No abstract

The mechanisms by which mixed-lineage leukemia (MLL) fusion products resulting from in utero translocations in 11q23 contribute to leukemogenesis and infant acute leukemia remain elusive. It is still controversial whether the MLL fusion protein is sufficient to induce acute leukemia without additional genetic alterations, although carcinogenesis in general is known to result from more than 1 genetic disorder accumulating during a lifetime. Here we demonstrate that the fusion partner-mediated homooligomerization of MLL-SEPT6 is essential to immortalize hematopoietic progenitors in vitro. MLL-SEPT6 induced myeloproliferative disease with long latency in mice, but not acute leukemia, implying that secondary genotoxic events are required to develop leukemia. We developed in vitro and in vivo model systems of leukemogenesis by MLL fusion proteins, where activated FMS-like receptor tyrosine kinase 3 (FLT3) together with MLL-SEPT6 not only transformed hematopoietic progenitors in vitro but also induced acute biphenotypic or myeloid leukemia with short latency in vivo. In these systems, MLL-ENL, another type of the fusion product that seems to act as a monomer, also induced the transformation in vitro and leukemogenesis in vivo in concert with activated FLT3. These findings show direct evidence for a multistep leukemogenesis mediated by MLL fusion proteins and may be applicable to development of direct MLL fusion-targeted therapy. IntroductionRecurrent translocations involving chromosome 11 band q23 (11q23) are frequent cytogenetic abnormalities observed in hematological malignancies, occurring in approximately 80% of infant and 10% of adult acute leukemias (1, 2). The mixed-lineage leukemia (MLL) gene (also called ALL1 or HRX) has been cloned in 11q23 translocations, such as t(4;11), t(9;11), and t(11;19) (3, 4), and more than 30 partner genes fused with MLL have been identified in various types of 11q23 translocations (5).MLL encodes a nuclear protein characteristic of several domains with assigned activities including an N terminus with 3 AT-hook motifs, a CXXC domain, 4 cysteine-rich zinc fingers, a transactivation domain, and a highly conserved C-terminal Su(var)3-9, Enhancer of zeste, and Trithorax (SET) domain with histone methyltransferase activity (6, 7). Recent studies demonstrated that MLL is cleaved by Taspase1, generating 2 fragments that heterodimerize to stabilize the complex (8) and assemble in a chromatin-modifying supercomplex (7).

Key Points MLL-ENL targets long-term HSCs exclusively to develop leukemia in a novel conditional transgenic mouse through upregulation of Plzf. Plzf is critically involved in the aberrant self-renewal program in HSCs induced by the MLL fusion gene.

Septins are evolutionarily conserved GTP-binding proteins that can heteropolymerize into filaments. Recent studies have revealed that septins are involved in not only diverse normal cellular processes but also the pathogenesis of various diseases, including cancer. SEPT6 is ubiquitously expressed in tissues and one of the fusion partner genes of MLL in the 11q23 translocations implicated in acute leukemia. However, the roles of this septin in vivo remain elusive. We have developed Sept6-deficient mice that exhibited neither gross abnormalities, changes in cytokinesis, nor spontaneous malignancy. Sept6 deficiency did not cause any quantitative changes in any of the septins evaluated in this study, nor did it cause any additional changes in the Sept4-deficient mice. Even the depletion of Sept11, a close homolog of Sept6, did not affect the Sept6-null cells in vitro, thus implying a high degree of redundancy in the septin system. Furthermore, a loss of Sept6 did not alter the phenotype of myeloproliferative disease induced by MLL-SEPT6, thus suggesting that Sept6 does not function as a tumor suppressor. To our knowledge, this is the first report demonstrating that a disruption of the translocation partner gene of MLL in 11q23 translocation does not contribute to leukemogenesis by the MLL fusion gene.