CHSJ

Curr Health Sci J, vol. 35, no. 4, 2009

Particularities of Real-Time Polymerase Chain Reaction Technique (RT-PCR) in the Monitoring of Chronic Myeloid Leukemia Patients – A Brief Overview

[Update]

K. TATAR(1), H. IONITA(1)


(1)Department of Hematology, University of Medicine and Pharmacy “Victor Babes”, Timisoara


Abstract:

Chronic myeloid leukemia (CML) is a myeloproliferative disorder, characterized by a specific chromosomal aberration, the Philadelphia [Ph] chromosome. The Ph chromosome is the result of a reciprocal translocation between the long arms of chromosomes 9 and 22, t (9;22)(q34;q11). The molecular consequence of this translocation is a novel fusion gene, BCR-ABL, which encodes a constitutively active tyrosine kinase, implicated in pathophysiology and development of CML [1,2]. Imatinib mesylate currently the golden standard for front line treatment of CML, is a selective inhibitor of the BCR-ABL tyrosine kinase activity. Responses to imatinib occur at hematologic, cytogenetic and molecular levels. IM therapy now allows the majority of patients with CML to reach CCyR - a confirmed good prognostic indicator [3]. In different studies, 87% of patients in chronic, 17% of patients in accelerated, and 7% of patients in blast phases reached the important clinical aim of Ph negativity [3-5]. Ph chromosome status is usually assessed by classical bone marrow cytogenetics and this method has the limitation of poor sensitivity. Patients who reveal 1012 leukemic cells at the time of diagnosis, in CCyR may still have as many as 1010 leukemic cells in their body. Once a patient has achieved CCyR, monitoring of residual cells is usually performed by estimating the BCR-ABL transcripts on a molecular level using quantitative real-time polymerase chain reaction (QRT-PCR). A 2-log reduction in BCR-ABL transcripts correlates with Ph negativity in CCyR. Patients achieving a 3-log reduction in BCR-ABL transcripts are defined as having a major molecular response (MMR), a surrogate marker closely correlating with the probability of disease free survival [8]. Patients failing to achieve this 3-log response, at any time during therapy, had significantly shorter progression-free survival [6]. After a 5 to 6 log reduction, BCR-ABL transcripts can no longer be detected by QRT-PCR and patients are designated as having complete molecular response (CMR). But even with the most sensitive QRT-PCR assay, CMR is consistent with the persistence in the patient’s body of up to 106 or 107 leukemic cells [7]. Due to the proliferation of such residual cells a significant fraction of those patients who have responded on a deep molecular level, lose this response and progress to advanced phase disease. The clinical advantage of the extremely sensitive method of QRT-PCR is to be alerted by rising transcript levels at a very early time point, usually weeks or even months before the onset of clinical symptoms, allowing early terapeutic intervention with a beneficial impact on survival.


Keywords: BCR-ABL mRNA , control gene, Lightcycler, TaqMan, major molecular response, minimal residual disease



Corresponding: Kinga Tatar MD , Department of Hematology, University of Medicine and Pharmacy "Victor Babes", City Hospital Timisoara, Gheorghe Dima Street, No.5Timisoara , tatarkinga@yahoo.com


DOI 10.12865/CHSJ.35.04.15 - Download PDF
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