Bone marrow fibrosis in myeloproliferative neoplasms-associated myelofibrosis: deconstructing a myth?

Bone marrow fibrosis in primary myelofibrosis: pathogenic mechanisms and the role of TGF-β

Primary myelofibrosis (PMF) is a Philadelphia chromosome negative myeloproliferative neoplasm (MPN) with adverse prognosis and is associated with bone marrow fibrosis and extramedullary hematopoiesis. Even though the discovery of the Janus kinase 2 (JAK2), thrombopoietin receptor (MPL) and calreticulin (CALR) mutations have brought new insights into the complex pathogenesis of MPNs, the etiology of fibrosis is not well understood. Furthermore, since JAK2 inhibitors do not lead to reversal of fibrosis further understanding of the biology of fibrotic process is needed for future therapeutic discovery. Transforming growth factor beta (TGF-β) is implicated as an important cytokine in pathogenesis of bone marrow fibrosis. Various mouse models have been developed and have established the role of TGF-β in the pathogenesis of fibrosis. Understanding the molecular alterations that lead to TGF-β mediated effects on bone marrow microenvironment can uncover newer therapeutic targets against myelofibrosis. Inhibition of the TGF-β pathway in conjunction with other therapies might prove useful in the reversal of bone marrow fibrosis in PMF.

Impact of bone marrow pathology on the clinical management of Philadelphia chromosome-negative myeloproliferative neoplasms

Philadelphia chromosome-negative myeloproliferative neoplasms include primary myelofibrosis (PMF), polycythemia vera (PV), and essential thrombocythemia (ET). Although these 3 entities share many pathogenic characteristics, such as dysregulated Janus kinase (JAK)/signal transducer and activator of transcription signaling, they differ substantially regarding prognosis, progression to myelofibrosis (MF), risk of leukemic transformation, and specific medical needs. Accurate diagnosis and classification of myeloproliferative neoplasms are prerequisites for appropriate risk-based therapy and should be based on an integrated approach following the World Health Organization guidelines that, in addition to clinical, molecular, and cytogenetic evaluation, includes the examination of bone marrow morphology. Reticulin fibrosis at presentation in ET and PV is associated with increased risk of myelofibrotic transformation, and higher fibrosis grade in patients with MF is associated with worse prognosis. Additional assessment of collagen deposition and osteosclerosis may further increase diagnostic and prognostic precision. Moreover, the evaluation of bone marrow pathology has become very important in the new era of disease-modifying agents. In randomized controlled phase 3 studies, the JAK1/JAK2 inhibitor ruxolitinib provided rapid and lasting improvement in MF-related splenomegaly and symptom burden as well as a survival advantage compared with placebo or best available therapy. Follow-up for up to 5 years of patients who participated in a phase 1/2 study of ruxolitinib, revealed stabilization or reversal of bone marrow fibrosis in a proportion of patients with MF. Combinations of JAK inhibitors with other therapies, including agents with antifibrotic and/or anti-inflammatory properties, may possibly decrease bone marrow fibrosis further and favorably influence clinical outcomes.

JAK2 mutants (e.g., JAK2V617F) and their importance as drug targets in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) mutant V617F and other JAK mutants are found in patients with myeloproliferative neoplasms and leukemias. Due to their involvement in neoplasia and inflammatory disorders, Janus kinases are promising targets for kinase inhibitor therapy. Several small-molecule compounds are evaluated in clinical trials for myelofibrosis, and ruxolitinib (INCB018424, Jakafi®) was the first Janus kinase inhibitor to receive clinical approval. In this review we provide an overview of JAK2V617F signaling and its inhibition by small-molecule kinase inhibitors. In addition, myeloproliferative neoplasms are discussed regarding the role of JAK2V617F and other mutant proteins of possible relevance. We further give an overview about treatment options with special emphasis on possible combination therapies.