Identification and surveillance of rare relapse-initiating stem cells during complete remission post-transplantation.

Relapse following complete remission (CR) remains the main cause of mortality after allogeneic stem cell transplantation for hematological malignancies and therefore improved biomarkers for early prediction of relapse remains a critical goal towards development and assessment of preemptive relapse treatment. Since the significance of cancer stem cells as a source of relapses remains unclear, we investigated whether mutational screening for persistence of rare cancer stem cells would enhance measurable residual disease (MRD) and early relapse-prediction post-transplantation. In a retrospective study of relapse patients and continuous-CR patients with myelodysplastic syndromes and related myeloid malignancies, combined flow cytometric cell sorting and mutational screening for persistence of rare relapse-initiating stem cells was performed in bone marrow at multiple CR time points post-transplantation. In 25 CR samples from 15 patients that later relapsed, only 9 samples were MRD-positive in mononuclear cells (MNCs) whereas flowcytometric sorted hematopoietic stem and progenitor cells (HSPCs) were MRD-positive in all samples, and always with a higher variant allele frequency than in MNCs (mean 97-fold). MRD-positivity in HSPCs preceded MNCs in multiple sequential samples, in some cases preceding relapse by more than 2 years. In distinction, in 13 patients in long-term continuous-CR, HSPCs remained MRD-negative. Enhanced MRD-sensitivity was also observed in total CD34+ cells, but HSPCs were always more clonally involved (mean 8-fold).In conclusion, identification of relapse-initiating cancer stem cells and mutational MRD-screening for their persistence consistently enhances MRD-sensitivity and earlier prediction of relapse after allogeneic stem cell transplantation.