Cytogenetic and Molecular Genetic Aspects of Chronic Myeloid Leukaemia

Abstract

Chronic myeloid leukaemia (CML) is caused by the product of the BCR-ABL oncogene, located on the Philadelphia (Ph) chromosome. BCR-ABL is generated as a result of a reciprocal t(9;22) chromosomal translocation. The mechanisms responsible for this illegitimate recombination event remain elusive but are presumed to require a close spatial association of the translocation partners (chromosomes 9 and 22). BCR-ABL fusion transcripts can be detected by a sensitive reverse transcription-polymerase chain reaction (RT-PCR) in the leucocytes of some healthy individuals suggesting that chromosomal translocations may occur frequently in the general population. The presence of BCR-ABL fusion transcripts does not imply that the individual will inevitably develop CML since other conditions must be favourable for expansion of the abnormal clone. Breakpoints in the ABL gene occur within a 5′ segment. BCR-ABL fusion transcripts lack ABL exon a1 and consist of BCR exons fused directly to ABL exon a2. The breakpoints in the BCR gene on chromosome 22 are found within three defined regions. Depending on the position of the BCR breakpoint, fusion genes are generated that encode 190-, 210- or 230-kD forms of the Bcr-Abl tyrosine kinase. Since the ABL component of the fusion gene is largely invariant, it follows that variability in disease phenotype may be due to protein sequences encoded by the translocation partner, BCR. Different disease phenotypes are associated with each of the three Bcr-Abl oncoproteins, p190^Bcr-Abl, p210^Bcr-Abland p230^Bcr-Abl. Mechanisms associated with malignant transformation include altered cellular adhesion, activation of mitogenic signalling pathways, inhibition of apoptosis and proteasomal degradation of physiologically important cellular proteins. CML is subject to an inexorable progression from an ‘indolent’ chronic phase to a terminal blast crisis. Disease progression is presumed to be associated with the phenomenon of genomic instability.

Flow cytometric immunobead assay for the detection of BCR–ABL fusion proteins in leukemia patients

“…Indeed, the immunobead assay was able to detect all BCR-ABL variants in a set of well-defined cell lines. On the basis of the position of the BCR and ABL epitopes that are recognized by the applied antibodies, it can be anticipated that all rare and novel BCR-ABL variants will be detected by the here presented immunobead assay, 42 as was proven for the novel e18-a2 BCR-ABL (p225) variant. 19 This is in contrast to the currently used PCR techniques, which focus on the m-bcr (p190) and M-bcr (p210) breakpoints and might easily miss the p225 and p230 variants.…”

“…[17][18][19] (b) The three well-defined breakpoint regions in the BCR gene can produce at least eight different fusion transcripts, because of alternative splicing in the ABL gene (splicing to exon 2 or exon 3) and because the M-bcr consists of two intronic regions (intron 13 and intron 14). 13,42 Additional BCR-ABL fusion transcripts have been described, but they are rare or only reported once. 13,[17][18][19] Flow cytometric immunobead assay for detection of BCR-ABL fusion proteins F Weerkamp et al gene (splicing to exon 2 or exon 3), and because the M-bcr consists of two intronic regions (intron 13 and intron 14), a total of at least eight different frequently occurring fusion sites can be identified (Figure 1b).…”

Bcr-Abl induces autocrine IGF-1 signaling

“…BC patients respond less frequently to imatinib (Druker et al, 2001) due to the acquisition of additional genetic or epigenetic aberrations that drive the transition from CP to BC (Ren, 2005). Several nonrandom chromosomal aberrations are associated with CP to BC transition (Barnes and Melo, 2002). We recently investigated the role of the putative tumor suppressor Riz1 in CML (Pastural et al, 2007), which is located at chromosome region 1p36 and undergoes loss of heterozygosity during the transformation from CP to BC (Mori et al, 1998).…”

Expression of the p210BCR-ABL oncoprotein drives centrosomal hypertrophy and clonal evolution in human U937 cells

“…The oncoprotein is capable of reprogramming the prior lineage commitment of hematopoietic stem and early progenitor cells, and induces illegitimate enlargement of clonal hematopoiesis and genetic instability that drives disease progression from chronic phase (CP) toward the fully transformed phenotype of blast crisis (BC). [1][2][3] The mechanisms responsible for the transition of CML CP into BC remain poorly understood, although it is generally accepted that an unrestrained BCR-ABL activity and accumulation of additional genetic alterations in hematopoietic stem/ progenitor cells are the primary determinants for expansion of increasingly malignant cell clones. [4][5][6] Molecular changes commonly observed in evolution to BC are oncogene mutations as well as additional nonrandom chromosomal abnormalities involving whole chromosomes.…”