Driver mutation‐specific clinical and genomic correlates differ between primary and secondary myelofibrosis
Authors: Andrew T. Kuykendall, Chetasi Talati, Eric Padron, Kendra Sweet, Jeffrey E. Lancet, Alan F. List, David Sallman, Rami S. Komrokji
Journal: American Journal of Hematology (2019)

Primary myelofibrosis (PMF) and secondary myelofibrosis (sMF) that has evolved from a prior diagnosis of polycythemia (PV) or essential thrombocythemia (ET) are myeloproliferative neoplasms (MPNs) characterized by the presence of a phenotypic driver mutation in one of three genes: Janus kinase 2 (JAK2), thrombopoietin receptor (MPL), and calreticulin (CALR). More than 90% of patients will exhibit a mutation in one of these three genes, with the remaining patients being deemed to have “triple-negative” (TN) disease.1 Pathogenic mutations in JAK2, MPL, and CALR invariably lead to upregulation of JAK/signal transducer and activator of transcription (STAT) signaling and a heterogeneous clinical phenotype that often features dysfunctional hematopoiesis, constitutional symptoms, extramedullary hematopoiesis, and an increased propensity to develop acute myeloid leukemia (AML).1 The presence of a JAK2, MPL, or CALR mutation supports a diagnosis of PMF or sMF; however, the clinical significance of the specific driver mutation is not fully understood. Prior studies have demonstrated CALR mutations to convey a prognostic advantage, though this appears to be limited to type 1 or type 1-like CALR mutations.2 At this point, a convincing biologic rationale for this favorable profile is lacking, though several studies have suggested the disease biology of MPNs may differ based on driver mutation.3 To evaluate the clinical role of driver mutations in myelofibrosis (MF), we separately analyzed PMF and sMF patients drawn from a large MF database. We stratified patients by driver mutation, and assessed the variability in clinical characteristics, genomic abnormalities, and survival. Inclusion to the current study required a diagnosis of PMF, PPV-MF, or PET-MF based on World Health Organization (WHO) 2016 or International Working Group - Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria, respectively.4 Included patients must have tested positive for a phenotype-driving mutation in JAK2, MPL, or CALR, or have tested negative for mutations in all three genes by either next generation sequencing (NGS) or polymerase chain reaction (PCR). Laboratory values were used from time of first presentation or time of diagnosis if this occurred after first presentation. Screening for high molecular risk (HMR) mutations in ASXL1, SRSF2, U2AF1 Q157, EZH2, IDH1, and IDH2 was performed by NGS testing in a (Clinical Laboratory Improvement Amendments) CLIA-certified laboratory as part of routine clinical care. In cases where multiple NGS assessments were performed on a single patient, the earliest assessment was used in analysis. Overall survival (OS) was defined as time from diagnosis to date of death, or censored at last known follow-up or allogeneic hematopoietic stem cell transplant (AHCT). Patients alive after 15 years of follow-up were censored at that time. Differences in the incidence of categorical variables between cohorts were analyzed by Fisher's exact test. OS curves were analyzed by Kaplan-Meier method and compared by log-rank test. Cox proportional hazard regression model was used for multivariable analysis. P values less than .05 were considered significant. GraphPad Prism v7.04 and SPSS 25 were used for statistical calculations. We identified 436 MF patients, among whom 291 (67%) had PMF and 145 (33%) had sMF. The sMF patients were comprised of 71 (49%) PPV-MF and 74 (51%) PET-MF patients. Full demographic, laboratory, and genomic characteristics of PMF and sMF patients stratified by driver mutation is shown in Table 1. In the PMF cohort, 206 (71%), 20 (7%), and 48 (16%) had mutations in JAK2, MPL, CALR, respectively, with 20 (7%) being TN. Three patients (1%) had mutations in both JAK2 and MPL. Compared to patients with CALR mutations, those harboring JAK2 mutations were more likely to be >65 years old [P = .024; Odds Ratio (OR) 2.2], have constitutional symptoms (P = .001; OR 3.4), have a platelet count <100 × 109/L (P = .006; OR 3.5), feature high-risk mutations in U2AF1 Q157 (P = .045; OR 6.7) and SRSF2 (P = .027; OR 6.7), and have ≥ two high-molecular risk (HMR) mutations (P = .048; OR 3.1). TN patients were more likely to harbor mutations in SRSF2 than patients with a JAK2 mutation (P = .014; OR 3.9). Median OS was estimated to be 60.5, 101.8, not reached, and 30.1 months for the JAK2, MPL, CALR, and TN cohorts, respectively. OS was significantly different between JAK2- and CALR-mutant cohorts (P < .001; HR 15.1), TN and JAK2-mutant cohorts (P = .029; HR 2.04), MPL- and CALR-mutant cohorts (P = .001; HR 8.9) and TN and CALR-mutant cohorts (P < .001; HR 21.8) (Figure S1). In the sMF cohort, 109 (75%), 7 (5%), and 27 (19%) had driving mutations in JAK2, MPL, and CALR, respectively, with 2 (1%) being TN. Compared to patients with CALR-mutant disease, those with JAK2 mutations were more likely to be >65 years old (P = .018; OR 2.9) and have a platelet count <100 × 109/L (P = .045; OR 6.7). There was no significant variability in the incidence of high-risk mutations based on driver mutation, due to a comparatively low incidence of high-risk splicing mutations (ie, SRSF2 and U2AF1 Q157) in JAK2-mutant sMF compared to JAK2-mutant PMF (P = .003; OR 3.3). Median OS was estimated to be 81.8, 20.4, and 161 months for the JAK2, MPL, and CALR cohorts, respectively. Median OS was not estimated for TN patients due the very small number of these patients (n = 2). Median OS was significantly different between JAK2- and CALR-mutant cohorts (P = .049; HR 3.0) and MPL- and CALR-mutant cohorts (P = .009; HR 6.02) (Figure S1). On multivariate analysis, controlling for anemia, thrombocytopenia, circulating blasts, constitutional symptoms and age, mutations involving ASXL1 (P < .001; HR 7.0) and U2AF1 Q157 (P = .003 HR 8.8) were independently associated with an inferior prognosis, while the presence of a CALR mutation was associated with a favorable prognosis (P = .033; HR 0.15). In this study, we validated the favorable prognostic impact of CALR mutations in both PMF and sMF in addition to the inferior prognosis of TN disease in PMF. Additionally, we validated previously described correlations including a decreased incidence of high-risk splicing mutations in CALR-mutant PMF compared to JAK2-mutant PMF.5, 6 This, in conjunction with a more favorable clinical phenotype in CALR-mutant PMF likely contributes to the favorable prognostic profile. Interestingly, we did not demonstrate the same molecular correlations in sMF, largely due to a decreased incidence of splicing mutations in JAK2-mutant sMF. This may explain why the prognostic impact of CALR mutations in sMF is not as marked as in PMF. We would like to acknowledge H. Lee Moffitt Cancer Center for their support and, specifically, the departments of Malignant Hematology and Hematopathology. The corresponding author has received honoraria from Incyte, Celgene, Abbvie and Janssen. This compensation was not, in any way, related to this manuscript and no third party had access or influence on the data contained and analyzed within this study. While the authors believe there is no significant competing interest, these relationships have been disclosed to eliminate any appearance of impropriety. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.