Minimal Residual Disease Detection at RNA and Leukemic Stem Cell (LSC) Levels: Comparison of RT-qPCR, d-PCR and CD26+Stem Cell Measurements in Chronic Myeloid Leukemia (CML) Patients in Deep Molecular Response (DMR)

Simple Summary The monitoring of the minimal residual disease (MRD) in Chronic Myeloid Leukemia is a key element in the management of affected patients because it reflects the response to therapy and allows for the selection of the best responders who may benefit from the suspension of the therapy. The gold-standard method of MRD monitoring is the quantification of the BCR::ABL1 transcript, the hallmark of the disease, by RT-qPCR. Considering that almost half of the patients who discontinue the treatment experience a molecular relapse, the identification of new approaches for the improvement of the selection of the best-responding CML patients is needed. In the present pilot study, we compared the gold standard with two additional MRD techniques: the quantification of the BCR::ABL1 transcript by digital PCR and the quantification of leukemic stem cells by flowcytometry. In fact, this cell population is one of the factors driving relapses. Although no linear regression was observed, a correlation between the lowest levels obtained using the three methods was noted. To the best of our knowledge, this is the first time that these methods have been compared in the CML setting. A Deep Molecular Response (DMR), defined as a BCR::ABL1 transcript at levels <= 0.01% by RT-qPCR, is the prerequisite for the successful interruption of treatment among patients with Chronic Myeloid Leukemia (CML). However, approximately 50% of patients in Treatment-Free Remission (TFR) studies had to resume therapy after their BCR::ABL1 transcript levels rose above the 0.1% threshold. To improve transcript detection sensitivity and accuracy, transcript levels can be analyzed using digital PCR (dPCR). dPCR increases BCR::ABL1 transcript detection sensitivity 10-100 fold; however, its ability to better select successful TFR patients remains unclear. Beyond the role of the immune system, relapses may be due to the presence of residual leukemic stem cells (LSCs) that are transcriptionally silent. Flow cytometry can be used to identify and quantify circulating bone marrow Ph+ LSCs CD34+/CD38- co-expressing CD26 (dipeptidylpeptidase-IV). To date, the significance of circulating Ph+ LSCs in TFR is unclear. The aim of this work is to compare and examine the values obtained using the three different methods of detecting minimal residual disease (MRD) in CML at RNA (RT-qPCR and dPCR) and LSC (flowcytometry) levels among patients in TFR or exhibiting a DMR. The twenty-seven patients enrolled received treatment with either imatinib (12), dasatinib (6), nilotinib (7), bosutinib (1), or interferon (1). Twelve patients were in TFR, while the rest exhibited a DMR. The TFR patients had stopped therapy for less than 1 year (3), <3 years (2), 6 years (6), and 17 years (1). Blood samples were collected and tested using the three methods at the same time. Both d-PCR and LSCs showed higher sensitivity than RT-qPCR, exhibiting positive results in samples that were undetectable using RT-qPCR (17/27). None of the patients tested negative with d-PCR; however, 23/27 were under the threshold of 0.468 copies/mu L, corresponding to a stable DMR. The results were divided into quartiles, and the lowest quartiles defined the lowest MRD. These data were strongly correlated in 15/27 patients, corresponding to almost half of the TFR patients. Indeed, the TFR patients, some lasting up to 17 years, corresponded to the lowest detectable DMR categories. To the best of our knowledge, this is the first attempt to analyze and compare DMRs in a CML population using standard (RT-qPCR) and highly sensitive (dPCR and LSCs) methods.