 patients, two PCR targets (128 patients) or three PCR targets (20 patients) were used for MRD detection. The PCR targets concerned 73 IGK—Kde rearrangements in 66 precursor-B-ALL, 126 TCRG gene rearrangements in 83 precursor-B-ALL and in 23 T-ALL, 186 incomplete TCRD gene rearrangements (Vδ2-Dδ3 and Dδ2-Dδ3) in 139 precursor-B-ALL, 21 TCRD gene rearrangements (5 Vδ2-Dδ3, 2 Dδ2-Jδ1, and 14 Vδ-Jδ1) in 17 T-ALL, and 2 TAL1 deletions in 2 T-ALL. In 379 (93%) of 408 of these
Discussion
This large PCR-based study of MRD in childhood ALL unequivocally demonstrates that monitoring patients serially gives clinically relevant insight into the effectiveness of treatment. For instance, during the three initial treatment blocks among SRG and MRG patients, the frequency of MRD-positive bone-marrow samples rapidly halved per treatment block down to about 10%. Maintenance treatment further reduced the percentage of MRD-positive patients to less than 5% at the end of treatment. Not only
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