Molecular Responses in Chronic Myeloid Leukemia with Atypical BCR-ABL1 Transcripts: Is Digital PCR the Answer?

Stephen E. Langabeer1, PhD, FRCPath

1 Cancer Molecular Diagnostics, St. James’s Hospital, Dublin, Ireland.

Conflict-of-interest statement: The author(s) declare(s) that there is no conflict of interest regarding the publication of this paper.

Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http: //creativecommons.org/licenses/by-nc/4.0/

Correspondence to: Stephen E. Langabeer, Cancer Molecular Diagnostics, Trinity Translational Medicine Institute, St. James’s Hospital, Dublin, D08 W9RT, Ireland.
Email: slangabeer@stjames.ie
Telephone: +353 14162413

Received: October 13, 2020
Revised: November 10, 2020
Accepted: November 16, 2020
Published online: November 19, 2020


Serial monitoring of the level of BCR-ABL1 transcripts is an integral part of chronic myeloid leukemia (CML) management. However, assay standardization is lacking for those patients who express atypical BCR-ABL1 transcripts. Given the increasing survival number of CML patients and the option of treatment-free remission, quantitation of atypical transcripts by digital PCR is emerging as an attractive alternative to real-time quantitative PCR in such patients.

Key words: Chronic myeloid leukemia; BCR-ABL1; Molecular response; Atypical transcripts; Treatment-free remission

© 2020 The Authors. Published by ACT Publishing Group Ltd. All rights reserved.

Langabeer SE. Molecular Responses in Chronic Myeloid Leukemia with Atypical BCR-ABL1 Transcripts: Is Digital PCR the Answer? International Journal of Hematology Research 2020; 5(1): 191-192 Available from: URL: http://www.ghrnet.org/index.php/ijhr/article/view/3008


In the last twenty years, chronic myeloid leukemia (CML) has become the paradigm for a malignancy targeted by rationally-designed tyrosine kinase inhibitors (TKI): overall survival of patients is now similar to individuals without the disease[1]. Furthermore, adhering to judicious criteria, TKI may be discontinued in selected CML patients with approximately half achieving and maintaining a treatment-free remission (TFR)[2]. Monitoring expressed BCR-ABL1 fusion transcripts, the molecular hallmark of CML, by real-time quantitative PCR (RT-qPCR) has become an integral part of patient management with international guidelines setting molecular milestones for optimal therapeutic outcome. Considerable global efforts have resulted in the harmonization of individual laboratory RT-qPCR assays with the widespread ability to report BCR-ABL1 responses on a standardized international scale[3]. While most CML patients express e13a2 or e14a2 BCR-ABL1 transcripts, a minority of approximately 2% of patients express uncommon BCR-ABL1 transcripts, usually due to alternative splicing of BCR or ABL1 exons[4]. However, the aforementioned efforts for harmonising molecular responses in CML applies only to those patients expressing the common variants with little or no harmonization of BCR-ABL1 RT-qPCR assays for these atypical standards. There remains a growing and unmet need to detect deep molecular responses in a standardized fashion for those patients harbouring the atypical BCR-ABL1 transcript types.

A recent advance in this area is the increasing use of digital PCR (dPCR). Advantages of dPCR are reported to be an increase in the detection limit achieved by partitioning the sample in thousands of independent PCR reactions increasing the effective concentration, that dPCR is less prone to variations in amplification efficiency by measuring the end point of amplification, and the lack of requirement for a calibrated standard curve enabling absolute quantification of target molecules[5]. Several studies have shown the applicability of dPCR in measuring the common BCR-ABL1 transcripts in CML patients with a similar, if not an improved sensitivity over RT-qPCR, in measuring deep molecular responses and predicting TFR[6]. However, it is monitoring those patients expressing the rare, variant BCR-ABL1 transcript types in which dPCR may be the most beneficial due to the current lack of standardization. Proof of principle for this approach came by measuring e6a2 BCR-ABL1 transcripts in a patient with blast phase CML[7]. This has been followed by a more recent study showing the usefulness of digital PCR in following CML patients expressing e13a3, e14a3 and e19a2 BCR-ABL1 transcripts[8]. This observation has been successfully expanded to encompass BCR-ABL1 monitoring by dPCR during TFR attempts[9].

While traditional RT-qPCR remains an option for monitoring the rare BCR-ABL1 transcript types, the lack of methodological harmonization, the increasing number and overall survival of CML patients that require monitoring, and the possibility of attempting TFR, all contribute to the appeal and potential expanding role of a dPCR approach for monitoring CML patients with the atypical BCR-ABL1 transcript types in the future.


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