Deletion of PTPN1 Promotes the Development of Myelofibrosis

Deletion of chromosome 20q [del(20q)] is a common chromosomal abnormality associated with myeloid neoplasms including myeloproliferative neoplasms (MPN), myelodysplastic syndrome (MDS), MDS/MPN overlap disorders and acute myeloid leukemia (AML). The del(20q) lesion is often associated with myeloproliferative features; it is present in patients with myelofibrosis (MF) at a high frequency (24%) and thus considered to be one of the most frequent cytogenetic abnormalities in MF (Wassie et al., Br J Haematol. 2015). The del(20q) lesion can also coexist with JAK2V617F mutation in MPN/MF. However, the target tumor-suppressor gene(s) within chromosome 20q involved in the pathogenesis of MF remains unknown.

The PTPN1 locus maps to human chromosome 20q13.1-q13.2. PTPN1 (also known as PTP1B) is a ubiquitously expressed non-receptor tyrosine phosphatase that has been linked to metabolism and cancer. Mice deficient in Ptpn1 exhibit resistance to diet-induced obesity and diabetes. Both oncogenic and tumor suppressor functions for PTPN1 have been suggested. PTPN1 can negatively regulate the JAK/STAT signaling, which is frequently found activated in MPN.

Here, we report the identification and functional consequences of PTPN1 deletion in the pathogenesis of MF. Deletion of PTPN1 was identified in 14% cases of MF. Conditional deletion of Ptpn1 in the mouse hematopoietic compartment resulted in significant increases in white blood cell and neutrophil counts in the peripheral blood and enlargement of spleen size. Flow cytometric analyses showed significant expansion of myeloid (Gr-1⁺/Mac-1⁺) precursors in the bone marrow (BM) and spleens of Ptpn1-deleted mice compared with control animals. Megakaryocytic (CD41⁺/CD61⁺) precursors were also significantly increased in the spleens of Ptpn1-deleted mice. Flow cytometric analyses also revealed significant increases in absolute numbers of LSK cells (Lin^-Sca1⁺c-kit⁺) and its subsets including long-term hematopoietic stem cells (LT-HSC), short-term HSC (ST-HSC) and multi-potent progenitors (MPP) in the spleens of Ptpn1-deleted mice. Hematopoietic progenitor colony assays showed significant increases in myeloid (CFU-GM) and megakaryocytic (CFU-Mk) colonies in the BM of Ptpn1-deleted mice compared with control mice BM. Histopathologic analysis demonstrated fibrosis (grade 2) in the BM and spleens of Ptpn1-deleted mice at 52 weeks after induction, whereas control animals did not exhibit fibrosis at that age. Together, these results suggest that deletion of Ptpn1 induces an MPN-like phenotype, which progresses to MF over time. Moreover, transplantation of Ptpn1-deficient BM into lethally irradiated wild-type animals resulted in fibrosis at 18 weeks after transplantation, demonstrating that the effect of Ptpn1 loss in the development of myelofibrosis is cell-intrinsic. Competitive repopulation assays using BM from control or Ptpn1-deficient (CD45.2⁺) mice with wild type congenic (CD45.1⁺) mice showed that deletion of Ptpn1 enhances the repopulation capacity of hematopoietic stem cells. Biochemical analyses revealed that depletion of Ptpn1 enhanced JAK2/STAT5, AKT and ERK signaling in the BM of Ptpn1-deleted mice. Furthermore, we observed that deletion of Ptpn1 in Jak2V617F knock-in mice accelerates the development of myelofibrosis. In conclusion, our results establish a tumor-suppressor function for PTPN1 in MF.