There is a presence of the Philadelphia (Ph) chromosome in the majority (90-95%) of cases of chronic myeloid leukemia. The Ph chromosome is the result of a translocation that took place between Chromosome 9 and Chromosome 22 in a process known as reciprocal translocation (9;22). In the process of this translocation, a segment of chromosome 9 that contains the oncogene ABL is moved to chromosome 22 and joined to the gene for BCR. The chimeric fusion gene BCR-ABL is responsible for the creation of the oncoprotein bcr-abl tyrosine kinase. This oncoprotein is responsible for causing cancer. The bcr-abl oncoprotein has an unregulated tyrosine kinase activity, which deregulates cellular proliferation, reduces the adhesion of leukemia cells to the stroma of the bone marrow, and shields leukemic cells from the usual programmed cell death that would otherwise occur (apoptosis). CML develops when an aberrant pluripotent hematopoietic progenitor cell begins excessive production of all myeloid lineage cells, usually in the bone marrow but sometimes in extramedullary locations. This excessive production of myeloid lineage cells may occur everywhere in the body (eg, spleen, liver). Despite the fact that granulocyte synthesis is the most prevalent, the neoplastic clone also contains red blood cells, megakaryocytes, monocytes, and even some T cells and B cells. After the CML clone has been drug-suppressed, normal stem cells are preserved and have the potential to develop.
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