A New Method of Retroviral Lineage Specific Expression Utilizing the Cre/lox System Induces a T-Lymphoid Malignancy in a Mouse Model of ALCL.

A t(2;5) (p23;q35) chromosomal translocation can be found in more than 50% of all cases of anaplastic large cell lymphoma (ALCL), which belong to the group of aggressive Non-Hodgkin-Lymphomas (NHL). The fusion of the nucleophosmin gene (NPM) on chromosome 5 to the anaplastic lymphoma kinase (ALK) on chromosome 2 leads to a deregulated, constitutively active kinase which has been shown to play an important role in lymphomagenesis of ALCL. Mouse models, where the NPM-ALK fusion oncogene is retrovirally expressed in whole bone marrow (BM) cells which are subsequently transplanted into lethally irrradiated recipients have shown that NPM-ALK is able to induce histiocytic and plasmacytoid malignancies. Since ALCLs are in the vast majority either of T- or ‘Null’-cell phenotype, we have now developed a murine model of T-lymphoid ALCL. By employing a retroviral vector incorporating a translational stop-cassette flanked by loxP recombination sites recognized by the Cre recombinase, we established a new method of lineage specific gene expression in-vivo. We tested the vector by infecting BM cells harvested from mice expressing Cre from different lineage specific promotors. We have infected BM cells from mice expressing Cre under the control of the lysozyme M-promotor (lysM-mice), which is active in the myeloid compartment, from mice expressing Cre from the granzyme B-promotor (GrzmB-mice), which is mainly active in T-cells, and finally we also used BM cells from C57B6 wildtype mice as a control. Mice transplanted with BM from lysM-mice succumbed to a histiocytic malignancy after 4–6 weeks, whereas mice transplanted with BM from GrzmB-mice developed a mixed T-NHL/histiocytic phenotype with a similar latency. These mice displayed infiltration of malignant, mostly CD4/CD8 DP T-cells in the thymus, lymphnodes and spleen, as well as a histiocytic infiltration in the spleen and BM. Mice transplanted with control BM from wt mice did not show signs of disease after up to 6 months. The results indicate that the hematopoetic lineage where an oncogene is expressed is paramount to the malignant phenotype

The described model represents a versatile tool for the analysis of NPM-ALK signaling pathways in-vivo. With the help of this method, any gene can be rapidly expressed in a defined lineage or in a developmental stage. Thus, also more subtle phenotypes may become amenable to investigation in-vivo.