Spliceosome Gene Mutations Are Frequently Found In JAK2 Negative Myelofibrosis and Associated With Worse Clinical Outcomes

Myelofibrosis (MF), a Philadelphia chromosome negative myeloproliferative neoplasm (MPN) requires clonal markers for accurate diagnosis according to the 2008 World Health Organization classification scheme. However, molecular mutations in Janus Kinase 2 (JAK2) are found in only 50% of MF patients (pts). Various somatic mutations identified in other myeloid cancers have also been found in MF with pre-existing JAK2 V617F mutations. We previously reported in 57 pts diagnosed with Triple Negative MF (TN-MF: JAK2 exon12/14 and wild type (WT) MPL) the absence of mutations in genes commonly identified in other myeloid malignancies, specifically TET2, DNMT3A, CBL, IDH1/2, SH3B2 (LNK), N/KRAS and EZH2. Only somatic mutations in ASXL1 were identified in a small cohort of TN-MF (16%). Mutations in components of the RNA splicing machinery have been recently identified in varying frequencies in different myeloid malignancies. We performed RNA-sequencing on 2 samples (JAK2 mutant (MUT) and 1 JAK2 WT) and found that 2 spliceosome genes (U2AF1 and SF3B1) had higher mRNA expression levels in the JAK2 WT (MFC=0.76 and 0.82, respectively) compared to JAK2 MUT patient. We hypothesized that somatic mutations in RNA splicing factor genes occur in TN-MF pts and may be helpful in the biological characterization of this group of pts. Therefore, we isolated DNA from bone marrow (BM) or peripheral blood mononuclear cells from a cohort of MF pts (N=132; JAK2 MUT:75 and JAK2 WT:57) and performed Sanger sequencing for SF3B1 (exons 13-16), U2AF1 (exons 2, 6 and 7), and SRSF2 (exons 1 and 2). Baseline characteristics of pts and clinical data including hematologic parameters, BM results, and presence of splenomegaly by palpation were collected. Pts were stratified based on the Dynamic International Prognostic Scoring System-Plus risk as high=40, Int-2=51, Int-1=11 and low-risk=5 pts. The mean duration of follow-up was 16.5 months in JAK2 MUT and 12.8 months in JAK2 WT. Spliceosome mutations were found in 35/132 (27%) (SRSF2=17%, U2AF1=8% and SF3B1=2%) of pts. Interestingly, the frequency of spliceosome mutations in JAK2 WT was higher (31%; SRSF2=26%, U2AF1= 5%, SF3B1=0) compared to JAK2 MUT (18%; SRSF2=10%, U2AF1=6%, SF3B1=2%) pts. Of note, the 2 pts with SF3B1 mutations (K700E) had ring sideroblast in the BM (occasional and 50%). A large number of WT JAK2 patients (50%) had concomitant SRSF2 and ASXL1 mutations. Both SRSF2 and ASXL1 have been associated with poor prognosis in MF. Indeed, in our JAK2 WT cohort, pts who harbored SRSF2 mutations had a higher mean percentage of BM blasts compared to the ones that were SRSF2 WT (5.6% vs 1.8%; P=.006). Moreover, JAK2 WT pts carrying spliceosome mutations had more severe anemia as shown by lower hemoglobin (8.98 g/dL vs 10.5) and higher leukocyte counts (27.5 x10⁹/L vs 20.5) compared to WT cases. Furthermore, we noticed that they also had higher frequency of RBC (Red Blood Cells) transfusions compared to WT (60% vs 40%) cases. Spleen examination by palpation below the left sub-costal margin showed that JAK2 WT cases carrying spliceosome mutations had larger splenomegaly compared to WT cases [10.1 cm (range 4-26), vs 8.7cm (range 2-20); P=0.05]. In conclusion, molecular alterations in the spliceosome machinery are frequently found in MF pts who are WT for JAK2/MPL and are associated with higher BM blast percentage, more severe anemia, higher leukocyte counts, transfusion dependence and more prominent splenomegaly. The observation of appreciable frequency of spliceosome mutations in JAK2 WT MF pts opens the possibility of using spliceosome inhibitors in the management of this disease group.