Quantitatively, neoplasia is a disease more commonly seen in older individuals (Chapter 9), and there is ample evidence to indicate that both point mutations (Lee et al., 1994) and chromosomal aberrations increase with age (Nisitani et al., 1990; Liu et al., 1994). Despite this correlation, the genome of most species is reasonably stable throughout their lifetime, and there is little if any evidence that the aging process is itself carcinogenic (Chapter 8). Even so, the search for so- matic mutations that are characteristic of neoplasia amidst the apparent randomness of back- ground mutations has been the goal of many investigators since the time of Boveri. This search was initially rewarded by the demonstration of a specific chromosomal change associated with a specific neoplasm, chronic myeloid leukemia in humans, by Nowell and Hungerford (1960). This condition is a neoplasm of polymorphonuclear leukocytes, is chronic in that the neoplastic cells are reasonably well differentiated, and the disease has a relatively protracted course. The original observation of an abnormally small G group (chromosomes 21 and 22) led to later stud- ies that confirmed it to be chromosome 22 with an abnormal shortening of the long arm of the chromosome. The resultant chromosome has been termed the “Philadelphia” chromosome (Ph1) after the city where it was first described. The Ph1 chromosome occurs in over 90% of patients with chronic myeloid leukemia (CML). It has also been reported in about 20% of adults with acute lymphoblastic leukemia (ALL), 5% of children with ALL, and about 2% of adults with acute myeloid leukemia (AML) (Barton and Westbrook, 1994).
In 1973 Rowley reported that the “shortening” of the long arm of chromosome 22, resulting in the Ph1 chromosome, was actually the result of a translocation of this chromosomal segment to the long arm of chromosome 9. In Figure 6.4 may be seen an artist’s representation of this trans- location. Later studies by Geraedts and Van der Ploeg (1980) showed that the translocation is not
Figure 6.4 Diagram of chromosome translocation in chronic myeloid leukemia. The break occurs in chromosomes 9 and 22 in the area of the dotted lines. The translocation chromosome 9 is labeled 9q+ because of the additional genetic information on the long arm (q), extending it to a greater length than normal. Conversely, the translocation chromosome 22q-(Ph1) exhibits a shortening of the long arm because the translocated segment of chromosome 9 is smaller than that translocated from chromosome 22 to chromosome 9. (Adapted from Rowley, 1990b, with permission of the author and publisher.)
accompanied by any measurable loss in the DNA of the two chromosomes involved. Studies after Rowley’s initial observation indicated that, while a majority of translocations resulting in the Ph1 chromosome are to chromosome 9q, translocations to other chromosomes have been seen, in- cluding 12q, 17q, and 19q as well as more complex rearrangements (Sandberg, 1980).
In 1982 the Abelson (abl) proto-oncogene was localized to chromosome 9 by DeKlein and his associates. In 1984 Groffen and his associates cloned the translocation breakpoint. In these studies it was demonstrated that the abl gene translocated from chromosome 9 to 22, and the region of the breakpoint on chromosome 22 was limited to a small 5.8-kb region termed bcr (breakpoint cluster region). Thus, the gene on chromosome 22 in which this breakage occurs bears the terminology, bcr. As a result of the translocation, there is produced a chimeric bcr-abl fusion gene. A diagram of the genes at the breakpoints and on the translocated chromosomes is seen in Figure 6.5, which shows two translocation schemes. In A, the resultant protein has a molecular weight of 210,000, while in B the resultant protein is smaller, 190,000 Da (Rowley,
1990a; Barton and Westbrook, 1994). As noted, the breakpoints involve the 5′ region of the abl proto-oncogene, but the bcr gene is cleaved nearer the middle of the gene in the small p210 fusion gene. In the smaller p190 fusion gene, the bcr breakpoint is in the first intron of the bcr gene (Barton and Westbrook, 1994). Although the normal abl protein has relatively weak ty- rosine kinase activity, the protein product of the fused gene exhibits a much stronger tyrosine kinase activity (cf. Barton and Westbrook, 1994). Nearly 98% of CML cases exhibit the break- point patterns seen in Figure 6.5. In the remaining 2%, other aberrant transcripts have been re- ported (cf. Barton and Westbrook, 1994). None of the different CML fusion genes examined to date have a breakpoint in the c-abl gene of chromosome 9 at exactly the same location. The breakpoints appear to be distributed more or less at random within the 5′ region of the c-abl gene (cf. Groffen and Heisterkamp, 1989).
Figure 6.5 A. Map of the BCR/ABL fusion gene seen in CML and some adult patients with ALL. The breakpoint has occurred between the third and fourth exons included in the BCR region. These are equiva- lent to exons 11 and 12 in the bcr gene. The chimeric mRNA is diagramed below the gene. B. Map of the fusion gene in some ALL patients showing the breakpoint in the first intron of the bcr gene. The breakpoint in the abl gene is identical with that seen in CML in this example. The mRNA is much smaller than that seen in CML. (Adapted from Rowley, 1990b, with permission of the author and publisher.)
The importance of the presence of the BCR/ABL fusion gene in CML is exemplified by the finding that even in patients lacking an identifiable Ph1 chromosome, a number do exhibit the fusion gene probably representing an interstitial insertion or bcr rearrangement (cf. Dobrovic et al., 1991). Patients not exhibiting the fusion gene in the absence of a Ph1 chromosome usually exhibit atypical disease features and a more rapid and progressive course (cf. Dobrovic et al.,
1991; Barton and Westbrook, 1994). The exact mechanism by which the BCR/ABL fusion gene is related to the neoplastic transformation in CML is not clear. Recently, the function of the bcr gene was reportedly to encode a GTPase-activating protein whose function is involved with that of several proto-oncogenes, especially the ras gene family (Diekmann et al., 1991). Thus, the BCR/ABL protein may function to form a complex of an intrinsic GTPase-activating signal transduction pathway with an activated protein tyrosine kinase, both of which functions have been implicated in carcinogenesis (Chapter 16).