Monitoring of the therapy of chronic myeloid leukemia
[CML] [quantitative BCR-ABL diagnostics]
This test is supported by the German health insurance companies.
The fusion gene BCR-ABL has been shown to be present in most CML patients and some ALL (acute lymphocytic leukemia) patients. By measuring the BCR-ABL gene expression in the leukocyte population (minimal residual disease, MRD), the development of the disease can be reliably followed even after other tests have long passed the limits of their sensitivity. Any relapse can thus be recognized early and reacted to appropriately.
Other than measuring BCR-ABL gene expression, no further means exist for verifying conformance with the so called three log criterion with the Gleevec® Therapy (see below)
What is required? How is the diagnosis performed? Scientific Background
What is required?
Monovettes with a total of 10 ml EDTA-Blood should be sent together with the necessary documentation (see below) within 24 hours after blood is drawn to the following address:
Oncoscreen GmbH
Postfach 100 864
07708 Jena
Germany
Please enclose:
- the completed request form
To print this document you will need Acrobat Reader (Version 4.0 and up). - For patients within the German public health insurance system we need the “Überweisungs- Abrechnungsschein für Laboratoriumsuntersuchungen (Nr. 10)"
Please do not forget to print the name and telephone number of the doctor treating the patient, for the purposes of our reply.
The specimen can be sent in a padded envelope without the need for cooling; however, it needs to arrive at the laboratory not later than two days after blood has been taken. Within Germany, we guarantee a reply within 6 working days after arrival of the parcel at Oncoscreen. The reply includes details of all data of the patient in question, and provides an overview of the progression of the disease from a molecular biological point of view.
A knowledge of the subtype of the BCR-ABL translocation is required for correct quantification. Oncoscreen can also reliably perform this test; even for very seldom subtypes (Genotyping).
top
How is the diagnosis performed?
The routine diagnostics based on PCR developed by Oncoscreen enable a precise quantification of the BCR-ABL mRNA expression in peripheral blood (leukocyte population) even if the tumor load is extremely low.
In the Oncoscreen laboratories the leucocyte RNA is isolated and transformed into the more stable cDNA. Afterwards the degree of expression of the BCR-ABL transcript in the cDNA preparation is measured with help of the so-called quantitative real-time PCR (polymerase chain reaction).
The BCR-ABL gene exists in various versions (genotypes). For precise quantification, an exact knowledge of the version is necessary. Oncoscreen can determine the version, if required, via a genotyping test, which only has to be performed once per patient. The genotyping is also based on the cDNA preparation from peripheral blood.
top
Scientific Background
It can be proven that genetic changes occur continually in tumor tissue. Some tumor diseases are characterized by very particular genetic changes, as in, for example, chronic myeloid leukemia (CML). The affected white blood cells of 95% of all CML patients are characterized by the so-called Philadelphia-Chromosome, which develops due to a "reciprocal translocation". In this process two sub-units of genetic material (chromosomes) break apart and fuse into new chromosomes. At the location of the fusion, a new chimeric gene appears, the BCR-ABL gene, which is present only in leukemia cells and contributes decisively to tumor development. The BCR-ABL gene has also been shown to be present in about one-quarter of all adult patients with acute lymphatic leukemia (ALL) and about 5% of all child ALL patients.
The Philadelphia-Chromosome/BCR-ABL gene can be used to monitor the success of the therapy. If the quantity of Philadelphia-Chromosomes/BCR-ABL genes in the blood diminishes, this suggests a successful treatment. If, on the other hand, the quantity rises, then this is an important indication that a patient's condition has deteriorated, information to which the doctor can react with an appropriate therapeutic adjustment.
The Philadelphia-Chromosome has been used for many years to provide so-called cytogenetic evidence of the sickness, whereby the changed chromosome is detected by means of a microscope. Recently the quantitative PCR technology has been established which is 3 magnitudes more sensitive than classical methods and, thus, appropriate to investigate the tumor load in the state of so-called complete hematological and cytogenetic response, respectively. Peripheral blood is sufficient for this quantitative PCR analysis.
Recent published data of a clinical trial (IRIS Study, phase 3) show that the kinetics of the molecular response has prognostic significance with Gleevec therapy. Currently, a favorable prognosis is accepted when the molecular tumor load decreases by a factor of 1000 within 12 months of first line Gleevec therapy (the so-called “three log criterion"). For patients treated with Interferon it has been proposed that a BCR-ABL/ABL value below 0,045% was indicative of a good prognosis. Tumor loads below 1% are not accessible with cytogenetic methods and can be quantified by means of the PCR technology only.
If the aim of a 1000- fold reduction of the tumor load is not achieved with Gleevec therapy this may come from i) unsteady taking of the medicament ii) insufficient dose iii) suboptimal response to Gleevec. Quantitative BCR-ABL diagnostics help to identify patients with a less than optimal course of the therapy.
In addition, quantitative PCR technology can be used to detect CML relapse early. Therefore, continual PCR examinations of the patients blood are recommended every three months.
Several variants of the chimeric BCR-ABL gene have been published and new variants are continually being identified. We currently are screening each patient initially for eleven different variants. Nonetheless it is advisable to combine the quantitative PCR assay with the cytogenetic analysis which is indispensable for the initial diagnosis of the disease. This helps to avoid false-negative PCR results which may arise when extremely rare genetic variants are analyzed.
Literature
top
Oncoscreen®
Loebstedter Str. 93
07749 Jena
Germany
Phone: +49 (0) 3641 / 5074-0