This study provides a comprehensive analysis of the MLL recombinome in acute leukemia and demonstrates that the establishment of patient-specific chromosomal fusion sites allows the design of specific PCR primers for minimal residual disease analyses for all patients.
Intriguingly, clonally expanded non-leukemic cells expressing MLL-ELL during consolidation therapy were found to be eradicated after allo-HSCT during the monitoring of minimal residual disease in one patient; this indicates that allo-HSCT is efficacious for eliminating pre-leukemic cells resistant to chemotherapy.
The expression of the MLL gene had a higher specificity and sensitivity than WT1 or MRD monitored by FCM for predicting the relapse of the patients with MLL + AL.
A higher specific hazard of relapse was independently associated with postinduction MRD level ≥10(-4) and unfavorable genetic characteristics (ie, MLL gene rearrangement or focal IKZF1 gene deletion in BCP-ALL and no NOTCH1/FBXW7 mutation and/or N/K-RAS mutation and/or PTEN gene alteration in T-cell ALL).
Most sensitive methodology to detect MRD is molecular polymerase chain reaction (PCR) but its applicability is restricted to AML with leukemia-specific molecular targets (e.g.AML1-ETO, CBFB-MYH11, MLL, FLT-3).
From a clinical practice standpoint, this case illustrates the importance of detection of MLL rearrangement due to its prognostic implication and the effectiveness of flow cytometry immunophenotyping in diagnosing MPAL and monitoring minimal residual disease.
Furthermore, the genomic fusion site in MLL rearrangements represent a unique and reliable molecular marker that allows the tracing of minimal residual disease (MRD) in these patients before, during, and after therapy.
The precise characterization of the MLL-AF9 transcript is important to carry out the minimal residual disease analysis during the follow-up of the patients.
Molecular analysis of the 11q23/MLL rearrangement was used to evaluate minimal residual disease, which became undetectable in repetitive FISH analyses.
MRD was evaluated by real-time quantitative polymerase chain reaction (RQ-PCR) using probes derived from fusion chimeric genes (BCR/ABL and MLL/AF4) (n=22) or rearrangements of the T-cell receptor or immunoglobulin genes (n=21).
Levels of minimal residual disease (MRD) at the end of induction appeared to be high in infants with ALL compared with older children, and although the number of infant cases studied was small, there were no differences in MRD levels after induction therapy in infant ALL with or without MLL gene rearrangements (P = 0.41) and quantitative MRD assessment at the early time points may not be predictive of outcome.
A partial tandem duplication within the MLL-gene (MLL-PTD) can be found in 8% of all patients with karyotypically normal acute myeloid leukemia (AML), a group in which polymerase chain reaction-(PCR) based minimal residual disease analysis has not, so far, been possible.
Reverse transcriptase-polymerase chain reaction (RT-PCR) used to detect MLL/LTG4 chimeric mRNA showed no minimal residual disease (MRD) in the graft or bone marrow at the transplantation.