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

Abstract

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.