Expression of BCR/ABL and BCL-2 in myeloid progenitors leads to myeloid leukemias

Abstract

Chronic myelogenous leukemia is a myeloproliferative disorder (MPD) that, over time, progresses to acute leukemia. Both processes are closely associated with the t(9;22) chromosomal translocation that creates the BCR/ABL fusion gene in hematopoietic stem cells (HSCs) and their progeny. Chronic myelogenous leukemia is therefore classified as an HSC disorder in which a clone of multipotent HSCs is likely to be malignantly transformed, although direct evidence for malignant t(9;22)+ HSCs is lacking. To test whether HSC malignancy is required, we generated hMRP8p210BCR/ABL transgenic mice in which expression of BCR/ABL is absent in HSCs and targeted exclusively to myeloid progenitors and their myelomonocytic progeny. Four of 13 BCR/ABL transgenic founders developed a chronic MPD, but only one progressed to blast crisis. To address whether additional oncogenic events are required for progression to acute disease, we crossed hMRP8p210BCR/ABL mice to apoptosis-resistant hMRP8BCL-2 mice. Of 18 double-transgenic animals, 9 developed acute myeloid leukemias that were transplantable to wild-type recipients. Taken together, these data indicate that a MPD can arise in mice without expression of BCR/ABL in HSCs and that additional mutations inhibiting programmed cell death may be critical in the transition of this disease to blast-crisis leukemia.

Fig. 1.

(A) hMRP8p210^BCR/ABL construction. (B) RT-PCR analysis of BCR/ABL transcript expression from whole bone marrow (BM), HSCs, CMPs, neutrophils (PMN), B lymphocytes (B), and T lymphocytes (T). HPRT, hypoxanthine phosphoribosyltransferase.

Fig. 2.

hMRP8p210^BCR/ABL animals develop a CML-like MPD. (A) Morphology of cells from leukemic and control animals. (B) Differential cell counts in chronic phase (n = 6), accelerated phase (n = 3), and wild-type (n = 5) mice. B, myeloblasts; P, promyelocytes; M, myelocytes; N, neutrophils; Mo, monocytes; L, lymphocytes. (C) Incidence of CML-like MPD in hMRP8 transgenic mice decreased after each backcross onto the C57BL/Ka genetic background. (D) Nonleukemic hMRP8p210^BCR/ABL mice (n = 4) have higher WBC counts throughout life than littermate controls (n = 5). Error bars represent ±SD.

Fig. 3.

Double-transgenic hMRP8p210^BCR/ABL × hMRP8BCL-2 mice develop AML. (A) Morphology of cells from leukemic and control mice. NCAE, α-naphthyl chloroacetate esterase. (B) Morphology of leukemic blasts in bone marrow (Left) and spleen (Right) of lethally irradiated recipients transplanted with leukemic cells from double-transgenic mice. (C) Incidence of AML-like disease in hMRP8p210^BCR/ABL × hMRP8BCL-2 mice (n = 18) and lethally irradiated congenic recipients transplanted with leukemic cells (n = 10). Db Tg, double transgenic. (D) FACS profile of bone marrow cells from leukemic animals. (E and F) Staining for myeloid progenitors in healthy (E) versus leukemic (F) hMRP8p210^BCR/ABL × hMRP8BCL-2 mice. Note the substantial increase in the GMP subpopulation in leukemic animals. FSC, forward scatter.