Myeloproliferative neoplasms (MPNs) are clonal hematopoietic disorders that cause over-proliferation of specific myeloid lineages. More than 95% of patients with polycythemia vera, one type of MPN, have a point mutation in the JAK2 gene (JAK2V617F), which generates a constitutively-active JAK2 that hyperactives the JAK/STAT pathway. In Drosophila, JAK/STAT signaling is conserved but simplified, with a single JAK Hopscotch (Hop) and a single STAT (Stat92E). hopTum-l is a dominant, temperature-sensitive allele of hop that hyperactivates the JAK/STAT pathway. Similar to JAK2V617F-bearing MPN patients, hopTum-l mutants have dramatically increased number of myeloid-like cells, which leads to melanotic tumors. The hopTum-l tumor phenotype is dependent on JAK/STAT signaling, and reducing one copy of Stat92E in hopTum-l flies strongly suppresses the tumor burden. Based on this dominant modification of the tumor burden by reduction in Stat92E, we hypothesize that reducing the dose of genes involved in JAK/STAT signaling or in hematopoiesis will enhance or suppress the tumor incidence or burden in hopTum-l flies. Given the conservation of the pathway, our screen may identify genes that modify the hopTum-l phenotype and that have human homologs which may be involved in MPNs.
We conducted a dominant deficiency screen of the right arm of the second chromosome. We tested 90 deficiencies in the Bloomington kit, which together uncovers 98.5% of 2R euchromatin. We have identified 11 enhancer and 8 suppresser deficiencies. A deficiency that uncovers Enhancer of Polycomb [E(Pc)] significantly enhances melanotic tumor burden and larval/pupal lethality in hopTum-l animals. This enhancement was recapitulated by heterozygosity for E(Pc) alleles. To test if E(Pc) has a role in blood development, we depleted E(Pc) from hematopoeitic tissue. E(Pc) depletion promoted differentiation of lamellocytes, which are absent in control animals. Additionally, larval hemolymph bleeds of E(Pc) depleted animals revealed microtumors and small melanotic tumors, that are not present in in control larvae. E(Pc) is a highly conserved protein and regulates transcription through the Polycomb Group (PcG) factors. Recently, E(Pc) also was shown to function as part of the Tip60 complex, a chromatin remodeling complex. To further characterize the E(Pc) phenotype, we are first assessing if heterozygosity for PcG and Tip60 complex genes also enhance the tumor burden in hopTum-l animals. Second, we are hematopoietically depleting PcG or Tip60 complex genes in control larvae to assess if their depletion also leads to blood cell abnormalities. Our current data support the model that E(Pc) represses genes necessary for lamellocyte differentiation.