Quantitative Monitoring of T315I BCR-ABL Mutation by the Invader Assay.

Although imatinib mesylate has a highly inhibitory effect against BCR-ABL kinase, primary and secondary refractoriness have been observed. Point mutation within the kinase domain of BCR-ABL is one of the most important drug-resistant mechanisms. Of 25 reported point mutations, T315I BCR-ABL mutation might be dismal in clinical settings because it mediates clinical resistance to imatinib, nilotinib, and dasatinib, and the only established therapeutic option is hematopoietic stem cell transplantation at this moment. However, there is no information about the kinetics of this mutated clone. Here we developed quantitative Invader assay to monitor T315I BCR-ABL transcript. By using fluorescent resonance energy transfer system, the amount of released 5′ flap is measured as signal intensity, which enables us to calculate the percentage of T315I products with standard curve. Using this assay, we serially monitored T315I BCR-ABL transcript in a CML patient whose BCR-ABL transcript was still detectable (more than 100 copies/microgram RNA) 3 months after starting imatinib therapy. Although we have continued to monitor T315I BCR-ABL transcripts in 13 patients (chronic phase) up to 10 months, there was no patients who appear to be apparently resistant to imatinib due to a given T315I mutant, so far. In contrast, in a case of Ph+ ALL was being treated with chemotherapy including imatinib, we serially monitored both wild type and T315I BCR-ABL transcripts and observed the increased level of T315I transcript during relapse (0%at the time of diagnosis and 54.8% at relapse). Thus, our new strategy could be a useful tool to study the kinetics of the mutant clone and the pharmacokinetics of the drug resistant to T315I mutation.