Detection of Relapse after Hematopoietic Stem Cell Transplantation in Childhood By Monitoring of Minimal Residual Disease and Chimerism

Measurement of minimal residual disease (MRD) and the analysis of chimerism after hematopoietic stem cell transplantation (HSCT) have been used for the detection of impending relapse. Here, we present the clinical relevance of both methods. We investigated MRD in 135 consecutive children with ALL (n=62), AML (n=42), MDS (n=24), or CML (n=7) who underwent HSCT. The Wilms’ tumor gene (WT1) expression was used for the detection of MRD because WT1 is overexpressed in the vast majority of patients with leukemia. For the present study, we investigated if the MRD status prior to HSCT has any prognostic value. We observed a significantly reduced cumulative incidence of relapse (7% versus 48%; p<0.001) and a significantly increased 5-year event-free survival (84% versus 35%; p<0.001) in patients who were MRD negative before transplantation. All 99 patients who were continuously MRD negative remained in complete remission at a median of 2432 days after HSCT. In contrast, all 34 patients who suffered from hematological relapse after a median of 167 days presented with high levels of WT1 gene expression (p<0.001). In 21 patients, we observed an increase of WT1 expression levels at a median of 31 days before hematological relapse. In 17 of these 21 patients, DNA was available at the time of increase of WT1 expression level to perform analysis of hematopoietic chimerism using a quantitative short-tandem-repeat PCR. Interestingly, only 9 of the 17 patients (53%) showed a mixed chimerism, whereas 8 of the 17 patients (47%) revealed a complete donor chimerism. In the remaining 13 patients with relapse, no increase of WT1 expression levels could be detected because the time interval between the last sample harvest and hematological relapse was to long (median, 71 days). In five of these patients, DNA was available at the time of the last sample harvest before relapse and showed a complete donor chimerism. In two patients, we diagnosed a molecular relapse using WT1 gene expression. At that time, both children revealed a complete donor chimerism. In both patients, long-term molecular remission was achieved by immunotherapy. In conclusion, quantitative analysis of WT1 gene expression is a valuable tool for monitoring of MRD before and after HSCT. This approach is very useful for early diagnosis and treatment of molecular relapse after HSCT. MRD measurement using WT1 gene expression is more sensitive for the detection of impending relapse than the analysis of chimerism.