The AF10 leucine zipper is required for leukemic transformation of myeloid progenitors by MLL-AF10.

The t(10;11)(p12;q23) chromosomal translocation in human acute myeloid leukemia results in the fusion of the MLL and AF10 genes. The latter codes for a novel leucine zipper protein, one of many MLL fusion partners of unknown function. In this report, we demonstrate that retroviral-mediated transduction of an MLL-AF10 complementary DNA into primary murine myeloid progenitors enhanced their clonogenic potential in serial replating assays and led to their efficient immortalization at a primitive stage of myeloid differentiation. Furthermore, MLL-AF10-transduced cells rapidly induced acute myeloid leukemia in syngeneic or severe combined immunodeficiency recipient mice. Structure/function analysis showed that a highly conserved 82-amino acid portion of AF10, comprising 2 adjacent alpha-helical domains, was sufficient for immortalizing activity when fused to MLL. Neither helical domain alone mediated immortalization, and deletion of the 29-amino acid leucine zipper within this region completely abrogated transforming activity. Similarly, the minimal oncogenic domain of AF10 exhibited transcriptional activation properties when fused to the MLL or GAL4 DNA-binding domains, while neither helical domain alone did. However, transcriptional activation per se was not sufficient because a second activation domain of AF10 was neither required nor competent for transformation. The requirement for alpha-helical transcriptional effector domains is similar to the oncogenic contributions of unrelated MLL partners ENL and ELL, suggesting a general mechanism of myeloid leukemogenesis by a subset of MLL fusion proteins, possibly through specific recruitment of the transcriptional machinery.