PTPN1 Deficiency Accelerates the Progression to Myelofibrosis in JAK2V617F-Driven Myeloproliferative Neoplasm

The deletion of chromosome 20q (del20q) is frequently associated with myeloid malignancies including myeloproliferative neoplasms (MPN), myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). The del20q cytogenetic abnormality was observed most frequently in patients with myelofibrosis (MF). However, the key target genes within 20q involved in the pathogenesis of myelofibrosis has remained elusive. We previously reported that PTPN1 gene encoding protein tyrosine phosphatase non-receptor type 1 is a potential tumor suppressor within 20q in MPN/MF (Jobe et al. Leukemia 2017). The del20q involving PTPN1 deletion was more commonly found in MPN/MF in comparison to other myeloid neoplasms. We also observed significant co-occurrence of del20q involving PTPN1 deletion with JAK2V617F mutation in patients with MF. So, we hypothesize that PTPN1 deficiency may cooperate with JAK2V617F in the pathogenesis of MF.

In this study, we investigated the effects of heterozygous and homozygous PTPN1 deletion in JAK2V617F-induced MPN using a conditional PTPN1 knockout allele and a JAK2V617F knock-in mouse model of MPN. We previously reported the generation of a conditional JAK2V617F knock-in mouse. Mice expressing heterozygous JAK2V617F (JAK2^VF/+) exhibit a PV-like MPN characterized by increase in red blood cells (RBC), white blood cells (WBC), neutrophils and platelets in the peripheral blood. Heterozygous deletion of PTPN1 significantly increased WBC, neutrophil and platelet counts but reduced RBC and hemoglobin levels in JAK2^VF/+ mice. Flow cytometric analysis showed that heterozygous deletion of PTPN1 significantly increased myeloid (Gr-1⁺Mac-1⁺) and megakaryocytic (CD41⁺CD61⁺) precursors in the BM and spleens of PTPN1-deficient JAK2^VF/+ mice compared with JAK2^VF/+ mice. Additionally, deletion of PTPN1 significantly increased hematopoietic stem cells and myeloid progenitors in the BM and spleens of PTPN1-deficient JAK2^VF/+ mice compared to JAK2^VF/+ mice. Spleen weight was significantly increased in JAK2^VF/+ mice, while deletion of PTPN1 further enhanced splenomegaly in PTPN1-deficient JAK2^VF/+ mice. Furthermore, deletion of PTPN1 significantly enhanced bone marrow fibrosis in JAK2^VF/+mice. Homozygous deletion of PTPN1 resulted in more robust and accelerated development of lethal myelofibrosis in JAK2^VF/+ mice. The cooperative effects of JAK2V617F and PTPN1 deficiency in myelofibrotic transformation is associated with increased production of proinflammatory cytokines including IL-1beta, IL-6 and TGF-beta. We also observed enhanced activation of STAT1, STAT3, ERK1/2 and NF-kB in PTPN1-deficient JAK2^VF/+ mice bone marrow compared with JAK2^VF/+ or WT mice bone marrow. CRISPR/Cas9-mediated deletion of PTPN1 also resulted in enhanced activation of these signaling molecules and increased proliferation in JAK2V617F-positive mouse and human hematopoietic cells. Furthermore, deletion of PTPN1 significantly reduced ruxolitinib sensitivity against JAK2V617F-positive hematopoietic cells, suggesting that del20q involving PTPN1 deletion may cause ruxolitinib resistance in MPN. Overall, this study demonstrates that PTPN1 is an important target in del20q-associated MPN, and deficiency of PTPN1 collaborates with JAK2V617F in the progression to myelofibrosis.

Mohi:Erasca Inc.: Research Funding.