Chronic myeloid leukemia (CML): a rare disease

Leukemias are diseases whose main characteristic is the accumulation of abnormal young cells in the bone marrow. The cytogenetics applied to the leukemia clinic started in 1960 with the discovery made by Peter Nowell and David Hungerford, of the Philadelphia (Ph) chromosome, a biomarker of Chronic myeloid leukemia (CML).

Ph is formed by a translocation, where the DNA of chromosome 9 and 22 mix to form on chromosome 22 the chimeric gene BCR-ABL. This aberrant gene is transcribed into an mRNA molecule, which in turn is translated into the protein resulting from this gene fusion (oncoprotein BCR-ABL) that leads to the overproduction of white blood cells, a phenomenon that characterizes CML.

The quantification, by Real Time Polymerase Chain Reaction (qPCR), of the BCR-ABL translocation RNA reflects the amount of oncoprotein that the myeloid lineage is producing and whether antineoplastic drugs are eliminating positive Ph cells in CML patients in chronic and accelerated phase.

The oncoprotein BCR-ABL adds groups of phosphates to the amino acids tyrosine of other proteins, these phosphate groups are removed from the molecule adenosine triphosphate (ATP) which is linked to the oncoprotein BCR-ABL, whose activity, called tyrosine kinase, is able to promote the cell proliferation and inhibit DNA repair causing genomic instability and making the cell susceptible to developing new genetic abnormalities, which can be visualized by cytogenetic analyzes.

Available forms of treatment

Multiple treatment options are available to patients with CML. The choice of treatment and the combinations of new antineoplastic drugs to be used depend of the genetic and clinical analyzes that aim to act directly on the aberrantly expressed genes and prevent the formation of new resistant clones.

The discovery of imatinib revolutionized the treatment of CML, beginning in 1999. Its action results from its ability to inhibit the tyrosine kinase action of the oncoprotein BCR-ABL through the competitive link with the ATP binding site. Now, approximately 90% of patients survive for at least five years. Imatinib can eliminate Ph positive cells in CML patients.

However, there are patients who are resistant or intolerant to imatinib, in these cases other options of antineoplastic drugs that also have the ability to inhibit the tyrosine kinase action of the oncoprotein BCR-ABL are being used for the treatment of CML, such as nilotinib and dasatinib.

Detection of gene mutations

Molecular biology can predict the success of treatment in CML patients by detecting mutations in the chimeric BCR-ABL gene that confer resistance to imatinib, such as the T315I mutation that results in amino acid substitution at position 315 of the BCR oncoprotein -ABL, from a threonine to an isoleucine. The T315I mutation is one of the most common changes and represents about 20% of cases of CML resistant to imatinib.

The detection of the T315I mutation, by Next-Generation Sequencing (NGS), is in the planning of the Hospital Ophir Loyola [1] for the year 2020 in partnership with Varstation. It is hoped that the management of complex genetic data can provide increasingly accurate results for the diagnosis and treatment of CML patients. In this way, the characteristics of a patient are analyzed in isolation and the therapeutic modalities are applied in a personalized way, which provides an accurate and early assessment, a safer prognosis, a therapeutic direction, a reduction in treatment time and costs and an increase in life expectancy and healing.

Author: Dr. Rommel Burbano

References:
[1] The Molecular Biology Laboratory at Hospital Ophir Loyola, a reference in cancer treatment in the North of Brazil, is coordinated by Dr. Rommel Burbano, and provides analyzes of tumor biomarkers validated by scientific societies, which can be used as diagnostic tools, prognosis and monitoring of therapy used by cancer patients. It also offers, to the patients of the Unified Health System, the characterization by immunophenotyping the cell line and the analysis of the maturation of hematological cells. Additionally, in biological samples from patients with hematological neoplasms, an analysis of chromosomal changes is performed, which provides information that is applied in diagnosis, classification, staging and prognosis.

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