We evaluated minimal residual disease (MRD) in 91 children with acute lymphoblastic leukemia (ALL) by PCR amplification of clonal rearrangements, immunoglobulin (IgH; VDJ rearrangement, CDR3 region) and T-cell receptor (TCRdelta). Sequential monitoring of MRD was performed at different time points during and after chemotherapy and was correlated to patient outcome. In total, 792 bone marrow samples were assessed for MRD at the end of induction, and during and after treatment completion. MRD positivity at the end of induction was detected in 12% of patients and was associated with high incidence of relapse, 54.55% (p = 0.0002), at 5 years. On the other hand, 88% of patients were MRD-negative at the end of induction and the relapse rate at 5 years was very low, 5%. The frequency of MRD decreased to 16% in the first 6 months of chemotherapy; however, the incidence of relapse in MRD-positive patients remained high, 42.8%. After treatment completion (24-36 months from diagnosis), 32% patients were MRD-positive and the relapse rate was 36.5% (p = 0.0009). Our results indicated that monitoring of MRD constituted an essential prognostic marker, and detection of MRD particularly at the end of induction and after treatment completion was strongly predictive for patient outcome.

We analyzed the CDR3 region of 80 children with B-cell acute lymphoblastic leukemia (B-ALL) using the ImMunoGeneTics Information System and JOINSOLVER. In total, 108 IGH@ rearrangements were analyzed. Most of them (75.3%) were non-productive. IGHV@ segments proximal to IGHD-IGHJ@ were preferentially rearranged (45.3%). Increased utilization of IGHV3 segments IGHV3-13 (11.3%) and IGHV3-15 (9.3%), IGHD3 (30.5%), and IGHJ4 (34%) was noted. In pro-B ALL more frequent were IGHV3-11 (33.3%) and IGHV6-1 (33.3%), IGHD2-21 (50%), IGHJ4 (50%), and IGHJ6 (50%) segments. Shorter CDR3 length was observed in IGHV@6, IGHD7, and IGHJ1 segments, whereas increased CDR3 length was related to IGHV3, IGHD2, and IGHJ4 segments. Increased risk of relapse was found in patients with productive sequences. Specifically, the relapse-free survival rate at 5 years in patients with productive sequences at diagnosis was 75% (standard error [SE] ±9%), whereas in patients with non-productive sequences it was 97% (SE ±1.92%) (p-value =0.0264). Monoclonality and oligoclonality were identified in 81.2% and 18.75% cases at diagnosis, respectively. Sequence analysis revealed IGHV@ to IGHDJ joining only in 6.6% cases with oligoclonality. The majority (75%) of relapsed patients had monoclonal IGH@ rearrangements. The preferential utilization of IGHV@ segments proximal to IGHDJ depended on their location on the IGHV@ locus. Molecular mechanisms occurring during IGH@ rearrangement might play an essential role in childhood ALL prognosis. In our study, the productivity of the rearranged sequences at diagnosis proved to be a significant prognostic factor.

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