Application of germline IGH probes in real-time quantitative PCR for the detection of minimal residual disease in acute lymphoblastic leukemia

Minimal residual disease in leukaemia patients

Because of developments in diagnosis of haemopoietic malignant diseases during the past two decades, routine and reliable identification of very low numbers of malignant cells, known as minimal residual disease (MRD), is now possible. Several large-scale studies have shown that monitoring of MRD in haemopoietic malignant disease predicts clinical outcome. In acute lymphoblastic leukaemia, MRD detection is useful for evaluating early response to treatment and consequently for improving stratification, including treatment reduction. In acute promyelocytic leukaemia and chronic myeloid leukaemia, MRD information at specific time points enables effective early treatment intervention. MRD monitoring is also possible in other leukaemia subtypes, but in these disorders the clinical value of MRD detection is not yet known.

Molecular discrimination between relapsed and secondary acute lymphoblastic leukemia: proposal for an easy strategy

BACKGROUND

Discrimination between late relapse of acute lymphoblastic leukemia (ALL) and secondary ALL might be clinically important, because the former might still respond favorably to chemotherapy and/or bone marrow transplantation, whereas secondary ALL is associated with poor prognosis.

PROCEDURE

We present a pre-B-ALL patient in whom disease recurred 2 years after completion of treatment. Differences in cytomorphology and immunophenotyping raised a suspicion of secondary ALL. We performed detailed molecular studies of immunoglobulin and T-cell receptor genes for discrimination between relapsed and secondary ALL.

RESULTS

Southern blot analysis showed an oligoclonal immunoglobulin heavy chain (IGH) gene configuration at diagnosis and a monoclonal configuration at relapse. The size of one of the rearranged bands at relapse was identical to one of the faint rearranged bands at diagnosis. However, heteroduplex PCR analysis demonstrated that none of the clonal IGH gene rearrangements at diagnosis and at relapse was fully identical. Sequencing of several clonal PCR products revealed an identical DH6-13<-->JH6b junction shared by two different rearrangements at diagnosis and one rearrangement at relapse, thereby proving the clonal relationship between diagnosis and late relapse in this patient.

CONCLUSIONS

We propose a stepwise molecular approach for discrimination between relapsed and secondary ALL based on the rapid and cheap heteroduplex PCR technique, including mixing of clonal (homoduplex) PCR products identified at diagnosis and at relapse. Direct sequencing and comparative sequence analysis of IGH gene rearrangements at diagnosis and at relapse should be regarded as an ultimate standard, but can be limited to the rare cases, in which no identical clonal PCR products at diagnosis and at relapse were detected with the mixed heteroduplex PCR analyses.

Combining nucleic acid amplification and detection

Major recent advances in molecular amplification in the past year were initial validation of two new amplification technologies (rolling circle amplification and Invader), a significant increase in the number of molecular diagnostic assays, achievement of amplification directly on microarrays (by strand displacement amplification and rolling circle amplification), and description of two new read-out probes (Scorpions and nanoparticles).

Minimal residual disease studies are beneficial in the follow-up of TEL/AML1 patients with B-precursor acute lymphoblastic leukaemia

The t(12;21)(p13;q22) translocation has been identified as the most common chromosomal abnormality in childhood acute lymphoblastic leukaemia (ALL). Initially, several investigators reported an excellent prognosis in paediatric leukaemias with this translocation, but other studies showed a 20% incidence in relapsed ALL. We performed an extensive analysis of 90 ALL patients. In 17 (19%) cases a TEL/AML1 fusion was found. However, this group was not representative as it included a high number of relapsed patients compared with the normal incidence in B-precursor ALL [54 in continuous complete remission (CCR) and 36 relapsed patients] and only a slightly better prognosis for TEL/AML1-positive patients was found (not significant) (four relapses in 17 TEL/AML1-positive patients vs. 32 relapses in 73 TEL/AML1-negative patients). Comparison of known prognostic factors (age, sex, ploidy, white blood cell count and immunophenotype) between relapsed TEL/AML1-positive and TEL/AML1-positive patients in CCR did not reveal differences, except that the white blood cell count was significantly higher in the relapsed group (P = 0.001). Time between diagnosis and relapse was not different for the relapsed TEL/AML1-positive group vs. the relapsed TEL/AML1-negative group. In 11 TEL/AML1-positive patients, the minimal residual disease (MRD) level at the end of induction therapy was quantified in a limiting dilution assay using IGH or TCRD junctional regions as polymerase chain reaction (PCR) targets. In all four relapsed patients, the level of MRD at the end of induction therapy was high (range 0.24-1.2%), whereas in all seven CCR patients, the MRD level was extremely low (0.02 to < 0.001%). In agreement with previous studies in which MRD levels at the end of induction therapy were found to be the strongest risk factor independent of other risk factors, in the present study we show that the MRD level remains a risk factor independent of the presence of a TEL/AML1 fusion gene.