Loss of CDKN1B induces an age-related clonal hematopoietic disorder, via Notch2 activity dysregulation

The tumor suppressor gene CDKN1B, encoding for the p27Kip1 protein, defines the smallest region of deletion on chromosome 12p13 described in Myelodysplastic syndromes (MDS). In mice, Cdkn1B gene has been associated to the regulation of hematopoietic progenitors and lymphoid organs proliferation. Long-term effects of Cdkn1B absence on hematopoiesis have never been investigated. MDS are age-related, clonal hematopoietic disorders characterized by defective maturation of blood cells, recurrent genetic abnormalities and high risk of acute myeloid leukemia (AML) evolution.Methods. We studied one-year old (1Y) Cdkn1B wild type (WT) and knock out (KO) mice using biochemical and pathological approaches to identify the molecular basis of their clonal hematopoietic disorders. Molecular biology approaches were used to define the role of p27Kip1 protein in regulating Notch2 signaling. Human MDS samples were analyzed using NGS, droplet digital PCR and immunohistochemistry to evaluate mutation and copy number alteration in CDKN1B gene and alterations in p27Kip1 protein expressionResults. Cdkn1B KO animals develop an age-dependent progressive clonal hematopoietic disorders associated with profound architectural subversion of the spleen, reduction of B-cell follicles in the white pulp and expansion of the red pulp, infiltrated by immature cells. This phenotype was likely due to the ability of p27Kip1 to regulate the transcriptional activity of Notch2, a critical regulator of B-cell differentiation in the marginal zone of the spleen. Accordingly, spleen from KO mice displayed a reduced expression of Notch2 targets, associated with altered proliferation of hematopoietic progenitor cells.In human age-dependent clonal hematopoietic disorders like MDS and AML, over 40% of patients presented a p27Kip1 alteration, due to alterations in CDKN1B gene and/or decreased protein expression. Interestingly, 70% of patients acquired a CDKN1B alteration during the course of disease and presence of these alterations was associated with more frequent conversion of MDS to AML.Conclusion. We identify p27Kip1 as an important regulator of normal hematopoiesis and Notch2 activity in mice, showing high significance in age-related clonal hematopoietic disorders such as MDS and AML.