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PLoS Med. 2006 Jul;3(7):e270.

MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia.

Pikman Y, Lee BH, Mercher T, McDowell E, Ebert BL, Gozo M, Cuker A, Wernig G, Moore S, Galinsky I, DeAngelo DJ, Clark JJ, Lee SJ, Golub TR, Wadleigh M, Gilliland DG, Levine RL.

Source

Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, United States of America.

Abstract

BACKGROUND:

The JAK2V617F allele has recently been identified in patients with polycythemia vera (PV), essential thrombocytosis (ET), and myelofibrosis with myeloid metaplasia (MF). Subsequent analysis has shown that constitutive activation of the JAK-STAT signal transduction pathway is an important pathogenetic event in these patients, and that enzymatic inhibition of JAK2V617F may be of therapeutic benefit in this context. However, a significant proportion of patients with ET or MF are JAK2V617F-negative. We hypothesized that activation of the JAK-STAT pathway might also occur as a consequence of activating mutations in certain hematopoietic-specific cytokine receptors, including the erythropoietin receptor (EPOR), the thrombopoietin receptor (MPL), or the granulocyte-colony stimulating factor receptor (GCSFR).

METHODS AND FINDINGS:

DNA sequence analysis of the exons encoding the transmembrane and juxtamembrane domains of EPOR, MPL, and GCSFR, and comparison with germline DNA derived from buccal swabs, identified a somatic activating mutation in the transmembrane domain of MPL (W515L) in 9% (4/45) of JAKV617F-negative MF. Expression of MPLW515L in 32D, UT7, or Ba/F3 cells conferred cytokine-independent growth and thrombopoietin hypersensitivity, and resulted in constitutive phosphorylation of JAK2, STAT3, STAT5, AKT, and ERK. Furthermore, a small molecule JAK kinase inhibitor inhibited MPLW515L-mediated proliferation and JAK-STAT signaling in vitro. In a murine bone marrow transplant assay, expression of MPLW515L, but not wild-type MPL, resulted in a fully penetrant myeloproliferative disorder characterized by marked thrombocytosis (Plt count 1.9-4.0 x 10(12)/L), marked splenomegaly due to extramedullary hematopoiesis, and increased reticulin fibrosis.

CONCLUSIONS:

Activation of JAK-STAT signaling via MPLW515L is an important pathogenetic event in patients with JAK2V617F-negative MF. The bone marrow transplant model of MPLW515L-mediated myeloproliferative disorders (MPD) exhibits certain features of human MF, including extramedullary hematopoiesis, splenomegaly, and megakaryocytic proliferation. Further analysis of positive and negative regulators of the JAK-STAT pathway is warranted in JAK2V617F-negative MPD.

PMID:: 16834459; [PubMed - indexed for MEDLINE]
PMCID:: PMC1502153

Free PMC Article

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Figure 4

MPLW515L and MPLWT Bone Marrow Transplant Model Histopathology

Histology of MPLW515L-transduced and MPLWT-transduced Balb/C mice showing images of peripheral blood (A and B) and histopathology in representative sections of bone marrow (C and D), spleen (E–H), and liver (I–L). Peripheral blood smear (B) (600×, Wright-Giemsa) of a representative MPLWT animal displays an unremarkable white blood cell and platelet count. In contrast, peripheral blood smear (A) (600×, Wright-Giemsa) of a representative MPLW515L mutant animal reveals marked thrombocytosis and leukocytosis comprising a predominant population of maturing myeloid cells as well as frequent nucleated erythroid forms. Bone marrow images from MPLWT animals display preserved marrow architecture with maturing trilineage hematopoiesis (D) (600×, hematoxylin and eosin [H&E]). Comparatively, bone marrow sections from MPLW515L mutant animals demonstrate marrow elements comprising a prominent population of maturing myeloid cells with increased numbers of megakaryocytes including atypical and dysplastic forms occurring in frequent clusters (C) (600×, H&E) and showing emperipolesis of neutrophils in megakaryocyte cytoplasm. Spleen sections from MPLW515L mice display complete effacement of normal splenic architecture (E) (40×, H&E) with a marked expansion of red pulp that is composed of an admixture of maturing myeloid and erythroid elements and numerous numbers of atypical megakaryocytes (G) (600×, H&E) compared with MPLWT spleens (F and H) (40× and 600×, H&E), which display a relative preservation of normal spleen architecture and the presence of only maturing erythroid forms in the red pulp. Liver images from MPLW515L mice illustrate evidence of extensive extramedullary hematopoiesis in a perivascular and sinusoidal distribution (I) (100×, H&E) composed predominantly of a population of maturing erythroid elements with frequent large atypical megakaryocytes and occasional admixed myeloid forms (K) (600×, H&E). In comparison, only small, focal areas of nucleated erythroid cells were observed in livers from MPLWT animals (J and L) (100× and 600×, H&E).

MPLW515L Is a Novel Somatic Activating Mutation in Myelofibrosis with Myeloid Metaplasia

PLoS Med. 2006 July;3(7):e270.