A novel complex mutation affecting two codons in the exon 10 hotspot region of the MPL gene

Philadelphia chromosome-negative (Ph−) classical myeloproliferative neoplasms (MPNs), including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), are clonal hematopoietic stem cell disorders characterized by aberrant proliferation of one or more myeloid lineages. The majority of patients harbor somatic driver mutations, and their identification represent one of the major diagnostic criteria for classical MPNs as defined by the World Health Organization (WHO) [1]. Indeed, molecularly, MPNs result from dysregulation of cytokine signaling pathways. One of the central axes involved is the thrombopoietin (TPO) signaling pathway through its receptor MPL (myeloproliferative leukemia virus proto-oncogene). Upon TPO binding, MPL activates Janus kinase 2 (JAK2), initiating a downstream signaling cascade that promotes the survival, proliferation, and differentiation of hematopoietic cells [2]. Among the three main driver genes (JAK2, CALR and MPL) implicated in MPNs, MPL is the less frequently mutated. Gain-of-function mutations in MPL, particularly within exon 10, are observed almost exclusively in patients with ET and PMF, accounting for approximately 4–8% of cases [2]. Among these, the most reported are missense mutations at codon 515, especially W515L and W515K, which lead to ligand-independent activation of the TPO-R and constitutive signaling through the JAK2/STAT pathway. Other MPL mutations such as W515A, W515R, W515S, and S505N have also been described, though less frequently [3]. Structural modeling has shown that residues from 513 to 517 form the transitional 'cap' at the beginning of the intracellular domain, marking the boundary between the transmembrane segment and the intracellular domain of the MPL receptor. Within this short segment, residue 515 and its neighboring amino acids constitute a well-recognized hotspot for gain-of-function mutations. Alterations at these positions can induce displacement of the transmembrane domain and trigger conformational changes in the adjacent intracellular region, ultimately leading to ligand-independent activation of MPL and its kinase partner, JAK2 [4].