Another patient with stroke and a pulmonary embolism was heterozygous for the p.Pro106Leu variant of the MPL gene, which has been associated with thrombocytosis.
The recently characterized protein-protein interaction of CALR mutants and MPL receptor has advanced our knowledge on the functional role of CALR mutants in thrombocythemia but it has also uncovered limitations of the current established research models.
The thrombopoietin receptor (MPL) has been shown to be mutated (MPLW515L) in myelofibrosis and thrombocytosis yet new approaches to treat this disorder are still required.
The identification of somatic calreticulin (CALR) mutations can be used to confirm the diagnosis of a myeloproliferative disorder in Philadelphia chromosome-negative, JAK2 and MPL wild type patients with thrombocytosis.
Recently, germline mutations in Janus kinase 2 (JAK2) and MPL, two genes frequently mutated in sporadic MPD, have been shown to cause inherited thrombocytosis.
Genotyping for CALR mutations represents a novel useful tool for establishing a clonal myeloproliferative disorder in JAK2 and MPL wt patients with thrombocytosis and may have prognostic and therapeutic relevance.
Refractory anemia with ring sideroblasts (RARS-T) associated with marked thrombocytosis is a myelodysplastic/myeloproliferative neoplasm associated with both SF3B1 and JAK2 or MPL mutations.
Given their diagnostic relevance, it is also beneficial and relatively straightforward to screen JAK2 V617F negative patients for JAK2 exon 12 mutations (in the case of erythrocytosis) or MPL exon 10 mutations (thrombocytosis or myelofibrosis) using appropriate assays.
The ability to routinely assess both JAK2 and MPL mutations would be beneficial in the differential diagnosis of unexplained thrombocytosis or myelofibrosis.
A decrease in expression of the Mpl protein can cause thrombocytosis even in the absence of mutations in the coding sequence, due to a shift in the balance between stimulation of signaling in megakaryopoiesis and removal of thrombopoietin by receptor mediated internalization in platelets.
The recently discovered mutations in patients with CMD (V617F and exon 12 of JAK2 gene, MPL gene), and those identified in hereditary erythrocytosis and in hereditary thrombocytosis have improved our ability to discriminate these conditions.
By contrast, MPL gene mutations were not associated with erythrocytosis, but segregated primarily with the phenotypes of thrombocytosis, extramedullary disease, myelofibrosis, and osteosclerosis.
K39N represents an identified functional Mpl polymorphism and is associated with altered protein expression of Mpl and a clinical phenotype of thrombocytosis.
A unique point mutation, serine 505 to asparagine 505 (Ser505Asn), was identified in the transmembrane domain of the c-MPL gene in all of the 8 members with thrombocythemia, but in none of the other 8 unaffected members in this FET family.