HSC Hierarchy in Primary Myelofibrosis

Primary Myelofibrosis (PMF) is a Myeloproliferating Neoplasm (MPN) of hematopoietic stem cell origin, characterized by expansion of aberrant myeloid progenitors in the chronic phase that can lead to bone marrow fibrosis development and/or osteosclerosis. In 20-25% of PMF cases transformation to acute myeloid leukemia (AML) is observed. Identification of multiple molecular lesions suggests complex clone dynamics that indicates exceeding sub-clone dominance as PMF progresses. Our aim is to determine the HSC hierarchy orchestrating initiation and development of PMF.

In our previous studies we reported a CD133+ HSC population in PMF peripheral blood that represents the aberrant long-term stem cell fraction responsible for PMF reproduction of chronic and acute phases in vivo. Molecular analysis of PMF xenografts indicates sustenance of genetically different HSC clones exhibiting variation in both their engraftment potential and reproduction of PMF parameters in the first mouse model of the disease.

To further characterize the succession of molecular lesions determining HSC clone propagation we performed targeted exon sequencing of PMF HSC from 100 PMF patients for 54 genes. Mutations in the epigenetic regulators ASXL1 and EZH2 were detected in 38% and 15% of PMF patients respectively. HSC clonal evolution was determined by single cell molecular and phenotypic analysis in vitro and graft analysis in vivo.

Mutations detected in the epigenetic regulators ASXL1 and EZH2 represent founding molecular lesions at the top of PMF HSC hierarchy. ASXL1 mutations precede and are connected with sustenance of clonal hematopoiesis without any significant influence on the HSC differentiation potential. EZH2 mutations are connected with impaired erythropoiesis in vitro and anemia, high engraftment and expansion of pre-leukemic clones in vivo. Occurrence of JAK2 and CALR mutations in the background of mutated epigenetic regulator genes is connected with expansion of HSC subclones and reproduction of chronic phase PMF as atypical myelopoiesis, splenomegaly and induction of fibrosis.

Our results indicate genetic heterogeneity of PMF neoplastic HSC is comprised of three different mutational clusters. Mutations in epigenetic regulator genes (Group 1) precede and shape the epigenetic landscape conferring genetic instability to sustain the expansion of pre-leukemic clones (Group 3 mutations). JAK2 and CALR mutations (Group 2) occur later on and are connected with aberrant myelopoiesis of chronic phase disease. HSC clonal dynamics reflect genotype related phenotypes as determinants of chronic and acute phases of PMF.