The CALM-AF10 Fusion Gene Expressed in Transgenic Mice Produces Acute Leukemia.

The rare but recurring translocation [t(10;11)(p13;q21)] occurs in patients with both acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML); a more recent analysis of clinical cases showed that of the T-ALL cases, t(10;11) are restricted to the TCRγδ lineage. This translocation was initially described in immature T-cells; subsequently the molecular features were characterized in the U937 cell line and shown to result in the production of a CALM-AF10 fusion gene. In order to study the leukemic effects of a CALM-AF10 fusion in mice as a model of human disease, a CALM-AF10 fusion cDNA was cloned from the U937 cell line and inserted into an expression vector that utilized Vav 5′ and 3′ regulatory elements. The Vav gene is expressed in all hematopoietic tissues and is essential for lymphocyte development; Vav regulatory elements have been used by a number of groups to direct transgene expression to the hematopoietic compartment. We generated transgenic Vav-CALM-AF10 mice by pronuclear injection, and followed offspring from two founder mice that were shown to express the CALM-AF10 transgene in hematopoietic tissues for 18 months. Mice were clinically healthy for the first 9 months of life, but had T and B cell developmental abnormalities, including decreased CD4+/CD8+ cells and increased CD4−/CD8− cells in the thymus. In addition, CALM-AF10 mice had decreased B220+/IgM+ cells in the spleen, as well as an unusual Mac1+/B220+ population of cells in the spleen. Eight of 20 offspring from founder C10, and 7 of 16 from founder E6 developed acute leukemia between 9 and 18 months-of-age. Additionally, 5 of 20 mice from the C10 founder and 3 of 16 mice from the E6 founder were found dead unexpectedly but were too necrotic for analysis. Animals that developed acute leukemia were either found dead without premonitory symptoms or presented with ruffled fur, hunched posture and dyspnea. Gross findings included splenomegaly and hepatomegaly. CBC analysis indicated a leukocytosis and anemia and peripheral blood and bone marrow contained increased numbers of blasts and immature forms. Spleens from leukemic mice were effaced by leukemic blasts, which also invaded peripheral tissues including liver, lung, and kidney. Tissue sections from most of the leukemias were myeloperoxidase (MPO) positive and negative for CD3, F4/80, and B220, and immunophenotype was typically Mac1+, Gr+/−, CD4−, CD8−, and B220−. Spleen and/or liver from animals with leukemia were evaluated for clonal T-cell receptor (TCR) β or δ rearrangements. Two of 11 had clonal TCR β rearrangements, one of which also had a clonal TCR δ rearrangement. These findings suggest that CALM-AF10 produces a predominately myeloid leukemia in mice, some of which also have T-cell features. The long latency period and incomplete penetrance suggests that collaborative events may be needed to initiate leukemogenesis.