MLL Translocations: Fusion Proteins and Stem Cell Program

Coordinator:    Prof. Dr. Rolf Marschalek
Institution: Institut für Pharmazeutische Biologie, Goethe-Universität Frankfurt/Main, Biozentrum
Our research is mainly focussed on the t(4;11) chromosomal translocation that is associated with the development of proB acute lymphoblastic leukemia (proB ALL) in infants and early childhood. This type of leukemia is hard to treat and displays a dismal outcome. Due to the chromosomal translocation, two reciprocal fusion genes are generated in a single genetic event: MLL-AF4 and AF4-MLL, respectively.
In the last years, we combined clinical and basic research to unravel the secrets behind this particular disease. We started with gene expression profiling (GEP) experiments that identified unique signatures of deregulated genes in these patients. For this purpose, we developed a database program that allows in principle a simultaneous analysis of patient GEPs linked to experimental lab data of the same patient. Surprisingly, we discovered two independent patient signatures that subdivided the investigated cohort into 2 subgroups with deregulated genetic programs [1]. Remarkably, all patients shared also a unique signature of about 200 deregulated genes, present in every investigated leukemia patient. This "core signature" was nearly identical with a signature that we obtained from a mouse model where we modelled t(4;11) leukemia [2]. Mice displayed a proB ALL disease phenotype when we ectopically expressed one of the two fusion proteins, namely the AF4-MLL fusion protein.
Another aspect of our work was directed to our finding that key embryonic stem cell genes (NANOG, OCT4, SOX2) seemed to be expressed in t(4;11) cells [3]. Therefore, we investigated the transcriptional properties of NANOG1 and NANOG2 in leukemic cells. We could demonstrate that t(4;11) leukemia cells perform a specific (re-)activation of NANOG2, which may help to establish a cellular program needed by cancer stem cells [4].
Finally, we wanted to understand the epigenetic mechanism that occur in t(4;11) leukemia cells. We discovered that t(4;11) leukemia is mainly driven by two major mechanisms: (1) ectopic activation of transcriptional elongation mediated by P-TEFb, and (2) extended epigenetic signatures of histone H3 in transcribed gene regions (H3K4me3 and H3K79me2/3) [5]. Both mechanisms are triggered by the AF4-MLL fusion protein.
All these studies have led to a new pictures how these tumor cells arise. Not single point muations rather than a reshaping of the epigentic layer seems to be the driving force for these leukemias. All these data were reviewed in an article [6].

1.    Trentin L, Girodan M, Dingermann T, Basso G, te Kronnie G, Marschalek R (2009). Two independent gene signatures in pediatric t(4;11) acute lymphoblastic leukemia patients. Eur J Haematol 83, 406-419.

2.    Bursen A, Schwabe K, Rüster B, Henschler R, Ruthardt M, Dingermann T, Marschalek R (2010). AF4-MLL is capable of inducing ALL in mice without requirement of MLL-AF4. Blood 115, 3570-3579.

3.    Gaussmann A, Wenger T, Eberle I, Bursen A, Bracharz S, Herr I, Dingermann T, Marschalek R
(2007). Combined effects of the two reciprocal t(4;11) fusion proteins MLL•AF4 and AF4•MLL confer resistance to apoptosis, cell cycling capacity and growth transformation.Oncogene 26, 3352-3363.

4.    Eberle I, Pless B, Jacobi M, Dingermann T, Marschalek R (2010). Transcriptional properties of human NANOG1 and NANOG2 in acute leukemic cells. Nucl Acids Res 38, 5384-5395.

5.    Benedikt A, Baltruschat S, Scholz B, Bursen A, Arrey TN, Meyer B, Varagnolo L, Müller A, Karas M, Dingermann T, Marschalek R (2011). The leukemogenic AF4-MLL fusion protein causes P-TEFb kinase activation and altered epigenetic signatures. Leukemia 25, 135-144.

6.    Marschalek R (2011). Mechanisms of leukemogenesis by MLL fusion proteins. Br J Haematol 152, 141-154.

Additional relevant Internet links:
Diagnostic Center of Acute Leukemia DCAL
Forschergruppe "Pathologische Genprodukte und ihre Wirkmechanismen"
Goethe Universität Frankfurt am Main
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