Quantitative monitoring of BCR/ABL1 mutants for surveillance of subclone-evolution, -expansion, and -depletion in chronic myeloid leukaemia

Background: In chronic myeloid leukaemia (CML), clonal evolution with resistance to tyrosine kinase inhibitors (TKIs) is often triggered by BCR/ABL1 mutations. However, in the context of the complex pro-oncogenic signalling networks which ultimately lead to clonal expansion and disease progression, the exact contribution of BCR/ABL1 mutants remains uncertain. Recent data indicate that detection of BCR/ABL1 mutant subclones does not permit prediction of their expansion dynamics and their potential to become drivers of resistant disease.

Methods: To determine the patterns of clonal evolution and the distinct proliferation kinetics of individual BCR/ABL1 mutants during treatment, we employed ligase-dependent polymerase chain reaction (LD-PCR) analysis for quantitative surveillance of CML subclones with various tyrosine kinase domain (TKD) mutations including M244V, L248V, G250E, E255K, T315I, F317L-A/G, M351T and F359V.

Findings: Inadequate treatment responses were observed in 27 of 100 patients investigated and 16 were found to bear one or more BCR/ABL1 TKD mutations in separate subclones. Rapid subclone expansion upon onset or switch of TKI treatment was common and sometimes preceded corresponding changes in BCR/ABL1 transcript levels. Mutant subclones were found to respond differentially and sometimes unexpectedly to various treatment modalities. Decline and persistent depletion of specific mutation-bearing subclones in response to treatment could be documented by LD-PCR surveillance.

Interpretation: The observations show that quantitative monitoring of mutant BCR/ABL1 subclones by LD-PCR is a powerful tool for detection of clonal evolution, subclone-expansion and subclone-depletion and can contribute to optimised management of patients with CML.