| The Bcr gene was orginally identified by its presence in the chimeric Bcr-Abl oncogene (1). The amino-terminal region of Bcr contains an oligomerization domain, a serine/threonine kinase domain, and a region that binds SH2 domains. The middle of the protein has a PH domain and a region of sequence similarity to the guanine nucleotide exchange factors for the Rho family of GTP binding proteins. The carboxy-terminal region may be involved in a GTPase activating function for the small GTP-binding protein Rac (2,3). The function of wild type Bcr in cells remains unclear. PDGF receptor may use Bcr as a downstream signaling mediator (4). Research studies have shown that the Bcr-Abl fusion results in production of a constitutively active tyrosine kinase, which causes chronic myelogenous leukemia (CML) (5). Tyr177 of Bcr is phosphorylated in the Bcr-Abl fusion protein, which plays an important role in transforming the activity of Bcr-Abl (6). Phosphorylated Tyr177 provides a docking site for Gab2 and GRB2 (7,8).The fusion protein encoded by Bcr-Abl varies in size, depending on the breakpoint in the BCR gene. Three breakpoint cluster regions have been characterized to date: major (M-bcr), minor (m-bcr) and micro (mu-bcr). The overwhelming majority of CML patients have a p210 Bcr-Abl gene (M-bcr), whose mRNA transcripts have a b3a2 and/or a b2a2 junction. The smallest of the fusion proteins, p190 Bcr-Abl, (m-bcr breakpoint) is principally associated with Ph-positive ALL. Rare cases of CML are due to a p190-type of Bcr-Abl gene and in these, the disease tends to have a prominent monocytic component, resembling CMML. CML resulting from a p230 Bcr-Abl gene (mu-bcr breakpoint) is also rare, and has been associated with the CNL variant and/or with marked thrombocytosis. Exceptional CML cases have been described with Bcr breakpoints outside the three defined cluster regions, or with unusual breakpoints in Abl (9). |
