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The level of minimal residual disease (MRD) early in treatment of acute lymphoblastic leukemia (ALL) strongly predicts the risk of marrow relapse. As a variety of methods of varying complexity have been separately used for detecting and quantifying MRD, we compared the prognostic utility of three methods measurement of blast percentage on day 14 of treatment, detection of monoclonality on day 14 or day 35, and measurement of MRD by PCR-based limiting dilution analysis on day 14 or day 35. The study group comprised 38 children aged 1-15 with Philadelphia-negative B-lineage ALL who were uniformly treated and followed until relapse or for a minimum of 5 years. We also studied some of the technical factors which influence the ability to detect MRD. Measurement of blast percentage on day 14 by an expert morphologist, detection of monoclonality on day 35, and PCR-based measurement of MRD levels on days 14 and 35 all showed significant ability to divide patients into prognostic groups. Measurement of blast percentage on day 14 by routine morphology or detection of monoclonality on day 14 were not useful. The quality of DNA samples varied greatly, as determined by amplifiability in the PCR. However, virtually all amplifiable leukemic targets in a sample were detectable which suggests that the level of detection achieved by limiting dilution analysis is essentially determined by the amount of DNA which it is practicable to study. We conclude that quantification of MRD at the end of induction provides the full range of prognostic information for marrow relapse but is complex; detection of monoclonality on day 35 is simple and has good positive predictive value; and quantification of MRD on day 14 merits further study. PCR-based methods for measurement of MRD levels should incorporate a correction for variation in DNA amplifiability.

The Philadelphia translocation is associated with a poor prognosis in adults and children with acute lymphoblastic leukemia, even though the majority of patients achieve remission. To test the hypothesis that the translocation leads to drug resistance in vivo, we studied 61 children and 20 adults with acute lymphoblastic leukemia and used the level of minimal residual disease at the end of induction as the measure of drug resistance in vivo. In children the presence of the translocation was associated with a significant increase in residual disease, indicating higher drug resistance in vivo; five of seven Philadelphia-positive children but only five of 54 Philadelphia-negative children had a minimal residual disease level >10(-3), a level which is associated with a high risk of relapse in childhood acute lymphoblastic leukemia of standard risk. By contrast, in adults, residual disease and hence drug resistance was already higher than in children, and the presence of the Philadelphia translocation in seven patients had no obvious additional effect. We conclude that the Philadelphia chromosome may increase resistance to drugs in vivo in children, but not detectably in adults.

Sensitive quantification of minimal residual disease (MRD) using the polymerase chain reaction (PCR) is strongly predictive of outcome in childhood acute lymphoblastic leukemia (ALL), with MRD levels at the end of induction therapy of >10(-3) predicting a poor outcome. Methods for sensitive quantification are, however, complicated and time-consuming. Detection by PCR of monoclonal immunoglobulin heavy chain (IgH) and T cell receptor (TCR) gene rearrangements is simple and can be used in routine laboratories but is non-quantitative and of lower but uncertain sensitivity. The aim of this study was to determine the value of detection of monoclonality in identification of different levels of MRD. We looked for monoclonality in 64 bone marrow aspirates which had been obtained from 31 patients with B lineage ALL at various times during induction therapy and for which levels of MRD had been determined by limiting dilution analysis using patient-specific PCR primers. Detection of monoclonality identified levels of MRD of > or =10(-3) during induction with a sensitivity of 78% and a specificity of 93%. The positive and negative predictive values were 0.86 and 0.88, respectively. The sensitivity of detection of a monoclonal IgH rearrangement was greater than that for the TCRgamma locus during induction as an IgH rearrangement was detected more often than a TCRgamma rearrangement in patients who had both IgH and TCRgamma rearrangement at diagnosis. Detection of monoclonality is therefore a simple and quick test applicable to the majority of patients with ALL and it may be useful in identifying high-risk patients at the end of induction and in identifying relapsing patients later during therapy.

Techniques based on the polymerase chain reaction (PCR) to detect rearrangement of the immunoglobulin or T-cell receptor genes can detect residual disease in leukemia and hence have the potential to improve prognosis and treatment. Such techniques may involve either detection of monoclonality, which is simple and quick but has limited sensitivity, or specific detection of the leukaemic clone, which is complex and time-consuming but has high sensitivity. The PCR was used to detect monoclonal rearrangements of the immunoglobulin heavy chain and/or T-cell receptor gamma chain genes in archival marrow specimens from 185 children with acute lymphoblastic leukemia who achieved remission during two consecutive Australasian trials of treatment. A monoclonal rearrangement was detected at diagnosis in 152 (84%) patients and in these patients detection of the same rearrangement in the remission marrow at the end of induction therapy was highly significantly correlated with outcome. There were nine patients in whom polymerase chain reaction showed only the monoclonal rearrangement and eight (89%) relapsed; there were 26 patients in whom PCR showed the leukemic monoclonal rearrangement as well as polyclonal rearrangements from normal lymphocytes and 12 (46%) relapsed; and there were 117 patients in whom only polyclonal rearrangements could be detected and only 29 (25%) relapsed. In patients who relapsed, remissions were shorter in those patients in whom the leukemic rearrangements had been detected in the remission marrow. Treatment in the later trial was more intensive than in the earlier trial, the results were better and the PCR detected the leukemic rearrangement in the remission marrow in significantly fewer patients. We conclude that detection by PCR of the monoclonal gene rearrangement of the leukemic clone in remission marrow indicates that numerous leukemic cells have survived induction therapy and is a good predictor of relapse. However, due to limited sensitivity of the test, failure to detect the leukemic clone by PCR is not a sufficiently good predictor of ultimate cure.