Monitoring of minimal residual disease by quantitative WT1 gene expression following reduced intensity conditioning allogeneic stem cell transplantation in acute myeloid leukemia

WT1 is well-known to be a panleukemic marker that is expressed in 90% of acute myeloid leukemias (AML). Quantification of WT gene expression in bone marrow (BM) samples may be useful as a marker of minimal residual disease (MRD) during and after treatment for early prediction of relapse. We evaluated the validity of this AML-MRD marker after reduced intensity conditioning (RIC) allogeneic stem cell transplantation (SCT). The quantitative assessment of WT1 expression by real-time quantitative PCR (RQ-PCR) was measured in 25 patients (pts) with AML at diagnosis, at the time of RIC-SCT and after transplant at precise time points. All pts showed high WT1 levels at diagnosis with a mean of 4895 (SD 4462) and a median of 3679 (range 454-16 853) copies WT1/104 ABL. At transplant, 18/25 pts (72%) were in complete cytologic remission (CcR) and 7/25 (28%) had refractory AML. At the pre-SCT evaluation, BM samples from pts transplanted in CcR showed significantly lower WT1 expression levels compared to the samples from pts with refractory AML (p = 0.002). Median follow-up after RIC-SCT was 18 months (range 2-54). On 18 pts transplanted in CcR, those (17/18) who maintained CcR after RIC-SCT displayed a WT1 copy number persistently low during all the follow-up period. In patients who received RIC-SCT with active disease obtaining a sustained CcR after transplant (3/25), WT1 levels decreased to normal range in the first two months after RIC-SCT and remained low through the entire study period. All pts who relapsed after RIC-SCT (4/25) had a high WT1 copy number before the cytologic relapse. In 50% of these cases, an increase in WT1 expression was documented before molecular chimerism decreasing. With this experience, taking into account the limited number of pts, we confirmed a concordance between WT1 expression levels (measured by RQ-PCR at precise and sequential time points) and status of AML before and after RIC-SCT and we found a concordance between WT1 expression levels and hematopoietic chimerism status. Our data suggest that, in the RIC-SCT setting, the sequential and quantitative analysis of WT1 may be useful as a leukemia marker for monitoring MRD and as a predictor of overt AML cytologic relapse.