BCR-ABL1 fusion transcript is the minimal residual disease marker in chronic myeloid leukemia; 2% of patients show unusual breakpoints generating atypical transcripts, not quantifiable by standardized real-time PCR (RT-PCR). Response monitoring is performed by non-quantitative NESTED PCR, useless for evaluating patients' molecular remission, excluding them from treatment-free-remission protocols. Droplet digital PCR (ddPCR) is highly sensitive technology, allowing an absolute quantification independent of standard curves. Based on this, we have developed assays able to evaluate the molecular response in atypical patients. We designed new ddPCR-based molecular assays able to quantify atypical BCR-ABL1 transcripts, with a detection limit of 0.001%, validated in a cohort of 65 RNA from 11 patients. Fifty samples were identified congruently by ddPCR and NESTED PCR (40 positives and 10 negatives for atypical BCR-ABL1 transcript), while 11 positive samples were detected only by ddPCR. Our results highlight ddPCR usefulness, primarily when the BCR-ABL1/ABL1 level is less than 1.5% and NESTED PCR results are often inaccurate. Furthermore, we identified 3 patients who maintained a deep molecular response for at least one year, who could be considered good candidates for treatment-free remission approaches. Here, we describe a new promising molecular approach, highly sensitive, to monitor atypical BCR-ABL1 patients, paving the foundation to include them in treatment-free remission protocols.
Keywords: BCR–ABL1; MRD monitoring; atypical transcripts; chronic myeloid leukemia; digital PCR; treatment-free remission.