MLL Translocation in a Multipotent Progenitor Causing Acute Lymphoblastic Leukaemia - Two-Step Model of the Disease.

Leukaemias with MLL gene rearrangement are usually considered prognostically unfavourable and the clinical symptoms typically follow the translocation formation rapidly. MLL rearrangement is thus thought to be a major hit in leukaemogenesis that is either sufficient to cause the disease or it is a very strong and rapid inducer of the subsequent hit(s) required for the malignant transformation. We report an unusual presentation of secondary acute lymphoblastic leukaemia (sALL) with MLL rearrangement. Our patient was diagnosed originally with acute myeloid leukaemia (AML-M3) characterised by PML/RARα fusion and an internal tandem duplication of FLT3 (FLT3/ITD). After 30 months of complete remission of AML, she developed sALL with MLL/FOXO3A fusion gene. Bone marrow (BM) samples taken during AML therapy were analysed for the presence of these aberrations. Both the PML/RARα fusion and FLT3/ITD disappeared shortly after AML onset and did not reappear. However, FISH and quantitative RT-PCR showed the presence of the MLL/FOXO3A fusion 20 months before the diagnosis of sALL, present in 10–90% of BM cells. Morphological examination showed no blast infiltration of the BM at this time. Experiments combining FISH and morphology confirmed the presence of an MLL rearrangement in myeloid as well as lymphoid cells, indicating that the fusion arose in a multipotent progenitor. In order to identify potential secondary genetic events precipitating sALL in this patient, we used Affymetrix 50K single nucleotide polymorphism (SNP) array analysis on DNA from the diagnostic sALL sample versus the "preleukaemic" (remission AML) sample taken 16 months before. This analysis revealed a 10 Mb amplification on 19q13.32 in the sALL sample, not present in the preleukaemic sample: this was confirmed by FISH with a BAC from the amplified region. A difference between the pre-leukaemic and leukaemic cells is also demonstrated by the incomplete rearrangement of IgH gene (DH1/JH) present only at the diagnosis of sALL. There are about 450 genes in the amplified region on 19q and several of them might be involved in deregulation of the preleukaemic cell if overrepresented (e.g. FLT3 ligand, interleukin 11, Ras interacting protein 1, Stem cell growth factor, Aurora C). The long latency period prior to the onset of the secondary leukaemia in our case resembles the mouse model of MLL/FOXO3A. However, in contrast to the animal model and also to the previous reports of MLL/FOXO3A patients (2 cases described so far, both secondary AMLs after Hodgkin’s disease), our child developed leukaemia from the lymphoid lineage. Taken together, these results indicate that the MLL/FOXO3A fusion alone is not sufficient to cause leukaemia and that second hit is required to the onset of the disease. A responsible gene is possibly located on the telomeric part of the 19q.