Molecular monitoring of chronic myeloid leukemia patients using robust BCR-ABL1 tests standardized to the International Scale (IS) is key to proper disease management, especially when treatment cessation is considered. Most laboratories currently use a time-consuming sample exchange process with reference laboratories for IS calibration. A World Health Organization (WHO) BCR-ABL1 reference panel was developed (MR1–MR4), but access to the material is limited. In this study, we describe the development of the first cell-based secondary reference panel that is traceable to and faithfully replicates the WHO panel, with an additional MR4.5 level. The secondary panel was calibrated to IS using digital PCR with ABL1, BCR and GUSB as reference genes and evaluated by 44 laboratories worldwide. Interestingly, we found that >40% of BCR-ABL1 assays showed signs of inadequate optimization such as poor linearity and suboptimal PCR efficiency. Nonetheless, when optimized sample inputs were used, >60% demonstrated satisfactory IS accuracy, precision and/or MR4.5 sensitivity, and 58% obtained IS conversion factors from the secondary reference concordant with their current values. Correlation analysis indicated no significant alterations in %BCR-ABL1 results caused by different assay configurations. More assays achieved good precision and/or sensitivity than IS accuracy, indicating the need for better IS calibration mechanisms.

Objective Molecular monitoring of treatment response in patients with chronic myelogenous leukemia is performed using the Europe Against Cancer (EAC) qPCR assay using the International Scale (IS). The assay amplifies both e13a2 and e14a2 BCR‐ABL1 transcript variants. Observing distinct variant‐dependent amplification curves during qPCR, we aimed to determine if this affected quantitation of BCR‐ABL1. Methods We investigated the qPCR efficiency at three Danish diagnostic centers (Zealand University Hospital [ZUH], Aarhus University Hospital [AU], and Rigshospitalet [RH]) on cell lines expressing either the e13a2 or e14a2 BCR‐ABL1 transcript variants and compared %IS values from 219 chronic myeloid leukemia patients from the centers with either the e13a2 (n = 113) or e14a2 (n = 106) transcript variants obtained by qPCR with absolute quantitation by droplet digital PCR (ddPCR). Results Although no significant differences were found in amplification efficiencies of the transcript variants, Bland‐Altman analysis of qPCR vs ddPCR values for patient samples revealed a significant average difference in the bias between variants (e3a2/e14a2) of 4.6‐, 6.5‐, and 1.8‐fold for ZUH, AU, and RH, respectively. Furthermore, qPCR %IS values of diagnostic patient samples revealed a significant 4.7‐fold difference between the e13a2 and e14a2 variants. Conclusion Our findings suggest that the EAC qPCR assay may underestimate the e14a2 variant compared to the e13a2 variant.

Point mutations of bcr-abl tyrosine kinase are the most frequent causes of imatinib resistance in chronic myeloid leukaemia (CML) patients. In most CML cases with BCR-ABL mutations leading to imatinib resistance the second generation of tyrosine kinase inhibitors (TKI- e.g. nilotinib or dasatinib) may be effective. Here, we report a case of a CML patient who during imatinib treatment did not obtain clinical and cytogenetic response within 12 months of therapy. The sequencing of BCR-ABL kinase domains was performed and revealed the presence of a F359I point mutation (TTC-to-ATC nucleotide change leading to Phe-to-Ile amino acid substitution). After 1 month of nilotinib therapy a rapid progression of clinical symptoms was observed. In the presence of the F359I point mutation only dasatinib treatment overcame imatinib and nilotinib resistance.

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In our opinion, a detailed evaluation and appropriate interpretation of clinical and laboratory data in such a category of patients seem to be extremely important, especially when a decision about the TKI change due to therapy failure is considered.