Sources of error in measurement of minimal residual disease in childhood acute lymphoblastic leukemia

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Introduction The level of minimal residual disease (MRD) in marrow predicts outcome and guides treatment in childhood acute lymphoblastic leukemia (ALL) but accurate prediction depends on accurate measurement. Methods Forty-one children with ALL were studied at the end of induction. Two samples were obtained from each iliac spine and each sample was assayed twice. Assay, sample and side-to-side variation were quantified by analysis of variance and presumptively incorrect decisions related to high-risk disease were determined using the result from each MRD assay, the mean MRD in the patient as the measure of the true value, and each of 3 different MRD cut-off levels which have been used for making decisions on treatment. Results Variation between assays, samples and sides each differed significantly from zero and the overall standard deviation for a single MRD estimation was 0.60 logs. Multifocal residual disease seemed to be at least partly responsible for the variation between samples. Decision errors occurred at a frequency of 13–14% when the mean patient MRD was between 10−2 and 10−5. Decision errors were observed only for an MRD result within 1 log of the cut-off value used for assessing high risk. Depending on the cut-off used, 31–40% of MRD results were within 1 log of the cut-off value and 21–16% of such results would have resulted in a decision error. Conclusion When the result obtained for the level of MRD is within 1 log of the cut-off value used for making decisions, variation in the assay and/or sampling may result in a misleading assessment of the true level of marrow MRD. This may lead to an incorrect decision on treatment

Inhibition of the PCR by genomic DNA.

AimsqPCR, is widely used for quantifying minimal residual disease (MRD) and is conventionally performed according to guidelines proposed by the EuroMRD consortium. However it often fails when quantifying MRD levels below 10-4. By contrast, HAT-PCR, a recent modification designed to minimise false-positive results, can quantify MRD down to 10-6.MethodsThe factors leading to failure of conventional qPCR to quantify low levels of MRD were studied by analysing PCR reagents, protocol and primers and by testing for inhibition by adding primers to a plasmid amplification system. Complementary primers, ending in either G/C or A/T, were used to determine the effect of the 3' end of a primer.ResultsInhibition of conventional PCR resulted from interaction of primers with genomic DNA leading to exponential amplification of nonspecific amplicons. It was observed with approximately half of the EuroMRD J primers tested. Inhibition by a primer was significantly related to primer Tm and G/C content and was absent when extension at the 3' end was blocked. Nonspecificity and inhibition were decreased or abolished by increasing the annealing temperature and inhibition was decreased by increasing the concentration of polymerase. Primers terminating with G/C produced significantly more nonspecificity and inhibition than primers terminating with A/T. HAT-PCR produced minimal nonspecificity and no inhibition.ConclusionsInhibition of the PCR may result from the presence of genomic DNA and resultant exponential amplification of nonspecific amplicons. Factors contributing to the phenomenon include suboptimal annealing temperature, suboptimal primer design, and suboptimal polymerase concentration. Optimisation of these factors, as in HAT-PCR, enables sensitive quantification of MRD. PCR assays are increasingly used for sensitive detection of other rare targets against a background of genomic DNA and such assays may benefit from similar improvement in PCR design

Sensitive detection and quantification of minimal residual disease in chronic myeloid leukaemia using nested quantitative PCR for BCR-ABL DNA

Increasing numbers of patients with chronic myeloid leukaemia (CML) treated with tyrosine kinase inhibitors achieve undetectable levels of BCR-ABL mRNA using sensitive quantitative real-time reverse transcriptase PCR (RT-qPCR) methods and a method to measure minimal residual disease (MRD) in patients with low levels could be of value. Following isolation and sequencing of the patient-specific BCR-ABL breakpoint, a DNA-based nested qPCR assay was established, and MRD was measured by this method and one-round RT-qPCR in 38 samples from 24 patients with CML. Mixing experiments using patient DNA in normal DNA indicated that DNA qPCR could detect BCR-ABL sequences at a limit of approximately 10⁻⁶. In 22 samples in which MRD was detectable by both methods, comparison of the results of DNA qPCR with the results obtained on the same sample by RT-qPCR showed good correlation. In another 16 samples, BCR-ABL mRNA was not detectable by RT-qPCR. In 8 of the 16 samples, BCR-ABL DNA was detected at levels ranging from 1.1 × 10⁻⁵ up to 2.8 × 10⁻⁴ and in the remaining eight samples BCR-ABL was not detected by either method. In one patient, who had stopped imatinib, an almost 1000-fold rise in MRD, to 5.2 × 10⁻⁴ was observed in sequential samples. Nested DNA qPCR was more sensitive than one-round RT-qPCR and could be used for the monitoring of patients with CML with very low levels of MRD.P. A. Bartley, D. M. Ross, S. Latham, M. H. Martin-Harris, B. Budgen, V. Wilczek, S. Branford, T. P. Hughes, A. A. Morle

Analysis by ANOVA of sources of error in relation to MRD level.

<p>Results are expressed in log₁₀ mode and the SD rather than the variance in S2 ANOVA is shown. All values of SD differed significantly (p<0.01) from 0 except that asterisked.</p

Incorrect decisions grouped by the mean MRD level of the patient.

<p>The percent of assays is the percent of decision errors in the assays from patients with MRD between 10⁻² and 10⁻⁵. The results shown are data from all 41 patients.</p

Percentage of decision errors in relation to difference between the assayed MRD value and the cut-off value used for deciding on intensification of treatment.

<p>There were 957 differences and the percentage of errors is a 45-point moving average. The results shown are data from all 41 patients.</p

Difference between the left and right sides in the mean MRD value for each side.

<p>Each mean was the result of 2 assays on each of 2 samples. The results are for the 29 patients analysed by ANOVA.</p

Analysis of variance in all 29 patients.

<p>Results are expressed in log₁₀ mode.</p