First-hit SETBP1 mutations cause a myeloproliferative disorder with bone marrow fibrosis.

SETBP1 mutations are found in various clonal myeloid disorders. However, it is unclear whether they can initiate leukemia, as SETBP1 mutations typically appear as later events during oncogenesis. To answer this question, we generated a mouse model expressing mutated SETBP1 in hematopoietic tissue: this model showed profound alterations in the differentiation program of hematopoietic progenitors and developed a myeloid neoplasm with megakaryocytic dysplasia, splenomegaly, and bone marrow fibrosis, prompting us to investigate SETBP1 mutations in a cohort of 36 triple-negative primary myelofibrosis (TN-PMF) cases. We identified two distinct subgroups, one carrying SETBP1 mutations and the other completely devoid of somatic variants. Clinically, a striking difference in disease aggressiveness was noted, with SETBP1-mutated patients showing a much worse clinical course. As opposite to myelodysplastic/myeloproliferative neoplasms, where SETBP1 mutations are mostly found as a late clonal event, single-cell clonal hierarchy reconstruction in three TN-PMF patients from our cohort revealed SETBP1 to be a very early event, suggesting that the phenotype of the different SETBP1+ disorders may be shaped by the opposite hierarchy of the same clonal SETBP1 variants.