BCR/ABL inhibition by an escort/phosphatase fusion protein.

Cellular transformation by the BCR/ABL oncogene depends on the ABL-encoded tyrosine kinase activity. To block BCR/ABL function, we created a unique tyrosine phosphatase by fusing the catalytic domain of SHP1 (SHP1c) to the ABL binding domain (ABD) of RIN1, an established binding partner and substrate for c-ABL and BCR/ABL. This fusion construct (ABD/SHP1c) binds to BCR/ABL in cells and functions as an active phosphatase. ABD/SHP1c effectively suppressed BCR/ABL function as judged by reductions in transformation of fibroblast cells, growth factor independence of hematopoietic cell lines, and proliferation of primary bone marrow cells. In addition, the leukemogenic properties of BCR/ABL in a murine model system were blocked by coexpression of ABD/SHP1c. Both the "escort" function provided by ABD and the inhibitor function provided by the phosphatase of SHP1c were necessary for effective BCR/ABL interference. Expression of ABD/SHP1c also reversed the transformed phenotype of K562, a human leukemia-derived cell line. These results have direct implications for leukemia therapeutics and suggest an approach to block aberrant signal transduction in other pathologies through the use of appropriately designed escort/inhibitors.