Presence of JAK2 Mutations in MDS/MPD-u WHO Classified Patients and Not Other Forms of MDS Suggests Their Derivation from Classical Myeloproliferative Syndrome.
Authors: Szpurka Hadrian , Tiu Ramon , Hsi Eric , Lichtin Alan E. , Sekeres Mikkael A. , Theil Karl S. , Maciejewski Jaroslaw P. 
Published: April 2020

Abstract
The WHO classification of myeloid neoplasms recognizes a category of myeloid disorders that overlaps traditional myelodysplastic syndromes (MDS) and myeloproliferative disorders (MPD) as the entity MDS/MPD disease. This disease category includes CMML, JMML, atypical CML and MDS/MPD unclassifiable (MDS/MPD-u). The provisional entity termed RARS associated with marked thrombocytosis (RARS-t) is currently best classified as MDS/MPD-u until further information is available regarding its pathogenesis. Recently a JAK2 mutation V617F (G→T) was identified as a pathogenetic lesion in typical myeloproliferative disorders (Kralovics et al, NEJM, 2005). Subsequent studies of the JAK2 mutation concentrated on more proliferative forms of MDS or MDS/MPD such as CMML and CNL (Steensma et al, Blood, 2005). These studies demonstrated that homo- and heterozygous JAK2 mutants are present in a rather small proportion of these patients. Based on similar clinical features, we theorized that JAK2 mutants might be also found in patients with MDS/MPD-u. We have collected a cohort of these patients (N=202) and analyzed them for the presence of JAK2 mutation using a molecular allele-specific PCR assay. Positive cases were confirmed by sequencing with a sensitivity of about 20% of mutated cells. A group of patients with PV, MF and ET served as positive controls (N=66). In agreement with previous reports, the detection rate of JAK2 mutants for PV, MF and ET was 92%, 55% and 55%, respectively. Our experimental group included 104 patients with MDS or MDS/MPD (53 RA/RS, 14 RAEB, 22 RAEB-t/sAML, and 15 CMML). Most significantly, the RA/RS group contained 13 patients with WHO-defined MDS/MPD overlap and 3 with RARS-t (among these patients, 3 were JAK2 mutants; all of them were heterozygous - 18.8%). Within 16 patients with CMML1 or 2 we found only 2 heterozygous for JAK2 mutation (12.5%). The remaining cohort of 73 patients in MDS categories revealed only 1 patient to be heterozygous for JAK2 mutation (RARS, 1.4%). As expected MDS/MPD patients with JAK2 mutation showed various degrees of BM fibrosis, splenomegaly and less pronounced cytopenias. Except for one patient, JAK2 mutants had normal cytogenetics. All had normal MCV and ANC. In 4/5 increased megakaryocytes with/or without atypia was seen. One CMML patient with abnormal cytogenetics showed an unusual translocation t(8;9)(q22;p24). JAK2 is located at 9p24 so it is possible that the JAK2 gene was involved in the translocation generating a novel fusion protein in addition to an activating JAK2 mutant. Our results showed that JAK2 mutations are rarely found in typical cases of MDS or CMML. However, further analysis of JAK2 mutational status in patients with MDS/MPD-u is warranted. The reported detection rate may suggest that the pathogenesis of these entities is more akin to myeloproliferative than myelodysplastic syndromes.