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Editorials
cells that are more vulnerable to chemotherapy because the presence of AF4-MLL was associated with a much better treatment outcome.1
Lastly, expression of AF4-MLL alone was shown to be necessary and sufficient to cause acute leukemia.14 Our group used a low expression retroviral vector backbone (PIDE vector) to express MLL-AF4, AF4-MLL or both in LSK cells purified from C57BL/6 mice. Empty vector or MLL-AF4 alone did not result in the development of leukemia, while AF4-MLL or the expression of both fusion genes resulted in full-blown pro-B-acute lym- phoblastic leukemia and mixed-lineage leukemia. The latency was 9 months and the penetrance was only 35%. However, this could be attributed to a low infection rate with about 1/1,000 cells for the AF4-MLL and 1/10,000 cells for the MLL-AF4 construct because these constructs were oversized for in vitro packaging (11.3 kb for MLL- AF4 and 13.3 kb for AF4-MLL). Therefore, an estimated 200 cells in 200,000 non-tranduced cells were transplant- ed into primary mice, which nevertheless caused a dis- ease outbreak (MLL-AF4 with only 20 cells did not work). It is noteworthy that all "leukemic cells" subsequently tested positive for the transcription and integrity of the appropriate transgenes, while the investigated white blood cells of mice who did not develop leukemia remained negative in reverse transcriptase and genomic polymerase chain reaction experiments. This indicates that the leukemia-negative mice either never received or lost the cells carrying the corresponding transgene (nega- tive selection of MLL-AF4 alone).
We, therefore, assume that, in humans, AF4-MLL and MLL-AF4 are both necessary, but AF4-MLL could pre- sumably be shut off after "preparing the ground" for MLL-AF4, and that this process of shutting down AF4- MLL makes the leukemic disease even more aggressive (positive selection). This explains in part the molecular situation diagnosed in human patients with leukemia, regardless of whether they are infants or adults. It would be of interest to compare primary diagnostic material with relapsed material from the same patient, and deter- mine whether AF4-MLL expression is lost in the relapse, in order to have another argument in favor of the above mentioned hypothesis.
The study by Agraz-Doblas et al. adds another, impor- tant piece to the puzzle of the molecular mechanism of t(4;11) leukemia.1 It is to be hoped that the precise mech- anism of this disease can be understood soon, because the full picture is needed in order to develop new drugs that can really help patients with t(4;11) leukemia.
References
1. Agraz-Doblas A, Bueno C,2 Bashford-Rogers R, et al. Unraveling the cellular origin and clinical prognostic markers of infant B-cell acute lymphoblastic leukemia using genome-wide analysis. Haematologica 2019;104(6):1176-1188.
2. Trentin L, Giordan M, Dingermann T, et al. Two independent gene signatures in pediatric t(4;11) acute lymphoblastic leukemia patients. Eur J Haematol. 2009;83(5):406-419.
3. Stam RW, Schneider P, Hagelstein JA, et al. Gene expression profil- ing-based dissection of MLL translocated and MLL germline acute lymphoblastic leukemia in infants. Blood. 2010;115(14):2835-2844.
4. Kang H, Wilson CS, Harvey RC, et al. Gene expression profiles pre- dictive of outcome and age in infant acute lymphoblastic leukemia: a Children's Oncology Group study. Blood. 2012;119(8):1872-1881.
5. Kühn A, Löscher D, Marschalek R. The IRX1/HOXA connection: insights into a novel t(4;11)- specific cancer mechanism. Oncotarget. 2016;7(23):35341-35352.
6. Guzman ML. CDK6 is a regulator of stem cells "Egr" to wake up. Blood. 2015;125(1):7-9.
7. Okuda H, Kanai A, Ito S, Matsui H, Yokoyama A. AF4 uses the SL1 components of RNAP1 machinery to initiate MLL fusion- and AEP- dependent transcription. Nat Commun. 2015;6:8869.
8. Okuda H, Takahashi S, Takaori-Kondo A, Yokoyama A. TBP loading by AF4 through SL1 is the major rate-limiting step in MLL fusion- dependent transcription. Cell Cycle. 2016;15(20):2712-2722.
9. Nilson I, Reichel M, Ennas MG, et al. Exon/intron structure of the human AF-4 gene, a member of the AF-4/LAF-4/FMR-2 gene family coding for a nuclear protein with structural alterations in acute leukaemia. Br J Haematol. 1997;98(1):157-169.
10. Benedikt A, Baltruschat S, Scholz B, et al. The leukemogenic AF4- MLL fusion protein causes P-TEFb kinase activation and altered epi- genetic signatures. Leukemia. 2011;25(1):135-144.
11. Caslini C, Serna A, Rossi V, Introna M, Biondi A. Modulation of cell cycle by graded expression of MLL-AF4 fusion oncoprotein. Leukemia. 2004;18(6):1064-1071.
12. Lin S, Luo RT, Ptasinska A, et al. Instructive role of MLL-fusion pro- teins revealed by a model of t(4;11) pro-B acute lymphoblastic leukemia. Cancer Cell. 2016;30(5):737-749.
13. Mück F, Bracharz S, Marschalek R. DDX6 transfers P-TEFb kinase to the AF4/AF4N (AFF1) super elongation complex. Am J Blood Res. 2016;6(3):28-45.
14. BursenA,SchwabeK,RüsterB,etal.TheAF4-MLLfusionproteinis capable of inducing ALL in mice without requirement of MLL-AF4. Blood. 2010;115(17):3570-3579.
haematologica | 2019; 104(6)