The chimeric BCR/ABL protein is characteristic of Philadelphia (Ph)+ leukemia because it is the direct product of the Ph translocation and it has been shown to play a causal role in the genesis of leukemia. The BCR/ABL protein exhibits a deregulated tyrosine-kinase activity capable of phosphorylating different cellular substrates in vivo and in vitro. CRKL, an adaptor protein consisting of SH2 and SH3 domains in the absence of a catalytic domain, is one potential in vivo substrate of BCR/ABL. Previous experiments have shown that CRKL is phosphorylated on tyrosine in the chronic myelogenous leukemia (CML) cell line K562 and that CRKL is a substrate for ABL and for BCR/ABL in COS-1 cells. In the current study, we show that in peripheral blood cells a direct correlation exists between the presence of BCR/ABL and the phosphorylation status of CRKL. In Ph- peripheral blood cells, CRKL is present only in the nonphosphorylated form. In contrast, all BCR/ABL+ CML and acute lymphoblastic leukemia patient samples examined showed clear tyrosine-phosphorylation of CRKL. This result strongly suggests that CRKL is a biologically significant substrate for BCR/ABL and is likely to play a major role in the development of Ph+ leukemia.

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