Chronic Myelocytic Leukemia - Part I: History, Clinical Presentation, and Molecular Biology

Clinical Laboratory Science, Winter 2005 by Randolph, Tim R

BCR/ABL FUSION GENES

As can be seen in Figure 2b, the 9;22 translocation brings together the 5' portion of the BCR gene with the 3' end of the ABL gene. It has been shown that there exists at least four primary versions of the BCR/ABL translocation resulting from four distinct breakpoints in the BCR gene. These four breakpoints create three different protein products, p210 (Figure 3b), p 190 (Figure 3c), and p230 (Figure 3d), which may account for some of the differences in leukemogenesis between CML, Philadelphia chromosome positive de novo ALL, and chronic neutrophilic leukemia (CNL), respectively.15,16 The four known breakpoints on the BCRl gene are illustrated in Figure 2a by the upward arrowheads. The most common breakpoint region in the BCR 1 gene is called major BCR (M-BCR) and is located in the middle of the BCR 1 gene. Major BCR contains five exons (labeled 1-5) and two of the four known breakpoints. These five exons within M-BCR correspond to exons 12-16 of the BCRl gene.

The majority of breakpoints that occur in the BCR gene are between exons 2 and 3 (arrowhead #1) or between 3 and 4 (arrowhead #2). There are only two known breakpoints in the ABL gene occurring either between exons 1 and 1 ' or between exons 1' and 2. The fact that there are two breakpoint options in the ABL gene is irrelevant because RNA splicing always results in the lead exon being number 2 (Figure 2a). Therefore, the genetic contribution of the ABL gene remains constant. The two common breakpoint possibilities in M-BCR, coupled with the only possibility in ABL, results in the two ' transcripts illustrated in Figure 2b. The upper fusion gene represents the first 13 exons of the BCR and the last 10 exons of ABL, while the lower fusion gene represents the first 14 exons of BCR and the same 10 exons from the ABL gene. In both cases the protein product is a 210 kilodalton protein that is either 902 or 927 amino acids in length. This is the protein product associated with classical CML.

An alternative breakpoint region on the BCRl gene is termed minor BCR (m-BCR), and is located 5' of the major BCR.15 This breakpoint is associated with the majority of cases of Philadelphia positive ALL and in rare cases of CML that tend to produce a monocytosis.17 In this case, only exon 1 is joined to the same 10 exons of the ABL gene, translating into a smaller 18 5/190 kilodalton protein shown in the upper panel of Figure 2c. The last breakpoint region on BCR occurs between exon 19 and 20 creating a longer fusion protein, 230 kilodaltons in size, illustrated in the lower panel of Figure 2c.18 This version is rarely observed in CML, but when identified seems to produce a version of CML called chronic neutrophilic leukemia (CNL), that is characterized by an abundance of more mature neutrophils and thrombocytosis. Other fusion products of the t(9;22) translocation have been described but rarely occur.19

BCR/ABL FUSION PROTEIN

The understanding of the composition of the BCR/ABL fusion protein and the functions of die corresponding wild-type BCR and ABL proteins, allows us to predict the function of the fusion protein and its ultimate role in leukemogenesis. The ABL moiety of the fusion protein contributes to the transforming capability of the protein in at least three ways. First, and most important, the ABL moiety of the fusion protein exhibits alterations in the normal function of the SH2 and SH3 domains that control the tyrosine kinase activity of the SHl domain (Figure 3b). It is well established that tyrosine kinase functions to add phosphate groups to othet proteins. In signal ttansduction padiways designed to control cell proliferation, increased phosphorylation promotes proliferation, and dephosphorylation inhibits proliferation. In the wild-type ABL protein, the SH2 domain normally up regulates tyrosine kinase activity, and the SH3 domain down regulates tyrosine kinase activity. The t(9;22) translocation event has created a fusion gene and a resultant protein product that has lost die amino terminus of the ABL gene designed to regulace tyrosine kinase activity and has gained BCR genes that will affect the tyrosine kinase activity of the SH2 and SH3 domains. The result is constitutive tyrosine kinase activity.