PCR approaches to distinguish between the physiologic background of WT1 and its “bona fide” over-expression in leukemic blasts. Whereas the role of FLT3 mutations in MRD monitoring is still unproven as a consequence of its instability at diagnosis and relapse

Transl Pediatr 2013;2(1):43-45 www.thetp.org In acute myeloid leukemia (AML), achievement of morphologic complete remission (RC) still remains the “gold standard” to make a judgment on treatment response and yet hematologists wonder about its utility in the era of minimal residual disease (MRD) detection. In fact, whatever the techniques used, a number of studies has convincingly demonstrated that the persistence in the bone marrow of leukemic cells below the threshold of conventional morphology MRD identifies patients at a significantly higher risk of relapse (1). Owing to the superior potential for sensitivity, flow cytometry and polymerase chain reaction (PCR) are the most favored methods to track residual leukemic cells that survived cytotoxic chemotherapy. Although using different timepoints and quantitative thresholds, several studies from different worldwide groups came to the converging conclusion that MRD detection by flow cytometry does have a prognostic role in AML, especially in term of relapse prediction (2-4). On the other hand, more controversial data have been generated about the role of PCR. Qualitative reverse trascriptase-PCR (RT-PCR) in core binding factor positive AML (AML carrying RUNX1-RUNX1T1 formerly AML1-ETO or CBFB-MYH11 transcript) has a limited clinical applicability since a persistent positivity has been observed in long survivors even after allogeneic stem cell transplantation. Using quantitative RT-PCR, Corbacioglu et al. (5) identified clinically relevant time-points at which persistence of CBFB-MYH11 transcript positivity was associated with a significantly increased risk of relapse, suggesting that this technique should be preferred over qualitative RT-PCR. In a recent publication of the National Cancer Research Institute (formerly United Kingdom Medical Research Council) (6), the role of quantitative RT-PCR in MRD monitoring was confirmed not only for CBFB-MYH11 positive AML but also for cases expressing the RUNX1-RUNX1T1 fusion gene. The role of WT1 over-expression as a universal marker for MRD detection in AML still await a conclusive validation. On the behalf of LeukemiaNet, Cilloni et al. (7) have made available a common standard protocol to measure WT1 copy number with MRD purpose. However, they reported that only in 46% and 13% of peripheral blood and bone marrow samples, respectively, were the levels of WT1 sufficiently over-expressed, compared with normal samples, to allow MRD determination and risk-assessment. Therefore, concerns still remain about the power of the present PCR approaches to distinguish between the physiologic background of WT1 and its “bona fide” over-expression in leukemic blasts. Whereas the role of FLT3 mutations in MRD monitoring is still unproven as a consequence of its instability at diagnosis and relapse (8), Schnittger et al. (9) have demonstrated that mutations of NPM1 are very stable at relapse and that the persistence of a mutated signature at different time-points after achievement of morphologic CR significantly predicts disease recurrence. One of the key-point in the MRD scenario is that nothing is really known of the relation between the two main opponents: flow versus PCR. What is the relation between MRD results obtained by flow and those by PCR? Answering this question is central to further developments of our skills in MRD surveillance. In their manuscript, Inaba et al. (10) tried to address exactly this issue by examining the relation Perspective