Reversible Transplantable Chronic Phase CML-Like Disease in SCLtTA/BCR-ABL Transgenic Mice.

Chronic myeloid leukemia (CML) is a disorder arising from the transformation of hematopoietic stem cells (HSC). Treatment with kinase inhibitors eradicates BCR-ABL positive progenitors but spares quiescent leukemic HSC (Copland et al. Blood 2006, Hu et al. PNAS 2006). The exact mechanism of this discrepancy is unknown. To better characterize the biology of CML stem cells in vivo, we have previously generated an inducible transgenic mouse model in which stem-cell specific expression of BCR-ABL leads to chronic phase CML-like disease. Here, we followed these mice non-invasively using positron emission tomography (FDG-PET) and abdominal high-resolution ultrasound. Moreover, we performed bone marrow transplants to analyze whether the disease is cell-autonomous and whether the phenotype of the disease is affected by the scheduling of BCR-ABL induction or the donor cell type. Splenomegaly was detectable as early as day 7 in induced double-transgenic SCLtTA/BCR-ABL mice, with an increase of the percentage of Gr-1⁺/Mac-1⁺ myeloid cells in spleen and bone marrow. Splenomegaly and myeloid cell proliferation progressed, and there was a close correlation between in vivo ultrasound measurements of the spleen and splenic weights upon autopsy. FDG-PET analysis demonstrated enhanced glucose uptake in the bone marrow suggestive of hyperproliferation. In addition, both FDG-PET and ultrasound revealed abnormalities of the small intestine, characterized by increased FDG uptake and distension of the intestinal wall. Upon autopsy, the small intestine showed an increased infiltration by granulocytic cells. These phenotypic changes were also evident in mice transplanted with cells from the bone marrow of double-transgenic sibling mice and were reversible upon tetracycline re-administration, demonstrating that this abnormality arises from bone marrow cells and is not due to expression of the oncogene outside of the hematopoietic system. We analyzed whether pre-transplant induction of BCR-ABL affected the repopulation potential of HSC or the disease phenotype. When recipient mice receiving unfractionated bone marrow cells from 3-week induced donor mice were compared with non-induced donors, there was no difference in the development of neutrophilia, myeloproliferation, or splenomegaly. However, when FACS-sorted Lin^negSca-1⁺c-kit⁺ HSC were used as donor cells, the disease latency increased from 8 to 11 weeks post-transplant, and the increase of Gr-1⁺Mac-1⁺ cells in the spleen was less pronounced than in mice receiving unfractionated bone marrow. In conclusion, this model reliably and efficiently demonstrates transplantable reversible chronic phase CML-like disease and may thus be valuable for the in vivo analysis of CML stem cell biology and susceptibility to stem-cell directed anti-leukemic therapies.