Architecture of the Stem Cell Compartment and Clonal Evolution in Myelodysplastic Syndromes

Background and Aim: Myelodysplastic syndromes (MDS) arehematopoietic stem cell (HSC) disorders of the elderly characterized byineffective hematopoiesis and a major risk of evolution to acute myeloidleukemia (AML). The WHO classification recognizes eight subgroups,based on 5q deletion, uni/multilineage dysplasia, ring sideroblasts orSF3B1 mutation, the percentage of blasts and. The phenotypic diversity ofMDS has driven the variegation of combined genetic events. To addressthe heterogeneity of clonal hematopoiesis at diagnosis and under diseasemodifyingtreatment, we investigated the architecture of hematopoieticstem/progenitor cell (HSPC) compartment.Methods: Mutation status of 40 genes were obtained in a cohort of 60non-del(5q) MDS by next-generation sequencing. The hematopoietichierarchy was analyzed by multiparametric flow cytometry. The clonalarchitecture was assessed by the genotyping of single CD34+CD38-cell-derived colonies, long-term colony-initiating cells (LTC-IC) andclonogenic progenitors. Disease-initiating cell was identified bygenotyping human cells engrafted in immunodeficient NSG mice after8–16 weeks. Clonal evolution of patients receiving lenalidomide was followed at evaluation after 4 cycles and further when they responded totreatment (GFM LenEpo2008, clinicaltrials.gov, EudraCT number 2008-008262-12).Results: In single HSPCs, the founding mutations usually affect epigeneticregulators (TET2, DNMT3A) or splicing factors (SF3B1, SRSF2). Mutationsaccumulate in a dominant subclone and generate few minor subclones,with a linear or branched architecture. The dominant clone is detected inthe LTC-IC compartment and repopulates the bone marrow of NSG micewith a myeloid and lymphoid progeny, showing that the cell of originis a genuine HSC. A refined flow cytometry analysis using a 14-colorpanel demonstrates that hematopoiesis in low-risk MDS is characterizedby a myeloid bias of multipotent progenitor (MPP) compartment atthe expense of pre-B progenitors. This results in the accumulation ofcommon myeloid (CMP) at the expense of granule-monocytic (GMP)and megakaryocytic-erythroid (MEP) progenitors, except in sideroblasticanemia where MEP are preserved. We noticed that mutation contentsmay differ along the hematopoietic hierarchy. For instance, STAG2mutations not detected in single HSCs, were present as neo-mutationsin MPP-like or GMP-like progenitors that drove the amplification of aclone dominant at the time of AML transformation. Finally, mutationsaccumulate with time along disease evolution, whatever the therapeuticstrategy. We found that lenalidomide by inducing an immune response,transiently reduced the size of the dominant clone in responding patientswhereas new mutations could emerge in non-responding patients.Conclusion: We identified MDS-initiating cells and clones driving theleukemic transformation. Mutation combination and selection duringevolution contribute to the heterogeneity of MDS. Defining the hierarchyof driver mutations provides insights into the process of transformation,and may guide targeted therapies.

کلیدواژه ها:

myelodysplastic syndrome ، hematopoietic stem cell ، hematopoiesis ، clonal architecture ، lenalidomide