Targeted next generation sequencing of <scp>PDGFRB</scp> rearranged myeloid neoplasms with monocytosis
Authors: Mrinal M. Patnaik, Terra L. Lasho, Christy M. Finke, Animesh Pardanani, Ayalew Tefferi
Journal: American Journal of Hematology (2015)

To the Editor: The 2008 WHO (World Health Organization) criteria define chronic myelomonocytic leukemia (CMML) as a disorder characterized by: (a) persistent peripheral blood (PB) monocytosis > 1 × 109/L, (b) absence of the Philadelphia chromosome and the BCR-ABL1 fusion oncogene, (c) absence of the PDGFRA or PDGFRB gene rearrangements, (d) less than 20% blasts and promonocytes in the PB and bone marrow (BM), and (e) dysplasia involving one or more myeloid lineages 1. The platelet derived growth factor receptors alpha and beta (PDGFRA-chromosome 4q12 and PDGFRB-chromosome 5q31-q32) are type III receptor tyrosine kinases. Chromosomal translocations involving PDGFRA/B have been associated with myeloid neoplasms, often characterized by prominent eosinophilia and responsiveness to imatinib 2-4. At times, PDGFRB rearranged myeloid neoplasms are associated with monocytosis and BM dysplasia (in the absence of prominent eosinophilia), however, given their unique responsiveness to imatinib, these are no longer classified as CMML, but instead as part of a unique clinicopathologic entity as per the 2008 WHO classification 1, 5. PDGFRB rearrangements are encountered in <1% of patients with a morphological diagnosis of CMML, with >20 fusion partner genes having been described in myeloid neoplasms thus far 5-7. In the presence of accompanying eosinophilia, t(5;12)(q31-q32;p13), giving rise to the ETV6(TEL)-PDGFRB fusion oncogene can be encountered 4, 7. Gene mutations are frequent in CMML (>90%) and involve: epigenetic regulators (TET2 ∼60%, ASXL1 ∼40%), spliceosome components (SRSF2 ∼50%), transcription factors (RUNX1 ∼15%), and cell signal pathways (RAS ∼30%, CBL ∼15%) 8. The prevalence of other myeloid-relevant gene mutations in PDGFRB rearranged myeloid neoplasms with monocytosis is currently unknown. We carried out this study to assess this question. Three patients with PDGFRB rearranged myeloid neoplasms with monocytosis were identified from the Mayo Clinic myeloid database (Table 1). All three patients had BM biopsies, cytogenetics and FISH (fluorescence in situ hybridization) studies performed at diagnosis. Two patients had PDGFRB rearrangements, with unknown partner genes, while one had the classical t(5;12)(q31-q32;p13), giving rise to the ETV6(TEL)-PDGFRB fusion oncogene. All three PDGFRB rearrangements were identified on metaphase cytogenetics and confirmed by FISH studies. Targeted capture assays were carried out on BM DNA specimens obtained at diagnosis for the following genes; TET2, DNMT3A, IDH1, IDH2, ASXL1, EZH2, SUZ12, SRSF2, SF3B1, ZRSR2, U2AF1, PTPN11, Tp53, SH2B3, RUNX1, CBL, NRAS, JAK2, CSF3R, FLT3, KIT, CALR, MPL, NPM1, CEBPA, IKZF, and SETBP1. Paired-end indexed libraries were prepared from individual patient DNA using the NEBNext Ultra Library prep protocol on the Agilent Bravo liquid handler. Capture libraries were assembled according to Nimblegen standard library protocol. Base-calling was performed using Illumina's RTA version 1.17.21.3. Genesifter software was utilized to analyze targeted sequence data. Specific variants were deemed as mutations if they were associated with a hematological malignancy (as identified by COSMIC database), or if they were not associated with a dbSNP. All three patients were male and presented with monocytosis and clinical suspicion for CMML. Patient 3, with the ETV6(TEL)-PDGFRB fusion oncogene, was the only one who had absolute eosinophilia. The clinical, laboratory and molecular characteristics of these patients are outlined in Table 1. Unlike in CMML, none of the three patients with PDGFRB rearrangements had mutational abnormalities in the above mentioned 27 gene targeted capture assay. All three patients were treated with imatinib mesylate and had an excellent response, achieving complete cytogenetic remissions. One patient in this group died secondary to refractory clostridium difficile colitis related toxic megacolon. In a recent review of 26 patients with PDGFRB rearranged myeloid neoplasms, after a median 6.6 years of exposure to therapy with imatinib, the 10-year over-all survival was 90% and the 6 year progression-free survival was 88%; highlighting the unique sensitivity of these disorders to imatinib mesylate 4. Our study shows that although PDGFRB rearranged disorders with monocytosis display a significant morphological overlap with CMML, they do not share the same genetic structure as CMML. However, we did not identify additional mutations (Case no. 3) that could potentially explain the eosinophilic phenotype. Overall, these molecular insights confirm that mutant PDGFRB is indeed the driver mutation in PDGFRB rearranged myeloid neoplasms, consistent with the robust treatment responses with imatinib, thereby constituting a unique stem cell disorder that should continue to be classified separately. Mrinal M. Patnaik, Terra L. Lasho, Christy M. Finke, Animesh Pardanani, Ayalew Tefferi* Division of Hematology, Department of Medicine, Mayo Clinic, Rochester, Minnesota