| Clinics and Pathology |
| Disease | CML is a malignant chronic myeloproliferative disorder (MPD) of the hematopoietic stem cell. |
| Phenotype / cell stem origin | Evidence exists for the involvement of the most primitive and quiescent hematopoietic stem cell compartiment (CD34+/CD38-, Thy1+): t(9;22) is found in myeloid progenitor and in B-lymphocytes progenitors, but, involvement of the T-cell lineage is extremely rare. The existence of a highly quiescent stem cell population has been demonstrated in patients with CML. More recently, the presence of Ph chromosome has been demonstrated in vascular endothelium of CML patients at diagnosis. Ph+ stem cells with stem markers of endothelial cells have been shown to be present at the level of single stem cells. These findings have suggested that a putative "hemangioblast" giving rise to both hematopoietic and endothelial cells could be present on adult marrow and be a target of t(9;22) translocation. |
| Epidemiology | Annual incidence: 10/106 (from 1/106 in childhood to 30/106 after 60 yrs); median age: 30-60 yrs; sex ratio: 1.2M/1F. |
| Clinics | The disease is currently discovered after a routine blood count revealing hyperleucocytosis and circulating immature white blood cells. A splenomegaly might be present. Bone marrow aspirate with cytogenetic analysis is required, as well as molecular evaluation for the detection of BCR-ABL oncogene which is quantified by calculating BCR-ABL / ABL ratio. The disease is classified most commonly using Sokal or Hasford scores. The natural history of the disease including classically three phases (Chronic phase, accelerated phase and blast crisis) has been profoundly modified by the current therapy regimens using tyrosine kinase inhibitors. For instance, recent update of the IRIS study demonstrates the progressive reduction of secondary events over time with no blast crisis occurring after 6 years. Most patients now seen in accelerated phase or in blastic phase are those who relapse after IM and/or dasatinib/Nilotinib therapies. The major problems in CML are the resistance encountered as first line therapy as well as intolerance to TKI therapy leading to discontinuation of the drugs. In the IRIS trial, approximately 30% of patients discontinue imatinib for reasons of resistance (15%) or intolerance. Finally, it is clear that currently available TKI therapies do not eradicate the most primitive stem cells, explaining relapses occurring after discontinuation of treatment (see below). |
| Cytology | Hyperplastic bone marrow; myeloid proliferation with maturation; some myelodysplastic features can be seen on Imatinib therapy after disappearance of the Ph clone. The significance of these abnormalities is not known but very rare patients evolve into myelodysplastic or leukemic phase. The typical AL cytology can be seen when patients evolve into accelerated or blast phase (see: t(9;22)(q34;q11) in ALL, t(9;22)(q34;q11) in ANLL). |
| Treatment | The treatment of CML has been revolutionized by the introduction of targeted therapies to the clinical practice. The first of these drugs was imatinib mesylate (IM) targeting the tyrosine kinase activity of BCR-ABL. In a large international multi-center trial, the use of IM as a first line therapy has been compared to standard IFN-ARA-C regimen (IRIS trial). This trial has clealy shown the advantages of IM in terms of complete haematological response, (CHR) complete cytogenetic response (CCR) and molecular response (MR) as compared to IFN-ARA-C regimen. Interestingly, the outcome on IM therapy has been shown to be correlate with the Sokal score at diagnosis. The most recent update of this trial has shown for the first time, a reduction of secondary events (accelerated phase, blast crisis) over time, with no patients progressing towards blast crisis after 6 years of therapy. However, several questions remain to be solved before demonstration of a "cure" under IM therapy: 1-The attempts to interrupt IM therapy has been followed in most cases by a cytogenetic and haematological relapse; 2-The most primitive stem cells seem to be resistant to IM therapy at least in vitro ; 3-The resistance to IM-therapy has been found to be associated, especially in patients who received it as second line treatment, with the occurrence of mutations in the ABL-kinase domain, impeding the binding of the drug to its target. Some of these mutations, occurring in the P-loop or in the ATP binding pocket ("gatekeeper" mutation T315I ) lead to a total resistance to IM; requiring the interruption of the drug. If the mutations reside outside these regions (C-lobe of the SH1 kinase domain), the increase of IM dose could lead to molecular responses. Detection of ABL-kinase mutations has therefore become a clinically useful molecular test. The majority of ABL-kinase mutations other than T315I have been shown to be targetable by second generation of TK inhibitors, essentially Dasatinib (Sprycell) and Nilotinib (Tasigna). Dasatinib is a combined SRC and ABL-inhibitor which is indicated as a second line therapy in CML patients failing on IM therapy or intolerant to IM-therapy. Nilotinib is a TK inhibitor with high affinity for ABL-tyrosine kinase. Novel therapies to eliminate the stem cells with T315I mutation remain a major futur challenge. |
| Prognosis | TKI therapies have changed the prognosis of CML as well as the natural history of the disease. Results from the IRIS trial suggest that the majority of patients with CML at first chronic phase will attain complete cytogenetic and major molecular responses with however persistence of minimal residual leukemic stem cells. Although it is difficult to qualify CML as an indolent disease, the estimated median survival with the available therapies is 25 years. |
| Cytogenetics |
| Cytogenetics Morphological | all CML patients have a t(9;22), at least at the molecular level (see below); but not all t(9;22) are found in CML: this translocation may also be seen in ALL, and in ANLL (see: t(9;22)(q34;q11) in ALL, t(9;22)(q34;q11) in ANLL), t(9;22) is also the hallmark of chronic neutrophilic leukemia which presents clinically with absence of circulating immature myeloid cells. In this translocation BCR breakpoint occurs in the micro-BCR region, with the e19-a2 junction at the DNA level and a large protein (BCR-ABL p230) with increased TK activity. It should be noted that the product of the reciprocal translocation ABL-BCR can also be detected in CML cells, the signification of which is unclear. |
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| t(9;22)(q34;q11) G- banding (left) - Courtesy Jean-Luc Lai and Alain Vanderhaegen (3 top) and Diane H. Norback, Eric B. Johnson, and Sara Morrison-Delap, UW Cytogenetic Services (2 bottom); R-banding (right) top: Editor; 2 others Courtesy Jean-Luc Lai and Alain Vanderhaegen); diagram and breakpoints (Editor). | |
| Cytogenetics Molecular | Is a useful tool for diagnostic ascertainment in the case of a 'masked Philadelphia' chromosome, where chromosomes 9 and 22 all appear to be normal, but where cryptic insertion of 3' ABL within a chromosome 22 can be demonstrated. |
| Additional anomalies | Deletion of the derivative chromosome 9: Detected at diagnosis (n 10% of patients), probably indicating a genetic instability phenotype, this finding has been associated to the more aggressive behavior of the disease, a poor prognostic factor potentially reversed by the use of imatinib mesylate. Since the introduction of imatinib mesylate to the clinical practice, chromosomal abnormalities have been detected in Ph-negative metaphases in patients in CCR. The most frequent abnormalities are +7, +8 and in very rare cases they are associated to the development of MDS or AML. The direct role of TKI therapies in these abnormalities is difficult to assess in vivo. However, in vitro, it has been demonstrated that imatinib induces centrosome and chromosome aberrations in normal human dermal fibroblasts. |
| Variants | Chromosome, are found in 5-10% of cases; however, 9q34-3'ABL always joins 22q11-5'BCR in true CML; the third chromosome and breakpoint is, at times, not random. In a way, masked Philadelphia chromosomes (see above) are also variants. |
| Genes involved and Proteins |
| Gene Name | ABL |
| Location | 9q34 |
| Dna / Rna | Alternate splicing (1a and 1b) in 5'. |
| Protein | Giving rise to 2 proteins of 145 kDa; contains SH (SRC homology) domains; N-term SH3 and SH2 - SH1 (tyrosine kinase) - DNA binding motif - actin binding domain C-term; widely expressed; localisation is mainly nuclear; inhibits cell growth. |
| Gene Name | BCR |
| Location | 22q11 |
| Dna / Rna | Various splicings. |
| Protein | Main form: 160 KDa; N-term Serine-Treonine kinase domain, SH2 binding, and C-term domain which functions as a GTPase activating protein for p21rac; widely expressed; cytoplasmic localisation; protein kinase; probable role in signal transduction. |
| Result of the chromosomal anomaly |
| Description | 1. The crucial event lies on der(22), id est 5' BCR/3' ABL hybrid gene is pathogenic, while ABL/BCR may or may not be expressed; 2. Breakpoint in ABL is variable over a region of 200 kb, often between the two alternative exons 1b and 1a, sometimes 5' of 1b, or 3' of 1a, but always 5' of exon 2; 3. Breakpoint in BCR is in a narrow region, therefore called M-bcr (for major breakpoint cluster region), a cluster of 5.8 kb, between exons 12 and 16, also called b1 to b5 of M-bcr; most breakpoints being either between b2 and b3, or between b3 and b4 |
| Transcript | 8.5 kb mRNA, resulting in a 210 KDa chimeric protein with increased tyrosine kinase activity. |
| Detection | RT-PCR for minimal residual disease detection. Current detection methods of BCR-ABL make use of quantitative PCR analyses, using essentially TaqMan or SybR green technologies. The amount of BCR-ABL is compared to an endogenous gene, most of laboratories using ABL or BCR. The ratio of BCR-ABL / ABL at diagnosis is then followed during the treatment which induces a reduction of this ratio which is expressed by two different manners: 1-An absolute number resulting from the BCR-ABL/ABL ratio is quantified, and the reduction of this ratio by 3-log under therapy represents "major molecular response" or MMR. Or 2- The BCR-ABL /ABL ratio is quantified at diagnosis, representing 100% of BCR-ABL amount detected for a given patient using the RQ-PCR technology used in the laboratory. This ratio expressed as a percentage is calculated at each timepoint during the follow-up on therapy, a percentage of 0.1% (3-logreduction from the diagnosis) signifies MMR. The introduction of this international scale allows the comparison of results from different laboratories. |
| Description | P210 with the first 902 or 927 amino acids from BCR; BCR/ABL has a cytoplasmic localization, in contrast with ABL, mostly nuclear. The shuttling of ABL between the nucleus and cytoplasm is accomplished by the presence of import and export signal sequences in the COOH-terminal region of ABL. Interestingly, these sequences are conserved in BCR-ABL. It has been shown that the inhibition of BCR-ABL export from the nucleus has been shown to induce apoptosis in BCR-ABL-expressing cells. This inhibition is accomplished by the use of export inhibitors such as leptomycin. |
| Oncogenesis | A- Major molecular pathways activated by BCR-ABL.
|
| To be noted |
| 1. Most CML patients are nowadays diagnosed in an early stage but blast crisis could still be seen at the first onset of CML, and those cases may be undistinguishable from true ALL or ANLL with t(9;22) and P210 BCR/ABL hybrid; 2. JCML (juvenile chronic myelogenous leukaemia) is not the juvenile form of chronic myelogenous leukaemia: there is no t(9;22) nor BCR/ABL hybrid in JCML, and clinical features (including a worse prognosis) are not similar to those found in CML; 3. So called BCR/ABL negative CML should not be called so! 4. P53 is altered in 1/3 of BC-CML cases 5. Recent data suggest that Rac pathway could be a therapeutic target. |
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| Contributor(s) |
| Written | 12-1997 | Jean-Loup Huret |
| Genetics, Dept Medical Information, University of Poitiers, CHU Poitiers Hospital, F-86021 Poitiers, France | ||
| Updated | 10-2000 | Ali G Turhan |
| Translational Research - Cell Therapy, Laboratory, Institut Gustave Roussy, INSERM U. 362, 1 - 39, rue Camille Desmoulins, 94805 Villejuif Cedex, France | ||
| Updated | 08-2008 | Ali G Turhan |
| Pole de Biologie-Sante - 40 avenue du Recteur Pineau - 86022 Poitiers Cedex, France |
| Citation |
| This paper should be referenced as such : |
| Huret JL . Chronic myelogenous leukaemia (CML). Atlas Genet Cytogenet Oncol Haematol. December 1997 . URL : http://AtlasGeneticsOncology.org/Genes/CML.html |
| Turhan AG . Chronic myelogenous leukaemia (CML). Atlas Genet Cytogenet Oncol Haematol. October 2000 . URL : http://AtlasGeneticsOncology.org/Genes/CML.html |
| Turhan AG . Chronic myelogenous leukaemia (CML). Atlas Genet Cytogenet Oncol Haematol. August 2008 . URL : http://AtlasGeneticsOncology.org/Genes/CML.html |
| © Atlas of Genetics and Cytogenetics in Oncology and Haematology | indexed on : Wed Sep 24 21:05:28 2008 |
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