Minimal residual disease analysis in children with t ( 12 ; 21 )-positive acute lymphoblastic leukemia : comparison of Ig / TCR rearrangements

Minimal residual disease (MRD) has evolved as an independent prognostic parameter in childhood acute lymphoblastic leukemia (ALL) and is currently used in clinical trials for treatment stratification. Development in technologies has made it possible to quantify the residual tumor load accurately. Immunoglobulin/Tcell receptor (Ig/TCR) gene rearrangements are the most commonly used DNA targets for the molecular monitoring of treatment response because of their high prevalence in lymphoid leukemia. Although they provide a fingerprint for the leukemia clone these rearrangements are not causally related to the leukemia development and can undergo clonal selection and/or evolution processes leading to potential loss of the marker. In contrast, genomic fusion sites generated by chromosomal translocations are not only patient-specific but also appear to be identical in the translocation target cell and the evolving progeny. The ETV6-RUNX1 fusion, also known as TEL-AML1, which results from the t(12;21)(p13;q22) chromosomal translocation, is an early or even initiating event in leukemia development. It is, however, insufficient to produce a clinically overt leukemia. Secondary events, as proposed for the deletion of the non-rearranged ETV6 gene, are necessary for the manifestation of the disease. Differences in the boundaries of these ETV6 deletions as well as changes of the Ig/TCR rearrangement between initial and relapsed leukemias support the hypothesis that at least some of the relapses of this leukemia subtype derive from the persistent ETV6-RUNX1 pre-leukemic master clone by independent secondary events. The detection of diverse Ig/TCR rearrangements at relapse of ETV6-RUNX1 positive ALL provided the unique opportunity of tracking the different subclones, i.e. the dominant clone at initial diagnosis and the relapse clone, by their unique clonotypic rearrangements in two children with a late recurrence of leukemia. We demonstrated that the dominant clone from initial diagnosis responded rapidly to chemotherapy while the relapsed clone that was present as a small subclone at initial manifestation reacted more slowly. Consequently, we wondered whether a fusion gene positive clone, which differs in its Ig/TCR rearrangements from the dominant clone at diagnosis, would be detectable in children with ETV6-RUNX1 positive ALL. We thus evaluated response to treatment in 12 children using real-time quantitative polymerase chain reaction (RQ-PCR) of the genomic ETV6-RUNX1 specific sequence as well as of clonotypic Ig/TCR rearrangements.