A 58-year-old man with chronic myeloid leukemia (CML) in chronic phase is found to have a BCR-ABL1 transcript on initial diagnosis. After 2 years of imatinib therapy, he develops resistance. Which investigation is most appropriate to detect the mechanism of resistance and guide salvage therapy?
A. Conventional cytogenetics for Philadelphia chromosome
B. Bone marrow aspirate for cytomorphology and blast count
C. Reverse transcription PCR (RT-PCR) for BCR-ABL1 kinase domain mutations
D. Flow cytometry for blast percentage and immunophenotype
Explanation
Detecting Acquired Resistance in CML
Key Point
RT-PCR with sequencing of the BCR-ABL1 kinase domain is the gold standard for identifying point mutations that confer tyrosine kinase inhibitor (TKI) resistance, enabling selection of second-line TKIs.
Mechanism of TKI Resistance in CML
1.
BCR-ABL1 kinase domain mutations (~50% of resistance cases)
T315I (most common, confers resistance to imatinib, dasatinib, nilotinib)
Other mutations: M244V, G250E, F359V
Detected by RT-PCR + Sanger or NGS sequencing
2.
Non-mutational resistance (~50%)
BCR-ABL1 overexpression
Clonal evolution (loss of Ph chromosome, acquisition of secondary mutations)
Reduced drug bioavailability
Investigation Comparison
Table
Investigation
Detects
Clinical Use
RT-PCR + sequencing
BCR-ABL1 kinase mutations
Identifies specific mutation; guides TKI selection (ponatinib for T315I)
Flow cytometry
Blast percentage, immunophenotype
Detects blast crisis, not mutation-based resistance
Bone marrow aspirate
Morphology, blast count
Assesses disease phase; does not identify molecular mechanism
Conventional cytogenetics
Ph chromosome, clonal evolution
Detects loss of Ph or secondary chromosomal changes; not point mutations
High-YieldNEET PG
T315I mutation = "gatekeeper" mutation; confers pan-TKI resistance; only ponatinib is effective
Dasatinib/nilotinib resistance mutations = ponatinib or second-generation TKI with activity against specific mutation
RT-PCR is quantitative and can also monitor BCR-ABL1 transcript levels (>1% on international scale = warning for resistance)
Mnemonic: TIKI — T315I is the "gatekeeper" mutation in BCR-ABL1 kinase domain; ponatinib overcomes it.
Clinical Pearl
Patients with imatinib resistance should undergo RT-PCR sequencing BEFORE switching to second-line TKI. If T315I is present, ponatinib is the only approved option; other mutations may respond to dasatinib or nilotinib.
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