EZH2 mutations in chronic myelomonocytic leukemia cluster with ASXL1 mutations and their co-occurrence is prognostically detrimental
Authors: Mrinal M. Patnaik, Rangit Vallapureddy, Terra L. Lasho, Katherine P. Hoversten, Christy M. Finke, Rhett Ketterling, Curtis Hanson, Naseema Gangat, Ayalew Tefferi
Journal: Blood Cancer Journal (2018)

Chronic myelomonocytic leukemia (CMML) is an aggressive hematological malignancy characterized by sustained peripheral blood monocytosis and an inherent risk for leukemic blast transformation 1,2 .Patients with CMML have between 10-15 mutations per kilobase of coding DNA regions; with majority of these (>90%) involving epigenetic regulator genes (TET2 60%, ASXL1 40%), splicing machinery (SRSF2 40%), and cell signaling (oncogenic RAS pathway 30%) 3-5 .The polycomb group proteins play an important role in transcriptional repression by regulating chromatin modifications and consist of two canonical polycomb repressive complexes (PRC) PRC1 and PRC2 6 .PRC2, comprises EED (embryonic ectoderm development protein), EZH2 (enhancer of zeste homolog 2), SUZ12 (suppressor of zeste 12 protein homolog), and RBBP4/7 (retinoblastoma binding protein), is recruited to chromatin and results in the trimethylation of lysine 27 of the histone 3 mark (H3K27me3), a repressive mark silencing gene transcription.In addition, ASXL1 (additional sex combs like 1) has been shown to associate with PRC2 and results in global reductions in H3K27me3, suggesting a regulatory role in the PRC2 7 .ASXL1 mutations (frameshift and nonsense) result in the truncation of the ASXL1 protein, are seen in ~40% of CMML patients, and independently and adversely impact overall survival (OS) 4,8 .EZH2 mutations (chromosome 7q36.1) in CMML, unlike in epithelial malignancies and lymphoproliferative disorders, are loss-of-function mutations and are uncommon (<5%), with an indeterminate prognostic impact 4,9 .Given that the co-occurrence of these mutations have been documented in myeloid neoplasms and that theoretically these could further impact the repressive role of the PRC2, we examined a large and informative CMML data set to assess the prognostic impact of ASXL1 and EZH2 co-mutations in CMML 10 .Patients with 2016 WHO (World Health Organization)defined CMML were identified from the institutional database 1 .Al.(BM) biopsies and cytogenetic studies performed at diagnosis.A 29 gene panel next-generation sequencing assay was carried out on BM DNA specimens on all 277 patients obtained at diagnosis for the following genes: TET2, DNMT3A, IDH1, IDH2, ASXL1, EZH2, SUZ12, SRSF2, SF3B1, ZRSR2, U2AF1, PTPN11, Tp53, SH2B3, RUNX1, CBL, NRAS, KRAS, JAK2, CSF3R, FLT3, KIT, CALR, MPL, NPM1, CEBPA, IKZF, ETNK1, and SETBP1, by previously described methods 5,9 .All statistical analyses considered parameters obtained at time of CMML diagnosis.Differences in the distribution of continuous variables between categories were analyzed by either Mann-Whitney or Kruskal-Wallis tests.Patient groups with nominal variables were compared by χ 2 test.OS was calculated from the date of first referral to date of death or last contact.Leukemia-free survival (LFS) was calculated from the date of first referral to date of leukemic transformation or death/last contact.Overall and LFS curves were prepared by the Kaplan-Meier method and compared by the log-rank test.Cox proportional hazard regression model was used for multivariable analysis.