Various Mutations In JAK2 Exon 12 Induce Divergent Bone Marrow Alterations
Authors: Hallas Cora , Schulte Christoph , Heidorn Klaus , Tiemann Markus 
Published: October 2019

Abstract
Abstract 4096
The V617F gain-of function mutation in the Janus kinase 2 (JAK2) gene has been detected in over 95% of patients with polycythemia vera (PV). Additionally, a growing number of different mutations in JAK2 exon 12 has been described in PV patients, comprising deletions, point mutations, and the occasional duplication. In contrast to patients carrying the V617F mutation, patients positive for exon 12 mutations present mainly with erythrocytosis, but with few or no abnormalities regarding thrombopoiesis and granulopoiesis (Scott et al. N Engl J Med. 2007;356:459-68). As part of our diagnostic work we routinely screen patients with persisting haemoglobin levels above 165 g/L (women) and 185 g/L (men) or bone marrow alterations, but a negative test for the JAK2 V617F mutation, for mutations in JAK2 exon 12. To date, RNA from peripheral blood leukocytes of over 400 patients was extracted and JAK2 exon 12 was sequenced. 7 of these patients showed known point mutations and short deletions, however in 4 other patients we found a duplication of 33 bp at the end of exon 12 described before in only 3 patients. In 2 of our patients this duplication turned out to be a faithful in frame duplication F537-F547dup11, in the other 2 patients the mutation was F537-F547dup11 + F547L.Bone marrow histologies of the 7 patients with point mutations or small deletions in JAK2 exon 12 revealed a major increase together with prominent pathological aberrations in erythropoiesis but usually little alterations in the other two myeloid lineages. Only in two cases an additional significant increase in granulopoiesis was detected as asserted by an elevated amount of immature cells. Megakaryocytes of all patients showed only moderate aberrations and only one of the patients presented with thrombocytosis (671 × 109 platelets/L) in the peripheral blood. Alltogether, the histo-pathological picture is in concordance with an isolated erythrocytosis as described for JAK2 exon 12 mutated PV (Scott et al. N Engl J Med. 2007;356:459-68). On the other hand, bone marrow biopsies of all 4 patients carrying the 11 aa duplication in JAK2 exon 12 diplayed a strikingly different picture. Increase in erythropoiesis was slight, but instead an unusually pronounced expansion of granulopoiesis was found, evidenced by a major increase in immature cells and complete destruction of the usual bone marrow structure. An extensive granulopoiesis like this is usually not seen in JAK2 V617F mutated PV either. Megakaryocytes presented with only moderate aberrations, comparable to the picture seen in patients with other JAK2 exon 12 mutations. Interestingly, the extensive increase in granulopoiesis was not reflected in the peripheral blood with only moderate or no elevation of white blood cell count. Instead, 2 of the 4 patients presented with splenomegaly. In the other 2 patients the disease seemed to be in an earlier stage as indicated by less severe pathological alterations.The difference in erythropoiesis between patients carrying JAK2 exon 12 duplications and short deletions or point mutations was again not reflected in haemoglobin levels in the peripheral blood. All patients carrying any JAK2 exon 12 mutation displayed similarly elevated levels. These data indicate that granulocytes and erythrocytes, respectively, are to a large extent retained in the bone marrow or the spleen of the patients.Taken together, the various JAK2 mutations all present with a different histological immage. In JAK2 V617F mutated PV thrombocytosis is usually prominently affected, whereas short deletions and point mutations in JAK2 exon 12 seem to lead to major aberrations in erythropoiesis. Evidence from the few known cases of 33 bp duplications in JAK2 so far indicates that in these cases granulopoiesis may be the predominantly affected lineage. Alltogether, these data hint that the various mutations of JAK2 may lead to different biochemical modifications of JAK2 function in the cell.

Disclosures:
No relevant conflicts of interest to declare.