#608232
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A number sign (#) is used with this entry because chronic myeloid leukemia is most frequently caused by a translocation between chromosomes 22 and 9, creating a BCR/ABL fusion gene encoding a tyrosine kinase (see 151410).
Clinical Features
Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder of a pluripotent stem cell with a specific cytogenetic abnormality, the Philadelphia chromosome (Ph), involving myeloid, erythroid, megakaryocytic, B lymphoid, and sometimes T lymphoid cells, but not marrow fibroblasts. Silver (2003) reviewed the hematologic and clinical aspects of chronic myeloid leukemia. Geary (2000) presented a historical review of CML.
CML has a biphase or triphase clinical course (Medina et al., 2003). Approximately 90% of patients are diagnosed in the chronic phase, but the disease eventually evolves to a blastic phase unless successfully treated. Approximately two-thirds of patients manifest an accelerated phase. A distinct feature of disease progression is the appearance of additional cytogenetic abnormalities in the Ph-positive cells. This phenomenon, known as clonal evolution, frequently involves a second Ph, trisomy of chromosome 8, and isochromosome 17 and other abnormalities of chromosome 17 (Kantarjian et al., 1988), although other abnormalities have been described. Clonal evolution is considered a criterion of accelerated phase, although when it represents the only criterion of transformation, it is associated with a better prognosis than other criteria of accelerated phase (Cortes et al., 2003).
Sawyers (1999) reviewed the clinical aspects of chronic myeloid leukemia.
Inheritance
Lillicrap and Sterndale (1984) reported 3 cases of CML in 3 successive generations with a myeloproliferative disorder in a 4th member of the kindred.
Cytogenetics
Nowell and Hungerford (1960) identified the Philadelphia chromosome of chronic myeloid leukemia (a G group chromosome with part of its long arm missing). It was called the Philadelphia chromosome because it was thought to be useful to follow the practice of hemoglobinologists and name anomalous chromosomes after the city of discovery. It was presumed to be a deleted chromosome 21, the same chromosome as that which is trisomic in Down syndrome (190685). The elevated alkaline phosphatase activity in Down syndrome and depressed activity in CML was viewed as consistent with this interpretation. Using the improved definition provided by 'banding' methods, Rowley (1973) showed that in fact there is a translocation of the distal part of chromosome 22 (not 21) onto another chromosome, usually 9q.
De Klein et al. (1982) demonstrated that the Abelson oncogene (ABL; 189980) is translocated from chromosome 9 to chromosome 22 in the formation of the Philadelphia chromosome. This indicated that the translocation is reciprocal and suggested a role for the ABL gene in the generation of CML. Not only is the ABL oncogene translocated from 9q to 22 but the SIS oncogene (190040) is presumably translocated from 22 to 9 since it is situated distal to the breakpoint that creates the Philadelphia chromosome (Swan et al., 1982). What is involved in the variant 22q- Philadelphia chromosomes with translocations to other chromosomes? Some 18 different ones were found by Mittelman and Levan (1978). Do all these other chromosomes contribute oncogenes to the 22q- chromosome? Does the translocation of SIS to the recipient chromosome play some role in the usual CML and the variant forms? These were questions raised by Klein (1983), who also asked, Will the microevolutionary process that leads tumor cells towards increased independence from host control, usually referred to as tumor progression, turn out to depend on the sequential activation of multiple oncogenes by genetic rearrangement?
Prakash and Yunis (1984) located the breakpoints in CML to subbands 22q11.21 and 9q34.1. Although the position of the breakpoint in chromosome 9 is quite variable, the breakpoint in chromosome 22 is clustered in an area called BCR for 'breakpoint cluster region.' Shtivelman et al. (1985) referred to BCR as a gene and stated that the ABL oncogene is transferred 'into the BCR gene of chromosome 22.' They found that an 8-kb RNA specific to CML is a fused transcript of the 2 genes. The fused protein is presumably involved in the malignant process. The protein has BCR information at its amino terminus and retains most but not all of the normal ABL protein sequences. Since the breakpoint in 9 may be as much as 30 or 40 kb 5-prime to ABL, a large amount must be 'looped out' in the fusion process. The fusion protein has tyrosine kinase activity.
About 10% of patients with acute lymphocytic leukemia (ALL) have the translocation t(9;22)(q34;q11) indistinguishable from that of CML. Erikson et al. (1986), however, found in 3 of 5 such cases of ALL that the BCR region was not involved and that the 22q11 chromosome breakpoint was proximal (5-prime) to the BCR region. Furthermore, the BCR and ABL transcripts were of normal size in an ALL line carrying the t(9;22) translocation. The breakpoints of the t(9;22) CML, the t(9;22) of ALL, and the t(8;22) of Burkitt lymphoma fall into 22q11 and are cytologically indistinguishable. By chromosomal in situ hybridization, however, they can be distinguished (Emanuel et al., 1984; Erikson et al., 1986). To this experience, Griffin et al. (1986) added observations on t(11;22), both constitutional and tumor-related (see 133450). In the hybrid gene of CML, the BCR contribution is 5-prime to the ABL contribution. In the creation of the gene, splicing can occur across an interval as great as 100 kb. The product of the BCR/ABL hybrid gene is a 210-kD protein. It was suggested that the hybrid gene and its product protein be designated PHL.
Ganesan et al. (1986) found that the BCR gene was rearranged in 7 cases of Philadelphia chromosome-negative CML. In 5 cases hematologic findings were indistinguishable from those of patients with the Philadelphia chromosome.
Mes-Masson et al. (1986) isolated overlapping cDNA clones defining the complete coding region for the hybrid protein generated from the ABL and BCR genes.
Schaefer-Rego et al. (1987) found that the breakpoints in 8 of 9 patients in blast crisis were in the 3-prime portion of BCR, whereas the breakpoints in 17 patients in the chronic phase were clustered in the 5-prime portion.
Mills et al. (1988) found a striking correlation between the site of the breakpoint within BCR and the length of time between presentation and onset of acute phase in CML patients: on average, patients with a 5-prime breakpoint had a 4-fold longer chronic phase than those with a 3-prime breakpoint. The median times were 203 weeks versus 52 weeks. Grossman et al. (1989) also presented evidence consistent with but not proving a relationship between the site of the breakpoint in BCR and the length of the clinical course before onset of blast crisis. Patients with 3-prime breakpoints progressed to acute disease after a shorter period (average 36.6 months) than did patients with 5-prime breakpoints (average 56.1 months), although the difference was not statistically significant.
Demiroglu et al. (2001) described 2 patients with a clinical and hematologic diagnosis of CML in chronic phase who had an acquired t(8;22)(p11;q11). They confirmed that both patients were negative for a BCR-ABL fusion gene and that both had an in-frame mRNA fusion between BCR exon 4 and FGFR1 (136350) exon 9. Thus, a BCR-FGFR1 fusion may occur in patients with apparently typical CML. The possibility of successful treatment with specific FGFR1 inhibitors was suggested.
Saglio et al. (2002) found that a patient with a typical form of chronic myeloid leukemia carried a large deletion on the derivative chromosome 9q+ and an unusual BCR-BCL transcript characterized by the insertion, between BCR exon 14 and ABL exon 2, of 126 bp derived from a region located on chromosome 9, 1.4 Mb 5-prime to ABL. This sequence was contained in a bacterial artificial chromosome (BAC), which in FISH experiments on normal metaphases was found to detect, in addition to the predicted clear signal at 9q34, a faint but distinct signal at 22q11.2, where the BCR gene is located, suggesting the presence of a large region of homology between the 2 chromosome regions. A BLAST analysis of the particular BAC sequence against the entire human genome revealed the presence of a stretch of homology, about 76 kb long, located approximately 150 kb 3-prime to the BCR gene, and containing the 126-bp insertion sequence. Evolutionary studies using FISH identified the region as a duplicon, which transposed from the region orthologous to human 9q34 to chromosome 22 after the divergence of orangutan from the human-chimpanzee-gorilla common ancestor about 14 million years ago. Saglio et al. (2002) noted that sequence analyses reported as part of the Human Genome Project had disclosed an unpredicted extensive segmental duplication in the human genome, and the impact of duplicons in triggering genomic disorders is becoming more and more apparent. The discovery of a large duplicon relatively close to the ABL and BCR genes and the finding that the 126-bp insertion is very close to the duplicon at 9q34 open the question of the possible involvement of the duplicon in the formation of the Philadelphia chromosome translocation.
Goldman and Melo (2003) tabulated 15 cytogenetic abnormalities leading to the expression of deregulated tyrosine kinases in chronic myeloproliferative disorders, beginning with BCR-ABL, which can cause either CML or ALL.
