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

Abstract

The reciprocal chromosomal translocation t(4;11) is correlated with infant, childhood, adult and therapyrelated high-risk acute leukemia. Here, we investigated the biological effects of MLL . AF4, AF4 . MLL or the combination of both reciprocal fusion proteins in a conditional in vitro cell culture model system. Several parameters like cell growth, cell cycling capacity, apoptotic behavior and growth transformation were investigated under physiological and stress conditions. Co-transfected cells displayed the highest resistance against apoptotic triggers, cell cycling capacity and lossof-contact inhibition. These analyses were complemented by gene expression profiling experiments and specific gene signatures were established for each of the three cell lines. Interestingly, co-transfected cells strongly upregulate the homeobox gene Nanog. In combination with Oct4, the Nanog homeoprotein is steering maintenance of pluripotency and self-renewal in embryonic stem cells. Transcription of Nanog and other stem cell factors, like Oct4 and Bmi1, was verified in biopsy material of t(4;11) patient cells which express both reciprocal t(4;11) fusion genes. In conclusion, the presence of both reciprocal MLL fusion proteins confers biological properties known from t(4;11) leukemia, suggesting that each of the two fusion proteins contribute specific properties and, in combination, also synergistic effects to the leukemic phenotype.

The heterodimerization domains of MLL—FYRN and FYRC—are potential target structures in t(4;11) leukemia

“…10,11 From these data it was concluded that AF4 behaves like a protooncoprotein. Recent efforts have unravelled the function of the AF4 protein.…”

“…9 By contrast, we have demonstrated that the reciprocal AF4-MLL fusion protein causes growth transformation in murine embryonal fibroblasts. 10,11 Additionally, infection of murine hematopoietic progenitor cells (Lin À Sca1 þ ) with a low titer retrovirus coding for the AF4-MLL fusion gene led to the development of predominantly proB ALL in the transplanted mice. 12 Hence, we investigated the molecular actions of the AF4-MLL oncoprotein in a subsequent study.…”

Insights into the cellular origin and etiology of the infant pro-B acute lymphoblastic leukemia with MLL-AF4 rearrangement

“…The mice developed exclusively B-cell lineage neoplasias, but of a more mature phenotype than normally observed in childhood leukemia. 92 Alternatively, it has been very recently suggested that the presence of both reciprocal MLL fusion proteins (MLL-AF4 and AF4-MLL) 93 or AF4-MLL alone 94 confers biological properties known from t(4;11) leukemia, suggesting that each of the two fusion proteins contribute specific properties and, in combination, also synergistic effects to the leukemic phenotype. This reproducible but confusing in vitro and in vivo data suggest that these mouse models are missing some essential ingredients of leukemogenesis in the human embryo/fetus.…”

The leukemogenic AF4–MLL fusion protein causes P-TEFb kinase activation and altered epigenetic signatures

“…This notion is supported by the observed changes in gene transcription after short-term expression of AF4-MLL fusion protein. 41 Of interest, different complexes seem to be formed by AF4 family members, like for example, the recently described AF5 complex which contained AF4, ENL, AF10, DOT1L, AF6, AF17 and several ELL's. 42 We have not identified AF6, AF17 or ELL1-3 in our purified complexes (data not shown), suggesting that cells are able to form different complexes by AF4 family members that exert, however, highly similar functions.…”