Blood, Vol. 94 No. 4 (August 15), 1999:
pp. 1486-1488
CORRESPONDENCE
Monitoring of Residual Disease in Chronic Myelogenous Leukemia by
Quantitative Polymerase Chain Reaction and Clinical Decision Making
 |
LETTER |
To the Editor:
In the May 1, 1999 issue of Blood, Faderl et
al1 discuss the possible clinical role of
polymerase chain reaction (PCR) analysis in the surveillance of
residual disease in patients with chronic myelogenous leukemia (CML).
Faderl et al1 raise questions regarding the significance of
positive PCR reactions in patients who are in clinical and cytogenetic
remission and express doubts about the possibility of using PCR
analysis as a basis for therapy decisions. They indicate that a major
drawback of current PCR studies is the lack of quantification of PCR
data and that the short follow-up duration of published studies makes
it difficult to interpret the results. The conclusions made are
questionable in view of the fact that the investigators do not discuss
any of the reports published by different groups on the monitoring of
the dynamics of residual disease in CML patients by serial quantitative
PCR analysis.2-7
Numerous investigators have demonstrated that detection of persisting
residual cells expressing the chimeric BCR-ABL message by sensitive
techniques such as PCR does not provide reliable information on
impending relapse in individual patients with CML. During treatment
with interferon-
(IFN-), sensitive PCR assays generally show
continuous presence of cells expressing the BCR-ABL fusion transcripts,
and it is not possible to predict imminent relapse by qualitative PCR
testing. Patients after allogeneic bone marrow transplantation (BMT)
have a statistically increased risk of relapse if PCR positivity
persists beyond 6 months after allografting,8 but several
patients have been reported who remained in unmaintained remission for
several years after BMT, despite continuous detectability of BCR-ABL
mRNA by PCR. Taken together, the data available indicate that mere
detection of PCR positivity in CML does not permit reliable prediction
of the course of disease in individual patients. This fact has been
appreciated by several investigators already a number of years ago and
has provided an impetus for the development of quantitative PCR (Q-PCR) assays facilitating the assessment of disease activity. The first techniques based on competitive PCR permitting quantification of
BCR-ABL transcripts were published approximately 7 years
ago,9-11 and different groups have addressed the clinical
impact of serial Q-PCR analysis in CML. In the first report on the
surveillance of residual disease by a quantitative PCR approach in CML
after allogeneic BMT,2 28 patients were presented who had
been monitored in a largely retrospective study over a period of up to
106 months (median, 37 months). It has been shown that detection of
increasing levels of BCR-ABL expression in serial samples from
peripheral blood preceded hematologic relapse by a median of 6 months.
Introduction of the term "PCR relapse" has been proposed based on
the dynamics of BCR-ABL expression. PCR relapse has been defined as a
10-fold or greater increase in the relative expression of the marker
gene detected and confirmed by a minimum of 3 independent, consecutive Q-PCR analyses. The proposed definition has been designed to account for the possibility of transient changes, such as fluctuating BCR-ABL
expression, and the inaccuracy inherent in the technique. The
possibility of predicting relapse in CML patients after BMT by the
detection of increasing levels of BCR-ABL transcripts by serial Q-PCR
analysis has been strengthened by a prospective study in 91 CML
patients.3 The observations indicating that quantitative monitoring of BCR-ABL mRNA expression permits early prediction of
disease recurrence have been further confirmed by prospective studies
performed by the above-noted groups and by other
investigators.5-7,12 Similar findings were also reported in
CML patients on IFN- therapy.4 We have shown that
increasing levels of BCR-ABL transcripts before disease progression do
not necessarily reflect an increase in the proportion of leukemic cells
in the samples analyzed, but show elevated steady-state levels of the
chimeric mRNA in the malignant cells.13 The fact that
increasing BCR-ABL expression can be detected before proliferation of
the leukemic clone underlines the value of quantitative analysis of the
chimeric BCR-ABL mRNA for early prediction of relapse.
Based on the results of the early studies using serial quantitative
PCR, the European Group of Investigators on CML (EICML) has provided
guidelines for the clinical use of Q-PCR monitoring in CML
patients.14 Hence, a number of investigators regard the quantification of BCR-ABL transcripts directed at the monitoring of the
dynamics of residual disease as a useful parameter for therapy
decisions. It seems, therefore, that the data provided by the reports
discussed above should be considered when the potential benefit of PCR
analysis in CML is critically evaluated.
Thomas Lion
Children's Cancer Research Institute (CCRI)
St. Anna
Children's Hospital
Vienna, Austria
| |
REFERENCES |
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Faderl S, Talpaz M, Kantarjian HM, Estrov Z:
Should polymerase chain reaction analysis to detect minimal residual disease in patients with chronic myelogenous leukemia be used in clinical decision making?
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Competitive polymerase chain reaction to estimate the number of BCR-ABL transcripts in chronic myeloid leukemia patients after bone marrow transplantation.
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Lion T, Gaiger A, Henn T, Horth E, Haas OA, Geissler K, Gadner H:
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Response
We thank Goldman et al and Lion for their letters to the editor in
response to our commentary.1 We agree with both that polymerase chain reaction (PCR) technology has undoubtedly added valuable information to our current knowledge and understanding of
minimal residual disease (MRD) in chronic myelogenous leukemia (CML)
and other hematologic malignancies. We are also pleased that both
letters address 2 basic issues regarding the clinical use of PCR: the
operational (ie, clinical utility of such testing) and the biological
(ie, the correlation of PCR results with the biology of MRD in CML).
However, we believe that their enthusiasm for the clinical use of PCR
is not supported by the available data.
Regarding the operational issue, even Goldman et al admit that PCR
assays do not routinely achieve high sensitivity in practice and that
their sensitivity can vary between specimens. Furthermore, qualitative
PCR assays, especially at single time points, are not useful for
predicting relapse. Meanwhile, quantitative PCR assays, such as those
used for the serial evaluation of MRD in CML, have yet to meet the
criteria expected of any laboratory test applied in clinical medicine:
(1) standardization of the test across different laboratories; (2)
reproducibility of positive and negative predictive values; and (3)
reliable sensitivity and specificity. Because quantitative PCR testing
is cumbersome, technically demanding, and highly dependent on internal
standards and on the personnel conducting these tests,2
such assays should especially not be used to guide clinical treatment
until they are standardized and proven to be acceptably reproducible,
specific, and sensitive.
The biological nature of MRD in hematologic malignancies is intriguing.
Residual disease is a dynamic process under the control of as yet
unidentified mechanisms and characterized by significant variability
among individuals. Furthermore, fluctuations in the level of measurable
MRD do exist and, despite the technical pitfalls associated with PCR,
such fluctuations have to be followed serially and quantitatively to
allow any meaningful interpretation.3 However, does the
lack of detection of BCR-ABL transcripts in a patient with MRD
in CML mean that the patient is cured, or conversely, does a
BCR-ABL-positive PCR assay herald imminent relapse? The answer
is most likely no to both questions.1,3,4 Kurzrock et
al,5 for example, found that BCR-ABL transcripts
were not detected by reverse transcriptase-PCR (RT-PCR) in
10 of 18 patients treated with interferon- and in complete
cytogenetic remission. Does PCR negativity correlate with improved
survival? The answer is unclear. Talpaz et al4 analyzed
patients with CML in complete cytogenetic remission who were being
treated with interferon- and were also BCR-ABL negative by
RT-PCR. Using clonogenic assays combined with RT-PCR, Talpaz et
al4 could still identify individual colonies that expressed
BCR-ABL fusion transcripts. More recently, at the American
Society of Hematology meeting in December 1998, Talpaz presented an
update of these 7 patients with CML in whom long-term interferon-
treatment was discontinued and who were in unmaintained complete
cytogenetic remission at a median follow-up duration of 10 years
(range, 7 to 12 years). Four of the patients had detectable residual
disease (30, 387, 73, and 20 BCR-ABL transcripts/µg protein,
respectively). Other investigators reported similar
observations.6,7 Thus, patients can apparently remain in
long-term remission or probably even be cured and still express high
levels of BCR-ABL transcripts (severalfold higher than those
reported by Biernaux et al8 in healthy individuals).
Can a threshold of residual disease detected by PCR be defined above
which relapse appears likely? Cross et al9 analyzed 91 patients by qualitative PCR. Twenty-eight patients with a positive PCR
result were further studied by competitive PCR for quantitation of the
BCR-ABL transcript. Among the 17 patients without cytogenetic relapse, BCR-ABL transcript levels ranged from 10 to 800 µg/mL. Among the 11 patients who relapsed, transcript levels ranged
from 1,600 to 700,000 µg/mL, although these levels were determined at
the time of cytogenetic relapse. In another study, Hochhaus et
al10 quantified residual BCR-ABL transcripts by
RT-PCR in 31 patients with CML who had achieved a complete cytogenetic
remission after interferon- therapy. BCR-ABL transcripts
were detected in all patients with BCR-ABL transcript levels
ranging from less than 10 to greater than 14,000/µg RNA. Moreover,
BCR-ABL/ABL ratios for all patients ranged from 0.00075% to
3.6%, with a median of 0.024%. Among the 6 patients who later had a
relapse (25 patients with residual disease did not have a relapse),
BCR-ABL/ABL ratios were greater than 0.024%. Whether a similar
threshold can be confirmed in other studies remains to be determined.
Is there any evidence that early intervention based on PCR monitoring
is beneficial? Studies by van Rhee et al11 and Raanani et
al12 suggest so, because both reported that molecular
monitoring identified early relapse and that therapeutic intervention
before the onset of hematologic relapse was associated with an
increased probability of a response to donor lymphocyte infusions.
However, both studies can be criticized, because they were not
randomized, had small patient numbers, and did not clearly define early
relapse. In fact, patients they considered to be in early relapse were, for the most part, patients in cytogenetic relapse, and only 2 in each
study were in molecular relapse. Therefore, the studies by van Rhee et
al11 and Raanani et al12 provide no evidence that initiation of therapy at molecular relapse is any better than
initiation of therapy at cytogenetic relapse. We are still left with a
lack of data as to the usefulness of therapy that is based on positive
PCR results.
In conclusion, we again agree that PCR studies aimed at detecting,
quantifying, and characterizing MRD have contributed valuable information to our understanding of MRD in hematologic malignancies. The presence of BCR-ABL transcripts in patients who are
probably cured is undoubtedly interesting. If these transcripts
represent residual disease, then the identification of the mechanism
responsible for its suppression is very important. However, we again
assert that, thus far, it has not been established that detection of MRD in CML by PCR can be used clinically. Until PCR testing has been
standardized and proven reliable and until randomized studies have
proven that treating molecular relapse is indeed beneficial for
patients with CML, PCR-based results should be interpreted only within
the confines of well-defined clinical trials. Consequently, any
generalized guidelines for using PCR tests to guide treatment appear
premature and should be viewed critically.
Stefan Faderl
Moshe Talpaz
Hagop M. Kantarjian
Zeev Estrov
Departments
of Leukemia and Bioimmunotherapy
The University of Texas M.D.
Anderson Cancer Center
Houston, TX
| |
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